Tapered Edge Readout Wiring for Transition Edge Sensor Calorimeter Arrays Using Ion Milling

We are developing transition edge sensor (TES) arrays using multilayer superconducting readout wiring for future X-ray astronomical missions. The TES pixels are formed on the multilayer readout wiring composed of hot and return wires sandwiching an insulation layer. The interface between the wiring and the TES is formed by patterning the topmost wiring layer followed by deposition of the TES layer. To mitigate problems found in a past device, edges of the upper readout wiring are tapered with an ion milling method to enhance an electrical contact between the TES film and the upper readout wiring. To verify this design, we fabricated a 400-pixel test array with the TES pixels on the tapered upper readout wiring. Good physical contact between the TES film and the readout wiring was confirmed with scanning electron microscope. Furthermore, a low transition temperature ( $\sim$ 160 mK), a sharp transition edge and a low residual resistance $(\sim\!\! 1\ \hbox{m}\Omega)$ were observed in $RT$ measurements of representative pixels. The critical current was small $10\sim 30\ \mu\hbox{A}$ , although it is still larger than the previous device $( . These results suggest that this method will enable a multilayer wiring TES array with good electrical properties.