Fabrication and characterization of a cryogenic AC-DC converter

At present, the AC voltage measurements at the 1 mV level are limited by the low sensitivity of the AC-DC transfer devices. We report the fabrication and characterization of a transition edge sensor on a silicon nitride membrane to perform direct measurements in this voltage range. The sensor consists of a Cr strip where AC power is dissipated and a superconducting-Nb thin film biased into its resistive transition that measures the temperature change. Deposition parameters of Cr film are chosen in order to have a resistance close to 50 ohm that minimize the mismatching with the AC supply. By second and third harmonic methods a thermal conductance of 6/spl middot/10/sup -6/ W/K and a time constant of 4 ms are evaluated for a device on a membrane of 3.5/spl times/3.5 mm/sup 2/. Analysis of the data indicates that a reduction of the noise equivalent power of 4-5 order of magnitude on that of traditional thermal converter is feasible.