Passive signal amplification via series-piezoelectric read-out

This paper describes an innovative method to passively amplify the output voltage in MEMS-based piezoelectric accelerometers through a series read-out scheme. In a conventional piezoelectric or capacitive MEMS sensor, output charge is collected by a single capacitor. Here, we present a monolithic accelerometer employing 15-element array of smaller piezoelectric capacitors formed between patterned top and bottom electrodes. By connecting the top electrode of the capacitor to the bottom electrode of the subsequent capacitor, the read-out capacitors are connected in series, and thus the output voltages are added in series, increasing the output voltage. Devices were fabricated on 200 mm wafers in a commercial foundry process based on wafer bonding of CMOS to MEMS. The close integration of MEMS with CMOS results in low electrical parasitics, an essential feature to enable impedance matching of the front-end amplifier to the much higher impedance series read-out capacitors. Using the method, the sensor's sensitivity is amplified 6-fold. The sensor with series-connected capacitors has a sensitivity of 4.8 mV/g, noise floor of 2.6 mg/VHz, and resonant frequency of 2570 Hz.