Design and Fabrication of 90 GHz TES Polarimeter Detectors for the South Pole Telescope

We present information about the design and fabrication of 90 GHz Transition Edge Sensor (TES) detectors deployed in the SPTpol camera for investigation of the cosmic microwave background (CMB) polarization signal. The 90 GHz portion of the camera consists of 180 individual feedhorn modules with dual polarization-sensitive detectors. We discuss microfabrication details and the characterization of detector elements. Each detector incorporates a dipole-like Pd-Au absorber and Mo/Au TES thermometer, suspended together on a rectangular silicon nitride (SiN) membrane via 6 long (640 μm) and narrow (10 μm) legs. The geometry of the SiN legs was optimized to provide the target thermal conductance of 200 pW/K in combination with mechanical robustness and reliability. The proximity effect in superconductor (Mo) and normal metal (Au) bilayers was utilized to obtain a TES operating temperature between 520 and 540 mK. Excellent superconducting properties (transition width <; 1 mK) and T_c uniformity (<; 3 mK) across 2 ^´´ wafers were achieved by sputtering in a confocal system under a single vacuum using an independent RF bias applied to the substrate. Superconducting Nb dots patterned on the TES surface provided controllable broadening of the transition width. We report the results of transition measurements, along with characterization of film morphology.