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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,

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1.fabrication yieldTask Goal: ～ 90% with uniform

IV characteristics

85%<1A@0.4mVNb 100nm

Nb 100nm

Al:50nmAl:50nm

Development of 100 STJ array detector

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4751

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Wide Distribution of the leak current in 100 STJ array1nA ～ 1μA

11mm

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Leak currents of STJ elementsLeak currents of STJ elements

Leak current level

Isub<150nA:good 1000nA<Isub150nA<Isub<1000nA

Point contact Short Open

Low leakage STJ : Center part of a 100 array

Large leakage STJ: Outer part of a 100 array

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Causes the leak current ? 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

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Stereograph of surface of STJs

Stereograph of surface of STJs

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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 ？

Origin of the stepped surface ？

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Displacement of curve of Si wafer at each steps

Displacement of curve of Si wafer at each steps

Top and Bottom Al wet etch

Bottom Nb RIE

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Two processes

for removing the stepped surface Two processes

for removing the stepped surface Decrease of the stress displacement ofNb/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

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Improvement of leak currentsImprovement of leak currents

Leak current : positive correlation to stepped surface area size

Large stepped surface

1

10

100

1000

10000

0 0.1 0.2 0.3 0.4 0.5 0.6

Currnet(V)

Cur

rent

(nA

)

Voltage(mV)

Large size stepped surface

~1000nA

1

10

100

1000

10000

0 0.1 0.2 0.3 0.4 0.5 0.6

Currnet(V)Current(nA)

Cu

rre

nt(

nA

)

Voltage(mV)

Large size stepped surfaceSmall size stepped surface

~100nA~100nA

~1000nA

Small stepped surfacePerfect surface

1

10

100

1000

10000

0 0.1 0.2 0.3 0.4 0.5 0.6

Currnet(V)Current(nA)Current(nA)

Cu

rre

nt(

nA

)

Voltage(mV)

Large size stepped surfaceSmall size stepped surface

~100nA~100nA

~1000nA

~1nA

No stepped surface

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ConclusionsConclusions

Distribution of leak current: 1nA ～ 1A

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 ～mm2

Start

End

Keep curveHigh fabrication yield

and reproducibility