Thin Gold Covered Titanium Transition Edge Sensor for Optical Measurement

A thin-gold-film-covered titanium transition edge sensor is newly developed for highly reliable optical photon detection. The aim of the gold film is to prevent a formation of a surface oxidation layer (typically 2.8?nm) on titanium that causes severe degradation of the titanium superconductivity. Optical properties for the gold-covered titanium TES embedded in an optical cavity are calculated, and we find that the maximum absorptance and absorption bandwidth will be reduced with increasing a thickness of the gold film. However, more than 99% absorptance can be possible for the gold (10?nm in thickness) and titanium (30?nm) if 11 dielectric layers are used in an anti-reflection coating. A depth profile of a chemical state for the fabricated device was analysed by an X-ray photoelectron spectroscopy. The profile shows no evidence of TiO₂ existence in photoelectron spectrum. Superconducting critical temperature covered with the 10?nm gold were in the range of 200?mK to 320?mK depending on the titanium thickness of 18?nm to 26?nm.