Structure of the interfacial region between polycarbonate and plasma-deposited SiN1.3 and SiO2 optical coatings studied by ellipsometry

Abstract

Fabrication of optical films on plastic substrates presents a considerable challenge due to the necessity of controlling adhesion and the optical properties of the interface. In the present work, the surface of polycarbonate (PC) substrates was pretreated in a microwave plasma in N2 and He gases, and amorphous hydrogenated silicon nitride (SiN1.3) and dioxide (SiO2) films were deposited in a dual-mode microwave/radiofrequency plasma. Using ex situ variable angle spectroscopic ellipsometry (VASE), spectrophotometry, and x-ray photoelectron spectroscopy, it was found that the plasma-treated PC contains a crosslinked surface layer which can extend to a depth of more than 50 nm, and which presents a refractive index increase of about 1%–2%. When PC is plasma treated and coated with SiN1.3 or SiO2, the interfacial region (interphase) is structured: it comprises the crosslinked layer, and a less dense transition region between the polymer and the film. The interphase may extend up to 100 nm. Some of the results suggest that the plasma pretreatment stabilizes the surface, leading to a lower thickness of the interphase. The accuracy of ex situ VASE analysis is discussed and the importance of the interface structure on the design and mechanical properties of optical systems on plastic substrates is emphasized.