Measurements of Optical Scatter Versus Annealing Temperature for Amorphous Ta2O5 and TiO2:Ta2O5 Thin Films

Abstract: Optical coatings formed from amorphous oxide thin films have many applications in precision measurements. The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo use coatings of SiO$_2$ (silica) and TiO$_2$:Ta$_2$O$_5$ (titania doped tantala) and post-deposition annealing to 500$^\circ$C to achieve low thermal noise and low optical absorption. Optical scattering by these coatings is a key limit to the detectors' sensitivity. This paper describes optical scattering measurements for single-layer ion-beam-sputtered thin films on fused silica substrates: two samples of Ta$_2$O$_5$ and two of TiO$_2$:Ta$_2$O$_5$. Using an imaging scatterometer at a fixed scattering angle of 12.8$^\circ$, in-situ changes in the optical scatter of each sample were assessed during post-deposition annealing to 500$^\circ$C in vacuum. The scatter of three of the four coated optics was observed to decrease during the annealing process, by 25-30$\%$ for tantala and up to 74$\%$ for titania-doped tantala, while scatter from the fourth sample held constant. Angle-resolved scatter measurements performed before and after vacuum annealing suggests some improvement in three of four samples. These results demonstrate that post-deposition high-temperature annealing of single-layer tantala and titania-doped tantala thin films in vacuum does not lead to an increase in scatter, and may actually improve their scatter.