U.S. patent application number 10/751091 was filed with the patent office on 2004-12-30 for high-temperature superconductor devices and methods of forming the same.
Invention is credited to Char, Kookrin, Moeckly, Brian H..
Application Number | 20040266627 10/751091 |
Document ID | / |
Family ID | 34794679 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040266627 |
Kind Code |
A1 |
Moeckly, Brian H. ; et
al. |
December 30, 2004 |
High-temperature superconductor devices and methods of forming the
same
Abstract
An electronic device including a crystalline substrate, an
electrode formed on and epitaxial to the substrate, the electrode
including a first superconductive oxide, an insulator formed on and
epitaxial to the electrode, a barrier that includes an ion-treated
surface of the first superconductive oxide, and a counter-electrode
formed on and epitaxial to the electrode and the barrier, the
counter-electrode including a second superconductive oxide, whereby
a Josephson junction is formed between the electrode and the
counter-electrode. A superconductor device that includes an oxide
superconductor having a surface exposed to ambient environment, and
a passivation layer covering at least a portion of the surface of
the oxide superconductor that is exposed to the ambient
environment. Methods of forming the above devices are also
included.
Inventors: |
Moeckly, Brian H.; (Menlo
Park, CA) ; Char, Kookrin; (Palo Alto, CA) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
34794679 |
Appl. No.: |
10/751091 |
Filed: |
January 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10751091 |
Jan 2, 2004 |
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10704215 |
Nov 6, 2003 |
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10704215 |
Nov 6, 2003 |
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09082486 |
May 20, 1998 |
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60047555 |
May 22, 1997 |
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60437781 |
Jan 3, 2003 |
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Current U.S.
Class: |
505/100 |
Current CPC
Class: |
H01L 39/2496 20130101;
H01L 39/247 20130101; H01L 39/225 20130101 |
Class at
Publication: |
505/100 |
International
Class: |
C10F 005/00 |
Goverment Interests
[0002] This invention was made with United States Government
support under Contract No. N0014-96-C-2095 awarded by the Naval
Research Laboratory. The United States Government has certain
rights in the invention.
Claims
What is claimed is:
1. An electronic device comprising: (a) a crystalline substrate;
(b) an electrode formed on and epitaxial to the substrate, the
electrode comprising a first superconductive oxide; (c) an
insulator formed on and epitaxial to the electrode; (d) a barrier
comprising an ion-treated surface of the first superconductive
oxide; and (e) a counter-electrode formed on and epitaxial to the
electrode and the barrier, the counter-electrode comprising a
second superconductive oxide, whereby a Josephson junction is
formed between the electrode and the counter-electrode.
2. The device of claim 1, wherein the barrier is a surface formed
by treating the first superconductive oxide with a plasma
comprising a gas selected from the group consisting of argon,
xenon, oxygen, and halogen.
3. The device of claim 2, wherein the gas is argon gas.
4. The device of claim 2, wherein the gas is a 1:1 mixture of argon
and oxygen.
5. The device of claim 1 wherein the first superconductive oxide
has an a-b plane and a step-edge junction is formed in the a-b
plane of the first superconductive oxide.
6. The device of claim 1 wherein the first superconductive oxide
has an a-b plane, the a-b plane is epitaxial to the substrate, and
the second superconductive oxide is on and epitaxial to the first
superconductive element, whereby a junction is formed perpendicular
to the a-b plane of the first superconductive oxide.
7. The device of any one of claims 1-6, wherein the first and
second superconductive oxide is YBCO.
8. The device of claim 1, the device having an I.sub.cR.sub.n value
of at least about 0.3 mV at a temperature of 4.2 K.
9. The device of claim 2, the device having an I.sub.cR.sub.n value
of at least about 0.3 mV at a temperature of 4.2 K.
10. The device of claim 3, the device having an I.sub.cR.sub.n
value of at least about 0.3 mV at a temperature of 4.2 K.
11. The device of claim 4, the device having an I.sub.cR.sub.n
value of at least about 0.3 mV at a temperature of 4.2 K.
12. The device of claim 5, the device having an I.sub.cR.sub.n
value of at least about 0.3 mV at a temperature of 4.2 K.
13. The device of claim 6, the device having an I.sub.cR.sub.n
value of at least about 0.3 mV at a temperature of 4.2 K.
14. The device of claim 7, the device having an I.sub.cR.sub.n
value of at least about 0.3 mV at a temperature of 4.2 K.
15. The device of claim 1, the device having an I.sub.cR.sub.n
value of at least about 0.5 mV at a temperature of 40 K.
16. The device of claim 2, the device having an I.sub.cR.sub.n
value of at least about 0.5 mV at a temperature of 40 K.
17. The device of claim 3, the device having an I.sub.cR.sub.n
value of at least about 0.5 mV at a temperature of 40 K.
18. The device of claim 4, the device having an I.sub.cR.sub.n
value of at least about 0.5 mV at a temperature of 40 K.
19. The device of claim 5, the device having an I.sub.cR.sub.n
value of at least about 0.5 mV at a temperature of 40 K.
20. The device of claim 6, the device having an I.sub.cR.sub.n
value of at least about 0.5 mV at a temperature of 40 K.
21. The device of claim 7, the device having an I.sub.cR.sub.n
value of at least about 0.5 mV at a temperature of 40 K.
22. A process for making a Josephson junction device comprising the
steps of: (a) preparing a substrate; (b) depositing an electrode
comprising a first layer of a superconductive oxide on the
substrate; (c) depositing an insulating layer on the first layer of
superconductive oxide; (d) patterning to form a pre-device having
an exposed surface of the first superconductive oxide; (e) placing
the pre-device into a deposition chamber; (f) forming a barrier on
the exposed surface of the first layer of superconductive oxide by
treating the exposed surface with ions; and (g) depositing a second
layer of a superconductive oxide on the pre-device, whereby a
Josephson junction is formed between the first and the second
superconductive oxides at the barrier.
23. The process of claim 22, wherein the treating with ions is
accomplished with a plasma of Ar gas at a pressure of between 10
and 100 mTorr.
24. The process of claim 22, wherein the treating with ions is with
a mixture of Ar and O.sub.2 gas at a pressure of between 10 and 100
mTorr.
25. The process of any one of claims 22-24, further comprising the
step of vacuum annealing the pre-device prior to depositing the
second superconductive oxide.
26. A superconductor device, comprising: a) an oxide superconductor
having a surface exposed to ambient environment; and b) a
passivation layer covering at least a portion of the surface of the
oxide superconductor that is exposed to the ambient
environment.
27. The device claim 26, further comprising a buffer layer at least
partially between the passivation layer and the oxide
superconductor.
28. The device of claim 26, wherein the passivation layer
originates from the superconductor.
29. The device of claim 28, wherein the passivation layer is an
ion-modified layer of the superconductor.
30. The device of claim 26, wherein the oxide superconductor
comprises YBa.sub.2Cu.sub.3O.sub.7-.delta., wherein
.delta..gtoreq.0.
31. The device of claim 26, wherein the passivation layer is an
electrical insulator.
32. The device of claim 26, wherein the passivation layer is
epitaxial and crystalline.
33. The device of claim 26, wherein the passivation layer covers
the entire surface of the oxide superconductor that is exposed to
the ambient environment.
34. The device of claim 26, further comprising a layer of a
superconductive oxide on the passivation layer, whereby a Josephson
junction is formed between the superconductive oxides.
35. A method of providing a passivation layer on the surface of an
oxide superconductor, the method comprising vacuum annealing and
ion treating at least a portion of the surface of the oxide
superconductor that is exposed to ambient environment.
36. The method of claim 35, further comprising additional vacuum
annealing after the ion treatment.
37. The method of claim 35, further comprising heating in an
oxygen-rich environment after the ion treatment.
38. The method of claim 35, comprising vacuum annealing and ion
treating the entire surface of the oxide superconductor that is
exposed to ambient environment
39. A method of making a superconductor device, the method
comprising: a) forming a layer of oxide superconductor on a
substrate, the layer of oxide superconductor having a surface that
is exposed to ambient environment; and b) passivating at least a
portion of the surface of the oxide superconductor that is exposed
to ambient environment.
40. The method of claim 39, comprising passivating the entire
exposed surface of the oxide superconductor.
41. The method of claim 39, wherein the passivating step comprises
bombarding the exposed surface portion with ions.
42. The method of claim 41, further comprising annealing the layer
of oxide superconductor between steps (a) and (b).
43. The method of claim 42, further comprising annealing the layer
of oxide superconductor after step (b).
44. The method of claim 42, wherein the bombarding step comprises
treating the exposed surface portion with plasma.
45. The method of claim 39, wherein step (a) comprises forming a
layer of YBa.sub.2Cu.sub.3O.sub.7-.delta., wherein
.delta..gtoreq.0.
46. The method of claim 42, further comprising heating the oxide
superconductor in oxygen after step (b).
47. The method of claim 46, further comprising cooling the oxide
superconductor to room temperature in oxygen after heating the
oxide superconductor in oxygen.
48. The method of claim 41, further comprising maintaining the
layer of oxide superconductor at a temperature of between about
300.degree. C. and about 650.degree. C. while bombarding the
exposed surface portion with ions.
49. The method of claim 46, wherein the heating step comprises
maintaining the layer of oxide superconductor at a temperature of
between about 700.degree. C. and about 800.degree. C. after
treating the exposed surface portion with plasma.
50. The method of claim 39, wherein the passivation step comprises
changing a surface layer of the oxide superconductor to a material
different from the oxide superconductor.
51. The method of claim 50, wherein the changing step comprises
changing the surface layer of the oxide superconductor to a
material having an oxygen mobility that is lower than the oxygen
mobility in the oxide superconductor.
52. The method of claim 39, further comprising forming a layer of
oxide superconductor on at least a portion of the passivated
surface portion, whereby a Josephson junction is formed between the
oxide superconductors.
53. A passivation layer comprising an ion-modified layer on an
oxide superconductor, the ion-modified layer covering at least a
portion of the surface of the oxide superconductor that would
otherwise be exposed to ambient environment, and the ion-modified
layer having an oxygen mobility that is lower than an oxygen
mobility of the oxide superconductor.
54. The passivation layer of claim 53, wherein the ion-modified
layer is formed by material originating from the oxide
superconductor.
55. The passivation layer of claim 53, wherein the ion-modified
layer is an externally applied layer that is bonded to the oxide
superconductor.
56. The passivation layer of claim 55, wherein the ion-modified
layer is quasi-cubic and is not YBa.sub.2Cu.sub.3O.sub.7-.delta.,
wherein .delta..gtoreq.0.
57. The passivation layer of claim 53, wherein the ion-modified
layer is epitaxial and crystalline.
58. The passivation layer of claim 53, the ion-modified layer
covering the entire surface of the oxide superconductor that would
otherwise be exposed to ambient environment
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part of U.S. patent application
Ser. No. 10/704,215, filed Nov. 6, 2003, which is a continuation of
U.S. patent application Ser. No. 09/082,486, filed May 20, 1998,
which claims the benefit of U.S. Provisional Patent Application
Ser. No. 60/047,555, filed May 22, 1997; and claims the benefit of
U.S. Provisional Patent Application Ser. No. 60/437,781, which
applications are hereby incorporated by reference in their
entirety.
FIELD OF THE INVENTION
[0003] This invention relates generally to superconductor devices.
More particularly, the invention relates to high-temperature
superconductor devices, for example high-temperature superconductor
("HTS") Josephson junctions.
BACKGROUND OF THE INVENTION
[0004] Since the 1986 discovery of the new class of oxide
superconductors, also known as high temperature superconductors
(HTS), cuprate superconductors, and perovskite superconductors,
many attempts have been made to fabricate useful junctions,
devices, circuits, and systems. This discovery promised to bring
the many benefits of superconductors to electronic circuits at
practically attainable temperatures. Achieving these benefits,
however, has been less than straightforward due to the nature of
the materials, which is quite different from the metals and
semiconductors normally used in electronics applications.
[0005] The first obstacle, now largely overcome, was the
polycrystalline nature of these new ceramic superconductors.
Traditional low temperature superconductors, having a
superconducting transition temperature T.sub.c<23 K, are metals,
metal alloys, or intermetallic compounds. Metals are usually
polycrystalline, but metallic bonding is so delocalized that the
grain boundaries in these materials are not electrically active.
Furthermore, coherence lengths in these superconductors are on the
order of 100 nm, which is much larger than the size of a single
grain, i.e., a single crystalline making up part of the
polycrystalline body. This means that the superconducting electron
pairs are affected by the average environment produced by many
individual grains and are therefore not extremely sensitive to
inhomogeneities at grain boundaries or other regions whose size is
much less than a coherence length.
[0006] The cuprate superconductors are ceramic materials with ionic
and covalent bonds that are more directional and localized than
metallic bonds. Across grain boundaries atoms are displaced with
respect to their normal positions in the ideal crystal. Chemical
bonds between these displaced atoms are stretched, bent, broken,
and sometimes vacant, depending on the atoms considered and their
relative displacements in distance and angle from their ideal
positions. This sort of disruption of the electronic structure of
the material, much more severe with directional bonding than with
isotropic metallic bonding, can cause corresponding disruptions in
the transport properties of the material. It is for this reason
that bulk polycrystalline specimens of the cuprate superconductors
typically have critical current densities, which are reduced by an
order of magnitude or more when compared to well oriented epitaxial
films of the same chemical composition.
[0007] Another exacerbating factor is the very small and
anisotropic coherence length of the superconducting perovskites.
The coherence length in these materials has been estimated at about
1.5 nm in the a-b plane and about ten times less (0.15 nm) in the
c-direction. These distances are much smaller than the dimensions
of a typical grain, and are of the order of the lattice constant in
the c-direction in YBa.sub.2Cu.sub.3O.sub.7-.delta.
(0.ltoreq..delta..ltoreq.1). The result is that the electrical
properties of these superconductors are strongly influenced by the
microstructure as well as the local environment of defects,
including impurity atoms, vacancies, voids, dislocations, stacking
faults, and grain boundaries.
[0008] With such a small coherence length, virtually any deviation
from perfection can interrupt the flow of supercurrent enough to
form a junction. Early thin films were so full of grain boundary
junctions, due to their poor in-plane epitaxy, that the inherent
properties of the material were masked by the behavior of thousands
of weak-link junctions occurring naturally in the polycrystalline
layers. By the early 1990s, however, the crystal growth technology
had progressed to a state in which high-quality, well oriented
epitaxial layers of high temperature superconductors could be grown
by a variety of techniques and on a variety of substrates, so that
well characterized junctions could be made in several ways. Most of
these junctions, however, were deficient in one or more
characteristics desirable for use in digital electronics or
superconductive quantum interference devices ("SQUIDs").
[0009] The Josephson junction is one of the basic elements of
superconductor electronic devices, and is well-developed in low
temperature superconductors. For high-temperature superconductors,
however, development of a technology for reproducible junctions has
been difficult. The first reported, intentionally fabricated,
junctions were of the weak-link type. They are characterized by a
critical current density J.sub.c, a critical current I.sub.c, an
effective device cross-sectional area A, junction resistance,
R.sub.n, and normalized junction resistance R.sub.nA. Later,
junctions with an interlayer of an insulating material (SIS
junctions) or normal metal (SNS junctions) were developed. However,
few of these approaches were commercially useful, and none met all
of the requirements for a useful technology. To make good
electronic devices and circuits from the oxide superconductors, a
manufacturable junction technology must be developed.
[0010] A manufacturable technology is one that gives reproducible
and predictable results when a defined series of processing steps
is carried out. The devices perform as designed, and the processes
are robust, that is, are sensitive to small changes in processing
parameters. A particular requirement of the technology is that all
necessary processing steps should be compatible, so that one step
does not destroy the results of a step that must be performed
earlier in the flow.
[0011] The junctions formed by this technology should meet design
criteria as specified by the user. The junctions must perform
reliably at a specified temperature. They must carry a current
density of 100 to 100,000 A/cm.sup.2, at the designer's discretion,
and must do so for the foreseeable lifetime of the device.
Fluctuations in the critical current of each junction, as well as
variations from junction to junction in a circuit, must be
minimized. Noise must be reduced to a level at which random signals
due to noise are much smaller and less common than the true signals
the circuit is designed to detect.
[0012] For useful superconducting quantum interference devices
(SQUIDs) it is necessary to fabricate matched pairs of junctions in
a predetermined geometrical relationship. Not only must each
junction have predictable qualities, but they must be easy to
position at will. In practical terms, this implies that all of the
materials used in a circuit should be patterned using similar
techniques.
[0013] Development of a HTS circuit technology has remained elusive
because of the difficulty of fabricating reproducible, uniform
Josephson elements that possess suitable electrical properties for
applications such as single flux quantum (SFQ) logic and SQUIDs.
The state of the art is the ramp-edge process employing a Co-doped
YBCO barrier layer. This process is described in Char, et al., U.S.
Pat. No. 5,696,392 "Improved Barrier Layers for Oxide
Superconductor Devices and Circuits," which is incorporated herein
by reference. The devices disclosed by Char, et al. and further
refined as disclosed in W. H. Mallison, S. J. Berkowitz, A. S.
Hirahara, M. J. Neal, and K. Char, "A multilayer
YBa.sub.2Cu.sub.3Ox Josephson junction process for digital circuit
applications," Appl. Phys. Lett., vol. 68, pp. 3808, 1996, have
spreads injunction parameters approaching that which is needed to
make multi-junction circuits. However, these junctions appear to
operate as true proximity-effect elements--their values of R.sub.n
are quite low due to the low resistivity of the barrier material in
their SNS configuration. Therefore, most of their usefully high
I.sub.cR.sub.n product derives from a relatively high I.sub.c,
outside the range that is useful for SFQ devices. SFQ technology
holds promise in high speed switching. Moreover, difficulty with
the deposition of reproducible Co-YBCO films has limited the
exploitation of those junctions.
[0014] Other types of high-T.sub.c SNS geometry junctions have
clearly been plagued by an excess resistance that does not
correlate with that of the barrier material. This resistance has
been shown to exist at the YBCO/barrier interface, most likely
arising from oxygen disorder due to mismatches in lattice and
thermal expansion coefficients. Unfortunately, although the R.sub.n
of these devices is in a useful range, their uncontrollable excess
resistance makes them unsuitable for a reproducible junction
technology. Indeed, we speculate that the primary weak-link effect
in many of these devices arises specifically because of the
weakened superconductivity at the interface, not due to the
intended proximity effect.
[0015] Successful manufacture of Co-YBCO junctions has required
great care in order to insure elimination of an excess interface
resistance. If we are to increase the R.sub.n of these devices, two
obvious options become apparent: (1) add an excess interface
resistance, or (2) increase R.sub.n of the barrier layer, while
preserving a negligible interface resistance. Unfortunately, we
don't know how to perform the first item uniformly, and the second
task requires the deposition of a high-resistivity,
lattice-matched, pinhole-free barrier on the scale of a few nm.
Such material expertise is presently beyond our capability, and
thus, ideal junctions using Co-YBCO are not yet commercially
practicable. Furthermore, it is not at all clear that
I.sub.cR.sub.n remains high for high-resistivity barriers.
[0016] Josephson junctions have also been reported using
surface-treated YBCO as the barrier. For example, R. B. Laibowitz,
R. H. Koch, A. Gupta, G. Koren, W. J. Gallagher, V. Foglietti, B.
Oh, and J. M. Viggiano, Appl. Phys. Lett. 56, 686 (1990) used ion
milling to damage the interface followed by ex-situ,
low-temperature plasma oxyfluoridation to repair the damage. K.
Harada, H. Myoren, and Y. Osaka, "Fabrication of all-high-Tc
Josephson junction using as-grown YBa2Cu3Ox thin films," Jap. J.
Appl. Phys., vol. 30, pp. L1387, 1991, report ion plasma treated
YBCO surfaces exhibiting Josephson behavior. C. L. Gia, M. I.
Faley, U. Poppe, and K. Urban, "Effect of chemical and ion-beam
etching on the atomic structure of interfaces in
YBa.sub.2Cu.sub.3O.sub.7/PrBa.sub.2cu.sub.3O.sub.7 Josephson
junctions," Appl. Phys. Lett., vol. 67, pp. 3635, 1995, report that
ion milling produces a surface phase of PBCO consistent with a
cubic structure. However, these surface methods fail to achieve a
sufficiently reproducible modified-surface barrier high-T.sub.c
junction technology.
[0017] Moreover, many oxide superconductors have crystal structures
and compositions that accommodate an appreciable range of oxygen
content. For example, in the well-known "1-2-3" oxide
superconductor, YBa.sub.2Cu.sub.3O.sub.7-.delta. ("YBCO"), the
oxygen content may deviate from stoichiometry (i.e., where
.delta.=0) by a significant amount. Oxygen deficiency, .delta., may
be has high as 1. Experience has shown that both normal and
superconducting properties, such as transition temperature, depend
strongly on oxygen content and atomic arrangement. Generally,
T.sub.C decreases as oxygen deficiency increases.
[0018] Unfortunately, oxide superconductors have a tendency to lose
their oxygen content to the surrounding atmosphere, even at normal
storage conditions. The change in oxygen content results in changes
in the electronic structure of the oxide material. As an example,
FIG. 8 shows a series of X-ray photoelectron spectra (XPS) taken at
the surface of a YBCO film after the film was formed. Initially,
and for a few days after the film was made, the XPS of the Ba 4d
emission shows three peaks. See, for example, spectra 110, 120,
130, 140, 150, 160 and 170, taken immediately, 15 minutes, 1 hour,
2 hours, 4 hours, 1 day and 3 days, respectively, after the film
was made. At three weeks (spectrum 180), the low-energy peak 112,
which is associated with Ba in the bulk, superconducting phase, had
begun to disappear. At three months (spectrum 190), the low-energy
peak 112 has entirely disappeared. The higher-energy peaks, 114 and
116, which are associated with Ba in a non-conducting surface
phase, remained. This change in XPS indicates that the
superconducting YBCO phase has disappeared over time, at least from
the near-surface regions. It is believed that this is due to oxygen
loss and from corrosion resulting from, for example, Ba reacting
with carbon to form barium carbonates.
[0019] The loss of oxygen and the associated structural changes
lead to change, and often degradation, of certain properties in the
superconductor. This variation or degradation is highly
undesirable. In telecommunication applications, for example, oxide
superconductors have been used in thin-film devices such as
high-performance, high-precision filters. These filters are often
placed inside sealed vacuum chambers and are difficult to tune once
sealed. Oxygen loss over time or as a result of bake-out and other
procedures at elevated temperatures can produce shifts and
degradation of filter characteristics.
[0020] Additionally, the performance of oxide superconductor
devices may also suffer as a result of corrosion due to chemical
reactions between the superconductor and its environment.
SUMMARY OF THE INVENTION
[0021] One embodiment of the invention provides a Josephson
junction having reproducible properties with a high R.sub.n.
Another embodiment of the invention provides a method of
fabricating a Josephson junction having a high, reproducible, and
controllable I.sub.cR.sub.n product.
[0022] An electronic device is provided that includes a crystalline
substrate; a first superconductive element formed on and epitaxial
to the substrate, the superconductive element including a
superconductive oxide having a surface including a barrier means, a
second superconductive element formed on and epitaxial to the first
superconductive element, whereby a Josephson junction is formed
between the first superconductive element and the second
superconductive element. In contrast to prior art Josephson
junctions which relied on grain boundaries at which crystalline
lattice changed direction or on metallic or insulating barrier
layers, the edge-junction of the present invention can be formed
without deposition of any barrier at all.
[0023] The invention takes advantage of a property of the
notoriously complex YBCO material; that its electrical properties
are tunable over a wide range, from an insulator to a
superconductor, by altering its oxygen content and order, changing
its crystal structure, or by adding dopants. The invention uses
this property to create a thin layer of high-resistivity material
on, for example, the junction edge of a Josephson junction, by
altering the structure or chemistry of YBCO only at the surface. In
the case of a Josephson junction, if this is done prior to
deposition of the YBCO counterelectrode, a high R.sub.n, device is
formed.
[0024] In the invention, the surface of a first layer of YBCO is
modified by using a combination of vacuum annealing and plasma
treatment. Unlike previous attempts in which a junction was formed
ex situ via ion milling or etching, the current invention does not
lead to weakened superconductivity in the second layer due to
lattice mismatch at the interface.
[0025] The invention also provides an oxide superconductor that has
a protective layer on the surface thereof. In one embodiment the
protective layer is a passivation layer. The passivation layer can
prevent deterioration or degradation of structural and physical
properties of the oxide superconductor due to oxygen migration or
corrosion.
[0026] In one embodiment, the passivation layer covers at least a
portion of the surface of the superconductor. In another
embodiment, the passivation layer covers all portions of the
surface of the superconductor that would otherwise be exposed to
the ambient environment.
[0027] The passivation layer can be formed by modifying the surface
of the superconductor so that the passivation layer is formed at
least partially by material originating from the oxide
superconductor. In another embodiment, the passivation layer is an
ion-modified layer of the oxide superconductor.
[0028] The oxide superconductor, which can be YBCO, can be
deposited onto a suitable substrate and at least a portion of the
surface of the superconductor is then passivated. Optionally, a
buffer layer can be deposited onto the substrate prior to
deposition of the oxide superconductor. After the oxide
superconductor is deposited, it is then annealed in some
embodiments of the invention. Following annealing, the surface of
the oxide superconductor is treated by ion bombardment, followed by
a further optional annealing, heating at elevated temperature in an
oxygen-rich environment, and cooling in an oxygen-rich
environment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic representation of a side view of a
junction of the invention;
[0030] FIG. 2 shows the processing steps used to form the
interface-engineered junction;
[0031] FIG. 3 illustrates an I-V curve for an interface-engineered
junction over the temperature range 4.2 K to 60 K. R.sub.n of this
4 .mu.m.times.0.15 .mu.m device is 3 .OMEGA.;
[0032] FIG. 4 illustrates examples of the dependence of I.sub.c on
temperature for several junctions of the invention;
[0033] FIG. 5 illustrates the dependence of I.sub.c on applied
magnetic field at 40 K for a junction with R.sub.n of 2.9
.OMEGA.;
[0034] FIG. 6 shows the range of I.sub.cR.sub.n and R.sub.n which
is presently attainable at 4.2 K for the junction process. The data
are plotted as a function of J.sub.c, and the lines represent
least-square fits to the data. The junction area is 4
.mu.m.times.0.15 .mu.m.;
[0035] FIG. 7 shows values of I.sub.c and R.sub.n at 4.2 K for a
O-junction test chip. The 1.sigma. values are 7.8% for .degree. C.
and 3.5% for R.sub.n.
[0036] FIG. 8 is a graphic depiction of a series of X-ray
photoelectron spectra (XPS), taken over a period of time, of a
prior art YBCO film without the benefit of a passivation layer of
the invention;
[0037] FIG. 9 is a schematic cross-sectional view of a device
having a superconductor with a passivation layer in accordance with
the invention;
[0038] FIG. 10 lists the steps in a process for making a
superconductor with a passivation layer according to the
invention;
[0039] FIG. 11 lists the more detailed steps of the process
illustrated in FIG. 10;
[0040] FIG. 12 shows the XPS of the surface of a YBCO film without
the benefit of a passivation layer;
[0041] FIG. 13 shows the XPS of the surface of a YBCO film with a
passivation layer; the films studied in FIGS. 12 and 13 were made
on the same day, and both spectra were taken after the same number
of days after the films were made; and
[0042] FIG. 14 shows a comparison between the XPS of the passivated
surface of a YBCO film and YBa.sub.2Cu.sub.3O.sub.6 single
crystal.
DETAILED DESCRIPTION OF THE INVENTION
[0043] Formation of the Junctions of the Invention
[0044] FIG. 1 is a schematic cross-sectional representation of a
junction in accordance with one embodiment of the invention. The
drawing is not necessarily to scale. FIG. 2 is a flow schematic of
the preferred process for forming the superconducting device of the
invention.
[0045] A suitable crystalline substrate 10 for the structure is
selected. Any substrate suitable for supporting epitaxial growth of
an oxide superconductor, either alone or with an intermediate
buffer layer may be used. Suitable substrates include MgO,
LaAlO.sub.3, sapphire, yttrium-stabilized zirconia, strontium
titanite and the like. In one embodiment, the substrate is
LaAlO.sub.3. In another embodiment, the substrate is (001)
LaALO.sub.3. As used herein, the term crystalline substrate refers
to a support material having major crystallographic axes and having
a lattice structure suitable for the growth of a superconducting
oxide.
[0046] A buffer layer (not depicted in FIG. 2) may optionally be
deposited on the substrate prior to deposition of the
superconductive oxide. Buffer layers are generally used to provide
chemical isolation from the substrate or to provide an improved
lattice match between the substrate and the superconductive oxide.
Suitable buffer layers include CeO.sub.2, SrTiO.sub.3 and
CaTiO.sub.3. The buffer layer and the superconductive oxide may be
deposited using any of several known methods for achieving
epitaxial growth of the oxide, including laser ablation, and
reactive coevaporation. In the embodiment depicted in FIG. 1, a
buffer layer of CeO.sub.2 (not depicted in FIG. 1) and a first
superconducting layer 12 of YBa.sub.2Cu.sub.3O.sub.7-.delta.
(YCBO), about 150 nm thick in this case, were deposited on the
substrate. However, the invention should not be viewed as limited
to YBCO. Other oxide superconductors may also be used.
[0047] Any suitable methods known to those of skill in the art,
having read this specification, for depositing these layers can be
utilized. In one embodiment, deposition conditions for these
materials, when deposited by laser ablation, are: substrate
temperatures about 780.degree. C..+-.20.degree. C., oxygen pressure
100-600 mT. In another embodiment, about 300-400 mT, laser energy
about 1-5 J/cm.sup.2 at the target. More detailed process
conditions for epitaxial film growth are described in many
publications.
[0048] Next an insulator 14, in this case a layer of PBCO capped
with a layer of epitaxial SrTiO.sub.3 16, is deposited to prevent
contact between the first superconducting layer 12 and the second
superconducting layer 18. Other insulators having good insulating
properties, suitable lattice structures and chemical compatibility
with the superconductor may also be used.
[0049] The first superconducting layer 12 and the overlying
insulators 14, 16 are then patterned as desired. For the devices
whose test results are shown, patterning was accomplished with
standard photolithography and inert ion etching.
[0050] Prior to insertion into the laser ablation vacuum chamber,
the edge is prepared via an Ar ion mill clean at 200 V for about
one minute. Other voltages and times of Argon milling are within
the scope of the invention. The sample is then heated under vacuum.
In one embodiment, it is heated to about a range of 10.sup.-5 to
10.sup.-7 Torr to between 400 and 500.degree. C. In another
embodiment, it is heated to about 450.degree. C., for a period up
to about one hour. In yet another embodiment, it is heated for
about 30 minutes. At temperatures above about 500.degree. C. in
vacuum, bulk orthorhombic YBCO films may be completely converted to
a quasi-cubic phase.
[0051] Following this anneal, the pre-device is treated, in situ,
to uniformly convert only the exposed YBCO surface 20 to a slightly
different phase, structure, or chemical configuration. This is
generally referred to as ion treating the surface of the
superconductor. This can be accomplished, for example, by using a
background gas of Ar and/or O.sub.2 plasma generated by biasing the
substrate heater with an rf source. In one embodiment, the
background gas is Ar. Any mixture is suitable, but in one
embodiment, a 1:1 mixture of Ar/O.sub.2 is utilized. In one
embodiment, a plasma treatment with a forward power of about
100-400 watts in a background total pressure of about 10 to 100
mTorr is sufficient to form an interface material whose properties
allow fabrication of Josephson devices. In another embodiment, the
background total pressure of about 20-50 mTorr is utilized. The
typical reflected rf power is less than about 10 W and the dc
self-bias on the heater is typically about 600-900 V. Specific
conditions of the plasma can be adjusted to achieve junctions with
the desired parameters. The ion treatment, of which a plasma
treatment is one example, is generally done for several minutes to
an hour. In one embodiment the plasma treatment is done for about
20 minutes, followed by a further optional vacuum anneal, for about
30 minutes at about 400.degree. C., in one embodiment. Other gasses
may be used as the background gas, including noble gasses such as
Xe and halogens, such as F.sub.2 for example. If other gasses are
used, the conditions may need to be adjusted to achieve the desired
parameters. Persons skilled in the art would be able to make such
adjustments, having read this specification.
[0052] In one embodiment, following the vacuum anneal, about
300-400 mTorr of O.sub.2 is introduced and the temperature is
increased to about 785.degree. C. Immediately thereafter, the YBCO
counter-electrode is deposited under standard conditions. The
bottom YBCO is re-oxygenated before and/or during the deposition,
as its T.sub.c remains quite high. In one embodiment, chips are
patterned to form 5 junctions with widths of about 4 .mu.m
each.
[0053] Junction Properties
[0054] Devices of the invention display RSJ-type I-V
characteristics over the entire temperature range of operation;
examples are shown in FIG. 3. This behavior may be contrasted with
Co-YBCO edge junctions, whose temperature range of operation is
limited due to their exponential I.sub.c(T) dependence and the
eventual onset of superconductivity in the barrier layer.
[0055] FIG. 4 displays the I.sub.c(T) dependence for a few
junctions of the invention. Note that the behavior is quasi-linear
in accord with many types of high-T.sub.c devices. Also, note that
the temperature at which a critical current decrease is begun to be
seen with decreasing I.sub.c. This behavior is not due to thermal
noise rounding alone, however, but also depends on the value of
R.sub.n of the device.
[0056] These junctions can respond quite strongly to an applied
magnetic field. The critical current can modulate to zero in a few
gauss at high temperatures, and I.sub.c(H) displays a
Fraunhofer-like pattern, as shown in FIG. 5. The periodicity is
consistent with the physical width of the device if flux focusing
is taken into account. Even at 4.2 K where excess current is
notable, these junctions continue to modulate by 80 to 90% in the
best cases.
[0057] In FIG. 6, the ranges of I.sub.cR.sub.n and R.sub.n that are
attainable at 4.2 K are displayed. Note that I.sub.cR.sub.n
products from 0.3 to 5 mV are possible, with corresponding R.sub.nA
values of 3.times.10.sup.-7 to 1.times.10.sup.9 .OMEGA.cm.sup.2. A
clear scaling relation exists between I.sub.cR.sub.n and J.sub.c
similar to that observed for grain boundary weak links, although
the dependence is somewhat different for these interface-engineered
junctions.
[0058] Even at higher temperatures, the parameters of these
junctions may make them quite attractive for applications. For
example, at 40 K, I.sub.cR.sub.n values between 0.1 and 2 mV can be
obtained. Thus embodiments of the invention may include junctions
with I.sub.cR.sub.n products of 500 .mu.V and corresponding I.sub.c
and R.sub.n values of 500 .mu.A and 1 .OMEGA. for a size of 4
.mu.m.times.0.15 .mu.m.
[0059] Of course, these junctions may not be useful for
multi-junction circuits unless they can be made uniformly. The
spread of these junctions on 10- and 20-junction test chips have
been preliminary studied. A result from one of these tests is
displayed in FIG. 7. This 10-junction chip displayed a I.sub.c
spread in I.sub.c of 7.8% and a spread in R.sub.n of 3.5%. The
spreads over 20 junctions have been as low as 12% in I.sub.c thus
far.
[0060] The junctions of the invention may find uses in a variety of
applications. Josephson junctions are an essential feature of
Superconducting Quantum Interference Devices ("SQUID(s)"), which
are useful in magnetic sensing applications and as amplifiers. The
junctions are also useful in digital logic devices, such as in
high-speed switching and clock recovery circuits.
[0061] The invention provides methods of fabricating all-YBCO
Josephson junctions that can avoid the deposition of a barrier
layer. These devices are uniform and reproducible. The tested
devices have worked as a resistively shunted junction. Their
electrical characteristics are easily adjustable within a range
suitable for electronics circuit technology. For example, an
I.sub.c of several hundred .mu.A and an R.sub.n of 2 .OMEGA. at 40K
is ideally suited for RSFQ technology. The junctions are also well
suited to making 1 .OMEGA. SQUIDs with an I.sub.c of 1 mA at
40K.
[0062] It would be apparent to one skilled in the art, having read
this specification, that variations of the process are also within
the scope of the invention. Examples of such variations include the
use of other superconductive oxides, variation of the background
gas (and could include other noble gasses or halogens, alone or in
combination), and variations in time and temperature for various
steps, for example.
[0063] Formation of a Passivation Layer of the Invention
[0064] The invention also provides an oxide superconductor having a
protective surface layer that substantially prevents deterioration
of structural and physical properties of the superconductor due to
oxygen migration or corrosion. The protective surface layer is
referred to herein as a passivation layer.
[0065] Referring to FIG. 9, a superconductor device 200 includes an
oxide superconductor layer 220, which is typically formed on a
substrate 210. For example, the oxide superconductor layer 220 can
be epitaxially grown on the substrate 210. An example of a suitable
oxide superconductor is YBCO. However, it is to be realized that
the invention could be implemented using other oxide
superconductors.
[0066] Any substrate suitable for supporting growth of an oxide
superconductor can be used. For example, materials such as MgO,
LaAlO.sub.3, sapphire, yttrium-stabilized zirconia (YSZ), and
SrTiO.sub.3 can be used as the substrate 210. The substrate 210
typically has a chosen crystallographic orientation for epitaxial
growth of the oxide superconductor layer 220. For example, an (001)
LaAlO.sub.3 substrate can be used.
[0067] The substrate can also be a noncrystalline or
polycrystalline substrate with a suitably aligned crystalline
buffer layer deposited on top by various methods known to those of
ordinary skill in the art, having read this specification, such
that an epitaxial superconductor thin film may be deposited.
[0068] The device can also include an intermediate layer 230, which
can be a buffer layer that provides chemical isolation or improved
lattice match between the rest of the substrate 210 and the oxide
superconductor 220. Examples of suitable buffering materials
include, but are not limited to, CeO.sub.2, SrTiO.sub.3, and
CaTiO.sub.3. The intermediate layer 230 can also include other
layers in a multi-layer circuit. As illustrated in FIG. 9, the
intermediate layer 230 is between only a portion of the substrate
210 and the superconductor 220. However, the intermediate layer 230
can be between the entire substrate 210 and the superconductor
220.
[0069] The oxide superconductor layer 220 also includes a
passivation surface layer 240. A passivation surface layer can
function to substantially reduce or prevent oxygen out diffusion
(i.e. oxygen diffusing out of the layer 220) and/or corrosion. That
is, the passivation layer can be characterized by a substantially
lower oxygen mobility as compared to that of the oxide
superconductor. A passivation layer can also be characterized as
isolating the portion(s) of the superconductor covered thereby from
the ambient atmosphere or other non-conducting surroundings. The
passivation layer 240 can additionally be an electrical insulator,
as well as being corrosion resistant (i.e. the layer 240 prevents
reaction of the constituent atoms with other elements).
[0070] The passivation layer 240 need not cover the entire portion
of the superconductor surface. In another embodiment, the
passivation layer 240 covers all surfaces of the superconductor 220
that would otherwise be exposed to the ambient, as illustrated in
FIG. 9.
[0071] The passivation layer 240 can be a native layer to the oxide
superconductor 220, that is, a layer formed at least partially by
the material originating from the oxide superconductor 220. Native
passivation layers may provide a number of advantages over
externally applied coatings. For example, because the native
passivation layer is grown from the superconductor it covers, the
bonding between the two can be more intimate than that between a
superconductor and an externally applied coating. It is believed
that the native passivation layer is thus mechanically more robust
than externally applied coatings. In addition, the native
passivation layer may also have a more uniform thickness and
construction, i.e. be free of holes. The lack of holes can be an
advantage because holes can provide spots for corrosion to occur.
Uniformity in thickness can also be important if an additional
layer(s) is to be grown on top of the passivation layer 240.
[0072] Specifically, the passivation layer 240 can be an
ion-modified layer of the oxide superconductor 220. For example,
the passivation layer 240 can be a plasma-treated surface layer of
the oxide superconductor 220. The passivation layer 240 can be of
any suitable thickness. For example, the layer 240 can be as thin
as between about 1-5 nm. In another embodiment, the passivation
layer 240 can be as thin as between about 2-3 mm.
[0073] The passivation layer 240 can be covered by one or more
additional layers of material if desired. For example, one or more
protective layers could be disposed on the passivation layer 240 to
protect against scratches and other damage to the passivation
layer. In addition, because the passivation layer 240 is epitaxial
and crystalline, one or more epitaxial layers can be grown on top
of the layer 240. For example, a Josephson junction having a
junction layer produced by ion-bombardment of an oxide
superconductor film could be produced as discussed above.
[0074] A method for passivating the surface of an oxide
superconductor includes modifying the surface. The modification can
be accomplished by bombarding the surface with ions, including
argon and oxygen ions. Ion bombardment can be accomplished by
treating the surface of the oxide superconductor with a plasma of
ions, for example argon or oxygen or a mixture of both. Ion
bombardment can also be accomplished by using any other type of ion
source known to one of skill in the art, having read this
specification, including ion beam sources. The ion bombardment can
be performed at elevated temperatures, with vacuum annealing at
elevated temperature, along with a high temperature oxygen anneal
as described further below.
[0075] Referring to FIG. 10, a method of making a superconductor
device includes depositing (step 310) an oxide superconductor film
on a substrate and passivating the surface of the superconductor
film (step 320). Another embodiment of a method in accordance with
the invention can be seen in FIG. 11, this method includes first
depositing (step 410) an oxide superconductor film on a substrate.
A buffer layer is optionally deposited on the substrate prior to
deposition of the oxide superconductor. The oxide superconductor
film and buffer layer can be deposited using any of several known
methods, including but not limited to laser ablation, sputtering,
and reactive coevaporation. For example, for epitaxial deposition
by laser ablation, the substrate temperatures can be about
780.degree. C..+-.20.degree. C., the oxygen pressure between about
100-600 mTorr. In another embodiment, the oxygen pressure is
between about 300-400 mTorr, and laser energy about 1-5 J/cm.sup.2
at the target. Further details regarding process conditions for
epitaxial film growth are known in the art. See, for example K.
Char and V. Matijasevic, "HTS film growth," in Engineering
Superconductivity, ed. by P. J. Lee (Wiley, New York), 2001, which
is incorporated herein by reference.
[0076] Next, the oxide superconductor film can be annealed (step
420), either in the same deposition chamber or after being
transferred to another vacuum chamber. For a YBCO film, for
example, the background pressure can range from about 10.sup.-7 to
about 10.sup.-5 Torr. Inert gas annealing could also be used. The
annealing temperature can be between about 400 and 500.degree. C.
In another embodiment, it is between about 450.degree. C. Annealing
time can vary from about 15 minutes to about one hour. In another
embodiment, the anneal time is about 30 minutes. Choices of
alternative and other annealing parameters are well within the
capabilities of persons skilled in the art, having read this
specification.
[0077] Following the annealing, the device is treated to convert
the exposed YBCO surface to a different phase, structure, or
chemical configuration. This can be accomplished by ion bombardment
(step 430) followed by annealing (step 440) of the surface. The ion
bombardment (step 430) can be performed by treating the surface
with a plasma of Ar or an Ar--O.sub.2 mixture, for example, a 1:1
mixture of Ar and O.sub.2. For example, the Ar background pressure
can range from about 10 to about 100 mTorr. In one embodiment, the
background pressure ranges between about 20-50 mTorr. In another
embodiment, the background pressure is about 20 mTorr. Noble gasses
such as Xe and halogens such as F.sub.2 could also be used in place
of Ar and O.sub.2.
[0078] The plasma can be generated by an RF source in a manner
known to those of skill in the art, having read this specification.
The forward RF power that is used can be between about 100 and
about 400 watts. In another embodiment, it is about 300 watts. The
typical reflected RF power is less than about 10 W and the DC
self-bias on the heater can be set at between about -600 to about
-1000 V. In one embodiment, the DC self-bias on the heater is set
at about -800 V.
[0079] The device can be heated by a substrate heater, which can be
electrically biased by being electrically connected to a suitable
source. The temperature of the device can be held at a temperature
between about 300.degree. C. to about 650.degree. C. In one
embodiment, the temperature is held at about 400.degree. C., while
the surface of the oxide superconductor is subjected to the plasma
treatment. The plasma treatment can last between two minutes and an
hour. In one embodiment, it lasts between about five and twenty
minutes. In yet another embodiment, it lasts about ten minutes.
Specific plasma treatment parameters can be adjusted by one of
skill in the art, having read this specification, to achieve the
desired characteristics of the passivation layer.
[0080] The ion bombardment can be followed by a further vacuum
annealing (step 440). The annealing conditions can be similar to
the first annealing (step 420). For example, the annealing step 440
can last about 30 minutes at temperatures between about 300.degree.
C. to about 600.degree. C. In another embodiment, the temperature
is about 400.degree. C., at a pressure of from about 10.sup.-7 to
about 10.sup.-5 Torr. Other annealing conditions can also be used,
as would be apparent to persons skilled in the art, having read
this specification.
[0081] Following the vacuum annealing step 440, the oxide
superconductor film is heated in an oxygen-rich environment (step
450). It is thought, but not relied upon, that this heating step
results in a passivation layer that has a different phase, or
crystal structure, than the oxide superconductor film. It has been
found that the passivation layer is quasi-cubic and its composition
is probably not "1-2-3". It is thought that the heating may anneal
and remove atomic defects from the passivation layer already
partially formed during ion bombardment to "perfect" the final
passivation layer, or cause the new phase to form by allowing the
atoms in the ion-bombarded regions to rearrange. The oxygen
pressure can be set at between about 100 to about 600 mTorr. In one
embodiment between about 300 to about 400 mTorr, and in anther
embodiment at about 350 mTorr. The temperature can be set at
between about 700.degree. C. to about 800.degree. C., and in
another embodiment at about 785.degree. C. The heating step 450
lasts about 10 minutes in one embodiment.
[0082] The device is then cooled down to room temperature in an
oxygen-rich environment in step 460. For example, the oxygen
pressure can be increased to about 600 Torr after the heating step
450.
[0083] The process discussed above can be varied in other ways to
produce a variety of devices without departing from the spirit and
scope of the invention. For example, the surface layer 240 can be
doped with various elements to achieve device properties depending
on the specific applications in which the device is to be used. In
addition, it is contemplated that the first annealing step could be
left out.
WORKING EXAMPLES
Example 1
[0084] Two YBCO films were grown under substantially the same
conditions on the same day. The first film did not undergo surface
treatment, whereas the second film was plasma-treated as described
above.
[0085] The first film was grown by pulsed laser ablation using a
KrF excimer laser operating at a repetition frequency of 10 Hz;
fluence on the YBCO target was about 1 J/cm.sup.2. The
single-crystal LaAlO.sub.3 substrate was attached to the heater
with silver paste and was located about 5 cm from the target. A
20-nm-thick CeO.sub.2 buffer layer was first deposited at a
substrate temperature of 785.degree. C. and a background oxygen
pressure of 100 mTorr. A 200-nm-thick YBCO film was deposited
immediately thereafter at a substrate temperature of 785.degree. C.
in a background oxygen pressure of 350 mTorr. Following deposition,
the vacuum chamber was backfilled to an oxygen pressure of 600
Torr, and the substrate temperature was decreased to room
temperature at a rate of 30.degree. C./min. This fabrication
procedure is standard. Similar deposition procedures by pulsed
laser deposition are well-known to those skilled in the art. The
YBCO thin films may be deposited by any technique, however, and the
surface treatment discussed here does not depend on the thin film
deposition method.
[0086] The second YBCO thin film was deposited in a manner
identical to that outlined above, except that immediately following
deposition, the surface treatment procedure of Applicant's
invention was employed. Following growth of the YBCO film, the
deposition chamber was backfilled to 600 Torr of oxygen and the
substrate temperature was reduced to 450.degree. C. The chamber was
then pumped to vacuum (<10.sup.-5 Torr) and held at this
temperature and pressure for 30 minutes. The substrate temperature
was then decreased to 400.degree. C. Argon gas was introduced to a
pressure of 20 mTorr, and a plasma was generated by biasing the
heater with an RF source. The forward power of the RF source varied
from 300 to 342 W, the reflected power varied 8 to 9 W, and the DC
self-bias was fixed at -900 V. This plasma treatment was performed
for 10 minutes. The RF source was then turned off, the deposition
chamber was pumped to vacuum, and the sample was annealed at
400.degree. C. for 31 minutes. Oxygen was then introduced into the
chamber to a pressure of 350 mTorr, and the substrate was heated to
785.degree. C. at 30.degree. C./min, and was held at this
temperature for 10 minutes. The oxygen pressure was then increased
to 600 Torr, and the substrate temperature was decreased to room
temperature at a rate of 30.degree. C./minute.
[0087] Both films were then stored in a desiccator. As shown in
FIG. 12, without the surface treatment of the invention, after four
months, the Ba 4d XPS (510) of the film surface contains only two
peaks 520 and 530 of higher binding energies; the lower energy peak
(peak 112 in FIG. 8) has disappeared, indicating that the
superconducting YBCO phase had deteriorated, at least near the
surface.
[0088] In contrast, as shown in FIG. 13, with the surface treatment
according to the invention, the XPS (610) taken at substantially
the same time as that shown in FIG. 12 still includes all three
peaks 620, 630 and 640, indicating that the superconducting YBCO
has been preserved. Thus, the plasma-modified surface phase of the
YBCO superconductor film has been shown to be stable over time,
structurally robust and substantially impervious to oxygen
migration.
Example 2
[0089] The photoemission spectra of the surface of an YBCO film
treated with plasma were compared with those of an
YBa.sub.2Cu.sub.3O.sub.6 single crystal.
[0090] The plasma treated YBCO film was prepared as follows. A
CeO.sub.2 buffer layer was deposited onto a (100) LaAlO.sub.3
single-crystal substrate by pulsed laser ablation to a thickness of
approximately 200 nm. The substrate temperature was 785.degree. C.,
and the background oxygen pressure was 100 mTorr. The excimer laser
conditions were identical to those mentioned previously. The YBCO
thin film was deposited immediately thereafter at a substrate
temperature of 785.degree. C., oxygen pressure of 350 mTorr, and to
a thickness of approximately 200 nm. Following deposition, the
deposition vacuum chamber was backfilled with O.sub.2 to a pressure
of 600 Torr, and the substrate temperature was decreased to
450.degree. C. at a rate of 30.degree. C./min. The chamber was then
pumped to a vacuum pressure of 9.times.10.sup.-7 Torr and held at
this pressure and substrate temperature of 450.degree. C. for 30
minutes. The substrate temperature was then decreased to
400.degree. C., and Ar gas was subsequently introduced into the
chamber to a pressure of 20 mTorr. A plasma was struck and the YBCO
surface was treated for 10 minutes. The forward power of the RF
source used to generate the Ar plasma was 302 W at the beginning of
this period, and increased to 350 W by the end of the 10 minute
treatment time. The reflected power varied from 7 to 8 W, and the
DC bias on the heater was fixed at -900 V. Following the plasma
treatment, the chamber was pumped to a vacuum pressure of
5.times.10.sup.-7 Torr, and the sample was held at this pressure
and a temperature of 400.degree. C. for 31 minutes. O.sub.2 gas was
subsequently introduced into the system to a pressure of 350 mTorr,
and the substrate temperature was increased to 785.degree. C. at a
rate of 30.degree. C./min. The sample was held at this pressure and
temperature for 8 minutes. The chamber was then backfilled with
O.sub.2 gas to a pressure of 600 Torr, and the substrate
temperature was decreased to room temperature at a rate of
30.degree. C./min.
[0091] Plot (a) in FIG. 14 shows a spectrum from the plasma-treated
YBCO film for energies around -2 eV relative to the Fermi energy,
E.sub.F. The spectrum exhibits a kink 710 near -2.2 eV. This kink
feature is similarly shown in the spectrum from the
YBa.sub.2Cu.sub.3O.sub.6 single crystal, as illustrated in plot
(b), which also exhibits a kink 720 at about -2.2 eV. This feature
is not present in untreated YBCO films, whether the films were
annealed in oxygen (ozone) or vacuum.
[0092] Because YBa.sub.2Cu.sub.3O.sub.6 is an insulator, the
similarities in the photoemission spectra in FIG. 14 suggests that
the plasma-treated YBCO superconductor surface according to the
invention also has insulating properties.
[0093] The device and process disclosed above are particularly
applicable to oxide superconductors such as YBCO in which oxygen
migration is significant and detrimental. However, the device and
process can also be used for other superconductors to produce
passivated surfaces to prevent or reduce atomic migration or
corrosion.
[0094] While the foregoing disclosure contains many specificities,
it should be understood that these are given by way of example
only. The scope of the invention should not be limited by the
specific examples given above, but only by the appended claims and
their legal equivalents.
* * * * *