Electrostatic frequency tuning of thermal piezoresistive MEMS resonators

Abstract

This paper presents a new technique for electrostatic tuning of the resonant frequency of in-plane thermal piezo-resistive micro-electromechanical resonators. The tuning method is based on applying a DC bias voltage between the underlying SOI handle layer and the suspended resonant structure. The DC bias voltage produces an electrostatic force which causes the bending of the resonant structure in the out of plane direction. The resulting stress in the structure causes a shift in its resonant frequency. The experiments were taken place under both atmospheric pressure and partial vacuum. Maximum upward and downward frequency shifts of +580ppm and -430ppm have been demonstrated. As a side product, due to minimized air damping, more than 2X resonator Q improvement was observed while tuning the resonator frequency. I. INTRODUCTIONWith the advances in micromachining technologies, various types of MEMS resonators with a wide range of frequencies from tens of kHz to a few GHz with high quality factors have been demonstrated [1,2]. A variety of microscale electromechanical resonators have been considered/used in various applications such as sensitive and highly integrated chemical [3] and environmental sensors [4], filters [5] and gyroscopes [6]. Arrays of such resonators can be batch fabricated at high quantities at a very low cost. The resonance frequency of silicon micromechanical resonators is dependent on the physical dimensions of the resonating structure. Process variations across the substrate (e.g. non-uniformities in photolithography, etching and film thickness) leading to variations and mismatch in the mechanical resonant frequencies are a major challenge for batch fabrication of micromechanical resonators. Postfabrication trimming is one of the ways to solve this issue. The post-fabrication frequency trimming methods make irreversible permanent changes to the frequency of the devices. Different frequency trimming techniques for MEMS silicon based micromechanical resonators have been presented. The techniques range from pulsed-laserdeposition/trimming [7], gold diffusion into the bulk of the resonator [8] and localized thermal oxidation [9].

Active Frequency Tuning for Magnetically Actuated and Piezoresistively Sensed MEMS Resonators

“…3, where f 0 is the resonant frequency of the device at T 0 . Equation (4) indicates that the device's resonant frequency decreases nonlinearly when increasing I DC or V AC . In fact, the amplitude of the AC voltage is fixed during the experiments, so it does not change the resonant frequency of the device.…”

“…The active methods are able to maintain frequency matching through the lifetime of the device. However, active methods are demonstrated only for electrothermally [1], [3] and electrostatically actuated [4]- [6] MEMS resonators. Electrothermal actuation is considered a good solution to overcome major drawbacks of electrostatic actuation such as Manuscript critical fabrication steps, impedance matching, and relatively high actuation voltages, but it suffers due to slow response and high power consumption [7].…”