a-Si:H/SiO_2 multilayer films fabricated by radio-frequency magnetron sputtering for optical filters

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Abstract
We examined the optical properties of a-Si:H/SiO2 multilayer films fabricated by radio-frequency magnetron sputtering for optical bandpass filters (BPFs). Because of the high refractive-index contrast between a-Si:H and SiO2, the total number of layers of an a-Si:H/SiO2 multilayer can be relatively small. We obtained an a-Si:H refractive index of 3.6 at λ = 1550 nm and its extinction coefficient k < 1 × 10-4 and confirmed by Fourier-transform infrared spectroscopy that such small k is influenced by the Si-H bonding in the film. We fabricated a-Si:H/SiO2 BPFs by using in situ optical monitoring. Thermal tuning of a-Si:H/SiO2 BPF upon a silica substrate was also performed, and a thermal tunability coefficient of 0.07 nm/°C was obtained.