LIGHT-SCATTERING FROM TRANSPARENT SUBSTRATES - THEORY AND EXPERIMENT

To produce high-quality, low-scatter optical interference coatings, substrates with extremely smooth surfaces are required for deposition. For successful production surface roughness characterization of the substrates before coating is essential. Light-scattering measurement is a nondestructive and fast surface characterization technique that determines statistical surface properties such as the power-spectral-density function (PSD) and the rms roughness delta(rms). Scatter measurement is an accepted tool to characterize opaque surfaces and optical interference coatings. Its application to characterize the surfaces of transparent substrates - commonly used for deposition - is difficult due to the low scatter intensities of high-quality substrates and the scattering contribution of the substrate's back surface. In this paper the total scattering distribution of a transparent substrate is calculated, considering the scattering contributions of the front and the back surface. With the theoretical result obtained, PSD's of the substrate interfaces can be cakulated from angle-resolved-scattering measurement and rms roughness from the total integrated scattering measurement.