Effects of the annealing of amorphous Ta2O5 coatings produced by ion beam sputtering concerning the effusion of argon and the chemical composition

We investigate the effects of thermal treatments in vacuum or air on amorphous Ta2O5 deposited on crystalline Si by Ion Beam Sputtering, mimicking the coatings of the mirrors of the gravitational-wave revealers of the Advanced Virgo and Advanced LIGO detectors. Argon is present in the whole thickness of the coating, likely introduced during the synthesis, and it can be released above 200°C in vacuum, while less discharge of Ar is detected during heating in air. Formation of sub-stoichiometric Ta oxides is evidenced in the surface layers, after in-vacuum annealing. The macroscopic structure of the samples is not affected by the thermal treatments. However, the length of the interference fringes, due to the coating, decreases after thermal treatments both in air and in vacuum, suggesting an increase of the Ta2O5 thickness of ~1-2%, caused by an increase of the volume of the pores present in the structure.