Thermally-actuated piezoresistively-sensed mechanical silicon oscillator

Abstract

In the past two decades, Microelectromechanical (MEMS) resonators have emerged as front runners for RF front-ends, high frequency filters, and frequency sources in various applications. The prospect of seamless integration with CMOS has provided a significant boost to displace Quartz, which for long has been the go-to option for timing sources. To construct an oscillator, a MEMS resonator is operated with an active feedback amplifier, the design of which can be a major challenge at high frequencies. In this work we implement a self sustaining mechanical Si oscillator that has an internal feedback mechanism. The oscillator is based on a thermal actuation mechanism due to the Joule heating effect caused by running currents through narrow channels. These narrow channels when oriented along the <100>direction in an n-doped Si wafer, show large negative piezoresistance coefficients. Beyond significant threshold DC current densities (GA/m 2 ), the thermal-actuation and piezoresitive-feedback loop excite the mechanical structure, causing spontaneous oscillations. We begin with the investigation of scaling trends based on an equivalent circuit model of the device. Targeting high frequency oscillators, we design suitable geometries and discuss the microfabrication processes used to fabricate these devices. Finally, we report the experimental results of the fabricated devices.