Effect of interface on the characteristics of functional films deposited on polycarbonate in dual-frequency plasma

Functional coatings are used in increasingly demanding applications that require specific optical characteristics, resistance to damage and good adhesion to different types of substrate materials, including polymers. In the present work we investigate thin films fabricated by low pressure plasma-enhanced chemical vapor deposition, using a dual-mode microwave/radio-frequency (MW/RF) plasma approach. The substrates are exposed to the principal dense microwave (MW, 2.45 GHz) plasma, while applying a RF-induced negative bias voltage. This technique provides independent control of the energy and flux of bombarding ions, and it allows one to deposit dense films at ambient substrate temperature at a high rate over large areas. We optimized the deposition process for amorphous hydrogenated silicon nitride (SiN1.3) obtained from SiH4/NH3 mixture. Using an average ion energy of about 150 eV, we obtained low-stress (<100 MPa) films with a refractive index of 1.89, at deposition rates around 30 Å/s. MW plasma pretreatments with different gases have been investigated in order to further enhance adhesion of the SiN1.3 coatings on polycarbonate (PC) substrates—the highest adhesion, determined by the microscratch- and the adhesive-tape peel tests, was found when PC was pretreated in N2 plasma. The adhesion is related to the thickness and chemical structure of the interface. In fact, spectrophotometry and x-ray photoelectron spectroscopy analysis suggest the presence of a graded interface region, up to 40 nm thick, part of which contains Si–C, Si–O–C, and Si–N–C chemical links.