Characterization of polarization-sensitive MKID arrays to be deployed in BLAST-TNG (Conference Presentation)

Abstract

Microwave Kinetic Inductance Detectors (MKIDs) have held promise as the focal plane sensing elements in large-format imaging arrays for over a decade and have now found application in several ground-based instruments. In this presentation, we discuss the first implementation of MKIDs for a suborbital instrument in the Balloon-borne Large Aperture Submillimeter Telescope – The Next Generation (BLAST-TNG), a suborbital imaging array designed to study the role magnetic fields play in star formation and bridge the angular scales between Planck’s low resolution all-sky maps and ALMA’s ultra-high resolution narrow fields. BLAST-TNG is scheduled to launch from Antarctica in December 2018. This experiment will utilize 8 times as many polarization sensitive detectors and will have 16 times greater mapping speed compared to its predecessor BLASTpol. This will also be a demonstration for future MKID instruments for ground based telescopes, e.g. TolTEC arrays on the LMT, as well as proposed space based missions. We have built three, large-format MKID arrays for BLAST-TNG. Each monolithic 100mm diameter array is sensitive to a different waveband centered at 250 micron, 350 micron, or 500 micron; together comprising 3318 individual polarization-sensitive detectors. The detector arrays are read out with high levels of multiplexing, with each microwave feedline addressing between 466 and 938 unique resonators depending on the array. Designing for space-like low photon loads, polarization-sensitivity, different frequency bands, and 275 mK operation all pose unique challenges. We address these challenges by employing feedhorn-coupled, dual-polarization sensitive pixels fabricated from TiN/Ti multilayers that combine high responsivity, high uniformity, low loss, and a tunable superconducting Tc. Here, we present the detailed design and fabrication of these arrays, which includes an optimized quarter wavelength silicon backshort for each band realized by micromachining a silicon on insulator (SOI) wafer, aluminum patching of the TiN/Ti absorbing inductor to increase response and tune the effective optical coupling impedance, and a semi-automated layout scheme to make a stepper-compliant lithography process that tiles identical stepper images across the array and then trims them individually to minimize their frequency scatter and crosstalk. This results in high quality, easily reconfigurable, and uniform arrays of MKIDs. We show measurements that demonstrate high pixel yield, > 98% polarization isolation, and a noise equivalent power (NEP) limited by photon noise at the expected in-flight photon load.