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BROADBAND ARRAY SIS MIXERS FOR 780–880 GHz WITH ALUMINUM TUNING CIRCUITS

As a building block for an 8-pixel 800 GHz array receiver at KOSMA we developed a low noise SIS mixer. The mixerblock is tunerless and similar to the "stamped waveguide cavity" mixers we have used from 345 GHz to 800 GHz before [1]. We embedded the SIS junction into a standard broadband tuning circuit, commonly used at lower frequencies. The circuit consists of a short end-loaded stub in series with two impedance transforming sections. For the 780-880 GHz band, we fabricated the electrodes of the tuning circuit as a combination of niobium and aluminum layers in different sequences. First heterodyne tests were performed on devices with a tuning microstrip circuit ground plane consisting of 40nm Nb with 200nm of Al on top and a top conductor consisting of a 70 nm thick Nb layer with a 280 nm thick Al layer on top. The Nb-Al2O3-Nb junction has an area of about 0.5 μm2 and is fabricated with optical lithography. For anisotropic Nb etching, we use a RIE gas mixture of CCl2F2 and NF3. The Al2O3 barrier is removed with Ar sputter etching. Fabrication yield improved considerably when we introduced a light CMP-step after self-aligned SiO dielectric liftoff. For a SIS junction with RnA equal to 14 Ωμm2, the RSubgap/Rn value of 15.4 is remarkably good. We measured uncorrected receiver noise temperatures around 400 K over the whole band from 780 to 880 GHz, with a best noise temperature of 370 K at 804 GHz.