Variable Refractive Index Polymer Thin Films Prepared by Plasma Copolymerization

We have developed plasma copolymerization techniques to fabricate photonic films with two (or more) monomers, whose composition can be controlled to manipulate the optical properties of the films. Spectroscopic ellipsometry confirmed that by changing monomer feed ratios (benzene and octafluorocyclobutane), films in the nanometer thickness range with refractive indices between those of the homopolymerized films could be fabricated. Spectroscopic analysis of the chemical structure of the copolymerized films using FTIR and XPS confirmed a nonlinear variation of chemical composition as a function of the monomer feed ratios. However, the refractive index of these films changes linearly with the film composition (characterized as the fluorine/carbon ratio). Facile control of optical thickness was demonstrated by fabricating multilayer antireflecting (AR) coatings on substrates of differing refractive indices. Very good agreement between the design and experimental spectra was obtained. This growth process creates a new way to prepare gradient-index (GRIN) films in the nanometer range, with a (delta)n greater than 0.3 and with well-controlled properties.