Fabrication of Comb-Driven Resonators with Submicrometer Features Patterned Using Electron Beam Lithography

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

To obtain submicrometer features in Si, direct write electron beam lithography was used to lift off an evaporated Ni mask. Polymethylmethacrylate (PMMA) with 950 K molecular weight in a 6% anisol solution was spun on a Si wafer. The resist was baked at 150 degrees C for 30 min giving a final resist thickness of 300 nm. The resist was then exposed to a 50 kV electron beam with a typical dose of 182.7 uC/cm2. Proximity correction was used in order to account for electron scattering and secondary electrons. The exposed resist was then developed in a 1:1 solution of methylisobutylketone (MIBK) and isopropyl alcohol (IPA) for 1 min followed by a 1 min rinse in IPA. Next a 5/170 nm Ti/Ni layer was evaporated onto the wafer in order to serve as the dry etch mask. The metal was then lifted off in acetone to leave a patterned metal mask. The samples were finally etched in the ICP source at 80 W source power and 100W stage power at 1 mTorr with 10 sccm of Cl2 flow and a source to sample distance of 13 cm.

An etch condition has been developed in an inductively coupled plasma (ICP) source in order to etch submicrometer features in Si. A Cl2 chemistry was used to etch the small trenches. Chlorine does not spontaneously react with Si and therefore etching of Si is thought to proceed by an ion assisted reaction. Species of chlorine adsorb onto the surface of the Si and the incoming ions impinge on the surface, providing enough energy for the reaction products to desorb, which removes the Si. This chemistry does not rely on passivation of the sidewalls of the trench such as often is required in fluorine chemistries due to the high lateral etch rate as fluorine spontaneously reacts with the Si. For narrow trenches, it is difficult to maintain passivated sidewalls without plugging up the trench completely. This advantage of chlorine chemistry has allowed very narrow trenches to be etched in Si to relatively large depths. Figure 1 shows the etch rate and selectivity variation with stage rf power. The samples were etched with 70 W source power at 1 mTorr with 10 sccm of Cl2 flow and a source to sample distance of 13 cm while the rf power to the stage was varied. As the stage power is increased from 80 to 200 W, the etch rate increases from 32 to 81 nm/min while the selectivity to a Ni mask decreases from 36 to 19. The |Vdc| increased from 200 to 330 V as the stage power was increased from 80 to 200 W. The increase in stage power increases the energy of the ions impinging on the substrate. This causes an increase in the physical sputtering component of the etch relative to the chemical component. Since the ion energy increases, the etch rate of the Si increases. However, since the physical component of the etch increases with increasing stage power, the selectivity decreases also. The selectivity of the etch is an extremely important factor because it determines the depth of trenches which can be etched, which in turn determines the thickness of the resonators that are fabricated. Ni masks as thick as 0.17 um have been lifted off with 0.1 um features. However, as the mask thickness is increased, it gets harder to lift off small features. Also, high stress in the evaporated Ni film makes it difficult to lift off a homogenous Ni mask greater than about 0.2 um. For Ni masks thicker than this, alternating layers of Ti and Ni must be evaporated so that the stress in the Ni film can be reduced.

Released cantilevered beams and resonators were fabricated. Dry etching was done in the ICP source using 70 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. Released cantilevered beams that are 500 um long and 3.1 um thick with gaps between beams of 0.1 um have been fabricated and are shown in Fig. 2. This large depth with small gaps gives an aspect ratio of 31. Comb driven clamped-clamped beam resonators have been fabricated and a micrograph of a 3.1 um thick resonator is shown in Fig. 3. The resonant beam is 400 um long and 2 um wide with an electrostatic comb drive attached to one side of the beam for driving the resonator. A close-up of the comb drive is shown in Fig. 4. The comb fingers are 3.1 um thick and 1.7 um wide with 0.2 um wide gaps between comb fingers for an aspect ratio of 15.5. As can be seen from the micrograph, a vertical profile is achieved which allows the capacitor formed between comb fingers to be easily modeled as a parallel plate.

There are many tradeoffs in choosing lateral over vertical micromechanical resonators for signal processing applications. With vertical resonators, which resonate out of the plane of the substrate, the gap between the resonant element and transduction electrodes is usually determined by the thickness of the sacrificial layer. It is relatively straightforward to grow thin films in the integrated circuit industry and vertical resonators with capacitive gaps smaller than 20 nm have been demonstrated. However, careful measures must be taken to prevent the structure from sticking down to the substrate such as release at the critical point and careful cleaning to prevent particles from shorting out the gap. The topography of the resonators can also be affected by the layers below them and simple beam geometries cannot always be easily achieved. Also, with lateral resonators, the capacitive gap used to drive and sense motion in the structure is set by dry etching. Dry etching can achieve very vertical profiles and structures can be made stiff in the vertical direction while compliant springs can be fabricated in the lateral direction which can reduce stiction problems. Research still must be done in order to be able to fabricate devices with vertical submicrometer capacitive gaps and reasonable thicknesses to maximize the output signal.

Figure 1. Etch rate and selectivity variation with stage power for 0.5 um wide trenches. The etch condition in the ICP source was 80 W source power, 1 mTorr, 13 cm, 10 sccm Cl2.

Figure 2. 500 um long, 3.1 um thick released cantilevered beams with 0.1 um spaces in between.

Figure 3. Micrograph of 3.1 um thick released resonator with 1.7 um wide fingers and 0.2 um wide gaps.

Figure 4. A close-up of the electrostatic comb drive fingers showing the high aspect ratio (15.5) dry etch.

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 

Back to Home Page