Monolithic z-axis CMOS MEMS accelerometer

This paper describes the design, fabrication, and characterization of a monolithic z-axis capacitive torsional accelerometer fabricated in a 0.18-μm one-polysilicon six-metal layer complementary metal-oxide-semiconductor (CMOS) micro-electro-mechanical-system (MEMS) multi-project wafer process. After completion of the CMOS process, an additional aluminum layer and a thick photoresist masking layer are employed to achieve etching and microstructural release. The sensing electrodes are composed of stacked metal layers isolated by silicon dioxide layers in the CMOS process, with the metal-5 to metal-6 layers as the top electrodes and the metal-1 to metal-3 layers as the bottom electrodes. The simulated capacitance sensitivity of the sensor device is 0.8 fF/g within the range of 0-6 g. A three-phase switched-capacitor sensing circuit is used to read the sensing capacitances of the accelerometer. The measured sensitivity is 205 mV/g, and the nonlinearity is 0.9% over 0-6 g. The cross-axis sensitivities with respect to the x-axis and y-axis are 1.36% and 1.55%, respectively. The measured output noise floor is 630 μg/Hz1/2.