Structure and optical properties of boron nitride thin films deposited by radio-frequency sputtering on polycarbonate

Thin optical protective coatings of boron nitride, similar to1 mum thick, w ere deposited at low substrate temperature on polycarbonate (PC) and fused silica substrates by r.f. magnetron sputtering using a mixture of Ar + N-2 as the sputtering gas. The structure and composition of the deposited layer s were studied by Fourier transform infrared spectroscopy (FTIR) and Ruther ford backscattering spectroscopy (RBS). The optical characterization of the coated sample was carried out by measuring the ultraviolet-visible transmi ttance at different wavelengths and the strength of adhesion of the deposit ed layers to the plastic substrate was analysed by a scratch test. The depo sited layers grow with hexagonal structure, the resulting layers being near ly stoichiometric, and are highly transparent in the visible range on both substrates examined; the measured optical band-gap of the coating was 4.6 /- 0.1 eV. The strength of adhesion to the polycarbonate was found to be de pendent on the substrate pre-deposition treatment: ion etching of the PC su rface before coating deposition gives rise to films with good adherence as a consequence of the formation of a mixed interface layer where C=N chemica l bonds were observed by FTIR.