MEMS angular rate and angular acceleration sensors with CMOS switched capacitor signal conditioning.

Abstract

Microelectromechanical sensors were designed to measure angular rate and angular acceleration signals. Sensors were fabricated in micromachined polysilicon and silicon on insulator (SOI) structural films using semiconductor wafer fabrication equipment and processes. Fabricated sensor performance was measured and documented in terms of sensitivity, resolution, and noise. The sensors output a differential capacitance signal based on the displacement of a seismic mass in reference to an externally applied rotation. The angular rate sensor utilizes normal mode energy coupling transfer caused by Coriolis acceleration applied to the resonant seismic mass while exposed to an external rotating frame of reference. The angular acceleration sensor is comprised of a non-resonant spring suspended seismic mass. Displacement of the seismic mass as a function of angular acceleration is sensed by dual differentially referenced arrays of interdigitated variable capacitors. Both angular rate and acceleration sensors require an electrical transduction scheme to convert their differential capacitance outputs to a voltage representing the applied external reference frame inertial condition. A complimentary metal oxide semiconductor (CMOS) based switched capacitor architecture was designed to convert the capacitive sensor signal into a voltage output. This circuit architecture is based on a novel charge redistribution technique used to expose multiple sensor nodes to single reference voltage source. Correlated double sampling of the sensor is used to redistribute and sum charge prior to first stage integration and subsequent cascaded stage amplification. Characterization results of the CMOS switched capacitor chip are presented as the signal conditioned sensor output and are compared the theoretical prediction and computer simulation where applicable. The surface micromachined polysilicon angular rate sensor was designed to be robust with respect to eliminating in-use process stiction yield loss by implementing multiple compression posts on both the movable proof mass and fixed beam electrodes. Undesirable cross axis sensitivity was reduced on both the angular rate and angular acceleration sensors using an interleaved folded beam spring suspension. In conclusion, an SOI based sensor process flow is demonstrated which provides substrate electrical contact and mechanical anchor insulation using polysilicon and silicon nitride via chemical vapor deposition LPCVD trench refill techniques.