Development of a transition-edge sensor bilayer process providing new modalities for critical temperature control
Joel C. Weber, Kelsey M. Morgan, Daikang Yan, Christine G. Pappas, Abigail L. Wessels, G. C. O’Neil, D. A. Bennett, G. C. Hilton, Daniel S. Swetz, Joel N. Ullom, D. R. Schmidt
Abstract
Abstract Transition-edge sensors (TESs) are thermal detectors in which a superconducting film that is electrically biased in the superconducting-to-normal transition is used as a thermometer. In most TESs, the film is a superconductor-normal metal bilayer where the two materials and their thicknesses are chosen to achieve various specifications including the transition temperature T c . Traditionally, the materials in the bilayer are deposited in sequence without breaking vacuum in order to achieve a clean, uniform bilayer interface at the wafer-scale. This approach leads to constraints in material properties, fabrication techniques and, ultimately, TES designs. To overcome these constraints, we have developed a bilayer fabrication process that allows the layers to be deposited and patterned separately with an exposure to atmosphere between the deposition steps. We demonstrate better than 6% transition-temperature uniformity across a 7.6 cm (3 in) substrate and present satisfactory spectra from TES x-ray detectors fabricated in this fashion. We show how the new hybrid additive-subtractive TES fabrication process creates new design possibilities, including broad tuning of T c across a substrate with a single bilayer thickness.