Chemical vapor deposition and functionalization of fluorocarbon-organosilicon copolymer thin films

Abstract

Neural prostheses are micron-scale integrated circuit devices that are under development for the treatment of brain and spinal cord injuries. A key challenge in the fabrication of these silicon- based devices is the protection of the electronic components from the ambient body environment. There is a need for a biopassivation coating on these devices that is chemically inert and electrically insulating with good adhesion to the underlying silicon substrate. Fluorocarbon-organosilicon copolymers are of interest for this application because they have the desirable attributes of both fluorocarbon and organosilicon polymers, such as low dielectric constant, thermal stability, and good adhesion to silicon. Chemical vapor deposition (CVD) is an attractive synthetic technique for this application because it is single-step, requires no solvent, and allows conformal coatings to be deposited on substrates with complex topographies and small dimensions. Fluorocarbon-organosilicon copolymers have been synthesized by hot-filament CVD, a thermal CVD technique. Control over deposition rate and chemical structure is achieved by precursor choice and variation of filament temperature. Chemical characterization by infrared (FTIR), x-ray photoelectron (XPS), and solid-state nuclear magnetic resonance (NMR) spectroscopies indicates that the copolymer films range from highly cross-linked films to flexible films comprised mostly of linear polymer chains. This variation in chemical composition influences physical properties such as thermal stability and flexibility. The possibility of creating bioactive surface coatings has been explored by using the techniques of CVD and solution chemistry in combination. Chains of poly(acrylamide) have been grafted onto fluorocarbon-organosilicon films as a first step towards the design of bioactive coatings that could potentially enhance the performance of medical implants.