Modelling and testing of a MEMS accelerometer controlled and read-out beyond the pull-in instability limit

Abstract

Electrostatic forces caused by the readout circuit and parasitic capacitances have much relevance in the scaling considerations of Micro and Nano electro mechanical (M/NEMS) parallel-plate motion sensors. The reduction of minimum geometrical dimensions (spring width, air gap), aimed at an improvement of the sensitivity/area ratio, is indeed limited by the pull-in instability determined by the electrostatic attraction between parallel plates. In this paper it is presented a compact MEMS accelerometer built in the ST Microelectronics ThELMA technology, together with a 0.35μm CMOS switched-capacitor circuit, suitably designed to avoid the pull-in. In particular we focus on the model of the MEMS, written in a Hardware Description Language file to be used directly in the VLSI circuit simulator. Although the MEMS is unstable at the chosen biasing conditions when read-out at fixed voltage, the experimental results show that the closed-loop circuit allows the biasing beyond the pull-in voltage, in good agreement with the simulation prediction. The sensing range is larger than ±9 g with a resolution of 2mg/Hz.