ist - 1999 – 11806

IST – 1999 – 11806 – NANOLITH FINAL REPORT p. 1 / 5518

NANOLITH

IST - 1999 – 11806

Arrays of microguns for parallel e-beam nanolithography

Final ReportCovering period 1.1.2000-1.6.2003

Report Version:

Report Preparation Date: August 2003

Classification:

Contract Start Date: 1.1.2000 Duration: 3 years + 6 month extension

Project Co-ordinator : Thales R&T

Partners : University of Cambridge, University of Lyon and Fraunhofer ISiT

Project funded by the EuropeanCommunity under the “InformationSociety Technologies” Programme (1998-2002)


TABLE OF CONTENTS

Executive summary

Questionnaire on Nanolith achievements

Work progress overview

WP1 : CNT electron source

WP 1 – 1 : Nanolithography localised growth of individual CNTs

WP 1 – 2 : Field emission properties of CNT arrays

WP 1 – 3 : Field emission from individual CNTs

WP2 : Fabrication and emission properties of CNT based cathodes

WP 2 – 1 : Self aligned fabrication process for CNTs

WP 2 – 2 : CNT based cathode fabricated with 1 µm lithography

WP 2 – 3 : Single CNT based cathode

WP 2 – 4 : Alternative process for self aligned cathodes

WP3 : Design of efficient microguns and fabrication process

WP 3 – 1 : Design of efficient microguns

WP 3 – 2 : Fabrication of 4 CNT microguns on a 10 x 10 mm chip

WP4 : Parallel lithography

T4.1 : Lithographic environment requirements

T4.2 : Active matrix design

T4.3 : Design of the future lithographic equipment

T4.4 : Design of a testvehicle for future lithographic equipment

and estimated performance of the Nanolith approach

WP6 : Parallel lithography

Deliverables table

Deliverable 3 summary sheet



Annexe : Expression of interest NESFEA

p. 3

p. 16

p. 21

p. 21

p. 21

p. 34

p. 39

p. 44

p. 44

p. 45

p. 46

p. 51

p. 56

p. 56

p. 61

p. 65

p. 65

p. 72

p. 99

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p. 112


EXECUTIVE SUMMARY

OBJECTIVES of NANOLITH during the life of the project

The overall objective of the three years Nanolith project is to demonstrate that parallel e-beamLithography using an array of electron microguns driven by an active matrix can comply with thefuture requirements of lithographic mask fabrication, that is a high resolution (10 nm range), highthroughput and low cost technology (the writing head is fully integrated on a single silicon wafer).

The initial Nanolith objective (see Annex 1 of Nanolith) was to demonstrate parallel lithographywith an array of microguns, each delivering 10 pA and independently driven with integrated dosecontrol circuits.

For this purpose, the Nanolith team worked on the design of the electronic writing head, the activepixel array made of CMOS circuits and the array of electron microguns. It appeared, during thisproject, that the fabrication of the array of electron microguns including the array of electronsources, was the most critical part of Nanolith.

1St year : study of different types of electron sources

In 2000, the mechanisms in order to explain the excellent field emission properties from carbonfilms were not clearly established. Three different models were described in the literature :

Local change of the electron affinity

For this purpose, we have studied the effect of low energy electrons and ions on the fieldemission properties of different carbon films

Conductive sp2 channels in an insulating matrix

During the first year, we attempted to engineer emission sites by artificially creating the sp2sites/channels directly. For this purpose, high energy electron and ion beams have been usedin order to transform a predominantly sp3 material (tetrahedral amorphous carbon) into sp2.

Simple geometrical effect (tip effect)

In this case, emission is due to the presence on the surface of nanostructures andnanoclusters which are characterised by a large geometrical enhancement factor. For thispurpose, nanocluster films obtained directly in the deposition system have been studied.

Among the results obtained during this first year (see 1st year report), one can note the fabrication ofsp2 channels with 1 GeV Uranium ions as shown on Figure 1.This figure shows that each ion track lead to the formation of ~ 1.5 nm height and ~ 10 nm diameterprotrusion which corresponds to the position of conductive channels. The conductivity of thesechannels is around 5 orders of magnitude larger than that of the insulating matrix. The exactdiameter of the channels determined by TEM is close to 5 nm.


Figure 1 : Topography (left) and conductivity (right) image obtained with a ta-C film irradiated with1 GeV Uranium ions using a dose of 1. 1010 cm-2

However, it was not possible to obtain field emission from these channels even for fields up to 200V/µm using a pulse mode (see first year report).On the other hand, the nanocluster carbon films showed good field emission properties. These filmscontain a lot of nanoprotusions and field emission can be simply explained by the simplegeometrical effect (tip effect).

Due to the complexity of the study of conductive channels and due to the excellent field emissionproperties of carbon Nanotubes (CNTs) published in the literature, the consortium has decided withthe agreement of our officer R. Compano that CNTs should be studied as an alternative to EPSduring this first year. Figure 2 shows the patterned growth of CNTs.

Figure 2: Patterned growth of carbon nanotubes.

It was decided at the end of this first year that CNTs will be the electron emitters of the Nanolithwriting head. One can note that CNTs are considered as the ultimate tips as they exhibit geometricalheight to radius ratios from 100 to 1000.


2nd and 3rd year : NANOLITH and NANOTUBES

The choice of nanotubes for the electron source implied a complete reorganisation of the work ofeach partner. One can note that the CNT growth temperature is 700°C instead of room temperaturefor the fabrication of conductive channels. Moreover CNT growth process has to be the lastfabrication step as the nanotubes are degraded if further process is performed.

It was agreed that CUED would study the Nanolithographically localised growth of CNTs, UCBLwould design the microguns to obtain by simulation sub 100 nm beam sizes, Thales would study anew fabrication process adapted to CNTs, and ISiT would investigate s high voltage (instead of lowvoltage) CMOS circuits and redesign the electronic writing head.

Recommendations according to the 2nd Review reportFirst concentrate all resources on achieving deliverable 10 : 100 nm 4 pA beam sizes with arrays ofmicroguns. In other terms, demonstrate that a focused beam can be obtained with a CNT basedmicrogun.Second the consortium should get in contact with relevant e-beam manufacturers, in order to maketheir results known. It was also noticed that there is no principle barrier that this e-beammanufacturer can be a start-up-type enterprise.

6 month extension :Due to the parallel study of two types of electron sources during the first year, the consortium askedthe commission for a 6 month extension to demonstrate the Nanolith approach. This was acceptedby the commission.

During the 2nd and 3rd year, the consortium worked on :

- the highly uniform growth of individual nanotubes (WP1)- Fabrication and FE properties of individual CNT based microcathodes (WP2)- an improved microgun design and fabrication process (WP3)- an efficient writing strategy (WP4)- a large dissemination of the project results and on the exploitation of the Nanolith results (WP6)

WP1 – 1 : Nanolithographically localised growth of individual and aligned CNTs

The conditions required for the growth of an individual CNT were determined by varying thecatalyst dot width (w) lithographically from 100nm to 800nm. We found that single nanotubescould be nucleated for patterned catalyst width of 300nm and below. The yield of single nanotubesat catalyst dot width 100nm was ~100%.


Figure 3 : Highly uniform growth of individual nanotubes. Sample tilt 55°.


WP1 – 3 : Field emission properties of individual CNTS

Field electron emission behaviour of individual multiwalled carbon nanotubes (MWNTs), that areelements of a vertically aligned array grown on a Si wafer, were analysed with a scanning anodefield emission microscope (see Figure 4). The electron emission of each MWNT followed theconventional Fowler-Nordheim field emission mechanism after their apexes were freed from theerratic adsorption species using a conditioning process at room temperature. The conditioningprocess led to stable emission currents and reduced their variations ∆I/I to less than 30% betweendifferent MWNTs of the array. This indicates that the 100% yield in the fabrication of the MWNTarray, was obtained also for field emission. This confirms the possibility for using MWNTs in anarray as independent electron sources for the massively parallel microguns.

-54

-52

-50

-48

1,6 10-8

2 10-8

2,4 10-8

2,8 10-8

MWNT 1MWNT 2MWNT 3MWNT 4

ln (

I /

F2 )

1 / F (cm / V)

(ii)

Figure 4 : (i) Scanning FE current distribution over an array of four MWNTs for a VApp= 260 V. (ii)Corresponding F-N plots . (iii) SEM of a 5 µm-spacing MWNT array.


WP 2 : Fabrication and field emission properties of CNT based cathodes

Arrays of CNT based microcathodes have been fabricated using a self aligned process. This processguaranties a perfect alignment between the CNT and the gate aperture. An array of 100 x 100microcathodes exhibit excellent field emission properties i.e. a turn on voltage of 22 Volts and apeak emission current density of 1.4 mA/cm2 at 40V. If all microcathodes emit, the averageemission current per nanotube is 1 nA at 40V. Therefore the current specification of 10 pA is fullycompleted.

Figure 5 : Individual CNT grown in amicrocathode

Figure 6 : Field emission properties of an arrayof 100x100 microcathodes. The average

emission current per microcathode is 1 nA at40V

SAFEM analysis performed on an array of 100x100 microcathodes have shown that stable andreproducible behavior of field emission from these CNT microguns is possible after a conditioningprocess.


WP 3 : Design of efficient microguns and corresponding fabrication process

WP 3 – 1 : Design of efficient microgun

Sub 100 nm beam size resinChromium layer

0.3 µm

φ = 0.5 µm

1 µm

0.5 µm

5 µmSi n+ (0.2 µm)

(0.2 µm)

500 µm

500 µm

200-300 µm

Vgrid~ 50 V

V60-70 V

PAS

Vfocusing~0 V

Vemitter0 - 50 VHV CMOS circuits

Vmask+1-2 kV

PAS

50 µm

Figure 7 : Schematic representation of the writing head showing the different biases

We have studied the effect of the non axial position of the CNT on the focus spot size. A new 3Dmodel has been designed with the possibility to place the nanotube axis at a distance x from theµgun axis. The results show the strong influence of the nanotube position on the focus spot size. Foran aperture angle of +/- 5° , the focus diameter is over the limit of 100 nm as soon as x>0.18 µm.If we take an aperture angle of +/-10 °, we must have x <0.05 µm to be under the 100 nm limit.Therefore all the fabrication processes developed implies a self alignment between the CNT and thegate aperture.

WP 3 – 2 : Corresponding fabrication process

Figure 8 shows a cross section of a microgun before CNT growth showing the different electrodesand insulating layers. One can see the substrate (oxidised Si wafer), the emitter layer (The CNTswill grow on this layer), the extraction grid with a 0.8 µm diameter aperture and the focusingelectrode with a 5 µm diameter aperture. Insulation between the extraction grid and both emitterand focusing electrodes is obtained with the two 1 µm thick SiO 2 films. Figure 9 is an opticalpicture of a 10 x 10 mm chip with four independent microguns. This figure shows the connectionpads of the 4 emitters (and therefore the 4 CNTs) and also the two equivalent connection pads of theextraction grid. The focusing electrode is the 2 mm diameter electrode at the centre of the chip andis connected through the conductive substrate.


insulators

emitter extraction gridwith 0.8 µm aperture

Focusing electrodewith 5 µm aperture

Si substrate

0.3 µm SiO2

1 µm SiO2

1 µm SiO2

0.1 µm TiWSi

0.25 µm Si n+

0.25 µm TiWSi/Si n+

Figure 8 : Cross section of a microgun before CNT growth showing the different electrodes and theinsulating layers.

Emitter 1 Emitter 2

Emitter 4Emitter 3

Testpad

Testpad

Extractiongrid

Focusing electrode

Extractiongrid

Figure 9 : 10 x 10 mm chip with four independent microguns. This figure shows the connectionpads of the 4 emitters (or the 4 CNTs) and those of the extraction grid. The focusing electrode is the2 mm diameter electrode at the centre of the chip. It is connected through the conductive substrate.


WP 4 : Parallel lithography

T4.1 : The lithographic environment requirements

The amount of gas leaving the resist surface as a possible source of microgun contamination hasbeen measured at Fraunhofer-ISiT in two ways :

1. Mass spectrometric investigation of released gas from the resist coated wafers in vacuum,2. Quartz balance measurements of resist outgassing under e-beam irradiation.

As standard DUV resists can also be suitable e-beam resists, we have chosen two chemicallyamplified DUV resists from Shipley and Clariant with different photo acid generators (PAG):

Shipley, UV II HS : Sensitivity for 30 keV electrons 5 µC/cm2

Resolution 100 nm in 1 µm resist at 15 µC/cm2

Clariant, AZ DX 3301 P: good e-beam performance anticipated.

As a reference we have selected the well known e-beam resist PMMA (Poly methyl methacrylate),which is not chemically amplified:

Merck PMMA, Selectilux EB 250 A: Sensitivity for 1.0 keV - 3.0 keV electrons2 - 40 µC/cm2,Resolution below 40 nm.

For the UV II HS resist the strongest outgassing component was C2H5O with a pressure of 4.6 x10-7

mbar at a total pressure of 4x10-6 mbar. This relates to a relatively large area of five 4” wafers at80°C. For the AZ DX 3301 P resist the largest outgassing peak was C3H7 with a partial pressure of1x10-6mbar. For an estimate of the contamination by these gases the sticking coefficient has to beknown.Special mass peaks indicating outgassing of PAGs were not found until mass 300. In earlierinvestigations there was concern , but with the development of long chained PAGs replacing shortchained PAGs, this problem seems to be solved.


T4.2 : Active pixel array and 100% efficient writing head concept

The active pixel ASIC was designed, simulated and fabricated. Two different types of circuits havebeen realised.

Based on electrostatic simulation of the microgun arrangement it was decided that at least a 50Vhigh voltage CMOS process is required for the CNT microgun circuitry. First ASICs werefabricated by MPW (Multi-Project-Wafer) service of X-Fab wafer foundry (Figure 10) The MPWapproach is fully compatible with the hybrid integration concept and the best choice for feasibilityinvestigation.

Figure 10 Microscope picture of the analog circuit with high voltage transistor and integration capacitor and the charge- and current comparator.

Figure 11 Switching behavior of signal Start (Ch1), Delay (Ch2), and signal Logic_out (Ch3)caused by the charge comparator with different Vref_charge values.

high voltagetransistor

analog circuit

capacitorC1

chargecomparatorCOMP1

currentcomparatorCOMP2


For a general test of the microgun ASIC the functioning of the current- and charge-control circuitsfor one pixel was demonstrated firstly. It was the strategy to test the current- and charge- controlcircuits independently by appropriate adjustment of the comparator reference voltages.The functioning of the charge comparator was verified by the course of the Start-, Delay- andLogic-out signals as illustrated in Figure 11. In order to demonstrate the ability of charge controltwo different values for the reference voltage Vref charge were adjusted for two different dose values.The time interval between the Start signal (channel 2, green) and the response of the Logic-outsignal (channel 3, red, blue) gives the measure for the dose calculation under the assumption ofconstant emitter current. In this example the reference voltage was adjusted for an exposure time of2,97µs (red) and 4,76µs (blue), respectively.

The general function of the current-control could be verified with current values below 10nA withintime intervals in the µs-range, as predicted by simulation.The exact value of the emitter current during charge -control cannot be given yet, and have to bevalidated under real CNT emission condition. As already mentioned for the current control we alsoestimate an emitter current <10nA for the charge control test.

The functions of the ASICs were measured in detail by using an optimised set up that showed morestable measurements due to less noise influence and that was automatically measuring by usingIEEE-bus and steering software testpoint on a PC.The working functions of ASIC1 could be verified showing results comparable with thesimulations, allowing the circuits to be used as specified. Measurements of CNTs with ASIC werenot possible, since CNT microguns have not been produced by the partner.


T4.3 Design of a test vehicle for future lithographic equipment

The aim of working theme T 4.3 was planned to be the preparation of a first demonstrator byhybrid integration, as well as the adaptation to an SEM as a tool for later exposure experiments. Thedemonstrator should consist out of 3 components, the CNT-chip, the active pixel electronics and thepost acceleration system PAS. The assembly should be carried out according to the schematic witha total demonstrator size of approx. 10mm x 10mm.. The CNT-wafer gives the reference plane forthe CMOS-chips, being wire connected with the CNT bond pads. The PAS has to be aligned withreference to the CNT-wafer with an accuracy in the µm-range.This hybrid process would allows us to fabricate and test independently the HV-CMOS chips, themicrogun arrays and the PAS system.

CM

OS

circ

uits

connexion between microgunsarray and CMOS circuitsrealised by wire bonding

connexion pitchbetween 2 pads

= 200 µm

2 lines of 16 microgunswith a pitch of 100 µm

PAS

Cross section

Top view

PAS = 2.5 x 8 mm Si substrate with 50 µm diameter holes sticked on 200-300 µm thick pillars

and biased to 60-70 V

200-300 µm thick pillars

10 mm

2.5 mm

1 mm

10 m

m

4 m

m

CM

OS c

ircuits

Figure 12 :The hybrid process: the HV-CMOSchips, the CNT tip and the PAS system.

The combination of the CNT microguns and theactive pixel electronics did not come into existencebecause of the lack of a functional CNT microgunfor this purpose. Therefore ISiT built a separatemicrogun test vehicle based on the e-beam induceddeposition of a tungsten carbon needle inside a freestanding aperture array (Figure 13). Electrical testsare in preparation.

Figure 13 ISiT test vehicle: W-C needleinside a free standing aperture array.

CNT tip

W-C needle


WP6 : DUP and TIP work package

Dissemination of results :

A large dissemination of the results has been performed during the whole life of the project :

24 presentations at different conferences

18 articles

3 patents

Exploitation plan for the Nanolith approach

It appeared to the consortium that the best strategy to exploit the Nanolith approach was to build anintegrated project. So we made an expression of interest named :

NESFEA : Novel Electron Sources for Field Emission Applications (see Annex 1)

The goal was to integrate in a same project the following technological skills : Nanotubetechnology, cathode technology, circuit technology, system technology.

For this purpose, the NESFEA partners were :Large companies : Thales, Philips, FEI,Universities : Cambridge (UK), Lyon (France), Fribourg (Switzerland), Delft (TheNetherthelands),R&D institutes as Fraunhofer ISiT (Germany) and Leti (France),SMEs as Raith (Germany) and Mapper (The Netherthelands).

One of the main goals of this IP was to exploit the Nanolith results due to the size of an IP and dueto the presence of new partners as SMEs : Raith (Manufacturer of Ebeam systems for Lithography),Mapper (Parallel e-beam lithography using a different approach) and large companies as FEI (e-beam based equipment).

Unfortunately, according to different officers, NESFEA did not fit the call NMP nor IST and no IPwas submitted.

It should be noticed that the Nanolith results have lead to the project Canvad concerning the use ofNanotubes for Microwave applications and should lead to a new project about Nanofocus X raysources.

We plan to build a second Nanolith project with the SME Raith. In fact Raith is able to integrate theNanolith writing head into one of their e-beam systems


QUESTIONNAIREON

NANOLITH ACHIEVEMENTS

This questionnaire is designed to gather information on project’s achievements in order to set up animpact assessment study on FET funded projects.

The questionnaire is divided into 5 sections:

1 - Scientific and technological achievements2 – Impact on science and technology (i.e.: publications)3 – Impact on innovation and micro-economy (i.e.: patents, starts-ups…)4 – Other effects (i.e.: attendance to conferences, training …)5 – Information on project’s participants.


1. Scientific and technological achievements of the project (Breakthrough)

2. Impact on S/T

2.1 Publications from one partner

2.2 Collaborative publications

1. K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, W.I. Milne, D.G. Hasko, G. Pirio, P.Legagneux, F. Wyczisk, and D. Pribat, “Uniform patterned growth of carbon nanotubeswithout surface carbon”, Applied Physics Letters 79, 1534 (2001).

2. K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, W.I. Milne, G. Pirio, P. Legagneux, F.Wycisk, and D. Pribat, “Preferential growth of carbon nanotubes/nanofibers usinglithographically patterned catalysts”, Materials Research Society Proceedings 675, W9.1(2001).

3. G. Pirio, P. Legagneux, D. Pribat, K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, and W.I.Milne, “Fabrication and electrical characteristics of a carbon nanotube field emissionmicrocathode with an integrated gate electrode”, Nanotechnology 13, 1 (2001).

4. W.I. Milne, K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, J. Yuan, J. Robertson, P.Legagneux, G. Pirio, D. Pribat, K. Bouzehouane, W. Bruenger and C. Trautmann, “Carbonfilms for use as the electron source in a parallel e-beam lithography system”, New Diamondand Frontier Carbon Technology 11, 235 (2001).

5. W.I. Milne, K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, J. Yuan, J. Robertson, P.Legagneux, G. Pirio, D. Pribat, K. Bouzehouane, W. Bruenger and C. Trautmann,“Investigating carbon materials for use as the electron emission source in a parallel electron-beam lithography system”, Current Applied Physics 1, 317 (2001).

6. J. Yuan, K.B.K. Teo, W. I. Milne, J. Robertson, P. Legagneux, G. Pirio, D. Pribat, K.Bouzehouane, W. Bruenger and C. Trautmann, “Fabrication and analysis of conductivechannels in diamond-like amorphous carbon films”, Electron Microscopy and Analysis 2001(IOP conference proceedings), 287 (2001).

7. K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, W.I. Milne, G. Pirio, P. Legagneux, F.Wyczisk, J. Olivier, and D. Pribat, “Characterisation of plasma enhanced chemical vapourdeposition carbon nanotubes by Auger electron spectroscopy”, Journal of Vacuum Scienceand Technology B 20, 116 (2002).

8. K.B.K. Teo, G. Pirio, P. Legagneux, F. Wyczisk, M. Chhowalla, D. G. Hasko, H. Ahmed, D.Pribat, G. A. J. Amaratunga and W. I. Milne, “Field emission from dense, sparse andpatterned Arrays of Carbon Nanotubes”, Applied Physics Letters 80, 2011 (2002).

9. K.B.K. Teo, G. Pirio, S.B. Lee, M. Chhowalla, P. Legagneux, Y. Nedellec, D.G. Hasko, D.Pribat, H. Ahmed, G.A.J. Amaratunga, and W.I. Milne, “Plasma enhanced chemical vapourdeposited carbon nanotubes for field emission Applications”, Materials Research SocietySymposium Proceedings 706, Z5.9 (2002).


10. W.I. Milne, K.B.K. Teo, M. Chhowalla, P. Legagneux, G. Pirio, and D. Pribat, “Electronemission from self aligned carbon nanotube field emission microcathodes”, Society forInformation Displays Technical Digest 33, 1120 (2002).

11. W.I. Milne, K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, D. Pribat, P. Legagneux, G.Pirio, Vu Thien Binh, and V. Semet, “Electron emission from arrays of carbonnanotubes/nanofibers”, Current Applied Physics 2, 509 (2002).

12. V. Semet, Vu Thien Binh, P. Vincent, D. Guillot, K.B.K. Teo, M. Chhowalla, G. A. J.Amaratunga, W. I. Milne, P. Legagneux, and D. Pribat, “Field electron emission fromindividual carbon nanotubes of a vertically aligned array”, Applied Physics Letters 81, 343(2002).

13. P. Legagneux, G. Pirio, E. Balossier, J.-P. Schnell, D. Pribat, K.B.K. Teo, M. Chhowalla,D.G. Hasko, G.A.J. Amaratunga, W.I. Milne, V. Semet, Vu Thien Binh, W. H. Bruenger, H.Hanssen, and D. Friedrich, “A revisited concept for parallel e-beam lithography”, PhantomsNewsletter, issue 5, March 2002.

14. K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, W.I. Milne, P. Legagneux, G. Pirio, E.Balossier, D. Pribat, V. Semet, Vu Thien Binh, W.H. Bruenger, J. Eichholz, H. Hanssen, D.Friedrich, S.B. Lee, D.G. Hasko, H. Ahmed, “Fabrication and electrical characteristics ofcarbon nanotubes based microcathodes for use in a parallel electron beam lithographysystem”, Journal of Vacuum Science and Technology B 21, 693 (2003).

15. K.B.K. Teo, S.–B. Lee, M. Chhowalla, V. Semet, V.T. Binh, O. Groening, M. Castignolles,A. Loiseau, G. Pirio, P. Legagneux, D. Pribat, D.G. Hasko, H. Ahmed, G.A.J. Amaratungaand W.I. Milne, “Plasma enhanced chemical vapour deposition carbon nanotubes/nanofibers– how uniform do they grow?” Nanotechnology 14, 204 (2003).

16. D. Pribat, G. Pirio, P. Legagneux, K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, W.I.Milne, and D.G. Hasko, “Field emission from triode type microcathodes fabricated withvertically aligned carbon nanotubes”, Proceedings of the 2nd International DisplayManufacturing Conference, 435 (2002).

17. W.I. Milne, K.B.K. Teo, S.B. Lansley, M. Chhowalla, G.A.J. Amaratunga, V. Semet, V.T.Binh, G. Pirio and P. Legagneux, “Growth of aligned multiwall carbon nanotubes and theeffect of adsorbates on the field emission properties” AIP Conference proceedings, to bepublished.

18. W.I. Milne, K.B.K. Teo, S.P. Lansley, M. Chhowalla, S.B. Lee, D.G. Hasko, H. Ahmed, O.Groening, P. Legagneux, G. Pirio, L. Gangloff and D. Pribat, “Field Emission Applications ofAligned Carbon Nanotubes/fibers”, Proceedings of the International Display Workshop, 1061(2002).

2.3 Review papers

19. K.B.K. Teo, C. Singh, M. Chhowalla and W.I. Milne, “Catalytic synthesis of carbonnanotubes and nanofibers” Encyclopedia of Nanocience and Nanotechnology, to bepublished.


3. Impact on Innovation and Micro-economy

3.1 Patents

Patent title: Steuerschaltung zum Steuern einer ElektronenemissionsvorrichtungPatent filed: July 03, 2002 (pending, not granted, EU-registration is in progress)

P. Legagneux, G. Pirio, D. Pribat, W.I. Milne, K.B.K. Teo,Procédé de croissance localisée de nanotubes et procédé de fabrication de cathode autoalignéeutilisant le procédé de croissance de nanotubes, N° 01 12222, 20th of September 2001.

P. Legagneux, D. Pribat, Y. Nedellec,Procédé de croissance catalytique de nanotubes ou nanofibres contenant une barrière de diffusion detype alliage NiSi, N° 01 156 47, déposé le 4 Décembre 2001.

3.2 Patents awarded

3.3 Patents sold

3.4 Creation of start-up

3.5 Creation of new department of research

Thales : Creation of a new research team named Advanced analysis and Nanostructures Laboratorywith 5 equivalent full time persons working on nanotube based devices.

3.6 Collaboration/partnership with a company

4. Other effects

4.1 Conferences attended in Europe and worldwide

Cambridge :Diamond Films, 2002, Granada, Sept 2002IWEPNM Kirchberg, March 2003Gordon Conference, Cambridge,July 2003Leverhulme Meeting, Cambridge, June 2002FE 2001, Alicante, Nov 2001.Oral presentation at EuroFE 2001, Alicante, Spain 2001Poster presentation at TNT2001, Segovia, SpainOral presentation at Materialsweek 2001, Munich, Germany

Thales :Oral presentation at IVMC Lyon, France July 2002Oral presentation at EMRS Strasbourg 10-13 June 2003Oral Phantoms Conference, Grenoble, September 2001

4.2 Conferences in USA

Cambridge :SID , Boston May 2002MRS Meeting San Francisco, April 2003


Poster and talk at MRS Spring 2001, San FranciscoTalk at MRS Fall 2001, BostonPoster at NT02, Boston

Thales :Oral presentation at TNT 9-13 September 2002Oral presentation at ECS Philadelphia 12-17 May 2002Invited presentation at MRS San Francisco 22-25 April 2003

4.3 Other countries

Cambridge :International Display Workshop 02, Hiroshima, December 2002QTSM III Seoul, Korea, May 2002.Melbourne, Australia,21-26 July 2002ICMAT 2001, Singapore, July 2-6th 2001.

Thales : Invited presentation at IVMC OSAKA 7-11 July 2003

4.4 Number of PhD students

Cambridge : 2 PhD studentsThales : 3 PhD students

4.5 Media appearances

Electronic times 17/10/01 : Nanotubes for e-beam sourcesElectronic times 5/9/01: Carbon nanotube layout achieves precision levels on silicon substrate

K. Teo, “Home grown nanotubes”, IEE Review 49, 38 (April 2003).G. Amaratunga, IEEE Spectrum, Sep 2003 issue.

4.6 Good pictures : Please see report

4.7 Good video : To be presented by W.I Milne at 2003 IST conference in Milan.

4.8 Outstanding presentation

MRS Graduate Student Gold Award for oral presentation of paper:K.B.K. Teo, G. Pirio, S.B. Lee, M. Chhowalla, P. Legagneux, Y. Nedellec, D.G. Hasko, D. Pribat,H. Ahmed, G.A.J. Amaratunga, and W.I. Milne, “Plasma enhanced chemical vapour depositedcarbon nanotubes for field emission Applications”, Materials Research Society SymposiumProceedings 706, Z5.9 (2002).

4.9 Any spill-over to national programs

Cambridge : Helped us put together the UK Consortium for Carbon Based Electronics

4.10 Any spill-over to another part of EU IST programme

Project IST FET open Canvad : Carbon nanotubes for vacuum devices (Microwave amplifiers)


WORK PROGRESS OVERVIEW

This work progress overview summaries the work of the Nanolith consortium after the first year i.e.after it was decided that CNT will be the electron source of Nanolith.

WP1 : CNT electron source

WP1 – 1 : Nanolithographically localised growth of individual and aligned CNTs

The work here describes the technological requirements in producing vertically aligned carbonnanotubes with great uniformity, on a conductive layer and inside microcathodes.

Deposition of vertically aligned carbon nanotubes

Multiwalled carbon nanotubes were grown by plasma enhanced chemical vapour deposition ofC2H2 and NH3 gases in the presence of Ni catalyst. The Ni catalyst was prepared on a siliconsubstrate covered with a thin diffusion barrier. The diffusion barrier is necessary as it prevents thediffusion of Ni into the silicon substrate to form NiSix (this reaction consumes the catalyst andreduces the yield of carbon nanotubes). Different diffusion barrier materials were studied andoptimised. The substrates were then transferred to the deposition chamber which was evacuated to10-2 Torr using a rotary pump. The substrates were then heated via a tungsten resistive heater to thegrowth temperature of 700°C. Previous work showed that at this temperature the Ni thin film breaksup into nanoclusters. If the size of the patterned Ni dot was sufficiently small, only a singlenanocluster would form from each Ni dot due to surface tension effects – we use this to achieve thegrowth of an individual nanotube (see later). These nanoclusters then catalytically seed the PECVDnanotube growth. The feedstock gas for nanotube growth was acetylene (C2H2), and the etching gasfor reducing surface amorphous carbon was ammonia (NH3). A d.c. plasma discharge wasgenerated by applying a bias of -600V between the heated substrates and a grounded anode abovethe substrates. The bias applied to the substrates created the electric field which aligned thenanotubes during growth (see Figure 14 et Figure 15 comparing thermal CVD to PECVD).

Figure 14 :

Non-aligned, curly nanotubes obtained bythermal CVD.

Figure 15 :

Vertically aligned nanotubes obtained by dc-PECVD


The plasma characteristics of the deposition system are now investigated in detail. Figure 3 plots theI-V characteristics of the cathode whilst maintaining the stage at a constant temperature of 550°Cand 700°C. The glow discharge was started at –350V when current began to be drawn from theplasma power supply. The currents drawn at a substrate temperature of 700°C were higher than thatat 550°C, indicating that the temperature contributes to the degree of ionization in the plasma.

Figure 3: I-V characteristic of the cathode (stage) at 550°C and 700°C

The voltage variation in a typical dc glow discharge plasma is depicted in Figure4(a). The plasmahas potential (Vp) in the glow region. A sheath, extending distance (S), is formed at the cathodeacross which the cathode voltage (-600V) plus Vp is dropped. Thus, there is an electric field in thesheath and it is proposed that this field, which is perpendicular to the cathode (ie. sample stage),aligns the nanotubes during growth.

A cylindrical langmuir probe (0.5mm radius wire, 5mm in exposed length) was inserted into theglow discharge plasma under typical deposition conditions of 40:200sccm C2H2:NH3, -600Vcathode bias and at 700°C. The I-V characteristic of the probe was measured and plotted inFigure(b). A sheath formed around the probe, and thus, in order to estimate the effective collectinginterface area of the probe, the geometrical construction of the inset of Figure 4(b) was used.Approximately, the collection area was assumed to be a cylinder 3x the radius of the probe (ie. r =0.15 cm) and extending twice its length (ie. l = 1 cm) as depicted, giving a total exposed area (A) of(πr2 + 2πrl) = 1cm2.

In the plasma, there are equal densities (n) of electrons and ions, and the electron and ion current (Ie

and I+) are expressed as:

ee cAneI41= and ++ = cAneI

41

(Eqn 1)

where, the particle mean speed e

ee m

kTc

π8

= and +

++ =

mkT

cπ8

, (Eqn 2)

e = 1.6 x 10-19C (electron charge),k = 1.38 x 10-23 J/K (Boltzmann constant),Te and T+ = electron and ion temperature in K, and,me and m+ = electron (9.1 x 10-31 kg) and ion mass.

The equations for Ie and I+ (equation 2.1) represent the flux density of charged particles crossingthrough an area. The ¼ factor appears because only ½ the particles move in the necessary direction,and the other ½ factor results from averaging over the hemisphere the cosine of the angle betweenthe direction of the particle velocity and the normal of the area element. The equations for the


particle mean speed (equation 2.2) are from assuming a Maxwell-Boltzmann distribution in particlespeeds.

Figure 4: (a) Typical voltage variation in a glow discharge plasma, and (b) the I-V characteristics ofa cylindrical wire langmuir probe (inset) placed in the plasma.

If a voltage (V) which is less than the plasma potential (Vp) is applied to the probe, the electronswould experience a retarding potential (Vp – V). Thus, the electron current (Ie) must be modifiedwith an exponential term ie. exp [-e(Vp – V)/kTe] which is the fraction of electrons with sufficientenergy to overcome the retarding potential in the Maxwell-Boltzmann distribution. The net currentto the probe (I) for V < Vp is given by the sum of electron (Ie) and ion currents (I+). But for V ≈ Vp,I+ is negligible as m+ >> me and Te >> T+ and so the probe current can be expressed as:

( )

−−≈

e

pe kT

VVeII exp (Eqn 3)

or ( ) VkTe

kT

eVII

ee

pe +−= lnln (Eqn 4)

The probe’s log I vs V plot (Figure 4(b)) indeed satisfies the equation 4 as a straight line can befitted at low voltages where the current is unsaturated (ie. for V < Vp). Thus, the inverse of the slopeof this line gives the electron temperature (kTe/e) as 1.3V, and Vp and Ie are obtained directly fromthe ‘knee’ of the plot (where the log I vs V deviates from the straight line) as 3.5V and 0.25Arespectively. Using equation 2, the mean electron speed is derived to be ce = 7.6 x 107 cm/sec, andusing equation 1, the electron density is calculated to be n = 8.2 x 1010 /cm3, which are very typical


values for a glow discharge plasma. Also, note that for V>Vp, the electron saturation current actuallyincreases to V1/2 (inset of 4(b)) due to the motion of electrons under the attractive positive potentialof the cylindrical probe, in agreement with plasma probe theory.

The characteristic electron Debye length (λ)of this plasma is now given by:

== 20

nekTeε

λ 0.0029 cm or 29 µm

where ε0 is the permittivity of free space (8.85x10-12 F/m).

And hence the sheath width (S), assuming a typical Child’s Law sheath, is given by:4/3

0232

=

ekTeV

S λ = 2.3x103 µm or 2.3 mm

where V0 is the voltage across the sheath = Vp + 600V = 603.5V.

The electric field in the sheath directly above the cathode or sample surface for a Child’s Lawsheath is given by (4/3) x (603.5V/2300µm) = 0.35 V/µm, with direction pointing downwardstowards the cathode. Hence, the growing nanotubes, which are negatively biased at the cathode,would experience a force opposite to the direction of the field (ie. vertically upwards) at their tipwhich would tend to guide them vertically during growth. The electric field generated here(0.35V/µm) is in good agreement with Bell labs microwave-CVD work which showed a field of0.1V/µm was necessary for CNT alignment, and also work at Stanford which showed that electricfields of 0.13-0.5V/µm were needed to align single-wall nanotubes laterally.

Structure of vertically aligned carbon nanotubes

The transmission electron micrograph of a typical CNT grown by dc PECVD is shown in Figure5(a). The Ni catalyst was observed at the tip of all CN investigated, which suggests that thedeposition and catalyst support (ie. substrate) conditions favour the tip-growth diffusionmechanism. The CNT contained 20-40 well crystallised graphene walls which were parallel to theaxis of the CNT. This suggests that these structures have excellent conductivity along their axis andindeed electrical conductivity measurements on individual PECVD CN revealed that each exhibits aroom temperature bulk resistivity of 10-6-10-5Ωm, comparable to multiwall carbon nanotubes by arcdischarge, and have a maximum current carrying capability of 107-108A/cm2. Graphitic bamboo-like fringes were also observed along the length of the CN, as have been observed by others usingPECVD. At the termination of growth when the catalyst cools down, it becomes covered bydisordered carbon which is expelled due to the reduction in solubility of carbon in the catalyst as thetemperature drops. Note from Figures 5(b) and (c) that the structures produced by this process are infact tubular and mostly hollow, even for extremely large diameter (300nm) CNT.


Figure 5:(a) TEM of PECVD CNT and (b,c) SEM showing the hollow CNT structure.


Development of the Conductive Diffusion Barrier

A variety of diffusion barriers (metals, conductive metal oxides or nitrides, silicides) wasinvestigated to prevent Ni diffusion into the Si substrate or underlying metal electrode in the CNTcathode. Note that using Ni directly on Si or Ni directly on TiW electrodes (as in the cathodedesign) produces no nanotube growth.

The testing methodology involved preparing a Ni/barrier/SiO2 stack and annealing it to 750°C, andthen taking the depth profile of the stack as seen in Figure 6. For a good diffusion barrier, such asTiN in Figure 6(a), the Ni can be seen to remain on top of the TiN after annealing. For a poordiffusion barrier, such as WN in Figure 6(b), it is clear that the Ni has diffused into the WN, andfurther more, some N has been lost from the barrier as the WN is no longer stoichiometric.

Figure 6: Auger elemental composition of (a) Ni on TiN layer and (b) Ni on WN layer afterannealing at 700°C. An Ar ion gun was used to sputter the surface, and hence the sputter time (x-

axis) is related to the depth of the layer under investigation. The C and O are probably due tochamber contaminants from the annealing.

Thus, TiN was chosen as the diffusion barrier in this project. The TiN was produced by rfmagnetron sputtering. Additionally, we found that a 15 nm layer of TiN was an effective diffusiononly if it was patterned appropriately to manage the stress and thermal expansion mismatch of theTiN film and the underlying layer (eg. Si). As seen in Figure 7, large cracks were observed in the


continuous TiN film after nanotube growth, whereas the patterned TiN film did not suffer from thisphenomenon. Thus, when the microcathodes are fabricated, the TiN is patterned at the same time asthe Ni catalyst.

Figure 7: (a) Cracking of the continuous TiN film after nanotube growth at 700°C. (b) By patterningthe TiN film with holes or into islands, stress is released and the cracks are eliminated.

Study of Ni silicides as diffusion barriers for Ni (Thales and Cambridge patent)

Ni silicides have also been studied as an alternative to TiN.Catalysts such as Nickel react with almost every conductive material. For example Ni reacts withthe Si substrates that we usually use and forms silicides (NiSi, NiSi2, …).Rather than avoiding the reaction between the Ni film and the Si substrate, we have formed Nisilicides which are stable at a temperature larger than 750°C and therefore stable at the CNT growthtemperature i.e. 700°C. The main idea is that subsequent Ni film deposited on the silicide will notinteract with it and will induce the formation of the Ni clusters necessary for CNT growth.

This relatively complex study is presented in the Second Year Nanolith report.

As a conclusion, we have determined conditions which lead to the formation of a continuous Nisilicide film (see Figure 16 and Figure 17). However, further experiments are needed to improve thehomogeneity and the roughness of these films. One can note that NiSi is characterised by a goodconductivity as films of ~1 Ohms per square have been obtained.


Figure 16 :

SEM picture of the sample surface after anneal at750°C during 5 min of a stack of :

30 nm Ni / 1.5 nm Pt / 56 nm Si / oxidised Sisubstrate

Figure 17 :

Same sample, points 1 and 2 correspond to Nisilicides areas. The difference in grey level

between 1 and 2 can be due to differentcompositions in the Ni silicide

Growth of Highly Uniform Individual Carbon Nanotubes

When micron-sized patterned areas of thin film Ni catalyst are used, closely packed multiplenanotubes are nucleated because the catalyst thin film forms multiple clusters at the growthtemperature of 700°C as shown in Figure 8. If the width of the catalyst (w) is reduced below acritical value, a single nanocluster would form due to surface tension effects and this leads to thedesirable nucleation of a single nanotube as shown in Figure 8.

Figure 8 : Multiple and single CNT growth by PECVD

In the following experiment, the influence of the patterned width of the catalyst (w) was studied byvarying w from 100nm to 800nm. Arrays of square Ni dots, 5µm in pitch, were patterned usingelectron beam lithography. The Ni film thickness was fixed at 7nm. The nanotubes were grownunder typical conditions of C2H2:NH3 flow of 40:200sccm, 700°C substrate temperature and –600Vsubstrate bias. Figure 9 (low magnification) and Figure 10 (high magnification) show the carbonnanotubes which were nucleated on these Ni dot arrays ranging from 100 - 800nm in size. It isevident that for Ni catalyst dots with sizes of 300nm and below, significant numbers of singlenanotubes per dot were nucleated. There were no instances when a catalyst dot did not nucleate atleast one nanotube. The height of the nanotubes was ~5.5µm and this is controlled by the depositiontime. The full cone angle at the nanotubes’ apex were 2-4° indicating that the nanotubes werewhisker-like in shape. At 100nm dot size, 100% yield of single nanotubes is obtained and this yieldreduces to 88% at width 300nm (see Figure 11)


Figure 9 : Nanotubes obtained with catalystdot sizes of 100-800nm. Scale bar is 5µm, tilt

45°.

Figure 10 : High magnification image, scalebar is 400 nm, tilt 40°.

Figure 11 : Yield of number of CNT as thecatalyst size is varied from 100-800nm.


Next, the tip diameter and height distribution of the arrays of individual nanotubes wereinvestigated. The diameters of single nanotubes, which were produced using 100nm to 300nm sizedcatalyst dots, were measured (altogether ~150 nanotubes) and the histograms showing theirdistributions are shown in 12(a)-(c). For a particular dot size, a tight distribution in the nanotubediameters was observed and the standard deviation was ~4% of the average nanotube diameter. Theaverage diameter was 49nm, 81nm and 90nm for the 100nm, 200nm and 300nm patterned Ni dotsrespectively. 12(d) shows the distribution in diameters (60 nanotubes measured) from a 100nmpatterned catalyst line which nucleated multiple nanotubes. In this case, due to the ‘random’coalescence of the catalyst, the standard deviation was much larger at 18% of the average nanotubediameter.

Figure 12: Distribution in nanotube diameters from (a) 100nm, (b) 200nm and (c) 300nm catalystdot size. The distribution from multiple nanotubes nucleated from a 100nm wide catalyst line is

provided in (d).

Note that in all cases the observed diameter of the nanotube was actually smaller than the catalystdot size. This is because the catalyst forms a nanocluster of equal volume during growth. If it isassumed that the catalyst becomes a spherical nanocluster, the expected diameters of each nanotubeafter growth could be calculated by equating the volume of the patterned catalyst (eg. for 100nm‘square’ dot, 7nm thick = 100x100x7nm3) with the volume of a sphere (ie. 4/3xπx(diameter/2)3)due to conservation of the catalyst. The calculated nanotube diameters are thus 53nm, 85nm and111nm for 100nm, 200nm and 300nm patterned catalyst dots. The observed diameters for the100nm patterned dots (49nm) and 200nm patterned dots (81nm) do in fact correspond well to thecalculated diameters. The observed diameters are actually slightly smaller than the calculateddiameters because the nanocluster tends to elongate to form a droplet rather than maintain itsspherical shape. For the ‘large volume’ 300nm patterned catalyst, the catalyst clusters formedaccentuated droplets which were 1.5x-2x longer than their diameter (see the large diameter


nanotubes of Figure 10), thus producing smaller diameter nanotubes (90nm) than predicted by the‘spherical’ nanocluster assumption (111nm).

The distribution in nanotube heights (~150 nanotubes observed in total) for the single nanotubesnucleated from 100nm to 300nm catalyst dots are shown in Figure 13(a)-(c). A relatively tightdistribution was observed and the standard deviation was ~6% of the average height (~5.8µm). Forcomparison, the height distribution of 60 nanotubes deposited from a 100nm catalyst line, ie.multiple nanotube nucleation case, is shown in Figure 13(d)-(e). Note that the distribution issignificantly wider (standard deviation 18% of average) in this case because of the ‘random’ naturein which the patterned catalyst line broke up to form multiple nanoclusters.

Figure 13: Distribution in heights for nanotubes grown from (a) 100nm, (b) 200nm, (c) 300nmcatalyst dots and (d,e) multiple nanotubes from a 100nm wide catalyst line.


Figure 14: Highly uniform growth of individual nanotubes. Sample tilt 55°.


Growth of CNT inside Microcathodes

Due to the dc-PECVD process, a problem in the fabrication of the microcathodes was the chargingof the various electrodes during PECVD. Note that the dc-PECVD technique for the growth ofvertically aligned nanotubes required the sample (ie. microcathode) to be biased at –600V. Thiscaused the floating electrodes of the microcathode to charge up in the plasma and destructivelybreakdown in an explosive arc discharge as shown in Figure 15(a).

Figure 15: (a) A destructive arc occurred due to the charging of the floating electrodes in the

microcathode. (b) The solution was this modified chip layout in which the gate and emitter wereshorted with a fuse. The microcathode was connected to the substrate using Ti/Pt metallisation.

Thus, the chip layout was modified with two extra connections in order to prevent electrodecharging during growth, namely a gate to emitter fuse and metallisation (Ti/Pt) to connect themicrocathode to the substrate as shown in Figure 15(b). After nanotube growth, the microcathodewas electrically isolated from the substrate by cutting the metallisation with a scratch. The gate toemitter fuse could be electrically severed during field emission testing by applying a voltage (eg. inthe form of a pulse) from the gate to emitter electrode.

Conclusions for WP1-1

• Deposition by DC-PECVD produces a 0.35V/µm electric field perpendicular to the substratesurface and this is believed to be needed for the alignment of the CNT structures.

• The CNT produce have 20-40 walls parallel to the substrate and are mainly hollow with abamboo-like structure.

• TiN is a good diffusion barrier for both Si and metal provided that the TiN layer is patterned toreduce stress due to the high temperature growth of CNT.

• CNT produced have good uniformity in terms of diameter and height.

• Additional fuses/metallisation in the design of the microcathode make it possible to directlygrow CNT inside devices.


WP1 – 2 : Field emission properties of CNT arrays

FE properties of CNT arrays in a DIODE configuration (Cambridge)

A patterned array of CNTs was fabricated on a 5 nm thick SiO 2 on doped silicon. The SEM imageof the array and the corresponding field emission image are shown in Figure 18. A total fieldemission current of 1 mA is obtained at 14 V/µm. This corresponds to an average current of 14 nAper CNT.

Figure 18 : CNT array SEM picture (left) and FE image of the patterned CNT array

Figure 19 : Cambridge FE measurement of the above array consisting of 7x104 CNTs.


FE properties of 100 x 100 CNT arrays in a TRIODE configuration (Thales)

The samples used for these experiments consisted of 100x100 arrays of individual multi-wallednanotubes (5µm tall and 60nm in diameter) at spacing of 10 µm prepared on a highly n-dopedsilicon substrate as shown in Figure 18. Field emission experiments were performed on this samplein a UHV measurement chamber evacuated to 10-10 mbar. A triode configuration was used as shownin Figure 20. An n-doped silicon micromachined grid, displaced at 100µm from the substrate,extracted the electrons which were collimated and filtered for low voltage secondary electronsbefore being collected by the anode. The extraction field and anode current were measuredautomatically by a PC acquisition system. The pressure of the chamber was measured concurrentlyduring the field emission measurements. The extraction grid (transparency ~30%) ensures a goodvacuum in the measurement cell. At the same time, a quadrapole mass spectrometer was used tomonitor the yield of the different outgased species from the sample. A background spectra of theoutgased species (mostly water) was recorded at the beginning of the experiment and this wassubsequently removed from the spectra obtained during field emission measurements. Thesensitivity of the instruments used allowed us to record partial pressures of gases at ~10-13 mbar.We have also performed measurements with other types of emitters (namely tetrahedral amorphouscarbon and spindt Mo tips on silicon substrates) and these do not yield significant outgassing resultscompared with nanotube emitters.

Figure 20 :Schematic view of the set up for the field

emission measurements of the hybridcathodes

R

Phosphor

Filter Grid

Cathode

+

-

-

-

+

+Ve

pA

Vf

Va

UHV Chamber

IEEE Interface

DC or pulse generator

Aquisition program

pA

PC E

J

Figure 21 shows the field emission characteristics of the as-deposited array of carbon nanotubes.The current-voltage behaviour can be fitted to the Fowler-Nordheim theory at low fields. However,above 10V/µm, there is significant deviation from the expected Fowler Nordheim curve whichpredicts more than a magnitude more current at the higher applied fields. Increasing the appliedelectric field beyond ~15 V/µm led to outgasing which was time and current dependent. Forexample, if the field is quickly increased to a constant value of 20 V/µm, the pressure rises to ~10-

9 mbar during for 5 s before it decreases to the base pressure. During this outgasing period, theemission current exhibited a lot of fluctuation (+/-20% variation). However, after the outgasing wascompleted, stable emission current (+/-3% variation) was observed at constant field. This leads usto believe that the adsorbed species on/in the nanotube cause current instabilities. On a separate, as-deposited sample, a reverse field of 20 V/µm was applied to the substrate to examine if theoutgassing was field-induced. No outgassing nor emission current was observed under reverse fieldconditions. Thus, outgassing only occurs when emission current is observed.


Figure 21 :

Field emission of 100 x 100 CNTs array

4 6 8 1 0 1 2 1 41E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

Em

itte

d c

urr

en

t (A

)

App l ied f i e ld (V /µm)

M e a s u r e d c u r r e n t o f a r r a y Fowler -Nordhe im law ( f i t a t low f ie ld )

Figure 22 shows how the field emission characteristics of another as-deposited nanotube array arealtered during a slow (rise rate of ~0.5 V/µm per second) upward and downward electric fieldsweep. During the upward sweep, outgasing was observed when the field was above ~15 V/µmwhich produced an emission current of ~10-7 A. The total pressure rose from 1.35x10-10 mbar to3.0x10-10 mbar and remained nearly constant (+/-3%) for the rest of the upward electric field sweep.The slow electric field ramp allowed us to measure the spectra of the ougasing species. Theoutgassed species contained mainly hydrogen (more than 95 %) with traces of N2, CO2, CO andH2O as shown in Figure 22. We are unable to resolve the differences between N2 and CO as theyshare the same 28 amu and 14 amu relative peak heights. However, we believe both species arepresent because N2 is a possible product from the deposition process and CO is well knownadsorbant from the samples exposed to the atmosphere. Similarly, we believe that the H2O and CO2

detected were due to air-exposure of the sample. The H2 and N2 were probably produced during thegrowth which involved the dissociation of C2H2 and NH3 in the deposition plasma. After reachingthe maximum field of ~24V/µm, a downward sweep was performed and the turn-on field wasobserved to have increased to ~10 V/µm.

outgasing Emission-induced outgasingAbove 15 V/µmMainly hydrogen (90 %)Other species: CO2, CO, H2O, N2

Modification of the current-fieldbehaviour

Figure 22 : Emission induced outgasing from the CNTs


A strong 30 minute emission stress at a constant electric field of ~28 V/µm was then used to driveout all the remaining gaseous species in the nanotube. The new emission current characteristicswere then obtained and these new emission characteristics followed Fowler-Nordheim behaviourover a large field range (see Figure 23). This is in contrast to the as-deposited curve which deviatedfrom Fowler Nordheim at high field and saturated. The new emission behaviour was reproduciblefor repetitive field sweeps. Note that the turn-on field has actually increased to 9V/µm afteroutgassing. This indicates that the adsorbed species are responsible for an apparent reduction in theturn on field but limits the emission current at high field. We can imagine these adsorbates ascontaminants which probably cause an increase in field enhancement at the tips of the nanotubes,but ultimately reduce the current carrying capability of the nanotubes due to their high resistancecompared with that of the nanotube1. SEM pictures did not show any modification of the substratesurface or of the nanotubes after the experiment.

Figure 23 : Stress process for the achievement of a stable and reproducible Fowler-Nordeimbehaviour

It is possible to roughly estimate the number of molecules outgased during the upward sweep offigure 4, knowing the pumping speed in the chamber (~100 L/s), the hydrogen partial pressure(1.65x10-10 mbar) and the time during which outgasing occurred (roughly 20 s to go from 15 to24 V/µm in the upward sweep). We estimate that 8x1012 molecules were desorbed during this time.As there were 10,000 nanotubes in the sample, a rough estimate is that each nanotube contained8x108 molecules of adsorbates. This is a large number of molecules compared with the surface areaof the nanotubes (9.4x10-9 cm2 for a 5 µm tall, 60 nm diameter nanotube). This is consistent withthe high ability of carbon nanotubes for hydrogen storage. Further than that, it has been shown2 thatthe hydrogen could be stored inside the nanotubes. As the gaseous species were introduced during

1 Seung-Beck Lee, K.B.K. Teo, M. Chhowalla, D. G. Hasko, G.A.J. Amaratunga, W.I. Milne and H. Ahmed,Microelectronics Engineering, submitted

2 Yan Chen, David Shaw, X. D. Bai, E. G. Wang, C. Lund, W. M. Lu and D. D. L. Chung, Appl. Phys. Lett. 78, 2128(2001)


the growth, it is likely that hydrogen could be inside the nanotube. The current flowing through thenanotubes during field emission drives out this hydrogen, an effect which is readily seen forhydrogenated semi-conductors such as our previous experiments in a-C:H materials have shown.

In a final experiment to determine whether surface hydrogen or hydrogen inside the nanotube wasresponsible for the emission current-saturation behaviour, an outgassed sample was exposed to apartial pressure of hydrogen of 10-6 mbar. This causes hydrogen to be adsorbed on the surface of theoutgassed nanotubes. However, the emission characteristics were not affected by the introduction ofhydrogen into the chamber. Thus, the hydrogen inside the nanotube due to the deposition was themain cause of the current saturation observed from as-deposited samples. One explanation is thathydrogen limits the current transport across the nanotube. The emission stress process describedabove removes a significant amount of hydrogen as observed, thus enabling a larger current to flowacross the nanotube without limitation. The structural modification of the nanotube induced by theoutgasing process is currently under investigation gain a better understanding of how the currentcarrying capability was improved. In addition, we will also investigate the reversibility of theobserved changes after exposition to high hydrogen pressure, since it has been shown to store alarge amount of hydrogen molecules on the nanotube surface.

In conclusion, we observed current saturation in field emission from as-deposited arrays of multi-walled carbon nanotubes. High emission current stress at room temperature produced an outgasingof species containing mainly hydrogen. Stable Fowler Nordheim type emission characteristicswithout current saturation was then observed after all the outgassing occurred. This work shows theimportance of performing an outgassing step, in this case a simple emission current stress at roomtemperature in order to have stable, reproducible electron emission characteristics.


WP1 – 3 : Field emission from individual CNTS of a vertically aligned array (UCBL)

The aim of this work, done in Lyon after having received the carbon nanotube array in October2001, was to assess the suitability of mutiwalled carbon nanotubes (MWNTs) arrays, grown byplasma-enhanced chemical vapour deposition (PECVD) at Cambridge, as field emission (FE) coldcathodes for an “on-chip” arrayed microguns As each of these microguns are to be used asindependent electron columns, we need to answer the two following questions:(1) Can the emission characteristics from a single MWNT be made stable and reproducible.Answering this allows us to determine what is required to obtain reproducible emission behaviorfrom a single MWNT emitter.(2) What is the variation of the emission current between different MWNTs within a verticallyaligned array of nanotubes obtained from the same fabrication process ? The adoption of MWNTsas electron sources for on-chip arrayed microguns is only possible if the variation between nanotubeemitters is small and the nanotube emitters can be fabricated with a yield near 100%.Electron emission studies of carbon nanotubes, in particular for those deposited on a planar surfaceencounter the two following main difficulties, compared to conventional metallic tips in FE studies.Namely, it is not possible to clean the surface by controlled thermal treatments at high temperaturesand it is difficult to determine the exact geometry of the actual emitter during the emission unlessthe emission is from an isolated, individual nanotube. In experiments in which the nanotubes caninteract with each other (eg. in a ‘forest’ of nanotubes), there is a great uncertainty in the exactdetermination of the geometrical β factor which converts the applied voltage Vapp to the local fieldat the apex, Flocal = β × Vapp, which acts in the tunneling process to extract the electrons from thesurface. Because of this uncertainty, it is very difficult to find a correlation or to even agree upon acommon emission mechanism using the experimental data in the literature for field emission from‘forests’ of nanotubes.

In this work, scanning anode field emission microscope (SAFEM) (Figure 24 and Figure 25(i))analyses were performed on individual vertically aligned MWNTs of an array grown on a Si waferby a dc PECVD at ~700°C using a C2H2 and NH3 gas mixture with Ni catalyst. The growth processis described in detail by the partner CUED. In order to achieve the growth of an individualnanotube, high resolution electron beam lithography was used to pattern the Ni catalyst . Thevoltage drop in the plasma sheath during PECVD generates an electric field perpendicular to thesurface, and this causes the nanotubes to align vertically on the substrate during growth. The sampleused for this study was an array of 40x40 individual MWNTs (~5µm tall and ~60nm in diameter) ata spacing of 100 µm. Figure 25(ii) shows a similar array with smaller spacing (10µm). The value of100 µm for the spacing is chosen to ensure that individual nanotubes were probed during SAFEManalyses. The diameter of the scanning Pt-Ir probe ball was ≈ 200 µm and it was attached to a 5-degree liberty (X,Y,Z,θ,φ) piezo-driven mechanical displacement system with a resolution step of 1nm. The working pressure of 10-8 to 10-9 Torr was obtained without baking the analysis chamber.FE measurements were performed under continuous bias conditions.

Here, we used a methodology which coupled experimental measurements (IFE, VApp and distancesbetween MWNT and the probe-ball) with systematic numerical simulations to determine Flocal. Theactual FE distance was measured by a controlled retraction of the probe-ball after an initial non-destructive contact between the probe-ball and the MWNT apex. Flocal at each distance wasdetermined using numerical simulations based on electron optics with the following assumptionsconcerning the MWNTs: they are cylinders with hemispherical cap, 5µm in height, 60 nm indiameter and perpendicular to the plane substrate. The exact geometry of the probe ball used in thesimulations was obtained by observing the probe with an optical microscope. Using this method, wecan convert the current-voltage (I-Vapp) measurements into current-local field (I-Flocal) data, rule outthe uncertainties in β , and determine the work function φ of the nanotube emitting surface.


Figure 24 : Experimental set-up of the Scanning Anode Field Emission Microscope (SAFEM)head ; showing the probe-ball in front of the Si wafer (1cm x 1cm).

Figure 25 : (i) Schematic drawing of the SAFEM set up. (ii) SEM observation of a verticallyaligned array of MWNT (≈5 µm height) with a spacing of 10 µm. In this photo the spacing is 10 µm

in order to have on the same photo the array and the MWNTs (CUED array).

The emission characteristics which follow are common to more than thirty different individualMWNTs distributed over the array. Each MWNT required a ‘cleaning’/’conditioning’ procedurebased on FE in order to obtain reproducible emission characteristics. Figure 26a are examples of (I-Vapp) measurements obtained before and after such a conditioning process on the same MWNT.

After the conditioning process the FE characteristics are:(1) Flocal ≈ 3000 V/µm for IFE = 1 pA.(2) The (I-Vapp) measurements followed strictly the conventional Fowler-Nordheim (F-N) equation,i.e. plotting ln(I/Vapp

2) vs. 1/Vapp resulted in a straight line ( Figure 26a plot 3).(3) After conversion of (I-Vapp) into (I-Flocal), φ was determined from F-N plot giving a value of φ ≈4 eV [Work function values measured on diamond (thermionic emission experiments of W.A.Mackie, J.E. Plumlee, A.E. Bell, J. Vac. Sci. Technol. B14, 2041 (1996)) and on ta-C films (Kelvin-probe experiments of A. Ilie, A. Hart, A.J. Flewitt, J. Robertson and W.I. Milne, J. Appl. Phys. 88,6002 (2000)) are in the range of 4 to 5 eV]..


(4) For IFE ≤ few µA, the emissions were very stable, with fluctuations ∆I/I ≤ 10 % (Figure 26b).This means that the adsorption on the MWNT apex during the emission was negligible even withina vacuum between 10-8 to 10-9 Torr.(5) After a long period without emission in vacuum, the FE characteristics of the MWNTs changed,but a recovery to the same F-N plot was obtained after a conditioning process. This means that theconditioning process generally led to the same status of the surface at the apex.

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5 10 -3 6 10 -3 7 10 -3 8 10 -3 9 10-3 10 10-3

ln (

I /

V2 )

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plot 1before seasoning

plot 2

plot 3after seasoning

(work function = 4.26 eV)

(a) I-V evolution during a seasoning process (from plot1 to plot 3)

0

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emis

sion

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Figure 26 : (a). Evolution of the I-V characteristics showing the cleaning of the MWNT apex duringthe conditioning process. (b). Current stability corresponding to plot 3 for VApp= 195 V. (c) Current

stability corresponding to before the data of the plot 1 were taken for VApp= 130 V.

Before the conditioning process, i.e. using the nanotube sample in the as-growth condition, wefound that:(1) The emission onset/threshold Vapp to obtain 1 pA was 2 to 3 times less than after theconditioning process.(2) The FE currents were very unstable with high fluctuation rate (Figure 26c).(3) The (I-V) characteristics always exhibited current saturation (Figure 26a plot 1), a behaviorwhich was also observed by other authors, and these plots were not reproducible. This means thatirreversible modifications of the surface occurred during the emission.

The room temperature conditioning process we used, is the following three-step procedure:(1) Just after the start of emission (1pA), the emission current is steadily increased until a suddendecrease in the current is observed. This sharp drop in IFE generally occurred when it reached ~0.1µA(2). After this sharp decrease, the (I-V) plots which showed saturation (Figure 26a, plot 1) evolvedtowards a straight line as the maximum conditioning currents were increased gradually from 0.1 µA


to a few µA (Figure 26a, evolution from plot 1 to plot 2). Concurrently, there was a noticeabledecrease in both the occurrence and amplitude of the observed fluctuations in the emission current(∆I/I at fixed Vapp). (3) Thereafter, by increasing the conditioning current to 5 µA or more, reproducible straight linesfor the F-N plots (Figure 26a plot 3) with very stable emission current (Figure 26b) was obtained.

We interpret the conditioning process as a “cleaning” of the surface at the apex of the MWNT. Justafter the introduction of the sample into vacuum, surface adsorbates were always present at the apexof the MWNT - this is a very well known phenomena observed for various types of tips using fieldemission microscopy. FE with an adsorbate-covered apex begins at small localized areas whichhave the smallest work function. This causes the rapid formation of nanoprotrusions due to electricfield-driven surface diffusion of the adsorbates. From the measurements point of view, thiscorresponds to the observed instabilities and large fluctuation rate in the emission current. At theseprotrusions, local heating by Nottingham effect occurs causing an increase in the local temperatureas a function of the current. The local increase in temperature was possibly enhanced by Jouleheating along the MWNT. At large current densities, the local temperature becomes high enough tofield evaporate some of the adsorbates, and consequently an increase of the global work function atthe apex occurs. From the measurements point of view, this corresponds to the observed evolution(ie. non-reversible/shifting) of the I-V plots with initially current saturation. After IFE ≥ 5 µA, mostof the adsorbates were field evaporated and those that remained were strongly bonded to thesurface. From the measurements point of view, this corresponds to a stable emission with reversibleF-N characteristics, with φ ≈ 4 eV.

So far, we have shown that the proposed conditioning process leads to reproducible FEcharacteristics for a single MWNT. The following observations deal with the variation of FEcharacteristics between each vertically aligned MWNT in an array grown simultaneously on thesame substrate:

(1) Before the conditioning process, the first constant-voltage scan at a fixed distance over the arrayyielded large differences in the emission currents from one MWNT to its neighbours. In someinstances, the variation in emission current between emitters was 1000.(2) Nevertheless, for all the scanning measurements performed for this study (ie. 3 lines having 6aligned MWNTs and 2 matrices of 3×3 MWNTs), emission currents have been observed from allthe nanotubes in their expected positions. This implies that the 100% yield in the fabrication of theMWNT array, as shown in Figure 25(ii), was obtained also for field emission.(3) After the conditioning process, most of the MWNTs exhibited very similar emissioncharacteristics. Figure 27(i) shows the current variation over four MWNTs located at the corners ofa 100 µm-square array, with their corresponding F-N plots in Figure 27(ii). For this array, ∆I/I ≤30% for a same applied voltage and the values for φ obtained were 3.90, 4.12, 4.21 and 4.04 eVrespectively. Such a similar φ between emitters suggest that the apex of the different MWNTs weremostly identical after the seasoning process. This could be possible since the same geometry of thenanotubes obtained by PECVD were almost identical (Figure 27(iii)), i.e. the same apex radius.Furthermore, the graphitic nature of the surface at the apex favors physical adsorption, and so thefield evaporation of the adsorbates during the conditioning process leaves the apex surface in asame relatively clean state.(4) For measurements with the probe-ball in close vicinity to the MWNT, a deviation from the F-Nbehavior for currents exceeding 1 µA was observed for approximately 20% of the MWNTs tested.The emission current increased more rapidly with voltage than expected from the F-N theory. Thisindicates some mechanical/physical changes to the MWNTs. We attributed this behavior to thestraightening of some slightly bent MWNTs (eg. nanotube ‘b‘ in Figure 27(iii)) under the


electrostatic force from the probe to the nanotube. Taking the field for 1 µA, we have calculated thisforce to be ≈ 4×10-7 N.(5) For IFE ≥ 20 µA, corresponding to an electrostatic force ≥ 8×10-7 N, we have observed for someMWNTs a sudden shortening of the nanotube length (sharp drop in emission current) that we couldattribute to either a fracture of the nanotube under the electrostatic force (probably at somecrystallographic defect as seen at the middle of the nanotube ‘d’ in Figure 27(iii)) or to a rapid fieldevaporation of the MWNTs during FE.

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2 10-8

2,4 10-8

2,8 10-8

MWNT 1MWNT 2MWNT 3MWNT 4

ln (

I /

F2 )

1 / F (cm / V)

(ii)

Figure 27 : (i) Scanning FE current distribution over an array of four MWNTs for a VApp= 260 V.(ii) Corresponding F-N plots . (iii) SEM of a 5 µm-spacing array showing the presence of defects

for some MWNTs (CUED array).

This preliminary study shows that each MWNT from a PECVD-deposited nanotube array acts as aconventional FE cathode up to 20 µA. The proposed cleaning procedure led to reproducible F-Ncharacteristics with a φ ≈ 4 eV. The initial deviation from F-N type emission are due to artifactsfrom surface adsorption at the nanotube apexes. More importantly, we observed a variation ∆I/I ofonly 30% in the FE currents between the different MWNTs of the array after conditioning. Thisconfirms the possibility for using MWNTs as independent FE sources for massively parallelmicroguns.


WP2 : Fabrication and emission properties of CNT based cathodes

A self aligned process has been studied in order to ensure a perfect alignment between the electronsource (CNT) and the gate aperture.

WP2 –1 : Self aligned cathode fabrication process for CNTs

We have utilised a self-aligned process to produce CNTs in the centre of the extraction grid (seeFigure 28). The fabrication process began with the deposition of a sandwich structure on a dopedsilicon substrate comprising 250 nm of doped polysilicon (gate) on 500 to 1000 nm of silicondioxide (insulator) on base metal electrode. Depending of the lithographic equipment used (Opticalor e-beam lithography), an array of 1 µm diameter (optical) or 200 nm diameter (e-beam) at a pitchof 4 µm was then patterned (Figure 28 (a)). A reactive ion etching (RIE) step using SF6 gas wasthen used to isotropically etch the polysilicon gate (Figure 28 (b)). Wet chemical etching in bufferedhydrofluoric acid was used to isotropically etch the silicon dioxide insulator, in order to form anarray of microcavities (Figure 28 (c)). Both the gate and insulator were deliberately overetched toproduce an undercut. A 15 nm thick TiN layer was then deposited by sputtering. This was followedby evaporation of 7 nm of nickel which is the catalyst for CNT growth (Figure 28 (d)). The role ofthe TiN layer is to prevent nickel diffusion into the back metal electrode during the CNT growthwhich occurs at 700°C. The unwanted TiN and nickel over the gate were then removed bydissolving the photoresist in acetone (lift-off process, Figure 28 (e)). The vertically aligned carbonnanotubes were deposited using PECVD with acetylene and ammonia gases at 700°C (see WP1).This process produces vertically-aligned carbon nanotubes which are catalytically nucleated on thenickel particles inside the gated device structure (Figure 28 (f)).

If 1µm optical lithography is performed, the TiN dot diameter (where CNT growth occurs) is 1 µmand the grid aperture was chosen to be 2 µm (due to RIE underetching). This leads to the growth ofmultiple CNT per gate aperture (step f1).

If 200 nm e-beam lithography is performed, the TiN dot diameter (where CNT growth occurs) is200 nm and the grid aperture was chosen to be 0.5 or 1 µm (due to RIE underetching). This leads tothe growth of single CNT per gate aperture (step f2).

Substrate

Resist

a d

b e

c f1 f2

Ni/TiN

CNTs CNTs

SiO2

Doped Si

Metal

1 µm Lithography 200 nm Lithography

Figure 28 : Self aligned cathode process for CNTs


WP2 – 2 : CNT based cathode fabricated with 1 µm resolution lithography

The Scanning Electron Micrographs of Figure 29 and Figure 30 show selectively grown CNTsinside each gate aperture. The isotropic etching of the polysilicon has been performed to enlargegate aperture diameter to ~2 µm. The silicon dioxide cavity was further undercut beneath the gateaperture in order to prevent the silicon dioxide from being charged during field emission. The CNTsare well defined within a 1 µm area in the centre of the gate aperture. Thus, possible short-circuitsbetween the CNTs after growth and the gate are avoided. The CNTs have diameters between 30 and100 nm. The growth time was chosen to obtain CNTs up to ~ 0.4 µm heights - this is approximatelyequal to the base electrode to gate distance (0.5 µm). This configuration has been shown to beoptimal for Spindt-type cathodes and should improve the field emission characteristics of ourmicrocathodes.

(a) 3 µm1 µm

gate

base (metal)

CNTs

Figure 29 :SEM view of an array of CNT based cathodes

with an integrated extraction electrode.

Figure 30 :Tilted (~ 80°) view of microcathode with

vertically aligned CNTs as the emission source.

After CNT growth, the device was tested in a field emission measurement system evacuated to abase pressure of 10-9 mbar. Figure 31 shows the test configuration used during field emissioncharacterisation. Either a negative DC voltage or square-wave voltage pulses with a low duty cyclewere applied to the base electrode (CNTs) relative to the grounded gate electrode, in order to extractelectrons from the CNT emitters. Pulse mode operation allows high peak emission currents to beextracted whilst limiting the overall power which prevents the degradation of the device. The fieldemitted electrons were then filtered through two grids which were biased so as to repel the lowenergy secondary electrons generated by field emitted electrons bombarding the gate. A positivelybiased phosphor anode was used to collect and measure the average field emission current by meansof a picoammeter. The field emission measurements were performed automatically via PC-controlled voltage sources and ammeters. The turn-on voltage is defined as the voltage required toproduce detectable field emission from our device (in our case, an average emitted current densityof ~10-10 A/cm2).


The field emission measurement results performed with a duty cycle of 0.5 % and a frequency of100 Hz are shown in Figure 32. The initial turn-on voltage was 9 V. However, after recurrentmeasurements by cycling the gate voltage, the turn-on voltage was observed to shift to 15 V, afterwhich stable and reproducible field emission characteristics were obtained. The shift in emissioncharacteristics during the initial turn-on phase can be due to the destruction of some of thenanotubes or to hydrogen desorption (see WP 1 – 2). The average current density measured at avoltage of 40 V using a duty cycle of 0.5 % was 3.0 µA/cm2, which corresponds to a peak emissioncurrent density of 0.6 mA/cm2.

R

Phosphor

Filter Grid

Microcathode

+

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IEEE Interface

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1/V (V-1)

ln(I/

V2)

Figure 31 :

Schematic view of the set up for the fieldemission measurements of the cathodes

Figure 32 :

Field emission I-V measurements obtained inpulsed mode with a duty cycle of 0.5 %

An average current per microcathode of 0.1 nA has been obtained at 40V. The low current isprobably due to the high electrostatic shielding between the bunches of CNTs in each microcathodewhich degrades the field emission performance of the nanotubes. This has been improvedsignificantly when single CNTs are produced within each hole (see below).

WP2 – 3 : Single CNT based cathode fabricated with 200 nm resolution lithography

According to the simulation performed in WP3, the requirements for the Nanolith cathodes are :

- one single CNT per microcathode- a SiO2 thickness of 1µm- a gate aperture of 0.5 µm- a CNT height of 0.3 to 0.5 µm

So we have fabricated new arrays of microcathodes in order to respect these specifications. Inparticular, this implied the use of 200 nm e-beam lithography fort the step f2 of Figure 28.


A

CNT height : 0.3 µm

Gate aperture : 0.5 µm

B

CNT height : 1 µm

Gate aperture : 1 µm

C

CNT height : 0.5 µm

Gate aperture : 1 µm

Figure 33 : Individual CNT grown in a microcathode


Figure 33 shows different examples of CNTs grown in microcathodes. Example A corresponds to amicrocathode which respect exactly the specifications described above. Example B correspond to alonger deposition time and C to a microcathode with a gate aperture of 1 µm and a CNT height of0.5 µm..

Field emission properties of 100 x 100 microcathodes using a planar anode

First we measured the field emission properties using a planar anode and the set-up described onFigure 31.Figure 34 shows the emission current from an array of 100 x 100 microcathodes corresponding toFigure 33 C. A threshold voltage of 22 V and a peak current density of 1.4 mA/cm2 at 40V has beenobtained. If all microcathodes emit, this leads to an average emission current of 1 nA/microcathode.Therefore the 10 pA specification is fully completed.

Figure 34 : Field emission properties of an array of 100 x 100 microcathodes.The average emission current per microcathode is 1 nA at 40V


Field emission from single CNT based cathodes by SAFEM

The field emission properties from CNT micro cathodes described in the paragraph WP 2-3 havebeen studied using the SAFEM configuration presented before (WP 1-3 fig.21). The measurementswere done under a vacuum of 10-9 Torr and the measurement configuration with the differentapplied potential is presented in the following figure:

Figure xx: Measurement configuration with the different applied voltages

Due to the present growing process, different possibilities exits:1) The tube is perfectly centred under the extracting electrode2) The CNT is not centred under the extracting electrode3) The CNT is in contact with the extracting electrode4) For each of the three cases before length of the CNT can be larger than the distance between

the base and the extracting electrodes, i.e. the apex of the CNT is over the extractingelectrode.

The objectives of the present study with the CNT microcathodes given by our parteners is to findout a recipe to obtain stable and reproducible field emission from the CNT’s of thesemicrocathodes. The chosen approach is to eliminate the microguns presenting the structuresdescribed in the upper possibilities 2 and 3.

The conditioning mechanism is then based on a two step process.The first one must destroy the CNT’s which were in contact with the extracting electrode.A potential is gradually applied between the CNT base and the extracting electrode in order todestroy the conctating CNT’s by Joule effect. (up to 80 Volts and 100 µA of current)

It can be noticed that during this process by increasing the voltage between the cathode and theextracting grid this contact disappeared for voltage well under 80 V. Nevertheless, it appeared againunder field emission current measurement.This can be interpreted by the progressive destruction of the few nanotubes in contact with theextracting grid and by the fact that few nanotubes not at the centre of the microcathode could bedeformed under the electric field and then established again an electrical contact with the extractinggrid during the field emission current measurement.

The second one is to clean the CNT that were now centred compared to the extracting electrode bya process similar to the one described in the Work Program WP 1-3.In details:The field emission current measurements were then performed under continuous bias.


The seasoning voltage for these cathodes reached values of 90 Volts given a field emission currentof about 10 µA. By repeating this seasoning process, stable FE current was then observed. Theevolution of the I(V) characteristics is shown in the underneath figures:Figure A shows the Field emitted current evolution during the first 35 minutes of the conditioningprocess for an applied Voltage of 40 Volts.Figure B shows the current stability at the end of the conditioning process.Figure C and D show examples of Fowler Nordheim curves obtained before and after theconditioning process.

Figure A Figure B

Figure C Figure D

The figure D is an example of the reproducible FE from CNT’s after the conditioning process. Inother words the I(V) characteristics were stable and remained mostly the same for the same valuesof applied field emission voltages.

In conclusion, with this first sample of CNT’s within the configuration of an array of 100x100microguns we have shown that stable and reproducible behavior of field emission from these CNTmicroguns is possible after a conditioning process. This allowed a positive discrimination of thedifferent microguns in an array, leaving then only the operational ones.

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WP2 – 4 : Alternative process for self aligned microcathodes

As shown in WP3 – 1, the alignment between the CNT and the hole in the extraction grid is highlycritical. In order to obtain a beam diameter below 100 nm, we must have ∆x <50 nm if α = +/-10 °.Moreover it is already known that future Ebeam lithography equipment will have to deliver sub 50nm diameter beams. Consequently, we need to use a fabrication process which guarantees a perfectalignment (misalignment below 30 nm).

For the fabrication processes we uses, the Ni catalyst must be evaporated. For a classicalevaporator, the distance between the source and the substrate is less than 1m. The abovespecification limit the size of the wafer to 2 inches.

Consequently, we studied a new fabrication process compatible with large surface silicon wafers.

Figure 35 shows that the process we use works only if the evaporated material flux is strictlyperpendicular to the substrate. Step 4 of Figure 35 shows clearly that this condition can not befulfilled for large surface Si substrates.

Figure 35 :State of the art for self

aligned CNT based cathodes

Ebeam litho < 300 nmanisotropic etch of the grid ,chemical etch of the insulator

Catalyst evaporation

Evaporation source

Liftoff

Localised growth of CNTs

Insulator

Extraction grid

Resin

Diffusion barrier

Conductor Substrate

1

2

3

4


Description of the new self aligned process (Thales and Cambridge patent)

This new self aligned process, that have been patented by Cambridge and Thales, is based on a newmethod for the localisation of the catalyst cluster.Two different alternatives of this new method are shown in Figure 36 (step 1a and 2a, or step 1band 2b). Step 1a corresponds to the realisation by Ebeam lithography of 100-300 nm diameteropenings in a resin deposited on a stack M / catalyst / diffusion barrier / conductor / substrate. Step2a consist in etching selectively the M material. Step 1b corresponds to the etching of 100-300 nmdiameter openings in the M material deposited on a stack diffusion barrier / conductor / substrate.The catalyst is then deposited (step 2b). M is a material that forms an alloy with the catalyst when itis annealed at the temperature used for CNT growth. We have mainly worked with silicon as the Mmaterial because Si etching processes are already available.The thermal anneal, which is performed before CNT growth, leads to the formation of an alloy inareas where the M material was not etched. In the 100-300 nm openings, spherical shaped clustersare formed and catalytic growth of the CNTs occurs.

Lithographyand M etching

Resin etching andcatalyst deposition

Thermal anneal

Localised growth of CNTs

Resin

M film

M-catalyst alloy

Diffusion barrier

Conductor

Catalyst

Substrate Substrate

1b

2b

3

4

Ebeam litho < 300 nm

M and resist etching

1a

2a

Figure 36 : New method for the catalyst localisation and CNT growth


Figure 37 : Localised growth of Nanotubes using the alternative method

Figure 37 shows localised growth of CNTs using steps 1b, 2b, 3 and 4. As 1 µm lithography wasperformed, mulriple CNT growth occured in the 1 µm aperture in the absorbing layer. Furtherexperiments are currently performed to determine the lithographic conditions that lead to singleCNT growth.

Figure 38 and Figure 39 correspond to the two different self aligned processes that can be used withthis method. Step 1 of Figure 38 shows 100-300 nm diameter openings obtained by lithography in aresin deposited on the stack : Grid / Insulator / M / catalyst / diffusion barrier / conductor / substrate.During step 2, is performed an anisotropic etch of the extraction grid and a chemical etching of theinsulator. Step 3 consist in etching openings in the M material that are self aligned with theopenings in the resin i.e. with the apertures in the extraction grid. This operation can be performedin a reactive ion etching (RIE) equipment operated at low pressure (< 10 mTorr). Because of itsfunctioning principle, this type of equipment delivers an ion flux which is perfectly perpendicular tothe substrate and that for large surfaces. During step 4, the thermal anneal induces the formation ofthe alloy M – catalyst as well as the formation of a cluster of pure catalyst in the openingspreviously etched in the M material. Consequently, this method leads to the growth of CNTs thatare perfectly aligned with the apertures in the extraction grid of each microcathode.


Figure 39 shows an alternative process. In this case, the stack is :Extraction grid / insulator / M / diffusion barrier / conductor / substrate. Step 2 corresponds to theanisotropic etching of the grid, the chemical etching of the insulator and the RIE etching ofopenings in M that are self aligned with the resin openings. Then, the catalyst is evaporated (step 3).During this operation, different rotations of the substrate holder can be performed in order to obtainan homogeneous deposit. This leads to an uniform catalyst dot with a diameter larger than the resinopening. The resin is then suppressed by lift off. The thermal anneal performed during step 4induces the formation of the alloy M-catalyst as well as the formation of a catalyst cluster in theopenings previously performed in the M film. The CNT growth is consequently perfectly selfaligned with the hole in the extraction grid of each microcathode.


Anisotropic etching of Mresin etching

Thermal anneal

Localised growth of nanotubes

Insulator

Grid

Resin

M film

Diffusion barrier

Conductor

Catalyst

Substrate

Anisotropic etching of the grid, Isotropic etching of the insulator

1

2

3

4

5

M-catalyst alloy


Anisotropic etching of the grid, Chemical etching of the insulatorAnisotropic etching of M

Homogeneous evaporationof the catalyst

Thermal anneal

M film

M-catalyst alloy

Substrate

1

2

3

4

Insulator

Extraction grid

Resin

Diffusion barrier

Conductor

Croissance localisée nanotubes

5

Figure 38 : First version of the new self alignedfabrication process for CNT based cathodes

Figure 39 : Second version of the new selfaligned fabrication process for CNT based

cathodes


First fabrication of cathodes using this alternative process

We have worked on the new self aligned fabrication process using the procedure described inFigure 39. The M material was a 10 nm thick Si film and the diffusion barrier was a 8 nm thickSiO2 layer.Even if SiO2 will not be used in the future fabrication process, it is an efficient diffusion barrier andhas allowed us to study and evaluate the novel self aligned process

Figure 40 shows a tilted SEM view of a cathode corresponding to the step 2 of Figure 39. For thefirst technological tests, we have used 1 µm diameter openings in the extraction grid i.e. the 0.2 µmthick Si n+ layer. One can see the perfect alignment between the aperture in the extraction grid andthe opening in the 10 nm thick Si film.After evaporation of a 5 nm thick Ni film and liftoff of the layer deposited on top of the grid, wehave performed a 750°C thermal anneal during 5 min. This treatment allows us to simulate thethermal anneal which is performed before CNT growth. One can see Ni clusters that are onlyformed in the openings previously etched in the 10 nm thick Si film.

SiO2

10 nm Si 8 nm SiO2

1 µm

Si n+

Figure 40 :Tilted SEM view of a cathode showing the perfect

alignment between the aperture in the extraction gridand the opening in the 10 nm thick Si film

Figure 41 :Tilted SEM view of a cathode showing the

formation of Ni clusters only in the openingspreviously etched in the 10 nm thick Si film

Further experiments are currently performed to demonstrate this new approach.


WP 3 : Design of efficient microguns and corresponding fabrication process

WP3 – 1 : Design of efficient microguns

two objectives :1. Estimation of the focusing degradation when the CNT emitter is not localized on the µgun axis.

This numerical simulation study has been performed with the potential of the PPMA layer at 3kV, in order to make comparison with already obtained on-axis results.

2. Determination of the focusing properties when the potential of the PPMA layer will be at 1 kV.

Moreover, following our midterm meeting, a new design of the writing head has been defined (seeC.)

A. Estimation of the focusing degradation when the CNT emitter is not localised on the µgunaxis.

We study the effect of the non axial position of the CNT on the focus spot size. A new 3D modelhas been designed with the possibility to place the nanotube axis at a distance ∆x from the µgunaxis.

Figure 1 : Schematic representation of the µgun with the PAS system having a CNT as emitter

The methodology

• The geometrical parameters of the µgun is taken from the best simulation parameters obtainedwith our past on-axis simulations (Table 1).

• We keep the CNT perpendicular to the base plane.• The electron emission is radial and uniform from the hemispherical apex.• The simulations have been performed for four values of ∆x, distance between the CNT axis and

the µgun axis. (∆x = 0 µm, 0.05 µm, 0.15 µm and 0.25 µm)


CNT length L 250 nm

CNT diameter 30 nm

Hole diameter 1000 nm

CNT apex to extraction Grid dextr 1000 nm

PAS diameter 100 µm

µgun - PAS distance 200 µm

PAS - Focus Plane distance 85 µm

PAS length 500 µm

Table 1: Characteristics values of the system used in the numerical simulations

The results

The results show the strong influence of the nanotube position on the focus spot size.For α α = +/- 5° , the focus diameter is over the limit of 100 nm as soon as ∆∆ x>0.18 µµmIf we take αα = +/-10 °, we must have ∆∆ x <0.05 µµm to be under the 100 nm limit.

B. Determination of the focusing properties when the layer potential is 1 kV

CNT length 300 nm

CNT diameter 30 nm

Hole diameter 500 nm

Grid to CNT base 1000 nm

CNT Voltage 0

Extraction electrode Voltage -50 V

Focus Plane Voltage ~ 1 kV

The distance between the µgun and the PAS, the PAS radius, the distance between the PAS and theobject plane as well as the different voltages have been optimized to obtained the minimum focusspot size taking into account the previous simulation results. Focus spot size under 30 nm has beenobtained for the new model.A study of the influence of the nanotube position on the focus spot size has been done showing thestrong influence of the nanotube position on the focus spot size.


C. New writing head design defined after midterm meeting

Following the midterm meeting, the distance between the PAS to focus plane has been increasedfrom 85 µm to 500 µm. The goal is to ease the mask displacement above the writing head. The newdesign proposed to be numerically simulated by UCBL is presented on Figure 2.

Sub 100 nm beam size resinChromium layer

0.3 µm

φ = 0.5 µm

1 µm

0.5 µm

5 µmSi n+ (0.2 µm)

(0.2 µm)

500 µm

500 µm

200-300 µm

Vgrid~ 50 V

V60-70 V

PAS

Vfocusing~0 V

Vemitter0 - 50 VHV CMOS circuits

Vmask+1-2 kV

PAS

50 µm

Figure 2 : Schematic representation of the writing head showing the different biases

A systematic study has been done in order to assess the role and the limits of the differentparameters to have the best focus within the proposed design. The carbon nanotube was consideredto be in the µgun axis.

C.1. Role of the Post Acceleration (PAS) aperture RPAS

Variation of the focus spot diameter vs the Rpas

The following parameters were kept fixed:distance of µgun to PAS d1 = 200 µmthickness of the PAS e= 500 µmdistance of PAS to resin d2 = 500 µm


0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0 20 40 60 80 100 120

PAS Radius (micron)

focu

s sp

ot d

iam

eter

(m

icro

n)

Figure 3 : Variation of the focus spot for different values of the PAS aperture radius.

The results were presented in Figure 3. This plot shows that an aperture radius ≤ 50 µm will notmodified the focusing property.

C.2. Role of the distance d1 between the µgun and the Post Acceleration System (PAS)

The following parameters were kept fixed:radius of the PAS aperture RPAS = 25 µmthickness of the PAS e= 500 µmdistance of PAS to resin d2 = 500 µm

0

5

10

15

20

25

30

35

40

45

200 250 300 350 400 450 500

d1 (micron)

focu

s sp

ot d

iam

eter

(nm

)

Figure 4 : Variation of the focus spot for different values of d1.

The results were presented in Figure 4. This plot shows a continuous decrease of the diameter of thefocus spot with d1. However, the diameter is less than 50 µm for d1 > 200 µm.


C.3. Role of the thickness, e, of the PAS

Variation of the focus spot diameter vs the thickness e of the PASThe following parameters were kept fixed:distance of µgun to PAS d1 = 300 µmradius of the aperture of the PAS RPAS = 50 µmdistance of PAS to resin d2 = 500 µm

PAS thickness e Focus spot diameter500 µm 29 nm600 µm 29.3 nm700 µm 29.7 nm

Table 3: Variation of the diameter of the focus spot with the PAS thickness.

The results presented in table 3, indicated that the PAS thickness parameter is not a sensitiveparameter for the focusing results.

C.4. Conclusions about the new writing head design defined after midterm meeting

Focus spot with diameter < 50 nm can be achieved, if the following experimental conditions areused :

Parameters Limits Proposedd1 ≥ 200 µm 300 µmd2 ≥ 500 µm 500 µme ≥ 500 µm 500 µmRPAS < 50 µm 50 µmVPAS < 100 V 73.2 VVfoc ≈ -5 V -3.96 VVplane ≈ 1 kV 1055 VFocus spot diameter < 50 nm 29 nm

Table 4: New writing head characteristics for a focus spot diameter < 50 µm.

In table 4 we have summarized in the second column of table 4 the limit values of the differentparameters in considering the new writing head proposed during the mid-term meeting.In the third column of table 4, we proposed a set of characteristic values for this new writing headsystem that give a focus spot diameter of 30 nm.

The value of VPAS and Vfoc should be optimised in function of the real size of the different elementsof the microgun to obtain a focus spot diameter < 50 nm. The main parameter having greatinfluence in the focusing property is the alignment of the carbon nanotube within the axis of themicroelectrodes.


WP3 – 2 : Fabrication of 4 CNT microguns on 10 x 10 mm chip

Figure 5 shows the fabrication process of the CNT microguns that were fabricated. The substrate weused is an oxydised Si wafer with a 0.3 µm thick SiO 2 layer.Step a : First we deposit a stack of a metal (0.1 µm TiWSi), a 1 µm thick SiO 2 film, a 0.25 µmdoped Si layer (the extraction grid), a second 1µm thick SiO 2 layer, then a 0.25 µm thick TiWSi/Sin+ film (the focusing electrode). Then a lithographic step is performed to obtain a photoresist withan aperture of 5 µm.Step b : Reactive ion etching of the focusing electrode to obtain a 5 µm diameter aperture.Step c : Chemical etching of the insulator (1 µm SiO 2)Step d : After deposition of a PMM layer, e-beam lithography to obtain a 200 nm diameter opening.Step e : Isotropic etching (by RIE) of the polysilicon gate to obtain a 0.5 to 1 µm diameter aperturein the extraction grid.Step f : Chemical etching of the insulator (1 µm SiO 2)

Figure 6 Shows a cross section of the microgun after step f showing the different electrodes and theinsulators.

One can see the substrate (oxidised Si wafer), the emitter layer (The CNTs will grow on this layer),the extraction grid with a 0.8 µm diameter aperture and the focusing electrode with a 5 µmdiameter aperture. Insulation between the extraction grid and both emitter and focusing electrodes isobtained with the two 1 µm thick SiO 2 films.

Step g : Deposition of the diffusion barrier (15 nm thick TiN layer) and perpendicular evaporationof the 7 nm thick Ni film.Step h : The unwanted Ni/TiN stack over the different electrodes is then removed by dissolving thePMM in acetone (Lift off process).Step i : Growth of individual CNT inside each microgun.

Different lithographic processes are also performed to define connection pads to the different CNTsand electrodes. Figure 8 shows a 10 x 10 mm chip with four independent microguns. One can seethe connection pads of the 4 emitters (and therefore the 4 CNTs) and the two equivalent connectionpads of the extraction grid. The focusing electrode is the 2mm diameter electrode at the centre ofthe chip. This electrode is connected through the conductive substrate.


SubstrateS Oi 2

S Oi 2

TiWSi

Si n+

Resist

Lithography 5 µm

RIE of the 5 µm diameter aperture in the focusing electrode

Chemical Etching of the insulator

200 nm E-beam lithography

RIE of the polysilicon gateIsotropic etch

Chemical Etching of the insulator

TiN deposition by sputteringNi evaporation

Lift-off

CNT growth by PECVD

Si n+

a

b

c

d

e

f

g

h

i

Figure 5 : Fabrication process of CNT microguns


insulators

emitter extraction gridwith 0.8 µm aperture

Focusing electrodewith 5 µm aperture

Si substrate

0.3 µm SiO2

1 µm SiO2

1 µm SiO2

0.1 µm TiWSi

0.25 µm Si n+

0.25 µm TiWSi/Si n+

Figure 6 : Cross section of a microgun before CNT growth showing the different electrodes and theinsulating layers.

Figure 7 Tilted view showing the aperturein the focusing electrode and also in the

extraction grid


Emitter 1 Emitter 2

Emitter 4Emitter 3

Testpad

Testpad

Extractiongrid

Focusing electrode

Extractiongrid

Figure 8 : 10 x 10 mm chip with four independent microguns. This shows the connection pads ofthe 4 emitters (or the 4 CNTs) and those of the extraction grid. The focusing electrode is a 2 mm

diameter electrode at the centre of the chip and is connected through the conductive substrate.


WP4 : Parallel Lithography

T 4.1 : Lithographic environment requirements

T 4.1.1 Introduction

In the planned array-cathode low energy e-beam system the resist will be in close proximity to theelectrostatic electron optics. Any contamination resulting from resist outgassing could createcharging on the metal electrodes or leakage currents on the insulators as well as emissioninstabilities of cold field emitters. Resist outgassing is also of concern for DUV steppers where theglass lenses can be affected. The problem seems to be solved for 193 nm lithography but extensivetests are running for 157 nm and EUV lithography (Ref. 1-3).

In the case of low energy e-beam writing the first step for an estimate of a possible contaminationbuilt up is to measure the amount of gas leaving the resist surface in vacuum and under e-beamexposure. This has been performed at Fraunhofer-ISiT in two ways :

1. Mass spectrometric investigation of resist coated wafers in vacuum

2. Quartz balance measurements of resist outgassing under e-beam irradiation.

As standard DUV resists can also be suitable e-beam resists and no specially developed low energye-beam resist is available yet we have chosen two chemically amplified DUV resists from Shipleyand Clariant with different photo acid generators (PAG):

Shipley, UV II HS : Sensitivity for 30 keV electrons 5 µC/cm2

Resolution 100 nm in 1 µm resist at 15 µC/cm2 (Ref.4)

Clariant, AZ DX 3301 P: good e-beam performance anticipated.

As a reference we have selected the well known e-beam resist PMMA (Poly methyl methacrylate),which is not chemically amplified:

Merck PMMA, Selectilux EB 250 A: Sensitivity for 1.8 keV electrons 5 µC/cm2,Resolution below 40 nm (Ref.5).

From these investigations we expect insight into the relative outgassing properties of differentresists to find ways for the minimisation of this effect.


T 4.1.2 Mass spectrometer measurements of resist outgassing

The chemical composition of molecules desorbing from a resist surface under vacuum has beendetermined by a quadrupole mass spectrometer (QMS). The set up of the test apparatus is shown infigure 4.1.1 . It consists of a test chamber separated by a gate valve from the mass spectrometer,which is pumped by a 110 l/sec turbo molecular pump. In order to increase the amount ofoutgassing species to be analysed, 5 wafers with 4 inch diameter have been stored in the testchamber. Reproducible clean vacuum conditions have been guaranteed by venting with nitrogenand keeping the whole vacuum system at 80° C. The quadrupole mass spectrometer Balzers QMS-311 has a mass range 1-300 and a sensitivity with secondary electron multiplier of 103A/mbar. Themass spectra are recorded on an x-y chart recorder.

Standard resist treatment has been applied:

UV II HS : spin coat 5000 rpm resist thickness 550 nm prebake 130°C, 60s on hotplate

AZ DX-3301 P: spin coat 6500 rpm resist thickness 410 nm prebake 110°C, 60s on hotplate

PMMA : spin coat 5000 rpm resist thickness 1110 nm prebake 150°C, 30 min on hotplate

Fig. 4.1.1 Quadrupole mass spectrometer with blow up of test chamber.


After resist treatment the test chamber has been vented with clean N2 ,filled with five wafersmounted at a separation of 1 cm, and immediately pumped down with a separate roughing pump for30 sec to 10 –2 mbar. Then the gate valve has been opened after opening beforehand two throttledbypass valves to avoid too much pressure rise in the area of the quadrupole. The first mass spectrumhas been recorded appr. 5 min after start of pumpdown at a total pressure of appr. 5x10-6 mbar.

The characteristic outgassing spectrum of the Shipley UVIIHS resist are depicted in figure 4.1.2together with background spectrum which had been recorded in a separate experiment with fiveuncoated wafers. Characteristic peaks for the different resists have been identified as molecularfragments of more complex hydro-carbon compounds. The peak heights have been calibrated bycomparing the sum of all peaks with the total pressure measured with a separate Penningmanometer.

Fig. 4.1.2: Outgassing mass spectrum of Shipley UVIIHS resist at 80°C without e-beamexposure, ptot=5x10-6mbar.Background spectrum for uncoated wafers

The same procedure has been applied for the two other resists AZDX-3301P and PMMA.


T 4.1.3 Resist outgassing under e-beam exposure

Resist outgassing under e-beam exposure has been investigated at ISiT with a quartz micro balancemounted onto the probe stage of a standard scanning electron microscope AMRAY 1830.A desorption experiment is shown in Fig. 4.1.3, where the e-beam hits the resist which has beendirectly spin coated onto the quartz crystal. Only the total amount of transported molecules ismeasured without a mass spectroscopic identification of the species.

Fig. 4.1.3 Desorption experiment in SEM (AMRAY 1830 scanning microscope) for outgassingexperiments under e-beam

T 4.1.3.1 Desorption experiment

In this case the three selected resists have been directly spin coated onto the quartz crystal disks andprebaked as described before. It was not possible to measure the resulting thickness on the crystalsbecause of their surface roughness of many micro meters. Therefore the thickness has beendetermined on a 4” wafer coated under same conditions of spin rotation and prebake.

Coated crystals have been mounted into the water cooled ( T=21°C) crystal holder on the probestage of the AMRAY 1830. After a pump down time of 5 min the readings of the quartz balancereached a nearly stable value indicating that adsorbed atmospheric gasses had been given off. Onlya small change remained of 0.5 Angstroem /min due to room temperature outgassing of the resists.Then the electron beam has been switched on at an energy of 20 keV. The energy was higher thanthe planned 2 keV for the array cathode, but was necessary as we see later in chapter 4.1.3.2 wherethe energy dependence has been studied. The irradiated area on the resist was 4x4 mm2 and the totalcurrent 0.2x10-7 A, the scan rate 10 frames/sec.

The resulting desorption rates in Angstroem per minute for the resist UV II HS, thickness 550 nmunder the assumption of a resist density of 0.95 g/cm3 are plotted in figure 4.1.4. The mostpredominant outgassing under 20 keV e-beam exposure was found for PMMA although PMMAhad the highest prebake temperature of 150°C for 30 min.

r e s i s t

e-

Q u a r t z - O s c i l l a t o r

M i c r o b a l a n c e


Fig. 4.1.4 Desorption rate of UVIIHS resist under 20keV e-beam exposure,e-beam current: 0.2x10-7A.

T 4.1.3.2 Variation of e-beam energy in desorption experiment

In order to check the outgassing of resist at lower e-beam energies, the acceleration voltage hasbeen varied in steps from 20 to 5 keV. As a resist PMMA has been selected because it gave thestrongest effect of all three resists. The outgassing effect gets weaker with lower e-beam energiesand is below detction limit at 5 keV. The beam current was kept constant at 0.2x10-7 A down to 7.5keV but dropped to 0.1x10-7 A at 5 keV. At the moment it is not clear, whether the decreasingoutgassing is due to the e-beam energy or to the lower deposited power.

UV II HS

0,00

0,50

1,00

1,50

2,00

2,50

13:35 13:40 13:45 13:50 13:55 14:00 14:05 14:10 14:15 14:20 14:25 14:30 14:35 14:40 14:45 14:50 14:55

time

des

orp

tio

n r

ate

(Å/m

in)

20 kV20 kV


T 4.1.4 Discussion of results

Mass spectrometric investigations have shown, that standard DUV resists, candidates also for e-beam exposure, do outgas in vacuum and that characteristic mass peaks appear in the spectrum,which can be identified as fragments of more complex hydrocarbon compounds.

An example of the reaction principle of a chemically amplified DUV resist is demonstrated infigure 4.1.5. The resist consisting of a polymer matrix ( Acetal type ) and a photo acid generator(PAG) is coated on a wafer. A radiation exposition generates an acid out of the PAG, whichtransforms in a catalytic way the insoluble resist matrix into a soluble compound, which is dissolvedin the base mostly 0.26 n TMAH ( tetra methyl ammonium hydroxide).

Fig. 4.1.5 The reaction principle of a chemically amplified DUV resist from Clariant.

The used UV II HS resist is of a so called ESCAP type (environmentally stable chem. ampl. photoresist). and the AZ DX 3301 P resist is a hybrid type ( Acetal+ESCAP ). More information aboutthe tested resists was not available.

Special mass peaks indicating outgassing of PAGs were not found until mass 300. In earlierinvestigations there was concern , but with the development of long chained PAGs replacing shortchained PAGs, this problem seems to be solved.

For the UV II HS resist the strongest outgassing component was C2H5O with a pressure of 4.6 x10-7

mbar at a total pressure of 4x10-6 mbar. This relates to a relatively large area of five 4” wafers at80°C. For the AZ DX 3301 P resist the largest outgassing peak was C3H7 with a partial pressure of1x10-6mbar. For an estimate of the contamination by these gasses the sticking coefficient has to beknown.


In comparison to the chemically amplified resists the outgassing of PMMA without e-beamexposure was very small, possibly because of the high baking temperature (150°C, 30 min), whichdrives off remaining solvents.

On the contrary under e-beam exposure PMMA showed the highest desorption rate of up to5 Angstroem/min, may be because the radiation sensitivity of this non chem. ampl. resist dependson easy bond scission.

The energy dependence of e-beam resist outgassing has been investigated. It dropped toimmeasurable levels at 5 keV.

T 4.1.5 Conclusion

In order to reduce outgassing of future low energy e-beam resists the baking conditions have to beoptimised without destroying the PAGs, and long chained chemical compounds must be used. It ishoped that remaining outgassing of hydrocarbons will not severely damage the emission stability offield emitting nanotubes. In a real lithographic environment the residual gas pressure must be below10-8 mbar. In this comparative investigation the detection limit for small mass peaks has beenincreased by working in the 10-6mbar range.

References:

1 P.M.Dentinger, Sandia Nat. Labs,Outgassing of Photoresist Material at EUV Wavelengths, J. Vac. Sci. Technol. B 18 (6), 3364 Nov/Dec (2000)

2 M-Chauhan et al. Univ. of Wisconsin, Outgassing of Photoresist in EUV, J. Vac. Sci. Technol. B 18 (6), 3402 Nov/Dec (2000)

3 R. R. Kunz and D. K. Downs, MIT, Outgassing of organic vapours for 193nm photoresists, J. Vac, Sci. Technol. B 17 (6), 3330 (1999)

4 W.H. Bruenger et al.,DUV resist UV II HS applied to high resolution electron beam lithography and masked ion beam proximity and reduction printing, Microelectronic Engineering 41/42, 237 (1998)

5 W.H. Bruenger et al. , Low energy e-beam lithography; energy control and variable energy exposure, Microelectronic Engineering 27,135 (1995)


T 4.2 Active Matrix Design and Fabrication

T 4.2.1 Introduction

Within the scope of the project the work package T4.2 : Active matrix design and fabrication wasaddressed to the definition of specification, circuit conception, simulation, design, fabrication andcharacterisation of the emitter control circuitry. In the following all topics will be introduced anddescribed in detail. The specifications were jointly defined within the consortium. Special emphasiswas laid on the characterisation, especially the development of an appropriate computer controlledmeasurement set up. The chips were realized by MPW (Multi-Project-Wafer) fabrication using ahigh voltage CMOS technology. It was the goal to make available a robust electronic test circuitproviding the most important features for a controlled active matrix array.

T 4.2.2 Specifications

Due to the fact, that the conductive channel approach was more difficult than initially expected, itwas jointly agreed to favour a nanotube concept. Since the carbon nanotube (CNT) process needs adeposition temperature of about 700°C a hybrid integration instead of monolithic integration wasfavoured for the microgun array and the pixel electronics, due to thermal budged incompatibility.

Concerning the choice of an appropriate IC-process for the electronic circuitry a 0,8µm highvoltage CMOS process has been chosen with maximum voltages of 80V and 40V for NMOS andPMOS, respectively. The implemented low voltage CMOS part requires a supply voltage of 5V.The decision for the HV-CMOS process has been made because of the voltage requirement of 50Vfor the extraction electrode within the electron-gun arrangement.

The emitter current for the CNT was specified in the range of 1-10nA based on first experimentalCNT results. The maximum exposure time has been kept in the microsecond range as initiallydefined (2-10µs). It is obvious, that even in the case of 1nA CNT current for an exposure time of2µs and an exposure area of 100nm x 100nm the dose value amounts to 20µC/cm2. This givessufficient margin even for non sensitive electron beam resists. The exposure doses were specified inthe range of 1- 40µC/cm2.

T 4.2.3 Circuit concept

In the previous reports short descriptions of the CNT circuits were presented. This documentationis of more details and in addition describes how to use the ASICs.

The main part of the ASIC is a small analog circuitry that- switches the CNT to the electronic- measures the current through the gun and switches off if the current is below a specified

value (comperator 2)- integrates the current on a capacitor and switches off if the required dose is reached

(comperator1)

For the CNT electronic two different circuits were realized based on the same general concept. Thespecific difference between the two solutions will be explained in the following. The descriptionstarts with the general circuit concept shown in Fig. 4.2.1.


Fig. 4.2.9 Principle schematic of the CNT anlog electronics to control emitter current andexposure dose.

Part A of the circuit shown in figure 4.2.1 is used for selection and activation of the circuit Part B.T1 is the high voltage transistor for switching the CNT emitter current. The transistor will be on-state with a 5V bias at the gate and off-state with 0V. In case of a current flow through T1 a voltagedrop occurs across T2. This voltage leads to on-state of T4 building up a voltage divider togetherwith R1 . The higher the current through T1, the higher is the voltage drop across T2 and the loweris the resistance of T4. The voltage level of the node T4-R1 depends on the emitter current andcontrols the current comperator COMP2. If a minimum emitter current is not reached COMP2changes the output and via logic gatter and S3 interupting the CNT current. The switching level isgiven by Vref2.

Fig. 4.2.10 Simulated schematic of CNT circuit 1.

T2 and T3 are building a current mirror, i.e. the current through T2 is identical with the currentthrough T3 which discharges the capacitor C1. If the voltage across C1 is less than the reference

T1

T2

T4

R1

C1

S3

Part A Part B

Area 1

T5

S3S4

C1current

current

Ibias Imirror

I1 I2

charge


voltage of the comparator COMP1 the digital output changes from VDD to ground (gnd). Theoutput of both comparators are connected to a logic gatter. If one of both comparators detects thestop condition of either “minimum emitter current” or “exposure dose completed”, S3 a switchingtransistor shorts the gate voltage of the high voltage transistor and interups the current through theCNT emitter.

The realised two circuits are very close to this general concept mentioned above. In the followingthe circuit 1 is described shown in figure 4.2.2. The schematic was directly taken from thesimulation. Compared to the general concept of figure 4.2.1 similar components are available. Fromleft to right it can be seen that e.g.- transistor T5 corresponds to switch S2- two transistors S3 and S4 are used for stopping the CNT emitter current- integration for charge control is realized by capacitor C1- comparators COMP1 and COMP2 are controlling exposure dose and CNT current with

repect to the reference voltages, respectively.

For the simulation of the complete circuit the current from the CNT was modelled by a diode havinga threshold voltage of 50V. The main change in circuit 1 is the implementation of additionaltransistors for special current mirrors located in area 1. These transistors are necessary to ensure awell defined static current through all branches and nodes. This constant current accelerates thetime the circuit needs to settle in steady state when switching the gun current on and it reducesdrastically the overshoot of the node voltages, also. Every current mirror is built by four transistors,so called cascaded mirror, to enhance the precision of the mirrored current.

Fig. 4.2 11 Simulation results of circuit1 showing the capacitor current of C1, the voltage nodes (current and charge) and the charge comperator output signal

The current comparator controls the voltage of the cascaded transistors at the current node.Without having the CNT switched on all currents I1, I2 and Imirror are identical with Ibias. Thecurrent through branch I2 is directly mirrored by the p-type transistors from Ibias and by the n-type

I (capacitor C1)

Voltage node (charge)

Voltage node (current)

Voltage comparator (charge)

1 nA

5 nm

10 nA

15 nA


transistors from I1. With this construction a stable voltage of node charge is defined. Under thiscondition the integrating capacitance C1 is fully loaded. When the CNT is now switched on thecurrent I1 will be enlarged by the CNT current and this will be mirrored to I2.Since the current through the p-type transistors in branch 2 remains constant, the charge stored byC1 will be reduced, that means C1 is reloaded by the value of the CNT-current. The logic circuitrybehind both comparators has become a little bit more complicated but has the same function asdescribed before.

Figure 4.2.3 shows the most important voltage nodes and currents of a simulation of circuit1. Hereafter 100ns the CNT has been switched on. This can be seen by the corresponding voltage jump atthe Voltage node current. Due to this voltage increase the current from C1 ( I capacitor C1 )temporarily changes to large negative values. After roughly 1µs a new steady state is reached, i.e.the Voltage node current as well as the current from C1 are constant and the Voltage node chargedecrease linearily by reloading via constant current. After approximately 9µs the Voltage nodecharge is below the reference voltage of the charge controlling comparator and the output changesfrom gnd to VDD, as shown by the output signal of the Voltage comparator charge.

In case of a larger mismatch of any transistor inside the loops in Area1 by e.g. technological reasonsa resulting mismatch of the currents will cause an integration error of charges on the capacitor C1.Therefore, a second different circuit approach is realised that will be explained below.The schematic of the circuit 2 is depicted in Fig. 4.2.12.

Fig. 4.2.12 Simulated schematic of the CNT circuit 2

In this circuit only the transistors inside Area 1 differ from the circuit 1 mentioned before. Here I1is always identical with Ibias. In case the CNT is switched off all currents Ix, Imirror and I2 areidentical with Ibias, too. When the CNT is switched on by the signal start, Ix will be reduced by thecurrent of the CNT emitter. Since Ix is mirrored to the branch I2 the upper p-type transistors will bedriven by this reduced current, also. In time the CNT is switched on via start signal transistor 6 inthe branch Imirror is switched off by the same signal. Under this condition capacitor C3 keeps thevoltage of node 1 and Imirror constant. Hence, I2 remains identical with Imirror at the value beforethe start signal was given i.e. the Ibias value. In order to balance the different current values in theupper and lower branch of I2 the capacitor C1 will be discharged.

Area 1

T5

S3S4

C1

current

Ix

ImirrorI1 I2

charge

currentIbias

startT6

node 1C1


With this change in the working principle the effect of a larger mismatch of transistors should besmaller. A possible drawback can be arise by additional switching states of transistors within Area 1that may introduce unwanted effects like spikes in node voltages and branch currents.Since simulations of both circuits behave nearly identical no additional results of circuit 2 will beshown here.


T 4.2.4 High voltage-CMOS process

Based on electrostatic simulation of the micro gun arrangement it was decided by all partners, thatat least a 50V high voltage CMOS process is required for the CNT driver circuitry. Since this typeof process is not available at ISiT production site a MPW (Multi-Project-Wafer) service of X-Fabwas used for the first ASIC fabrication. The MPW approach is fully compatible with the hybridintegration concept and the best choice for feasibility investigations.

Special technological features of the X-Fab 0,8µm HV-CMOS process are two metal levels and anoptional double poly module for stacked capacitors. The maximum voltage for the HV-CMOS partamounts to 80V, so different types of NMOS and PMOS transistors are implemented in this processfor 5V and 80V circuitry. The figure 4.2.5 illustrates principle cross sections for 80V NMOS andPMOS devices, respectively.

Fig. 4.2.5 Schematic of a cross section of the 0,8µm HV-CMOS process showing 80VNMOS and PMOS transistors

For the microgun ASIC two different circuit variants have been realized, each consisting out of 16microgun control circuits which can be addressed individually.In the following microscope picture of figure 4.2.6a three control units are shown with analog anddigital components (circuit 1). The upper part of each unit, which is magnified in figure 4.2.6b,shows the analog circuit with the high voltage gun driver, the current mirrors, the integrationcapacitor and the comparators for charge- and current control. In the lower part (below the testpads) the logic circuit of each unit and the overall multiplexer for unit selection is shown.


a) b)

Fig. 4.2.6 a) Microscope picture of 3 microgun control units (circuit 1).b) Microscope picture of the analog circuit with high voltage transistor and integration capacitor and the charge- and current comparator

high voltagetransistor

analog circuit

capacitorC1

chargecomparatorCOMP1

currentcomparatorCOMP2


T 4.2.5 Documentation of NANOLITH-ASIC

Both ASICs are nearly identical, the only difference is the core circuitry, which measures thecurrent coming from the nanotube.

This part of the documentation is identical to both ASICs. For the user no difference can be realisedbetween the two circuitries, neither for the handling, i.e. the pins, nor the programming. The twodifferent ASICs were designed due to the fact that the electrical behaviour of the nanotubes is stillunknown. So the results of the simulations are depending on the electrical model used for thenanotube.

This documentation will start with the pad arrangement of the test package. The bonding for bothcircuitries is identical, so that no variation for the PCB (printed circuit board)is necessary. Thepackage used is a standard plastic package with 40 pins.

Fig. 4.2.7 Top view of the pads of the ASICs


Name of pad testsocket

testpackage

Kind of pad Use for

Emitter_0 - 15 33 -18 13 – 1,40 - 38

Analog IN To be connected to CNT

VDDa 38 18 Analog IN Positive analog supply voltage = 5VVSSa 39 19 Analog IN negative analog supply voltage = 0V = groundBias 40 20 Analog IN Constant current for measuring circuitry

160nA < Bias < 1mAVref_charge 1 21 Analog IN Reference voltage for comparator 1.5V <

Vref_charge < 3VVref_current 2 22 Analog IN Reference voltage for comparator 1.5V <

Vref_current < 3VSel_0 - 3 3 – 6 23 - 26 Digital in with

pull down4 Pins to choose measuring circuitry 0 – 15 towork and to be connected to Logic_out

EN 7 27 Digital in/active withrising edge

Pin to start selection of circuitry

Start 8 28 Digital in/active high

Defines start of measuring

Reset 9 29 Digital in/active low

Reset signal for starting conditions

Delay 10 30 Digital in/active low

Defines begin the duration of charging Cswitch

Delay1 11 31 Digital in/active high

Defines end of window for measuring thecurrent

Logic_out 12 32 Digital out Output of analog circuitryVDDd 13 33 Analog IN Positive digital supply voltage = 5VVSSd 14 34 Analog IN Negative digital supply voltage = 0V = ground

Table 4.2.1 List of all pins of the two ASICs

In the following pages all signals of the ASICs written in column 1 of Table 4.2.1 are written bold.

To start measuring with the ASIC, first the pins have to be used in the right manner. In the first rowone can find the pin names as they are used in the schematic. The second row gives the connectionsof these pins to the test package. Both ASICs are bonded identically. The third row shortlydescribes the different kinds of pads which are used. Digital pads either INPUTs or OUTPUTs areonly valid for digital values, i.e. VSS=0V or VDD=5V. A non digital value of an output pad is notpossible since the voltage is directly formed by an high amplifying inverter. Consequently, the inputvoltage of an digital input will be fed to an inverter which creates a digital voltage from it. To loadan digital input with an analog voltage will therefore give an unknown input and is notrecommended. The fourth row very roughly describes the meaning of the pads and in several casesthe range of analog input values.

The Emitter_0-15 have to be connected to the carbon nanotubes (CNT). For the packaged ASICsan equivalent electrical solution has to be found. The supply pads for the analog and the digital partare totally separated, VDDa and VDDd, VSSa and VSSd. This is due to the fact that the switchingof the digital elements often causes a spike on the supply voltage that may influence the sensitivesensing of the analog circuitry. To reduce this risk they should be connected as close as possible tothe supply unit.


The Bias pad has to be fed with the constant bias current for the analog circuitry. Since this currentis splitted for all 16 circuits, the current amount has to be 16 times the value, i.e. can be variedbetween 160nA and 1.6uA.

Vref_charge and Vref_current are the pins which defines the threshold voltage for the twomeasuring comparators of every analog circuit. The circuit has detected one of the stop conditions,either the charge is reached or the current is too low, when the logic_out changes from 0 to 1. Thenthe measurement will be stopped, so that the circuitry can’t fall back to the working conditions. Tostart again the complete start and reset procedure must be done again.All other pins declared hereafter will be only connected to the circuit 0 – 15 that is chosen with theSel0_3 pins. This means that only the selected analog circuit will run, although the referencevoltages are defined for and the bias current runs through every other circuit, too.Sel0_3 defines a 4 bit word for selecting one of the 16 circuits that can be selected, i.e. the output ofthis circuit will be fed to the pin Logic_out. These 4 pins are pads with an pull down resistor. Thismeans that if the pins are not connected to any input or to a high ohmic one the voltage is 0V. Sel3is the most significant bit (MSB) and Sel0 the last significant bit (LSB).With EN the values of Sel0_3 will be taken over. More precisely: with the rising edge of thevoltage connected to EN. When switching the ASIC on the first time the content of the memorycells is not defined, so one of the analog circuits is connected to the pin Logic_out. To set the firstanalog circuitry, only the EN pin has to be connected with a rising edge, without having the pinsSel0_3 connected (due to the pulldown). The EN is a pull down pad.

The next four digital pins are connected only to the analog circuit chosen and define the time framesof every circuit. These four pins Start, Delay, Delay1, and Reset are connected to either pull downresistors or pull up resistors. This is done in a way that with these resistors the conditions forstarting an experiment is correct. I.e. if the pins are not connected then Start = 0, Delay = 1, Delay1= 0, Reset = 1 (0 = VSS = gnd and 1 = VDD). The next lines describe in more detail the four digitalsignals.

The Start pin initiates the time frame for measuring. The pin is connected to a pull down resistorand must be actively started with a VDD signal. Setting Start back to gnd directly stops the currentcoming from the gun, i.e. it opens the high-voltage transistor and disconnects the CNT from thecircuit. From figure 4.2.8 it can be seen that XStart = Start signal inverted directly closes S4 anddisconnects the CNT current from the circuitry. Switching Start to 1 closes the high-voltage-switchand connects the CNT to the circuit. In addition to this, Start connects the output from bothcomparators to the following logic part.

The Delay pin defines the time the capacitor Cswitch is connected to VDD, the capacitor Cswitchlocated left to S4 in figure 4.2.2. The pin Delay is connected to a pull up resistor and must be set tognd after the start signal is set to VDD. Either with setting start again to gnd or with the signallogic_out from one of comparators the current flow can be stopped. This will be done by shorteningthe capacitor Cswitch to gnd. The second function of Delay is that with connecting it to gnd themeasuring starts. The capacitor Cswitch can be connected via a switch to either VDD or VSS by T5and S4, respectively. The small logic circuits prevent a possible shortage that may happen byclosing both switches at the same time. Loading the capacitor with VDD is enabled if signal start isVDD and delay is gnd. The current will be stopped if either the start signal is reset to gnd or one ofthe comparators detects the stop condition. In the beginning of a measurement of the current from aCNT this stopping condition is true for the current control, because no current flows and that is trulyless than any meaningful current value. Now a small time is needed to let the CNT-current flow andto settle the new equilibrium. This time is defined by the time difference between connecting startis VDD and delay is gnd. Different solutions were tested to physically realise the start of


controlling current and charge after having the CNT connected to the detecting circuitry. Theproblem was that any switching of the comparator or the signal led to an unacceptable distortion.The solution realised with the reset signal simply sets the output of both comparators to gnd,without creating a shortage in the output stage or a spike somewhere, i.e. it allows a continouslymeasuring of current and charge. With the reset signal set back to VDD the comparator outputs willbe unlocked.

With Delay1 the stop time of the window will be defined in which the amount of current, that flowsfrom the CNTs through the circuits, is measured. When switching on with the signal Start andDelay, the CNTs are connected to the circuits and then current should flow. A part of the circuitcontrols whether the amount of current is underneath a specific threshold that is defined by thevoltage connected to pin Vref_current. Clearly this control must start after having the currentswitched on. Since simulations showed that the current can decrease during the experiment, theDelay1 gives the possibility to limit the time frame of the current control. Connecting Delay1 toVDD stops the time frame. Since Delay1 is connected to a pull down resistor, measuring of thecurrent will not be stopped when the pin is not connected. Permanently connected to the positivesupply voltage suppresses the current measurement.

The Reset pin resets, as the name indicates, the analog circuit to its initial starting values, i.e. itpulls the output of both comparators to gnd, i.e. to the starting position. The pin is connected with apull up resistor, this means that the circuits are actively resetted. Gnd connected to reset enables thecomparators and the current and charge measurements start. The reset signal has not be set to VDDbefore the start signal is set to VDD, too. For simple tests the signals delay and reset can beconnected together.

A typical set of digital input signals should look like this:

Fig. 4.2.8 Typical set of input signals

In all simulations the same set of digital input signals is used, if there are changes, it is mentioned inthe text. This set is shown in the figure above and the exact times for switching are:


Start from 0 to 1 at: t= 100ns in 10nsReset from 1 to 0 at: t= 200ns in 10nsDelay from 1 to 0 at: t= 500ns in 10nsDelay1 from 0 to 1 at: t= 3,11µs in 10ns individually settable

T 4.2.6 Improved setup for the measurement

The measurement results mentioned in the 2Year report were done with a first simple breadboard,in a quite noisy environment and only limited results were archived. To systematically demonstratethe function of the ASICs the complete setup is improved in the following points:

- new designed PCB with well defined wires and an optimised separation of analog and digitalwires

- precise measurement units were used- PC controlled measurement setup for programming and data collection- Several programmes were written for automated testing

Equipment used:

Component Test Fixture 8006 from KeithleyThis box is a multipurpose measurement box for electronic devices packaged in all kind of standardpackagings with various possibilities to connect any pin to different connectors and plugs. The boxprotects the circuits against light and electromagnetic noise coming from outside.

SMU 236 from KeithleyThis is a very versatile measurement instrument which can be used to generate a current and tomeasure the voltage or vice versa. Both functions can be set and measured very precisely and over awide range. With the SMUs the bias current was set as well as the reference voltages Vref_chargeand Vref_current.

Oscilloscope TDS 744A from TektronixThis four channel oscilloscope has a high sample rate of 500 MHz and includes a variety ofmathematical functions for analyzing the measured data. It was used to measure the delay timesbetween setting the signal start and measuring the output signal_out.

Signal generator AFG 2020 TektronixThe generator can be used to create complex signals in analog and digital form. The signalgenerator delivered the mentioned digital signals: start, reset, delay and delay1.

A small micro controller board was used to choose one of the 16 circuits available in each ASIC.A standard supply was taken to provide the controller board with VDD and VSS.

The complete measurement setup consisted of 5 SMUs to control the following inputs:

- To adjust the CNT voltage- To adjust the bias current- To adjust the reference voltage for the charge controlling comparator Vref_charge- To adjust the reference voltage for the current controlling comparator Vref_current- To supply the ASICs with VDD = 5V and VSS = gnd = 0V.


To achieve and record all the data a complex measurement programme was written using thesoftware TESTPOINT from Keithley. This software is especially written to automate single as wellas series measurements. To do so, every used test equipment needs to have a IEEE-488 buscontroller (also known as GPIB-bus) so that the computer can control the measurement and save thedata. The surface of every control unit can be programmed to the monitor so that the user cancontrol the equipment with the mouse from the monitor. With this tool very complex and longrunning measurement tasks with large amounts of data can be realised.

Controller signals

Start, Delay,Delay1, Reset

Choice of ICGnd

MSB

2.Bit

3.Bit

LSB

EN

Fig. 4.2.9a Schematic of connection of component test fixture

Fig. 4.2.9b: Top view of component test fixture


Fig. 4.2.9a and Fig. 4.2.9b show the schematic and a photograph of the fixture. The followingFigure 4.2.10 shows the measurement setup with the opened test fixture, the controller and thepower supply. On the left hand side the tower with the five SMUs can be seen and on the right handside the oscilloscope with the signal generator on top.

Fig. 4.2.10 View of the measurement equipment

To explain the measurement results a more detailed documentation of the circuits than given in thelast report is written here including simulation, measurements and their interpretation.


T 4.2.7 Strategy to measure the ASIC

T 4.2.7.1 DC-current of power supply

In order to check the power consumption of the ASIC, DC-measurements have been done withVDD and VSS=gnd connected , only. The current is mainly dominated by the comparators and hasto be in the range of 0,5mA to 1mA.All measured ASICs showed a normal current consumption.

T 4.2.7.2 Measurement of bias current depending voltages of the nodes „current“ and„charge“

For both circuits comparators are controlling the node voltages of the „current“ and the „charge“node. The „zero-level“, i. e. the potential of these nodes without the CNT emitter current flowingshould be known as a function of the bias current. This dependency is a precondition for theaccurate functioning of the circuit, since the bias current defines the optimal point of operation. Thefollowing figures are showing the simulation and measurement results of the zero-level referencevoltage versus bias current. In figure 4.2.11 and 4.2.12 simulation and measurement characteristicsare depicted from circuit1 in the resolution from 1nA < Ibias < 1000nA, respectively. The samecharacteristics in a lower bias current range 1nA < Ibias < 50nA are shown in figures 4.2.13 and4.2.14. It has to be mentioned that the measurement characteristics are demonstrating the excellentfunctioning of the comperator based charge and current control.

VDDa=5V, VSSa =0V, Bias with input current (NOTE: bias current will be fed parallel for all 16circuits)VDDd=5V, VSSd =0V,

Fig. 4.2.11 Simulation of the reference voltage as a function of the bias current 1nA < Ibias < 1000nA


0,0 200,0n 400,0n 600,0n 800,0n 1,0µ 1,2µ

0,0

0,2

0,4

0,6

0,8

1,0V

ref_

Cu

rre

nt (

V)

I_Bias (A)

Vref_curr(Chip1.1IC3) Vref_curr(Chip1.2IC1) Vref_curr(Chip1.2IC2) Vref_curr(Chip1.2IC6) Vref_curr(Chip1.3IC8) Vref_curr(Chip1.3IC9) Vref_curr(Chip1.4IC12) Vref_curr(Chip1.4IC13) Vref_curr(Chip1.4IC14) Vref_curr(Chip1.5IC7)

0,0 200,0n 400,0n 600,0n 800,0n 1,0µ 1,2µ

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

2,2

2,4

Vre

f_C

ha

rge

(V

)

I_Bias (A)

Vref_chr(Chip1.1IC3) Vref_chr(Chip1.2IC1) Vref_chr(Chip1.2IC2) Vref_chr(Chip1.2IC6) Vref_chr(Chip1.3IC8) Vref_chr(Chip1.3IC9) Vref_chr(Chip1.4IC12) Vref_chr(Chip1.4IC13) Vref_chr(Chip1.4IC14) Vref_chr(Chip1.5IC7)

Fig. 4.2.12 Measurement characteristic of the reference voltages as a function of the bias current for the different pixel circuits 1nA < Ibias < 1000nA

The measurement was done according to the following procedure. The emitter was not connected sothat no current could flow through the high voltage transistor. The four digital input nodes were set


as indicated in Fig. 4.2.2. The voltages Vref_charge and Vref_current were increased stepwisewith incremental steps of 1mV considering a holdtime of 10µs in between. This procedure wasused, because the circuits tend to slightly vary all input nodes, due to their high impedance. After ashort time all nodes have to be reset to their initial voltage. The zero level reference voltage wastaken when within this measuring period the signal_out arises. At this state the voltagesVref_charge and Vref_current were stored, respectively.

The principle behaviour of both curves matches the simulation results quite well, although for verysmall bias current up to 100nA the steepness is higher than predicted. Nevertheless, it has to bementioned that the absolute values of the reference voltages differ from the measurement results.The reason therefore is expected by the high sensitivity of the circuit to technological variations.Here it has to be considered that the node voltages „current“ and „charge“ are generated byconnecting two current sources in series with both having a low output conductance, as shown inFig. 4.2.10. One current source consists out of two n-channel FETs whereas the other is made out oftwo p-channel FETs. A small technological variation may result in a change of the outputconductance resulting very sensitively in node voltage changes. Nevertheless the function of thecircuits are not depending on these values and on the following pages the good functionality will beshown.

Fig. 4.2.13 Simulation of the reference voltage as a function of the bias current 1nA < Ibias < 50nA

In order to get a better resolution of the reference voltage versus bias current dependence thesimulations and the measurements were repeated as described before for a smaller bias currentrange 1nA < Ibias < 50nA. Figure 4.2.13 and figure 4.2.14 exhibit the simulation and measurementcharacteristics of the zero reference voltage (Vref_charge and Vref_current ) in dependence on thebias current, respectively.


0,0 10,0n 20,0n 30,0n 40,0n 50,0n

0,0

0,2

0,4

0,6

0,8V

ref_

Cu

rre

nt

(V)

I_Bias (A)

Vref_curr(Chip1.1IC3) Vref_curr(Chip1.2IC1) Vref_curr(Chip1.2IC2) Vref_curr(Chip1.2IC6) Vref_curr(Chip1.3IC8) Vref_curr(Chip1.3IC9) Vref_curr(Chip1.4IC12) Vref_curr(Chip1.4IC13) Vref_curr(Chip1.4IC14) Vref_curr(Chip1.5IC7)

-10,0n 0,0 10,0n 20,0n 30,0n 40,0n 50,0n

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

Vre

f_C

harg

e (V

)

I_Bias (A)

Vref_chr(Chip1.1IC3) Vref_chr(Chip1.2IC1) Vref_chr(Chip1.2IC2) Vref_chr(Chip1.2IC6) Vref_chr(Chip1.3IC8) Vref_chr(Chip1.3IC9) Vref_chr(Chip1.4IC12) Vref_chr(Chip1.4IC13) Vref_chr(Chip1.4IC14) Vref_chr(Chip1.5IC7)

Fig. 4.2.14 Measurement characteristic of the reference voltages as a function of the bias current for the different pixel circuits 1nA < Ibias < 50nA


T 4.2.7.3 Measurement of charge control function

The main function of the CNT pixel circuit is the exposure dose control, i.e. charge control. Sincethe value of the CNT current is by current mirroring identical with the current change at theintegrating capacitor C1 the voltage at the node charge is the measure for charge control. In thefollowing figure 4.2.15 a simulation of the charge control is given under the condition ofIbias=10nA.The upper plot displays the reference voltages for charge and current control as dashed lines and thecorresponding node voltages as solid lines in orange and red colour, respectively. It can be seen thatVref_current is clearly below the node voltage which means the CNT current level is accepted forcharge control. Due to the discharging of the capacitor C1 the node voltage charge decreases untilthe value of Vref_charge is reached. At this time both comperators change the output signalscharge-out and current-out to the opposite voltage levels, shown in second plot. Both is indicatingthat the required dose value was reached and the CNT current was switched off. The resultingsignal logic-out for switching off the CNT high voltage transistor can be seen in the lower plot offigure 4.2.15. The simulation clearly indicate the correct functioning of the circuit.

Fig.4.2.15 Example of a charge control simulation result of the voltage of node „charge“for a bias current of 10nA

The simulation results shown in figure 4.2.15


- 1st plot: the two reference voltages Vref_charge and Vref_current as dashed lines and theresulting nodes „charge“ and „current“ as solid lines with corresponding colours orange andred

- 2nd plot: the resulting output signal of both comparators charge-out and current-out- 3rd plot: the output signal logic_out derived from the logic circuit by the comperator output

signals current-out and charge-out .

A more complex simulation is demonstrated in figure 4.2.16 illustrating the charge control fordifferent values of Vref_charge with a bias current of 100nA. Here the plot shows the logic_outsignals (y-axis left) having different delay times according to the corresponding reference voltageand the node voltages “ charge” (y-axis right) which linearly decrease over time. Each pair ofcurves corresponds to a different reference voltage Vref_charge.

Fig. 4.2.16 Simulation of signal logic_out as a function of Vref_chargewith a bias current of 100nA

Even in an ideal simulation it can be seen that the gradient of the node voltage “charge”, i.e. thesubtraction of the charge stored on capacitor C1, is different although the bias current was changed,only. This gives an indication of the circuit sensitivity.


Figure 4.2.17 summarises the dependency of signal logic-out versus Vref_Charge for different biascurrent values. The values for ∆∆ (Vref-Charge) / ∆∆ (Delaytime) are in the range of 330 mV/µs forsmall bias currents down to 220mV/µs. This is factor of 30 less compared to the measured value.

0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6

0,0

500,0n

1,0µ

1,5µ

2,0µ

2,5µ

3,0µ

3,5µ

4,0µ

4,5µ

5,0µ

5,5µ

6,0µ

6,5µ

De

lay

of

Sig

na

l lo

gic

_o

ut

(s)

Vref_charge (V)

Delay@IBias40nA Delay@IBias100nA Delay@IBias200nA Delay@IBias400nA Delay@IBias800nA

Fig. 4.2.17 Characteristic of the signal logic-out versus the reference voltage Vref_charge for different values of bias currents

At the real ASIC only the output signal logic-out can be measured, whereas in simulation everynode voltage and current through a component can be monitored.

Concerning the ASIC characterisation it has to be mentioned that the CNT electronic is highlysensitive since currents of a few nano Amperes have to be controlled and measured in a range ofmicro seconds. So it was not reasonable to improve the signal-noise ratio by increasing themeasurement integration time, i.e. taking the mean values of a series of measurements, since theexperiments of charge control are time consuming with long stabilisation time needed.

For the circuit measurements it was necessary to find a substitution for the CNT-emitter. Bestresults were obtained with a voltage of 20V and a 100MOhm resistor between the voltage sourceand the chip. The first measurements mentioned in the midterm report were carried out with a300MOhm resistor connected to a 50V source. Since in all measurements noise played an importantrole it was decided to use the 100MOhm, because the noise of a resistor increases according to theNyquist equation: bandwidthkTRU noise ∗= 4 . In fact the modified measurement set up exhibit morestability.


The following diagram of figure 4.2.18 showing measurement results of the CNT exposure time(delay logic-out) versus the charge reference voltage Vref_charge. The values of Vref_chargecorrespond to different dose values which are correlated to different exposure times. Thisdependencies are shown for the bias currents 100, 200 and 400nA. It can be clearly seen that thecharacteristic is non linear. The voltage range for dose adjustment via Vref_charge becomesbroader with decreasing bias current. For a bias current of 100nA the differential quotient approx.amounts to ∆ (VRef-Charge) / ∆ (Delaytime) ~ 9 mV/µs.A comparison with the simulation shows again that the realised circuits are more sensitive thansimulated ones.

550,0m 600,0m 650,0m 700,0m 750,0m

0,0

2,0µ

4,0µ

6,0µ

8,0µ

10,0µ

De

lay

of

Sig

na

l lo

gic

_o

ut

(s)

Vref_charge (V)

Delay(Chip1.5IC1)@IBias=100nA Delay(Chip1.5IC1)@IBias=200nA

Delay(Chip1.5IC1)@IBias=400nA

Fig. 4.2.18 Measurement of signal logic_out as a function of Vref_charge at different biascurrents 100, 200 and 400nA for Chip1.5 C1

With the increase of the bias current the reference voltage must be set to higher values. Since thevalue of CNT charge is subtracted from the capacitor charge the node voltage decreases with themeasurement time, resulting in shorter time frames with increasing VRef-charge. Therefore, it canbe stated that optimal operation condition are reached with bias current values below 100nA.

A further illustration of the charge control functioning is given by the transient course of the Start-,Reset, Delay- and Logic-out signals as shown in figure 4.2.19. In order to demonstrate the ability ofcharge control two different values for the reference voltage Vref_charge were adjusted for twodifferent dose values (a, b). The time interval between the Delay signal (channel 2) and the responseof the Logic-out signal (channel 4) gives the measure for the dose calculation under the assumptionof constant emitter current. In this example the reference voltage was adjusted for an exposure timeof 2,22µs (a) and 98ns (b), respectively.


a b

Fig. 4.2.19 Measurement of the signals Start, Reset, Delay and logic_out as a function ofVref_charge. The Delay-Logic-out amounts to a) 2.2µs and b)98ns

Start

Reset

Delay

Logic-out


T 4.2.7.4 Measurement of current control function

The other function of the ASIC is the measurement of the current that is flowing through the CNTinto the chip. To be sure that only the current control function is switched on the Delay1 isconnected to gnd for permanently measuring, delay, reset and Start are used as usual.

The idea of this function is that it should be possible to stop the current through the CNT if theamount of charge can’t be delivered in the timeframe given. This function is realised by controllingthe voltage of the node called “current”, see figure 4.2.2 .The simulation of this feature is veryproblematic, since under ideal simulation conditions no instability of the current exists. Therefore, itwas planned to study the results of the measurements with real CNTs to model such a decrease ofcurrent, in case of it’s occurrence.

Fig. 4.2.20 Simulation of the current control function. The transient voltage characteristics of thenode voltages “current”, “charge” and “emitter” are shown.

Figure 4.2.20 shows the effect of switching on the high voltage transistor at t= 100ns. In this casethe node “emitter” (green curve) is connected with the node “current” (red curve) via the transistor.The current flows as predicted and reduces the voltage of node “charge” (brown curve). Figure4.2.21 now show in detail the transient voltage characteristic at the nodes “emitter” and “current”.With the high voltage transistor acting as a shortage, both curves are now nearly identical. Thesimulation shows that a stable situation is reached after 200ns, i.e. at t=300ns. Thereafter, the circuitworks under steady state condition. The relaxation of the node voltage from transistor on state tothe steady state at t= 300ns can be simply described by the behavior of a RC low pass filter. In this


case we can find a time constant τ = R x C = 20ns. Since the function of the current control wasverified in the first measurements no additional simulations were carried out.

Fig. 4.2.21 Details of the simulation of the current control according to fig.

Nevertheless for one circuit the function of this feature could be measured, as shown in figure4.2.22 . For this experiment the outside connections as well as the applied voltages were the same asfor other measurements. Here we can see the delay time of the logic-out signal versus the referencevoltage Vref_current which is identical to the voltage of node “current” for the switching conditionof the comperator. For each point in the diagram a complete measurement was done, so thatsimulation and this plot are not directly comparable. It can be seen that the settling of the node„current“ took much longer than simulated, here we have a time constant τ = 2µs, i.e. 100 timeslonger. There is currently no interpretation about the causes of this behaviour but for the sake ofcompleteness the result is mentioned.


1,7 1,8 1,9 2,0 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

-2,0µ

0,0

2,0µ

4,0µ

6,0µ

8,0µ

10,0µ

12,0µ

14,0µ

16,0µ

18,0µ

20,0µ

Del

ay o

f Sig

nal l

ogic

_out

(s)

Vref_current (V)

Delay(Chip1.1IC1)@IBias=400nA

Fig 4.2.22 Measurement of the current control function


T 4.2.8 Summary

The goal of this workpackage was to realize an ASIC for dose and current control in active matrixCNT microgun application. For this purpose specifications have been defined which were the basisfor concept development and circuit simulation.Two different circuit designs were simulated and realized. The basic concept of both circuits issimilar and hence the simulation behaviour. The designed circuits were fabricated in a 0,8µm highvoltage CMOS-technology with maximum voltage up to 80V.

For the ASIC characterisation a special measurement set up was realized. All measurements werecarried out under computer control. Special testprogramms were developed based on “testpoint”software. Due to the advanced circuit requirements with current values between 1nA and 10nAwithin a time frame of 2 – 10µs, it was clear that measuring the ASICs would be a complicated taskneeding a special set up using high precision test equipment.

The functionality of the design1 circuit was good, in contrast design 2 circuit exhibited an instablebehaviour. All required features of the circuit such as charge and dose control could be verifiedsuccessfully with regard to the specifications. A special feature of the circuit is the high sensitivity,higher than predicted by simulation. It has to be mentioned that a patent is pending for the principleof the circuit.

As an outlook for an improved redesign the following can be stated:Assuming that the specifications will be unchanged an improved circuit should be less sensitive tovariations of the reference voltages and to the bias current. A circuit that can be adapted to a largerrange of inputs would result in a more flexible ASIC. This can be realised by some additionalbuilding blocks. For the realisation of a more dense writing head an area optimized layout has to berealized. Further, a more advanced CMOS technology with smaller feature sizes can be optionallyenvisaged.

T 4.2.9 PatentPatent title: Steuerschaltung zum Steuern einer ElektronenemissionsvorrichtungPatent filed: July 03, 2002 (pending, not granted, EU-registration is in progress)


T 4.3 Fabrication of microguns based on e-beam induced W-C emitter deposition

In the original project plan the task for ISiT was to receive a microgun, combine it with theemission control electronics and perform first primitive lithography tests. At the time when ISiTcould foresee that it would not get a functional testgun because of delays before the end of theproject, the decision was made to produce a test vehicle at ISiT. The fabrication process is based one-beam induced deposition of W-C needles from a precursor Gas (W(CO)6). Compared to theparallel growth of nanotubes this is a slow serial process but it allows the construction of a limitednumber of test structures to test the switching circuitry. It is well known from literature that W-Cneedles can be deposited which are conducting and can also perform field emission (Brünger[Ref.1], Koops [Ref.2]). The tip surface is extremely smooth and tip radii can be as small as 15 nm(Matsui [Ref.3]) depending on e-beam spot size and the emission area of secondary electrons whichplay the main role in the decomposition of the adsorbed precursor gas. The deposits consist of acomposite of tungsten clusters in a diamond like carbon matrix (Matsui [Ref.4]).

For the deposition a standard scanning microscope (Amray 1645) has been equipped with a gasinlet system consisting of a heated gas chamber and a manipulator to adjust the gas nozzle intoclose vicinity of the electron beam. In this way a confined area of high gas pressure is created abovethe deposition site without disturbing the e-beam too much ( see Fig. 4.3.1).

a) b)

c)Fig. 4.3.1 Gas inlet system for e-beam induced needle deposition

a: with manipulator to adjust the gas nozzle to the e-beam.b: with heated gas reservoir,c: gas nozzle close to e-beam axis.

For the microgun structure a design was used consisting of a base metal layer of 300 nm thick Pt –Au-Pt connected to a Au contact pad, a free standing 1 µm thick Ni layer with an array of 5 µm

Gas nozzle with80µm opening


wide apertures, and a gap of 2,8 µm to the base Pt layer ( see Fig. 4.3.2 ). No leakage current wasmeasured up to a voltage of 50 V between aperture array and Pt-base.( isolation resistance : 22 MOhm ).The emitter needles have been deposited through the apertures by positioning the electron beam inthe centre.Deposition parameters have been: e-beam energy 20 keV

e-beam current 200 pAdeposition time 15 minchamber vacuum 4x10 exp –5 mbargas reservoir at 45°C

Fig. 4.3.2 Arrangement of free standing aperture grid 2,8 µm above Pt base layer.


The image of microguns fabricated in this way is shown in Fig. 4.3.3. The needles are 1,5 µm highand 0,2 µm wide at the base ,the tip radius is appr. 30 nm (Fig.4.3.4).

Fig. 4.3.3 Microguns with e-beam deposited W-C needles as emitter tips.

Fig. 4.3.4 Close-up of microgun with tip radius of appr. 30nm.


Fig. 4.3.5 Side view of W-C emitter tip under free standing extraction grid.

First electrical tests of W-C needles are under way with SPM methods to demonstrate conductivityand first results about emission current stability.

Fig. 4.3.6 SPM image of W-C needles with reduced height to demonstrate tip sharpness.

The needles appear too broad at the base because the SPM image gives the convolution of theneedle shape with the shaft of the SPM tip.

A microgun test structure different from the carbon nanotube approach is now available, butbecause of lack of time we will not be able to give results about functional tests in this report.


Ref. 1 W.H.Brünger, K.Kohlmann and A.Umbach:Optimization of Deposition Parameters for E-Beam induced X-Ray Mask RepairJ.J.A.P. Series 3, Proc. of Micro Process Conference, 1989, p.313

Ref. 2 H.W.P. Koops et al:Conductive dots, wires and supertips for field emitters produces by electron induceddepositionJ.Vac.Sci.Techn. B14(6), Nov./Dec. 1996, p.4105

Ref. 3 S. Matsui et al:Electron beam induced selective etching and deposition technology, Superlattices andMicrostructures.Vol.7,No.4, 1990, p.2219-2223

Ref. 4 S. Matsui et al:In situ observation on e-beam induced chemical vapour deposition by Auger electronspectroscopyAppl.Phys.Lett. 51(9), 31 p.646 (1987)


T 4.4 Design of a testvehicle for future lithographic equipment and estimated performance ofthe Nanolith approach

T 4.4.1 Future lithographic equipment

The workhorse of present integrated circuit production is optical lithography. With decreasingwavelength the achieved resolution is well below the 100 nm range:

Timothy Brunner, IBM, EIPBN Conference, Tampa, Florida, May 2003

Significant litho wavelength – resolution: Wmin assumes k1 = 0.3 and NA = 0.9

Source λλ ∆∆λλ /λλ Wmin

I-Line 365 nm 16 % 122 nmKrF 248.3 32 83ArF 193.4 22 64F2 157.6 19 53EUV 13.5 91 16 (EUV:NA = 0.25)

With shrinking dimensions production times and mask costs will increase dramatically resulting inunacceptable manufacturing costs for low volume production. This applies for structuring on thewafer as well as for mask making. One solution is massively parallel lithography with a multitudeof parallel beams individually controlled. There are a number of different concepts for such amask-less parallel lithography ( ML2).

ML2 concepts

• Photon based – single source – multiple mirror• Electron beam – projection optics – multiple source• Electron beam – proximity optics – single source• Electron beam - proximity optics – converter plate multiple source• Electron beam – proximity optics – carbon nano tube multiple source• Electron beam – proximal probe – multiple source• Ion beam – projection optics – single plasma, multiple source

One of them is the Nanolith approach with carbon nanotubes as emitters. A similar concept hasbeen investigated by the Oak Ridge Nat. Lab. Group.

As pointed out by Bill Oldham at the EIPBN-2003, Tampa, Florida, technical bottlenecks for theintroduction of maskless lithography today are:

• lack of reliable multi-beam modulator• lack of suitable particle or photon source


• the challenge of implementing an extremely wide-band data path.• control of CD and placement accuracy at a “1x” technology.

The difficulty of getting a uniform emission current from a multitude of field emitting nanotubeshas been addressed in the Nanolith approach by using current control circuitry at every singleemission tip.

T 4.4.2 Estimated performance of Nanolith approach for mask writing

Assuming a chip size of 10 x 10 mm2 for a 50 nm technology the mask size would be at 4 timesdemagnification 4 x 4 cm2. Again assuming that the complete control electronics for one tip can beplaced in a 100 µm x 100 µm field, the writing head of 4 x 4 cm2 would have 400 tips in a line and400 lines of tips, totally 160 000 tips. The writing head has to be structured with a deep UV Stepperby stitching, eventually the less advanced connecting layers with a wide field stepper.

The wafer moves continuously along a line. At 25 nm spotsize 4000 steps/100 µm and 4000 linesare needed = 16 Mio. steps. Assuming a displacement speed of the wafer stage of 2.5 mm/s = 25 nmeach 10 µs it would take 160 s for the whole 4 cm x 4 cm field, plus 4000 x 100 ms = 400 s for thedelays at the return points of the lines, which means approximately 600 s = 10 min per 4 cm x 4 cmmask field. During this time a resist could be exposed with a sensitivity of

(1 to 2 nA) x 100 tips/cm2 x 160 s ≈ 300 x 100 nAs/cm2 = 30 µC/cm2

which is reasonable.

T 4.4.3 Comment on emitted beam current determination in the presence of a grid current

It would be very difficult to have a separate lead to every individual grid section over an emissiontip to do the subtraction

Ibeam = ICNT - Igrid

also because of the potential difference of appr. 1kV. Therefore we have to approach this problemdifferently. As mentioned by Muray et al (J.Vac.Sci.Technol. B,Vol 10, No6, p.2749,Nov/Dec1992) the relation between Iemitter and Igrid is for constant geometry of the grid aperturedepending on the tip height above it (see Fig. 4.4.1). So for constant emitter height this relation canbe determined theoretically or experimentally. Then the transmission efficiency 1-Igrid/Iemitter

would be known and form a calibration factor to be applied to the emitted current to get the realbeam current.


a) b)

Fig. 4.4.1 a) Total field emission current Ie and Igrid to first electrodeas a function of tip heightb) Transmission efficiency (1-Igrid/Iemitter )After Muray et al: J.Vac.Sci.Technol. B,Vol 10, No6, p.2749, Nov/Dec1992


WP6 : DUP and TIP work package

Dissemination of results :

A large dissemination of the results has been performed during the whole life of the project :

24 conferences

18 articles

3 patents

Exploitation plan for the Nanolith approach

It appeared to the consortium that the best strategy to exploit the Nanolith approach was to build anintegrated project. So we made an expression of interest named :

NESFEA : Novel Electron Sources for Field Emission Applications

The goal was to integrate in a same project the following technological skills : Nanotubetechnology, cathode technology, circuit technology, system technology.

For this purpose, the NESFEA partners were :Large companies : Thales, Philips, FEI,Universities : Cambridge (UK), Lyon (France), Fribourg (Switzerland), Delft (TheNetherthelands),R&D institutes as Fraunhofer ISiT (Germany) and Leti (France),SMEs as Raith (Germany) and Mapper (The Netherthelands).

One of the main goals of this IP was to exploit the Nanolith results due to the size of an IP and dueto the presence of new partners as SMEs : Raith (Manufacturer of Ebeam systems for Lithography),Mapper (Parallel e-beam lithography using a different approach) and large companies as FEI (e-beam based equipment).

Unfortunately, according to different officers, NESFEA did not fit the call NMP nor IST and no IPwas submitted.

It should be noticed that the Nanolith results have lead to the project Canvad concerning the use ofNanotubes for Microwave applications and should lead to a new project about Nanofocus X raysources.

We plan to build a second Nanolith project with the SME Raith. In fact Raith is able to integrate theNanolith writing head into one of their e-beam systems


DELIVERABLES TABLE

Project Number: IST – 1999 - 11806

Project Acronym: NANOLITH

Title: Arrays of microguns for parallel e-beam nanolithography

Del. No. Revision Title Type1 Classifi-cation2

DueDate

IssueDate

3 Lithographic environment requirements R Int 03/01 03/01

10 100 nm beam size with one microgun R Int 2003 2003

11 Active pixel array and 100% writing head concept R Int 2002 2002

1 R: Report; D: Demonstrator; S: Software; W: Workshop; O: Other – Specify in footnote

2 Int.: Internal circulation within project (and Commission Project Officer + reviewers if requested) Rest.: Restricted circulation list (specify in footnote) and Commission SO + reviewers only IST: Circulation within IST Programme participants FP5: Circulation within Framework Programme participants Pub.: Public document


DELIVERABLE 3 SUMMARY SHEET

Lithographic environment requirements



Title: Arrays of microguns for parallel e-beam nanolithography

Deliverable N°: 3

Due date: 03 / 01

Delivery Date: 03 / 01

Short Description:

Resist outgassing, which could contaminate the arrayed microguns, has been investigated

with two methods:

1) Mass spectrometric investigation to determine the chemical compounds leaving the resist

surface in vacuum without e-beam exposure,

2) Quarz balance measurements of resist outgassing under e-beam exposure to determine the

total amount of desorption.

Tests have been performed after standard prebake conditions with chemically amplified DUV-

resists (also e-beam sensitive) from Shipley and Clariant with different acid generators as

possible outgassing sources.

Results for five 4” wafers at 80°C at a total pressure of 4x10-6mbar:

Shipley UV II HS, 550 nm thick, strongest outgassing component C2H5O of 4.6x10-7mbar

Clariant AZDX3301P, 410 nm thick, strongest outgassing C3H7 of 1x10-6mbar.

The total desorption rates under e-beam exposure at 20 keV energy and 0.2x 10-7A current

into a 4x4 mm2 field were about 1 Angstroem/min for the chemically amplified resists and

up to 5 Angstroem/min for PMMA,1100nm thick. This value dropped to unmeasurable values

below 5 keV e-beam energy.

Partners owning:

Partners contributed: ISiT

Made available to: March 2001



100 nm beam size with one microgun



Title: 100 nm beam size with one microgun

Deliverable N°: 10

Due date: 2003

Delivery Date: 2003

Short Description:

UCBL have determined by 3D simulation the design of microguns able to deliver sub 50 nm beamsize (see WP3 – 1)This study implied three imperative specifications for the microgun fabrication process :

One individual and vertically oriented CNT per catalyst dot localised by NanolithograpgyCambridge has determined the lithographic conditions that lead to the growth of one single CNTper catalyst dot with a yield of 100% (see WP1 – 1).

A perfect alignment (< 50 nm) between the CNT and the gate aperture in each microcathodeThales and Cambridge have determined two self aligned process that guaranties this specification :see WP2 - 3 and WP2 - 4 .

Fabrication of CNT microgunThales and Cambridge have fabricated CNT based microguns (see WP3-2)

CNT microgun demonstration should be performed during review meeting

Partners owning:

Partners contributed: CUED, UCBL and Thales

Made available to: 2003



Active pixel array and 100% writing head concept



Title: Active pixel array and 100% writing head concept

Deliverable N°: 11

Due date: 2002

Delivery Date : 2002

The active pixel ASIC was designed, simulated and fabricated. Two different types of circuits have been realised whichdiffer in the concept of the charge- and current-control circuit.

The purpose of the microgun electronics is to select a CNT-pixel and control an emission current flow within a fixedtime interval. It is the goal to ensure a constant current-time-product, which is equivalent to a specific dose value forresist exposure. Depending on the height of the emission current the exposure interval is adjusted within a fixed timeframe. That means, a fixed time interval per pixel corresponds to a minimum current value in order to reach a requiredexposure dose for the resist. These features are realized by charge- and current control circuits based on a comparatorprinciple. After simulating the electrostatics of the microgun arrangement, it was decided that at least a 50V highvoltage CMOS process is required for the CNT microgun circuitry. First ASICs were fabricated by MPW (Multi-Project-Wafer) service of the X-Fab (Erfurt) wafer foundry. The MPW approach is fully compatible with the hybridintegration concept and the best choice for a feasibility investigation.

Special technological features of the 1,0µm HV-CMOS process are two metal levels and an optional double polymodule for stacked capacitors. The maximum voltage for the HV-CMOS part amounts to 80V, so different types ofNMOS and PMOS transistors are implemented in this process for 5V and 80V circuitry.

For a general test of the microgun ASIC the functioning of the current- and charge-control circuits for one pixel wasdemonstrated at first. It was the strategy to test the current- and charge- control circuits independently by appropriateadjustment of the comparator reference voltages. The general function of the charge- and current-control could beverified with current values below 10nA within time intervals in the µs-range, as was predicted by simulation.

For a detailed analysis of the ASICs a new test setup was build which reduced the disturbance of the measurementssignificantly. In addition to this the setup was equipped with high precision measurement units which allow animproved characterisation. To achieve this the setup was connected via IEEE-488 bus and controlled by a PC with acomplex control program (TESTPOINT).

All measurements were done without CNT microguns attached, because no sample was available It could be shown thatASIC1 worked correctly for the implemented functions, especially the charge-control function which was the main goalof controlling a constant current-time-product.. A charge measurement of a 200nA current including switch-off in atime frame down to a few 100ns was possible allowing the control of the targeted value of 10nA in 10µs.

As a replacement for the missing CNT microgun a separate testvehicle has been produced by ISiT based on e-beaminduced tungsten-carbon needle deposition..

Partners owning:

Partners contributed: CUED, ISiT

Made available to: 2002


ANNEX :

EXPRESSION OF INTEREST

Integrated project

NESFEA

Novel electron source based devicesfor

field emission applications

Prepared by :

P. Legagneux, Thales R&T, FranceP. Micheli, Thales Electron devices, France

W.I. Milne, Cambridge University, UKF. De Jong, FEI, The Netherlands

N. De Jonge, Philips, The NetherlandsR. Meyer, Leti, France

D. Pribat, Ecole Polytechnique, FranceVu Thien Binh, Lyon University, France

O. Groening, Fribourg University, SwitzerlandD. Friedrich, Fraunhofer ISiT, GermanyL. Piraux, Louvain University, Belgium

P. Kruit, Delft University, The NetherthelandsE. Jede, Raith, Germany

M. Wieland, Mapper, The Netherthelands


Novel electron source based devices for field emission applications

ACRONYM : NESFEA

1. Aim of the work

This proposal brings together an outstanding consortium of industrial, academic and SME partners with theaim of fabricating field emission devices based on novel electron sources (e.g. nanotubes) for the followingapplications : Electron microscopy, Parallel e-beam Nanolithography, Microwave triodes, electron Torchesand digital X-ray tubes. The aim of an integrated project is to combine our expertise in key field emissiondevice technologies within a system approach which can in the medium/long term be successfully employedin the above mentioned applications.

1.1 Contribution to priority thematic of framework 6

The proposed work will contribute directly or indirectly to several sub-thematic priorities:• 1.3.1 “Nanotechnologies and Nanosciences”. The proposed work will address several of the most vital

problems in nanotechnology, which are:The massively parallel fabrication or growth of nm size objects with controlled geometries andproperties.The integration of these nano-objects in micro-systems.Breakthrough applications by using these novel e- sources

• 1.3.3 “New Production Processes and Devices”. The mastering of the controlled growth of carbonnanotubes, metallic nanowires and nanotips and their integration in microsystems will set the basis forthe development of five new applications from microscopy to X ray tubes.

The results of the proposed work will have a profound impact in other sub-thematic priorities:• 1.4.2 “Space”. The development of field emission based tubes for microwave amplification will lead to

communication satellites with enhanced performance and reduced size due to lower power consumption.• 1.2.3 “Components and microsystems”. The development of field emitter arrays opens the way for

parallel e-beam lithography, pushing the structure size of economic lithography into the nanometerrange.

1.2 Contribution to the European Research area

Advances in stable and cold electron source technology may be at the origin of a whole swathe of newapplications including : Parallel e-beam Microscopy and Nanolithography, Portable electron microscope,Microwave triodes, electron Torches and digital X-ray tubes.Recently, different European laboratories have demonstrated its abilities to develop field emission devicesbased on new cold and stable electron sources. But there is no single organisation within Europe which hasthe resources or expertise to demonstrate alone one of these five applications. However Europe is fortunatein having companies and universities that can form the key building blocks of a world-class consortium,which would have the capability to successfully undertake such applications. This includes one of theworld’s leading manufacturer of electronic tubes and X-ray tubes (Thales), of electron sources andinstrumentation (FEI), academic groups with world leading expertise on field emission source fabricationand/or characterisation (Cambridge Univ., Lyon Univ., Fribourg Univ., Delft Univ., Louvain Univ), CMOSpiloting of field emission devices (Fraunhofer ISiT), and SMEs for fabrication of the microscopy andlithography related devices (Raith, Mapper). Whilst American and mainly Asian companies lead the world infield emission applications, several Europe institutions have been recognised to be key innovators in thedevelopment of the next generation of field emission electron source. The proposed consortium is the idealvehicle to pull together the currently fragmented research activities within Europe into a cohesive andfocused entity. This will allow Europe to become a dominant leader in field emission application as it istoday for the development of new electron sources. In this context the EUROFE network initiative funded in1999 by the European Science Foundation has been a catalyser to bring in contact many groups representednow in this EOI.


Whilst there is much basic R&D to be undertaken, this consortium has a clear focus on the development,demonstration and delivery of practical prototypes as listed in section 3. The commercialisation of thetechnologies developed by the consortium is ensured by the participation of industrial organisations, such asThales who is Europe’s leader for electronic and X ray tubes, as well as established SMEs, such as Raith.The presence of these industrial partners ensure that the R&D will be directed towards a market-drivenproduct and that a smooth transition from R&D to the eventual realisation of successful products takes place.

2. Back ground to the proposed work

Currently the most widely exploited electron source is the thermionic cathode, used for example in cathode-ray tubes and microwave amplifiers and the thermal field or Schotky tip used for electron columns (electronmicroscopy, instrumentation,...). However, due to their high temperature operation (T > 900°C), it is notpossible to fabricate arrays of such electron sources and they can not be integrated on a substrate that alsoholds electronic lenses or CMOS circuits.The new concepts of parallel e-beam microscopy, parallel e-beam nanolithography, microwave triode,electron torches and digital X-ray tubes requires cold and stable electron sources that can be fabricated inparallel on a substrate. Cold cathodes can, moreover, generate e-beams with higher current densities and canbe turned fully on and off instantly. Furthermore, they can produce electron beams in the nanometer range.

Cold cathode electron emission mechanism relies either on a strong local field enhancement due to ananometric protruding geometry (microtips metallic or Si based, carbon nanotubes, nanowires, …), or on ahuge lowering of the surface barrier resulting from the deposition of thin or ultra-thin films with specificproperties on the cathode surface. European laboratories are currently pioneers in these two orientations. Inparticular, this consortium includes the partners of two IST FET European projects based on carbonnanotube field emitters for parallel e-beam lithography (Nanolith) and for microwave amplification(Canvad).

The development of instruments using parallel nano beams, electron sources for microscopy,nanolithography, microwave applications or for free electron lasers requires a strong “system” approach.Because the expertise has to be shared between engineers and researchers from domains lying far away, thesuccess of these new applications imply a need to establish a critical mass of research and development skillsin the field of Nanometric size electron sources, planar electron sources, field emission device design andfabrication, field emission characterisation, and the final integration of these expertise in a prototype. Theseneed to be performed hand-in-hand with the participation of industrial companies so that the technologiescould be exploited effectively in the future.

The recent advances in novel electron sources (see 4.2) and the long experience in industrial application offield emission devices shared by some partners provide a unique opportunity to establish Europe as the worldleader in the development and exploitation of next generation electron source devices in these fiveapplication areas of major importance to the European Union.

The five principal objectives are the practical demonstration of parallel e-beam microscopy, compact andportable electron microscope, parallel e-beam lithography, RF and microwave amplifiers which are wellsuited to future satellite (and multimedia) communication or general high frequency amplification, electrontorches for low energy e- beams and intelligent X-ray tubes for circuit analysis, medical and civilengineering applications.


3. Expected results from the proposed work

The following applications and end users have already been identified.

Expected Result Users of the Results within the EU1 Electron microscopy Performances of electron microscopes depends mainly on the

quality of the electron source. The new type of nm size electronsource based device developed in this IP would improve theperformances of electron microscope, allows parallel microscopyand the concept of compact and portable microscope. Results wouldbe used by companies like FEI, Raith.

2 Parallel e-beam nanolithography Parallel e-beam lithography is a valid candidate for high throughputmask fabrication, low volume maskless prototype circuit fabricationand maskless direct writing as an alternative to EUV. Results wouldbe used by companies like Raith.

3 30-100 GHz microwave amplifier Suppliers of RF amplifiers (such as Thales) would use thesedevices for 1-10 W amplifiers in the 30-100 GHz bandwith

4 Digital Xray sources New low, medium or high energy X-ray tubes could be used forcircuit analysis, control of tags, imaging of very low Z materialsusing arrays of microscopic X-ray sources. Thales who is on of theEurope’s leader for Xray tube would use these results

5 Low energy Electron torches The use of low energy electron beams allows to “softly” sendelectrons on gases or solid materials in order to characterise them(cathodoluminescence of Martian rocks) or modify them(polymerisations) or excite (Ultra violet light generation in solid orgaseous lasersl

4. Activities

4.1 Integration Activities

The team assembled here combines unique expertise in Nanometric size electron sources, planar electronsources, field emission cathode design and fabrication, field emission characterisation, electron optics, circuitpiloting of field emission devices, fabrication of low series, production and system integration with existingtechnologies and components. This integration will transform novel electron source based devices from aninteresting research curiosity to a product in wide use by 2007. Integration activities will include:

(i) A dedicated web site will be established for dissemination of the activities and resultsobtained in this project.

(ii) Workshops will be promoted for dissemination of the results to all interested parties and alsoto bring outside expertise into the project.

(iii) Workshops and brainstorming sessions to define milestones and routes to success.(iv) Researcher mobility will be encouraged between the participating institutions

4.2 Research Activities

The partners in the integrated project bring together a proven track record, a wealth of research experienceand considerable resources. This expertise is essential for advancing the new research activities proposed,and is beyond any one of the partners working alone. There are three main areas:

Development of novel electron source

The most common technological gambits seek either reduce the barrier height of the cathode by using lowwork function materials and reduce the voltage needed to generate the intense fields associated withtunnelling by fabricating well-controlled nanometric size objects for field emission sources.The well controlled nm size objects include : vertically aligned carbon nanotubes (see fig. 1), verticallyaligned metallic nanowires (see fig. 2), nm size metallic or silicon tips.


Low work function materials can be obtained by the deposition of an ultra-thin layer of wide-band gapsemiconductor on top of a metal which acts as an electron reservoir. The technique, known as solid-statefield-controlled electron emission, gives stable emission using low electric fields, even in a poor vacuum.This activity will deliver the field emission properties, brightness, energy distribution, life time for each ofthese new electron sources.

Fig. 1 : Array of 5 µm height, 50 nm diameter carbonnanotubes (University of Cambridge)

Fig. 2 : Array of 1 µm height, 50 nm diametermetallic nanowires (University of Louvain)

Design, fabrication and characterisation of field emission cathodes

Depending on the application and SMEs or Industrials requirements, specific microcathodes will be designedby 3D simulation of the electronic trajectories, fabricated (see examples on fig. 3) and tested. Thecharacteristics of these microcathodes or arrays of microcathodes (emission current, current densities, energydistribution,….) will give clear input for their use in industrial application.

1 µm

gate

base (metal)

CNTs

Fig. 3 : Multiple and single nanotube based microcathodes (Thales and Cambridge)

Development of demonstrators for the five industrial applications

The aim of this integrated project is to combine the expertise and technologies developed in the two firstactivities with the experience and fabrication knowledge of industrials and SMEs involved in this project inorder to demonstrate the new concepts of parallel or portable electron microscopy, parallel e-beamNanolithography, Microwave triode, electron torches and digital Xray tube.

5.0 Expertise Needed to Achieve Objectives


5.1 Critical Mass Required + Multidisciplinary Skills

Europe leads the world of new electron sources. The understanding of nanometric size objects, field emissioncharacterisation and field emission device fabrication are the foundations for the success of our five definedapplication. This consortium has world leading expertise in synthesis and characterisation of novel electronsources (nanotubes, nanotips, nanowires, planar sources, smart microtips), Nanotube technology, fieldemission device fabrication and circuit piloting, system engineering and end users to test the devices in thefield, and device design, fabrication and packaging.

5.2 Proposed consortium

We expect the integrated project to include at the very minimum the following participants. Other memberswith particular need or expertise will be encouraged to join.

Organization Names Area of excellence1 Thales R&T P. Legagneux F Field emission device fabrication process, device

characterisation2 Thales Electron

devicesP. Micheli F Electronic tubes for microwave amplification and

Xray generation3 Cambridge

UniversityW.I. Milne UK Nanolithography localised growth of CNTs

4 FEI/Philips F. De Jonge / N.De Jonge

N Electron emission sources

5 EcolePolytechnique

D. Pribat F CNT growth

6 UCBL Vu Thien Binh F Novel e- sources fabrication and characterisation,7 Fribourg O. Groening S CNT growth and field emission characterisation8 ISiT D. Friedrich G Circuit piloting of field emission devices9 Louvain L. Piraux B Nanowire growth10 Delft P. Kruit Nanosize electron source and electronic design11 Onera A. Loiseau F Nanocharacterisation, NT growth from new materials12 Raith R. Jede G High precision stages, Lithography Pattern

Generators, Lithography and Defect ReviewSoftware

13 Mapper M. Wieland H Parallel e-beam lithography


6.0 Promotion of Results Outside of the Consortium

The combined technical leadership of the consortium members and pioneering nature of the R&D to beundertaken will generate many opportunities for scientific publications and presentations at majorinternational conferences (for example EUROFE and IVMC). These opportunities will be actively pursued,particularly when they provide the chance to educate end users of the availability of next generation fieldemission devices and the outstanding performance they offer. This is important groundwork necessary toensure the wider adoption of these devices for new applications.

An internet web site will be run and maintained in order to publicize a high level roadmap of the plannedactivities of the project and also allow rapid dissemination of key results to groups and individuals outsidethe consortium. The web site will be used as forum to seek input and involvement from non-consortiummembers.

7.0 The role of SMEs in the Proposed Work

The groundbreaking research envisaged in this project will undoubtedly generate many spins off that willprovide excellent exploitation opportunities for a number of SMEs within Europe. It is a priority of thecurrent consortium to identify suitable SMEs and seek their involvement in the proposed project. In additionto providing an outstanding source of competitive advantage to these SMEs (eg. unique access to worldbeating technology and R&D expertise) this will also provide valuable early opportunities for small-scalecommercialisation of novel e- source based devices. The experienced gained here will be invaluable when itcomes to commercialize the next generation e- source based devices that will be developed as a result of thisproject. It is our intention that several SME’s will have joined the consortium and become active membersby the time the final proposal for this project is submitted.

8.0 How will the Project be Managed

The core members of the consortium will form a governing council who take overall responsibility for themanagement of the project. The council will set high level goals and objectives for the IP and ensure thateffective operational management is in place to achieve these targets. The council will regularly reviewoverall project progress, ensure that objectives remain timely and relevant and where appropriate invite newmembers to join the consortium to undertake specific tasks. The governing council will be defined later.

Operational management of the project will be delegated to an experienced project manager (to beappointed) who will ensure that all project activities are effectively co-ordinated and that appropriateprogress is made in meeting defined milestones and objectives.

ist - 1999 – 11806

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