Hydrogen concentration and mechanical dissipation upon annealing in zirconia-doped tantala thin films for gravitational wave observatory mirrors

Sensitivity in instruments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) is limited by a noise originating from fluctuations linked to internal mechanical dissipation (IMD) in the amorphous thin films of their Bragg reflectors. We investigate the correlation between IMD, characterized by a loss angle, and the hydrogen concentration in a thin film made of Zr-doped tantalum oxide after annealing at different temperatures. The film was deposited by magnetron sputtering and the loss angle measured by gentle nodal suspension. The concentration of heavier elements was obtained by Rutherford backscattering spectrometry (RBS). The hydrogen concentration in the as-deposited and annealed samples was obtained by elastic recoil detection (ERD). We observe that the hydrogen atomic concentration gradually decreases from 2.0 ± 0.1% down to the detection limit at 0.2 ± 0.1% as we anneal to 650°C. We also find that the loss angle decreases by a factor of two over the same annealing temperature range, suggesting that the loss angle is correlated with the hydrogen concentration. However, the loss angle remains relatively high even when most of the hydrogen is desorbed. We conclude that the presence of hydrogen is not the main limiting factor for further reducing the IMD.