Impact of Annealing on T_C and Structure of Titanium Thin Films

Abstract

Transition-edge sensors (TES) are superconducting devices used for detecting particles and electromagnetic radiation, ranging from γ-ray to mm wavelengths. A fundamental parameter for operations of TES detectors for the desired application is the superconducting critical temperature TC. We are developing TES based bolometers made of metallic Ti films, with an operating temperature of 500 mK, to be used for cosmic microwave background (CMB) measurements. We have observed that electronbeam evaporation grown Ti films can reach critical temperatures higher than 500 mK when the substrate temperature is kept below some temperature threshold. Discordant critical temperatures of Titanium TES are found in literature and very little information is available about the various TC and relative fabrication process. Critical temperature of Ti films is generally known to be affected by deposition methods, substrates, processing conditions and heating. In the past we tried to tune the critical temperature of titanium thin films by means of post-annealing and we found a regular decrease of TC from 540 mK to 360 mK, in this work we confirm this effect. Further, we have found that titanium film grows with a most stable hcp structure. The annealing process until 260 °C, does not modify the morphological features of the film. We find also evidences of a shift in XRD peaks that indicate structural changes of the lattice parameters, which could play a role in the TC modification.