Optical properties of a photoformable polymer
The optical properties of a novel, crosslinkable, photoformable polymer, in the form of thin, spin coated, films were investigated. The effect of material processing parameters on the film index of refraction, index anisotropy and absorbance properties were measured. Prism coupling into the film waveguides was used to measure the index of refraction properties of the polymer. Attenuation in the waveguides was measured using a two prism technique.
The measured average index of refraction and the anisotropy between the index normal to and perpendicular to the film surface were found to depend strongly on the extent of the crosslinking reaction. The anisotropy is shown to be a stress birefringence phenomena arising from tension in the films due to contraction during curing. Index anisotropy measurements on softbaked films prior to curing indicate that the films are under compression. This is explained by a non uniform stress distribution across the film due to incomplete solvent evaporation, which places the solvent free surface under tension and creates a compression zone in the bulk film. Perturbation calculations show that the calculated index will be weighted toward the properties in the compression zone at the film-substrate interface.
It has been found that the dye Disperse Red, which has a large linear polarizability along the molecule's backbone, can be used as an orientation marker for evaluating stress effects in polymer films. By comparing the index anisotropy measured for a dye doped film to that of a dye free control film the average orientation of the dye can be calculated. The dye in the soft baked film is initially oriented normal to the surface, confirming that compressive stresses are present. During curing the average orientation shifts to be parallel to the surface, as the film contracts during crosslinking and goes under tension. Therefore it is demonstrated that doping with low concentrations of a dye like Disperse Red coupled with accurate index anisotropy measurements by prism coupling may be used as a general tool for probing orienting forces, such as mechanical stress, which may be present in polymer films.