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Electron beam lithography and ion implantation techniques for fabrication of high-Tc Josephson junctions
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 30 (1996) 407-410
Electron Beam Lithography and Ion Implantation Techniques for Fabrication of High-Tc Josephson Junctions R. Barth, A.H. Hamidi, B. Hadam, J. Hollkott, D. Dunkmann, J. Auge, H. Kurz Institut fiir Halbleitertechnik II, Rheinisch-Westf'alische Technische Hochschule Aachen, SommerfeldstraBe 24, D-52074 Aachen, Germany
Established semiconductor process technologies are demonstrated to be suitable for the fabrication of high temperature superconductor (HTS) Josephson junctions. Single YBCO bridges were modified by local oxygen ion irradiation through a narrow slit in a PMMA mask which was formed by electron beam lithography. The influence of slit dimension and irradiation dose was investigated. The critical current and normal resistance oI the modified microbridges can be controlled by these two parameters. Proximity coupling across the modified region is observed up to a slit width of 250 nm. When exposed to microwave irradiation the microbridges exhibited Shapiro steps. In dc SQUIDs a voltage modulation as a function of an applied magnetic flux is observed.
1. INTRODUCTION Josephson junctions are the key elements of superconductor based electronics. Normally tunnel type junctions are used because of their higher quality and reproducibility compared to other types of weak links. As a consequence of the short coherence length of high temperature superconductors (HTS) and the rather complex growth techniques related with this materials the fabrication of HTS tunnel junctions is difficult. Therefore most work on HTS Josephson junctions has been concentrated on artificial induced grainboundaries in thin film bridges (e. g. step edge, bicrystal) [1, 2]. There are several advantages related with these junctions like simple preparation technique and the correlation between the junction properties and the grain boundary angle. On the other hand one has to deal with restrictions like the fixed grain boundary angle and geometrical aspects of the positioning of the junction within one chip. The fabrication of planar SNS proximity coupled junctions might be an alternative. There are different solutions for such devices [3-5]. Proximity coupling can be achieved across narrow trenches etched into a HTS bridge and subsequently filled with a noble metal like silver or gold [5]. These trenches can be realized easily by a combination of
electron beam lithography and dry etching. The main problem of this junction type is the quality of the interface between the superconductor and the metal. Monolithic SNS-junctions offer the possibility to overcome these problems. A normal conducting barrier is induced into the same HTS film by altering locally its electrical properties. Several authors report on excellent HTS junctions by direct electron beam modification. They suffer, however, from poor long term stability [6, 7]. Modifications by focused ion beam techniques [8] or ion irradiation using masks [9] are reported to give similar results with good stability. In this paper we describe the modification of HTS microbridges by irradiation with 100keV oxygen ions. The influence of the irradiation dose and width of the modified region on the electrical properties of microbridges will be discussed.
2. EXPERIMENTAL
The YBa2Cu307 (YBCO) films used in this work were prepared b y DC magnetron sputtering on (100) SrTiOa-substrates [10]. X-ray diffraction measurements (XRD) revealed that the c-axis of the YBCO films was perpendicular to the surface of the substrate. They also proved the epitaxial structure of the films. Critical temperatures T c up to 91 K are
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All fights reserved. S S D I 0167-9317(95)00274-X
408
R. Barth et al. / Microelectronic Engineering 30 (1996) 407-410
measured. The critical current density Jc at 77 K in zero magnetic field exceeded 106A/cm2. The thickness of the YBCO films was 60 rim. In a first step the degradation of T c upon ion exposure is studied. We concentrated on the implantation of oxygen ions with 100 keV energy. In this energy regime the mean projected range of the oxygen ions is larger than the film thickness. After implantation at different ion fluences we checked the temperature dependence of the electrical resistance. For an ion fluence in the range of 1012/cm 2 only a slight reduction in the transition temperature was observed while the specific resistance increased. In the fluence range of lxl013/cm 2 to lxl01/cm 2, T c of the samples was reduced significantly until the superconductivity disappeared (figure 1). After implantation with doses higher than 7x1013/cm 2 the samples showed semiconducting behavior and the resistance rose rapidly with decreasing temperature. The ion fluence appears to be very critical for the control of the specific resistance. i
70
x l O -1 k
60
l ",.
.
50 :
E ~
- -
\ X, X,
4O :30
:
as grown
........ 3x1013/om 2 ............. 6 x 1 0 1 3 / c m 2
",.. ......... 9x1013/cm 2
onto the YBCO film to protect it against any chemical degradation due to contact with the PMMA resist [11]. Within these experiments the ion fluence was kept constant at 1014/cm 2 and the resist thickness was increased from 200nm to 1 pro. The transition temperature of the samples was checked before and after ion implantation. Using PMMA masking layers thinner than 300 nm the superconductivity vanished. Increasing the resist thickness, a declination in the Tc reduction was observed. From these experiments we determined that a resist thickness of at least 500 nm will be necessary to prevent significant degradation of superconductivity in the masked regions. This experimental result is in good agreement with Monte Carlo based calculations [12]. In a final step the YBCO microbridges are modified by local oxygen ion implantation. The microbridges have been patterned by optical lithography and ion-beam etching. Consecutively 80 nm amorphous carbon are deposited and a layer of 500 nm PMMA is spin coated onto the sample. 10 nm wide lines are exposed by 35 keV electron beam lithography to ensure local ion implantation. The samples are developed in a solution of 1 : 3 Methyl Isobutyl Ketone (MIBK) : IsoPropyl Alcohol (IPA) in an ultrasonic bath for times around 5 s, and rinsed in IPA. The samples were blown dry with pure nitrogen. After preparing the masks, the samples were irradiated by oxygen ions with an energy of 100 keV.
i
20
:"
10 0
: 0
2O
40
60
Temperature
80
1 O0
[K]
Figure 1. Temperature dependence of the specific resistance on 60nm thick YBCO films after implantation with 100 keV oxygen ions.
In a second step a series of tests was performed to determine the minimum required resist thickness for effective protection of the YBCO film during irradiation with 100keV oxygen ions. A 80nm thick amorphous carbon layer has been deposited
Figure 2. SEM micrograph (60 °) of a typical 200 nm wide trench in a 500 nm thick PMMA layer. The pattern is e-beam written at 35 keV.
409
R. Barth et aL / Microelectronic Engineering 30 (1996) 407-410
Because of the forward scattering of electrons within the resist the minimal achievable width of the trenches depends on the PMMA thickness. A SEM image of a 200 nm wide trench in a 500 nm thick PMMA-layer is depicted in Figure 2. The exposure is done at 35 keV. The undercut of the resist profile due to forward scattering of the electrons is clearly visible. Increasing the electron energy up to 100 keV will reduce forward scattering. The influence of the slit width on the electrical properties of the modified microbridges (8x1013/cm2) has been investigated in the range between 500 nm and 200 nm. The irradiated region of the microbridge certainly exhibits normal conducting behavior at all temperatures as shown in figure 1. A supercurrent across these microbridges due to proximity coupling will only be observed if the dimensions of the irradiated region will be in the order of the normal coherence length in the normal conducting barrier. Thus the width of the PMMA trenches is the critical parameter for the proximity coupling. 1,0;
•
-
i
i
I
T=20K
> ¢
0,5 • width of PMMA trench: 300nm ........... 250nrn . . . . . . 200nm ; .......... .( -0,0 / / //
O >
]
//
/. •
// /
/
....
0,2 T= 49 K f = 8,131GHz
n=;-
0,1
--
n~ n=4 n = 3--~
> n =1
t~ O >
0,0 ...... dB ..... 11dB ..... 9dB ..... 7dB ---4dB
-0,1
-0,2
,'
f
r
I'
.gIw
-0,4
i
i -0,2
i
i
I
0,0 Current
.;
-0,5
-1 ,.Q
the same dose of 8x1013/cm2. Proximity coupling across the implanted regions can be observed up to trench widths of 250 nm. Compared to unmodified structures the critical current density is reduced by more than two orders of magnitude. In the case of a 200 nm wide trench• supercurrents could be / observed up 74 K. Mlcrobridges which have been irradiated in a wider region exhibit resistive behavior in the whole temperature range.
I 0,2
I 0,4
[mA]
Figure 4. Current voltage curves at different microwave power levels (in dBm). The microwave frequency is 8.131 GHz.
,0
;
i
I
/
-0,5
,
I
0,0
i
0,5
,0
C u r r e n t [mA]
Figure 3. Influence of the width of the modified region in a microbridge on the current voltage characteristic.
In figure 3 the current voltage characteristics at 20 K is shown for three widths of the PMMAtrench. The structures have been irradiated with a
When exposed to microwave power modified microbridges exhibit current steps as shown in figure 4. These steps, commonly known as Shapiro steps, are totally absent before ion irradiation. Depending on the microwave power, clear steps up to the order n=8 were observed. They are noise rounded but appear at voltages Un=n~of consistent with the microwave frequency [13]. Shapiro steps could be measured in the range between 4 and 74 K. The response of the microbridges to microwave
410
R. Barth et al. / Microelectronic Engineering 30 (1996) 407-410
radiation gives unambiguous evidence for the Josephson nature of the structures investigated. In figure 4 Shapiro steps are visible for different levels of microwave power. Modified microbridges have been used to fabricate dc SQUIDs. We measured the flux to voltage transfer function dV/dO. Therefore the SQUIDs were current-biased and the corresponding dcvoltage drop across the SQUID was recorded as a function of the applied magnetic flux. Voltage modulations appear only at bias currents slightly above the critical current. Figure 5 gives the voltage modulation of a dc SQUID as a function of the applied magnetic flux at 200 ~tA bias current and 25 K. 0,353 . . . . . . . . . . . . . ......
,l°ot
0,352 0,351 >
E=_. 0,350 "5 > 0,349 T=25K IBias=2OOpA
0,348 n
I
m
a
.
I
.
n
-I0 -8 -6 -4 -2
.
I
0
.
=
2
,
t
4
.
I
6
•
I
8
,
I0
applied magnetic flux [arbitrary units] Figure 5. Voltage modulation of a dc SQUID as a function of the applied magnetic flux at 25 K.
3. CONCLUSION In conclusion we demonstrated the utilization of standard techniques which are well established in semiconductor technology for the modification of HTC microbridges. If the width of the modified region does not exceed 250nm the structures exhibit Shapiro steps under microwave irradiation. This gives clear evidence for the Josephson nature of these structures.
ACKNOWLEDGMENT The authors gratefully acknowledge C. Jaekel and R. Aguiar for stimulating discussions. This work was supported by the german BMBF under contract No 13N6402. REFERENCES 1. R.W. Simon, J.B. Bulman, J.F. Butch, S.B. Coons, K.P. Daly, W.D. Dozier, R. Hu, A.E. Lee, J.A. Luine, C.E. Platt, S.M. Schwarzbek, M.S. Wire and M.J. Zani, IEEE Trans. Magn. MAG-27 (1991) 3209. 2. D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoues Phys. Rev. Lett. 61 (1988) 219. 3. R.H. trio, J.A. Beall, M:W. Cromar, T.E. Harvey, M.E. Johansson, C.D. Reintsema and D.A. Rudman, Appl. Phys. Lett. 59 (1991) 1126. 4. J. Gao, W.A.M. Aarnink, G.J. Gerritsma and H. Rogalla, Physica C 171 (1990) 126. 5. B. Schwartz, P.M.Mankiewich, R.E. Howard, L.D. Jackel, B.L. Straughn, E.G. Burkhart and A.H. Dayem, IEEE Trans. Magn. MAG-25 (1989), 1298. 6. A.J. Pauza, A.M. Campell, D.F. Moore, R.E. Somekh and A.N. Broers, IEEE Trans. Appl. Supercond. 3 (1993) 2405. 7. S. Tolpygo, S. Shokhor, B. Nadgorny, A. Bourdillon, J.Y. Lin, S.Y. Hou, J.M. Phillips and M. Gurvitch, Appl. Phys. Lett. 63 (1993) 1696. 8. M.J. Zani, J.A. Luine, R.W. Simon and R.A. Davidheiser, Appl. Phys.Lett. 59 (1991) 234. 9. S.S. Tinchev, Supercond. Sci. Technol. 3 (1990) 500. 10.J. Auge, M. Jansen, H.G. Roskos and H. Kurz, Appl. Phys. Lett. 64 (1994) 3166. 11. J. Hollkott, R. Barth, J. Auge, B. Spangenberg, H. G. Roskos, and H. Kurz, Proceedings of the European Conference on Applied Superconductivity, 3-6 July 1995, in press. 12. P. Mazoldi and G.W. Arnold, Ion Beam Modification of Insulators, Elsevier, Amsterdam (1987). 13. A. Barone and G. Patemo, Physics and Applications of the Josephson Effect, John Wilex&Sons, New York (1982).
ELSEVIER
Microelectronic Engineering 30 (1996) 407-410
Electron Beam Lithography and Ion Implantation Techniques for Fabrication of High-Tc Josephson Junctions R. Barth, A.H. Hamidi, B. Hadam, J. Hollkott, D. Dunkmann, J. Auge, H. Kurz Institut fiir Halbleitertechnik II, Rheinisch-Westf'alische Technische Hochschule Aachen, SommerfeldstraBe 24, D-52074 Aachen, Germany
Established semiconductor process technologies are demonstrated to be suitable for the fabrication of high temperature superconductor (HTS) Josephson junctions. Single YBCO bridges were modified by local oxygen ion irradiation through a narrow slit in a PMMA mask which was formed by electron beam lithography. The influence of slit dimension and irradiation dose was investigated. The critical current and normal resistance oI the modified microbridges can be controlled by these two parameters. Proximity coupling across the modified region is observed up to a slit width of 250 nm. When exposed to microwave irradiation the microbridges exhibited Shapiro steps. In dc SQUIDs a voltage modulation as a function of an applied magnetic flux is observed.
1. INTRODUCTION Josephson junctions are the key elements of superconductor based electronics. Normally tunnel type junctions are used because of their higher quality and reproducibility compared to other types of weak links. As a consequence of the short coherence length of high temperature superconductors (HTS) and the rather complex growth techniques related with this materials the fabrication of HTS tunnel junctions is difficult. Therefore most work on HTS Josephson junctions has been concentrated on artificial induced grainboundaries in thin film bridges (e. g. step edge, bicrystal) [1, 2]. There are several advantages related with these junctions like simple preparation technique and the correlation between the junction properties and the grain boundary angle. On the other hand one has to deal with restrictions like the fixed grain boundary angle and geometrical aspects of the positioning of the junction within one chip. The fabrication of planar SNS proximity coupled junctions might be an alternative. There are different solutions for such devices [3-5]. Proximity coupling can be achieved across narrow trenches etched into a HTS bridge and subsequently filled with a noble metal like silver or gold [5]. These trenches can be realized easily by a combination of
electron beam lithography and dry etching. The main problem of this junction type is the quality of the interface between the superconductor and the metal. Monolithic SNS-junctions offer the possibility to overcome these problems. A normal conducting barrier is induced into the same HTS film by altering locally its electrical properties. Several authors report on excellent HTS junctions by direct electron beam modification. They suffer, however, from poor long term stability [6, 7]. Modifications by focused ion beam techniques [8] or ion irradiation using masks [9] are reported to give similar results with good stability. In this paper we describe the modification of HTS microbridges by irradiation with 100keV oxygen ions. The influence of the irradiation dose and width of the modified region on the electrical properties of microbridges will be discussed.
2. EXPERIMENTAL
The YBa2Cu307 (YBCO) films used in this work were prepared b y DC magnetron sputtering on (100) SrTiOa-substrates [10]. X-ray diffraction measurements (XRD) revealed that the c-axis of the YBCO films was perpendicular to the surface of the substrate. They also proved the epitaxial structure of the films. Critical temperatures T c up to 91 K are
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All fights reserved. S S D I 0167-9317(95)00274-X
408
R. Barth et al. / Microelectronic Engineering 30 (1996) 407-410
measured. The critical current density Jc at 77 K in zero magnetic field exceeded 106A/cm2. The thickness of the YBCO films was 60 rim. In a first step the degradation of T c upon ion exposure is studied. We concentrated on the implantation of oxygen ions with 100 keV energy. In this energy regime the mean projected range of the oxygen ions is larger than the film thickness. After implantation at different ion fluences we checked the temperature dependence of the electrical resistance. For an ion fluence in the range of 1012/cm 2 only a slight reduction in the transition temperature was observed while the specific resistance increased. In the fluence range of lxl013/cm 2 to lxl01/cm 2, T c of the samples was reduced significantly until the superconductivity disappeared (figure 1). After implantation with doses higher than 7x1013/cm 2 the samples showed semiconducting behavior and the resistance rose rapidly with decreasing temperature. The ion fluence appears to be very critical for the control of the specific resistance. i
70
x l O -1 k
60
l ",.
.
50 :
E ~
- -
\ X, X,
4O :30
:
as grown
........ 3x1013/om 2 ............. 6 x 1 0 1 3 / c m 2
",.. ......... 9x1013/cm 2
onto the YBCO film to protect it against any chemical degradation due to contact with the PMMA resist [11]. Within these experiments the ion fluence was kept constant at 1014/cm 2 and the resist thickness was increased from 200nm to 1 pro. The transition temperature of the samples was checked before and after ion implantation. Using PMMA masking layers thinner than 300 nm the superconductivity vanished. Increasing the resist thickness, a declination in the Tc reduction was observed. From these experiments we determined that a resist thickness of at least 500 nm will be necessary to prevent significant degradation of superconductivity in the masked regions. This experimental result is in good agreement with Monte Carlo based calculations [12]. In a final step the YBCO microbridges are modified by local oxygen ion implantation. The microbridges have been patterned by optical lithography and ion-beam etching. Consecutively 80 nm amorphous carbon are deposited and a layer of 500 nm PMMA is spin coated onto the sample. 10 nm wide lines are exposed by 35 keV electron beam lithography to ensure local ion implantation. The samples are developed in a solution of 1 : 3 Methyl Isobutyl Ketone (MIBK) : IsoPropyl Alcohol (IPA) in an ultrasonic bath for times around 5 s, and rinsed in IPA. The samples were blown dry with pure nitrogen. After preparing the masks, the samples were irradiated by oxygen ions with an energy of 100 keV.
i
20
:"
10 0
: 0
2O
40
60
Temperature
80
1 O0
[K]
Figure 1. Temperature dependence of the specific resistance on 60nm thick YBCO films after implantation with 100 keV oxygen ions.
In a second step a series of tests was performed to determine the minimum required resist thickness for effective protection of the YBCO film during irradiation with 100keV oxygen ions. A 80nm thick amorphous carbon layer has been deposited
Figure 2. SEM micrograph (60 °) of a typical 200 nm wide trench in a 500 nm thick PMMA layer. The pattern is e-beam written at 35 keV.
409
R. Barth et aL / Microelectronic Engineering 30 (1996) 407-410
Because of the forward scattering of electrons within the resist the minimal achievable width of the trenches depends on the PMMA thickness. A SEM image of a 200 nm wide trench in a 500 nm thick PMMA-layer is depicted in Figure 2. The exposure is done at 35 keV. The undercut of the resist profile due to forward scattering of the electrons is clearly visible. Increasing the electron energy up to 100 keV will reduce forward scattering. The influence of the slit width on the electrical properties of the modified microbridges (8x1013/cm2) has been investigated in the range between 500 nm and 200 nm. The irradiated region of the microbridge certainly exhibits normal conducting behavior at all temperatures as shown in figure 1. A supercurrent across these microbridges due to proximity coupling will only be observed if the dimensions of the irradiated region will be in the order of the normal coherence length in the normal conducting barrier. Thus the width of the PMMA trenches is the critical parameter for the proximity coupling. 1,0;
•
-
i
i
I
T=20K
> ¢
0,5 • width of PMMA trench: 300nm ........... 250nrn . . . . . . 200nm ; .......... .( -0,0 / / //
O >
]
//
/. •
// /
/
....
0,2 T= 49 K f = 8,131GHz
n=;-
0,1
--
n~ n=4 n = 3--~
> n =1
t~ O >
0,0 ...... dB ..... 11dB ..... 9dB ..... 7dB ---4dB
-0,1
-0,2
,'
f
r
I'
.gIw
-0,4
i
i -0,2
i
i
I
0,0 Current
.;
-0,5
-1 ,.Q
the same dose of 8x1013/cm2. Proximity coupling across the implanted regions can be observed up to trench widths of 250 nm. Compared to unmodified structures the critical current density is reduced by more than two orders of magnitude. In the case of a 200 nm wide trench• supercurrents could be / observed up 74 K. Mlcrobridges which have been irradiated in a wider region exhibit resistive behavior in the whole temperature range.
I 0,2
I 0,4
[mA]
Figure 4. Current voltage curves at different microwave power levels (in dBm). The microwave frequency is 8.131 GHz.
,0
;
i
I
/
-0,5
,
I
0,0
i
0,5
,0
C u r r e n t [mA]
Figure 3. Influence of the width of the modified region in a microbridge on the current voltage characteristic.
In figure 3 the current voltage characteristics at 20 K is shown for three widths of the PMMAtrench. The structures have been irradiated with a
When exposed to microwave power modified microbridges exhibit current steps as shown in figure 4. These steps, commonly known as Shapiro steps, are totally absent before ion irradiation. Depending on the microwave power, clear steps up to the order n=8 were observed. They are noise rounded but appear at voltages Un=n~of consistent with the microwave frequency [13]. Shapiro steps could be measured in the range between 4 and 74 K. The response of the microbridges to microwave
410
R. Barth et al. / Microelectronic Engineering 30 (1996) 407-410
radiation gives unambiguous evidence for the Josephson nature of the structures investigated. In figure 4 Shapiro steps are visible for different levels of microwave power. Modified microbridges have been used to fabricate dc SQUIDs. We measured the flux to voltage transfer function dV/dO. Therefore the SQUIDs were current-biased and the corresponding dcvoltage drop across the SQUID was recorded as a function of the applied magnetic flux. Voltage modulations appear only at bias currents slightly above the critical current. Figure 5 gives the voltage modulation of a dc SQUID as a function of the applied magnetic flux at 200 ~tA bias current and 25 K. 0,353 . . . . . . . . . . . . . ......
,l°ot
0,352 0,351 >
E=_. 0,350 "5 > 0,349 T=25K IBias=2OOpA
0,348 n
I
m
a
.
I
.
n
-I0 -8 -6 -4 -2
.
I
0
.
=
2
,
t
4
.
I
6
•
I
8
,
I0
applied magnetic flux [arbitrary units] Figure 5. Voltage modulation of a dc SQUID as a function of the applied magnetic flux at 25 K.
3. CONCLUSION In conclusion we demonstrated the utilization of standard techniques which are well established in semiconductor technology for the modification of HTC microbridges. If the width of the modified region does not exceed 250nm the structures exhibit Shapiro steps under microwave irradiation. This gives clear evidence for the Josephson nature of these structures.
ACKNOWLEDGMENT The authors gratefully acknowledge C. Jaekel and R. Aguiar for stimulating discussions. This work was supported by the german BMBF under contract No 13N6402. REFERENCES 1. R.W. Simon, J.B. Bulman, J.F. Butch, S.B. Coons, K.P. Daly, W.D. Dozier, R. Hu, A.E. Lee, J.A. Luine, C.E. Platt, S.M. Schwarzbek, M.S. Wire and M.J. Zani, IEEE Trans. Magn. MAG-27 (1991) 3209. 2. D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoues Phys. Rev. Lett. 61 (1988) 219. 3. R.H. trio, J.A. Beall, M:W. Cromar, T.E. Harvey, M.E. Johansson, C.D. Reintsema and D.A. Rudman, Appl. Phys. Lett. 59 (1991) 1126. 4. J. Gao, W.A.M. Aarnink, G.J. Gerritsma and H. Rogalla, Physica C 171 (1990) 126. 5. B. Schwartz, P.M.Mankiewich, R.E. Howard, L.D. Jackel, B.L. Straughn, E.G. Burkhart and A.H. Dayem, IEEE Trans. Magn. MAG-25 (1989), 1298. 6. A.J. Pauza, A.M. Campell, D.F. Moore, R.E. Somekh and A.N. Broers, IEEE Trans. Appl. Supercond. 3 (1993) 2405. 7. S. Tolpygo, S. Shokhor, B. Nadgorny, A. Bourdillon, J.Y. Lin, S.Y. Hou, J.M. Phillips and M. Gurvitch, Appl. Phys. Lett. 63 (1993) 1696. 8. M.J. Zani, J.A. Luine, R.W. Simon and R.A. Davidheiser, Appl. Phys.Lett. 59 (1991) 234. 9. S.S. Tinchev, Supercond. Sci. Technol. 3 (1990) 500. 10.J. Auge, M. Jansen, H.G. Roskos and H. Kurz, Appl. Phys. Lett. 64 (1994) 3166. 11. J. Hollkott, R. Barth, J. Auge, B. Spangenberg, H. G. Roskos, and H. Kurz, Proceedings of the European Conference on Applied Superconductivity, 3-6 July 1995, in press. 12. P. Mazoldi and G.W. Arnold, Ion Beam Modification of Insulators, Elsevier, Amsterdam (1987). 13. A. Barone and G. Patemo, Physics and Applications of the Josephson Effect, John Wilex&Sons, New York (1982).