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Integration is vital to the performance of many types of microsensors, since as dimensions decrease, sensitivity often falls off precipitously. The sensitivity of a torsional capacitive accelerometer, for example, scales as the fifth power of the lateral dimension. Adding proximally located amplification ensures that the coupling of external noise sources into the signal path is kept to a minimum. The impact of stray capacitance, which would otherwise diminish the sensor signal, may also be reduced in this way. Large arrays of transducers benefit from integration as well, since the number of bondpads and the external circuitry needed to control each device can be reduced by using on-chip multiplexers. LPCVD polysilicon has been the material of choice for integrated surface micromachined structures since the early 1980's and has been used for everything from inertial sensors to pressure sensors to electrostatic actuators. Unfortunately, the high deposition and annealing temperatures (~580-630 C and >900 C, respectively), have required either metallizing after the polysilicon mechanical layers have been deposited in etched and planarized pits, or using refractory metals such as tungsten instead of aluminum for the circuit metallization. Both approaches increase the overall fabrication complexity and may require re-engineering the CMOS process to accommodate the new thermal budget, metal layers, and/or lithography changes.

Sputtered films provide an alternative to both these approaches that can be deposited directly atop CMOS circuitry at room temperatures. Aluminum was examined first as a structural layer. It was found that the release parameters played a dominant role in the warping of the final structures and that there were large variations in the radius of curvature, even among adjacent cantilevers. Design and process techniques to reduce the impact of released curvatures on electrostatic parallel plate devices were explored and evaluated. These techniques included corrugating the structures, adding PECVD silicon nitride stiffening layers, subdividing the plates into arrays, and suspending them with path-retracing hinges. Additionally, a novel way to add mass to a structure without affecting its curvatures was developed and analyzed. Using these techniques, it was possible to build released aluminum-based capacitive switches with on-off ratios of greater than three to one, and pull-in voltages of less than 10 V. However, the voltages at which these switches operated was still quite variable.

Silicon, by virtue of its lower thermal expansion coefficient, higher melting temperature, and potentially greater yield strength was more promising than aluminum. Low stress sputtered silicon films were demonstrated without the high temperature anneals of prior micromachining work. Annealing the films at 350 C reduced the stress in 2.0 and 5.0 um thick samples and produced stress values that were still stable after three months. Annealing also reduced the RMS surface roughness of the sample films, which ranged from 3 to 6 nm, depending on thickness. The density of the sputtered films is slightly lower than bulk, indicating that voiding may be present. This is consistent with sputtered silicon's porosity to buffered HF, which was demonstrated at film thickness of up to 5 um on oxide sacrificial layers. The electrical conductivity of sputtered silicon films is low (on the order of 10s of MWs/q to 10s of GWs/q, depending on annealing), but can be increased by cladding in 50 nm of TiW.

Sputtered silicon was deposited atop oxide and polyimide sacrificial layers. Films deposited atop oxides were wet-released in buffered HF. Films deposited above polyimide were dry-released in oxygen plasma. This eliminated the need for critical point drying. Unlike aluminum cantilever beams, little difference in released curvature was observed between sputtered silicon cantilever beams on polyimide sacrificial layers released at 50W, 100W, or 200W. Wet and dry-released curvatures were dependent on thickness, however, and a physical and analytical model was developed to explain this dependency. The model is based on surface stresses acting against a homogenous bulk and predicts a released radius of curvature proportional to thickness squared, which matches empirical results. In dryreleased beams with TiW cladding layers, the stress in the TiW played a role in controlling released curvatures. The dry-release process did not affect the maximum saturation current of pre-fabricated CMOS transistors by more than 3%. No difference was apparent in the subthreshold operation of the transistors.

As a demonstration of the dry-released TiW-clad sputtered silicon process, an electrostatically deflected plate structure was made with integrated capacitance measurement circuitry. This type of device is representative of most integrated surface micromachined sensors which operate using some variation on capacitance measurement. This work represents the first application of sputtered silicon to integrated MEMS. Sputtered silicon may prove to be an attractive replacement for LPCVD polysilicon when integration with pre-fabricated CMOS is desired.
 

Honer, K. A., "Surface Micromachining Techniques for Integrated Microsystems," Ph.D. Thesis, Stanford University, CA, 2001.