Lateral Inverse Proximity Effect in Ti/Au Transition Edge Sensors

Abstract

We report measured $T_\mathrm{c}$ 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 $T_\mathrm{c}$ drops as W decreases and the declining trend almost perfectly follows $T_\mathrm{c}/[\mathrm{mK}] =-738.4[{\upmu } \mathrm{m}]^{2}/W^{2}+91.0$, where $T_\mathrm{c}(W=\infty )$ of 91 mK is consistent with the intrinsic $T_\mathrm{c}$ 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 $T_\mathrm{c}$ shift from the intrinsic $T_\mathrm{c}$ should be anticipated in addition to the longitudinal proximity effect from superconducting Nb leads when one designs Ti/Au TESs.