Development of MEMs Micro-Bridge Mechanical Resonators Interrogated By Microcavity Interferometry

Abstract

Miniature resonators such as microelectromechanical systems (MEMS) cantilevers have found multiple uses including development of mass resonant sensing. In this thesis, by the microcavity interferometer technique, we investigate the properties of bulk micromachined silicon nitride membranes that are wafer bonded to a substrate to form a Fabry-Perot cavity.

Photothermal actuation is used to drive the motion of the bridges by frequency modulating a 980nm diode laser, while the shift in Fabry-Perot resonance is detected by monitoring the intensity using of a continuous wave 1550nm tunable laser. Using this technique, 100 femtometer deflections can be easily monitored.

A variety of double clamped beam structures were investigated as a function of beam width and length. Depending on the dimensions of the micro-bridges, resonance frequencies from 10kHz to 5MHz were observed. For smaller bridges of dimensions 30μm length and 10μm width, resonance frequencies near 1MHz, quality factors of Q ˜ 150 were observed in air at room temperature. However in vacuum, the quality factors on the order of Q ˜ 7000 were observed, verifying air damping as the main source of energy loss.

Moreover, in air at room temperature, the Brownian motion of the bridge was observable without any photothermal driving force suggesting the possibility of using this structure as an un-powered chemical sensor and demonstrating the sensitivity of the optical system to measure small deflections.

The sensitivity of the bridge to chemical exposure was not quantified, but environmental perturbations were observed to change the resonant frequency of the bridge by several kilohertz indicative of a device structure suitable for sensor applications.