Control of Trenching and Surface Roughness in Deep Reactive Ion Etched 4H and 6H SiC
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0911-B10-15
Control of Trenching and Surface Roughness in Deep Reactive Ion Etched 4H and 6H SiC Glenn M. Beheim, and Laura J. Evans NASA Glenn Research Center, 21000 Brookpark Rd, Cleveland, OH, 44135 ABSTRACT An optimized deep reactive ion etching (DRIE) process for the fabrication of SiC microstructures has been developed. The optimized process enables the etching of 4H and 6H SiC to depths > 100 µm with the required characteristics of (1) high rate (>0.5 µm/min), (2) vertical sidewalls, (3) minimal microtrenching at the sidewall base, and (4) smooth etched surfaces. The optimized process was determined based on the results of an experiment (full factorial design) which determined how the etch characteristics are affected by four important process parameters: temperature of the wafer chuck, pressure within the chamber, and concentrations of O2 and Ar in a gas flow comprised of O2, Ar and SF6. This study is believed to be the first systematic investigation of the effect of temperature on SiC DRIE characteristics; substrate heating was found to be a key in producing the desired etch properties. INTRODUCTION Single-crystal silicon carbide (SiC) of the 4H and 6H polytypes has attractive characteristics for harsh-environment microelectromechanical systems (MEMS) such as high-temperature pressure sensors [1]. Fabrication of SiC MEMS frequently requires that SiC be etched to depths of 100 µm or greater. Various groups have demonstrated deep reactive ion etching (DRIE) of SiC with a high etch rate (>0.5 µm/min) [2], but a process has not yet been demonstrated which provides high rate together with other required etch characteristics: vertical sidewalls are needed so that lateral dimensions do not vary as a function of etch depth; microtrench depth and roughness of the etched surface must be minimized to achieve high strength. A microtrench, which is caused by a locally higher ion flux (and etch rate) at the base of the sidewall, can significantly weaken a pressure sensor diaphragm by concentrating stress. We report here the development of a SiC DRIE process which enables the high-rate fabrication of SiC MEMS that are largely free of performance-reducing micromachining defects. EXPERIMENTAL DETAILS Deep reactive ion etching was performed in a ST Systems Multiplex ICP inductively coupled plasma etcher. Because heating is not required to volatilize the etch products, DRIE of SiC is generally performed with the substrate cooled to about room temperature. Previously, experiments in which the SiC sample was thermally isolated and allowed to heat up by plasma action indicated that increasing the sample temperature might help minimize microtrenching. Therefore, the etcher was modified to provide substrate heating prior to the work reported here. An experiment was designed to determine how microtrench depth, sidewall slope, surface roughness and etch rate were affected by key process parameters. The process parameters (and the range of values studied) were chuck temperature (20 to 125 °C), pressure (5 to 25 mT), and concentratio
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