PECVD Silicon Carbide as a Thin Film Packaging Material for Microfabricated Neural Electrodes
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PECVD Silicon Carbide as a Thin Film Packaging Material for Microfabricated Neural Electrodes Allison Hess1, Rocco Parro1, Jiangang Du1, Jeremy Dunning2, Maximillian Scardelletti3, and Christian A. Zorman1 1 Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, 44106 2 Louis Stokes VA Medical Center, Cleveland, OH, 44016 3 NASA Glenn Research Center, Cleveland, OH, 44135 ABSTRACT This paper reports our effort to develop amorphous silicon carbide (a-SiC) films for use as hermetic thin film coatings for mechanically-flexible neural electrodes. In our work, the a-SiC films were deposited by plasma enhanced chemical vapor deposition (PECVD) using two distinct methods, namely a single precursor approach using trimethylsilane, and a dual precursor approach using methane (CH4) and silane (SiH4). The mechanical properties of films deposited on Si substrates were characterized using the wafer curvature and load-deflection methods. The effectiveness of the films as moisture barriers for polyimide substrates was characterized by measuring the leakage currents of SiC-coated interdigitated electrode structures soaked in PBS. A microfabricated prototype of the flat interface nerve electrode (FINE) based on a flexible polyimide substrate and a PECVD SiC capping layer was fabricated using a monolithic process based on conventional micromachining techniques. To facilitate this approach, a reactive ion etching process was developed that exhibited high etch rates and high selectively to the SiC films. INTRODUCTION SiC is an attractive material for microelectromechanical systems (MEMS) due to its outstanding mechanical, chemical and electrical properties. The preponderance of work in the development of SiC for MEMS has focused on high temperature applications using thin films deposited by APCVD and LPCVD at high substrate temperatures (> 800ºC). Recent interest in using SiC as a diffusion-barrier coating on temperature-sensitive substrates has motivated the development of PECVD-based processes for a-SiC. The PECVD approach has permitted the lowering of substrate temperatures below 400ºC, thus enabling the incorporation of buried metal electrode structures, the use of non-conventional substrates (i.e., polyimide) and furthering the integration of SiC with Si microelectronics [1-3]. A recent investigation also indicates that aSiC:H is biocompatible [4]. This paper reports our effort to develop a-SiC films suitable for thin film packaging of moisture-sensitive devices such as polymer-based neural electrode arrays. EXPERIMENTAL DETAILS AND RESULTS In this study, 50 nm-thick and 500 nm-thick films were deposited using two distinct methods differing primarily by the selection of precursors. SiC films were deposited using trimethylsilane,
a single C- and Si-containing precursor shown previously to produce SiC films suitable as structural layers for MEMS [5]. The films were deposited in an UltraDep1000 PECVD tool equipped with both low (400 kHz) and high (13.56 MHz) frequency power supplies
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