Optical properties of polymeric thin films grown by chemical vapor deposition
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Optical properties of polymeric thin films grown by chemical vapor deposition Justin F. Gaynor and Seshu B. Desu Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0237 (Received 22 November 1994; accepted 29 September 1995)
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 sl 632.8 nmd 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.
I. INTRODUCTION
Polyparaxylylene (PPX), polyparachloroxylylene (PPX-C), and polyparadichloroxylylene (PPX-D) have achieved modest commercial importance due to their unique ability to form polymeric thin films by chemical vapor deposition (CVD).1 Recently, the use of these films in planar thin-film waveguides or optical filters has been proposed.2 The films are transparent throughout the visible spectrum, tough, easily patterned, hydrophobic, resistant to solvent attack, and pinhole-free even when very thin. They can be grown on any substrate, including liquids and powders. There are small differences in the indices of refraction snd of PPX, PPX-C, and PPX-D. A much greater range of properties is available if the process of chemical vapor copolymerization is employed2,3 ; here, we present a low-index copolymer, poly(parachloroxylylene-co-perfluorooctyl methacrylate), or PPX-C/PFOMA, which would significantly increase the efficiency of a waveguide. This copolymer, which retains all the advantages of CVD processing, may also prove useful for antireflective coatings or as a low dielectric constant material for the microelectronics industry. The structures of the three homopolymers and copolymer are shown in Fig. 1. Until now, information about the homopolymers’ optical properties has been limited to UV/visible/IR transmission spectra, refractive indices of the three films at wavelengths of 589.9 and 632.8 nm, and birefringence of PPX. Here, the refractive indices of annealed films are reported through the wavelength spectrum from 236
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