Ultrasensitive TES bolometers for space based FIR astronomy

We present results from the development of a background limited transition edge sensor (TES) bolometer for the wavelength band 30 60 gm. The bolometer consists of a Ti/Au superconducting thermometer and a Ta radiation absorber deposited on a 200 mu m x 300 mu m membrane of SixNy suspended on long, narrow legs. This device is voltage biased and the current through the device is measured by a SQUID amplifier. The thermometer has transition temperature T-c = 108 mK and the device is operated from a 70 mK base plate. FIR radiation is coupled into a multimode horn with entrance aperture of 450 mu m, length 4.5 mm and exit aperture of 45 mu m, which feeds a metal integrating cavity containing the detector. The radiation band is defined by a pair of lowpass and highpass mesh filters in front of the horn. Here we present measurements of optical noise equivalent power (NEP), optical efficiency, dynamic range and time constant. The results show that measured TES detectors are close to meeting the requirement of the "Band 3" of SAFARI FTS imaging instrument [1] on the SPICA mission [2].