Accelerating Storage Lifetime Testing of Microcalorimeter Arrays for NewAthena X-IFU

The X-IFU instrument for ESA's upcoming flagship mission Athena will use Mo/Au bilayer transition-edge sensors (TES) with Au/Bi X-ray absorbers to form a kilo-pixel microcalorimeter array. Such detectors require careful calibration to maintain their best possible performance, but such calibrations can change over long periods of time when exposed to certain environmental conditions. We have previously reported the effects of humidity and elevated temperature on prototype array characteristics. In that study, storage of a prototype array at 60° C for 10 days under vacuum produced an unexpected shift in the TES critical temperature T꜀ (+13 to +19%) and normal resistance Rₙ (+4%). In this work, we continue investigating shifts in array characteristics after long-duration temperature exposures so that safe storage and handling conditions for X-IFU flight arrays can be set. A prototype array exposed to 35° C under vacuum for 26 days gradually increased in TES normal resistance. The energy scale of a second prototype array was measured before aging and then recovered to within 0.5 eV following a similar aging induced Rn increase. Aging of the TES was studied directly by applying the same thermal conditions exclusively to Mo/Au bilayers on Si substrate. These measurements allow a better understanding of the cause of the observed shift by decoupling the TES bilayer from the remaining device structures such as the SiNx membrane and the absorber. Four-point resistance measurements were used to readout the bilayer samples at 4.2 K. Bilayers aged in a vacuum generally saw no change or a slight increase in resistance, while bilayers aged in nitrogen gas decreased in resistance. Finally, we conclude with a comparative overview of all test results on prototype arrays and TES bilayers, elaborate on possible mechanisms for the observed shifts in characteristics, and make recommendations for maintaining the long-term stability of our devices.