High aspect ratio transition edge sensors for x-ray spectrometry

We are developing large Transition Edge Sensor (TES) arrays in combination with a frequency domain multiplexing readout for the next generation of X-ray space observatories. For operation under an AC-bias, the TESs have to be carefully designed and optimized. In particular, the use of high aspect ratio devices will help us to mitigate non-ideal behavior due to the weak-link effect. In this paper, we present a full characterization of a TES array containing five different device geometries, with aspect ratios (width:length) ranging from 1:2 up to 1:6. The complex impedance of all geometries is measured in different bias configurations to study the evolution of the small-signal limit superconducting transition parameters alpha and beta, as well as the excess noise. We show that high aspect ratio devices with properly tuned critical temperatures (around 90mK) can achieve excellent energy resolution, with an array average of 2.03 +/- 0.17eV at 5.9keV and a best achieved resolution of 1.63 +/- 0.17eV. This demonstrates that AC-biased TESs can achieve a very competitive performance compared to DC-biased TESs. The results have motivated a push to even more extreme device geometries currently in development.