Plasma-enhanced chemical vapor deposition is very attractive for the fabrication of optical filters on plastic substrates. We found, using ex situ and in situ spectroellipsometric analysis, that plasma deposition of silicon nitride as a high-refractive-index material, on bare and plasma-pretreated polycarbonate (PC) substrates, gives rise to the formation of a 50- to 100-nm-thick interfacial region (or interphase). We show that plasma pretreatment can reduce the thickness of this interphase by 50% from that obtained with deposition on untreated PC, possibly due to interface stabilization. An optical model based on real-time in situ measurements is proposed to describe the interphase refractive index profile. This model is fully supported by complementary cross-sectional transmission-electron-microscopy observations. Important consequences for the mechanical performance (adhesion and scratch resistance) and optical design of optical coatings on plastics are discussed.
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