*Corresponding author. Tel.: #359-2-7144680; Fax: #359-2-9753201.

E-mail addresses: vutova@ie.bas.bg (K. Vutova), mladenov@ie.bas.bg (G. Mladenov).

Vacuum 62 (2001) 273}278

Why light ions in future ion lithography?

K. Vutova*, G. Mladenov

Laboratory of Electron Beam Technologies, Institute of electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko shosee,1784 Soxa, Bulgaria

Received 27 June 2000; accepted 8 November 2000

Abstract

The ranges and the average quadratic deviation of some light ions (H�, He�, B�, Ga� and Ar�) in polymethylmethacrylate are calculated over a wide energy region using computer simulation of the accelerated ion penetration. Theelectronic and nuclear losses and other physical parameters concerning the irradiated polymer resist region in the caseof He� bombardment are also presented. Some peculiarities of the chemically ampli"ed resists used in ion nano-lithography are also discussed. � 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Projection ion lithography; Ion penetration ranges; Average quadratic deviation; Nuclear and electronic losses

1. Introduction

Some years ago it was concluded that heavy ions,and ions with high energies, were more appropriatein the "eld of ion lithography because the sensitiv-ity of the resist depends on the electronic losses[1}3]. It was also concluded [1}3] that future ionlithography systems would be accelerators (ionimplanters) with well collimated beams andbeam energy from some hundreds of keV toabout 2MeV. Scanning ion beam machines, witha nanometer diameter of the beam spot, generatedby a liquid gallium source and ion energies up to100keV, were predicted as inappropriate. This wasbecause the short penetration ranges of Ga� ions(or ions with similar mass and energy, used in these

nano-beam systems) in a polymer resist (less than1000As ) were less than the thickness of the resistsused that time (6000}7000As ) [2].Nowadays, lithography requirements have

changed. The range of dimensions less than 150 nmwill be achieved in the generation of microelec-tronics memories and microprocessors during thenext few years. The resist thickness required isabout 4000}5000As to achieve a reasonable value ofthe aspect ratio (about 3}4). Only light ions (

�H�,

�He�,

�Li�,

�Be� and

�B��) with energies from 60

to 100keV have an appropriate penetration rangefor patterns in resists of this thickness (see below).Consequently, it is time to design and try to

apply an ion projection lithography system, usinglight, well collimated ions with energies of 100 keVfor the manufacturing of critical levels of integratedcircuits. In order to overcome the di$culties con-cerning the production of transparent masks, theseprojection systems can become image reducingsystems.

0042-207X/01/$ - see front matter � 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 4 4 9 - 8

Fig. 1. Projected range R�and mean quadratic deviation �R

�for di!erent ions in PMMA over a wide energy region.

Fig. 2. Dependence of the mean projected rangeR�, of the mean

quadratic deviation �R�and of the transverse mean quadratic

deviation �> vs. the atomic number Z for an ion energy of100keV and a PMMA target.

In this paper some calculated data on the trans-verse, longitudinal and projected ranges, as well asthe electronic and nuclear losses in the case ofa polymer resist irradiation with light ions arepresented and discussed. Some data concerning thedevelopment process of the exposure resist aregiven.

2. Calculated data and discussion

The computer simulation of ion penetration inpolymers was made using our version of the TRIMcomputer program [2}4] based on the famousBiersack model [5]. Polymethyl methacrylate(PMMA) was chosen as a typical polymer resist.The chemical composition of this resist is C

�H

�O

�,

the e!ective atomic number is Z"3.6, the atomicmass M"6.7 and the density �"1.2 g/cm�. Thecalculations are done for 10 000 irradiated ions.Fig. 1 shows the projected penetration ranges

R�"�t

�� (the mean value of the projections t

�of

the ranges t on the axis x coincident with beam axisand with the sample depth; the brackets �� markthe average procedure) and the longitudinal (pen-etration) range deviation �R

�"�(t

�!�t

��)����

of the ions (H�, He�, B�, Ar� and Ga�) in a wideenergy region.Fig. 2 shows the projected penetration ranges

R�and the mean quadratic range deviation �R

�(square root of the dispersion assuming a normaldistribution of the penetration ranges R

�) as well as

the transverse ion range deviation �>"

�(t�!�t

��)����"�(t

�)���� (the projected length

t of the penetrated ion ranges between two colli-sions in the direction y parallel to the sample sur-face axis, characterising the width of the irradiatedline) vs. the atomic numbers Z of the irradiatedions.From these curves it is concluded that only H�,

He� and B� cover the requirements of the presentday ion lithography which aims to achieve 150 nmresolution patterns. The projected ion ranges R

�on x and their deviations �R

�are, respectively,

9938$558, 7964$762 and 5642$900As for anion energy of 100 keV. Note that R

�and the �R

�for positive ions of

�Li and

�Ba� accelerated to the

same energy, which are not included in the datashown in Figs. 1 and 2, are 7701$924 and6779$977As . Due to its chemical inactivity, He�

274 K. Vutova, G. Mladenov / Vacuum 62 (2001) 273}278

Fig. 3. Dependence of the mean range R, calculated through thebombarding ion trajectories, of the mean quadratic deviation ofthis range �R, and of the transverse mean quadratic deviation�> vs. the energy for He� ions and the resist PMMA.

Fig. 4. Dependences of the calculated moments of the spacedistributions in the case of He� ions, penetrating PMMA: curve1 is �t�

�/�>�� i.e., the transverse kurtosis �; curve 2 is �t

�t��/t���;

curve 3 is the kurtosis �"�t��/�R�

��; curve 4 is �t�

�t��/t�

��. The

curve under the horizontal axis (the energy) is the skewness�"�t�

�/�R�

��.

has some advantages among these ions, but all ofthem have satisfactory ranges. The values of thetransverse range deviation �> for 100 keV ions(Fig. 2) are: H�* 1030 and He�* 1150As , respec-tively (nearer to the maximum values, but still lessthan the desirable line width of 150 nm).Data calculated for the all other moments char-

acterising the spatial distribution of the He� pen-etration in the irradiation PMMA are shown inFig. 3. The upper curve represents the calculatedranges through the penetrated particle trajectoriesR

�"�t� (t are the straight line free paths between

the sequential collisions of the projectile ion withtwo of the resist atoms, chosen using the MonteCarlo procedure and an appropriate probabilityfor the chemical resist composition). The meanquadratic deviation �R

�"�(t!�t�)���� and the

transverse mean quadratic deviation �> are alsoshown in Fig. 3.The curve situated under the horizontal axis in

Fig. 4 represents the moment of the third ordercharacterising the distribution symmetry i.e. theskewness �"�t�

�/�R�

��. For example, �"0 means

that the distribution of the ranges is symmetrical.The next moment of the projected range distribu-tion is the kurtosis �"�t�

�/�R�

�� which character-

ises the tail of the projected range distribution(curve 3 in Fig. 4). For example, �"3 correspondsto the normal distribution; �"6 is in the case of anexponential distribution; the higher values of

K. Vutova, G. Mladenov / Vacuum 62 (2001) 273}278 275

Fig. 5. Electronic stopping powers for He� penetrating PMMA.The lower curve * the electronic losses �dE/dx�

���of second-

ary particles, moved after collisions with the projectile ion. Theupper curve* the electronic losses of primary ions as well as thetotal electronic losses: �dE/dx�

���+�dE/dx�

��� . Fig. 6. Nuclear stopping powers for He� in PMMA.

� means that longer distribution tails exist. Curve1 (Fig. 4) shows the transverse kurtosis concerningthe distribution symmetry in a width. Curves 2 and4 represent the moments �t

�t��/t�

�� and �t�

�t��/t���,

respectively.Fig. 5 shows calculated energy losses in the case

of He� bombardment of PMMAwhere �dE/dx� isthe mean electronic energy loss. Indices 1 and 2 cor-respond to the primary and secondary ions, respec-tively. The upper curve represents the electronicenergy losses of primary ions, whereas the lowercurve describes these losses for secondary ions. Thetotal electronic energy losses (the sum of theselosses of primary and secondary ions) coincide withthe upper curve (Fig. 5) since secondary ions havea small contribution to the total electronic energylosses.The index f of the mean nuclear energy losses

�dE/dx���

means losses of energy through nuclearcollisions (Fig. 6). The maximum value of theselosses is assumed to be equal to 20 eV. It is assumedthat these collisions create only phonons (i.e. heat-ing). The index d of �dE/dx�

��indicates the higher

portions of lost energy ('20 eV) through the nu-clear type of collisions, which are able to change the

resist structure completely by the creation of defects* curve �dE/dx�

��, Fig. 6. The total nuclear en-

ergy losses, �dE/dx��� , are also shown in Fig. 6.

A comparison between the electronic stoppingpower and the nuclear stopping power values in thecase of He� in PMMA con"rms the more generalconclusion, based on the experimental data, thatthe electronic stopping losses [1] determine thesolubility changes in the resists due to the fact thatthe values of �dE/dx�

��are higher by more than

two orders than (dE/dx'�in the ion energy

range studied. This conclusion can also be madeusing the solubility rate relation [6] of a resist withan initial molecular weightM

�, irradiated by an ion

dose D:

S"S�#B[(1/M

�)#(g

�E�#g

�E�)D/(�N

�)]�, (1)

where S�is the solubility rate of unexposed poly-

mer, B and A are constants for a given resist-developer couple, g

�and g

�are the radiation ef-

"ciencies in the resist from the electronic losses andfrom the nuclear losses of bombarding ions, E

�and

276 K. Vutova, G. Mladenov / Vacuum 62 (2001) 273}278

E�

are the adsorbed energies in the resist[eVC �cm �], � is the resist density and N

�is

Avogadro's number.Eq. (1) describes the relation S(D) for a given

resist thickness and energy. The calculated rangesand electronic stopping powers can easily convertthis dependence into a more physically clear de-pendence S(E

�/<), where E

�/< is the absorbed en-

ergy in one unit of resist volume. For everyresist}developer combination these dependencesgive the possibility to optimise the sensitivity used(or dose requirements) to achieve the desired con-trast for a chosen resist and developing system. Thetypical sensitivity is from 10 � to 10� �Ccm � thatis from 10 � to 10� J cm � and/or 10 to 10� J cm �

[1,4,9,10]. In Ref. [2] two cases of this relation S(D)are distinguished. In the "rst of them, a universaldependence S(D) is obtained during the develop-ment process with one and the same solubility rate.In the second case, the obtained dependence S(D) isa multi-valued function with a non-linear solubilityrate during the development process for di!erentdeveloping times. One can see, that the contrastparameter value is proportional to the slope of thedependence S(D). In both cases, one and the sameresist}developer couple can show a high sensitivityat a low contrast value and a high contrast value ata lower sensitivity for di!erent conditions. This isdue to the value of A in Eq. (1) which is higher than1 (A"1.5 for the resist PMMA and the solventmethyl isobutyl ketone: isopropyl alcohol(MIBK:IPA) (1 : 1)) [2].The values of radiation e$ciencies g

�and g

�,

that characterise the mean number of chemicalevents (chain scissions in PMMA) per 1 eV ab-sorbed energy, are less than 2 for PMMA in the0.5�m lithography region [6]. In the case of otherpolymer resists and resolutions this upper limit canbe about 10 (starting from minimum values of theorder of 0.1 provided, there are reasonable irradiat-ing #ows and times). The maximum value ofg (10�}10�) is limited by the minimum energy re-quired for the chemical changes and the thermalstability of the resist. This value is lower for higherresolution lithography. The electronic losses ofpenetrated ions are the more important phe-nomena because of (i) the values of nuclear lossesare two to three orders less than the values of the

electronic losses, (ii) and the small e$ciencies in thecase of ion irradiation in the studied energy range(the values of the total g, which is e!ectively g

�, are

in the range 0.006}0.023 [6], i.e., signi"cantly lessthan 1.0).The decreased energy and mass of the irradiated

particles is an advantage for the mask reliability,but it reduces the sensitivity (due to the lower valueof the radiation e$ciency g

�) of the traditional ion

resists (for example PMMA). The brightness valueof ion sources is also small due to space}chargee!ects. Therefore the application of chemically am-pli"ed resists (CARs) is convenient [7}10] insteadof simple positive resists such as PMMA and tra-ditional negative resists, used in the region of0.5}1.5�m. In this case, instead of the direct chainscissions (in PMMA), in the positive CARs (forexample APEX-EUV resist [11]) or crosslinking innegative CARs (SAL-601 [9]) there is a two-stepprocess of (i) generation of an acid by the sensitizerduring the exposure process, (ii) and di!usion andcatalytic solubility changes during the post-expo-sure bake-PEB (and exposure-PEB delay time).The use of photo- and electron-sensitive resists inion lithography will lead to preservation ofthe predominant role of acid formation by elec-tronic stopping losses. Only a general radiatione$ciency from the two-step process, depending onmore factors (PEB temperature and time are newfactors), can be determined and used in Eq. (1)for the solubility rate evaluation and for optimisa-tion by choosing an optimal sensitivity and con-trast for a given developer. The main task of suchoptimisation is to keep the maximum resolutionbased on a knowledge of a latent image transfer,minimisation of the transverse deviation of the pro-jectiles, the proximity e!ect during the exposureprocess of the sensitiser, the result of the generatedacid concentration and the related sensitiser de-composition, the acid transverse di!usion and anappropriate consumption of existing acid duringthe PEB.

3. Conclusions

In this paper, some features of projection ionlithography, which is one of the alternatives for

K. Vutova, G. Mladenov / Vacuum 62 (2001) 273}278 277

the realisation of sub 0.5�m lithography (deepUV-220/280nm photolithography, scanning orprojection electron beam lithography, X-ray andscanning ion lithography) were discussed. Aconclusion concerning an appropriate use of lightions at energies 60}100keV is reached. Resultsfrom computer simulation of the penetration oflight ions (mainly Helium) are shown and dis-cussed. A discussion on the relation between thetwo important parameters * the sensitivity (doserequirements) and the contrast for a speci"c re-sist}developer combination is presented. The needfor using chemically ampli"ed resists in the techno-logy is explained. During the next three}four yearsthe competition in the microelectronics manufac-turing of key integrated circuits will drive the selec-tion of future winners in this very expensive step ofnano-structuring.

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