Fabrication of Micro- and Nanoscale SiC Structures Using Selective Deposition Processes
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Fabrication of Micro- and Nanoscale SiC Structures Using Selective Deposition Processes L. Chen1, X. A. Fu, C. A. Zorman, and M. Mehregany 1 Dept. of Materials Science and Engineering Dept. of Electrical Engineering and Computer Science Case Western Reserve University Cleveland, OH 44106, USA ABSTRACT A method to fabricate nanometer scale SiC beams and nanoporous SiC shells using conventional microlithographic techniques combined with selective APCVD has been developed as an alternative to nanolithographic patterning and electrochemical etching. The process involves the selective deposition of poly-SiC films on patterned SiO2/polysilicon/SiO2 thin film multilayers on (100) Si substrates using a carbonization-based 3C-SiC growth process. This technique capitalizes on significant differences in the nucleation of SiC on SiO2 and polysilicon surfaces in order to form mechanically durable and chemically stable structures.
INTRODUCTION SiC is an attractive material for microelectromechanical systems (MEMS) due to its outstanding electrical, mechanical and chemical properties [1]. These properties also make SiC an attractive material for devices incorporating nanometer scale structures, as recently shown by the successful development of 3C-SiC based NEMS structures with resonant frequencies in excess of 1 GHz [2] and nanoporous membranes made from 6H-SiC wafers [3]. In the first example, a top-down approach based on electron beam lithography was used to define the dimensions of the 3C-SiC beams. Similar structures, such as SiC nanowires and nanorods, have been created using bottom-up synthesis techniques [4-6]. Porous SiC membranes up to 100 µm in thickness and with pore sizes of roughly 10 to 40 nm were fabricated using a photoelectrochemical etching technique. The purpose of this work was to explore the possibility of simplifying the steps required to fabricate SiC nanomechanical structures and nanoporous membranes using a process that requires only conventional thin film and micron-scale photolithography techniques. EXPERIMENTAL DETAILS Fabrication of SiC nanometer-scale beams A long, narrow, doubly clamped microbeam design was selected as the pattern for the fabrication of nano-structures. A plan-view schematic of the microbeam is shown in Figure 1, while a series of cross-sectional schematics that illustrate the fabrication process is shown in Figure 2. The process began with a standard RCA cleaning of 100 mm-diameter Si wafers. Each wafer was then thermally oxidized to form a 1 µm-thick SiO2 film atop the Si. A 0.5 µm-thick
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polysilicon film was then deposited on the SiO2 film, after which a 0.5 µm-thick low temperature SiO2 (LTO) film was deposited on the polysilicon film, thus completing a SiO2/polysilicon/SiO2 multilayer stack. Conventional photolithography was then used to pattern a photoresist layer on the surface of the multilayer. This pattern was transferred to the LTO layer using a buffered HF etching solution. The polysilicon layer was then etched using a Cl2-based plasma, with the bottom
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