Characterization of a Free-Standing Membrane Supported Superconducting Ti Transition Edge Sensor

Superconducting transition edge sensors (TES) based on a Ti microbridge on Si substrate have demonstrated a very low noise equivalent power. Their effective response time, however, is in the order of microseconds due to relatively high transition temperature (i.e., 300-400 mK) of the Ti microbridge, making it difficult to read out the signal of a large Ti TES array with a SQUID-based multiplexer. We propose a twin-slot antenna coupled superconducting Ti microbridge separated from the antenna feed and supported by a free-standing membrane. Its resistive transition (R-T) and current-voltage (I-V) curves are measured before and after KOH wet etching of the Si substrate underneath the Ti microbridge. The free-standing membrane supported Ti TES with a thickness of 40 nm shows slightly lower transition temperature and higher normal resistance. Its thermal conductance is reduced to ~800 pW/K from ~3000 pW/K. In addition, its effective response time measured with a current pulse signal is about 5 μs. The uniformity of the 8 × 8 TES array is studied by measuring the distribution of normal resistance and critical temperature. In order to improve the uniformity, we increase the Ti film thickness to 64 nm, and characterize its thermal and electrical features.