Scatter loss and surface roughness of hafnium oxide thin films

Abstract

The work presented in the thesis aims to characterize and improve the surface quality and scatter loss of HfO2 single layer thin films. Dual ion beam sputtered coatings of HfO2 produced at CSU have been shown to have a high damage threshold and low absorption. They have not been optimized for reduced surface roughness. Both surface quality and scattering of dual ion beam sputtered thin films depend on the growth conditions and substrate quality, so a study of growth parameters and substrate choice is conducted. The growth parameters selected in this work are beam voltage of the main ion source and sample thickness. Samples grown on standard optically polished substrates are compared to samples grown on two types of super-polished substrates. A multilayer coating is also examined to characterize how scatter loss scales with the number of layers. A device is built to measure scatter loss at 1064nm, 633nm and 405nm and an in depth analysis is conducted of the surfaces using atomic force microscopy and white light interferometry. The films scatter loss at 1064nm are shown to be sensitive to substrate choice, film thickness and main beam voltage. Scatter loss at 1064nm generally decreases when beam voltage is reduced. Scatter loss at lower wavelengths decreases much more significantly. Smoother substrates show improved scatter performance, but it is again much more noticeable at lower wavelengths. Thicker samples show increased scatter, especially at shorter wavelength. Surface scans are benchmarked using a variety of techniques, but power spectral density analysis is shown to be the best predictor of scatter loss for most samples. The best growth conditions and a super polished substrate yield a scatter loss of 6.7ppm for a single layer and 12.3ppm for an output coupler which is comparable to commercially available output couplers.