Exploration of co-sputtered Ta 2 O 5 -ZrO 2 thin films for gravitational-wave detectors

Abernathy, M and Amato, A and Ananyeva, A and Angelova, S and Baloukas, B and Bassiri, R and Billingsley, G and Birney, R and Cagnoli, G and Canepa, M and Coulon, M and Degallaix, J and Di Michele, A and Fazio, M A and Fejer, M M and Forest, D and Gier, C and Granata, M and Gretarsson, A M and Gretarsson, E M and Gustafson, E and Hough, E J and Irving, M and Lalande, É and Lévesque, C and Lussier, A W and Markosyan, A and Martin, I W and Martinu, L and Maynard, B and Menoni, C S and Michel, C and Murray, P G and Osthelder, C and Penn, S and Pinard, L and Prasai, K and Reid, S and Robie, R and Rowan, S and Sassolas, B and Schiettekatte, F and Shink, R and Tait, S and Teillon, J and Vajente, G and Ward, M and Yang, L (2021) Exploration of co-sputtered Ta 2 O 5 -ZrO 2 thin films for gravitational-wave detectors. Classical and Quantum Gravity, 38 (19). 195021. ISSN 0264-9381 (https://doi.org/10.1088/1361-6382/ac1b06)

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We report on the development and extensive characterization of co-sputtered tantala–zirconia (Ta2O5-ZrO2) thin films, with the goal to decrease coating Brownian noise in present and future gravitational-wave detectors. We tested a variety of sputtering processes of different energies and deposition rates, and we considered the effect of different values of cation ratio η = Zr/(Zr + Ta) and of post-deposition heat treatment temperature T a on the optical and mechanical properties of the films. Co-sputtered zirconia proved to be an efficient way to frustrate crystallization in tantala thin films, allowing for a substantial increase of the maximum annealing temperature and hence for a decrease of coating mechanical loss φ c. The lowest average coating loss was observed for an ion-beam sputtered sample with η = 0.485 ± 0.004 annealed at 800 °C, yielding φ¯c=1.8×10−4 rad. All coating samples showed cracks after annealing. Although in principle our measurements are sensitive to such defects, we found no evidence that our results were affected. The issue could be solved, at least for ion-beam sputtered coatings, by decreasing heating and cooling rates down to 7 °C h−1. While we observed as little optical absorption as in the coatings of current gravitational-wave interferometers (0.5 parts per million), further development will be needed to decrease light scattering and avoid the formation of defects upon annealing.