Frontside Release Process For High Aspect Ratio Si Reonators

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

Moveable microstructures fabricated out of Si using conventional IC processing techniques allow for the implementation of many kinds of devices including sensors and actuators. These structures often use the vertical parallel plates of a capacitor for sensing and actuation. The larger the plates of the capacitor are, the greater the sensitivity of the structure. In addition, with larger parallel plates, a lower driving voltage is needed for actuation of the structure. For this reason, thick Si microstructures with small gaps are desired.

Fast, deep etching technology has been developed using an electron cyclotron resonance (ECR) source to enable the fabrication of thick structures in Si. Trenches deeper than 100 um with a vertical profile have been etched at etch rates greater than 1 um/min using a Cl2 plasma and a Cl2/SF6 plasma as shown in Fig. 1. Figure 2 shows structures with aspect ratios >30 which have been achieved. High selectivity to a Ni mask allows these deep trenches to be etched.

Thick Si resonators have been fabricated using the deep etch-shallow diffusion process. This allows devices to be fabricated, using a short boron diffusion, which are then bonded to glass. Normally, the rest of the lightly doped Si substrate is then dissolved in a long etch in ethylenediamine pyrocatechol (EDP). A new frontside-release etch-diffusion process has been developed which eliminates the need for wafer bonding in addition to shortening the EDP etch to about 30 min. This process uses the EDP etch to undercut the structures from the frontside of the wafer, freeing the structure from the substrate. The structure is isolated from the substrate using the reverse bias of a p+/n diode. Because the structures are thick, they are very stiff, and stiction to the substrate is significantly reduced. Cantilevers longer than 1 mm have been fabricated with a negligible amount of bending, and with no danger of contacting the substrate as shown in Fig. 3. This process is very simple, requiring only one mask, and the final device is made only of Si. Resonators as thick as 55 um have been fabricated using this process and tested. A typical frontside released Si resonator is shown in Fig. 4. This resonator is 30 um thick with a 5 um wide, 400 um long center beam. There are 25 combs, and each comb is 4 um wide separated by a 2 um gap.

Figure 1. Deep trench etched in Si using Cl2 and SF6 plasma. The trench is 20 um wide, 106 um deep, and etched in 90 min.

Figure 2. High aspect ratio structures etched in Si using a Cl2 plasma. Features are 52 um deep with an aspect ratio of 34.

Figure 3. An array of cantilevered beams released from the substrate. The longest beam is 1 mm long and all beams are 25 um thick.

Figure 4. Micrograph of 30 um thick released resonator with 4 um wide fingers and 2 um wide gaps. The resonators and supporting beams are released, and the large pads are still anchored to the substrate.

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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