Thermal Annealing of AlMn Transition Edge Sensors for Optimization in Cosmic Microwave Background Experiments

Abstract

The 2020 decadal review recognized the measurement of the polarization of the cosmic microwave background (CMB) to be a top priority for the decade. CMB experiments including POLARBEAR2/Simons Array, Atacama Cosmology Telescope/Advanced-ACT, SPT-3G, the Simons Observatory, and CMB-S4 have or will use transition edge sensor (TES) bolometer fabricated with Aluminum doped with Manganese (AlMn). AlMn is a popular material choice as the superconducting transition temperature (\(T_c\)) and normal resistance (\(R_n\)) of the TES can be tuned with Mn concentration, geometric patterning, film thickness, and thermal annealing. In addition the conductivity is appropriate for both time division multiplexing and frequency division multiplexing that require 10 m\(\Omega\) and 1 \(\Omega\) sensors respectively. In this paper we present work on the ability to tune the \(T_c\) of a film based on its time and temperature thermal tuning profile combined with room temperature monitoring of film resistivity. Such control allows for the fabrication of a wide range of TES parameters from a single AlMn concentration. Scanning electron microscope (SEM) imaging shows that the AlMn film’s grain boundaries are changed by thermal annealing making the film more conductive and raising its superconducting transition temperatures, and that at high enough temperatures will eventually recover the \(T_c\) of bulk Al. We find that baking films at \(\sim\)200 \(^\circ\text{C}\) for tens of minutes yields a \(T_c\) that is suitable for 100 mK base temperature experiments and we present on the thermal tune profiles of several different thicknesses of AlMn.