Thickness-dependent crystallization on thermal anneal for titania/silica nm-layer composites deposited by ion beam sputter method.

Crystallization following thermal annealing of thin film stacks consisting of alternating nm-thick titania/silica layers was investigated. Several prototypes were designed, featuring a different number of titania/silica layer pairs, and different thicknesses (in the range from 4 to 40 nm, for the titania layers), but the same nominal refractive index (2.09) and optical thickness (a quarter of wavelength at 1064 nm). The prototypes were deposited by ion beam sputtering on silicon substrates. All prototypes were found to be amorphous as-deposited. Thermal annealing in air at progressive temperatures was subsequently performed. It was found that the titania layers eventually crystallized forming the anatase phase, while the silica layers remained always amorphous. However, progressively thinner layers exhibited progressively higher threshold temperatures for crystallization onset. Accordingly it can be expected that composites with thinner layers will be able to sustain higher annealing temperatures without crystallizing, and likely yielding better optical and mechanical properties for advanced coatings application. These results open the way to the use of materials like titania and hafnia, that crystallize easily under thermal anneal, but ARE otherwise promising candidate materials for HR coatings necessary for cryogenic 3rd generation laser interferometric gravitational wave detectors.