Demonstration of 3C-SiC MEMS Structures on Polysilicon-on-Oxide Substrates
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1246-B08-05
Demonstration of 3C-SiC MEMS Structures on Polysilicon-on-Oxide Substrates Christopher Locke1, Christopher L. Frewin1, 2, Luca Abbati1, 3, and Stephen Saddow1, 2 1 Electrical Engineering Dept., University of South Florida, Tampa, FL 33620, U.S.A. 2 Dept. of Molecular Pharmacology and Physiology, USF, Tampa, FL 33620, U.S.A. 3 Department of Electrical Engineering, University of Perugia, Perugia, Italy ABSTRACT Silicon carbide has robust mechanical, electrical, and chemical properties which make it an attractive material candidate for micro- and nano-electromechanical systems (MEMS and NEMS). 3C-SiC films grown via a polysilicon seed-layer CVD-deposited on an oxide coated (111) Si substrate offers an innovative method to overcome the residual film stress issues associated with 3C-SiC heteroepitaxy and the difficulties of fabricating structures from 3C-SiC films. The oxide plays a dual role by permitting film relaxation with respect to the supporting substrate and functioning as a MEMS release layer, allowing MEMS structures such as cantilevers and diaphragms, to be easily fabricated from the 3C-SiC film. The impact of the oxide layer on the relaxation of the film stress was investigated by comparing direction-sensitive MEMS stress sensors fabricated from 3C-SiC films grown via a polysilicon-on-oxide-coatedsubstrate and a polysilicon-on-crystalline Si substrate. Scanning Electron Microscopy (SEM) analysis of bridge structures fabricated on the polysilicon-on-oxide substrate revealed evidence of film strain relaxation when compared to bridge structures fabricated on the polysilicon-oncrystalline Si substrate. However, the upward-curled cantilever and comb structures fabricated on both substrates indicate the presence of a strain gradient in the 3C-SiC film grown on both substrates. INTRODUCTION Silicon carbide is a semiconductor material that is desirable for many power electronics and MEMS applications due to its wide band gap, mechanical resilience, thermal properties, and chemical inertness. However, many of these inherent properties create extreme difficulties when processing MEMS devices. SiC chemical resistance reduces the effectiveness of wet chemical etching and requires the use of dry etching techniques through reactive ion etching (DRIE/RIE). Fortunately, 3C-SiC is the one polytype of SiC that can be grown heteroepitaxially on Si substrates, and the addition of this Si layer allows for many more processing options in device manufacturing. For example, one can utilize the Si substrate as a sacrificial layer for the creation of freestanding 3C-SiC MEMS structures.1,2 However, the recipes used to etch Si in DRIE/RIE have a similar etch rate with SiC, thereby excluding selectivity and reducing accuracy for the desired structure.1-3 Freestanding SiC MEMS devices using the sacrificial Si layer have also encountered difficulties during device creation resulting from unetched Si preventing the release of the structure.3,4 SiO2 has been traditionally used as an etch-stop in Si processing involving DRIE/RIE
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