A new process for CMOS MEMS capacitive sensors with high sensitivity and thermal stability

Structure curling induces thermal instability into CMOS MEMS capacitive sensors. The charging effect during reactive ion etching damages the existing on-chip MOS transistors and drastically reduces the yield rate of chips. This paper presents a novel post-CMOS process that solves the problems and leads to CMOS MEMS capacitive sensors with high sensitivity and thermal stability. The novel process was demonstrated with a capacitive accelerometer in 0.35 mu m CMOS technology. The accelerometer contains a thermally stable MEMS sensor and an on-chip CMOS sensing circuit with a chopper stabilization scheme. The temperature stabilization was achieved by forming a thick single-crystal silicon (SCS) layer at the bottom of the multi-layer MEMS structure. No leakage current due to charge damage was ever observed in the sample chips. The proposed process also led to minimal undercut of the SCS layer after MEMS structure release. The sensitivity of the accelerometer is 595 mV g(-1), and the overall noise floor is 50 mu g Hz(-1/2) which corresponds to an effective capacitance noise floor of 0.024 aF Hz(-1/2). The zero-g temperature coefficient of the accelerometer output voltage is only 1 mV degrees C-1 in the temperature range from 0 to 70 degrees C, which corresponds to an effective acceleration variation rate of 1.68 mg degrees C-1.