We have fabricated Nb/AlOX/Nb Josephson tunnel junctions using a sputtering apparatus with a load-lock system. The junctions that had 50 mu m x 50 mu m area showed a V-m value (the product of the critical current and the subgap resistance at 2 mV) as high as 50 mV at a current density of 160 A/cm(2). Moreover, junctions having different thicknesses of the Al over-layer were concurrently fabricated on one wafer to study the dependence of the current-voltage characteristics on this Al over-layer. The I-V characteristics were also calculated by McMillan\u27s tunneling model and were compared with the measured I-V characteristics

Nagashima Haruo

Nakayama Akiyoshi

Okabe Yoichi

Shimada Junichi

中山 明芳

岡部 洋一

UTokyo Repository

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 5, NO. 2, JUNE 1995 2299 
Effects of Aluminum Over-layer Thickness on Characteristics 
of Niobium Tunnel Junctions Fabricated by DC Magnetron Sputtering 
Akiyoshi Nakayama, Haruo Nagashima, Jun'ichi Shimada 
Department of Electrical Engineering, Faculty of Engineering, Kanagawa University 
3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama, 221, Japan 
Yoichi Okabe 
Research Center for Advanced Science and Technology, University of Tokyo 
4-6-1, Komaba, Meguro-ku, Tokyo, 153, Japan 
Abstract-We have fabricated Nb/AIOx/Nb Josephson tunnel 
junctions using a sputtering apparatus with a load-lock system. 
The Junctions that had 50 pm x 50 pm area showed a Vm value 
(the product of the critical current and the subgap resistance at 
2 mV) as high as 50 mV at a current density of 160 A/cm2. 
Moreover,Junctions having different thicknesses of the AI over- 
layer were concurrently fabricated on one wafer to study the 
dependence of the current-voltage characteristics on this AI 
over-layer. The I-V characteristics were also calculated by 
McMillan's tunneling model and were compared with the 
measured I-V characteristics. 
I. INTRODUCTION 
Niobium Josephson tunnel junctions are potentially very 
useful in the realization of high-speed, low-power integrated 
circuits for many electronic applications, e. g. digital and 
analog circuits and sensitive magnetic sensors. Its excellent 
electrical characteristics, durability and thermal cyclability 
make it a promising device for application of 
superconductivity [ 11. Niobium/aluminum-oxide/niobium 
(Nb/AIOx/Nb) Josephson tunnel junctions have been 
fabricated by electron-beam deposition [2, 31 or magnetron 
sputtering [4, 51. In order to obtain Nb films with good 
superconducting properties by sputtering, Nb has to be 
deposited in very pure Ar gas, because niobium easily reacts 
with oxygen and degrades its superconducting properties. In 
a sputtering system, we can not measure the exact 
background pressure during the sputtering. Hence, both a 
background pressure better than 10-5 Pa and low outgassing 
from the chamber wall are needed for a sputtering system to 
fabricate Nb/AlO,/Nb junctions. 
This paper reports the fabrication of Nb/AIOx/Nb 
Josephson tunnel junctions by a sputtering apparatus with 
load-lock system. In sect. 11, the sputtering system and the 
fabrication process of the Nb junctions are mentioned. 
Fabrication technique for varing the thickness of the A1 over- 
layer is also presented. In sect. III.A, current-voltage 
characteristics of the Nb/AIOx/Nb junctions at 4.2 K are 
mentioned. In sect. III.B, the characteristics of the junctions 
with different Al thicknesses are studied. In sect. III.C, 
Manuscript received in October 18,1994. 
the I-V characteristics of the Nb junctions calculated with 
McMillan's tunneling model are mentioned. We obtained 
the density of states (DOS) of the electrodes of the Nb 
junctions by this model and calculated the tunneling current 
to obtain the current-voltage characteristics and to compare 
them with the measured I-V characteristics. Section IV 
presents our conclusions. 
11. FABRICATION 
Niobium/aluminum-oxide/niobium Josephson tunnel 
junctions were fabricated by a vacuum system with a load- 
lock chamber. This system consisted of four parts: a 
sputtering main chamber, a sputtering sub chamber, a 
chamber for oxidation, and a preparation chamber. The 
sputtering main chamber had sputtering guns for Nb and Al 
depositions, and was always maintained as low as 10 -6 Pa 
range by a 60 l/s triode ion pump, an 800 l/s Ti sublimation 
pump and a 1600 l/s cry0 pump. Nb and AI films were 
deposited by DC magnetron sputtering. 
The preparation chamber was used for a first entry lock 
chamber. Since the volume of the preparation chamber was 
as small as 1 liter, a 200 l/s turbo-molecular pump evacuated 
this chamber from the atmosphere to the Pa range within 
30 minutes. Hence, this sputtering apparatus with the load 
lock system shortened the fabrication time of 
Nb/AIOx/Nb junctions. 
In the sputtering sub chamber, there is another sputtering 
gun for A1 deposition. Many junctions that had different A1 
thicknesses were simultaneously fabricated. By moving the 
shutter 4 mm above the substrate during the deposition of AI 
film, we could fabricate the junctions having the different A1 
thicknesses on 2-inch Si wafer substrate. The size of the 
shutter was 5 cm X 3 cm and was also made of Si wafer. The 
shutter was moved as much as 50 mm by linear motion drive. 
The precision of the movement was 10 p. 
The fabrication process of the Nb junctions was as follows: 
After Nb base electrode was deposited on the substrate in the 
sputtering main chamber, the Al over-layer was deposited by 
sputtering in the sputtering sub chamber. The substrate was 
moved from one chamber to another by magnetic-coupled 
linear motion drives. The surface of the A1 over-layer was 
then oxidized in the preparation chamber by introducing pure 
oxygen. Oxidation was done in 2.7~103 Pa oxygen for 30 
minutes without heating the substrate. After the Nb counter- 
1051-8223/95$04.00 @ 1995 IEEE 
electrode was deposited in the main chamber, the junction 
areas of the Nb/Al&/Nb hi-layers were defined by selective 
niobium anodization process. Finally, the wiring Nb 
electrode was deposited and was patterned by chemical 
etching. Tunneling Model 
the tunnel oxide became thicker. The best I-V characteristics 
were obtained when the A1 thickness was 3 nm. 
C.  Calculation of I-V characteristics by McMillan's 
III. RESULTS AND DISCUSSIONS 
A. I-V characteristics 
Figure 1 shows the current-voltage characteristics of the 
fabricated Nb/AIOx/Nb tunnel junction at 4.2 K. The Vm 
value (the product of the critical current and the subgap 
resistance at 2 mV) of this junction was as high as 50 mV. 
The junction size was 50 pm x 50 pm. The Vm value of a 10 
pm x 10 pm junction and that of a 5 pm x 5 pm junction 
were 33 mV and 31 mV, respectively. 
We have calculated the current-voltage characteristics of 
the Nb tunnel junctions by McMillan's Tunneling Model 
(MTM) [6]. We consider that the Nb/AIOx/Nb junctions 
have a structure of superconductorlproximity-layer/barrier/ 
proximity-layer/superconductor, where the proximity layer 
of the base electrode is Al over-layer. We also think that 
there is a inter-diffusion layer between the AlG and the Nb 
counter-electrode as the proximity layer of the counter- 
electrode side. The density of states of the proximity layer 
(normal layer) and that of the superconducting layer are 
obtained from the equations: 
N") = Re( E/EE2-&(EJ]ln), (1) 
NsQ = Re(E/CE2-&4]ln), (2) 
where the A&) and As@) are obtained by the numerical 
calculation of the equations as follows: 
(3) 
Horiz. : 1.0 mV/div 
Vert. : 5.0 mA/div 
Fig. 1. Current-voltage characteristics of the Nb/AIOx/Nb junction 
at 4.2 K. The junction area was 50 p x 50 p. 
B .  Dependence on the AI over-layer thickness 
We studied the dependence of the junctions' I-V 
characteristics upon the thickness of the AI over-layer. Lots 
of junctions having different A1 thicknesses were 
concurrently fabricated on one Si wafer by moving the 
shutter during the deposition of Al over-layer. By fabricating 
the junctions on the same wafer, we can obtain the junctions 
having the same process conditions other than the thicknesses 
of the Al over-layer. Figure 2 shows the dependence of the 
Nb/AIOx/Nb junctions' characteristics upon the thickness of 
the A1 over-layer deposited on the Nb base electrode. 
Several junctions that lay in the perpendicular direction to 
the shutter's movement, had the same A1 thickness. The 
values of the characteristics of these junctions with the same 
A1 thickness had however a little spread as shown in this 
figure, probably because the A1 layer was not perfectly 
uniform in the perpendicular direction to the shutter's 
movement. 
When the Al thickness was less than 2 nm, the Rsg& ratio 
and the Vm value became low. It is possibly because the 
coverage of the AI over-layer was poor and the tunnel barrier 
could not be formed uniformly. When the A1 layer became 
thicker than 4 nm the Characteristics also became poor, 
because the proximity layer between the base electrode and 
Figure 3 shows the I-V characteristics calculated using 
MTM. The lifetime broadening parameters r N  and rs, and 
the fixed BCS potentials ANPh and ASPh assumed in the 
numerical calculation with MTM are mentioned in the figure 
captions in Fig. 3. The lifetime broadening parameters rN 
and Ts are inversely proportional to the thickness of the 
proximity layer and that of the supercondutting layer, 
respectively. Current-voltage curve with the different r N  
and rs parameters are showed in the figure, where the 
subscripts [l] and [2] represent the base electrode and the 
counter-electrode, respectively. r N  = 0.025 eV and r N  = 
0.25 eV are correspondent with the 4.0 nm and 0.4 nm 
proximity layer, respectively, and rs = 3.9 x 10-5 eV with 
200 nm superconducting layer. The gap voltage becomes 
small and the sub-gap leak-current becomes large when the 
thickness of the proximity layer becomes thicker. The I-V 
curve doesn't strongly depend on the thickness of the 
superconducting layer as shown in Fig. 3(c). Temperature 
dependence of the I-V curve is also obtained as shown in 
Fig. 3(d). Comparing with the measured I-V curve, the 
thicknesses of the proximity layers of the base and the 
counter-electrode are estimated to be 4.0 nm and 0.4 nm, 
respectively. 
2301 
1000 
10 
1 2 3 4 5 
Aluminum thickness [nm] 
20 
L 
E 
0 
0 
0 
0 
1 2 3 4 5 
Aluminum thickness [nm] 
30 
20 
Y 
E > 
10 
0 
3.0 
2.9 
E 
Y 
2.6 
40 
-5 30 
9 
E 
- 3 20 
-? 
Y - 
10 
1.2 
I I 
E 1.0 
8 
0 I 
0 
e 
0 
0 
.1. 
1 2 3 4 5 
I$ 0.8 
ai 
0.6 
2 0.4 
u - 
v) 
v) 
.- 
- 
0.2 
0.0 
z" 
1 2 3 4 5 
Aluminum thickness [nm] 
1 2 3 4 5 
Aluminum thickness [nm] 
1 2 3 4 5 
Aluminum thickness [nm] Aluminum thickness [nm] 
Fig. 2. Dependence of the I-V characteristics upon the thickness of the Al over-layer. Dependence of (a)critical current density, 
Jc, (b) RsglRn, (c) Vm, (d) gap voltage, Vg. (e) Ic/I(4 mV), and (f) normal resistance Rn of the I-V characteristics of the 
Nb/AIOx/Nb junctions at 4.2 K is shown, where the subgap resistance Rsg and the normal resistance Rn are measured at 
2 mV and at 4 mV, respectively, and I (4 mV) is the current value at 4 mV. The junction areas are 50 pm x 50 p. 
2302 
0.005 , 0.005 
I 
0.004 
T 0.003 
Y 
1 
g 0.002 
2 V 
0.001 
O.OO0 
O.Oo0 0.001 0.002 0.003 0.004 0.005 
VoltageM 
0.005 , I 
0.001 
o.Oo0 .:::._..__....I ............... J. ...... A., . ,,,,..._... .......: J 1 .  
O.Oo0 0.001 0.002 0.003 0.004 0.005 
VoltageM 
O.OO0 
O.Oo0 0.001 0.002 0.003 0.004 0.005 
Voltage[VI 
Fig. 3. Current-voltage characteristics calculated by McMillan’s tunneling madel. The critical tempexature of superconducting layer Cr 1 and 
that of normal layer are 9.23 K and 1.1% K. respectively. A~P’ =1.38x10JeV. &* =1.38x104eV. (a) dependence on rN~,,(rs=3.~lo-s eV. rN 
pl=25x104eV, T = 0.4 TJ. (b) dependence on ~Np1(rs=3.9x105 eV, rNI,1=2.5x10-2 eV, T = 0.4 TJ, (c) dependence on Ts (rN11,=2.5~10-2 eV, 
rNp1=2.5x10-1 eV. T= 0.4 Tc), (d) temperature dependence of I-V characteristics ( r,= 3.9x1Cs eV. TN1,,=2.5x10-2 eV, TNp1=25x10-1 eV, T= 0, 
0.2 T, 0.4 T, 0.6 T ,  0.8 Tc). 
w. CONCLUSIONS 
In summary, the characteristics of the Nb/AIOx/Nb 
Josephson tunnel junctions fabricated by a sputtering apparatus 
with a load-lock system have been presented. The sputtering 
apparatus had two chambers for sputtering, a sub chamber for 
oxidation and a sub chamber for preparation. The junctions 
that had 50 pm x 50 pn area successfully showed the Vm 
value as high as 50 mV at the current density of 160 Ned. 
Moreover, junctions having different thicknesses of the AI over- 
layer were concurrently fabricated on one wafer in der to study 
the dependence of the I-V characteristics upon the thickness of 
the AI over-layer. The I-V characteristics were also calculated 
by McMillan’s tunneling model and were compared with the 
measured I-V characteristics. 
REFERENCES 
Speed Josephson Processor Technology. ” Trans. on Mag. , vol. 
MAG-27, no. 2, pp. 2602-2609. Mar. 1991. 
[2] Broom. R. F.. Laibowitz, R. B., Mohr, Th. 0. and Walter, W., 
“Fabrication and Properties of Niobium Josephson Tunnel 
Junctions.” IBM J. Res. Develop., vol. 24, no. 2, Mar. 1980. 
[3] Inone. A., Nakaya~na, A. and Ohbe, Y.. “‘Nb/AlOx/Nb Josephson 
Tunnel Junctions Using Electron Beam Evaporation,” Jpn. J. Appl. 
[4] Gurvitch, M., Washington, M. A. and Huggins, H. A., “High 
Quality Refractory Josephson tunnel junctions utilizing thin 
aluminum layer%” Appl. Phys. Lett., vol. 42, no. 5, pp. 472-474, 
Mar. 1983. 
[5] Morohashi, S.. Shinoki. F.. Shoji. M. and Hayakawa, H., “High 
quality Nb/Al-AlOxJNb Josephson junctions,” Appl. Phys. Lett.. 
vol. 46, no. 12. pp. 1179-1181. Jun. 1985. 
[6] W.L.McMillan. “Tunneling Model of the Superconducting 
Proximity Effect,” Phys. Rev., vol. 175, no. 2, pp. 537-542, Nov. 
1968. 
Phys. vol. 27, IW). 7. pp. 1234-1239, Jul. 1988. 
[l] Hasuo. S.. Kotani, S..Inoue,A. and Fujimaki, N., “High 


EFFECTS OF ALUMINUM OVER-LAYER THICKNESS ON CHARACTERISTICS OF NIOBIUM TUNNEL-JUNCTIONS FABRICATED BY DC MAGNETRON SPUTTERING

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EFFECTS OF ALUMINUM OVER-LAYER THICKNESS ON CHARACTERISTICS OF NIOBIUM TUNNEL-JUNCTIONS FABRICATED BY DC MAGNETRON SPUTTERING

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