Investigation of effects of assisted ion bombardment on mechanical loss of sputtered tantala thin films for gravitational wave interferometers

Abstract

Reduction of Brownian thermal noise due to mechanical loss in high-reflectivity mirror coatings is critical for improving the sensitivity of future gravitational wave detectors. In these mirrors, the mechanical loss at room temperature is dominated by the high refractive index component, amorphous tantala (Ta 2 O 5) or tantala doped with titania (Ti∶Ta 2 O 5). Toward the goal of identifying mechanisms that could alter mechanical loss, this work investigates the use of assist ion bombardment in the reactive ion beam sputtering deposition of tantala single layers. Low-energy assist ion bombardment can enhance adatom diffusion. Low-energy assist Ar þ and Xe þ ion bombardment at different conditions was implemented during deposition to identify trends in the mechanical loss with ion mass, ion energy, and ion dose. It is shown that the atomic structure and bonding states of the tantala thin films are not significantly modified by low-energy assist ion bombardment. The coatings mechanical loss remains unaltered by ion bombardment within errors. Based on an analysis of surface diffusivity, it is shown that the dominant deposition of tantala clusters and limited surface diffusion length of oxygen atoms constrain structural changes in the tantala films. A slower deposition rate coupled with a significant increase in the dose of the low-energy assist ions may provide more favorable conditions to improve adatom diffusivity.

Low Mechanical Loss TiO2:GeO2 Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers

“…The SiO 2 layers can already be produced with low enough mechanical loss angle [22], so the main focus of the current research is on improving the high refractive index material. Several different approaches have been investigated, including deposition at elevated substrate temperatures [28,29] and with assist ion bombardment [30,31], doping and nanolayering of Ta 2 O 5 [32][33][34][35], and the use of nitrides [36,37]. Here we report results on amorphous oxide coatings based on mixtures of GeO 2 and TiO 2 .…”

“…As a first step toward the production of a full highreflectivity coating, and to better characterize this new material, we deposited single layers of SiO 2 and TiO 2 ∶GeO 2 , as well as a stack of 5 QWL layers of TiO 2 ∶GeO 2 alternated with 5 layers of SiO 2 , and 20 layers of TiO 2 ∶GeO 2 alternated with 20 layers of SiO 2 . The depositions were performed using a commercial Spector Ion Beam Sputtering system that can produce films with better optical quality [30] than the biased target system used for the initial parameter exploration. At the laser wavelength of 1064 nm, the transmission of the 40-layer structure was 190 ppm and the optical absorption was measured to be 3.1 ppm after annealing.…”

Structural Evolution that Affects the Room-Temperature Internal Friction of Binary Oxide Nanolaminates: Implications for Ultrastable Optical Cavities

“…At this temperature, Q –1 is (3.1 ± 0.2) × 10 –4 for the SiO 2 /Ta 2 O 5 nanolaminate. In both cases, the value is lower than the best value for a single-layer Ta 2 O 5 annealed at 500 °C, Q –1 ≈ 4.0 × 10 –4 . For comparison, Figure b plots the internal friction of the SiO 2 /Ta 2 O 5 and TiO 2 /Ta 2 O 5 mixtures with annealing up to the highest temperature before crystallization.…”

“…Full crystallization of the TiO 2 /Ta 2 O 5 and SiO 2 /Ta 2 O 5 nanolaminates is observed after annealing at 750 and 800 °C, respectively. The crystallization process is delayed to a higher annealing temperature in both nanolaminates compared to 675 °C, the crystallization temperature of a single-layer Ta 2 O 5 . The amorphous phase is stabilized to a higher temperature due to a greater contribution from the surface enthalpy to the total Gibbs free energy change for the amorphous-to-crystalline transition. ,, Diffraction patterns of the crystallized nanolaminates and the crystallized Ta 2 O 5 single layer are shown in Figure .…”

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

“…The lowest loss value measured on co-sputtered tantala-zirconia thin films has to be compared to ϕ c = (2.3-3.4) × 10 −4 rad of tantala-titania layers in current GW detectors [32,82], and to other high-index oxide coatings, either simple or co-sputtered [29,33,[59][60][61][62]. Further coating loss reduction, down to 1 × 10 −4 rad or lower, might be obtained with non-oxide coatings [33].…”