Optimizing Transition-Edge Sensor Design for High Count-Rate Applications

We are developing transition-edge sensor (TES) X-ray detectors optimized for high count-rate applications. These devices are fabricated on thick (300 km) Si substrates, resulting in a 20 times increase in thermal conductance to the heat sink compared to our conventional membrane isolated TES's. Operating a TES with higher heat sink conductance requires 4.5 times more bias current. This results in a 2.7 times increase in b, the logarithmic derivative of resistance with respect to current Noise measurements show a lower limit on the TES excess noise scales as (2b)(1/2), consistent with the near-equilibrium, non-linear expansion of the Ohmic Johnson noise. This is consistent with our membrane devices though the increased b means the theoretical best attainable resolution is degraded by 25-35%. We have tested devices with different contact geometries between the absorber, and the TES and substrate. This allows us to investigate the loss of athermal phonons to the substrate, which can degrade the resolution. Results show a correlation between the stem contact area and a low-energy tail in the spectral response at 5.9 keV due to the athermal phonon loss. In several devices tested we demonstrate a resolution of 4.1-5.6 eV, coupled with detector time constants as fast as 44 us, representing an increase in detector response by 7 times compared to the membrane devices.