J. A. Bonetti

Transition Edge Sensor Focal Plane Arrays for the BICEP2, Keck, and Spider CMB Polarimeters

We describe the transition-edge-sensor (TES) focal plane arrays used for three cosmic microwave background (CMB) polarimetry projects. The first is BICEP2. The focal plane array for BICEP2 has been delivered and the instrument is now taking data at the South Pole. Second is the Keck Array. The focal planes for Keck will be nearly identical to those of BICEP2, but will cover multiples frequencies (96, 150 and 225 GHz). The scheduled deployment of Keck is late 2010. Finally, there is Spider, a high-altitude balloon observatory, which will also cover 96, 150, and 225 GHz and will be deployed in 2012. The goal of all three experiment is detecting B modes in the polarization of the CMB.

Characterization of Antenna-Coupled TES Bolometers for the Spider Experiment

We describe the transition-edge-sensors (TESs) to be employed on the Spider experiment, a balloon-based observatory whose goal is detecting the imprint of gravitational waves by mapping the polarization of the cosmic microwave background (CMB). The devices consist of Ti and Al thermistors in series sitting on a thermally isolated suspended membrane. Also on the membrane is a termination resistor coupled through a superconducting microstrip line to an on-chip, polarization sensitive, 150 GHz slot-array antenna. Several important parameters were measured. Transition temperatures were deduced by measuring the Johnson noise in the Ti thermistor. The thermal conductance between the isolated TES islands and substrate was measured by obtaining current-voltage measurements at various temperatures. The Electrical noise equivalent power was measured to sub-Hertz frequencies with varying sample geometries including those with and without normal metal bars (zebra stripes). Finally, the time constant of the devices was measured within the Al and Ti transitions where electrothermal feedback speeds up the bolometer response. This time response is compared with the natural time constant measured just above the Ti transition temperature. The results of these measurements are within the design specifications for Spider.

Spatially selective and reversible doping control in cuprate films

We describe a reversible, spatially controlled doping method for cuprate films. The technique has been used to create superconductor-antiferromagnetic insulator-superconductor (S-AFI-S) junctions and optimally doped superconductor-underdoped superconductor-optimally doped superconductor cuprate structures. We demonstrate how the S-AFI-S structure can be employed to reliably measure the transport properties of the antiferromagnetic insulator region at cryogenic temperatures using the superconductors as seamless electrical leads. We also discuss applied and fundamental issues which may be addressed with the structures created with this doping method. Although it is implemented on a cuprate film (YBa2Cu3O7−δ) in this work, the method can also be applied to any mixed-valence transition metal oxide whose physical properties are determined by oxygen content.

Microfabrication and Device Parameter Testing of the Focal Plane Arrays for the Spider and BICEP2/Keck CMB Polarimeters

Spider and BICEP2/Keck are projects to study the polarization of the cosmic microwave background (CMB). The focal planes for both require large format arrays of superconducting transition edge sensors (TES’s). A major challenge for these projects is fabricating arrays with high uniformity in device parameters. A microfabrication process is described that meets this challenge. The results from device testing are discussed. Each focal plane is composed of 4 square wafers (tiles), and each wafer contains 128 membrane‐isolated, polarization‐sensitive, antenna‐coupled TESs. After processing, selected wafers are pre‐screened in a quick‐turn‐around, cryogen‐free, 3He fridge. The pre‐screening is performed with a commercial resistance bridge and measures transition temperatures (Tc) and normal state resistances (Rn). After pre‐screening, 4 tiles at a time are fully characterized in a testbed employing a SQUID readout and SQUID mulitplexing. The tests demonstrate the values of Tc, Rn, thermal conductance, g, and ...