Design, Fabrication, and Testing of High Sensitivity Submicrometer Si Resonators

S. W. Pang and J. W. Weigold
University of Michigan, Ann Arbor, Michigan 48109-2122, USA

An acceleration sensor with a minimum detectable acceleration in the µg range was designed in order to demonstrate and prove the performance gains provided by the dry etch conditions and fabrication processes. This accelerometer will benefit by the high aspect ratio gaps, providing increased capacitance as well as a larger number of sense fingers in the comb sense structure compared to sensors fabricated using other technologies. With submicrometer beam width and comb gaps, these devices should be able to achieve µg sensing with the added benefit that the simple process can result in high yields with very few processing steps. In addition, the process used is compatible with circuit integration so that circuitry can be included with the sensor.

These high sensitivity accelerometers were fabricated using a 3-mask high aspect ratio technology. (Figures 1 and 2) Nanofabrication technology with electron beam lithography and high density plasma etching in an inductively coupled plasma source was used to obtain resonators with aspect ratios >30. This makes possible beams with very small spring constants. Combining the ability to measure very small displacement of a proof mass due to narrow capacitive gaps between comb fingers, a highly sensitive accelerometer can be obtained.

The fabricated sensor has a mass of 1.7e-9 kg and had a proof mass that was 825e15 µm2. The measured spring constant for 1 µm wide beams was 0.127 N/m which was close to the simulated spring constant of 0.146 N/m. The measured sensitivity was 6.3 fF/g which was close to the calculated sensitivity of 7.0 fF/g for the 1 µm wide beams. For the device with 0.4 µm wide beams, a sensitivity of 39.62 fF/g for one set of combs or 79.2 fF/g for both sides was measured. (Figure 3) The accelerometer sensitivity can be further improved by fabricating even narrower beams. As shown in the calculations, 0.2 µm wide beams with 0.1 µm wide gaps can provide a sensitivity of 76 pF/g with a noise equivalent acceleration of 4.4 µg/Hz1/2.

Figure 1. Die photo of a 1 mm by 3 mm accelerometer with 970 µm wide, 1795 µm long proof mass and 5 µm x 5 µm holes.

Figure 2. Scanning electron micrograph of a released accelerometer after a 25 min EDP etch. The combs are 3 µm thick with 0.2 µm gaps in between and connected to the Al bond pads.

Figure 3. Measured capacitance change (after subtracting out junction capacitance variation) with applied voltage for fabricated accelerometer with 0.4 µm wide beams. The voltage is only applied to one of the two sets of comb fingers.

References

  1. 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).
  2. 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).
  3. 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).
  4. 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).
  5. 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).
  6. 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).
  7. 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.
  8. 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).
  9. 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.
  10. 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).
  11. 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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