Ti/Au Ultrathin Films for TES Application

Transition-Edge Sensors (TESs) are versatile superconducting microcalorimeters used as single photon detectors in a large range of electromagnetic wavelength, from X-rays to near-infrared. Among the many materials investigated in literature, Ti/Au is one of the most widely used bilayer to fabricate TES thanks to simple deposition process, long term stability material and protection against oxidation of Ti film by Au over layer. Moreover, the critical temperature of Ti/Au can be tuned by trimming the thickness of Ti and Au films. For low energy photon detection, the Ti/Au superconducting layer performs simultaneously as absorber and thermometer. This implies that a TES thickness reduction helps to significantly reduce the thermal capacitance, which has a direct impact on the detector energy resolution. In this paper we present a study of the superconducting properties of Ti/Au films, grown in UHV by thermal evaporation as a function of Ti thickness. The chemical state of Ti and Ti/Au were analyzed by X-ray photoelectron spectroscopy, to evaluate the protective effect of Au film. The film morphology, structure and optical properties were investigated by ellipsometry. The critical temperature showed a marked trend on film thickness and was strongly affected by Au cover layer. Ti film of only 12 nm thick covered with 10 nm Au film showed a remarkable critical temperature of about 300 mK.