Implementation of a monolithic capacitive accelerometer in a wafer-level 0.18 μm CMOS MEMS process

This paper describes the design, fabrication and characterization of a complementary metal-oxide-semiconductor (CMOS) micro-electro-mechanical-system (MEMS) accelerometer implemented in a 0.18 mm multi-project wafer (MPW) CMOS MEMS process. In addition to the standard CMOS process, an additional aluminum layer and a thick photoresist masking layer are employed to achieve etching and microstructural release. The structural thickness of the accelerometer is up to 9 mm and the minimum structural spacing is 2.3 mm. The out-of-plane deflection resulted from the vertical stress gradient over the whole device is controlled to be under 0.2 mm. The chip area containing the micromechanical structure and switched-capacitor sensing circuit is 1.18 x 0.9 mm(2), and the total power consumption is only 0.7 mW. Within the sensing range of +/- 6 G, the measured nonlinearity is 1.07% and the cross-axis sensitivities with respect to the in-plane and out-of-plane are 0.5% and 5.8%, respectively. The average sensitivity of five tested accelerometers is 191.4 mV G(-1) with a standard deviation of 2.5 mV G(-1). The measured output noise floor is 354 mG Hz(-1/2), corresponding to a 100 Hz 1 G sinusoidal acceleration. The measured output offset voltage is about 100 mV at 27 degrees C, and the zero-G temperature coefficient of the accelerometer output is 0.94 mV degrees C-1 below 85 degrees C.