Investigation of amorphous silicon carbide:hydrogen and Parylene-C thin films as encapsulation materials for neural interface devices

Neural interface devices have been developed for neuroscience and neuroprosthetics applications to record and stimulate nerve signals. Chronic use of these devices is prevented by their lack of long-term stability due to device failure or immune system responses. Fully integrated, wireless, silicon-based neural interface (INI) devices are being developed to address the main failure modes by eliminating the wired connections. Furthermore, chronic stable, conformal, hermetic, biocompatible, and electrically insulating coating materials that sustain chronic implantation and guarantee stable recording or stimulation are needed. Even though a large selection of materials has been proposed and tested for this purpose, to date, no encapsulation material or coating process presented in scientific literature has been thoroughly characterized or qualified as long-term hermetic encapsulation for silicon micro-electrode arrays.; In this work, hydrogenated amorphous silicon carbide (a-SiCx:H) and Parylene-C films were investigated as encapsulation materials. The deposition parameters and corresponding film properties were explored and correlated with the encapsulation characteristics.; The bond configuration of the deposited a-SiCx:H films was analyzed by FT-IR in order to develop films with strong chemical structure and low defect density. Film properties were optimized based on the bond configuration and process temperature requirements (<200°C). The long-term stability of optimized a-SiCx:H films was studied by electrochemical impedance spectroscopy (EIS) and dissolution tests. EIS showed electrical stability of a-SiCx:H films over a 6-month period. Dissolution tests indicated no discernible dissolution during a 4-week soaking in 90°C phosphate buffered saline (PBS). These accelerated aging tests make a-SiCx:H films a promising candidate as encapsulation.; A second material, Parylene-C, was studied as dielectric encapsulation. The electrically insulating properties of Parylene-C films were investigated using EIS and leakage current tests. Both tests in 37°C saline solution showed hermetic protection and a long-term stability (> 12 months). Oxygen plasma etching processes necessary for deinsulation of the electrode tips and the etching performance on the Parylene-C were investigated, and the relationship between tip exposure and electrode impedance was studied.; Excellent encapsulation properties of Parylene-C were demonstrated. The correlation between process parameters and Parylene-C properties was investigated, including surface topography, adhesion, and crystallinity.