Crystallization of amorphous Ta2O5 coatings for gravitational waves astronomy

In 1915, Albert Einstein published his theory of General Relativity, proposing the existence of ripples in space-time, known as Gravitational waves. It was not until 2015 when the U.S. National Science Foundation Laser Interferometer Gravitational-wave Observatory (NSF LIGO) detected for the first time, a gravitational wave. A Gravitational wave detector works similarly to a Michelson interferometer with two mirrors in each arm operating as a Fabry-Perot cavity, increasing the effective distance light travels and, in consequence, improving its sensitivity. These mirrors are composed of alternating layers of amorphous materials with different refractive index. Mirror coatings play a key role in the overall performance of the GW detector, since the most-sensitive frequency range of the interferometers lies between 40-400 Hz, same band where the coating Brownian thermal noise (CTN) sets up a limit in the detector. Thermal annealing has been proven as a way to reduce CTN. However, this treatment also induces the generation of crystals in the material, increasing its optical scattering. The aim of this thesis is to characterize and analyze the optical properties of tantalum oxide Ta2O5 coatings, manufactured using Ion Beam Sputtering Deposition technique and later treated in an oven at high temperatures for step-increasing hours, where a crystalization process takes place. Finally, given the changes of crystallization, the evolution of its properties are studied. A visual analysis is carried out using Atomic Force Microscopy (AFM) and Optical Microscopy, estimating surface roughness (RMS) and Grain Size Distribution (GSD) respectively. These examinations revealed the formation of single-crystalline grains of random orientations with a trend of increasing surface roughness and average crystal sizes. The optical scattering measurement was done using a home-build scatterometer device. Results demonstrated that at high crystallization levels, optical scattering increased.