Performance of SAFARI Short-Wavelength-Band Transition Edge Sensors (TES) Fabricated by Deep Reactive Ion Etching

We are developing the SAFARI/S-band transition edge sensor (TES) based on a superconducting Ti/Au bilayer on a suspended silicon nitride (Si_xN_y) membrane with narrow, thin, and long Si_xN_y legs. To develop a suitable fabrication process for a large TES array, we have recently replaced a wet KOH etching step for Si by a deep reactive ion etch (RIE) fabrication step. We measured the current-voltage characteristics, noise performance, and impedance of a few Ti/Au TESs fabricated by deep RIE at different bath temperatures. The measured critical temperatures (mbi Tc) are found to be around 105 mK and are in good agreement with the values obtained from an unprocessed Ti/Au bilayer. This shows that the new fabrication process does not affect the T_c. This is a positive result for the fabrication of large TES arrays. In addition, we find that a measured dark noise equivalent power (NEP_meas) approaches the phonon noise-limited NEP (NEP_ph) with a ratio NEP_meas NEP_ph=1.2, which is supported by observing clear bath temperature dependence of the NEPmeas. This result is extremely encouraging because we can realize a lower NEP with a larger possible dynamic range. A very low-G TES with low heat capacities of the Si_xN_y legs and Si_xN_y membrane compared with that of the TES is a promising route to realize such performance.