Thiol-based adhesion promoters for strong Parylene bonding on gold-coated substrates

Parylene coatings are effective in protecting biomedical devices and biological samples from each other. Strong and reliable adhesion of Parylene coatings is therefore crucial. For many substrates, proper adhesion is easy to obtain. For oxidizable substrates for example, the primer silane A-174 yields a sufficient adhesion. However, for gold-coated structures, adhesion is problematic. The purpose of this study was to find an adhesion promoter that chemically binds Parylene-C to gold. Two thiol-based primers were selected and tested. Gold-coated silicon wafers were soaked in an adhesion promoter solution containing 2-(methylthio)ethyl methacrylate or 4-chlorothiophenol, conformally coated with Parylene-C, and diced into 1 cm wide test strips. The adhesion force between the gold structures and Parylene-C coatings was measured by conducting 90° peel tests. In addition, the influence of post-treatment steps on the Parylene-C coated substrates was investigated. These steps included an autoclave procedure at 121 °C for 15 min, soaking in 37 °C phosphate-buffered saline for 24 h, and heat exposure in a vacuum chamber at 230 °C for 30 min. All performed peel tests showed strong adhesion between the gold substrates and Parylene-C coating. Both 2-(methylthio)ethyl methacrylate and 4-chlorothiophenol are effective adhesion promoters. A five-minute soaking step in the adhesion promoter solution just before Parylene deposition can easily be implemented in the conformal coating process flow. • Efficient method for strong adhesion between Parylene and gold-coated substrates • Strong chemical linkage by 2-(methylthio)ethyl methacrylate and 4chlorothiophenol • The thiol-based primer adhesion is resistant to standard sterilization steps. • Easy implementable step in conformal coating processes