Optical scattering and absorption losses at interfaces and in thin films

Abstract A key to improving the performance of optical thin films is to reduce scattering and absorption losses, both in the films and at the interfaces. Scattering from thin films can be measured by collecting the light scattered into a hemisphere (total integrated scattering (TIS) or as a function of angle. For opaque films the TIS can be simply related to the r.m.s. roughness using a scalar scattering theory. Angular scattering from thin films can be predicted using a vector theory which has as inputs the optical constants of the film(s), angles of incidence and scattering, wavelength, polarization of the incident and scattered light, and surface roughness parameters. A refinement of this theory allows the optical constants of the film to depend on the position along the surface. For multilayer films, the scattering depends on whether the roughness at the film interfaces are uncorrelated, partially correlated or fully correlated. Calorimetric techniques have been developed to measure film and interface absorption as well as absorption in the substrate. These techniques make use of wedged films and/or wedged substrates. Other useful thin film characterization techniques include measurements of reflection, transmission, optical constants and film thickness. Special techniques are also available; these include total internal reflection microscopy, photoacoustic spectroscopy, nuclear resonance for hydrogen detection, laser polishing, Mireau heterodyne profilometry and laser desorption of surface contaminants. By using the appropriate characterization techniques it is possible to measure the performance of thin films and then to improve their properties.

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