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
$\mbox{Nb}_{1-x}\mbox{Ti}_{x}\mbox{N} $
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
$(2{\Delta}/{k}_\mathrm{B}{T}_\mathrm{C})$
of the Nb
_0.8
Ti
_0.2
N films is larger than that of the Nb
_0.7
Ti
_0.3
N films. Since we obtained better sensitivities in the SIS mixers using the Nb
_0.8
Ti
_0.2
N films, they were used to produce all the receivers. These showed excellent performance complying with the stringent ALMA specifications.