Developing AlMn films for Argonne TES fabrication

Paper summary: E. M. Vavagiakis, N. F. Cothard, J. R. Stevens, C.L. Chang, M.D. Niemack, G. Wang, V.G. Yefremenko, J. Zhang 2019. “Developing AlMn Films for Argonne TES fabrication,” Journal of Low Temperature Physics (in review), arXiv:1910.10199.

The reference design for the next-generation cosmic microwave background (CMB) experiment, CMB-S4, relies on large arrays of transition edge sensor (TES) bolometers to map the CMB to near cosmic variance limits. These measurements will enable us to address science goals including characterizing dark energy and dark matter, searching for signatures of inflation in the early universe, measuring the sum of the neutrino masses, and mapping the universe’s matter distribution. To learn more about CMB-S4, visit my research page here, or visit the CMB-S4 webpage: https://cmb-s4.org.

The cryogenic cameras of the CMB-S4 effort will be cooled using dilution refrigerators with bath temperatures < 100 mK, which will reduce the thermal fluctuation noise as compared to the higher bath temperatures of other cryocooler options.

The signals we are measuring are small. TESes work thanks to the principal of superconductivity, which allows us to operate the detectors on their superconducting transition to measure a relatively large change in resistance when detecting a small change in temperature due to the light from the CMB. The critical temperature (Tc) of the superconducting TESes will need to be tuned for this low bath temperature, and the normal resistance (Rn) of the devices will also need to be optimized for the readout of the CMB-S4 experiment, which will be time domain multiplexing (TDM) or microwave-SQUID multiplexing (uMUX). Monolithic arrays of multichroic TESes have previously been fabricated at Argonne National Laboratory and deployed in the South Pole Telescope’s SPT-3G camera. In order to adjust Tc and Rn to the CMB-S4 requirements, we are informing Argonne’s new fabrication process by measuring films and devices produced using various materials, thicknesses, geometries, and thermal annealing temperatures.

We presented our measurements of device parameters for two Argonne AlMn TESes, along with Tc and Rn as measured for various film stacks, at the 18th International Workshop on Low Temperature Detectors in Milan, Italy. Through this iterative testing at Cornell, we’ve informed Argonne’s fabrication process to produce TESes with the desired critical temperature (150-200 mK) and normal resistance (10-20 mOhms) to be used in CMB-S4. Work is currently ongoing to fabricate and measure devices with higher saturation powers and lower time constants to optimize the TES performance for all of the CMB-S4 frequency bands.

Full paper as submitted to JLTP: https://arxiv.org/pdf/1910.10199.pdf

Featured image: AlMn TES film, V.G. Yefremenko, Argonne National Laboratory

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