Improved sensitivity in ellipsometry of thin biochemical films by employing sublayers

Ellipsometry is widely used for investigating the optical properties of thin films on planar substrates, including films of adsorbed proteins or polymers. The average thickness and effective refractive index of the adsorbed layer are calculated by measuring the A and ¥ ellipsometry parameters. Unfortunately the thickness of the adsorbed protein layers is often too thin to significantly affect the A and Y parameters. However, using a substructure consisting of an additional sublayer placed between the substrate and the adsorbed layer, we can improve the sensitivities of both A and 4* to changes in the adsorbed layer, provided that the thickness of the sublayer is optimized. We show that for a Si02 layer on a Si wafer, the optimum Si02 thickness is about 1350 A when the incident angle is 70 degrees and the wavelength is 6328 A. The materials of the sublayer can be metal, semiconductor and/or dielectric.