OBSERVATIONS OF FIXED AND MOBILE CHARGE IN COMPOSITE MEMS RESONATORS

This paper investigates dielectric charge in oxide-coated composite-beam silicon electrostatic resonators. The effects of charge in the oxide layers on resonant frequency are theoretically modeled. Evidence for the presence of both fixed and mobile charge is presented through experimental studies. It is shown that the motion of charge is controllable through the bias voltage applied to the resonator. These studies are relevant for improving the reliability of non-contacting MEMS where dielectrics may be present such as timing references, inertial sensors, and optical actuators. INTRODUCTION Dielectric charging in contacting structures such as RF capacitive switches has been studied extensively. In these structures capacitance measurements are used to infer charge through partial deflection or complete pull-in measurements [1, 2]. In stiff non-contacting structures such as resonators, accelerometers and gyroscopes, charge build-up can also affect the stability and reliability of the transducer. Pull-in measurements are often impractical for such structures and do not emulate standard operating conditions. Here, the use of resonant frequency measurements can complement pull-in measurements as a noncontact method to study the evolution of charge in dielectrics under low-field conditions. We utilize oxide-coated composite-beam silicon resonators designed for passive temperature compensation as test structures [3]. This paper presents evidence for trapped charge in the resonator oxides by two methods: observing frequency dependence on bias voltage to show the existence of built-in electric fields within the resonating device, and examination of transient behavior of frequency under controlled bias and temperature conditions.