Tuning Circuit Material for Mass-Produced Terahertz SIS Receivers

A total of 73 dual polarization superconductor-insulator-superconductor (SIS) receivers for the Atacama Large Millimeter/submillimeter Array (ALMA) have been successfully produced to observe astronomical signals in the frequency band of 787-950 GHz. For the mass production of waveguide SIS mixer chips used in the receivers, we have tested two different compositions of Nb_1-xTi_xN thin films for use in the mixer´s superconductor low-loss transmission lines. These components are being fabricated for mass-produced waveguide SIS mixer chips in receivers. One composition was prepared by employing a sputtering target with 20 wt.% Ti and 80 wt.% Nb (x = 0.2), and the other was prepared with a target composed of 30 wt.% Ti and 70 wt.% Nb (x = 0.3). Both films are deposited using dc magnetron sputtering on quartz substrates. We found that the ratio between the superconducting energy gap and the critical temperature (2Δ/k_BT_C) of the Nb_0.8Ti_0.2N films is larger than that of the Nb_0.7Ti_0.3N films. Since we obtained better sensitivities in the SIS mixers using the Nb_0.8Ti_0.2N films, they were used to produce all the receivers. These showed excellent performance complying with the stringent ALMA specifications.