Lateral Inverse Proximity Effect in Ti/Au Transition Edge Sensors

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 / W 2 + 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.