Polysilicon hollow beam lateral resonators

Abstract

This paper reports the first microfabrication of hollow polysilicon beams. Arrays of lateral resonators are designed, processed, and tested with resonant frequencies from 8 kHz to 0.5 MHz. The quality factor as a function of pressure of the hollow beam resonators is compared with solid beam resonators, and values as high as 34,000 are obtained in vacuum. The hollow beam resonators are verified with theory and compared to resonators with solid cross sections. I. INTRODUCTIOKIn recent years, polysilicon surface micromachining has been used to fabricate vertical and lateral resonators [l, 21, as well as a variety of other flexure-suspended microstructures [3, 41. A feature of this simple process is that the mechanical elements are defined by etching a single LPCVD polysilicon structural layer, which is usually around t = 2 pm in thickness. A typical minimum linewidth is W = 1 -2 p m , assuming optical lithography, reactive ion etching, and a photoresist etch mask. The minimum-geometry polysilicon beam therefore has an aspect ratio t / W = 1 -2 and consists of solid polysilicon, which is a major constraint on the mechanical design of the flexural suspension and the rigid elements of the microstructure. This paper reports on the fabrication of hollow micromeclianical structures, which have cross sections shown in Figure 1. The reasons for investigating hollow cross section microstructures are twofold. First, by making the entire structure hollow the stiffness-tc-mass ratio of the flexures increases, which increases the resonant frequency. A high frequency, high Q oscillator will have niany potential applicat,ions, such as in resonant microsensors, on-chip clocks, and niicromechanical filters [5, 61. The design of lateral resonators with solid suspensions becomes difficult for frequen- Figure 1: Perspective drawing of hollow beam lateral resonator. The entire structure is hollow, not just the suspension beams.The second reason for investigating hollow beams is that the devices are more sensitive to mass loading and to axial loading, simply because the masses and stiffnesses of the resonators are lower than for the solid cross section devices. In addition, the surface to volume ratio of the hollow devices is very large, indicating that surface effects, such as etchant attack of the polysilicon, might be observable.We first describe the fabrication process sequence required to fabricate the polysilicon hollow beam devices. Next, the theory behind these resonators is discussed, including the nonlinearities of the mechanical flexure and electrostatic comb drive as well as a comparison of hollow and solid beam devices. Experimental results that agree with the theory are then presented. Finally, the advantages and future applications of hollow beam devices are discussed. FABRICATIONcies over a few MHz, because the beam lengths required are very short, constraining the shuttle to small oscillations in order to maintain linearity. Calculations and experiments show that a hollow lateral resonator can have approximately 2-4X th...

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