Fabrication of superconducting direct detectors in submillimeter-wave bands

Abstract

Direct detectors of submillimeter-wave radiation based on Nb/Al/AlOx/Al/Nb superconducting tunnel junctions (STJs) will be powerful tools for astronomical continuum observations in submillimeter-wave bands. To realize the detectors based on photon-assisted tunneling with wide frequency coverage, it is necessary to fabricate extremely high-quality STJs with very small areas (4 sq micrometer), high Josephson critical current densities (1000 A/sq-cm), and very low leakage currents below 10 pico-ampere at 0.3 K. The noise equivalent power (NEP) of the detectors is expected to be an order of 10(exp -18) W/(square-root of Hz) at the operating temperature below 0.7 K. The development of the direct detectors have been started using the fabrication facility at RIKEN. By improving the fabrication process using anodization of the junction edge, it has been successful to fabricate the small and high-quality STJs with the high critical current density of about 1000 A/sq-cm, the low leakage current of less than 100 pA, and the high fabrication yield of more than 90 percent.