A nonplasma technique, hot-filament chemical vapor deposition ͑HFCVD͒, is an alternative method for producing organosilicon films of novel structure. Films are deposited onto room-temperature substrates from the precursors hexamethylcyclotrisiloxane (D 3) and octamethylcyclotetrasiloxane (D 4) at high rates ͑Ͼ1 m/min͒. Filament temperature can be used to control film structure, and the limited reaction pathways available via thermal decomposition make it possible to elucidate the chemistry of the growth process. During film growth, there appears to be competition between reaction pathways for the incorporation of cyclic and linear siloxane structures. For both D 3 and D 4 HFCVD films, infrared, Raman, and nuclear magnetic resonance spectroscopies indicate the incorporation of ring structures consisting of three siloxane units. The concentration of these structures increases as filament temperature is raised and is especially pronounced for films deposited from D 3. In comparison, films grown from D 4 show a greater degree of incorporation of linear, unstrained structures over the range of filament temperatures studied. In contrast to plasma-deposited organosilicon films, cross-linking in HFCVD films occurs predominantly via silicon-silicon bonding and not from siloxane bonds with tertiary or quaternary silicon atoms.

Thin films produced by plasma-enhanced chemical vapor deposition (PECVD) have potential application as conformal coatings on implantable devices with complex topologies and small dimensions. Coatings on such devices need to be biocompatible, insulating, and flexible enough to minimize static forces on the surrounding tissue. In this study, we describe the use of pulsed-PECVD to deposit thin films from hexamethylcyclotrisiloxane (D3). Pulsed-PECVD is a method in which plasma excitation is modulated to favor deposition from neutral and radical species. Thin, conformal coatings were demonstrated on nonplanar substrates suitable for implantation, such as copper wires and neural probes. Coatings were resistant to prolonged immersion in warm saline solution, and wire coatings produced by pulsed-PECVD showed more flexibility than analogous coatings deposited by continuous-wave (CW) excitation. Using Fourier transform infrared spectroscopy, it was demonstrated that the mode of plasma excitation is important in determining film structure. Both CW and pulsed-PECVD showed evidence of cross-linking via ternary and quaternary silicon atoms bonded to more than two oxygen atoms. Methylene groups were observed only in CW films, and may constitute part of a carbon cross-linking unit of the form Si(CH2) n Si, where n ≥ 1. Methylene was not detectable in the pulsed-PECVD films, suggesting that formation of carbon cross-links requires a longer plasma decomposition period. The presence of two distinct cross-linking structures in CW films leads to a highly networked structure and results in brittle coatings on thin wires. A higher proportion of terminal methyl groups was also observed in CW films, suggesting that pulsed-PECVD films may retain more precursor ring structure than CW films.

No abstract

Poly (trivinyl-trimethyl-cyclotrisiloxane) or polyV3D3 is a promising insulating thin film known for its potential application in neural probe fabrication. However, its time-consuming synthesis rate renders it impractical for manufacturing standards. Previously, the growth mechanism of polyV3D3 was shown to be affected by significant steric barriers. This article describes the synthesis of a copolymer of polyV3D3 via initiated chemical vapor deposition (iCVD) using V3D3 as the monomer, hexavinyl disiloxane (HVDS) as a spacer, and tert-butyl peroxide (TBP) as the initiator to obtain nearly a 4-fold increase in deposition rate. The film formation kinetics is limited by the adsorption of the reactive species on the surface of the substrate with an activation energy of −41.5 kJ/mol with respect to substrate temperature. The films deposited are insoluble in polar and non polar solvents due to their extremely crosslinked structure. They have excellent adhesion to silicon substrates and their adhesion properties are retained after soaking in a variety of solvents. Spectroscopic evidence shows that the films do not vary in structure after boiling in DI water for 1 hour, illustrating hydrolytic stability. PolyV3D3-HVDS has a bulk resistivity of 5.6 (±1) × 1014 Ω-cm, which is comparable to that of parylene-C; the insulating thin film currently used in neuroprosthetic devices.