Performance of High-Resolution Mo/Cu Transition Edge Sensors Fabricated Using a Lift-Off Process

• Published in: IEEE transactions on applied superconductivity Vol. 33; no. 5; pp. 1 - 7
• Main Authors: Jaeckel, Felix T., Ambarish, C. V., John, Elisa, Liu, Wei, McCammon, Dan, Roy, Avirup, Stueber, Haley R., Wang, Zelong, Yan, Thomas
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-08-2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Absorbers; astrophysics; Calorimeters; Energy resolution; excess noise; Extraterrestrial measurements; Fabrication; Galaxy distribution; High temperature; Intergalactic media; lift-off process; Metallicity; Metals; Microcalorimeters; Noise prediction; Sensitivity; Sensors; Soft x rays; Sounding rockets; Specific heat; Superconducting transition temperature; Temperature measurement; Thermal analysis; Thermal conductivity; thermal models; Thermal noise; Thermodynamic properties; transition edge sensors; X-ray detectors
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2023.3250163

The soft X-ray band below <inline-formula><tex-math notation="LaTeX">\approx 0.5</tex-math></inline-formula> keV is expected to provide rich information on metallicity, temperature, and distribution of hot gas in our galaxy and the intergalactic medium. Resolving these densely spaced emission lines requires energy resolution on the order of 1--2 eV. The low count rates from diffuse astrophysical sources also necessitate large collection areas, particularly for a sounding rocket instrument with short exposure times and limited resources. This translates to large pixel sizes (about 1 mm<inline-formula><tex-math notation="LaTeX">^{2}</tex-math></inline-formula>) for a reasonable number of read-out channels. To achieve the required energy resolution with 200 nm thin Au absorbers (heat capacity <inline-formula><tex-math notation="LaTeX">C</tex-math></inline-formula> about 1 pJ/K), we are developing TES microcalorimeters with very high temperature sensitivity (<inline-formula><tex-math notation="LaTeX">\alpha _{I}</tex-math></inline-formula>), low enough current sensitivity (<inline-formula><tex-math notation="LaTeX">\beta _{I}</tex-math></inline-formula>), and minimal excess noise (<inline-formula><tex-math notation="LaTeX">M</tex-math></inline-formula>). Out of the various fabrication approaches for the TES bilayer we have explored, the lift-off approach (as pioneered for Mo/Au TES by Joel Weber at NIST) so far is the simplest to execute. The simple square TES geometries we have tested to date show promising results for <inline-formula><tex-math notation="LaTeX">\alpha _{I}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">\beta _{I}</tex-math></inline-formula>. Measured complex impedance and noise for bare devices without absorbers are well described by two-body thermal models and their excess noise is found to be consistent with predictions for mixed-down Johnson noise by Wessels et al. More sensitive measurements with larger heat capacity are planned to accurately determine excess noise. In this paper, we give an overview of the process and summarize results of our detailed characterization measurements for thermal conductance <inline-formula><tex-math notation="LaTeX">G</tex-math></inline-formula>, <inline-formula><tex-math notation="LaTeX">\alpha _{I}</tex-math></inline-formula>, <inline-formula><tex-math notation="LaTeX">\beta _{I}</tex-math></inline-formula>, and noise for several devices across a range of bias points.