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 α = dlogR/dlogT. 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 μm and lengths of 5, 10, 20, 40, and 120 μm, and studied the variation of T_C, α, and approximate 80% resistance per square (R_0.8) with Ti (T_C ~ 500 mK) and TiN (T_C ~ 3.8 K) wiring to the devices. We found larger α values in general for the Ti wiring, where α was as high as 90 for 20-120 μm devices and decreased to 20 for 5-μm-wide devices. We then built arrays of TESs with bilayer thermistor lengths of 10 μm, 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) × 10^-19 W/Hz^1/2 and a response time τ of (360 ± 30)ms on our best device with a thermal conductance x^1/2 = (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 mΩ and value of α = dlogR/dlogT\n\n\t\t