Determination of Multiple Spring Constants, Gaps and Pull Down Voltages in MEMS CRAB Type Microaccelerometer Using Near Pull Down Capacitance Voltage Measurements

Abstract

A simple experimental method based on capacitance voltage (CV) measurements is presented to extract the spring constants (k) different actuation voltages and gaps, in crab type capacitive MEMS accelerometer sensors. It is shown that in addition to main spring action provided by the legs of the structure, the additional spring constants related to the interaction of main spring-proof mass joint and corner region of the proof mass also contribute to change in capacitance. The proposed approach is used in resolving and measuring these model parameters simultaneously because all of them can be extracted from the just one CV measurement. It is found that this additional k varies by more than a factor of 10 across the 6-inch wafer. Furthermore, zero bias capacitance C0, zero bias gap g0, main spring constant k and initial pull down voltage Vpd1 vary by factors of 2.4, 2.38, 30 and 3 respectively. The method also allows us to extract different values of pull down and spring constants associated with different regions of crab structure. The experimental results agree well with the theoretical predictions and reported trends in literatures. The method is routinely applied while fabricating the actual prototype sensors fabricated in our laboratory