Thomas'
- 'D.J. Goldie'
- 'D. Osman'
bibliography:
- 'References.bib'
title: Superconducting transition edge sensors with phononic thermal isolation
---

Introduction
============

There is considerable interest in developing superconducting Transition Edge Sensors (TESs)[@IrwinChapter] for astronomy and space science. For ground-based photometric measurements at long wavelengths (3 mm - 300$\mu$m), Noise Equivalent Powers (NEPs) of 10$^{-17}$WHz$^{-1/2}$ are required [@posada2015fabrication; @westbrook2016development; @AlMnAdvancedACTPol; @BICEP2KECKSPIDER]; for space-based measurements and Earth Observation at long wavelengths, NEPs of 10$^{-18}$WHz$^{-1/2}$ are necessary; for space-based measurements with cooled-aperture telescopes at FIR wavelengths, such as SPICA (200 - 30$\mu$m) [@SafariSPICA; @SafariNewImproved; @SPICANewFramework; @goldie2016performance; @goldie2012ultra], NEPs of 10$^{-19}$WHz$^{-1/2}$ and better are the necessary target. Time and energy resolved photon counting TESs are being developed for the x-ray space telescope Athena (0.2 - 12keV) [@gottardi2016development; @den2014requirements; @akamatsu2016tes; @TESMicrocalorimeterATHENA], and for general utilitarian applications at optical wavelengths (1550 - 400nm) [@cabrera1998SinglePhoton; @portesi2008OpticalPhoton; @eisaman2011invited; @SPDNatPhot2009; @QKDNoiseFree].

State-of-the-art TESs have many favourable characteristics, but they also have a number of shortcomings. To achieve low-noise operation, a low thermal conductance ($G =$ 0.1 - 10pWK$^{-1}$) is needed between the active elements of the device and the heat bath. TESs are usually fabricated on SiN$_{\rm x}$ membranes, and thin ($H =$ 200nm - 1$\mu$m), narrow ($W 