Phonon-cooled hot electron bolometers on freestanding 2ftm Si3N4 membranes for THz applications

KOSMA is working on the development of a heterodyne receiver channel (1.7 to 1.9 THz) for the GREAT instrument on SOFIA. We report on recent progress in fabricating and characterizing prototype mixers at 800 GHz on 21.1m thick SiN membranes. The HEB is fabricated from a 5-6 rim thin NbTiN film with T 8 K which was sputtered on a 20 nm thick MN buffer layer at 400° C substrate temperature. An individual membrane device is then "flip-chip" bonded to a separately fabricated silicon frame. The frame assembly is mounted into the waveguide mixer such that the membrane is suspended in a substrate channel crossing the waveguide. All device fabrication including the thin NbTiN films is done at KOSMA. Heterodyne measurements with receiver noise temperatures of 1000 K at 1 GHz IF and 4.2 K bath temperature at first go are very promising for phonon-cooled HEBs on thin membranes as THz mixers.