Low Noise SIS Mixer for Far Infrared Radio Astronomy

We present a low noise SIS mixer developed for the 1.2 THz band of the heterodyne spectrometer of the Herschel Space Observatory. The device of this design will be among the first SIS mixers flown in space at Herschel. The developed SIS mixer has a quasi-optical design, with a double slot planar antenna and an extended spherical lens made of pure Si. The SIS junctions are Nb/AlN/NbTiN with the critical current density of about 30 KA/cm2 and with the junction area of a quarter of a micron square. For the ease of the Josephson current suppression, the SIS junctions are diamond-like shaped. The simultaneous suppression of the Josephson currents in the two SIS junctions is a particular advantage of the used shape. A low loss Nb/Au micro-strip transmission line is used for the first time in the mixer circuit well above the gap frequency of Nb. Using the new mixer we demonstrated the minimum uncorrected Double Sideband receiver noise of 550 K (Y=1.34). The minimum receiver noise corrected for the local oscillator beam splitter and for the cryostat window is 330 K, about 6 hν/k, apparently for the first time in the THz frequencies range.