Micromachining Techniques for Advanced SiC MEMS
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Micromachining Techniques for Advanced SiC MEMS Mehran Mehregany and Christian A. Zorman Department of Electrical Engineering and Computer Science Case Western Reserve University Cleveland, Ohio 44106
ABSTRACT This paper reviews the development of a multilayer, micromolding-based surface micromachining process for SiC microelectromechanical systems (MEMS). The micromolding process uses polysilicon and SiO 2 thin films that are deposited onto polysilicon and SiO 2 sacrificial layers, patterned into micromolds by reactive ion etching, filled with polycrystalline SiC (poly-SiC), planarized by mechanical polishing, and eventually dissolved and released in selective wet chemical etchants. In addition, a SiC lift-off technique that exploits the microstructural differences between SiC films deposited on Si, SiO 2 and Si3 N4 surfaces has been developed. The micromolding and lift-off techniques are being used as the basic patterning processes for a four-layer, poly-SiC surface micromachining process that we call the MUSiC (Multi- User SiC) process. INTRODUCTION SiC is well known for its outstanding combination of mechanical, electrical, and chemical properties; making it an excellent material for microfabricated sensors and actuators designed for environments too harsh for Si-based devices. Many of these devices can be fabricated using a process called surface micromachining. An essential aspect of surface micromachining is the use of sacrificial thin films that serve initially as a platform for the deposition and patterning of the structural layers, but are later dissolved to release the free standing micromechanical components of the device. A robust, multi-layer surface micromachining process requires that the material for the structural layer adhere well to the sacrificial substrate layers while being chemically inert to the sacrificial layer etchants. SiC is particularly well suited as a structural material because polysilicon and silicon dioxide films, both commonly used in conventional polysilicon surface micromachining, meet these criteria. Reactive ion etching (RIE) is the primary technique used to pattern SiC thin films. Unfortunately, problems associated with SiC RIE, such as micromasking, low etch rates, and poor etch selectivity to Si and SiO 2 make surface micromachining of SiC a challenging task. Despite these problems, several single-layer, RIE-based, polycrystalline SiC (poly-SiC) surface micromachining processes using polysilicon and SiO 2 as sacrificial layer materials have been developed [1,2]. Unfortunately the difficulties with SiC RIE, especially in the area of etch selectivity, make the multilayer processing that is analogous to polysilicon microelectromechanical systems (MEMS) very difficult in SiC. In order to circumvent the problems associated with SiC RIE, we have recently developed a micromolding technique that uses microfabricated polysilicon molds in conjunction with polyH4.3.1
SiC film deposition and mechanical polishing to pattern poly-SiC films [3]. The micromolding process has been succe
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