We report measured T_c of superconducting Ti/Au bilayer strips with a width W varying from 5 to 50 µm. The strips were fabricated based on a Ti/Au bilayer that consists of a 41-nm-thick Ti layer to which a 280-nm-thick Au layer was added. We find that the T_c drops as W decreases and the declining trend almost perfectly follows T_c/ [mK] = - 738.4 [μ m] ²/ W²+ 91.0 , where T_c(W= ∞) of 91 mK is consistent with the intrinsic T_c of the bilayer. The result is interpreted as a consequence of the lateral inverse proximity effect originated in normal-metal microstructures, namely Au overhangs that exist at the edges of the Ti/Au bilayer. The T_c shift from the intrinsic T_c should be anticipated in addition to the longitudinal proximity effect from superconducting Nb leads when one designs Ti/Au TESs.

We are developing X-ray microcalorimeters as a backup option for the baseline detectors in the X-IFU instrument on board the ATHENA space mission led by ESA and to be launched in the early 2030s. 5 × 5 mixed arrays with TiAu transition-edge sensor (TES), which have different high aspect ratios and thus high resistances, have been designed and fabricated to meet the energy resolution requirement of the X-IFU instrument. Such arrays can also be used to optimize the performance of the frequency domain multiplexing (FDM) readout and lead to the final steps for the fabrication of a large detector array. In this work, we present the experimental results from tens of the devices with an aspect ratio (length-to-width) ranging from 1-to-1 up to 6-to-1, measured in a single-pixel mode with a FDM readout system developed at SRON/VTT. We observed a nominal energy resolution of about 2.5 eV at 5.9 keV at bias frequencies ranging from 1 to 5 MHz. These detectors are proving to be the best TES microcalorimeters ever reported in Europe, intending to meet the requirements of the X-IFU instrument, but also those of other future challenging X-ray space missions, fundamental physics experiments, plasma characterization and material analysis.