Design and fabrication of high-performance capacitive micro accelerometers

This thesis presents the development of capacitive high-performance accelerometers for sonar wave detection. Two different designs of in-plane and out-of-plane accelerometers are developed, micro-fabricated, and experimentally tested.The out-of-plane accelerometer is designed based on a continuous membrane suspension element. In comparison to beam-type suspension elements, the new design provides uniform displacement of the proof mass, lower cross-axis sensitivity, and lower stress concentration in suspension elements which could result in higher yield in the fabrication process. The out-of-plane accelerometer is fabricated using a novel microfabrication method which facilitates developing continuous membrane type suspension elements and full wafer thick proof mass for accelerometers. The designed accelerometer is fabricated on a silicon-on-insulator wafer with an 8 µm device layer, 1.5 µm buried-oxide layer, and 500 µm handle wafer. The developed accelerometer is proven to have resonance frequency of 5.2 kHz, sensitivity of ~0.9 pF/g, mechanical noise equivalent acceleration of less than 450 ng/√Hz, and an open loop dynamic range of higher than 130 dB while operating at atmospheric pressure.The in-plane single-axis accelerometer is designed based on a proposed mode-tuned modified structure. In this modified structure, the proof mass is substituted with a moving frame which also provides the area for increasing the number of sensing electrodes. This substitution contributes to widening the bandwidth of the accelerometer by locating the anchors and elastic elements both inside and outside of the moving frame. The designed accelerometer is fabricated on a silicon-on-insulator wafer with a 100µm device layer and high aspect ratio capacitive gaps of ~2 µm. The sensitivity of the accelerometer is measured as ~0.7 pF/g with the total noise equivalent acceleration of less than 500 ng/√Hz in the flat band region of the bandwidth. The resonance frequency of the devices is 4.2 kHz while maintaining a linearity of better than 0.7%. The open loop dynamic range of the accelerometer, while operating at atmospheric pressure, is higher than 135 dB, and the cross-axis sensitivity is less than -30 dB.