1. 1 Anomaly in fabrication processes for large-scale array detectors of superconducting tunnel junctions M. Ukibe, Y. Chen, Y. Shimizugawa, Y. Kobayashi, A. Kurokawa, M. Ohkubo National Institute of Advanced Industrial Science and Technology(AIST), Research Institute of Instrumentation Frontier,

2. 2 1.fabrication yield  Task Goal: ～ 90% with uniform IV characteristics 85%<1  A@0.4mV Nb 100nm Al:50nm Development of 100 STJ array detector Wide Distribution of the leak current in 100 STJ array 1nA ～ 1μA 11mm

3. 3 Leak currents of STJ elements Leak current level I sub <150nA: good 1000nA<I sub 150nA<I sub <1000nA Point contact Short Open Low leakage STJ : Center part of a 100 array Large leakage STJ: Outer part of a 100 array

4. 4 Causes the leak current ? After etching of bottom Nb layers Stepped surface! In the outer part of a 100 array Same with the the leak currents

5. 5 Stereograph of surface of STJs

6. 6 Change of the stress condition between Nb/Al layer and wafer After the bottom Nb RIE Almost Nb layer on wafer is removed Defects in the tunnel barrier ? Origin of the leak currents ？ Stress Free Nb film ? Origin of the stepped surface ？

7. 7 Displacement of curve of Si wafer at each steps Top and Bottom Al wet etch Bottom Nb RIE

8. 8 Two processes for removing the stepped surface Decrease of the stress displacement of Nb/Al films during the process Decrease of the etched Nb/Al area Liftoff of Nb/Al films Limited etching Keep curve Start End Start End Keep curve Liftoff method

9. 9 Improvement of leak currents Leak current : positive correlation to stepped surface area size Large stepped surface Small stepped surface Perfect surface

10. 10 Conclusions Distribution of leak current: 1nA ～ 1  A Distribution of stepped surface Variation of the curve of Nb/Al and the wafer Even stress free films has big influence!! In case of a large–size STJ array over the size of ～ mm 2 Start End Keep curve High fabrication yield and reproducibility