Dry Etching Technology for High Aspect Ratio Resonators Using an Inductively Coupled Plasma Source

S. W. Pang, J. W. Weigold, and W. -C. Tian

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

In order to investigate the characteristics of Si etching in an inductively Coupled Plasma (ICP) source, various etch conditions and their effects on Si etch rate, etch profile, surface roughness, and mask etch rate were studied. As the source power was increased from 250 to 1600 W, the Si etch rate increased from 219 to 563 nm/min because of the higher density of the etch species, as shown in Fig. 1.

An optimized etch condition was obtained by using 250 W source power with 70 W applied to the stage at 5 mTorr with 20 sccm of Cl2 flowing and a source to sample distance of 6 cm. This provided a vertical profile with a Si etch rate of about 219 nm/min while achieving smooth surfaces and high selectivity of 26 to a Ni-Ti mask. A vertical profile is desirable, because this profile defines the mechanical structure and it is easier to predict the device characteristics for resonators. A smooth surface is also favorable so that air flow past the moving surfaces is smooth and can be accurately modeled. Rough surfaces can degrade the quality factor of the fabricated resonators. Besides, thicker resonators can provide higher sensitivity due to the increased area of the vertical capacitive plates. Released cantilevered beams were fabricated using the condition optimized to etch deep trenches described previously. Figure 2 shows an array of released cantilevered beams which are 40 µm thick and 5 µm wide with lengths varying from 50 to 1000 µm.

A 40 µm thick clamped-clamped beam single crystal Si comb drive resonator is shown in Fig. 3. The comb fingers are 4 µm wide with 2 µm spaces between the combs. The resonant clamped-clamped beam is 400 µm long and 5 µm wide. The etch profile is vertical and the surface is smooth. These thick resonant devices do not bend due to stresses and therefore, the resonant behavior can be more easily predicted. Since the ICP source allows the fabrication of thick single crystal Si resonators with small gaps, high sensitivity can be obtained from these high aspect ratio structures with high quality factor.

For resonant devices required to operate at high frequencies, a small size high aspect ratio cantilever beam is needed. For high frequency operation, thin devices with small gaps between excitation and sensing plates are desirable. A gentler etch condition can be obtained by trading off the higher etch rates for the ability to etch very narrow trenches. Cantilevered beams that were 3.1 µm thick with gaps of 0.1 µm were etched with a vertical profile and smooth surface using an electron beam patterned resist to lift off a Ni mask. The optimized etch condition used to obtain submicrometer features was 80 W source power and 100 W stage power at 1 mTorr with 10 sccm of Cl2 flow and a source to sample distance of 13 cm. A 3.1 µm thick released clamped-clamped beam resonator with 1.7 µm wide comb fingers and 0.2 µm wide gaps between fingers is shown in Fig. 4. With the submicrometer gap of 0.2 µm in the fabricated resonator, there is an increased electromechanical coupling compared to larger gaps. This allows the resonator to be operated at high frequency with a low driving voltage and large output signal.

Figure 1. Dependence of Si and Ni mask etch rate and selectivity on source power. The plasma conditions were 70 W power to the stage, 20 sccm of Cl2 flow, chamber pressure of 5 mTorr, 8 cm separation between stage and ICP source, and etch time of 30 min.

Figure 2. Released cantilevered Si beams that were 40 um thick, 5 um wide, and lengths varying from 50 to 1000 um.

Figure 3. 40 um thick released clamped-clamped beam Si resonators with 4 um comb fingers and 2 um gaps between fingers.

Figure 4. A 3.1 um thick released Si clamped-clamped beam resonator with 1.7 um wide fingers and 0.2 um wide gaps. The resonant beam is 200 um long and 2 um wide.

References

  1. W. -C. Tian and S. W. Pang, "Released submicrometer Si microstructures formed by one-step dry etching", to be published in J. Vac. Sci. Technol. B (2001).
  2. W. -C. Tian, J. W. Weigold, and S.W. Pang, "Comparison of Cl2 and F-based dry etching for high aspect ratio Si microstructures etched with an inductively coupled source", J. Vac. Sci. Technol. B 18, pp. 1890-1896 (2000).
  3. J. W. Weigold, A.-C. Wong, C. T.-C. Nguyen, and S. W. Pang, "A Merged Process for Thick Single Crystal Si Resonators and Conventional BiCMOS Circuitry", IEEE J. of Microelectromech. Syst. 8, 221-228 (1999).
  4. J. W. Weigold, W. H. Juan, S. W. Pang, and J. T. Borenstein, "Characterization of Bending in Single Crystal Si Beams and Resonators", J. Vac. Sci. Technol. B 17, 1336-1340 (1999).
  5. J. W. Weigold, W. H. Juan, and S. W. Pang, "Dry Etching of Deep Si Trenches for Released Resonators in a Cl2 Plasma", J. Electrochem. Soc. 145, pp. 1767-1771 (1998).
  6. J. W. Weigold and S. W. Pang, "Fabrication of Thick Si Resonators with a Frontside-Release Etch-Diffusion Process", IEEE J. Microelectromech. 7, pp. 201-206 (1998).
  7. M. R. Rakhshandehroo, J. W. Weigold, W.-C. Tian, and S. W. Pang, "Dry Etching of Si Field Emitters and High Aspect Ratio Resonators Using an Inductively Coupled Plasma Source", J. Vac. Sci. Technol. B 16, pp. 2849-2854 (1998).
  8. J. W. Weigold and S. W. Pang, "High Aspect Ratio Single Crystal Si Microelectromechanical Systems", Proc. SPIE Conference on Micromachining and Microfabrication Process Technology 3551, pp. 242-251 (1998).
  9. J. W. Weigold, A.-C. Wong, C. T.-C. Nguyen, and S. W. Pang, "Thick Single Crystal Si Lateral Resonant Devices Integrated with a Conventional Circuit Process", in Late News Digest IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, June 1998.
  10. J. W. Weigold, W. H. Juan, and S. W. Pang, "Etching and Boron Diffusion of High Aspect Ratio Si Trenches for Released Resonators", J. Vac. Sci. Technol. B 15, pp. 267-272 (1997).
  11. J. W. Weigold and S. W. Pang, "A New Frontside-Release Etch-Diffusion Process for the Fabrication of Thick Si Microstructures", in Digest 9th Int. Conference on Solid-State Sensors and Actuators (Transducers'97), pp. 1435-1438, Chicago, June 1997.
  12. J. W. Weigold, W. H. Juan, S. W. Pang, and J. T. Borenstein, "Optical Interferometric Characterization of Membrane Curvature in Boron Doped Si Microstructures", Proc. SPIE Conference on Micromachining and Microfabrication Process Technology 3223, pp. 142-148 (1997).
  13. W. H. Juan, J. W. Weigold, and S. W. Pang, "Dry Etching and Boron Diffusion of Heavily Doped High Aspect Ratio Si Trenches", Proc. SPIE Conference on Micromachining and Microfabrication Process Technology 2879, pp. 45-55 (1996).

Last Updated: November 19, 2007

E-Mail: pang@eecs.umich.edu 

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