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

TL;DR

This study investigates how annealing temperature affects optical scatter in amorphous Ta2O5 and TiO2:Ta2O5 thin films, showing that high-temperature annealing can reduce scatter and improve optical quality for precision measurement applications.

Contribution

It provides the first in-situ measurements of optical scatter changes during annealing of these specific amorphous oxide thin films, demonstrating potential improvements in optical performance.

Findings

Scatter decreased by 25-30% for tantala and up to 74% for titania-doped tantala during annealing.

High-temperature annealing does not increase scatter; it may reduce it.

Some samples showed constant scatter, indicating variability in response to annealing.

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.