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Niobium superconductive tunnel diode integrated circuit arrays
So/id-State Nrwonics,
1972, Vol.
15, pp. 1167-I 173. PergamonPress. Printedin Great Britain
NIOBIUM
SUPERCONDUCTIVE
INTEGRATED
CIRCUIT
TUNNEL
DIODE
ARRAYS*
L. S. HOEL, W. H. KELLER, J. E. NORDMAN and A. C. SCOTT Department of Electrical Engineering, The University of Wisconsin, Madison, Wisconsin, U.S.A (Received
27 December
197 1; in revisedform
22 February
1972)
Abstract-A technology is described for building arrays of niobium based thin film superconductive tunnel junctions. Both Nb-Nb oxide-Sn negative resistance devices and Nb-Nb oxide-Pb Josephson devices were fabricated using a modified photoresist-sputter etching technique to make patterns in the Nb film. Good control over the volt-ampere characteristics of the Josephson junctions is demonstrated on single devices and is further illustrated by the use of this process to fabricate multistate oscillator arrays. 1. INTRODUCTION
of niobium as a base film for Josephson junctions is desirable because it can produce stable and reliable devices THE
USE
[ 12- 141. In Section 4, the theory of the multi-state oscillator array is briefly reviewed. The design and the operating characteristics of a (4 X 4) or 16 element array are described in Section 5. In Section 6, some other possible applications for the niobium photoresist technology are indicated. 2. NIOBIUM PHOTORESIST In this section
we describe
TECHNOLOGY in detail the fabrica-
tion procedures for Nb-NbO-Sn and Nb-NbOPb tunnel junctions in which the geometry of the niobium layer is specified by standard photolithographic techniques (contact printing through high resolution photographic plates). The major items 1167
L. S. HOEL, W. H. KELLER, J. E. NORDMAN and A. C. SCOTT
1168
of equipment
used in this research are:
(cl The aluminum is removed from exposed parts
(1) R.f. Sputtering
unit, 1000 W with adjustable bias and both Nb and Al 5 in. cathodes (MRCMARZ grade). Cathode to substrate spacing is about two inches. The substrate platform rotates under the targets. Substrate platform and cathodes are water cooled. (2) Photoresist spinner. (3) Mask aligner. (4) Evaporator for Sn and Pb (- lo-” Tort). The processing with reference Fig. 1.
steps can be most easily explained to the corresponding diagrams in
AL Nb KPR Nb
AZ-
1350
AI Nb NbO
The above steps will produce a pattern in the Nb without any apparent degradation in superconducting transition temperature or resistance ratio. The following steps indicate the way in which our tunnel junctions and arrays were made using either tin or lead top layers. Sputter cleaning steps described in parts (d) and (f) are crucial for obtaining repeatable results.
KPR
(b)
Nb Nb
/
using a phosphoric-acetic-nitric etch (77 crn:l 85% H,PO,; 15 cm” 36% acetic; 3 cm” 70% HN03; 5 cm” DI-H,O) at 55°C. After etching, the Nb surface should appear smooth and bright. The remaining photoresist is removed with acetone and the sample is cleaned with DI water and isopropylalcohol. (4 The exposed niobium is completely removed by sputter etching at 300 W for about 20 min. Some aluminum should remain after sputter etching and it is then again removed chemically as in (c). The Nb surface is now covered with a thick oxide which must be removed by sputter etching at 200 W for 6 min.
Substrate
(f) I
NbO
Cc) A( Nb
w
CdI
Fig. 1. Steps in photoresist
fabrication junctions.
of niobium
tunnel
(a) A chemically clean substrate of Corning 7059 glass is mildly sputter cleaned for about 2 min at 100 W. Then niobium is sputtered at 500 W for 25 min after a 10 min presputter. This produces a film about 1000 A thick. Without opening the system, aluminum is sputtered for at least 40 min at 500 W after a 10 min presputter. The substrate is protected by a shield during presputter. Film thicknesses are not critical, however the Al film should be significantly thicker than the Nb film. (b) Photoresist (SHIPLEY AZ-1350) is applied, spun for 30 set at 3000 revlmin and exposed and developed after a 5 min bake at 80°C.
(e) KPR photoresist is applied and spun for 15 set at 8000rev/min. It is exposed and developed in the usual way. (f) The sample is sputter cleaned at 50 W for 3 min. It is then oxidized in dry oxygen for about 5 min on a hot plate preheated to 150°C. The way in which a barrier is formed can be varied. _One can also use wet oxidation, or a deposited semiconductor [ 151. (g) Tin or lead is evaporated in the sample immediately after oxidation. Note that KPR serves as an insulator. An additional layer of KPR can be applied to protect the top layer against ambient humidity. Although we have not used photo-etching techniques to form the top lead or tin pattern one could do so using AZ 1350 again. 3. JUNCTION PROPERTIES The volt-ampere characteristic of a typical crossed-strip Nb-NbOx-Sn junction is shown in Fig. 2(a). This can be compared with a similar characteristic [Fig. 2(b)] of a device with junction area determined by the photoresist step (step e) as discussed in the previous section. The superconducting gaps of niobium and tin are sufficiently
I mV/cm
I
(b) Cd
Fig. 2. Comparison of V-Z characteristics of Nb-NbO-Sn junctions made with and without the photoresist steps. Area equals 2 X lo-* mm*. (a) Without photoresist (T = 3,3”K), (b) with photoresist (T = 3.35”K).
1 mV/cm0
DAYS
l4.P*K)
21 DAYS 279
Fig. 3. Volt-ampere
characteristics
DAYS
vs. lifetime for Nb-NbO-Pb 2 X IO-*mm*.
junctions.
Area equals
[Facing page 1168 ]
Fig. 7. Nb-NbO-Pb
multimode array. Substrate dimensions are 1 X $ in.
Fig. 9. Multimode array with bias resistor limited to two edges.
Fig. 10. Detail of the junction geometry in the multimode array.
ORIENTATION
FIRST
53 57 64 74 70 07
SECOND
0
5 p
seckm
ORIENTATION
-
Fig. Il. Multimode oscillations at various bias currents and orientations with respect to the laboratory magnetic field.
0.2
seclcm
d
Fig. 12. Multistate oscillation of the array obtained by a transient magnetic field. Two sweeps are presented for each state.
NIOBIUM
SUPERCONDUCTIVE
different to give a distinct negative conductance region of the type described by Giaever [ 161. With the photoresist step the characteristic is slightly more rounded, but the two curves have essentially the same shape. The higher current level and emergence of Josephson [ 171 current in the characteristic of Fig. 2(a) is due to a slightly thinner oxide layer and larger junction area. The Pb junctions described below have virtually identical characteristics to those made without photoresist processing. The superconducting energy gaps of niobium and lead are close enough together to make a Giaever type negative conductance difficult to observe for Nb-NbOx-Pb devices. However, by reducing the oxidation time Josephson type characteristics are readily obtained. Since these devices have a thinner oxide layer, they would be more interesting to investigate for reliability under repeated testing. One such test is indicated in Fig. 3. Four Nb-NbOx-Pb junctions were made on the same substrate with equal areas (2 X lo-’ mm2) defined by the photoresist step as described in the previous section. Two of the junctions (No. 1 and No. 3) have niobium leads of 10 pm in width and the other two (No. 2 and No. 4) have 100 pm leads. After the initial test (day 0) the entire surface was coated with a photoresist layer to prevent deterioration of the lead film. Between tests the junctions were stored in a laboratory dessicator and at room temperature. Volt-ampere characteristics measured at later times are shown in Fig. 3. The normal conductance of these junctions (high voltage slope of the characteristic) is plotted vs. time in Fig. 4. They exhibit a reasonable degree of stability over
Fig. 4. Normal conductance
TUNNEL
DIODE
1169
the test period. Hundreds of junctions and several multimode oscillators have been made using this photoetch fabrication technique. It was always possible to trace an incorrect processing step for niobium-lead junctions which did not function properly. The niobium-tin junctions were somewhat less reliable. Virtually all of the devices made using r.f. sputtered niobium films show an anomalous excess current just above the niobium energy gap[ 181. This appears to be caused by a proximity effect in the niobium surface. It will be discussed in detail in another publication [ 191. When biased as is indicated in Fig. 5(a), a Josephson junction should undergo a relaxation oscillation the frequency of which is controlled by the time constants associated with the inductor L[20]. This effect is readily observed for the Nb-NbOxPb Josephson junctions as indicated in Figs. S(b) and 5(c). Notice that the frequency increases by about an order of magnitude as inductance is decreased from that of the leads of a bias resitor external to the dewar to that of the inductance of a small nichrome bias resistor mounted close to the junction (L = I FH). To initiate the oscillation the bias current (I,) must exceed the zero voltage current of the Josephson junction. With such bias the junction appears to be an effective negative conductance. 4. REVIEW
OF MULTISTATE OSCILLATOR ARRAY THEORY
The multistate oscillator array is an example of a new class of circuits which presents interesting possibilities for large scale integration. The theory
vs. lifetime for the junctions
of Fig. 3.
L. S. HOEL, M. H. KELLER, J. E. NORDMAN
1170
(a)
E
50pseckm (b)
5psec/cm (c)
Fig. 5. Josephson junction relaxation oscillator. (a) Equivalent circuit, (b) oscillation with inductance due to external
bias leads, (c) oscillation
for L =
1PH.
has been considered in several recent publications [ 12-141; here we shall confine ourselves to a brief survey of the salient concepts. Consider first the linear and lossless two dimensional array shown in Fig. 6(a). This circuit is the electrical analog of a square mechanical membrane which is fixed at its four edges. If there are N unit cells (capacitors) in the array, there will be N modes of oscillation. For each mode the voltage will vary with position and time as u = V sin psx sin &y cos (wt + $) .
(1)
The propagation constants, PI and &,, in equation (1) must satisfy the requirement that the length of one side of the array (A) be an integral number of half wavelengths. Thus each mode is specified by a point in reciprocal space (fix - pU space) where ps and & satisfy
and A. C. SCOTT
two half wavelengths in the x-direction (m = 2) and one half wavelength in the y-direction (n = I) is sketched in Fig. 6(b) and the corresponding mode Point in reciprocal space is indicated. The number of mode points in reciprocal space is just equal to (A/a)* which is the number of unit cells in the array. Such a linear and lossless array can be used to store information. One bit of information is stored in each mode by either exciting or not exciting that mode. Thus, an array with N unit cells can store N bits of information. It is important to notice, however, that this information is distributed in real space but localized in reciprocal space. If the information was stored in an array of N switches it would be localized in real space and distributed in reciprocal space. Thus in the array of Fig. 6 we have a simple model for a distributed memory. Any practical linear circuit is only approximately lossless at non zero frequency. Thus the array in Fig. 6 could only store information for a time of the order of the decay times of its modes. To remedy this defect we can suppose that nonlinear elements are placed in parallel with the capacitors in order to supply power to the excited modes and maintain their amplitudes at constant values. The modes will tend to stabilize at amplitudes for which the power input from the nonlinear element is equal to the power being lost in the linear dissipation. One of the main objectives of the nonlinear analysis of this problem is to determine how the modes interact with each other in the presence of the nonlinearity. On the linear array 2” mode combinations or states can be excited. It is of particular interest to determine how many of these can be stably excited on the nonlinear array. This question has been investigated in some detail for the case of a conductive nonlinear element as shown in Fig. 2(b)[13]. The analysis of the case when the nonlinear element is a Josephson junction has not yet been carried out. However, the experimental observations in the following section indicate that the qualitative behavior is quite similar in the two cases. 5. NIOBIUM INTEGRATED CIRCUIT OSCILLATOR ARRAY We have built and tested several niobium integrated circuit oscillator arrays similar to that shown in Fig. 7. The corresponding equivalent
and m and IZare integers. For example, the spatial variation of the voltage for the mode which has
circuit for the array is given in Fig. 8. The inductor pattern is formed in the initial niobium layer. A
NIOBIUM SUPERCONDUCTIVETUNNEL
(b)
DIODE
1171
(cl
Fig. 6. (a) Linear multimode array, (b) location of mode points in reciproCal space, (c) spatial variation of a single mode.
lead layer provides the other contacts for the NbNbO-Pb junctions and serves as a ground blane. An aluminum film provides a low inductance bias resistance around the edge of the array. This bias resistance can be either completely around the edge, as indicated in Figs. 7 and 8, or around some restricted portion of the edge as indicated in Fig. 9. A magnified view of one of the junctions is shown in Fig. 10. This photograph was taken after development of the KPR while delineates the junction area (step e in Section 2 and Fig. 1). The junction diameter is 6.25 mil or 156 pm. The maximum zero voltage current is about 4 mA. Assuming an oxide thickness of 30 A and a dielectric constant of 12, the junction capacitance is about 700pF. The series inductance between diodes is
that of a transmission line of 1 pm in height (the thickness of the KPR), 0.5 mil or 12 pm wide, and 1.78 cm long. The corresponding inductance is about 1.8 nH. These arrays readily oscillate when the bias current is raised above the sum of the maximum Josephson currents for the sixteen individual junctions. The maximum current is inferred from that of a test junction on the same substrate. This oscillation is clearly multimode in character. Furthermore, the state of the oscillation depends upon the orientation of the array with respect to the residual magnetic fields in the laboratory. For example, in Fig. 11 the signal output waveforms are recorded at various levels of bias current for two different orientations of the array, The waveforms are single sweep with a bandwidth of 500 kc.
1172
L. S. HOEL,
M. H. KELLER,
J. E. NORDMAN
Junction capoclty
and A. C. SCOTT Nb- NbO-Pb
tunnel
-Ground plane ( lead film) Bias resistors (Aluminium film)
d
iianal
Bias
._~ outaut
current
Fig. 8. Equivalent
circuit
For these measurements the bias resistor was only on the two edges adjacent to the output edge as in Fig. 9. The array can be switched from one state of oscillation to another by moving the field of a small magnet (about I OG) near the dewar. Fig. 12 shows about nine different oscillatory states of the array which were obtained this way at a constant current bias of 100 mA. 6. OTHER
may niobium problems are listed ences are It
for multimode
array
ture cycling and room temperature further study.
storage deserves
6.2 Flux quantum shuttle Anderson has suggested the possibility of information processing systems in which bits of information are carried by individual flux quanta [23]. An elementary shift register employing this principle and fabricated by plasma oxidation of tin has recently been described [24].
APPLICATIONS
be interesting and useful to apply the integrated circuit technology to other in applied science. Several possibilities below and appropriate background referindicated.
6.1 Tunneling cryotron This computing device, originally suggested by Matisoo [21], uses the Josephson zero voltage current in its operation. Anacker has shown that the tunneling cryotron has considerable potential as the basic logic element in large capacity information processing systems [22]. He notes, however, that the problem of device stability during tempera-
6.3 Superconductive neuristor This device used Giaever type tunneling to achieve an electronic analog of a nerve axon. It has been discussed in considerable detail from a theoretical point of view[7,8,25,26], but the experimental work has lagged because of the difficulty in making tin-lead devices of large area. We feel that the niobium technology now makes such experiments much more interesting. 6.4 Josephson trnnsmission lines Quasi-linear equations have been developed to describe the envelope dynamics of a pulse containing many flux quanta on a Josephson transmission
NIOBIUM
SUPERCONDUCTIVE
line [27]. Stationary propagation of pulses with many flux quanta on a biased line with losses has also been considered[28,29]. Experimental observation of these effects would be most interesting. 6.5 Large circular Josephson junctions In a junction with dimensions large compared with the Josephson penetration depth (AJ = O.l0.0 I mm [ 1O]), the normalized flux obeys the two dimensional Sine-Gordon equation
Exact solutions of this equation can be obtained for circular boundary conditions[30]. These solutions should be studied experimentally. REFERENCES 1. J. E. Nordman, J. uppl. Phys. 40,2 I 11 (I 969). 2. L. 0. Mullen and D. B. Sullivan, J. appl. Phys. 40, 21 15 (1969). 3. W. H. Keller and J. E. Nordman, J. appl. Phys. 42, 137(1971). 4. K. Schwidtal, Bull. Am. Phys. Sot.. Ser. II, 16, 400 (1971). 5. W. Schroen, J. uppl. Phys. 39,267 1 (1968). 6. J. P. Pritchard, Jr., and W. H. Schroen, Truns. fnsf. Elec. Electron Enars. MAC-4, 320 (1968). 7. H. T. Yuan and ‘A. C. Scott. Solid-St.’ Electron. 9, 1149(1966). Solid-St. Electron. 12,287 (1969). 8. R. D. Parmentier,
TUNNEL
DIODE
1173
9. A. C. Scott and W. J. Johnson, Appl. Phys. Lett. 14, 316(1969). and A. Barone, J. uppl. Phys. 41, 10. W. J. Johnson 2958 (1970). 11. A. Barone. J. awl. Phvs. 42.2747 (197 1). kgrs. CT-17, 12. A. C. Scott, Trans. Inst. Elec. Ele&n 55 (1970). 13. A. C. Scott, Trans. Inst. Elm. Electron Engrs. SMC-1, 267 (1971). Trans. Inst. Elec. Electron Engr. 14. R. D. Parmentier, In press. To be published. 15. W.-H. Keller and J. E. Nordman, 16. 1. Giaever, Phvs. Rev. Lett. 5,464 (1960). Phys. Letf. 1,25 I (196i). 17. B. D. Josephson, and W. H. Keller, Phys. Lett. 36A, 18. J. E. Nordman 52(1971). To be published. 19. J. E. Nordman, J. uppl. Phys. 39, 20. F. L. Vernon and R. J. Pederson, 2661 (1968). Proc. Inst. Elec. Electron Engrs. 55, 21. J. Matisoo, 172 (1967). Trans. Inst. Elec. Electron Engrs. 22. W. Anacker, MAGd,968 (1969). Phys. Today 23,29 (1970). 23. P. W. Anderson, 24. P. W. Anderson, R. C. Dynes and T. A. Fulton. Bull. Am.Phys.Soc.l6,399(1971). 25. A. C. Scott, Solid-St. Electron. 7, 137 (1964). Proc. Inst. Elec. Electron Engrs. 26. R. D. Parmentier, 58, 1829 (1970). 27. A. C. Scott, II Nuovo Cimento 69B, 241 (1970). Ph.D. thesis, University ofWisconsin 28. W. J. Johnson, (1968) Ch. IV. 29. A. C. Scott, Act&e and Nonlinear Wave Propagation in Electronics. Chapter 5, Wiley (I 970). 30. A. Barone, F. Esposito, C. J. Magee and A. C. Scott, Rivistu de1 Nuoco Cimento. In press.
1972, Vol.
15, pp. 1167-I 173. PergamonPress. Printedin Great Britain
NIOBIUM
SUPERCONDUCTIVE
INTEGRATED
CIRCUIT
TUNNEL
DIODE
ARRAYS*
L. S. HOEL, W. H. KELLER, J. E. NORDMAN and A. C. SCOTT Department of Electrical Engineering, The University of Wisconsin, Madison, Wisconsin, U.S.A (Received
27 December
197 1; in revisedform
22 February
1972)
Abstract-A technology is described for building arrays of niobium based thin film superconductive tunnel junctions. Both Nb-Nb oxide-Sn negative resistance devices and Nb-Nb oxide-Pb Josephson devices were fabricated using a modified photoresist-sputter etching technique to make patterns in the Nb film. Good control over the volt-ampere characteristics of the Josephson junctions is demonstrated on single devices and is further illustrated by the use of this process to fabricate multistate oscillator arrays. 1. INTRODUCTION
of niobium as a base film for Josephson junctions is desirable because it can produce stable and reliable devices THE
USE
[ 12- 141. In Section 4, the theory of the multi-state oscillator array is briefly reviewed. The design and the operating characteristics of a (4 X 4) or 16 element array are described in Section 5. In Section 6, some other possible applications for the niobium photoresist technology are indicated. 2. NIOBIUM PHOTORESIST In this section
we describe
TECHNOLOGY in detail the fabrica-
tion procedures for Nb-NbO-Sn and Nb-NbOPb tunnel junctions in which the geometry of the niobium layer is specified by standard photolithographic techniques (contact printing through high resolution photographic plates). The major items 1167
L. S. HOEL, W. H. KELLER, J. E. NORDMAN and A. C. SCOTT
1168
of equipment
used in this research are:
(cl The aluminum is removed from exposed parts
(1) R.f. Sputtering
unit, 1000 W with adjustable bias and both Nb and Al 5 in. cathodes (MRCMARZ grade). Cathode to substrate spacing is about two inches. The substrate platform rotates under the targets. Substrate platform and cathodes are water cooled. (2) Photoresist spinner. (3) Mask aligner. (4) Evaporator for Sn and Pb (- lo-” Tort). The processing with reference Fig. 1.
steps can be most easily explained to the corresponding diagrams in
AL Nb KPR Nb
AZ-
1350
AI Nb NbO
The above steps will produce a pattern in the Nb without any apparent degradation in superconducting transition temperature or resistance ratio. The following steps indicate the way in which our tunnel junctions and arrays were made using either tin or lead top layers. Sputter cleaning steps described in parts (d) and (f) are crucial for obtaining repeatable results.
KPR
(b)
Nb Nb
/
using a phosphoric-acetic-nitric etch (77 crn:l 85% H,PO,; 15 cm” 36% acetic; 3 cm” 70% HN03; 5 cm” DI-H,O) at 55°C. After etching, the Nb surface should appear smooth and bright. The remaining photoresist is removed with acetone and the sample is cleaned with DI water and isopropylalcohol. (4 The exposed niobium is completely removed by sputter etching at 300 W for about 20 min. Some aluminum should remain after sputter etching and it is then again removed chemically as in (c). The Nb surface is now covered with a thick oxide which must be removed by sputter etching at 200 W for 6 min.
Substrate
(f) I
NbO
Cc) A( Nb
w
CdI
Fig. 1. Steps in photoresist
fabrication junctions.
of niobium
tunnel
(a) A chemically clean substrate of Corning 7059 glass is mildly sputter cleaned for about 2 min at 100 W. Then niobium is sputtered at 500 W for 25 min after a 10 min presputter. This produces a film about 1000 A thick. Without opening the system, aluminum is sputtered for at least 40 min at 500 W after a 10 min presputter. The substrate is protected by a shield during presputter. Film thicknesses are not critical, however the Al film should be significantly thicker than the Nb film. (b) Photoresist (SHIPLEY AZ-1350) is applied, spun for 30 set at 3000 revlmin and exposed and developed after a 5 min bake at 80°C.
(e) KPR photoresist is applied and spun for 15 set at 8000rev/min. It is exposed and developed in the usual way. (f) The sample is sputter cleaned at 50 W for 3 min. It is then oxidized in dry oxygen for about 5 min on a hot plate preheated to 150°C. The way in which a barrier is formed can be varied. _One can also use wet oxidation, or a deposited semiconductor [ 151. (g) Tin or lead is evaporated in the sample immediately after oxidation. Note that KPR serves as an insulator. An additional layer of KPR can be applied to protect the top layer against ambient humidity. Although we have not used photo-etching techniques to form the top lead or tin pattern one could do so using AZ 1350 again. 3. JUNCTION PROPERTIES The volt-ampere characteristic of a typical crossed-strip Nb-NbOx-Sn junction is shown in Fig. 2(a). This can be compared with a similar characteristic [Fig. 2(b)] of a device with junction area determined by the photoresist step (step e) as discussed in the previous section. The superconducting gaps of niobium and tin are sufficiently
I mV/cm
I
(b) Cd
Fig. 2. Comparison of V-Z characteristics of Nb-NbO-Sn junctions made with and without the photoresist steps. Area equals 2 X lo-* mm*. (a) Without photoresist (T = 3,3”K), (b) with photoresist (T = 3.35”K).
1 mV/cm0
DAYS
l4.P*K)
21 DAYS 279
Fig. 3. Volt-ampere
characteristics
DAYS
vs. lifetime for Nb-NbO-Pb 2 X IO-*mm*.
junctions.
Area equals
[Facing page 1168 ]
Fig. 7. Nb-NbO-Pb
multimode array. Substrate dimensions are 1 X $ in.
Fig. 9. Multimode array with bias resistor limited to two edges.
Fig. 10. Detail of the junction geometry in the multimode array.
ORIENTATION
FIRST
53 57 64 74 70 07
SECOND
0
5 p
seckm
ORIENTATION
-
Fig. Il. Multimode oscillations at various bias currents and orientations with respect to the laboratory magnetic field.
0.2
seclcm
d
Fig. 12. Multistate oscillation of the array obtained by a transient magnetic field. Two sweeps are presented for each state.
NIOBIUM
SUPERCONDUCTIVE
different to give a distinct negative conductance region of the type described by Giaever [ 161. With the photoresist step the characteristic is slightly more rounded, but the two curves have essentially the same shape. The higher current level and emergence of Josephson [ 171 current in the characteristic of Fig. 2(a) is due to a slightly thinner oxide layer and larger junction area. The Pb junctions described below have virtually identical characteristics to those made without photoresist processing. The superconducting energy gaps of niobium and lead are close enough together to make a Giaever type negative conductance difficult to observe for Nb-NbOx-Pb devices. However, by reducing the oxidation time Josephson type characteristics are readily obtained. Since these devices have a thinner oxide layer, they would be more interesting to investigate for reliability under repeated testing. One such test is indicated in Fig. 3. Four Nb-NbOx-Pb junctions were made on the same substrate with equal areas (2 X lo-’ mm2) defined by the photoresist step as described in the previous section. Two of the junctions (No. 1 and No. 3) have niobium leads of 10 pm in width and the other two (No. 2 and No. 4) have 100 pm leads. After the initial test (day 0) the entire surface was coated with a photoresist layer to prevent deterioration of the lead film. Between tests the junctions were stored in a laboratory dessicator and at room temperature. Volt-ampere characteristics measured at later times are shown in Fig. 3. The normal conductance of these junctions (high voltage slope of the characteristic) is plotted vs. time in Fig. 4. They exhibit a reasonable degree of stability over
Fig. 4. Normal conductance
TUNNEL
DIODE
1169
the test period. Hundreds of junctions and several multimode oscillators have been made using this photoetch fabrication technique. It was always possible to trace an incorrect processing step for niobium-lead junctions which did not function properly. The niobium-tin junctions were somewhat less reliable. Virtually all of the devices made using r.f. sputtered niobium films show an anomalous excess current just above the niobium energy gap[ 181. This appears to be caused by a proximity effect in the niobium surface. It will be discussed in detail in another publication [ 191. When biased as is indicated in Fig. 5(a), a Josephson junction should undergo a relaxation oscillation the frequency of which is controlled by the time constants associated with the inductor L[20]. This effect is readily observed for the Nb-NbOxPb Josephson junctions as indicated in Figs. S(b) and 5(c). Notice that the frequency increases by about an order of magnitude as inductance is decreased from that of the leads of a bias resitor external to the dewar to that of the inductance of a small nichrome bias resistor mounted close to the junction (L = I FH). To initiate the oscillation the bias current (I,) must exceed the zero voltage current of the Josephson junction. With such bias the junction appears to be an effective negative conductance. 4. REVIEW
OF MULTISTATE OSCILLATOR ARRAY THEORY
The multistate oscillator array is an example of a new class of circuits which presents interesting possibilities for large scale integration. The theory
vs. lifetime for the junctions
of Fig. 3.
L. S. HOEL, M. H. KELLER, J. E. NORDMAN
1170
(a)
E
50pseckm (b)
5psec/cm (c)
Fig. 5. Josephson junction relaxation oscillator. (a) Equivalent circuit, (b) oscillation with inductance due to external
bias leads, (c) oscillation
for L =
1PH.
has been considered in several recent publications [ 12-141; here we shall confine ourselves to a brief survey of the salient concepts. Consider first the linear and lossless two dimensional array shown in Fig. 6(a). This circuit is the electrical analog of a square mechanical membrane which is fixed at its four edges. If there are N unit cells (capacitors) in the array, there will be N modes of oscillation. For each mode the voltage will vary with position and time as u = V sin psx sin &y cos (wt + $) .
(1)
The propagation constants, PI and &,, in equation (1) must satisfy the requirement that the length of one side of the array (A) be an integral number of half wavelengths. Thus each mode is specified by a point in reciprocal space (fix - pU space) where ps and & satisfy
and A. C. SCOTT
two half wavelengths in the x-direction (m = 2) and one half wavelength in the y-direction (n = I) is sketched in Fig. 6(b) and the corresponding mode Point in reciprocal space is indicated. The number of mode points in reciprocal space is just equal to (A/a)* which is the number of unit cells in the array. Such a linear and lossless array can be used to store information. One bit of information is stored in each mode by either exciting or not exciting that mode. Thus, an array with N unit cells can store N bits of information. It is important to notice, however, that this information is distributed in real space but localized in reciprocal space. If the information was stored in an array of N switches it would be localized in real space and distributed in reciprocal space. Thus in the array of Fig. 6 we have a simple model for a distributed memory. Any practical linear circuit is only approximately lossless at non zero frequency. Thus the array in Fig. 6 could only store information for a time of the order of the decay times of its modes. To remedy this defect we can suppose that nonlinear elements are placed in parallel with the capacitors in order to supply power to the excited modes and maintain their amplitudes at constant values. The modes will tend to stabilize at amplitudes for which the power input from the nonlinear element is equal to the power being lost in the linear dissipation. One of the main objectives of the nonlinear analysis of this problem is to determine how the modes interact with each other in the presence of the nonlinearity. On the linear array 2” mode combinations or states can be excited. It is of particular interest to determine how many of these can be stably excited on the nonlinear array. This question has been investigated in some detail for the case of a conductive nonlinear element as shown in Fig. 2(b)[13]. The analysis of the case when the nonlinear element is a Josephson junction has not yet been carried out. However, the experimental observations in the following section indicate that the qualitative behavior is quite similar in the two cases. 5. NIOBIUM INTEGRATED CIRCUIT OSCILLATOR ARRAY We have built and tested several niobium integrated circuit oscillator arrays similar to that shown in Fig. 7. The corresponding equivalent
and m and IZare integers. For example, the spatial variation of the voltage for the mode which has
circuit for the array is given in Fig. 8. The inductor pattern is formed in the initial niobium layer. A
NIOBIUM SUPERCONDUCTIVETUNNEL
(b)
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(cl
Fig. 6. (a) Linear multimode array, (b) location of mode points in reciproCal space, (c) spatial variation of a single mode.
lead layer provides the other contacts for the NbNbO-Pb junctions and serves as a ground blane. An aluminum film provides a low inductance bias resistance around the edge of the array. This bias resistance can be either completely around the edge, as indicated in Figs. 7 and 8, or around some restricted portion of the edge as indicated in Fig. 9. A magnified view of one of the junctions is shown in Fig. 10. This photograph was taken after development of the KPR while delineates the junction area (step e in Section 2 and Fig. 1). The junction diameter is 6.25 mil or 156 pm. The maximum zero voltage current is about 4 mA. Assuming an oxide thickness of 30 A and a dielectric constant of 12, the junction capacitance is about 700pF. The series inductance between diodes is
that of a transmission line of 1 pm in height (the thickness of the KPR), 0.5 mil or 12 pm wide, and 1.78 cm long. The corresponding inductance is about 1.8 nH. These arrays readily oscillate when the bias current is raised above the sum of the maximum Josephson currents for the sixteen individual junctions. The maximum current is inferred from that of a test junction on the same substrate. This oscillation is clearly multimode in character. Furthermore, the state of the oscillation depends upon the orientation of the array with respect to the residual magnetic fields in the laboratory. For example, in Fig. 11 the signal output waveforms are recorded at various levels of bias current for two different orientations of the array, The waveforms are single sweep with a bandwidth of 500 kc.
1172
L. S. HOEL,
M. H. KELLER,
J. E. NORDMAN
Junction capoclty
and A. C. SCOTT Nb- NbO-Pb
tunnel
-Ground plane ( lead film) Bias resistors (Aluminium film)
d
iianal
Bias
._~ outaut
current
Fig. 8. Equivalent
circuit
For these measurements the bias resistor was only on the two edges adjacent to the output edge as in Fig. 9. The array can be switched from one state of oscillation to another by moving the field of a small magnet (about I OG) near the dewar. Fig. 12 shows about nine different oscillatory states of the array which were obtained this way at a constant current bias of 100 mA. 6. OTHER
may niobium problems are listed ences are It
for multimode
array
ture cycling and room temperature further study.
storage deserves
6.2 Flux quantum shuttle Anderson has suggested the possibility of information processing systems in which bits of information are carried by individual flux quanta [23]. An elementary shift register employing this principle and fabricated by plasma oxidation of tin has recently been described [24].
APPLICATIONS
be interesting and useful to apply the integrated circuit technology to other in applied science. Several possibilities below and appropriate background referindicated.
6.1 Tunneling cryotron This computing device, originally suggested by Matisoo [21], uses the Josephson zero voltage current in its operation. Anacker has shown that the tunneling cryotron has considerable potential as the basic logic element in large capacity information processing systems [22]. He notes, however, that the problem of device stability during tempera-
6.3 Superconductive neuristor This device used Giaever type tunneling to achieve an electronic analog of a nerve axon. It has been discussed in considerable detail from a theoretical point of view[7,8,25,26], but the experimental work has lagged because of the difficulty in making tin-lead devices of large area. We feel that the niobium technology now makes such experiments much more interesting. 6.4 Josephson trnnsmission lines Quasi-linear equations have been developed to describe the envelope dynamics of a pulse containing many flux quanta on a Josephson transmission
NIOBIUM
SUPERCONDUCTIVE
line [27]. Stationary propagation of pulses with many flux quanta on a biased line with losses has also been considered[28,29]. Experimental observation of these effects would be most interesting. 6.5 Large circular Josephson junctions In a junction with dimensions large compared with the Josephson penetration depth (AJ = O.l0.0 I mm [ 1O]), the normalized flux obeys the two dimensional Sine-Gordon equation
Exact solutions of this equation can be obtained for circular boundary conditions[30]. These solutions should be studied experimentally. REFERENCES 1. J. E. Nordman, J. uppl. Phys. 40,2 I 11 (I 969). 2. L. 0. Mullen and D. B. Sullivan, J. appl. Phys. 40, 21 15 (1969). 3. W. H. Keller and J. E. Nordman, J. appl. Phys. 42, 137(1971). 4. K. Schwidtal, Bull. Am. Phys. Sot.. Ser. II, 16, 400 (1971). 5. W. Schroen, J. uppl. Phys. 39,267 1 (1968). 6. J. P. Pritchard, Jr., and W. H. Schroen, Truns. fnsf. Elec. Electron Enars. MAC-4, 320 (1968). 7. H. T. Yuan and ‘A. C. Scott. Solid-St.’ Electron. 9, 1149(1966). Solid-St. Electron. 12,287 (1969). 8. R. D. Parmentier,
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9. A. C. Scott and W. J. Johnson, Appl. Phys. Lett. 14, 316(1969). and A. Barone, J. uppl. Phys. 41, 10. W. J. Johnson 2958 (1970). 11. A. Barone. J. awl. Phvs. 42.2747 (197 1). kgrs. CT-17, 12. A. C. Scott, Trans. Inst. Elec. Ele&n 55 (1970). 13. A. C. Scott, Trans. Inst. Elm. Electron Engrs. SMC-1, 267 (1971). Trans. Inst. Elec. Electron Engr. 14. R. D. Parmentier, In press. To be published. 15. W.-H. Keller and J. E. Nordman, 16. 1. Giaever, Phvs. Rev. Lett. 5,464 (1960). Phys. Letf. 1,25 I (196i). 17. B. D. Josephson, and W. H. Keller, Phys. Lett. 36A, 18. J. E. Nordman 52(1971). To be published. 19. J. E. Nordman, J. uppl. Phys. 39, 20. F. L. Vernon and R. J. Pederson, 2661 (1968). Proc. Inst. Elec. Electron Engrs. 55, 21. J. Matisoo, 172 (1967). Trans. Inst. Elec. Electron Engrs. 22. W. Anacker, MAGd,968 (1969). Phys. Today 23,29 (1970). 23. P. W. Anderson, 24. P. W. Anderson, R. C. Dynes and T. A. Fulton. Bull. Am.Phys.Soc.l6,399(1971). 25. A. C. Scott, Solid-St. Electron. 7, 137 (1964). Proc. Inst. Elec. Electron Engrs. 26. R. D. Parmentier, 58, 1829 (1970). 27. A. C. Scott, II Nuovo Cimento 69B, 241 (1970). Ph.D. thesis, University ofWisconsin 28. W. J. Johnson, (1968) Ch. IV. 29. A. C. Scott, Act&e and Nonlinear Wave Propagation in Electronics. Chapter 5, Wiley (I 970). 30. A. Barone, F. Esposito, C. J. Magee and A. C. Scott, Rivistu de1 Nuoco Cimento. In press.