A superconducting resonator with a hafnium microbridge at temperatures of 50–350 mK

A high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature $T_C$ $\approx$ 380 mK, and a plan size of 2.5 $\times$ 2.5 $\mu$m, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm ($T_C$ $\sim$ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity $G$ of the bridge was calculated and found to be $\sim$4 $\times$ 10$^{-13}$ W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP $\approx$ 8 $\times$ 10$^{-19}$ W/Hz$^{1/2}$ at a temperature of 150 mK.