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Focused ion beams in microsystem fabrication
-'
MICROELECTRONIC ENGINEERING ELSEVIER
Mieroelectronic Engineering 35 (1997) 431-434
Focused ion beams in microsystem fabrication J.H. Daniel a, D.F. Moore a, J.F. Walker b, J.T. Whitney b aCambridge University Engineering Department, Cambridge CB2 1PZ, UK bFEI Europe Ltd., Cottenham, Cambridge CB4 4PS, UK Focused ion beam (FIB) systems are widely used in microelectronics for prototype modification, device failure analysis and process monitoring. With the growing importance of micro-electro-mechanical systems (MEMS) the question arises whether FIB can be successfully and economically applied to prototyping and production in that area.
1. FIB FOR THE FABRICATION OF A MICROACCELEROMETER So far, focused ion beam (FIB) techniques have many applications in microelectronics [1]. Recently silicon MEMS is of growing importance in acceleration sensors and other systems [2-6]. To demonstrate the usefulness of FIB milling in this field, it was applied, in combination with bonded silicon-on-insulator (SOl) material, to the fabrication of micro-accelerometer structures [7,8]. As shown in figure 1, a square proof mass (perforated to allow underetching) is suspended on two arms. A narrow silicon beam fixes the structure at a third point during wet etching and is then cut at an angle using a 30keV Ga ion beam. In summary, the process steps are: • oxidation of the SOl wafer for masking layer • patterning of proof mass • anisotropic wet etching of the 3~tm top Si layer • wet etch in HF(conc.) to undercut the proof mass • focused ion beam to mill the readout gap and fully release the proof mass The operating mode would either be tunneling across a gap or a tapping mode in which the gap acts as a kind of micro-switch and the duty cycle of switching is measured. In both cases the acceleration causes the proof mass paddle to move and changes the size of the gap. FIB is used because of its ability to cut deep and narrow trenches at an angle. A narrow readout gap is required to keep the operating voltage low [7]. The deposition of an appropriate metal layer (e.g. Pt) will be necessary as a final step. 0167-9317(97)/$17.00 © 1997 ElsevierScience B.V. All rights reserved. PII: S0167-9317(96)00128-1
Figure 1. FIB image of an accelerometer structure with FIB cut. The silicon proof mass is 3p.m thick.
Figure 2. FIB cut of silicon beam at 5O°. wJtn an ~on current of 350pA, the cutting time was less than 5 minutes.
432
J.H. Daniel et al. /Microelectronic Engineering 35 (1997) 431-434
Figure 3. Silicon beam with a two-step FIB cut at 50 °. Initially an ion current of 2700pA was applied for 9 seconds, and the f'mal cut was done in 45 seconds with 150pA. 2. FIB P R O C E S S For the FIB cut we used a 30keV Ga ion beam. As FIB is a serial process, the economic aspect of the fabrication process for the devices is critical. The FIB process would add about $4 per device if the milling time were less than five minutes. Low currents can be used to produce the required narrow cuts but the cutting time is then long. In figure 2 an ion current of 350pA produced the cut in less than 5 minutes. Iodine enhanced etch can be used to increase the etch rate but the gap size then becomes greater. For deep trenches and low ion currents the etch time is still unacceptably high. As an improved process with comparatively short etch times, a two-step cut was introduced. First the beam is milled for several seconds at high ion current. Subsequently, a low current finely focused beam does the rest. This results in a gap of concave shape as shown in Figure 3. The gap size in the cen-
tre is small. However, stress in the currently used SOI material results in the suspended part lifting slightly and the gap size is larger than the actual cut. Methods to overcome this problem and the possible use of different SOl material are under investigation. 3. M E A S U R E M E N T S Measurements of the resonance frequencies of the accelerometer structures were carried out in the electron microscope. A 1V AC signal with variable frequency was applied between the proof mass and the substrate. For the small structure of figure 1 (proof mass size: 160pm x 160pm x 3~tm) a sharp resonance was observed at 75kHz drive frequency which corresponds to a mechanical resonance of 150kHz. On the bigger structure in figure 4 (220~tm x 220pm x 3pm) the lowest frequency mechanical resonance was at 90kHz (with a 45kHz drive). In figure 4 the structure is in resonance with a 1V AC drive and the vibrating part is blurred while the fixed part is sharp.
J.H. Daniel et al. /Microelectronic Engineering 35 (1997) 431-434
433
Figure 4. SEM image o f the accelerometer structure in resonance at 90kHz. The vibrating part appears blurred. 4. P R E V E N T I O N O F S T I C T I O N As wet etching techniques were used to fabricate the structures the common problem of stiction arises (figure 5) [9]. Our approach to that problem was to add small suspension beams and cut them afterwards with FIB. Figure 6 shows the success of this method for the accelerometer paddle. All structures with an additional beam had not stuck to the substrate. The time to cut the additional beams with FIB is relatively low as high ion currents can be chosen. For the beams which had a width o f 4~tm on the top the cutting time was around 20 seconds per beam, giving the structure in figure 7. These beams can also be useful in prototyping for gradually
Figure 5. Stuck down paddle after wet etch.
changing the dimensions or form of a mechanical structure. The usefulness of this method for making force transducers is shown for the structure in figure 8 which could be used as a magnetometer when a current is sent around the loop to produce a Lorentz force. In this structure one readout gap and four additional suspension beams can be seen. 5. C O N C L U S I O N S In microfabrication technology there are unique possibilities of using focused ion beam milling to produce oblique sub-micron cuts. Its usefulness in prototyping has been shown using silicon-on-insula-
Figure 6. Non sticking structure with additional beam.
J.H. Daniel et al. /Microelectronic Engineering 35 (1997) 431-434
434
411'
cut
41r
4P'
4P'
4D' 41'
,i, 41r
4p, ~
~ ~ 41' 'I' 41' ~r P' ~ all¢4 P' a t' , .4. .' ~m' ,
i, 41 '
i
Figure 7. Oblique cut for readout gap and fast (~20sec), high-ion-current cut of anti-stiction beam.
tor material. For manufacture it may be applicable for high value devices when the milling time is optimized.
Figure 8. The proposed magnetometer structure. A magnetic field will deflect the loop due to the Lorentz force when a current is sent round it. ACKNOWLEDGMENTS Helpful discussions and collaboration are greatly appreciated with A. Heaver, S.C. Burgess, H. Klaubert, T.J. Wilmshurst, N. Shibaike and other members of the EDC under K.M. Wallace.
REFERENCES
1. R.J. Young, Vacuum, 44 (1993) 353 2. K.E. Petersen, Proc. IEEE, 70 (1982) 420 3. S.B. Waltman and W.J. Kaiser, Sensors and Actuators, 19 (1989) 201 4. B. Diem, P. Rey, S.Renard, S.Viollet-Bosson, H. Bono, F. Michel, M.T. Delaye and G. Delapierre, Sensors and Actuators, A46 (1995) 8 5. N.C. MacDonald and A. Jazairy, SPIE, Vol. 2383 (1995) 125 6. S.J. Sherman, W.K. Tsang, T.A. Core, R.S. Payne, D.E. Quinn, K.H.-L.Chau, J.A. Farash and S.K. Baum, IEDM, 92 (1992) 501 7. J.F. Walker, D.F. Moore and J.T. Whitney, Microelectronic Engineering, 30 (1996) 517 8. D.F. Moore, S.C. Burgess, H-S. Chiang, H. Klaubert, N. Shibaike and T. Kiriyama, Proc. SPIE, Vol. 2639 (1995) 253 9. G.T. Mulhem, D.S. Soane and R.T. Howe, Transducers '93, Yokahama, (1993) 296
MICROELECTRONIC ENGINEERING ELSEVIER
Mieroelectronic Engineering 35 (1997) 431-434
Focused ion beams in microsystem fabrication J.H. Daniel a, D.F. Moore a, J.F. Walker b, J.T. Whitney b aCambridge University Engineering Department, Cambridge CB2 1PZ, UK bFEI Europe Ltd., Cottenham, Cambridge CB4 4PS, UK Focused ion beam (FIB) systems are widely used in microelectronics for prototype modification, device failure analysis and process monitoring. With the growing importance of micro-electro-mechanical systems (MEMS) the question arises whether FIB can be successfully and economically applied to prototyping and production in that area.
1. FIB FOR THE FABRICATION OF A MICROACCELEROMETER So far, focused ion beam (FIB) techniques have many applications in microelectronics [1]. Recently silicon MEMS is of growing importance in acceleration sensors and other systems [2-6]. To demonstrate the usefulness of FIB milling in this field, it was applied, in combination with bonded silicon-on-insulator (SOl) material, to the fabrication of micro-accelerometer structures [7,8]. As shown in figure 1, a square proof mass (perforated to allow underetching) is suspended on two arms. A narrow silicon beam fixes the structure at a third point during wet etching and is then cut at an angle using a 30keV Ga ion beam. In summary, the process steps are: • oxidation of the SOl wafer for masking layer • patterning of proof mass • anisotropic wet etching of the 3~tm top Si layer • wet etch in HF(conc.) to undercut the proof mass • focused ion beam to mill the readout gap and fully release the proof mass The operating mode would either be tunneling across a gap or a tapping mode in which the gap acts as a kind of micro-switch and the duty cycle of switching is measured. In both cases the acceleration causes the proof mass paddle to move and changes the size of the gap. FIB is used because of its ability to cut deep and narrow trenches at an angle. A narrow readout gap is required to keep the operating voltage low [7]. The deposition of an appropriate metal layer (e.g. Pt) will be necessary as a final step. 0167-9317(97)/$17.00 © 1997 ElsevierScience B.V. All rights reserved. PII: S0167-9317(96)00128-1
Figure 1. FIB image of an accelerometer structure with FIB cut. The silicon proof mass is 3p.m thick.
Figure 2. FIB cut of silicon beam at 5O°. wJtn an ~on current of 350pA, the cutting time was less than 5 minutes.
432
J.H. Daniel et al. /Microelectronic Engineering 35 (1997) 431-434
Figure 3. Silicon beam with a two-step FIB cut at 50 °. Initially an ion current of 2700pA was applied for 9 seconds, and the f'mal cut was done in 45 seconds with 150pA. 2. FIB P R O C E S S For the FIB cut we used a 30keV Ga ion beam. As FIB is a serial process, the economic aspect of the fabrication process for the devices is critical. The FIB process would add about $4 per device if the milling time were less than five minutes. Low currents can be used to produce the required narrow cuts but the cutting time is then long. In figure 2 an ion current of 350pA produced the cut in less than 5 minutes. Iodine enhanced etch can be used to increase the etch rate but the gap size then becomes greater. For deep trenches and low ion currents the etch time is still unacceptably high. As an improved process with comparatively short etch times, a two-step cut was introduced. First the beam is milled for several seconds at high ion current. Subsequently, a low current finely focused beam does the rest. This results in a gap of concave shape as shown in Figure 3. The gap size in the cen-
tre is small. However, stress in the currently used SOI material results in the suspended part lifting slightly and the gap size is larger than the actual cut. Methods to overcome this problem and the possible use of different SOl material are under investigation. 3. M E A S U R E M E N T S Measurements of the resonance frequencies of the accelerometer structures were carried out in the electron microscope. A 1V AC signal with variable frequency was applied between the proof mass and the substrate. For the small structure of figure 1 (proof mass size: 160pm x 160pm x 3~tm) a sharp resonance was observed at 75kHz drive frequency which corresponds to a mechanical resonance of 150kHz. On the bigger structure in figure 4 (220~tm x 220pm x 3pm) the lowest frequency mechanical resonance was at 90kHz (with a 45kHz drive). In figure 4 the structure is in resonance with a 1V AC drive and the vibrating part is blurred while the fixed part is sharp.
J.H. Daniel et al. /Microelectronic Engineering 35 (1997) 431-434
433
Figure 4. SEM image o f the accelerometer structure in resonance at 90kHz. The vibrating part appears blurred. 4. P R E V E N T I O N O F S T I C T I O N As wet etching techniques were used to fabricate the structures the common problem of stiction arises (figure 5) [9]. Our approach to that problem was to add small suspension beams and cut them afterwards with FIB. Figure 6 shows the success of this method for the accelerometer paddle. All structures with an additional beam had not stuck to the substrate. The time to cut the additional beams with FIB is relatively low as high ion currents can be chosen. For the beams which had a width o f 4~tm on the top the cutting time was around 20 seconds per beam, giving the structure in figure 7. These beams can also be useful in prototyping for gradually
Figure 5. Stuck down paddle after wet etch.
changing the dimensions or form of a mechanical structure. The usefulness of this method for making force transducers is shown for the structure in figure 8 which could be used as a magnetometer when a current is sent around the loop to produce a Lorentz force. In this structure one readout gap and four additional suspension beams can be seen. 5. C O N C L U S I O N S In microfabrication technology there are unique possibilities of using focused ion beam milling to produce oblique sub-micron cuts. Its usefulness in prototyping has been shown using silicon-on-insula-
Figure 6. Non sticking structure with additional beam.
J.H. Daniel et al. /Microelectronic Engineering 35 (1997) 431-434
434
411'
cut
41r
4P'
4P'
4D' 41'
,i, 41r
4p, ~
~ ~ 41' 'I' 41' ~r P' ~ all¢4 P' a t' , .4. .' ~m' ,
i, 41 '
i
Figure 7. Oblique cut for readout gap and fast (~20sec), high-ion-current cut of anti-stiction beam.
tor material. For manufacture it may be applicable for high value devices when the milling time is optimized.
Figure 8. The proposed magnetometer structure. A magnetic field will deflect the loop due to the Lorentz force when a current is sent round it. ACKNOWLEDGMENTS Helpful discussions and collaboration are greatly appreciated with A. Heaver, S.C. Burgess, H. Klaubert, T.J. Wilmshurst, N. Shibaike and other members of the EDC under K.M. Wallace.
REFERENCES
1. R.J. Young, Vacuum, 44 (1993) 353 2. K.E. Petersen, Proc. IEEE, 70 (1982) 420 3. S.B. Waltman and W.J. Kaiser, Sensors and Actuators, 19 (1989) 201 4. B. Diem, P. Rey, S.Renard, S.Viollet-Bosson, H. Bono, F. Michel, M.T. Delaye and G. Delapierre, Sensors and Actuators, A46 (1995) 8 5. N.C. MacDonald and A. Jazairy, SPIE, Vol. 2383 (1995) 125 6. S.J. Sherman, W.K. Tsang, T.A. Core, R.S. Payne, D.E. Quinn, K.H.-L.Chau, J.A. Farash and S.K. Baum, IEDM, 92 (1992) 501 7. J.F. Walker, D.F. Moore and J.T. Whitney, Microelectronic Engineering, 30 (1996) 517 8. D.F. Moore, S.C. Burgess, H-S. Chiang, H. Klaubert, N. Shibaike and T. Kiriyama, Proc. SPIE, Vol. 2639 (1995) 253 9. G.T. Mulhem, D.S. Soane and R.T. Howe, Transducers '93, Yokahama, (1993) 296