Ion-beam sputtered amorphous silicon films for cryogenic precision measurement systems

Abstract

Thermal noise resulting from the mechanical loss of multi-layer dielectric coatings is expected to impose a limit to the sensitivities of precision measurement systems used in fundamental and applied science. In the case of gravitational wave astronomy, future interferometric gravitational wave detectors are likely to operate at cryogenic temperatures to reduce such thermal noise and ameliorate thermal loading effects, with the desirable thermo-mechanical properties of silicon making it an attractive mirror substrate choice for this purpose. For use in such a configuration of precision instrument, appropriate coatings of low thermal noise are essential. Amorphous silicon (a-Si) deposited by e-beam and other techniques has been shown to have low mechanical loss. However, to date, the levels of mechanical and optical loss for a-Si when deposited by ion-beam-sputtering (the technique required to produce amorphous mirrors of the specification for gravitational wave detector mirrors) are unknown. In this paper results from measurements of the mechanical loss of a series of IBS a-Si films are presented which show that reductions are possible in coating thermal noise of a factor of 1.5 at 120 K and 2.1 at 20 K over the current best IBS coatings (alternating stacks of silica and titania-doped tantala), with further reductions feasible under appropriate heat treatments.

Optical absorption of ion-beam sputtered amorphous silicon coatings

“…5. At 1550 nm the aSi single layer has an absorption minimum for heat treatment at a temperature of 450 • C. At this temperature also a significant decrease of up to a factor of 4 in mechanical loss was observed following heat-treatment of an as-deposited coating [19]. …”

“…This paper presents absorption measurements on an aSi single layer and an aSi/SiO 2 bilayer produced with the ion-beam sputtering (IBS) deposition technique, which is currently used for coatings in GWDs as it provides low scattering and high homogeneity. The mechanical loss of these coatings at 10 K, which is the ET design temperature, has previously been shown to be a factor of 40 lower than the loss of Ta 2 O 5 [19]. The absorption of these coatings was measured at 1064 nm and 1550 nm for heat treatment of up to 1000 • C, in air and in vacuum and the optimal temperature and duration was investigated.…”

Development of Mirror Coatings for Gravitational Wave Detectors

“…However, the loss of silica films increases at low temperature, with loss peaks observed around 150 and 20 K [67], close to temperatures proposed for the operation of future GW detectors. At low temperature, the loss of the silica layers will limit the thermal noise performances of coatings using, e.g., aSi, as a high-index layer [68], and of coatings based on a multi-material design (see next section). Reducing the loss of the low-index material is, therefore, also critical for coatings for cryogenic application.…”

Effect of Stress and Temperature on the Optical Properties of Silicon Nitride Membranes at 1,550 nm

“…Amorphous silicon (aSi) shows a very low mechanical loss of ϕ Si <2 × 10 −5 at temperature below ≈ 30 K (Murray et al, 2015). In addition, the very high refractive index (n Si = 3.5 at 1,550 nm) would reduce the numbers of layers required to achieve high reflectivity as the reflectivity per pair of high and low refractive layers increases with the contrast between the refractive indices.…”