Delamination behavior study of AF4 parylene thin films on Si and SiO2 substrates by scratch testing

Abstract

In this work delamination behavior $1,6\ \mu\mathrm{m}$ thin AF4 coatings on silicon (Si) and glass (SiO2) substrates were tested by scratch testing. The coatings were deposited by vapor deposition polymerization (VDP) technique on untreated and polished 8-inch Si and SiO2 wafers, with root mean square (RMS) roughness of 4 and 2 nm, respectively. Scratch tests were performed in displacement and load controlled mode, where scratch depths were varied from 1 to $1,6\ \mu\mathrm{m}$. The indentation loads and subsequently depths, were varied to observe different delamination features over the penetration depth at the substrate-coating interface. Additionally, the scratches were observed in-situ with scanning electron microscopy (SEM) to better understand the changes in lateral force during the test. The results showed that cohesive failure in AF4 happens before adhesive failure at indentation depths down to $1,4\ \mu\mathrm{m}$, At $1,4\ \mu\mathrm{m}$ depth we can observe first adhesive failure features on the AF4 coating - SiO2 substrate interface. This shows that the AF4 coating on the SiO2 substrate has a slightly lower adhesion than on the Si substrate. The adhesion failures on both samples start to occur at scratches near the AF4-substrate interface, without chipping on the sides. The results also showed that if the scratch depth is not near the AF4-substrate, only the cohesive failure in AF4 can be seen, which indicates very good adhesion for both samples.