Precise thickness and refractive index determination of polyimide films using attenuated total reflection.

Surface plasmons generated at a silver-polyimide interface and the guided modes coupled into planar films cause dips in the reflectivity curve of a transparent dielectric-silver-polyimide-air structure. These minima in the reflectivity curve were used to measure the polyimide thickness as well as the real and imaginary parts of the polymer refractive index (extinction). Precision within 1% of the polymer coat thickness was achieved through the use of this technique over the range 0.5-10 µm. In some cases, this technique is capable of yielding a precision of ~ 10% on the imaginary part of the polymer refractive index and provides a useful method for determining the performance of a low-loss polymer waveguide. Techniques in fitting the experimental reflectivity data to obtain the optical constants are also described.