U.S. patent application number 10/320110 was filed with the patent office on 2003-08-07 for high temperature superconductor josephson junction element and manufacturing method for the same.
Invention is credited to Koshizuka, Naoki, Sato, Tetsuro, Tanaka, Shoji, Wen, Jian-Guo.
Application Number | 20030146432 10/320110 |
Document ID | / |
Family ID | 26487956 |
Filed Date | 2003-08-07 |
United States Patent
Application |
20030146432 |
Kind Code |
A1 |
Sato, Tetsuro ; et
al. |
August 7, 2003 |
High temperature superconductor Josephson junction element and
manufacturing method for the same
Abstract
In a method of manufacturing a Josephson junction, a first
superconductive layer is formed on a substrate. An insulating film
is formed on the first superconductive layer. The insulating film
is etched to have an inclination portion. The first superconductive
layer is etched using the etched insulating film as a mask, to have
an inclination portion. A barrier layer is formed on a surface of
the inclination portion of the first superconductive layer. A
second superconductive layer is formed on the barrier layer and the
inclination portion of the insulating layer.
Inventors: |
Sato, Tetsuro; (Tokyo,
JP) ; Wen, Jian-Guo; (Tokyo, JP) ; Koshizuka,
Naoki; (Tokyo, JP) ; Tanaka, Shoji; (Tokyo,
JP) |
Correspondence
Address: |
Scully, Scott, Murphy & Presser
400 Garden City Plaza
Garden City
NY
11530
US
|
Family ID: |
26487956 |
Appl. No.: |
10/320110 |
Filed: |
December 16, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10320110 |
Dec 16, 2002 |
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09385710 |
Aug 30, 1999 |
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6541789 |
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Current U.S.
Class: |
257/31 ;
257/E39.015 |
Current CPC
Class: |
H01L 39/225 20130101;
H01L 39/2496 20130101 |
Class at
Publication: |
257/31 |
International
Class: |
H01L 029/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 1, 1998 |
JP |
247418/1998 |
Jun 9, 1999 |
JP |
162025/1999 |
Claims
What is claimed is:
1. A method of manufacturing a Josephson junction comprising:
forming a first superconductive layer on a substrate; forming an
insulating film on said first superconductive layer; etching said
insulating film to have an inclination portion; etching said first
superconductive layer using said etched insulating film as a mask,
to have an inclination portion; forming a barrier layer on a
surface of said inclination portion of said first superconductive
layer; and forming a second superconductive layer on said barrier
layer and said inclination portion of said insulating layer.
2. A method according to claim 1, wherein said first
superconductive layer and said second superconductive layer are a
copper oxide system superconductive layer.
3. A method according to claim 2, wherein said copper oxide system
superconductive layer is YBaCuO superconductive layer.
4. A method according to claim 1, wherein said second
superconductive layer is formed in a same sample chamber without
being exposed to an atmosphere, after said inclination portion of
said first superconductive layer is formed.
5. A method according to claim 1, wherein said barrier layer has a
film thickness equal to or less than 2 nm.
6. A method according to claim 1, wherein said barrier layer has a
perovskite structure.
7. A method according to claim 6, wherein a material of said
barrier layer has a lattice constant from 0.41 nm to 0.43 nm.
8. A method according to claim 1, wherein said insulating film
contains at least one element selected from the group consisting of
La, Sr, Al, Ca and Ta.
9. A method according to claim 1, wherein a composition of said
barrier layer is Y.sub.1-xBa.sub.1Cu.sub.xO.sub.y (x being in a
range of 0 to 1).
10. A method according to claim 9, wherein said x is equal to or
less than 0.5.
11. A method according to claim 10, wherein said x is 0.
12. A method according to claim 1, wherein said barrier layer
includes at least one element selected from the group consisting of
La, Sr, Al, Ca and Ta which are supplied from said insulating layer
or said substrate.
13. A method according to claim 1, wherein said first
superconductive layer and said insulating layer are formed by a
pulsed laser deposition method.
14. A method according to claim 1, wherein said first
superconductive layer is formed at a first substrate temperature,
and said insulating layer is formed at a second substrate
temperature lower than said first substrate temperature.
15. A method according to claim 1, wherein said etching a first
superconductive layer includes: etching said first superconductive
layer while inert gas ions are irradiated.
16. A method according to claim 15, wherein said etching a first
superconductive layer includes: etching said first superconductive
layer while inert gas ions are irradiated and said substrate is
rotated.
17. A method according to claim 1, wherein said insulating layer is
etched while said first superconductive layer is etched such that
at least one of the elements of said insulating layer is
re-deposited on a surface of said inclination portion of said first
superconductive layer.
18. A method according to claim 17, wherein a surface portion of
said inclination portion where said at least one element is
re-deposited forms an amorphous layer.
19. A method according to claims 18, further comprising heating
said substrate in an oxygen ambience and crystallizing said surface
portion of said inclination portion where said at least one element
is re-deposited.
20. A method according to claim 19, wherein said crystallizing is a
part of said forming a second superconductive layer.
21. A method according to claim 1, further comprising: changing a
structure and composition of a surface portion of said inclination
portion of said first superconductive layer through an ion
irradiation damage.
22. A method according to claim 21, wherein said changing includes:
crystallizing said surface portion of said inclination portion of
said first superconductive layer, while a heating is carried out to
said first superconductive layer in an oxygen ambience.
23. A method of manufacturing a Josephson junction comprising:
forming a first superconductive layer on a substrate; forming an
insulating film on said first superconductive layer; etching said
insulating film and said first superconductive layer to have an
inclination portion; forming a barrier layer on a surface of said
inclination portion of said first superconductive layer; and
forming a second superconductive layer on said barrier layer and
said inclination portion of said insulating layer.
24. A method according to claim 23, wherein said first
superconductive layer and said second superconductive layer are a
copper oxide system superconductive layer.
25. A method according to claim 24, wherein said copper oxide
system superconductive layer is YBaCuO superconductive layer.
26. A method according to claim 23, wherein said second
superconductive layer is formed in a same sample chamber without
being exposed to an atmosphere, after said inclination portion of
said first superconductive layer is formed.
27. A method according to claim 23, wherein said barrier layer has
a perovskite structure.
28. A method according to claim 27, wherein a material of said
barrier layer has a lattice constant from 0.41 nm to 0.43 nm.
29. A method according to claim 23, wherein said insulating film
contains at least one element selected from the group consisting of
La, Sr, Al, Ca and Ta.
30. A method according to claim 23, wherein a composition of said
barrier layer is Y.sub.1-xBa.sub.1Cu.sub.xO.sub.y (x being in a
range of 0 to 1).
31. A method according to claim 30, wherein said x is equal to or
less than 0.5.
32. A method according to claim 31, wherein said x is 0.
33. A method according to claim 23, wherein said barrier layer
includes at least one element selected from the group consisting of
La, Sr, Al, Ca and Ta which are supplied from said insulating layer
or said substrate.
34. A method according to claim 23, wherein said first
superconductive layer and said insulating layer are formed by a
pulsed laser deposition method.
35. A method according to claim 23, wherein said first
superconductive layer is formed at a first substrate temperature,
and said insulating layer is formed at a second substrate
temperature lower than said first substrate temperature.
36. A method according to claim 23, wherein said etching a first
superconductive layer and an insulating layer includes: etching
said first superconductive layer and said insulating layer while
inert gas ions are irradiated.
37. A method according to claim 36, wherein said etching a first
superconductive layer and an insulating layer includes: etching
said insulating layer and said first superconductive layer while
inert gas ions are irradiated and said substrate is rotated.
38. A method according to claim 23, wherein said forming a barrier
layer includes: ion-etching a surface of said inclination portion
of said insulating film and a surface of said inclination portion
of said first superconductive layer, and wherein said ion-etching
includes: ion-etching said surface of said inclination portion of
said insulating film and said surface of said inclination portion
of said first superconductive layer, such that at least one of
elements of said insulating layer re-deposited on said surface of
said inclination portion of said first superconductive layer.
39. A method according to claim 38, wherein a surface portion of
said inclination portion where said at least one element is
re-deposited forms an amorphous layer.
40. A method according to claim 39, further comprising heating said
substrate in an oxygen ambience and crystallizing said surface
portion of said inclination portion where said at least one element
is re-deposited.
41. A method according to claim 23, further comprising: changing a
structure and composition of a surface portion of said inclination
portion of said first superconductive layer through an ion
irradiation damage.
42. A method according to claim 41, wherein said changing includes:
crystallizing said surface portion of said inclination portion of
said first superconductive layer, while a heating is carried out to
said first superconductive layer in an oxygen ambience.
43. A method of manufacturing the Josephson junction comprising:
forming a first super conductive layer on a substrate; forming an
interlayer insulating layer as a barrier layer on said first
superconductive layer; forming a second superconductive layer on
said barrier layer; etching said second superconductive layer and
said barrier layer to form 2 or more electrodes; forming an
insulating layer between said 2 or more electrodes.
44. A method according to claim 43, wherein said first
superconductive layer and said second superconductive layer are a
copper oxide system superconductive layer.
45. a method according to claim 44, wherein said copper oxide
system superconductive layer is YBaCuO superconductive layer.
46. A method according to claim 43, wherein said barrier layer has
a film thickness equal to or less than 2 nm.
47. A method according to claim 43, further comprising: carrying
out ion irradiation to a surface portion of said first
superconductive layer to give a damage to said surface portion of
said first superconductive layer before said barrier layer is
formed on said first superconductive layer.
48. A method according to claim 47, wherein said carrying out ion
irradiation includes: carrying out said ion irradiation at a
temperature of said substrate in a range of room temperature to
700.degree. C.
49. A Josephson junction comprising: a first superconductive layer
formed on substrate and having an inclination portion; an
insulating film formed on said first superconductive layer and
having an inclination portion following to said inclination portion
of said first superconductive layer; a barrier layer formed on said
inclination portion of said first superconductive layer; and a
second superconductive layer formed on said inclination portion of
said insulating film and said inclination portion of said barrier
layer.
50. A Josephson junction according to claim 49, wherein said
barrier layer has a film thickness equal to or less than 2 nm.
51. A Josephson junction according to claim 49, wherein said first
superconductive layer and said second superconductive layer are a
copper oxide system superconductive layer.
52. A Josephson junction according to claim 51, wherein said copper
oxide system superconductive layer is YBaCuO superconductive
layer.
53. A Josephson junction according to claim 49, wherein said
barrier layer has a perovskite structure.
54. A Josephson junction according to claim 53, wherein a material
of said barrier layer has a lattice constant from 0.41 nm to 0.43
nm.
55. A Josephson junction according to claim 49, wherein said
insulating film contains at least one element selected from the
group consisting of La, Sr, Al, Ca and Ta.
56. A Josephson junction according to claims 49, wherein a
composition of said barrier layer is
Y.sub.1-xBa.sub.1Cu.sub.xO.sub.y (x being in a range of 0 to
1).
57. A Josephson junction according to claim 56, wherein said x is
equal to or less than 0.5.
58. A Josephson junction according to claim 57, wherein said x is
0.
59. A Josephson junction device comprising: a first superconductive
layer formed on a substrate; 2 or more interlayer insulating films
provided on said first superconductive layer to be apart from each
other; and 2 or more second superconductive layers respectively
formed on said interlayer insulating layers.
60. A Josephson junction device according to claim 59, wherein each
of said interlayer insulating films has a film thickness equal to
or less than 2 nm.
61. A Josephson junction device according to claim 59, wherein said
first superconductive layer and said second superconductive layer
are a copper oxide system superconductive layer.
62. A Josephson junction device according to claim 61, wherein said
copper oxide system superconductive layer is YBaCuO superconductive
layer.
63. A method of manufacturing a Josephson junction, wherein a
material of an insulating layer which is formed on a first
superconductive layer is diffused into said first superconductive
layer to form a barrier layer on a surface portion of said first
superconductive layer, and a second superconductive layer is formed
on said barrier layer to form a Josephson junction.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a Josephson junction device
which uses a high temperature oxide superconductor and has small
spread in characteristic and a manufacturing method for the
same.
[0003] 2. Description of the Related Art
[0004] A high temperature oxide superconductor of a YBaCuO system
has a high superconductor critical temperature (Tc) of about 90 K.
Therefore, a non-expensive liquid-nitrogen having the boiling point
of 77 K can be used as refrigerant, and a small and handy freezer
that the low temperature of about 60 K can be easily achieved can
be used as a cooling means. Also, the facilities for the
maintenance of the low temperature can be simplified.
[0005] Conventionally, a superconductive electronic device and
circuit with a high speed and low power consumption have been
realized using a material such as Nb having a low superconductor
critical temperature Tc. For the above reasons, if the device and
circuit are possible to be realized using the high temperature
superconductor, it contributes greatly to the industry.
[0006] A superconductor of a YBaCuO system is a main object for the
study and development of application to the electronic device at
present. The YBaCuO system high temperature superconductor thin
film indicative of a good superconductive characteristic can be
obtained through vapor-phase growth under the condition of the high
substrate temperature of 600 to 800.degree. C. and the high oxygen
partial pressure in the order of 100 mTorr. One of the
superconductive thin films having especially good characteristic
has c-axis orientation in which a c-axis of its crystal structure
is perpendicular to the substrate surface. On the other hand,
depending on the growth condition, it is possible to form an a- or
b-axis orientation film. However, the a- or b-axis orientation film
is inferior in the superconductive characteristics such as the
superconducting critical temperature Tc and the superconducting
critical current value Ic, compared with the c-axis orientation
film.
[0007] A substrate formed of SrTiO.sub.3, MgO, LaAlO.sub.3 or
NdGaO.sub.3 is generally used for the growth of the superconductive
thin film in consideration of matching with YBaCuO in lattice
constant and thermal expansion coefficient and non-occurrence of
solid phase reaction. Also, LaSrAlTaO is used as the substrate,
since it has a good lattice matching with YBaCuO and has a low
dielectric constant and a relatively large substrate can be formed,
as shown in, for example, Journal of Crystal Growth, Vol. 109,
pp.447-456, 1991.
[0008] The YBaCuO system has a strong anisotropic characteristic
like the other high temperature superconductors. The
superconductive coherence length is longer in the a- or b-axis
direction than in the c-axis direction. The coherence length in the
c-axis direction is as very short as about 0.3 nm. Therefore, it is
desirable to flow a current in the a- or b-axis direction in wiring
sections and Josephson junctions in the superconductor circuit
using a high temperature superconductor.
[0009] For these reasons, when a high temperature superconductor
electronic circuit is manufactured using the high quality c-axis
orientation thin film, it is difficult to manufacture the Josephson
junction of the high quality in a sandwich type Josephson junction
in which it is necessary to flow current in the film thickness
direction, i.e., in the c-axis direction of the high temperature
superconductor crystal, unlike the conventional superconductor
circuit using Nb. In this case, an edge type Josephson junction
device using an edge part of the superconductive thin film is
suitable rather than the sandwich type Josephson junction device,
as shown in "IEEE TRANSACTIONS ON MAGNETICS, VOL.27, NO.2, MARCH,
1991, pp.3062-3065". Therefore, the study and development of the
high temperature superconductor circuit is mainly performed using
the edge type Josephson junction device.
[0010] In the high temperature superconductor Josephson junction
device, the technique is not established for forming a very thin
barrier layer as an Al.sub.2O.sub.3 barrier layer used in the Nb
system Josephson junction device having a low critical temperature
Tc. Therefore, a non-superconductive oxide film such as a PrBaCuO
system film and a superconductive oxide film such as a Co doped,
YBaCuO system film are deposited on a lower superconductive layer
as a barrier layer. The PrBaCuO system crystal has a similar
crystal structure to the YBaCuO system crystal and the PrBaCuO
system non-superconductor oxide is easy to hetero-epitaxially grow
on the YBaCuO layer. Also, the Co doped YBaCuO system
superconductive film has a critical temperature Tc lower than
YBaCuO superconductive film.
[0011] In the above, there is a problem in the spread of junction
characteristics such as a superconductive critical current value Ic
in a high temperature superconductor Josephson junction device. In
the barrier layer forming method using the thin film growth method,
it is difficult to grow the thin barrier layer sufficiently
uniformly. As a result, the coverage of the lower superconductive
layer by the barrier layer is low, so that the spread of the
characteristics is large between the junctions. For this reason,
the study of the high temperature superconductor Josephson junction
device is accomplished by use of an interface control technique
without using thin film growth technique, as shown in "APPLIED
PHYSICS LETTERS, VOL.71, NO.17, OCTOBER, 1997, pp.2526-2528". In
this method, after the lower superconductive layer is etched to a
predetermined shape, a combination of an annealing process in a
vacuum and the irradiating process of accelerated ions is
performed. Through the combination process, the surface portion of
the lower superconductive layer is changed in the crystal structure
to have a function as the barrier layer. However, there is the
spread of critical current value Ic of (1.sigma.=.+-.8%) even in
the 10 samples of the high temperature superconductor Josephson
junction device manufactured by the above method.
[0012] Also, as the superconductive material for the high
temperature superconductor Josephson junction device, a copper
oxide system material is used and is practicable in the high
temperature region. In this case, however, it is especially
difficult to form the barrier layer. Because the coherence length
is as short as about 1 to 2 nm in the copper oxide system material,
it is necessary that the barrier layer of the Josephson junction
also has the film thickness approximately equal to the coherence
length. When the film thickness of the barrier layer becomes
thicker than the above coherence length, the Josephson current
becomes difficult to flow and the quality of the Josephson junction
is degraded. As a result, the function sometimes becomes not
attained. However, in the conventional method, it is very difficult
to form the barrier layer of such a thin film in actual.
[0013] In conjunction with the above description, a Josephson
junction device and a manufacturing method for the same are
disclosed in Japanese Laid Open Patent Application (JP-A-Heisei
3-94486). In this reference, a high temperature oxide
superconductor of LnBa.sub.2Cu.sub.3O.sub.7-8 and an insulator of
Ln.sub.2BaCuO.sub.5 which is composed of the same elements as the
superconductor are continuously formed in a sputtering apparatus.
Then, an annealing process is carried out at a predetermined
temperature in an oxygen atmosphere to produce a Josephson junction
device.
[0014] Also, a method of manufacturing a barrier layer type
electronic device is disclosed in Japanese Laid Open Patent
Application (JP-A-Heisei 4-317381). In this reference a YBaCuO
oxide superconductor thin film is formed on a substrate. The
forming process is carried out by a CVD method, a sputtering method
or a vapor deposition method. Next, Fe ions are injected into the
oxide superconductor thin film, and the injected ions are diffused
in the internal direction of the thin film by a heating process to
form a barrier layer. Next, the surface of the barrier layer which
has received physical damage through the ion implantation is
removed. After that, a YBaCuO oxide superconductor thin film is
formed on the barrier layer as a counter electrode of the oxide
superconductor thin film.
[0015] Also, in Japanese Laid Open Patent Application (JP-A-Heisei
10-173246) is disclosed a high temperature SSNS, an SNS Josephson
junction device and a manufacturing method for the same. In this
reference, a first superconductive (HTS) layer of the high critical
temperature Tc is formed on a substrate, and a first dielectric
layer is formed on the HTS layer. The first HTS layer and the
dielectric layer have an inclined edge. A 3-layer SNS structure is
arranged on the inclined edge to form 4-layer SSNS junction.
[0016] Also, the study result of the factor which influences on the
resistance of the SNS edge junction using Co-doped YBaCuO as a
usual metal layer is described in "High-Resistance SNS Edge
Junctions" by Brian D. Hunt (6th International Superconductive
Electronics Conference 1997).
SUMMARY OF THE INVENTION
[0017] Therefore, an object of the present invention is to provide
a high temperature superconductor Josephson junction device in
which a barrier layer having a uniform film thickness and
composition can be formed with good reproducibility and with a
small spread and which is superior in characteristic.
[0018] Another object of the present invention is to provide a
Josephson junction device in which a good barrier layer can be
formed using a copper oxide system material as a superconductive
material, and which is practicable in a higher temperature.
[0019] In order to achieve an aspect of the present invention, a
method of manufacturing a Josephson junction include:
[0020] forming a first superconductive layer on a substrate;
[0021] forming an insulating film on the first superconductive
layer;
[0022] etching the insulating film to have an inclination
portion;
[0023] etching the first superconductive layer using the etched
insulating film as a mask, to have an inclination portion;
[0024] forming a barrier layer on a surface of the inclination
portion of the first superconductive : layer; and
[0025] forming a second superconductive layer on the barrier layer
and the inclination portion of the insulating layer.
[0026] In order to achieve another aspect of the present invention,
a method of manufacturing a Josephson junction include:
[0027] forming a first superconductive layer on a substrate;
[0028] forming an insulating film on the first superconductive
layer;
[0029] etching the insulating film and the first superconductive
layer to have an inclination portion;
[0030] forming a barrier layer on a surface of the inclination
portion of the first superconductive layer; and
[0031] forming a second superconductive layer on the barrier layer
and the inclination portion of the insulating layer.
[0032] In the above, the first superconductive layer and the second
superconductive layer preferably are a copper oxide system
superconductive layer. In this case, the copper oxide system
superconductive layer may be YBaCuO superconductive layer.
[0033] Also, the second superconductive layer may be formed in a
same sample chamber without being exposed to an atmosphere, after
the inclination portion of the first superconductive layer is
formed.
[0034] Also, the barrier layer preferably has a film thickness
equal to or less than 2 nm. Also, the barrier layer preferably has
a perovskite structure. In this case, a material of the barrier
layer may have a lattice constant from 0.41 nm to 0.43 nm.
[0035] Also, the insulating film may contain at least one element
selected from the group consisting of La, Sr, Al, Ca and Ta.
[0036] Also, a composition of the barrier layer may be
Y.sub.1-xBa.sub.1Cu.sub.xO.sub.y (x being in a range of 0 to 1).
Especially, x may be equal to or less than 0.5 or 0.
[0037] Also, the barrier layer preferably includes at least one
element selected from the group consisting of La, Sr, Al, Ca and Ta
which are supplied from the insulating layer or the substrate.
[0038] Also, the first superconductive layer and the insulating
layer are formed by a pulsed laser deposition method.
[0039] Also, the first superconductive layer may be formed at a
first substrate temperature, and the insulating layer may be formed
at a second substrate temperature lower than the first substrate
temperature.
[0040] Also, the etching of a first superconductive layer may
include etching the first superconductive layer while inert gas
ions are irradiated. In this case, the etching a first
superconductive layer may include etching the first superconductive
layer while inert gas ions are irradiated and the substrate is
rotated.
[0041] Also, the etching a first superconductive layer and an
insulating layer may include etching the first superconductive
layer and the insulating layer while inert gas ions are irradiated.
In this case, the etching a first superconductive layer and an
insulating layer may include etching the insulating layer and the
first superconductive layer while inert gas ions are irradiated and
the substrate is rotated.
[0042] Also, the insulating layer may be etched while the first
superconductive layer is etched such that at least one of the
elements of the insulating layer is re-deposited on a surface of
the inclination portion of the first superconductive layer.
[0043] Also, the forming a barrier layer may include ion-etching a
surface of the inclination portion of the insulating film and a
surface of the inclination portion of the first superconductive
layer. At this time, the ion-etching may include ion-etching the
surface of the inclination portion of the insulating film and the
surface of the inclination portion of the first superconductive
layer, such that at least one of elements of the insulating layer
is re-deposited on the surface of the inclination portion of the
first superconductive layer. In this case, a surface portion of the
inclination portion where the at least one element is re-deposited
may form an amorphous layer.
[0044] Also, the method may further include heating the substrate
in an oxygen atmoshere and crystallizing the surface portion of the
inclination portion where the at least one element is re-deposited.
In this case, the crystallizing may be a part of the forming a
second superconductive layer.
[0045] Also, the method may further include changing a structure
and composition of a surface portion of the inclination portion of
the first superconductive layer through an ion irradiation damage.
In this case, the changing may include crystallizing the surface
portion of the inclination portion of the first superconductive
layer, while a heating is carried out to the first superconductive
layer in an oxygen atmosphere.
[0046] In order to achieve still another object of the present
invention, a method of manufacturing the Josephson junction
include:
[0047] forming a first superconductive layer on a substrate;
[0048] forming an interlayer insulating layer on the first
superconductive layer;
[0049] forming a second superconductive layer on the interlayer
insulating layer as a barrier layer;
[0050] etching the second superconductive layer and the barrier
layer to form 2 or more electrodes; and
[0051] forming an insulating layer between the 2 or more
electrodes.
[0052] The first superconductive layer and the second
superconductive layer are a copper oxide system superconductive
layer. In this case, the copper oxide system superconductive layer
is YBaCuO superconductive layer. In this case, the barrier layer
has a film thickness equal to or less than 2 nm.
[0053] The method may further include carrying out ion irradiation
to a surface portion of the first superconductive layer to give a
damage to the surface portion of the first superconductive layer,
before the barrier layer is formed on the first superconductive
layer. In this case, the carrying out ion irradiation may include
carrying out the ion irradiation at a temperature of the substrate
in a range of room temperature to 700.degree. C.
[0054] In order to achieve yet still another aspect of the present
invention, a Josephson junction include a first superconductive
layer formed on a substrate and having an inclination portion, an
insulating film formed on the first superconductive layer and
having an inclination portion following to the inclination portion
of the first superconductive layer, a barrier layer formed on the
inclination portion of the first superconductive layer, and having
a film thickness equal to or less than 2 nm, and a second
superconductive layer formed on the inclination portion of the
insulating film and the inclination portion of the barrier
layer.
[0055] In the above, the first superconductive layer and the second
superconductive layer may be a copper oxide system superconductive
layer. In this case, the copper oxide system superconductive layer
may be YBaCuO superconductive layer.
[0056] The barrier layer may have a perovskite structure. In this
case, a material of the barrier layer may have a lattice constant
from 0.41 nm to 0.43 nm.
[0057] Also, the insulating film may contain at least one element
selected from the group consisting of La, Sr, Al, Ca and Ta.
[0058] Also, a composition of the barrier layer is
Y.sub.1-xBa.sub.1Cu.sub- .xO.sub.y (x being in a range of 0 to 1).
The value of x may be equal to or less than 0.5 or 0.
[0059] In order to achieve another aspect of the present invention,
a Josephson junction device includes a first superconductive layer
formed on a substrate, 2 or more interlayer insulating films
provided on the first superconductive layer to be apart from each
other and having a film thickness equal to or less than 2 nm, 2 or
more second superconductive layers respectively formed on the
interlayer insulating layers.
[0060] In the above, the first superconductive layer and the second
superconductive layer may be a copper oxide system superconductive
layer. The copper oxide system superconductive layer may be YBaCuO
superconductive layer.
[0061] In order to achieve still another aspect of the present
invention, a method of manufacturing a Josephson junction, wherein
a material of an insulating layer which is formed on a first
superconductive layer is diffused into the first superconductive
layer to form a barrier layer on a surface portion of the first
superconductive layer, and a second superconductive layer is formed
on the barrier layer to form a Josephson junction.
BRIEF DESCRIPTION OF THE DRAWING
[0062] FIG. 1 is a diagram of about the characteristic part of the
way of the present invention;
[0063] FIGS. 2A to 2G are diagrams of a manufacturing method of a
high temperature superconductor edge type Josephson junction
according to a first embodiment of the present invention;
[0064] FIGS. 3A to 3G are diagrams of a manufacturing method of the
high temperature superconductor edge type Josephson, junction
according to a second embodiment of the present invention; and
[0065] FIGS. 4A to 4G are diagrams of a manufacturing method of the
high temperature superconductor sandwich type Josephson junction
according to a third embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0066] A barrier layer in the present invention is a layer which is
arranged between upper and lower superconductive layers. The
barrier layer has a characteristic as semiconductor or insulator in
the temperature dependence. The resistivity of the barrier layer at
the room temperature (25.degree. C.) is equal to 1 m.OMEGA.cm or
more. It is desirable that the barrier layer has the film thickness
equal to or less than 10 nm in order to show the function. The
lower limit of the film thickness is not especially defined. The
barrier layer is typically sufficient to have the thickness of
about 1 atom layer although depending on the material of the
barrier layer. It should be noted that it is desirable for the film
thickness of the barrier layer to be equal to or less than about 2
nm, when the copper oxide system material is used for the
superconductive layer.
[0067] In the present invention, "diffusion" means that
non-superconductive material elements or ions given with a
predetermined kinetic energy are physically introduced into a layer
and that the elements or ions are thermally diffused in the
layer.
[0068] Also, in the present invention, "change in material" means
that the surface portion of the material layer is changed in
structure and is physically changed in composition by the
non-superconductive material elements or ions having the
predetermined kinetic energy given.
[0069] The superconductive material of the present invention is
preferable to be of a copper oxide system. When the copper oxide
system material is used as the superconductive material, the
Josephson junction device can be practicable in a high temperature
region. As mentioned above, when the copper oxide system material
is used, there is a peculiar problem that the forming of the
barrier layer becomes especially difficult because of the short
coherence length. In the present invention, the problem is solved
by at least one of the following ways. That is, {circle over (1)}
the non-superconductive material is re-deposited to the surface of
the lower superconductive layer and the barrier layer is formed
through the diffusion of the non-superconductive material elements
or ions. Or, {circle over (2)} the material which contains specific
elements is selected as the non-superconductive material for the
barrier layer. Otherwise, {circle over (3)} the surface portion of
the lower superconductive layer is changed in material through ion
irradiation. It should be noted that YBaCuO is preferable as the
material of the above copper oxide system. It is especially easy
for elements of the non-superconductive material such as La, Sr,
Al, Ta and Ca to diffuse into the YBaCuO layer. Therefore, the
barrier layer having a uniform film thickness and composition can
be formed in the YBaCuO layer or on the surface portion of the
YBaCuO layer.
[0070] In the present invention, it is preferable that the material
of the barrier layer has the perovskite structure. Epitaxial growth
becomes easier in this way. Especially, the epitaxial growth on the
YBaCuO layer becomes better. Thus, the uniformity of the
composition and film thickness of the barrier layer can be
improved.
[0071] Also, in the present invention, a mask material for ion
etching, an interlayer insulating layer and a substrate may contain
at least one of the elements of La, Sr, Al, Ta and Ca. Thus, at
least one of elements is diffused into the lower superconductive
layer when the edge portion of the lower superconductive layer is
formed through the ion etching. Also, in the present invention,
when the edge surface portion of the lower superconductive layer is
cleaned by the ion etching, any of the above elements such as La,
Sr, Al, Ta and Ca are exposed the surface portion of the substrate
or interlayer insulating layer. In this case, the exposed elements
are possible to be supplied to the lower superconductive layer
through the phenomena such as re-depositing the exposed elements on
the surface of the lower superconductive layer and the surface
diffusion. It should be noted that these elements may be supplied
from an external vapor source. For example, when YBaCuO is used as
the material for the lower superconductive layer, the
above-mentioned elements are mixed with the elements such as Y, Ba,
Cu and O in the surface portion of the lower superconductive layer
through the ion irradiation. The mixed elements are crystallized
through a heating process. In this way, the uniform barrier layer
is formed on the edge portion of the lower superconductive layer.
As a result, the high temperature superconductor edge type
Josephson junction device having high uniformity can be
manufactured.
[0072] Also, a thin film containing the elements such as La, Sr,
Al, Ta and Ca may be deposited as the barrier layer on the lower
superconductive layer and the upper superconductive layer may be
deposited on the barrier layer. In this way, the uniform barrier
layer can be formed on the lower superconductive layer, so that the
high temperature superconductor sandwich type Josephson junction
device having high uniformity can be manufactured. It should be
noted that accelerated ions are irradiated to the lower
superconductive layer surface or the barrier layer. At this time,
the material in the surface portion of the lower superconductive
layer and at least one of the above-mentioned elements can be
effectively mixed, so that the more uniform barrier layer can be
formed.
[0073] Hereinafter, the Josephson junction device of the present
invention will be described below in detail.
[0074] FIG. 1 shows a cross sectional view of the Josephson
junction device of the present invention. Referring to FIG. 1, the
Josephson junction device of the present invention is composed of a
lower superconductive layer 2, a barrier layer 3 and an upper
superconductive layer 4 which are provided onto a substrate 1 in
this order. The barrier layer is sandwiched between the lower
superconductive layer and the upper superconductive layer. The
barrier layer is composed of two layers: one layer being composed
of the material which contains at least one of elements such as La,
Sr, Ca, Al and Ta, and the other layer being formed with at least
one of the above elements diffused in the lower superconductive
layer. By adopting such a structure, the barrier layer having the
uniform film thickness and composition can be formed with
reproducibility. Therefore, the upper and lower superconductive
layers never forms a short-circuit when the barrier layer is very
thin. If the Josephson junction is manufactured to use such a
structure, the Josephson junction devices with well characteristics
can be manufactured.
[0075] FIGS. 2A to 2G show a method of manufacturing the high
temperature superconductor edge type Josephson junction device
according to the first embodiment of the present invention.
[0076] Referring to FIG. 2A, first, a bi-layer film growth process
will be described. The lower superconductive layer 2 of
YBa.sub.2Cu.sub.3O.sub.x is formed on the substrate 1 and the
insulating layer 5 of LaSrAlTaO is formed on the lower
superconductive layer 2. Thus, the 2-layer thin film is formed. The
two thin film layers are grown by a pulsed laser deposition method.
The laser is an excimer laser of KrF. The laser wavelength was 248
nm, the laser output was 900 mJ and the laser repetition frequency
was 7 Hz. As the thin film growth method, other methods such as a
sputtering method and various deposition methods such as an
electron beam deposition method and a deposition method using a
heat source may be used.
[0077] A LaSrAlTaO (100) single crystal is used as the substrate 1.
The composition of the substrate was
(La.sub.0.30Sr.sub.0.70)(Al.sub.0.65Ta.s- ub.0.33)O.sub.3. The size
of the substrate was the square of 15 mm.times.15 mm and the
thickness was 0.5 mm. The lower superconductive layer 2 of
YBa.sub.2Cu.sub.3 O.sub.x was first grown on the substrate 1 in a
film growing chamber to have the film thickness of about 300 nm. A
sintered target of the composition of YBa.sub.2Cu.sub.3O.sub.x was
used as a target. The substrate temperature during the growth was
about 700.degree. C., and, the ambience gas is an oxygen gas and
has the oxygen partial pressure of about 200 mTorr. The grown thin
film was a c-axis orientation thin film, there was hardly any phase
other than YBa.sub.2Cu.sub.3O.sub.x and the surface of the thin
film was flat. Also, the superconductor critical temperature Tc is
in a range of about 85 to 89 K.
[0078] After the lower YBa.sub.2Cu.sub.3O.sub.x superconductive
layer 2 was grown, the interlayer insulating layer 5 of LaSrAlTaO
was formed in the same film growing chamber to have the film
thickness of about 600 nm. At this time, a single crystal of
LaSrAlTaO was used as the target. The composition of the single
crystal target was (La.sub.0.30Sr.sub.0.70)(Al.-
sub.0.65Ta.sub.0.35)O.sub.3. Another composition may be used if the
composition is in a range of
La.sub.0.30.+-.0.20Sr.sub.0.70.+-.0.20)(Al.s-
ub.0.65.+-.0.20Ta.sub.0.35.+-.0.20)O.sub.3.+-.1. The substrate
temperature was set to a temperature lower by 10 to 20.degree. C.
than the substrate temperature of the growth of the
YBa.sub.2Cu.sub.3O.sub.x layer, and the oxygen partial pressure was
100 mTorr. After the bi-layer thin film layer has been grown, the
substrate was cooled for about 1 hour in the oxygen ambient gas of
500 Torr and then was taken out from the film growing chamber.
[0079] The film growth condition and the film thickness described
above may be changed if the good Josephson junction is attained.
Also, the target may be a sintered body but the single crystal
target is more desirable. This is because the sintered body is
lower in density than the single crystal. As a result, fine
particles are sometimes formed on the thin film peculiar to the
pulsed laser deposition method in a higher density.
[0080] Next, the forming process of the edge structure by an
etching method will be described. The etching process is classified
into a first process of forming an etching mask from the LaSrAlTaO
interlayer insulating layer, and a second process of forming a
superconductive edge portion using the mask.
[0081] First, the first etching process will be described below. A
photoresist layer was spin-coated onto the 2-layer thin film of
LaSrAlTaO/YBa.sub.2Cu.sub.3O.sub.x by 1.2 .mu.m. The photoresist
layer was patterned by use of the usual photolithography method. At
this time, a portion of the interlayer insulating layer pattern
which functions as the edge junction was etched to be parallel to
[100] or [010] direction of the substrate crystal structure.
[0082] As shown in FIG. 2B, after the patterning, a reflow process
of the photoresist layer was carried out for 20 minutes at
160.degree. C. As a result, the end portion of photoresist layer 6
was rounded to have a gentle inclination.
[0083] Next, as shown in FIG. 2C, the interlayer insulating layer 5
was etched by an ion milling method so that the LaSrAlTaO mask was
formed for the superconductive layer edge portion. In this process,
Ar ions 7 were irradiated at the angle of about 30 degrees from the
substrate surface while the substrate is rotated. This angle may be
an angle other than 30 degrees as far as the inclination angle of
the superconductive layer edge portion finally formed becomes too
large so that crystal grains having different orientations grow on
the inclined edge portion. Also, the ion acceleration voltage was
400 V and the ion current was 50 mA.
[0084] Next, as shown in FIG. 2D, after the etching process, the
substrate was supersonic-cleaned using acetone and then
isopropanol, so that the photoresist layer was dissolved and
removed. Finally, the substrate was cleaned for 30 minutes in an
oxygen plasma activated in 400-W RF.
[0085] Next, the second etching process will be described.
[0086] As shown in FIG. 2E, in this process, the lower
superconductive layer 2 of YBa.sub.2Cu.sub.3O.sub.x was etched
using the interlayer insulating layer 5 of LaSrAlTaO as a mask to
produce the superconductive layer edge portion. In this process, Ar
ions 8 were irradiated at the angle of about 45 degrees from the
substrate surface while the substrate was rotated. Because the
incident angle of the Ar ions does not have a large influence to
the angle of the edge portion in this etching process, the incident
angle may be in a range of approximately 20 to 60 degrees. Also,
the ion acceleration voltage was 200 V and the ion current was 15
mA. In order to prevent oxygen depletion from the surface of the
superconductive layer edge portion, the etching was carried out
while the oxygen activated with the microwave of 2.45 GHz and 120 W
was directed at the substrate.
[0087] The interlayer insulating layer 5 was etched in the second
etching process and the elements such as La, Sr. Al and Ta were
sputtered and re-deposited onto the surface of the lower
superconductive layer 2. The elements redeposited to the surface
such as La, Sr, Al and Ta were thermally diffused in the forming
process of the upper superconductive layer 4 and then the barrier
layer 3 containing at least one of the elements such as La, Sr, Al
and Ta was formed.
[0088] After the etching process, in order to reduce an etching
damage to the edge surface, a cleaning process by low acceleration
ions was carried out while the substrate is rotated. In this case,
the ion acceleration voltage was 50 V, the ion current was 10 mA,
and the cleaning time was 10 minutes. The activated oxygen was
introduced in the cleaning process, too, as described above. It
should be noted that this cleaning process is not an indispensable
process to manufacture the high temperature superconductor
Josephson junction device with a small spread.
[0089] Through the above two etching processes, the same
superconductive layer edge portions were formed in 4 directions
parallel to [100] or [010] direction of the substrate. The angle of
the completed edge portion is about 25 degrees.
[0090] Because the LaSrAlTaO interlayer insulating layer itself is
used as the mask in the second etching process, the mask removal
process is unnecessary. Therefore, after the etching process ends
in an etching chamber the sample can be transferred to a film
growing chamber and is possible to enter a film forming process.
Because the etching chamber and the film growing chamber are
connected through a transfer tube, the sample can be transferred
without exposing the surface of the superconductive layer edge
portion to the atmosphere, so that the contamination of the edge
portion surface can be prevented.
[0091] After the substrate temperature was increased to about
700.degree. C. again, the upper superconductive layer 4 of
YBa.sub.2Cu.sub.3O.sub.x was grown on this structure to have the
film thickness of about 400 nm. The condition at the growth of this
layer is the same as that at the growth of the lower
superconductive layer 2.
[0092] As shown in FIG. 2F, through the above process, the barrier
layer 3 was formed between the upper and lower superconductive
layers to be composed of the material contains at least four
elements of La, Sr, Al and Ta. There is a case where it is
requested that a barrier layer is thicker than the above barrier
layer. In this case, the barrier layer can be made thicker by
depositing the material containing at least one of elements of La,
Sr, Al and Ta, more preferably at least four of the elements on the
lower superconductive layer 2. This additional barrier layer may be
grown under the same condition as that at the time of growth of the
lower superconductive layer 2. In this case, the substrate
temperature may be in a range from the room temperature to
700.degree. C.
[0093] As shown in FIG. 2G, after the upper superconductive layer 4
of YBa.sub.2Cu.sub.3O.sub.x was grown, the sample was cooled for
about 1 hour in the oxygen ambience of 500 Torr and then taken out.
The upper superconductive layer 4 was processed by the usual
lithography method and the ion milling method so as to produce the
edge type Josephson junction device in which the upper and lower
superconductive layers were connected through the barrier layer 3.
Lastly, contact pads are formed through gold deposition and a
lift-off method.
[0094] The characteristics of the edge type Josephson junction
manufactured in this way were evaluated. Either of the samples
showed good Josephson characteristics. Shapiro steps appeared in
the theoretical voltage in response to microwave irradiation. Also,
the periodic modulation of the critical current by magnetic field
was observed. The current--voltage characteristic approximately
showed an over-dump type characteristic. However, it showed a
hysteresis in the low temperature up to about 40 K. The product
(IcRn) of the critical current and normal conductive resistance was
in a range of about 2 to 2.5 mV at 4.2 K and about 0.1 mV at 60 K.
A characteristic spread of the samples was small. The spread of the
12 samples in critical current density is 1.sigma.=.+-.6% and the
spread of the 100 samples in critical current density is
1.sigma.=.+-.8%. These values can be obtained through the
above-mentioned manufacturing processes, because the high
temperature superconductor edge type Josephson junction device is
manufactured to have high uniformity.
[0095] From the cross-sectional observation result of the Josephson
junction device by a high resolution transmission electron
microscope and the composition analysis result by an energy
dispersive X-ray analysis method, the following matters became
clear. That is, it was confirmed that the barrier layer was formed
at the interface between the upper and lower superconductive layers
to have the film thickness of about 2 nm. The structure of the
material of the barrier layer is inferred to be a perovskite
structure of a cubic symmetry. The lattice constant is in a range
of 0.41 to 0.43 nm.
[0096] Also, in the composition, Y increases from the composition
of YBa.sub.2Cu.sub.3O.sub.x as the composition of the lower
superconductive layer, and Cu shifts onto the decreasing side. An
average of analysis values on the 10 analysis points was
Y:Ba:Cu=30:43:27 (if being YBa.sub.2Cu.sub.3O.sub.x composition, it
is Y:Ba:Cu=17:33:50).
[0097] Because the barrier layer is very thin, information of the
upper and lower superconductive layers of YBa.sub.2Cu.sub.3O.sub.x
is contained in this analysis result. Therefore, this analysis
result must not be interpreted as composition of the barrier layer
itself.
[0098] Supposing that the barrier layer material has the perovskite
structure, the composition is represented as being ABO.sub.3. An
ion having relatively large radius occupies the site A and an ion
having relatively, small radius occupies the site B. Considering
from the analysis result for this time and the ion radii
(Y3.sup.30:0.89 .ANG., Ba2.sup.+:1.34 .ANG., Cu2.sup.+:0.72 .ANG.)
the barrier layer material is supposed to be
Y.sub.1-xBa.sub.1Cu.sub.xO.sub.y.
[0099] The edge surface immediately after the formation was
subjected to the similar observation and analysis. The state of
"Immediately after the formation" means the state in which an Au
layer as a dummy layer for the TEM sample is deposited at the room
temperature and the edge portion surface is not subjected to any
annealing process.
[0100] An amorphous layer with the film thickness in a range of 1
to 2 nm which was substantially the same as the barrier layer was
observed in the edge portion surface. Also, the composition of the
amorphous layer is substantially the same as that of the
above-mentioned barrier layer. Therefore, it became clear that
composition change occurred in the amorphous state. It should be
noted that there were some examples that Cu was hardly detected in
the composition analysis of the amorphous layer. Because the Au
layer was formed on the one side of the amorphous layer in this
sample, the influence of YBa.sub.2Cu.sub.3O.sub.x to the analysis
result is smaller than in the above case. From this result, there
is the possibility that Cu ions are not only decreased compared
with YBa.sub.2Cu.sub.3O.sub.x in the amorphous layer, but also Cu
ions are almost lost. This is also possible to say to the finally
formed barrier layer.
[0101] From the above result, a barrier layer forming mechanism is
supposed as follows.
[0102] That is, in case that the edge portion is formed by the ion
etching method, the edge portion surface suffers a damage by the
ion irradiation, when the edge portion is completed. As a result,
the edge portion is covered with the amorphous layer. In the
composition of this amorphous layer, Y is increased from the
composition of YBa.sub.2Cu.sub.3O.sub.x by the preferential
sputtering phenomena, and Cu shifts to the side where Cu decreases.
In this case, there is the possibility that Cu ions are
approximately completely lost. It could be considered that this
amorphous layer is heated through the substrate temperature
increase for growth of the upper superconductive layer, and
crystallized to form the barrier layer.
[0103] Last, the effect that Cu ions are decreased in the amorphous
layer on the edge portion surface would be considered.
[0104] In the case of the thin film growth of
YBa.sub.2Cu.sub.3O.sub.x, it is known that when the Cu composition
of the thin film composition becomes about 3 or more, a large
BaCuO.sub.x precipitates are produced in the thin film so that the
flatness of the thin film is degraded. From this viewpoint, because
the generation of these precipitates are restrained, it is
preferable that there are few Cu ions in the amorphous layer for
the barrier layer with the uniform film thickness.
[0105] When the substrate of SrTiO or MgO was used instead of the
substrate of LaSrAlTaO, the Josephson junction device was similarly
formed. However, the spread of critical current value Ic and the
critical current value were both large. Even when the substrate of
LaSrAlTaO was used, the critical current value Ic of the junction
was very large so that the Josephson junction were not formed, if
the material of SrTiO, MgO or SrAlTaO was used for the interlayer
insulating layer. When the formation of a Josephson junction is
tried in these cases, it is necessary to decrease the growth
temperature of the upper superconductive layer lower from being
general. Therefore, the superconductive characteristic of the upper
superconductive layer is degraded. In this way, it is important to
use the material containing the elements such as La, Sr, Al and Ta
for the interlayer insulating layer. The interlayer insulating
layer, which is used for the mask at the time of the ion etching
process for the purpose of manufacturing of the Josephson junction
device with the small spread of characteristics. If the material
containing the elements such as La, Sr, Al and Ta is used for the
substrate, it is still desirable.
[0106] It should be noted that if a (La.sub.0.20Ca.sub.0.80)
(Al.sub.0.60Ta.sub.0.40)O.sub.3 target is used instead of the
(La.sub.0.30Sr.sub.0.70) (Al.sub.0.65Ta.sub.0.35)O.sub.3 target, a
LaCaAlTaO interlayer insulating layer is grown instead of the
LaSrAlTaO interlayer insulating layer. In this case, the high
temperature superconductor Josephson junction with the small spread
of the characteristics can be manufactured in the same manner as
described above.
[0107] FIGS. 3A to 3G show the manufacturing method of the high
temperature superconductor edge type Josephson junction device
according to the second embodiment of the present invention. In the
second embodiment, the interlayer insulating thin film and the
lower superconductive layer are collectively etched.
[0108] Referring to FIG. 3A, first, a bi-layer film growth process
will be described. The lower superconductive layer 2 of
YBa.sub.2Cu.sub.3O.sub.x is formed on the substrate 1 and the
insulating layer 5 of LaSrAlTaO is formed on the lower
superconductive layer 2. Thus, the 2-layer thin film is formed. The
two thin film layers are grown by a pulsed laser deposition method.
The laser is an excimer laser of KrF. The laser wavelength was 248
nm, the laser output was 900 mJ and the laser repetition frequency
was 7 Hz. As the thin film growth method, other methods such as a
sputtering method and various deposition methods such as an
electron beam deposition method and a deposition method using a
heat source may be used.
[0109] A LaSrAlTaO (100) single crystal is used as the substrate 1.
The composition of the,substrate was
(La.sub.0.30Sr.sub.0.79)(Al.sub.0.65Ta.s- ub.0.35)O.sub.3. The size
of the substrate was the square of 15 mm.times.15 mm and the
thickness was 0.5 mm. The lower superconductive layer 2 of
YBa.sub.2Cu.sub.3O.sub.x was first grown on the substrate 1 in a
film growing chamber to have the film thickness of about 300 nm. A
sintered target with the composition of YBa.sub.2Cu.sub.3O.sub.x
was used as a target. The substrate temperature during the growth
was about 700.degree. C., and the ambience gas is an oxygen gas and
has the oxygen partial pressure of about 200 mTorr. The grown thin
film was a c-axis orientation thin film, there was hardly any phase
other than YBa.sub.2Cu.sub.3O.sub.x and the surface of the thin
film was flat. Also, the superconductor critical temperature Tc is
in a range of about 85 to 89 K.
[0110] After the lower superconductive layer 2 of YBa.sub.2Cu.sub.3
O.sub.x was grown, the interlayer insulating layer 5 of LaSrAlTaO
was formed in the same film growing chamber to have the film
thickness of about 300 nm. At this time, a single crystal of
LaSrAlTaO was used as the target. The composition of the single
crystal target was
(La.sub.0.30Sr.sub.0.70)(Al.sub.0.65Ta.sub.0.35)O.sub.3. Another
composition may be used if the composition is in a range of
(La.sub.0.30.+-.0.20Sr.sub.0.70.+-.0.20)((Al.sub.0.65.+-.0.20Ta.sub.0.35.-
+-.0.20)O.sub.3.+-.1. The substrate temperature was set to a
temperature lower by 10 to 20.degree. C. than the substrate
temperature of the growth of the lower superconductive layer 2 of
YBa.sub.2Cu.sub.3O.sub.x, and the oxygen partial pressure was 100
mTorr. After the bi-layer thin film layer has been grown, the
substrate was cooled for about 1 hour in the oxygen ambient gas of
500 Torr and then was taken out from the film growing chamber.
[0111] The film growth condition and the film thickness described
above may be changed if the good Josephson junction is
attained.
[0112] Next, the forming process of the edge structure by an
etching method will be described.
[0113] First, a photoresist layer was spin-coated onto the 2-layer
thin film of LaSrAlTaO/YBa.sub.2Cu.sub.3O.sub.x by 1.2 .mu.m. The
photoresist layer was patterned by use of the usual
photolithography method. At this time, a portion of the 2-layer
thin film pattern which functions as the edge junction was etched
to be parallel to [100] or [010] direction of the substrate crystal
structure.
[0114] As shown in FIG. 3B, after the patterning, a reflow process
of the photoresist layer was carried out for 20 minutes at
160.degree. C. As a result, the end portion of patterned
photoresist layer 6 was rounded to have a gentle inclination.
[0115] Next, as shown in FIG. 3C, the interlayer insulating layer 5
and the lower superconductive layer 2 of YBa.sub.2Cu.sub.3O.sub.x
were etched by an ion milling method so that the edge portion of
the superconductive layer was formed. In this process, Ar ions 7
were irradiated at the angle of about 30 degrees from the substrate
surface while the substrate is rotated. This angle may be an angle
other than 30 degrees as far as the inclination angle of the
superconductive layer edge portion finally formed becomes too large
so that crystal grains having different orientations grew on the
inclined edge portion. Also, the ion acceleration voltage was 400 V
and the ion current was 50 mA. In this process, the elements such
as La, Sr, Al and Ta were sputtered from the interlayer insulating
film 5 by etching the interlayer insulating film 5 and then
re-deposited on the edge portion surface of the lower
superconductive layer 2. The adhered elements such as La, Sr, Al
and Ta were thermally diffused through the forming process of the
upper superconductive layer 4 so that the barrier layer 3 was
formed to contain at least one of the elements such as La, Sr, Al
and Ta.
[0116] Next, as shown in FIG. 3D, after the etching process, the
sample was supersonic-cleaned using acetone and then isopropanol,
so that the photoresist layer was dissolved and removed. Finally,
the sample was cleaned for 30 minutes in an oxygen plasma activated
in 400-W RF.
[0117] The edge portion surface of the lower superconductive layer
was exposed to the atmosphere and organic solvent so that a
contaminated layer was formed on the edge portion surface through
the photoresist removing process which was carried out in the
atmosphere. The contaminated layer was removed through an ion
etching process. As shown in FIG. 3E, in this process, Ar ions 8
were irradiated at the angle of about 45 degrees from the substrate
surface while the substrate was rotated. Because the incident angle
of the Ar ions does not have a large influence to the angle of the
edge portion in this etching process, the incident angle may be in
a range of approximately 20 to 60 degrees. Also, the ion
acceleration voltage was 200 V and the ion current was 15 mA. In
order to prevent oxygen depletion from the surface of the
superconductive layer edge portion, the etching was carried out
while the oxygen activated with the microwave of 2.45 GHz and 120 W
was directed at the substrate.
[0118] After the etching process, in order to reduce an etching
damage to the edge portion surface, a cleaning process by low
acceleration ions was carried out while the substrate is rotated.
In this case, the ion acceleration voltage was 50 V, the ion
current was 10 mA, and the cleaning time was 10 minutes. The
activated oxygen was introduced in the cleaning process, too, as
described above.
[0119] It should be noted that this cleaning process is not an
indispensable process to manufacture the high temperature
superconductor Josephson junction device with a small spread.
[0120] Through the above processes, the same superconductive layer
edge portions were formed in 4 directions parallel to [100] or
[010] direction of the substrate. The angle of the completed edge
portion is about 25 degrees.
[0121] After the etching process for removing the contaminated
layer ends in an etching chamber, the sample can be transferred to
a film growing chamber and is possible to enter a film forming
process. Because the etching chamber and the film growing chamber
are connected through a transfer tube, the sample can be
transferred without exposing the surface of the superconductive
layer edge portion to the atmosphere, so that the contamination of
the edge portion surface can be prevented.
[0122] As shown in FIG. 3F, after the substrate temperature was
increased to about 700.degree. C. again, the upper superconductive
layer 4 of YBa.sub.2Cu.sub.3O.sub.x was grown on this structure to
have the film thickness of about 400 nm. The condition at the
growth of this layer is the same as that at the growth of the lower
superconductive layer 2. Through the above process, the barrier
layer 3 was formed between the upper and lower superconductive
layers to be composed of the material contains at least four
elements of La, Sr, Al and Ta.
[0123] As shown in FIG. 3G, after the upper superconductive layer 4
of YBa.sub.2Cu.sub.3O.sub.x was grown, the sample was cooled for
about 1 hour in the oxygen ambience of 500 Torr and then taken out.
The upper superconductive layer 4 was processed by the usual
lithography method and the ion milling method so as to produce the
edge type Josephson junction device in which the upper and lower
superconductive layers were connected through the barrier layer 3.
Lastly, contact pads are formed through gold deposition and a
lift-off method.
[0124] The characteristics of the edge type Josephson junction
manufactured in this way were evaluated. Either of the samples
showed good Josephson characteristics. Shapiro steps appeared in
the theoretical voltage in response to microwave irradiation. Also,
the periodic modulation of the critical current by magnetic field
was observed. The current--voltage characteristic approximately
showed an over-dump type characteristic. However, it showed a
hysteresis in the low temperature up to about 40 K. The product
(IcRn) of the critical current and normal conductive resistance was
in a range of about 2 to 2.5 mV at 4.2 K and about 0.1 mV at 60 K.
A characteristic spread of the samples was small. The spread of the
12 samples in critical current density is 1.sigma..+-.=6%. These
values can be obtained through the above-mentioned manufacturing
processes, because the high temperature superconductor edge type
Josephson junction device is manufactured to have high
uniformity.
[0125] When the substrate of SrTiO or MgO was used instead of the
substrate of LaSrAlTaO, the Josephson junction device was similarly
formed. However, the spread of critical current value Ic and the
spread of critical current values were both large. Even when the
substrate of LaSrAlTaO was used, the critical current value Ic of
the junction was extremely large so that the Josephson junction
were not formed, if the material of SrTiO, MgO or SrAlTaO was used
for the interlayer insulating layer. When the formation of a
Josephson junction is tried in these cases, it is necessary to
decrease the growth temperature of the upper superconductive layer
lower from being general. Therefore, the superconductive
characteristic of the upper superconductive layer is degraded. In
this way, it is important to use the material containing the
elements such as La, Sr, Al and Ta for the interlayer insulating
layer which is used for the mask at the time of the ion etching
process for the purpose of manufacturing of the Josephson junction
device with the small spread of characteristics. If the material
containing the elements such as La, Sr, Al and Ta is used for the
substrate, it is still desirable.
[0126] It should be noted that if a
(La.sub.0.20Ca.sub.0.80)(Al.sub.0.60Ta- .sub.0.40)O.sub.3 target is
used instead of the (La.sub.0.30Sr.sub.0.70)(A-
l.sub.0.65Ta.sub.0.35)O.sub.3 target, a LaCaAlTaO interlayer
insulating layer is grown instead of the LaSrAlTaO interlayer
insulating layer. In this case, the high temperature superconductor
Josephson junction with the small spread of the characteristics can
be manufactured in the same manner as described above.
[0127] Next, FIGS. 4A to 4G show the manufacturing process of the
high temperature superconductor sandwich type Josephson junction
device according to the third embodiment of the present invention.
The high temperature superconductor sandwich type Josephson
junction device according to the third embodiment of the present
invention will be described below with reference to FIGS. 4A to
4G.
[0128] First, a tri-layer thin film growth process will be
described. As shown in FIG. 4A, the lower superconductive layer 2
was formed on a substrate 1, the barrier layer 3 was formed on the
lower superconductive layer 2, and the upper superconductive layer
4 was formed on the barrier layer 3. Thus, the tri-layer thin film
is composed of 3 layers of the lower superconductive layer 2 of
YBa.sub.2Cu.sub.3O.sub.x, the barrier layer 3 contains elements
such as La, Sr, Al and Ta, and the upper superconductive layer 4 of
YBa.sub.2Cu.sub.3O.sub.x. These thin film layers were grown by the
pulsed laser deposition method. The laser is the excimer laser of
KrF. The laser wavelength was 248 nm, the laser output was 900 mJ
and the laser repetition frequency was 7 Hz. At this time, a
sintered target of YBa.sub.2Cu.sub.3O.sub.x was used for growth of
the superconductive layers. A single crystal target of LaSrAlTaO
was used for the growth of the barrier layer. The composition of
the single crystal target was (La.sub.0.30Sr.sub.0.70)
(Al.sub.0.65Ta.sub.0.35)O.sub.3. Another composition may be used if
the composition is in a range of
(La.sub.0.30.+-.0.20Sr.sub.0.70.+-.0.20)(Al.sub.0.65.+-.0.20Ta.sub.0.35.+-
-.0.20)O.sub.3.+-.1.
[0129] A LaSrAlTaO (100) single crystal was used as the substrate
1. The composition of the substrate was
(La.sub.0.30Sr.sub.0.70)(Al.sub.0.65Ta.s- ub.0.30)O.sub.3. Another
single crystal of SrTiO.sub.3, MgO, NdGaO.sub.3 or LaAlO.sub.3 may
be used for the substrate 1. The substrate surface may have one of
planes such as (110) plane displaced from the (100) plane. The size
of the substrate was the square of 15 mm.times.15 mm and the
thickness was 0.5 mm. The lower superconductive layer 2 of
YBa.sub.2Cu.sub.3O.sub.x was first grown on the substrate 1 in a
film growing chamber to have the film thickness of about 300
nm.
[0130] The substrate temperature during the growth was about
700.degree. C., and the ambience gas is an oxygen gas and has the
oxygen partial pressure of about 200 mTorr. The grown thin film was
a c-axis oriented thin film, there was hardly any phase other than
YBa.sub.2Cu.sub.3O.sub.x and the surface of the thin film was flat.
Also, the superconductor critical temperature Tc is in a range of
about 85 to 89 K.
[0131] Subsequently, the barriers layer 3 containing the elements
such as La, Sr, Al and Ta was grown on the lower superconductive
layer 2 in the same condition to have the film thickness from 1 to
10 nm. Last, the upper superconductive layer 4 of
YBa.sub.2Cu.sub.3O.sub.x was grown on the barrier layer 3 in the
same condition to have the film thickness of about 300 nm. Thus the
tri-layer thin film was formed. After the growth of the tri-layer
thin film, the sample was cooled to about 100.degree. C. in the
oxygen atmosphere of about 500 Torr.
[0132] The thin film growth condition, the thin film thickness, and
thee target maybe changed if the Josephson junction device with
good characteristics was obtained. Also, before the barrier layer 3
was deposited, it is preferable that any damage is given to the
surface of the lower superconductive layer 2 of
YBa.sub.2Cu.sub.3O.sub.x previously through the irradiation of ions
such as argon and oxygen. In this case, the ion acceleration
voltage is preferably in a range of 50 to 1000 V, more preferably
in a range of 50 to 200 V for the Josephson junction device with a
high critical current value Ic. The ion irradiation angle may be in
the range of 0 to 90 degrees from the substrate surface. In this
case, when a damaged region in the surface portion of the lower
superconductive layer 2 of YBa.sub.2Cu.sub.3O.sub.x should be
shallow, it is preferable to select an angle as small as possible.
The substrate temperature in case of ion irradiation may be in a
range of room temperature to 700.degree. C. When the ion
irradiation is carried out in the low temperature near the room
temperature, it is necessary to cool the sample under the condition
of the high oxygen partial pressure of about 500 Torr after the
lower superconductive layer 2 has been grown. In the case of the
sample heating for the growth of the barrier layer after that, it
is preferable to keep the high oxygen partial pressure equal to or
more than 200 mTorr.
[0133] Next, the manufacturing process of the sandwich type
Josephson junction device structure will be described.
[0134] First, as shown in FIG. 4C, a photoresist layer 9 was
spin-coated on the 3-layer structure to have the thickness of about
1.2 .mu.m and then was patterned by use of the usual
photolithography method. The upper superconductive layer 4, the
barrier layer 3 and the lower superconductive layer 2 were etched
by an ion etching method using the patterned photoresist as a mask.
Thus, lower superconductive layer wiring patterns were determined.
In this case, the patterned photoresist layer was removed.
Subsequently, another photoresist layer was spin-coated onto the
3-layer structure and then was patterned by the photolithography
method. The shape of the sandwich type Josephson junction device
was determined by the ion etching method using the patterned
photoresist layer as a mask. In this way, the Josephson junction
device having 2 or more Josephson junctions, 2 Josephson junctions
in this example were formed.
[0135] In this case, the lower superconductive layer 2 was
over-etched by about 50 nm in depth after the etching of the
barrier layer was ended.
[0136] As shown in FIG. 4D, after the etching ended, an interlayer
insulating layer 10 of SrTiO.sub.3 was deposited on the sample as
the patterned photoresist layer was left, to have the film
thickness of about 400 nm. The interlayer insulating layer 10 was
grown by the pulsed laser deposition method at a temperature near
the room temperature intentionally without substrate heating. The
laser was the excimer laser of KrF. The laser wavelength was 248
nm, the laser-output was 900 mJ and the laser repetition frequency
was 7 Hz. A single crystal of the composition of SrTiO.sub.3 was
used for the target. Another material such as MgO, LaAlO.sub.3,
NdGaO.sub.3, and Y.sub.2O.sub.3 may be used for the interlayer
insulating layer 10.
[0137] As shown in FIG. 4E, after the interlayer insulating layer
was deposited, the patterned photoresist layer which was used to
determine the junction shape was removed with solvent. As a result,
a part of the interlayer insulating layer 10 was removed by use of
the principle of a lift-off process.
[0138] Last, upper layer normal conductive wiring lines and contact
pads were formed of gold.
[0139] As shown in FIG. 4F, a gold thin film 11 was deposited on
the sample to have the film thickness of about 400 nm.
[0140] Subsequently, as shown in FIG. 4G, the gold thin film 11 was
patterned by the ion etching method using a patterned photoresist
as a mask, which was obtained by patterning a photoresist layer by
the usual photolithography process. Thus, a normal conductive
wiring line and contact pad 20 connected with the upper
superconductive layer 4 and a contact pad 21 connected with the
lower superconductive layer 2 were formed.
[0141] In this way, the sandwich type Josephson junction device is
completed.
[0142] The characteristics of the sandwich type Josephson junction
device manufactured in this way were evaluated. When the deposited
barrier layer had a film thickness equal to or smaller than 2 nm,
either of the samples showed good Josephson characteristics.
Shapiro steps appeared in the theoretical voltage in response to
microwave irradiation. Also, the periodic modulation of the
critical current by magnetic field was observed. The
current--voltage characteristic approximately showed an over-dump
type characteristic. However, it showed a hysteresis in the low
temperature up to about 40 K. The product (IcRn) of the critical
current and normal conductive resistance was in a range of about 1
to 2 mV at 4.2 K and about 0.1 mV at 60 K. A characteristic spread
of the samples was small. The spread of the 12 samples in critical
current density is 1.sigma.=.+-.5%. These values can be obtained
through the above-mentioned manufacturing processes, because the
high temperature superconductor sandwich type Josephson junction
device is manufactured to have high uniformity.
[0143] It should be noted that the analyzed results can be applied
to the second and third embodiments.
[0144] As described above, according to the present invention, it
is possible to form the barrier layer with the high uniformity
between the YBaCuO lower superconductive layer and the YBaCuO upper
superconductive layer. Thus, the high temperature superconductor
Josephson junction device showing the high uniformity in the
characteristic can be realized. Therefore, the application to
various superconductor devices and circuits is possible.
[0145] Also, according to the present invention, the process of
depositing the barrier layer is not always necessary in the high
temperature, superconductor Josephson junction device. Therefore,
it is possible to simplify the manufacturing processes.
* * * * *