1. Recent result of ~650-GHz SIS-device fabrication at NRO
K. Kikuchi 1, W. Shan 2, Y. Fujii 3, and J. Inatani 1
(1NASDA, 2 CRL, 3 Nittuki )

2. Introduction Purpose:
Development of ~650-GHz SIS mixer devices for JEM/SMILES,
B-SMILES, and SMA.
Difficulties:
- Just below the gap frequency of Nb.
- Junction size should be as small as ~1mm2.
Mixer performance is highly sensitive to fabrication parameters.
This work is …
- Improvement and optimization of the fabrication processes.
- Understanding of controllability of the SIS device parameters.

3. SIS-device and Fabrication Process
IF port
500 mm
GND
- SIS Junction: Nb/AlOx/Nb
- Junction Size:
~1 x 1 mm2 (for JEM/SMILES)
- Current Density:
~7 kA/cm2 (for JEM/SMILES)
- RF Matching:
PCTJ with Integrated Circuit

4. Cross-section Images by TEM
TEM image of junction portion
500 nm
Wire-Nb
Al2O3
Upper-Nb
SiO2
Nb2O5
Lower-Nb
Quartz substrate
Al-AlOX
20 nm
“T96” device:
- fabricated in 2001
- moderate RF performance
(TRX,DSB ~ 300 K)

5. Nb-film: Superconductivity
- Nb-film samples were fabricated with the parameter changing.
- Film having a good superconductivity seems to be obtained by
using LN2 shroud or gettering effect of sputtered Nb and Al.
TC ~ 9.0 ± 0.1 K
r10K ~ 5.0 ± 0.5 mW cm

6. Nb-film: Surface Roughness Measured by AFM
+0.22 GPa, 184 nm
Stress
-0.49 GPa, 173 nm
-0.52 GPa, 359 nm
Thickness
1mm

7. Optimization of Etching Processes
- Etching rates and stabilities are investigated in various conditions.
Example: introduction of Ar-cleaning process:
50mm
Un-developed photo-resist makes rough
surface in some conditions.
Ar-cleaning is effective to make flat.
(SiO2 : Nb ~ 5:1 for 100 W, 50 mTorr.)
Surface Profile (nm)
Relative Distance (mm)

8. Improvement of Junction Lift-off Process
Introduced paint brush,
instead of cotton swab

9. Controllability: Film Thickness
Surface profile is measured at each process to estimate the thickness.
JEM/SMILES SIS device
640-GHz PCTJs
f35
Test junctions
Surface profile measurement
(500-GHz PCTJs)

10. Controllability: Junction Size
Rn distribution of KS17 devices
ID = 1 76
Mean junction size with 1-s deviation in a wafer
L ∝ (Jc・Rn) -0.5
fc ∝ L-1

11. Peak Structure in PIF-V Curve
SIS having peak structure shows poor RF performance.
Model
Amplitude is affected by magnet.
- Position varies with temperature.

12. Mixer Performance
Mixer block for JEM/SMILES
TRF~60K, TIF~15K

13. Summary Improvement of processes (Ar-cleaning, lift-off, …)
Understanding of the film properties (TEM, AFM, superconductivity, …)
Improvement of processes (Ar-cleaning, lift-off, …)
Variation of device parameters:
- Superconductivity of Nb film: DTC ~ 0.1 K, Dr10K ~ 0.5 mW cm
- Film thickness: DT ~ 2-5 %
- Junction size: DL ~ 5-10 %
- Repeatability of JC is also poor (~ 10 %)
Rough estimation of yield:
- Survival rare in DC characteristics: pDC ~ 90 %
- No peak structure in PIF-V curve: pno-peak ~ 70 % (?)
If the band-width of SIS mixer DB/B = Db ~ 5 %, Db/DL ~ 1 s.
Then the maximum “yield” from the view point of RF performance is…
pDC x pno-peak x pGaussian(Db/DL) = 0.9 x 0.7 x 0.68 = 0.43