Thin-Film Ruthenium Microstructures For Transition Edge Sensors

The superconducting properties of ruthenium (Ru) thin films and microstructures are investigated. The microstructures are used as transition edge sensors (TES), working at He-3 evaporation cryostats' temperatures. Ruthenium is substantially inert, and the critical temperature T-c for bulk Ru samples is known from state of art to be 0.40-0.51 K. We investigated magnetron sputtered Ru thin films with thicknesses 13-300 nm on a Si substrate and electron lithography fabricated TES samples, based on the thin-film Ru microstructures. It has been found, that the T-c for the Ru thin films is 0.55-0.70 K, and the width of the transition region is 1-5 mK, and for the Ru TES T-c = 0.55 and Delta T = 4 mK. Furthermore, it was established that lithography process had no significant influence on the properties of the TES samples, so we were able to get consistent properties for several fabrication sessions. Therefore ruthenium is concluded to be a desirable material for transition edge sensors working at He-3 cryostats' temperatures.