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Preparation and lifetest of niobium Josephson junction tunnel diodes and arrays
Solid-State Electronics, 1974, Vol. 17, pp. 611~,15. Pergamon Press.
Printed in Great Britain
P R E P A R A T I O N A N D LIFETEST OF NIOBIUM JOSEPHSON JUNCTION T U N N E L DIODES A N D ARRAYS* PAUL RISSMANt Laboratorio di Cibernetica del C.N.R., Arco Felice, Napoli, Italy and THOMAS PALHOLMEN:~ Division of Electrical Engineering, Chalmers University of Technology, Gothenburg, Sweden (Received 22 June 1973; in revised form 5 November 1973)
Photoresist technology can be used to prepare superconductive tunnel junctions and arrays. Niobium thin films, which produce durable junctions, are chemically etched to form the base superconductor. Nb-NbOx-Pb junctions continue to function after more than 800 days of testing. The failure mechanism for these junctions is the decrease of conductance. The niobium photoresist technology is being used to investigate neuristor-type devices. Abstract
INTRODUCTION The use of Josephson tunnel junctions is predicated on the capability to produce durable devices and junctions arrays by simple fabrication techniques. Niobium has been shown to produce stable tunnel junctions [ I-5] when used as the base superconductor. It also can be processed by integrated circuit techniques to produce junction arrays [6]. The purpose of this paper is to supplement the data of Schwidtal[5] and Hoel, et al.[6] on the life of N b - N b O x - P b tunnel junctions and to describe a simplified procedure for fabrication of junction arrays with a high degree of junction definition. PHOTORESIST TECHNOLOGYFOR THE PREPARATION OF NIOBIUM JUNCTIONS
In [6] the niobium base film, prepared by RF sputter techniques, is covered by a second film of aluminium. Photoresist is applied to this second film and it is chemically etched to provide a mask for the sputter etching of the niobium film. Photoresist is then used to define the junction area, the *This work was supported in part by National Science Foundation Grant No. GK-12502. tOn research leave from the University of Wisconsin, Department of Electrical Engineering, Madison, Wisconsin, U.S.A. ~Work done while the author was on research leave at the University of Wisconsin, Madison, Wisconsin, U.S.A.
sample sputter cleaned, oxidized, and finally the top superconductor is evaporated. This method can also be used with a sputtered[7] or evaporated semiconductor [8]. The major drawbacks of the method described above are that it is both complex and dirty. The use of alminium film can lead to contamination of the niobium base layer. A simplified method has been used to produce N b - N b O ~ - P b and N b - N b O x - S n junctions. It will be described below in sufficient detail to permit duplication of the process by other laboratories. (1) Corning 7059 glass slides (1 x '~in.) are chemically cleaned by immerision in a bath of ammonia (NH4OH : H202 : H20; 1 : 1 : 5) at 80°C for 4 min and a bath of hydrochloric acid (HCI : H202 : H20; 1 : 5 : 25) at 80°C for 4 min, The substrates are then washed with di-ionized water and stored in methanol. (2) Just before RF sputter deposition of the niobium film, the substrates are dried with nitrogen gas. The niobium layers could be produced by electron beam evaporation and with ultra high vacuum techniques this would lead to higher purity films[9]. The samples here were prepared in an MRC dual cathode RF sputtering system. A thickness of approximately 1000 A was attained by sputtering twenty five minutes at 500 W of power with a MARZ grade niobium cathode. The substrate glass should not be sputter cleaned as is commonly done.
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Samples with sputter cleaned substrates could not be successfully etched. The niobium would flake off of the glass, presumably due to roughness of the substrate surface. (3) Immediately after removal from the vacuum system, S H I P L E Y AZ 1350 photoresist is applied to the samples and spun on for 30sec at 5000 rev/min. The samples are baked five minutes at 80°C and then the photoresist is exposed and developed. The samples are postbaked fifteen minutes at 130°C. (4) The niobium is etched in a solution of 25 ml of nitric acid (HNO3, 30%), 75 ml of hydrochloric acid ( H C L , 37%) and 7 ml of hydrofluoric acid (HF,, 50%). The etch rate is approximately 25 A per second, meaning that a 1000 .~ film will be etched in about 40 sec. The S H I P L E Y photoresist will hold for about 2 rain before lifting. K O D A K K T F R photoresist lifted after about fifteen seconds and therefore was unsuitable for use with this process. The sample is rinsed with DI water and the photoresist removed by cleaning with acetone and isQpropyl alcohol. Because there is hydrofluoric acid in the etch solution, some damage to the substrate glass will result. This can lead to difficulties in the top superconductor etch and should be minimized by removal of the sample from the acid bath as soon as the niobium is etched through. (5) The junction pattern is then defined by a K T F R photoresist step. The photoresist is applied and spun 12 sec at 6000 rev/min. It is baked 45 min at 80°C and then exposed and developed. The postbake is kept to a minimum (10min at 85°C) to inhibit niobium surface oxidation. Presumably S H I P L E Y positive type photoresist could also be used to define the junction geometry. (6) The junction surface must now be cleaned. Samples made without cleaning after the K T F R photoresist were open circuits with no tunneling behavior, indicating that the junction oxide was too thick. Samples have been cleaned by RF backsputtering 5rain at 200W in an atmosphere of argon. They can also be cleaned using an apparatus which is arranged for plasma oxidation [10]. Samples have been fabricated by bombarding the substrate with ions of oxygen at low power (300 V and 20 mA of current) for short periods of time (15-45 sec). This "ion cleaning" served the dual purpose of cleaning the surface and forming the junction oxide at the same time. (7) If the niobium surface was cleaned by RF backsputtering, the junction barrier must now be formed. It can be fabricated by plasma oxidation of the niobium[5, 11], by RF sputtering of a
semiconductor[7], by evaporation of a semiconductor [8], or by thermal oxidation. In the last case, the sample is oxidized in an atmosphere of dry oxygen on a hot plate at 130°C for 3-5 rain. (8) The top superconducting contact of tin or lead is then evaporated. Typically a third layer of photoresist (SHIPLEY-type) is applied to seal the junction from ambient humidity. In cases where complex junction geometry is used, this step defines a pattern for acid etching. In this case, baking times are kept to a minimum and only the low (80°C) temperature is used to inhibit change in the junction oxide. When the above steps have been followed, junctions with highly reproducible characteristics were obtained. I-V CHARACTERISTICS OF Nb-NbO~-Pb AND Nb-NbO~-Sn JUNCTIONS
Figures 1 and 2 show current-voltage characteristics of niobium-niobium oxide-lead and niobium-niobium oxide-tin junctions prepared by this technique. The junctions displayed in Fig. 1 were acid etched, sputter cleaned, thermally oxidized, as described in section 8 above, and then the lead layer evaporated. Those in Fig. 2 were acid etched, plasma oxidized-cleaned for 45 sec by the process described in section 6, and then the tin layer evaporated. The junctions in Fig. 1 have 95/~ m niobium lines which connect to circular contact pads 190 ~ m dia. The circular junction area defined by the photoresist window is 1.5 x 10 2 mm 2. Both junctions display similar I - V characteristics. Clearly evident is the anomalous negative resistance at the sum of the gap voltages which has been reported for R F sputter cleaned niobium junctions[12]. An attempt to measure the magnetic field dependence of the Josephson current did not yield the Bessel function of x over x pattern that is predicted for circular junctions [13]. This could be due to inhomogeneities of the junction interface. However the junction diameter, 140 ~ m, is about the same as the Josephson penetration depth, AI [14]. In this case the pattern will not be monotonic and not of the Bessel function form [ 15]. Certainly some of the non-regularity is due to interference effects and a non-uniform junction oxide. The same junction gives a remarkably regular pattern of self-induced Fiske steps, shown in Fig. 3116, 17]. In this case the steps are due to the earth's magnetic field as they are not evident when the sample was magnetically shielded. The junctions of niobium-oxide-tin shown in Fig. 2 are relatively large area (2-5 10-~-mm 2) and in this
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Fig. 1. Two niobium-niobium oxide-lead junctions on the same substrate. Niobium linewidth is 95/zm and junction area (circular) is 1"5 x 10 -z mm 2. Vertical scale: 10 mA/div. Horizontal scale: 1 mV/div. Temperature: 4"2°K. Fig. 2. Two niobium-niobium oxide-tin junctions. Vertical: junction 1-50 t~A/div. Junction 2-10 t~A/div. Horizontal: 0.5 mV/div. Temperature: approx. 3°K. (Difference in impedance is due to a change in junction area, not defined by photoresist.) [Facing p. 612
Fig. 3. Self-induced Fiske steps for junction 1 shown in Fig. 1. The steps disappear when the earth's magnetic field is shielded. Vertical: 0.5 mA/div. Horizontal: 0-1 mV/div. ; T = 4.2°K.
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Fig. 5.(a)l-Vcharacteristic for the Hoel device on day zero. Vertical: 5 mA/div. Horizontal: 1 mV/div. (b) 1 - V characteristics for the Hoel device on day 787. Vertical: junctions 1 and 4: 5 mA/div. Junctions 2 and 3:0"5 mA/div. Horizontal: 1 mV/div. (note slight change in oscilloscope calibration).
Fig. 7.(a) Neuristor strip line 0"2 m in length with linewidth of 150 tzm. (b) 1 - V characteristic for a Nb--NbOx-Sn strip line like the one in Fig. 2(a) made by the chemical etch process. Vertical: 10 mA/div. Horizontal: 1 mV/div; temperature, approx. 3°K.
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Fig. 8.(a) Josephson junction strip line 2 m in length with a line width o f 25/zm. (b) Detail of Fig. 8(a) shown at 36 x power. The niobium and lead lines are 75 p.m wide and the photoresist hole, shown light here, is one-third of that measure, and defines the junction.
Niobium Josephson junctions case the junction area is not defined by a photoresist hole. The difference in current level in the two junctions is due to different junction area. The absence of the anomalous bump in both junctions can be attributed to the reduction of surface damage when the low power oxidation cleaning procedure is used. It is presently thought that the presence or absence of the bump can be related to surface damage at the junction interface. This effect will be discussed m o r e thoroughly in a future publication [18].
JUNCTION LIFETIME
Matisoo [ 19] has shown that Josephson junctions can be switched in subnanosecond times with low power dissipation. Before switching circuits can be commercially produced, however, reliable devices which can be temperature cycled must be produced [20]. The data of Nordman[1] and Schwidtal[4] indicate that the failure mechanism for junctions of N b - N b O x - P b is the increase in junction conductance and eventual shorting. Shown in Fig. 4 is the data for the four junctions discussed in Hoel et al. [6] which has been extended to a period of more
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than 800 days. Note that all four junctions, after the initial time period, shown a decrease of junction conductance. The shape of the I - V characteristics has changed little over the period of more than two years, during which they were stored at room temperature on the laboratory shelf. This can be seen by examining Figs. 5(a) and (b), while noting that the current scale for junctions 2 and 3 in Fig. 5(b) has been amplified by a factor of 10. The discontinuity in the curves of Fig. 4 is due to a 15min, 100°C, baking step performed 787 days after the junctions were fabricated. The junctions were found to have developed parallel conductance which was significantly visible on the I - V characteristics of the high impedance junctions. The sample was recoated with S H I P L E Y AZ 1350 photoresist on day 786 and mildly baked. This improved the characteristics and prompted the further baking. Presumably the leakage current was caused by absorption of humidity in the junction defining photoresist layer, in this case K O D A K KPR.
Figure 6 shows the initial lifetest data for the niobium-niobium oxide-lead junctions shown in Fig. 1. As with the Hoel sample, after an initial increase in junction conductance, the junctions have
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Fig. 4. The data of Hoel, et al. [6] which has been extended to more than 800 days. Arrow indicates point of a 15 min, 100°C bake.
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PAUL RISSMAN and THOMAS PALHOLMEN
LINE WIDTH AREA
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Fig. 6. Lifetest data for the niobium-niobiumoxide-lead junctions shown in Fig. 1. shown a gradual decrease in conductance.* Note that the two junctions have been "tracking" exactly, which did not occur for the Hoel sample. The decrease of conductance could be explained by the diffusion of the oxide into the lead layer and gradual increase of the barrier width[l]. A more likely explanation, since the impedance change is not time but rather temperature cycle related, is that thermal stress causes increase in the barrier width [4]. This effect should be carefully examined by temperature cycling one junction while another is stored for a control. More durable devices can perhaps be made by changing the junction definition layer from KTFR- to SHIPLEY-type photoresist. Perhaps this would tend to equalize the stresses on the junction since the overcoat is also SHIPLEY. APPLICATIONSOF THE NIOBIUMTECHNOLOGY Present applications of this technology are for the investigation of neuristor type devices[21-24]. Devices of niobium-niobium oxide-tin which use the Giaever-type negative resistance to propagate pulses have been fabricated. Figure 7 shows the I - V characteristic for a strip line 0.2 m in length and 150~m wide. Long Josephson junction passive transmission *These junctions are especially durable since they continue to function, I - V characteristics unchanged, after accidentally being frozen in a dewar for three days!
lines are being fabricated with lead as the top layer[25]. The device shown in Fig. 8 is 2 m in length with a line width of 25/x m. It is hoped that propagation of pulses with many flux quanta can be observed on this line [26]. CONCLUSIONS
A process for fabrication of tunnel junctions and arrays which employ niobium as the base superconductor has been described. Although the junctions discussed here are of large dimensions, consideration should be given to the electron b e a m photoresist technology [27] which is presently being used to produce niobium Dayem bridges with dimensions of the order of 0.5/.~m[28]. This would enable fabrication of extremely small junctions, comparable to the 5 x 10-6 mm 2 area that has been reported [29]. The N b - N b O x - P b structures have nearly classic I - V characteristics, with low values of excess current below the gap voltages, and are extremely durable. Four junctions previously reported on [6] continue to function after 2 y r and more than 25 tests. The life of these devices, when subjected to constant operation and switching, should be investigated. The durability and reproducibility of these junctions is important for their use in many applications. The process described provides a means to utilize such junctions to attain structures of a high degree of complexity.
Niobium Josephson junctions T h i s is especially i m p o r t a n t f o r r e s e a r c h applications, s u c h as n e u r i s t o r s t r u c t u r e s , w h o s e s u c c e s s h a s b e e n limited b y t h e fallibility of soft m e t a l - t y p e junctions. Note added in proo[--The Hoel, et al. sample was recently tested on day 1091 (nearly 3 yr). All junctions continue to function with a high voltage conductance of 1.1, 0.075, 0-11, and 1.2 ~3 for junctions 1-4, respectively. Acknowledgements--The authors wish to thank Professor A. Barone and H. Guckel for their helpful suggestions. Gratitude is due to S. Reible for help in preparation of samples and C. Salinas for help in preparation of data.
REFERENCES 1. J. E. Nordman, J. appl. Phys. 40, 2111 (1%9). 2. L. O. Mullen and D. B. Sullivan, J. appl. Phys. 40, 2115 (1%9). 3. W. H. Keller and J. E. Nordman, J. appl. Phys. 42, 137 (1971). 4. K. Schwidtal, Bull. Am. Phys. Soc. Ser II, 16, 400 (1971). 5. K. Schwidtal, J. appl. Phys. 43, 202 (1972). 6. L. S. Hoel, W. H. Keller, J. E. Nordman and A. C. Scott, Solid-St. Electron. 15, 1167 (1972). 7. J. E. Nordman and W. H. Keller, J. appl. Phys. 44, 4732 (1973). 8. P. Rissman, J. appl. Phys. 44, 1893 (1973). 9. C. A. Neugebauer and R. A. Ekvall, J. appl. Phys. 35, 547 (1964).
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10. J. P. Pritchard and W. H. Schroen, Process for Preparation of Tunneling Barriers, U.S. Patent No. 3,673, 071, June 27, 1972. 11. R. Graeffe and T. Wiik, J. appl. Phys. 42, 2146 (1971). 12. J. E. Nordman and W. H. Keller, Phys. Letts 36A, 52 (1971). 13. J. Matisoo, J. appl. Phys. 40, 1813 (1969). 14. R. A. Ferrell and R. E. Prange, Phys. Rev. Letts 10, 479 (1%3). 15. K. Schwidtal, Phys. Rev. B2, 2526 (1970). 16. D. N. Langenberg, D. J. Scalapino, and B. N. Taylor, Proc. I E E E 54, 560 (1%6). 17. J. Matisoo, Phys. Letts 29A, 478 (1969). 18. J. E. Nordman, private communication, (1973). 19. J. Matisoo, Proc. Inst. Elect. Electron. Engrs. 55, 172 (1967). 20. T. D. Clark and J. P. Baldwin, Electron. Letts 3, 178 (1%7). 21. H. T. Yuan and A. C. Scott, Solid-St. Electron. 9, 1149 (1966). 22. R.D. Parmentier, Solid-St. Electron. 12, 287 (1969). 23. A. C. Scott, Solid-St. Electron. 7, 137 (1964). 24. R. D. Parmentier, Proc. Inst. Elect., Electron. Engrs. 58, 1829 (1970). 25. A. C. Scott, II Nuovo Cimento 69B, 241 (1970). 26. A. C. Scott, Active and Nonlinear Wave Propagation in Electronics, Chapter 5, Wiley, New York (1970). 27. M. Hatzakis, J. Electrochem. Soc. 116, 1033 (1%9). 28. R. B. Laibavitz, J. M. Viggiano, and M. Hatzakis, Int. Con. Detection and Emission of Electromagnetic Radiation with Josephson Junctions. Sept. 3-5, Perros-Guirec, France (1973). 29. W. Jutzi, T. H. O. Mohr, M. Gasser, and H. P. Gschwind, Electron. Letts 8, 591 (1972).
Printed in Great Britain
P R E P A R A T I O N A N D LIFETEST OF NIOBIUM JOSEPHSON JUNCTION T U N N E L DIODES A N D ARRAYS* PAUL RISSMANt Laboratorio di Cibernetica del C.N.R., Arco Felice, Napoli, Italy and THOMAS PALHOLMEN:~ Division of Electrical Engineering, Chalmers University of Technology, Gothenburg, Sweden (Received 22 June 1973; in revised form 5 November 1973)
Photoresist technology can be used to prepare superconductive tunnel junctions and arrays. Niobium thin films, which produce durable junctions, are chemically etched to form the base superconductor. Nb-NbOx-Pb junctions continue to function after more than 800 days of testing. The failure mechanism for these junctions is the decrease of conductance. The niobium photoresist technology is being used to investigate neuristor-type devices. Abstract
INTRODUCTION The use of Josephson tunnel junctions is predicated on the capability to produce durable devices and junctions arrays by simple fabrication techniques. Niobium has been shown to produce stable tunnel junctions [ I-5] when used as the base superconductor. It also can be processed by integrated circuit techniques to produce junction arrays [6]. The purpose of this paper is to supplement the data of Schwidtal[5] and Hoel, et al.[6] on the life of N b - N b O x - P b tunnel junctions and to describe a simplified procedure for fabrication of junction arrays with a high degree of junction definition. PHOTORESIST TECHNOLOGYFOR THE PREPARATION OF NIOBIUM JUNCTIONS
In [6] the niobium base film, prepared by RF sputter techniques, is covered by a second film of aluminium. Photoresist is applied to this second film and it is chemically etched to provide a mask for the sputter etching of the niobium film. Photoresist is then used to define the junction area, the *This work was supported in part by National Science Foundation Grant No. GK-12502. tOn research leave from the University of Wisconsin, Department of Electrical Engineering, Madison, Wisconsin, U.S.A. ~Work done while the author was on research leave at the University of Wisconsin, Madison, Wisconsin, U.S.A.
sample sputter cleaned, oxidized, and finally the top superconductor is evaporated. This method can also be used with a sputtered[7] or evaporated semiconductor [8]. The major drawbacks of the method described above are that it is both complex and dirty. The use of alminium film can lead to contamination of the niobium base layer. A simplified method has been used to produce N b - N b O ~ - P b and N b - N b O x - S n junctions. It will be described below in sufficient detail to permit duplication of the process by other laboratories. (1) Corning 7059 glass slides (1 x '~in.) are chemically cleaned by immerision in a bath of ammonia (NH4OH : H202 : H20; 1 : 1 : 5) at 80°C for 4 min and a bath of hydrochloric acid (HCI : H202 : H20; 1 : 5 : 25) at 80°C for 4 min, The substrates are then washed with di-ionized water and stored in methanol. (2) Just before RF sputter deposition of the niobium film, the substrates are dried with nitrogen gas. The niobium layers could be produced by electron beam evaporation and with ultra high vacuum techniques this would lead to higher purity films[9]. The samples here were prepared in an MRC dual cathode RF sputtering system. A thickness of approximately 1000 A was attained by sputtering twenty five minutes at 500 W of power with a MARZ grade niobium cathode. The substrate glass should not be sputter cleaned as is commonly done.
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Samples with sputter cleaned substrates could not be successfully etched. The niobium would flake off of the glass, presumably due to roughness of the substrate surface. (3) Immediately after removal from the vacuum system, S H I P L E Y AZ 1350 photoresist is applied to the samples and spun on for 30sec at 5000 rev/min. The samples are baked five minutes at 80°C and then the photoresist is exposed and developed. The samples are postbaked fifteen minutes at 130°C. (4) The niobium is etched in a solution of 25 ml of nitric acid (HNO3, 30%), 75 ml of hydrochloric acid ( H C L , 37%) and 7 ml of hydrofluoric acid (HF,, 50%). The etch rate is approximately 25 A per second, meaning that a 1000 .~ film will be etched in about 40 sec. The S H I P L E Y photoresist will hold for about 2 rain before lifting. K O D A K K T F R photoresist lifted after about fifteen seconds and therefore was unsuitable for use with this process. The sample is rinsed with DI water and the photoresist removed by cleaning with acetone and isQpropyl alcohol. Because there is hydrofluoric acid in the etch solution, some damage to the substrate glass will result. This can lead to difficulties in the top superconductor etch and should be minimized by removal of the sample from the acid bath as soon as the niobium is etched through. (5) The junction pattern is then defined by a K T F R photoresist step. The photoresist is applied and spun 12 sec at 6000 rev/min. It is baked 45 min at 80°C and then exposed and developed. The postbake is kept to a minimum (10min at 85°C) to inhibit niobium surface oxidation. Presumably S H I P L E Y positive type photoresist could also be used to define the junction geometry. (6) The junction surface must now be cleaned. Samples made without cleaning after the K T F R photoresist were open circuits with no tunneling behavior, indicating that the junction oxide was too thick. Samples have been cleaned by RF backsputtering 5rain at 200W in an atmosphere of argon. They can also be cleaned using an apparatus which is arranged for plasma oxidation [10]. Samples have been fabricated by bombarding the substrate with ions of oxygen at low power (300 V and 20 mA of current) for short periods of time (15-45 sec). This "ion cleaning" served the dual purpose of cleaning the surface and forming the junction oxide at the same time. (7) If the niobium surface was cleaned by RF backsputtering, the junction barrier must now be formed. It can be fabricated by plasma oxidation of the niobium[5, 11], by RF sputtering of a
semiconductor[7], by evaporation of a semiconductor [8], or by thermal oxidation. In the last case, the sample is oxidized in an atmosphere of dry oxygen on a hot plate at 130°C for 3-5 rain. (8) The top superconducting contact of tin or lead is then evaporated. Typically a third layer of photoresist (SHIPLEY-type) is applied to seal the junction from ambient humidity. In cases where complex junction geometry is used, this step defines a pattern for acid etching. In this case, baking times are kept to a minimum and only the low (80°C) temperature is used to inhibit change in the junction oxide. When the above steps have been followed, junctions with highly reproducible characteristics were obtained. I-V CHARACTERISTICS OF Nb-NbO~-Pb AND Nb-NbO~-Sn JUNCTIONS
Figures 1 and 2 show current-voltage characteristics of niobium-niobium oxide-lead and niobium-niobium oxide-tin junctions prepared by this technique. The junctions displayed in Fig. 1 were acid etched, sputter cleaned, thermally oxidized, as described in section 8 above, and then the lead layer evaporated. Those in Fig. 2 were acid etched, plasma oxidized-cleaned for 45 sec by the process described in section 6, and then the tin layer evaporated. The junctions in Fig. 1 have 95/~ m niobium lines which connect to circular contact pads 190 ~ m dia. The circular junction area defined by the photoresist window is 1.5 x 10 2 mm 2. Both junctions display similar I - V characteristics. Clearly evident is the anomalous negative resistance at the sum of the gap voltages which has been reported for R F sputter cleaned niobium junctions[12]. An attempt to measure the magnetic field dependence of the Josephson current did not yield the Bessel function of x over x pattern that is predicted for circular junctions [13]. This could be due to inhomogeneities of the junction interface. However the junction diameter, 140 ~ m, is about the same as the Josephson penetration depth, AI [14]. In this case the pattern will not be monotonic and not of the Bessel function form [ 15]. Certainly some of the non-regularity is due to interference effects and a non-uniform junction oxide. The same junction gives a remarkably regular pattern of self-induced Fiske steps, shown in Fig. 3116, 17]. In this case the steps are due to the earth's magnetic field as they are not evident when the sample was magnetically shielded. The junctions of niobium-oxide-tin shown in Fig. 2 are relatively large area (2-5 10-~-mm 2) and in this
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Fig. 1. Two niobium-niobium oxide-lead junctions on the same substrate. Niobium linewidth is 95/zm and junction area (circular) is 1"5 x 10 -z mm 2. Vertical scale: 10 mA/div. Horizontal scale: 1 mV/div. Temperature: 4"2°K. Fig. 2. Two niobium-niobium oxide-tin junctions. Vertical: junction 1-50 t~A/div. Junction 2-10 t~A/div. Horizontal: 0.5 mV/div. Temperature: approx. 3°K. (Difference in impedance is due to a change in junction area, not defined by photoresist.) [Facing p. 612
Fig. 3. Self-induced Fiske steps for junction 1 shown in Fig. 1. The steps disappear when the earth's magnetic field is shielded. Vertical: 0.5 mA/div. Horizontal: 0-1 mV/div. ; T = 4.2°K.
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Fig. 5.(a)l-Vcharacteristic for the Hoel device on day zero. Vertical: 5 mA/div. Horizontal: 1 mV/div. (b) 1 - V characteristics for the Hoel device on day 787. Vertical: junctions 1 and 4: 5 mA/div. Junctions 2 and 3:0"5 mA/div. Horizontal: 1 mV/div. (note slight change in oscilloscope calibration).
Fig. 7.(a) Neuristor strip line 0"2 m in length with linewidth of 150 tzm. (b) 1 - V characteristic for a Nb--NbOx-Sn strip line like the one in Fig. 2(a) made by the chemical etch process. Vertical: 10 mA/div. Horizontal: 1 mV/div; temperature, approx. 3°K.
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Fig. 8.(a) Josephson junction strip line 2 m in length with a line width o f 25/zm. (b) Detail of Fig. 8(a) shown at 36 x power. The niobium and lead lines are 75 p.m wide and the photoresist hole, shown light here, is one-third of that measure, and defines the junction.
Niobium Josephson junctions case the junction area is not defined by a photoresist hole. The difference in current level in the two junctions is due to different junction area. The absence of the anomalous bump in both junctions can be attributed to the reduction of surface damage when the low power oxidation cleaning procedure is used. It is presently thought that the presence or absence of the bump can be related to surface damage at the junction interface. This effect will be discussed m o r e thoroughly in a future publication [18].
JUNCTION LIFETIME
Matisoo [ 19] has shown that Josephson junctions can be switched in subnanosecond times with low power dissipation. Before switching circuits can be commercially produced, however, reliable devices which can be temperature cycled must be produced [20]. The data of Nordman[1] and Schwidtal[4] indicate that the failure mechanism for junctions of N b - N b O x - P b is the increase in junction conductance and eventual shorting. Shown in Fig. 4 is the data for the four junctions discussed in Hoel et al. [6] which has been extended to a period of more
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than 800 days. Note that all four junctions, after the initial time period, shown a decrease of junction conductance. The shape of the I - V characteristics has changed little over the period of more than two years, during which they were stored at room temperature on the laboratory shelf. This can be seen by examining Figs. 5(a) and (b), while noting that the current scale for junctions 2 and 3 in Fig. 5(b) has been amplified by a factor of 10. The discontinuity in the curves of Fig. 4 is due to a 15min, 100°C, baking step performed 787 days after the junctions were fabricated. The junctions were found to have developed parallel conductance which was significantly visible on the I - V characteristics of the high impedance junctions. The sample was recoated with S H I P L E Y AZ 1350 photoresist on day 786 and mildly baked. This improved the characteristics and prompted the further baking. Presumably the leakage current was caused by absorption of humidity in the junction defining photoresist layer, in this case K O D A K KPR.
Figure 6 shows the initial lifetest data for the niobium-niobium oxide-lead junctions shown in Fig. 1. As with the Hoel sample, after an initial increase in junction conductance, the junctions have
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Fig. 4. The data of Hoel, et al. [6] which has been extended to more than 800 days. Arrow indicates point of a 15 min, 100°C bake.
614
PAUL RISSMAN and THOMAS PALHOLMEN
LINE WIDTH AREA
95;,m
1.5x10"2 mm 2
,;o
2;0 TIME
3;°
IDAYS)
Fig. 6. Lifetest data for the niobium-niobiumoxide-lead junctions shown in Fig. 1. shown a gradual decrease in conductance.* Note that the two junctions have been "tracking" exactly, which did not occur for the Hoel sample. The decrease of conductance could be explained by the diffusion of the oxide into the lead layer and gradual increase of the barrier width[l]. A more likely explanation, since the impedance change is not time but rather temperature cycle related, is that thermal stress causes increase in the barrier width [4]. This effect should be carefully examined by temperature cycling one junction while another is stored for a control. More durable devices can perhaps be made by changing the junction definition layer from KTFR- to SHIPLEY-type photoresist. Perhaps this would tend to equalize the stresses on the junction since the overcoat is also SHIPLEY. APPLICATIONSOF THE NIOBIUMTECHNOLOGY Present applications of this technology are for the investigation of neuristor type devices[21-24]. Devices of niobium-niobium oxide-tin which use the Giaever-type negative resistance to propagate pulses have been fabricated. Figure 7 shows the I - V characteristic for a strip line 0.2 m in length and 150~m wide. Long Josephson junction passive transmission *These junctions are especially durable since they continue to function, I - V characteristics unchanged, after accidentally being frozen in a dewar for three days!
lines are being fabricated with lead as the top layer[25]. The device shown in Fig. 8 is 2 m in length with a line width of 25/x m. It is hoped that propagation of pulses with many flux quanta can be observed on this line [26]. CONCLUSIONS
A process for fabrication of tunnel junctions and arrays which employ niobium as the base superconductor has been described. Although the junctions discussed here are of large dimensions, consideration should be given to the electron b e a m photoresist technology [27] which is presently being used to produce niobium Dayem bridges with dimensions of the order of 0.5/.~m[28]. This would enable fabrication of extremely small junctions, comparable to the 5 x 10-6 mm 2 area that has been reported [29]. The N b - N b O x - P b structures have nearly classic I - V characteristics, with low values of excess current below the gap voltages, and are extremely durable. Four junctions previously reported on [6] continue to function after 2 y r and more than 25 tests. The life of these devices, when subjected to constant operation and switching, should be investigated. The durability and reproducibility of these junctions is important for their use in many applications. The process described provides a means to utilize such junctions to attain structures of a high degree of complexity.
Niobium Josephson junctions T h i s is especially i m p o r t a n t f o r r e s e a r c h applications, s u c h as n e u r i s t o r s t r u c t u r e s , w h o s e s u c c e s s h a s b e e n limited b y t h e fallibility of soft m e t a l - t y p e junctions. Note added in proo[--The Hoel, et al. sample was recently tested on day 1091 (nearly 3 yr). All junctions continue to function with a high voltage conductance of 1.1, 0.075, 0-11, and 1.2 ~3 for junctions 1-4, respectively. Acknowledgements--The authors wish to thank Professor A. Barone and H. Guckel for their helpful suggestions. Gratitude is due to S. Reible for help in preparation of samples and C. Salinas for help in preparation of data.
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