Abstract
We have developed antenna-coupled transition-edge sensor (TES) bolometers for a wide range of cosmic microwave background (CMB) polarimetry experiments, including BICEP2, Keck Array, and the balloon borne SPIDER. These detectors have reached maturity and this paper reports on their design principles, overall performance, and key challenges associated with design and production. Our detector arrays repeatedly produce spectral bands with 20%-30% bandwidth at 95, 150, or 220~GHz. The integrated antenna arrays synthesize symmetric co-aligned beams with controlled side-lobe levels. Cross-polarized response on boresight is typically ~0.5%, consistent with cross-talk in our multiplexed readout system. End-to-end optical efficiencies in our cameras are routinely 35% or higher, with per detector sensitivities of NET~300 uKrts. Thanks to the scalability of this design, we have deployed 2560 detectors as 1280 matched pairs in Keck Array with a combined instantaneous sensitivity of ~9 uKrts, as measured directly from CMB maps in the 2013 season. Similar arrays have recently flown in the SPIDER instrument, and development of this technology is ongoing.
Highlights
Cosmic microwave background (CMB) polarimetry is a key observable to further our understanding of cosmology in both the later and early universe
We pick the ratio of impedances on the ports closest to the slots to determine the illumination pattern. To accomplish this synthesis with low return loss, we must construct microstrips with correct impedances, which requires accurate knowledge of dielectric constants and the penetration depth into our superconducting niobium films. (See Section 5.) far, all detectors deployed for BICEP2, Keck Array, and SPIDER produce a uniform top-hat illumination where the power splits are in proportion to the number of slots on each side of the junctions
We DC-sputter titanium (Ti) and use inductively coupled plasma (ICP) etching to etch the Tc = 0.5 K TES for on-sky observing. Both films are immediately chemically passivated with RF-sputtered silicon dioxide (SiO2) with ICP etched via holes to allow DC electrical contacts with subsequent layers
Summary
Cosmic microwave background (CMB) polarimetry is a key observable to further our understanding of cosmology in both the later and early universe. A future space mission has the potential to measure large-scale polarization to astrophysical limits, for precise tests of inflation. These ambitious scientific goals require high sensitivity cameras with exquisite control of systematic errors. The key feature in our design is optical coupling through a planar antenna, allowing the entire design to be fabricated with scalable photolithographic techniques. This has allowed us to rapidly deploy arrays for BICEP2, Keck Array, and the balloon borne SPIDER, amounting to over 6000 detectors fielded as of this writing.
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