Low voltage MEMS actuation using Negative Capacitance

Electrostatic Micro-Electro-Mechanical-System (MEMS) actuators find use in a large spectrum of areas. Typical actuation voltages lie in the 10-100V range. However, CMOS integrated circuits typically operate with a supply voltage of around 1V. There is hence a lot of interest in the design and fabrication of actuators with sub-1V actuation voltage. A novel method to reduce the actuation voltage was proposed by Alam et al (NanoLett, 204) using the negative capacitance provided by a ferroelectric material connected in series with the MEMS actuator. No experimental demonstration of using negative capacitance to reduce the actuation voltage has yet been reported, due to the challenges in fabricating a high quality, single domain ferroelectric over a MEMS cantilever. This proposal takes a different approach to investigate the feasibility of low voltage MEMS actuation using negative capacitance. We propose to develop a circuit that mimics the terminal characteristics provided by a high quality ferroelectric layer. We propose to then use this circuit, as a black box, connected in series with a MEMS cantilever to experimentally investigate the behaviour of hybrid Negative Capacitance - MEMS structures. We also wish to analyze other MEMS devices, such as open-loop and closed-loop accelerometers, where negative capacitance can improve performance. We propose to fabricate aluminum cantilever based electrostatic actuators using surface micromachining. The circuit mimicing the ferroelectric will be connected in series with the actuator. The actuation voltage will be measured by monitoring either the capacitance of the structure or the current through the structure. We also propose to measure the dynamics of the actuation process using a laser doppler vibrometer and a lock-in amplifier. This will build on our recent simulation work looking at the dynamics of actuation, wherein we show that there is a net advantage in energy of actuation in the hydrid actuator, as compared with the stand-alone MEMS actautor. Successful completion of this project will experimentally prove that negactive capacitance can be used to reduce the actuation voltage of electrostatic actuators. Since fabrication of high quality, single domain ferroelectric films is very challenging and expensive, success in this project will provide sound justification for investment in the development of high quality ferroelectrics. This project should also lead to publications and patents on improving the performance of other MEMS devices using the idea of negative capacitance, which should in turn motivate experimental investigations in future.