IRES²

1 Introduction
Comparison of superconducting
tunnel junctions (STJs)

Scanning probe microscope (SPM)

Overview of the fabrication flow

SPM is a term for microscopes based on a scanning tunneling microscope (STM).

Rod development
2c-axis Bulk

XY drive circuit

Piezoelectricdevices

XYZ

New probe(This work)
Vacuum I
Tunnel current

rectangular Rod(40 40 10 mm3)

Z-voltage
feedback
circuit

Tunnel current
detection circuit

Fine cutter
machine(0.5 0.5 10 mm3)

GapProbe development

Sample surface

Focus
unknown sample(metallic probes)

Polishingmachine3

sharp RodProbe

Stage(Tip ~1 m)
Bias voltage(Tip ~100 m)

Focused ion beam
(FIB)

An STM equipped with an HTS probe is expected to realize SIS junctions
that are separated by a vacuum insulator.

2 Rod development(a) Bulk evaluation
RINT-2500 diffractometer(Rigaku Corporation, Japan)

SlicedFine cutter machine(HS-100, Heiwa Technica)

SampleYBCO
Cu-K

Source voltage

40 kV

current

100 mA

Sliced Bulk

1000 cps

Thin Film

Scan angle

10 ~ 70

step

0.02

speed4/min

Cooling water(013)

ICDD 01-079-0469(213)(001) (002) (003) (004) (005) (006) (007)10203040silver paint
10 mm

50ViseYBCO rod(0.5 0.5 10 mm3)

Machining conditions(103)

03 K cryostat

40 mm

20000 cps

Measuring conditions
X-ray source(c) Rod evaluation
YBCO bulk(schematic)

Grinding stone

caxis(0.5mmt)caxis(b) Rod fabrication

c-axis YBCO(CSYL-404010, CanSuperconductors)

Crystalline evaluation by X-ray diffraction

60

YBCO bulk showed the similar diffraction peaks to a sputtered film.

Abrasive grains

DiamondFeed speed

10 mm/minFeed amount (X)

1.1 mmFeed amount (Y)

40 mm

Feed depth [mm]1.2 mmRotation speed

3,000 rpm

Measuring conditions
Material

YBCO

Sample size

10 0.5(rod)

0.5 mm3

10 10 0.5 mm3(sliced bulk)

High fabrication yield of >90%
without breaking the rod

Refrigerator

Gifford-McMahon

Temperature

300 ~ 3 K

Rod has the same Tc as sliced bulk.
1No damage to superconductivity

3 Probe development(b) Probe evaluation(a) Probe fabrication

Analysis of influence of Ga-ion implantation

Focused Ga-ion beam system(NB5000, Hitachi High-Tech)(c) Probe cleaning
3Ar-ion milling system

A (Before Milling)
B (Before Milling)
C (Before Milling)A (After Milling)
B (After Milling)C (After Milling)

2.5

Energy dispersive X-ray spectroscopy in FE-SEM (EMAX ENERGY EX-250, Horiba)
Au-coated YBCO probe

2123

BC

1.520 m

20 m

A6 m

10.5

Machining conditions
Acceleration voltage

40 kV

Aperture diameter

550 m

Beam current

56 nAdensity

Fullwidth

1 pixelIrradiation time

20 minRadius ofcurvature~ 1.0 mYBCOprobe

Measuring conditions
Counting rate

~3 kcpsAcceleration voltage 30 kV
20 m

Sharp YBCO probe was processed by FIB.

Measurement time

100~120 sec

Working distance

15Magnification

0.1 mm10k

Ga-ion penetration depth < 1 m
The chemical composition
was conserved within the rod.

Experimental setup

YBCO probe
on a sapphire substrate

Pumped LN2(~61 K)
Au sputtered film
on a MgO substrate

StructureS
I S (or N)

Energy diagram

S (YBCO) - I (vac.) - N (Au) characteristics was evaluated.

2Milling time

10 minGa-ion removal ratio
Edge (No. 1) 51-67%
Surface (No. 2-3) 98-100%
Production of pure YBCO probes
possible by Ar-ion milling.

We proposed, fabricated, and evaluated a YBCO probe for realizing SIS junctions.

Rod development

Temperature dependence of thedensity of state for YBCO

Probe development

1050-5-10Voltage

Acceleration voltage 60 V

3A c-axis oriented bulk was cut into a rectangular rod using a fine cutter machine.
Tc of the rod was ~90 K, which was almost the same as that of the original bulk,indicating no critical damage to superconductivity owing to the cutting.

S. D. Adrian et al., Phys. Rev. B,
56, p.7878 (1997)

I-V curve

Milling conditions

2

Number of Analyzed Point [-]Theoretical work byother groups~1 eV(~80 K)

Two-gap model(~7 & 25 meV)

Tunnel current characteristics

Scanning tunneling microscope(JSPM-5200, JEOL Ltd.)

1

5 Summary

4 Installation to SPM (STM mode)

ariyoshi@tut.jp

04 mm

High V range
(~1V)

0.1

Low V range(<100 mV)

0-0.1

-3 -2 -1 0 1 2 3Bias Voltage [V]-0.6 -0.3 0 0.3 0.6
Bias Voltage [V]Noise reduction and YBCO stage will reveal the SIS nature.

One end of the rod was ground to a pyramid shape of ~70 m radius, and thenthe apex was further sharpened to ~1 m using a focused Ga ion beam.
The penetration depth of Ga ions from the YBCO surface was <1 m at 40 kV,
suggesting that the chemical composition was conserved inside the rod.

Installation to SPM (STM mode)
A YBCO probe was installed in a LN2-cooled STM system. Preliminary results
about the tunnel current characteristics were obtained. A sharp increase in tunnel
current from ~1 V was observed at the high V range, and SIS-like I-V curve wasalso acquired at the low V range.

Future works
Optimization of FIB process
Sharper probe tip, reduced Ga-ion damage
Atomic force microscope imaging in liquid nitrogen
Noise reduction in STM
YBCO-based SIS nano-junctions