Investigation of the Young's modulus and thermal expansion of amorphous titania-doped tantala films

Abstract

The current generation of advanced gravitational wave detectors utilize titania-doped tantala/silica multilayer stacks for their mirror coatings. The properties of the low-refractive-index silica are well known; however, in the absence of detailed direct measurements, the material parameters of Young's modulus and coefficient of thermal expansion (CTE) of the high refractive index material, titania-doped tantala, have been assumed to be equal to values measured for pure tantala coatings. In order to ascertain the true values necessary for thermal noise calculations, we have undertaken measurements of Young's modulus and CTE through the use of nanoindentation and thermal-bending measurements. The measurements were designed to assess the effects of titania-doping concentration and postdeposition heat-treatment on the measured values in order to evaluate the possibility of optimizing material parameters to further improve thermal noise in the detector. Young's modulus measurements on pure tantala and 25% and 55% titania-doped tantala show a wide range of values, from 132 to 177 GPa, which are dependent on both titania concentration and heat-treatment. Measurements of CTE give values of 3.9 0.1 × 10 −6 K −1 and 4.9 0.3 × 10 −6 K −1 for 25% and 55% titania-doped tantala, respectively, without dependence on post-deposition heat-treatment.

Internal Friction and Young's Modulus Measurements on SiO2 and Ta2O5 Films Done with an Ultra-High Q Silicon-Wafer Suspension

“…(4)), a tantala Young's modulus of about 110 GPa has been worked out. The difference may be explained by a different choice of deposition parameters between our samples and those of reference [23] from where the tantala Young's modulus used in the calculations and FEA [17] has been taken. Fig.…”

Apparatus to Measure Optical Scatter of Coatings Versus Annealing Temperature

“…aLIGO's core optics are superpolished fused silica substrates ion-beam sputtered with alternating near-quarterwavelength layers of amorphous silica, SiO 2 , and titania-doped tantala, Ti:Ta 2 O 5 , (low and high index, respectively) with the number and thickness of the layers chosen to give the optics their desired optical properties at 1064 nm while keeping the coating's mechanical loss and thus thermal noise low [3,7,8]. After coating, these optics are annealed in air with a ramp of roughly 1 • C/minute to 500 • C and held at that temperature (soaked) for 24 hours to improve their optical absorption and mechanical properties.…”

“…The onset of crystal nucleation and growth in amorphous Ta 2 O 5 thin films is well studied, but varied. Un-doped Ta 2 O 5 films crystallize between 600 • C and 850 • C [12], while Ti:Ta 2 O 5 shows no signs of crystallization at 600 • C (see [7] and references therein). It is not known whether the LIGO annealing process triggers crystal growth.…”

Residual stress and laser-induced damage of ion-beam sputtered Ta2O5/SiO2 mixture coatings

“…Metal oxide mixtures have gained a lot of attention at frontiers of the optical coatings in recent years. For example, in the most famous gravitational wave detectors Advanced Laser Interferometer Gravitational-Wave Observatory, titania-doped tantala is a very satisfying high refractive index material in the ultra-low loss mirrors to ensure low mechanical and optical losses to achieve limited sensitivity for the detector [1,2]. Meanwhile, it is very promising to deposit mixture thin films with lower residual stress compared to pure high/low refractive index material [3].…”