Optical properties of polymeric thin films grown by chemical vapor deposition

For the first time, the refractive index of polyparaxylylene films, the only polymers grown commercially by chemical vapor deposition (CVD), is reported throughout the visible spectrum. This information is required if optical components such as antireflective coatings or waveguides are to be fabricated with CVD polymers. These properties are compared to a low-index CVD copolymer, poly(parachloroxylylene-co-perfluorooctyl methacrylate), invented in our laboratory. The ellipsometric constants psi and delta were measured for wavelengths between 400 nm and 1000 nm using variable angle spectroscopic ellipsometry; many samples of each film were grown to improve statistics. The data were modeled assuming a birefringent Cauchy dispersion; excellent agreement between models and experimental data was obtained. The refractive index (lambda = 632.8 nm) of the copolymer in the film plane was 1.389, compared to 1.645-1.665 for the homopolymers. PPX, PPX-C, and the copolymer showed negative birefringence, while PPX-D showed positive bifringence. The optical properties of PPX showed little thickness dependence for films ranging from 36 nm to 2100 nm thick.