Effect of Mo/Cu Superconducting Bilayer Geometry on Ultra-Sensitive Transition-Edge Sensor Performance

We built membrane-isolated transition-edge sensors (TESs) for the background-limited infrared/sub-mm spectrograph using Mo/Cu superconducting bilayer thermistors of varying geometry and found that undesired proximity effects, including the so-called longitudinal proximity effect (LoPE) and the latitudinal inverse proximity effect, affect both the superconducting transition temperature T-C and the sharpness of the transition alpha = d log R/d log T. The LoPE and latitudinal inverse proximity effect arise because of unintentional proximity effects between the bilayer thermistors, the superconducting wiring of the TES circuitry, and normal metal decorations added to mitigate the LoPE. We examined Mo/Cu bilayer films with widths of 120 mu m and lengths of 5, 10, 20, 40, and 120 mu m, and studied the variation of T-C, alpha, and approximate 80% resistance per square (R-0.8) with Ti (T-C similar to 500 mK) and TiN (T-C similar to 3.8 K) wiring to the devices. We found larger a values in general for the Ti wiring, where a was as high as 90 for 20-120 mu m devices and decreased to 20 for 5-mu m-wide devices. We then built arrays of TESs with bilayer thermistor lengths of 10 mu m, Ti contacts, TiN wiring, and Au borders. The devices were expected to demonstrate a noise equivalent power less than or equal to 10(-19) W/Hz(1/2). We report a measured noise equivalent power at 87 mK of (0.95 +/- 0.2) x 10(-19) W/Hz(1/2) and a response time tau of (360 +/- 30)ms on our best device with a thermal conductance G = (15 +/- 5)fW/K, T-C = (120.5 +/- 3.5)mK, and stray power P-D = (135 +/- 85)aW. The thermistor had a value of R-N = 6 m Omega and value of alpha = d log R/d log T between 10 and 60 in the transition. We compare our measured performance with the performance specifications needed for ultrasensitive TESs on the Background-Limited Infrared/Sub-mm Spectrograph (BLISS) and discuss paths forward.