Thin Gold Covered Titanium Transition Edge Sensor for Optical Measurement

Abstract

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.