Fabrication and Characterization of X-ray TES Detectors Based on Annular AlMn Alloy Films

TL;DR

This work addresses X-ray detection with AlMn TES, proposing an annular TES geometry to independently tune normal resistance via the inner/outer radius while adjusting thermal conductance through the outer perimeter. The detectors employ an annular AlMn film (Tc tuned by annealing to around $100$–$120$ mK) coupled to Au absorbers on a $Si_3N_4$/$SiO_2$ membrane, with characterization performed in a dilution refrigerator. Key results include highly consistent I–V behavior across three devices and a best energy resolution of $11.0\pm1.0$ eV at $5.9$ keV, though purification toward the theoretical limit is limited by readout noise; measured $R_n$ values (~$27$ mΩ) exceed the design target ($19.7$ mΩ) due to uncertainties in film thickness and sub-surface AlMn under Nb electrodes. An analysis of noise sources shows that SQUID noise and excess noise dominate, suggesting that further reductions in readout noise are essential to approach the calorimeter’s theoretical performance. Overall, the study demonstrates the feasibility of annular AlMn TES detectors for X-ray applications and highlights the critical role of readout electronics in achieving ultimate energy resolution for space-based missions.

Abstract

AlMn alloy flms are widely fabricated into superconducting transition edge sensors (TESs) for the detection of cosmic microwave background radiation. However, the application in X-ray or gamma-ray detection based on AlMn TES is rarely reported. In this study, X-ray TES detectors based on unique annular AlMn flms are devel-oped. The fabrication processes of TES detectors are introduced in detail. The char-acteristics of three TES samples are evaluated in a dilution refrigerator. The results demonstrate that the I-V characteristics of the three annular TES detectors are highly consistent. The TES detector with the smallest absorber achieved the best energy resolution of 11.0 eV @ 5.9 keV, which is inferior to the theoretical value. The dis-crepancy is mainly attributed to the larger readout electronics noise than expected.

Figures (9)

• Figure 1: Structure diagram of TES detector. The left panel gives a three-dimensional schematic picture. The right panel is a top view of the TES detector without absorber.
• Figure 2: Relationship between critical temperature of AlMn film and baking temperature. Under atmospheric pressure, each AlMn film is thermally annealed on a hot plate for 10 minutes
• Figure 3: Schematic diagram of TES detector preparation process. (1) Etching the back of silicon nitride/silicon dioxide, (2) wet etching of AlMn film, (3) preparation of isolation layer, (4) sputtering Nb electrodes, (5) sputtering a Ti/Au film on TES, (6) defining plating zone, (7) electroplating to form an absorber, (8) acetone dissolving photoresist A-10, and etching Ti/Au seed layer, (9) using photoresist A-10 to protect the absorber, (10) making DRIE to release the membrane, (11) cleaning all photoresists to release the absorber.
• Figure 4: A side view of a TES detector. The gold absorber is square (240 $\mu$m x 240 $\mu$m) with a thickness of 2.7 $\mu$m. It is elevated about 2.5 $\mu$m from the substrate by five 10 $\mu$m-width square stems. The center stem doubles as a thermal link to the AlMn TES.
• Figure 5: TES detector test system and internal physical diagram. (a) depicts the structure of the whole test bench, including a $^{55}$Fe isotope, a TES test box, a dilution Refrigerator LD250 and a two-stage DC-SQUID read out electronics. (b) shows a photo of the TES detector test box, which is thermally connected to the mixing chamber. (c) shows eight different TES detectors, among which three detectors marked as $\#1$, $\#2$ and $\#3$ are tested here.