Superconducting-resistive-transition device for the precision measurement of low AC voltage

Single and multijunctions thermal converters are devices widely utilised fo r AC measurements at room temperature. However, low sensitivity and high no ise of these devices limits their applications at voltage level above 10 mV . A cryogenic thermal transfer sensor has been developed to realise AC meas urements at 1 mV voltage level. This device consists of a superconducting-r esistive-transition Nb thin film, which detects the AC power, dissipated on a Cr film resistor. To improve the sensitivity, the sensor has been fabric ated on a 500 nm thick square silicon nitride membrane with size ranging fr om 1x1 mm(2) to 3.5x3.5 mm(2). The device has been tested at liquid helium and a thermal conductance lower than 10(-6) W/K has been measured. From the se data we expect an increasing of responsivity higher than three order of magnitude and a noise limited by intrinsic thermal noise.