IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY— [1EPD-05/112 V1.7] 1 Stability of Al-Mn transition edge sensors for frequency domain multiplexing J. Hubmayr, J. E. Austermann, J. A. Beall, D. Becker, D. A. Bennett, B. A. Benson, L. E. Bleem, C. L. Chang, J. E. Carlstrom, H.-M. Cho, A. T. Crites, M. Dobbs, W. Everett, E. M. George, W. L. Holzapfel, N. W. Halverson, J. W. Henning, G. C. Hilton, K. D. Irwin, D. Li, P. Lowell, M. Lueker, J. McMahon, J. Mehl, S. S. Meyer, J. P. Nibarger, M. D. Niemack, D. R. Schmidt, E. Shirokoff, S. M. Simon, K. W. Yoon, E. Y. Young Abstract—We are developing arrays of 150 GHz transition edge sensor (TES) polarimeters for the South Pole Telescope polarime- ter (SPTpol). Prototype devices use an aluminum manganese (Al- Mn) alloy TES with a normal resistance Rn suited to frequency domain multiplexing (fMUX) used in SPTpol. Using the fMUX readout, the devices exhibit noise performance consistent with expectations when R > 0.8Rn. Below 0.8Rn, the detectors have high loopgain and become unstable, which is predicted by use of a compound TES model. We address this issue in a recent fabrication with increased TES heat capacity and normal metal structures on the TES to tune the temperature sensitivity. Index Terms—AlMn, CMB, polarimeter, transition edge sensor I. I NTRODUCTION Science targets achievable through cosmic microwave back- ground (CMB) polarimetry include observing signatures of inflation and measuring the sum of the neutrino masses. Achieving such goals requires instruments with high sen- sitivity and fine control over polarization systematic errors. To meet these demands, we are developing an all silicon focal plane composed of micro-machined silicon platelet feed- horn arrays [1] [2] coupled to monolithic arrays of transition edge sensor (TES) polarimeters for the South Pole Telescope polarimeter (SPTpol) [3]. SPTpol plans ∼ 600 feed-horn coupled polarimeters operating near 150 GHz. Contribution of NIST, not subject to copyright. Manuscript received August 3, 2010. Work at the University of Chicago is supported by the NSF through grant ANT-0638937 and NSF Frontier Center grant PHY-0114422 to KICP at the University of Chicago. Generous support also received from the Kalvi Foundation and the Gordon and Betty Moore Foundation. J. Hubmayr, J. A. Beall, D. Becker, D. A. Bennett, H.-M. Cho, G. C. Hilton, K. D. Irwin, D. Li, P. Lowell, J. P. Nibarger, M. D. Niemack, D. R. Schmidt, and K. W. Yoon are with the NIST Quantum Devices Group, 325 Broadway Mailcode 817.03, Boulder, CO 80305 J. E. Austermann, N. W. Halverson, J. W. Henning, S. M. Simon are with the Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences and Department of Physics, University of Colorado, Boulder, CO 80309 B. A. Benson, L. E. Bleem, J. E. Carlstrom, C. L. Chang, A. T. Crites, W. Everett, J. Mehl and S. S. Meyer are with the Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 M. Dobbs is with the Department of Physics, McGill University, 3600 Rue University, Montreal, Quebec H3A 2T8, Canada E. M. George, W. L. Holzapfel, M. Lueker, E. Shirokoff and E. Y. Young are with the Department of Physics, University of California, 366 LeConte Hall MC 7300, Berkeley, CA 94720 J. McMahon is with the Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan 48109 In these proceedings, we discuss the development of TES polarimeters suitable for frequency domain multiplexing (fMUX) [4] [5] used in SPTpol. Single pixel polarimeters with molybdenum copper (Mo:Cu) bilayer TESs used in time division multiplexing are well characterized [6]–[8]. We use the existing polarimeter design, substituting an Al-Mn film in place of the Mo:Cu bilayer, to obtain a normal resistance R n ∼ 1 Ω matched for fMUX. We characterize the device thermal conductivity and temperature sensitivity using fMUX. The onset of electro-thermal oscillations are observed when operating the TES at modest depths into the superconducting transition, which is predicted by use of a compound TES model. This instability limits the operating range of the sensor to above 0.8R n , decreasing the sensor dynamic range. We address this issue in our second round of fabrication by increasing the effective TES heat capacity with additional metal thermally connected to the TES [9] as well as by tuning the TES temperature sensitivity with normal metal structures on the Al-Mn film [10] [11]. II. POLARIMETER DESIGN Fig. 1 shows a photograph of the 6 mm SPTpol test pixel fabricated at the National Institute of Standards and Technology. The corrugated feed-horn (not shown) transitions to a 1.6 mm circular waveguide for operation at 145 GHz. Four Nb probes inside the circular waveguide comprise the orthomode transducer (OMT), which launches radiation in X and Y polarizations onto separate superconducting circuits. The power in X and Y is deposited on separate thermally isolated membranes and sensed with transition edge sensors (TESs). Details of the pixel design are described in [12] and [13]. The 305 × 300 µm island contains a lossy Au meander that deposits the power from the superconducting circuit onto the island. This power is sensed with a 1200 ppm by atomic percent Al-Mn TES [14] [15] with dimensions 0.05 × 48 × 92 µm 3 . The 0.05 × 48 × 68 µm 3 active area has R n ∼ 0.9 Ω and T c ∼ 525 mK. The calculated heat capacity of the Al-Mn TES is 25.6 fJ/K, assuming bulk aluminum and half the BCS enhancement [16]. The island also contains a ∼ 2 Ω Au heater for sensor calibration. The membrane island is thermally isolated and suspended by four SiN/SiO x legs with dimensions 0.5/0.45 × 9 × 350 µm 3 .