nano- and micro-technology applications of focused ion beam processing

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Microelectronics Journal 28 (1997) 465-473 © 1997 Elsevier Science Limited

All rights reserved. Printed in Great Britain 0026-2692/97/$17.00

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Nano- and micro- technology applications of focused ion beam processing D.F. Moore1 J.H. Daniel 1 I

and J.F. Walker 2 I Cambridge University Engineering Department, Trumpington Street, Cambridge CB2 1PZ, UK 2FEI Europe Ltd, Cottenham, Cambridge CB4 4PS, UK

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Advances in focused ion beam technology including higher current density, finely focused beams under computer control are making micromachining an excellent prototyping route for devices and a commercial possibility for production. This paper concentrates on new applications of focused ion beam processing in Si based microdevices and in sensors, including high-resolution patterning of YBaCuO superconducting Josephson junction devices. © 1997 Elsevier Science Ltd.

1. In t roduct ion

W ell-established applications of focused ion beam (FIB'.) technology in the micro-

electronics industry include: (i) the repair of prototype integrated circuits and photomasks; and (ii) device failure and process analysis [1]. Recently, FIB systems have shown promise for a surprising variety of new applications, based on the technology to use a finely focused (10nm spot size) gallium ion beam for imaging and precision milling [2-7]. For example, present Si micro-electro-mechanical systems (MEMSs) are

made largely using conventional semiconductor processing technology to produce ultra-minia- ture mechanical structures, from simple canti- levers and sensor devices to micro-motors, pumps and micro-fluidic systems. Adding FIB processing for nano- and micro-technology enables precision cuts to be made with great flexibility, including the possibility of cutting microstructures at oblique angles. The etch rate using FIB processing is relatively independent of the material to be etched. This is a great advantage in the precise fabrication and trimming of high temperature superconductors such as YBaCuO for superconducting quantum inter- ference devices (SQUIDs) applied to magnetic field sensors and small scale integrated circuits. Finally, the precise imaging and cutting now possible with FIB systems can be used to simplify and to extend the range of transmission electron microscope (TEM) sample preparation, which is vital in the quest to improve the yield of advanced integrated circuits [8].

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2. Micro-technologies

Several MEMSs have been proposed, demon- strated in research laboratories and, in the case of cantilever based accelerometers with capacitative sensing, have recently been put into mass product ion [9-17]. A commercially available chip has an integral 2/~m thick surface micromachined polysilicon proof mass anchored to a silicon substrate by a micromechanical suspen- sion, and it has integrated readout electronics. The fabrication is based on the well-established processes used in the micro-circuit industry: namely, photolithography, physical and chemical vapour deposition, wet and dry etching. The potential market is large, including automotive applications such as air-bag actuators, advanced braking systems, and navigation instrumentation, but many applications require increased precision. Recent reports show that a significant increase in the sensitivity of accelerometers can be obtained when a tunnel junct ion is used to sense the opening and closing of a gap between the p roof mass and a fixed electrode [18-23]. The high performance derives from the fact that the resistance of a tunnel junc t ion increases exponentially with electrode separation. The bulk micromachined tunnel devices reported to date have great sensitivity, but they require the assembly of two or more micromachined parts, and the manufacturing costs would be high.

The use of silicon on insulator (SOl) starting material promises a manufacturable acceleration sensor with tunnel junc t ion readout, fabricated using a single wafer process [24, 25]. As sketched in Fig. 1, the FIB is used to cut a Si cantilever at an oblique angle to form the gap g in the structure. In the proposed mode of operation, a voltage is to be applied between the p roof mass p and the substrate s to close the gap g and main- rain a tunnel current across it. Using force feed- back, when the sensor experiences an acceleration in the direction of the arrow in Fig 1, the proof mass is to be held in a fixed position by adjusting the voltage between p and

g

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Fig. 1. Cross-section of the proposed accelerometer showing the single crystal Si proof mass p, separated from the Si substrate s by the oxide layer o, and the focused ion beam

etched readout gap g.

s. This voltage is the readout signal correspond- ing to the acceleration.

For this structure a small gap that is not parallel with the cantilever mot ion must be cut, to ensure that the gap is closable. The use of FIB etching is crucial because conventional techni- ques cannot etch at an angle to the normal to the substrate with the large aspect ratio required to mill through a thick cantilever and leave a sub- micrometre gap. The use of a single crystal Si p roof mass is designed to reduce cross-sensitiv- ities because it ensures that the mechanical properties and thermal expansion coetticient o f the proof mass match those o f the substrate. The fabrication process used for prototypes is simple and employs a single photolithographic mask as outlined in the following sequence:

1. Deposit a hard mask layer on a commercial bonded SOl wafer.

2. Apply optical photoli thography and reactive ion etching to pattern the mask layer.

3. Wet etch to transfer the pattern into the p roof mass Si layer in K O H and H 2 0 .

4. Wet etch the underlying SiO2 to undercut the p roof mass using HF and H 2 0 .

5. Focused ion beam to etch (< 5 minutes) a readout gap and fully release the p roof mass.

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The starting material is commercially available bonded SOl with single crystal Si layers in the thickness range 3-7j~m, and SiO2 layers in thickness range 2-4/~m.

A masking layer, such as Si3N4, is deposited on the substrate and patterned as oulined in the above sequence. The shape of the proof mass is etched in the single crystal Si which is then undercut to give the structure shown in the secondary electron micrograph (SEM) in Fig. 2. The perforations in the proof mass ensure efficient undercutting during the SiO2 etch process, without excessive undercutting of the fixed bank to which the two canti- levers supporting the proof mass are anchored.

Figure 3 shows a secondary electron image in the ion beam system after cutting the beam in another accelerometer test structure. The sample was rotated just after the FIB cut to take this image. The micrograph in Fig. 3 shows that the stress in the SOl wafer is sufficiently low to allow large structures to be made without bending significantly out of the plane when they become free-standing. The FIB etch time required to make the oblique cut in the 5 #m wide 3 #m thick Si cantilever was less than 5 minutes with a 30keV 0.4nA gallium ion beam. The FIB milling time could be reduced by using a high current to cut through most of the Si, followed by a finer cut at a reduced ion beam current and beam diameter to mill the narrowest possible tunnel gap. In the ion beam

Fig. 2. Secondary electron scanning electron micrograph of the surface micromachined accelerometer structure after release of the Si proof mass, but before focused ion beam etching the readout gap. The silicon is 3 #m thick and stands 4/~m above the

substrate.

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Fig. 3. Secondary electron ion image ofa 3/,m thick single crystal Si accelerometer structure 4/~m above the Si substrate. The sub-micrometre readout gap was cut by a Ga focused ion beam.

processing, multiple cuts from different angles can be used to produce shaped electrodes. Figure 4 shows a 7 #m thick silicon beam that was cut using a gallium ion beam with iodine assist which more than doubles the etch rate. To produce durable tunnel contacts the final step in the process sequence must be to deposit a contact layer, e.g. a few nanometres thick Pt by sputter deposition.

This overall process sequence is simple and for high-value sensors it is not a serious disadvantage to use the serial FIB process to cut the beams.

The flexibility o f FIB processing is a great advantage for device prototyping: in the longer term, angled dry etch processes may be devel- oped to allow batches o f sensors on a wafer to be made in parallel.

3. Superconducting Josephson junction devices

The rate o f development o f electronic applications o f high superconducting transition temperature (high-To) materials is determined by progress in Josephson junction technology and

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Fig. 4. Secondary electron ion image of part of a 7/lm thick single crystal Si accelerometer structure 2 #m above the Si substrate. The readout gap was cut by a Ga focused ion beam using iodine to enhance the etch rate.

fabricating weak links [26-35]. Well-controlled junct ions suitable for SQUID sensors are made on bicrystal substrates; but for general applications such as A/D converters to operate at temperatures approaching that o f boiling nitro- gen (77K), sub-micrometre low inductance device structures are required. Focused ion beam machining is very useful both for fabricating and t r immimg these devices.

There are several junc t ion configurations under development and typical problems are junc t ion

uniformity and reproducibility. Interfaces and materials must be controlled on the relevant superconductor length scales (10nm) to make high-To Josephson junctions. One promising approach is to fabricate directly a weak link by irradiating YBaCuO with a focused electron beam to make the junct ion structure shown schematically in Fig. 5 [27, 28]. The high- freqency electrical behaviour of the junct ion under microwave irradiation fits well to the resitively shunted junct ion (RSJ) model, and the magnetic field dependencies o f the junc t ion

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g%.

Fig. 5. Cross-section showing the fabrication method to form an electron-beam damage Josephson junction in a YBaCuO superconductor s using a focused 350keV e- beam e which damages the dotted region of super-

conductor.

and operated above 70K. The design and process flexibility of e-beam junctions allows compact low inductance devices, particularly when combined with focused ion beam patterning of the YBaCuO. Figure 8 shows detail of one of the structures. Josephson junctions having widths different from the track width are produced by applying a heavy damage dose at the edges to destroy the superconductivity as seen in Fig. 8, leaving 300 nm lightly damaged junction regions. For sub-micrometre device applications it is desir- able to make precisely defined SQUID structures using focused ion beam cuts.

critical currents confirm the uniformity of the Josephson current density [28].

Figure 6 shows four e-beam damage junctions in series made in a 200 nm thick 3 pm wide superconducting YBaCuO track using a 350 keV electron beam to damage the superconductor directly and form the Josephson weak link. The topologi- cal contrast as seen in Figs. 6 and 7 where an e- beam damage line has been written, is due to a carbon containing layer from contamination in the e-beam cold-trapped vacuum system. Its presence is incidental and does not affect the electrical operation of the underlying device. It is useful that in the FIB system there is sufficient contrast in the 30keV Ga ion beam image to register FIB cuts to previously made junctions. This allows junctions to be trimmed and induc- tances to be defined with greater precision than is feasible with optical lithography and broad beam Ar ion etching. To make the structure in Fig. 6 the etching time was 1 minute with a 150pA 30 keV gallium beam. Cutting only the perimeters of shapes minimizes FIB time and debris. On insulating substrates such as MgO there is a clear end point when the ion image of the YBaCuO island changes contrast.

Superconducting interferometers have been both made and trimmed using FIB processing

At present, the details of the damage mechanism, the length scale and nature of the e-beam weak links are not yet well understood. Junction stability during room temperature storage can be improved by irradiating at 350 keV with a high fluence to reduce Tc below 77K, and then putting the devices through an anneal protocol to heal partially the damage. To establish a manufacturable technology, the junction uniformity and stability must be brought under better control.

4. Conclusions

Advances in focused ion beam technology towards finer focus and higher current beams have made micromachining by FIB both an attractive ideal and a commercial reality. While integrated circuit repair and process analysis are the main applications of focused ion beams, there are exciting new applications in the prototyping of nano- and micro-devices, and in the manufacture of high-value components. FIB- defined high temperature superconducting devices are promising for sensor applications and high-performance circuits. In microsystems technology there are interesting possibilities using the ability of focused ion beams to make oblique cuts and sub-micrometre structures. The general approach of using commercially available SOl wafers for machining single crystal

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Fig. 6. Secondary electron ion inaage ofa~3 #m wide 0.2/~m thick superconductor track with four e-beam damage Josephson junctions in series, identifiable by the carbon contamination layer on the surface of the YBaCuO from the focused e-beam. The subsequent focused ion beam process took 1 minute to trim the junction lengths by etching the perimeter of the two

rectangles with sub-micrometre cuts.

Si micromechanical structures is simple, flexible and has many possible applications in sensors and MEMS.

Acknowledgements

The authors have appreciated collaboration and helpful discussions with M. G. Blamire, S. C.

Burgess, T. Dingle, H. Fujita, M. I. Lutwyche, M. Miyazaki, A. J. Pauza, N. Shibaike, Y. Soutome, J. T. Whitney and R. J. Young.

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Fig. 7. Secondary electron ion image at a different angle of the 3 #m wide superconductor track with four series e-beam damage junctions shown in Fig. 6. After the focused ion beam etch process used to trim the junction lengths, the two isolated

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Fig. 8. Secondary electron ion image of a SQUID comprising two 300 nm long e-beam junctions in a superconducting loop defined by a focused ion bedim etched slit in a YBaCuO thin film. In this case the Josephson junction length is determined by using a heavy e-beam dose to destroy the superconductivity adjacent to the desired junction, giving a carbon layer of increased thickess as seen in the figure. The surrounding superconductor structures are defined by using a sub-micrometre focused ion

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nano- and micro-technology applications of focused ion beam processing

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