Dry Etching Technology for High Aspect Ratio and Submicrometer Sensors

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

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

Dry etching technologies using high density plasmas are very useful in fabricating microstructures with high aspect ratio features, smooth morphology, vertical profile, high etch rate, and low etch-induced damage. An inductively coupled plasma (ICP) source using rf power supplies and a simple source design can be used to meet most of these requirements. Both F- and Cl-based gases have been used to etch Si microstructures. While F-based gases can provide faster etch rate and high selectivity, Cl-based gases can produce submicrometer microstructures with vertical profile and smooth sidewalls.

Poly-Si trenches that were 0.1 µm wide and 4 µm deep with an aspect ratio of 40 have been demonstrated with Cl2 etching. After etching in BHF for 20 min, these 400 µm long, 2 µm wide cantilevered beams were released from the substrate, as shown in Fig. 1. A clamped-clamped beam resonator with 0.2 µm gaps between the resonating beam and electrodes has been fabricated with Cl2 etching, as shown in Fig. 2.

There were limitations when submicrometer trenches were patterned by the switching and non-switching F-based etching. As shown in Fig 3, while Cl2 etching provides vertical profile for trenches with 0.1 µm wide openings, the F-based etching often results in tapered etch profile. The etching eventually stopped when the top of the trenches was covered by the deposited polymer. One step dry etching to form released microstructures using doping dependent etching in Cl2 was demonstrated in Fig. 4. Since n++ Si has a higher lateral etch rate than undoped or p++ Si, microstructures above the n++ Si can be released from the substrate by undercutting the n++ layer. Besides, smooth etched surface without residues or enhanced lateral etching at the bottom of the trenches can be achieved using Cl2 etching.

Figure 1. Released cantilevered poly-Si beams with 0.1 µm gaps after Cl2 etching and 15 min release etch in BHF.

Figure 2. Micrograph of a 4 µm thick released poly-Si clamped-clamped beam resonator with 0.2 µm wide gaps between the beam and the electrodes. The resonant beam is 50 µm long and 3 µm wide.

Figure 3. Etch profile dependence on patterned trench width in Cl2 and F-based etching as indicated by the ratio of the bottom trench width over patterned trench width on top.

FFigure 4. Released poly-Si cantilevered beams formed by one step Cl2 etching. The 2 µm thick poly-Si released beams with 0.5/0.5 and 1/1 µm line/space were obtained after 30 min etching in Cl2.

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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