Exploration of co-sputtered Ta₂O₅–ZrO₂ thin films for gravitational-wave detectors

Abstract

We report on the development and extensive characterization of co-sputtered tantala-zirconia (Ta 2 O 5 -ZrO 2 ) 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.

Enhancing Titania-Tantala Amorphous Materials as High-Index Layers in Bragg Reflectors of Gravitational-Wave Detectors

“…Important examples include TiO 2 − Ta 2 O 5 , 14,21−23 SiO 2 −TiO 2 , 24 TiO 2 −Nb 2 O 5 , 25 GeO 2 −TiO 2 , 26 and ZrO 2 −Ta 2 O 5 . 20 Today, the physical mechanisms at work in mixing remain to be clearly understood, with the consequence that the identification of optimal binary mixtures largely rests on educated guesses and a trial-and-error process. 23 In particular, the oxide mixture currently used in GWD mirrors is TiO 2 − Ta 2 O 5 with a Ti/(Ti + Ta) ratio around 20% because this was found to yield the lowest mechanical and optical losses after thermal annealing at 500 °C in air.…”

“…A strategy of primary importance toward the improvement of mirror coatings therefore consists in identifying adequate high-index mixed oxides . , Indeed, it is known in many different contexts, ranging from metallic glasses to suspensions, that mixing strongly improves the stability of glasses against crystallization. − The goal is then to optimize a complex trade-off between refractive index, stability against crystallization, optical absorption, and thermal noise. This must be accomplished in consideration of the entire stack.…”

“…The simplest mixed oxides are typically envisioned as a combination of two pure oxides, one of which is stable at the relevant annealing temperatures and another one which has a higher index. Important examples include TiO 2 –Ta 2 O 5 , ,− SiO 2 –TiO 2 , TiO 2 –Nb 2 O 5 , GeO 2 –TiO 2 , and ZrO 2 –Ta 2 O 5 …”

Ar transport and blister growth kinetics in titania-doped germania-based optical coatings

“…Figure 6 presents the depth profile of the elements in a stack obtained using ERD-TOF measurements, as deposited and annealed at indicated temperatures. The depth scale, in at cm −2 , can be divided by the atom density in at cm −3 measured by other means [29] in order to obtain a depth scale in length units. However, TGO and SiO 2 not having the same atom density, the conversion of the depth scale is not straightforward.…”

Imaging Scatterometer for Observing Changes to Optical Coatings During Air Annealing

“…The titania dopant is added to further decrease the mechanical loss and the absorption of the Ta 2 O 5 layers [12]. We note that other dopants such as zirconia can be added to frustrate crystallization in tantala [13] and titania-doped-tantala [14], though the samples used here do not include zirconia. There is currently heavy research into identifying coatings for future detectors that will have even lower coating thermal noise [15] and as good optical properties as the current coatings.…”