Growth of Polycrystalline Silicon Carbide on Thin Polysilicon Sacrificial Layers for Surface Micromachining Applications
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Growth of Polycrystalline Silicon Carbide on Thin Polysilicon Sacrificial Layers for Surface Micromachining Applications
R.F. Wiser, J. Chung*, M. Mehregany, and C.A. Zorman Department of Electrical Engineering and Computer Science, *Department of Material Science and Engineering Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106 ABSTRACT Polycrystalline silicon carbide (poly-SiC) films were deposited by atmospheric pressure chemical vapor deposition (APCVD) at epitaxial growth temperatures on planar, 100 nm-thick polysilicon sacrificial layers using two recipes that included or excluded a pre-growth carbonization step. Poly-SiC films grown using the carbonization-based recipe exhibited a relatively high degree of (111) 3C-SiC texture and had uniform, well-defined, void-free polySiC/polysilicon interfaces. In contrast, poly-SiC films grown without carbonization were randomly oriented, had numerous poly-SiC inclusions that sometimes completely penetrated the polysilicon underlayer, and had a higher surface roughness than the films grown with carbonization. Analysis of micromechanical clamped-clamped (C-C) beam resonators fabricated from films grown using the two differing recipes shows that the carbonization step is needed to protect the thin polysilicon sacrificial layer from voids and inclusions and thus maintain the proper spacing between the drive electrodes and the resonant beams. INTRODUCTION Growing interest in using micromechanical resonators in RF devices has created the desire to push the operating frequencies of micromechanical oscillators to frequencies higher than can currently be achieved by using polysilicon as the structural material. Materials that have higher acoustic velocities (square root of Young’s modulus divided by density) than polysilicon, namely silicon carbide (SiC) [1] and diamond [2], are currently being developed for these applications. These materials are attractive alternatives because their high acoustic velocities translate to higher resonant frequencies for devices of the same geometry. Surface micromachining processes have been developed for both materials, but SiC is currently at the forefront due to the availability of more mature processing technologies that, in most instances, are quite similar to silicon. The purpose of this study was to determine suitable deposition recipes for the fabrication of poly-SiC C-C beam micromechanical resonators designed for MHz frequency applications. The designs for the poly-SiC C-C beam resonator were based on designs that have been successfully implemented using polysilicon as the structural layer [3]. Fabrication of these devices out of poly-SiC poses several significant challenges in the area of film growth. These challenges stem mainly from the fact that the poly-SiC resonating bridges are suspended over anchored poly-SiC electrodes with an air gap that is defined by a sub-micron thick sacrificial layer. For polysilicon, use of such thin SiO2 sacrificial layers is commonplace; however, issues related to the
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