Method for manufacturing an artificial grain boundary type Josephson junction device

A portion of a sufficiently thick insulating layer formed on a substrate is removed so that a recessed device region is formed and surrounded by masking wall portions left at both ends of the recessed device region. A first oxide superconducting thin film is deposited at angle of 30.degree. to the substrate so as to ensure that a c-axis oriented oxide superconducting thin film grows in such a way that a portion of the recessed device region is masked by one of the masking wall portions so that no thin film grows over the masked portion of the recessed device region. Then, another oxide superconducting thin film is deposited at angle of -30.degree. to the substrate so as to ensure that an a-axis oriented oxide superconducting thin film grows in such a way that another portion of the recessed device region is masked by the other of the masking wall portions, so that no thin film grows over the masked portion, but the a-axis oriented oxide superconducting thin film is in contact with the c-axis oriented oxide superconducting thin film, with the result that a grain boundary functioning a weak link is formed between the a-axis oriented oxide superconducting thin film and the c-axis oriented oxide superconducting thin film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for manufacturing an artificial 
grain boundary type Josephson junction device. More specifically, the 
present invention relates to a novel method for preparing an artificial 
grain boundary type Josephson junction device formed of an oxide 
superconducting thin film. 
2. Description of Related Art 
A Josephson junction device can be realized in various structures. Among 
the various structures, the most preferable structure in practice is a 
stacked junction realized by a thin insulating layer sandwiched between a 
pair of superconductors. However, Josephson junction devices, which are 
composed of a pair of superconductor regions weakly linked to each other 
by for example a point contact type junction, a Dayem bridge type 
junction, also exhibit Josephson effect, although they show different 
characteristics. In general, these Josephson junctions have fine 
structures in which the superconductor and/or the insulator are composed 
of thin films. 
In order to realize, for example, a stacked type Josephson junction device 
by using an oxide superconductor, a first oxide superconducting thin film, 
an insulator thin film and a second oxide superconducting thin film are 
stacked on a substrate in the named order. 
The thickness of the insulating layer of the stacked type Josephson 
junction device is determined by the coherence length of the 
superconductor. Since the coherence length of an oxide superconductor is 
very short, the thickness of the insulating layer must be about a few 
nanometers in the stacked type Josephson junction device formed of the 
oxide superconductor. 
Further, both of the point contact type Josephson junction device and the 
Dayem bridge type Josephson junction device require a very fine processing 
such as a fine etching and a fine patterning, which makes it possible to 
realize a weak link between a pair of superconductors. 
On the other hand, considering the operation characteristics of the 
Josephson junction device, each of the layers constituting the Josephson 
junction device has to have a high crystallinity and to be composed of a 
single crystal or a polycrystal having an orientation very close to that 
of a single crystal. 
In the above mentioned stacked type Josephson junction device, therefore, 
it is necessary to stack a first oxide superconducting thin film, an 
insulator thin film and a second oxide superconducting thin film, which 
are of high crystallinity, respectively. However, it is difficult to stack 
an extremely thin and high crystalline insulator thin film on an oxide 
superconducting thin film. Furthermore, it is very difficult to stack a 
high crystalline oxide superconducting thin film on this insulator thin 
film because of the characteristics of the oxide superconductor. 
Further, although the above mentioned stacked structure was realized, the 
interface between the oxide superconductor and the insulator was not in a 
good condition, so that a desired characteristics could not be obtained. 
On the other hand, it is very difficult to conduct a fine processing such 
as a fine etching and a fine patterning on an oxide superconductor, which 
permits it to realize a point contact type Josephson junction device or a 
Dayem bridge type Josephson junction device. Therefore, a Josephson 
junction device using an oxide superconductor and having a stable 
performance could not be produced with good repeatability. 
In view of the above mentioned problems, researches have been conducted for 
manufacturing a Josephson junction device taking advantage of the 
characteristics intrinsic to the oxide superconductor, while reducing the 
fine processing, such as a fine etching and a fine patterning, of the 
oxide superconductor to a possible extent. The oxide superconductor has a 
considerably different superconducting characteristics, dependently upon 
its crystalline direction. For example, if oxide superconductors having a 
crystalline direction different from each other are joined together, a 
grain boundary formed at the junction interface constitutes a barrier, so 
that a Josephson junction is formed. A Josephson junction device using 
this Josephson junction is called a artificial grain boundary type 
Josephson junction device, and can be manufactured without the fine 
processing as mentioned above. 
As an example of the above mentioned artificial grain boundary type 
Josephson junction device, there may be mentioned a device formed by a 
junction between a c-axis oriented oxide superconducting thin film having 
its c-axis of crystal perpendicular to the substrate and another oxide 
superconducting thin film having its c-axis of crystal in parallel to the 
substrate (called "a-axis oriented oxide superconducting thin film" for 
the clarity of the description in this specification). 
However, in order to manufacture a Josephson junction device comprising the 
artificial grain boundary having the structure as mentioned above, it is 
necessary to effect a processing of, for example, physically patterning a 
previously formed oxide superconducting thin film. In the course of this 
processing, the oxide superconducting thin film is exposed to the 
atmosphere so as to be degraded, with the result that an unnecessary 
Josephson junction is formed and/or a sharp grain boundary cannot be 
formed. Therefore, no process for manufacturing a Josephson junction 
device having a desired characteristics with good repeatability has been 
established yet. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
for manufacturing an artificial grain boundary type Josephson junction 
device, which has overcome the above mentioned defect of the conventional 
one. 
Another object of the present invention is to provide a method for 
manufacturing an artificial grain boundary type Josephson junction device, 
which can realize an excellent characteristics with good repeatability. 
The above and other objects of the present invention are achieved in 
accordance with the present invention by a method for manufacturing an 
artificial grain boundary type Josephson junction device which includes a 
substrate, a first superconducting region composed of a c-axis oriented 
oxide superconducting thin film which is deposited on the substrate and 
which has its c-axis of crystal perpendicular to the substrate, and a 
second superconducting region composed of an oxide superconducting thin 
film which is formed on the substrate in contact with the first 
superconducting region and which has its c-axis of crystal in parallel to 
the substrate, so that a weak link is constituted at an interface between 
the first and second superconducting regions, the method comprising the 
steps of; 
forming a sufficiently thick insulating layer on a principal surface of a 
substrate; 
removing a part of the insulating layer to form a recessed device region 
surrounded by wall portions having a sufficient height from a bottom of 
the device region, respectively; 
depositing a first oxide superconducting thin film on the insulating layer, 
under the condition that a c-axis oriented oxide superconducting thin film 
having its c-axis perpendicular to the principal surface of the substrate 
grows, and from a first upper oblique direction of the substrate that a 
first end of opposite ends of the device region is masked by a first one 
of the wall portions so that no thin film grows in the masked first end of 
the device region; and 
depositing a second oxide superconducting thin film on the insulating 
layer, under the condition that an oxide superconducting thin film having 
its c-axis in parallel to the principal surface of the substrate grows, 
and from a second upper oblique direction of the substrate that a second 
end of opposite ends of the device region is masked by the other one of 
the wall portions so that no thin film grows in the masked second end of 
opposite ends of the device region but the second oxide superconducting 
thin film partially overlaps the first oxide superconducting thin film so 
that a grain boundary is formed at an interface between the first oxide 
superconducting thin film and the second oxide superconducting thin film. 
The method in accordance with the present invention for manufacturing an 
artificial grain type Josephson junction device is essentially 
characterized in that an oxide superconducting thin film is deposited 
obliquely after forming a three-dimensional mask on the substrate, so that 
a crystalline grain boundary, which will constitute a barrier in the 
Josephson junction device, can be formed without a processing such as a 
fine etching and a fine patterning of the oxide superconducting thin film. 
According to the methods of the prior art, the processing such as the 
etching and the fine patterning of the oxide superconducting thin film 
have been effected by any means after depositing the thin film, in order 
to form a crystalline grain boundary. Therefore, the oxide superconducting 
thin film could have been degraded, particularly in the neighborhood of 
the junction interface in the course of the processing. 
According to the method of the present invention, the processing such as 
the etching and the patterning of the oxide superconducting thin film is 
no longer necessary in order to form a crystalline grain boundary, and the 
Josephson junction can be formed only by effecting the deposition 
operation from an upper oblique direction of the substrate after formation 
of a three-dimensional mask. Further, it is possible to successively 
effect a series of processes without destroying a vacuum in a deposition 
room. Thus, the surface of the thin film is not exposed to the atmosphere 
in the course of the manufacturing process, so that the surface of the 
thin film and the substrate are not degraded. 
It is possible to control the crystalline orientation of the oxide 
superconducting thin film by varying a substrate temperature in the 
deposition process. In the case of a typical Y.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x oxide superconducting thin film, an a-axis oriented film is 
deposited when the substrate temperature in the deposition process is 
relatively low, for example, less than 650.degree. C., while a c-axis 
oriented film is deposited when the substrate temperature in the 
deposition process is relatively high, for example, not less than 
650.degree. C. However, if the substrate temperature in the deposition 
process is higher than 750.degree. C., the amount of oxygen included in 
the oxide superconductor crystals is decreased and the superconducting 
properties are largely degraded. 
As mentioned above, the substrate temperature in the course of depositing a 
c-axis oriented thin film is higher than that in the course of depositing 
an a-axis oriented thin film. Thus, in the method of the present 
invention, it is preferred to first deposit a c-axis oriented thin film 
and then to deposit an a-axis oriented thin film. If an a-axis oriented 
thin film is first deposited and thereafter a c-axis oriented thin film is 
formed, there is a risk of losing the superconductivity of the a-axis 
oriented thin film previously formed. In addition, on an a-axis oriented 
thin film, an a-axis oriented thin film (not a c-axis oriented thin film) 
is apt to be easily grown regardless of the substrate temperature. 
In the method of the present invention, the above mentioned insulating 
layer can be formed of any insulator, such as MgO or SrTiO.sub.3, having a 
low reactivity with oxide superconductor. More particularly, it is 
preferably formed of Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y, since Pr.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-y is a non-superconducting oxide which is 
constituted by substituting Pr for Y of the Y.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x oxide superconductor and which has a crystalline structure 
almost identical to that of the oxide superconductor. Thus, even if 
Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y oxide is in contact with an oxide 
superconductor, not only it gives no adverse effect on the oxide 
superconductor, but also a portion of the oxide superconducting thin film 
in contact with the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y layer may show an 
improved superconductivity if the oxide superconductor layer and a 
Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y layer are stacked. 
The method of the present invention having the above mentioned features can 
be generally applied to the manufacture of an artificial grain type 
Josephson junction device. The oxide superconductor thin film is 
preferably formed of a high-T.sub.c (high critical temperature) oxide 
superconductor, more preferably, formed of a high-T.sub.c copper-oxide 
type compound oxide superconductor. As particularly preferable 
superconducting material. Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, Bi.sub.2 
Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 
O.sub.x, etc. can be exemplified. 
As the substrate material, a MgO(100) substrate, a SrTiO.sub.3 (110) 
substrate, a YSZ substrate, etc. can preferably used. However, it is not 
limited to these substrates. For example, a silicon substrate having an 
appropriate buffer layer on its deposition surface can be used. 
The above and other objects, features and advantages of the present 
invention will be apparent from the following description of preferred 
embodiments of the invention with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the process in accordance with method of the present 
invention for manufacturing an artificial grain boundary type Josephson 
junction device will be explained with reference to FIGS. 1A to 1E. 
First, as shown in FIG. 1A, a MgO(100) substrate is prepared. In this 
embodiment, a MgO(100) substrate having a size of 15 mm.times.8 mm and a 
thickness of 0.5 mm was used. Then, as shown in FIG. 1B, a Pr.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-y thin film was deposited on this substrate 1 by 
a sputtering process so as to form a mask layer 2. The conditions for the 
sputtering process were as follows: 
______________________________________ 
Substrate temperature 
750.degree. C. 
Sputtering gases 
Ar 9 sccm (90%) 
O.sub.2 1 sccm (10%) 
Pressure 5 .times. 10.sup.-2 Torr 
Thickness of film 700 nm 
______________________________________ 
Thereafter, as shown in FIG. 1C, a portion of the mask layer 2 was removed 
by an ion milling process using Ar ions so as to form a recessed device 
region 20 having a width of 1.5 .mu.m and a depth of 0.5 .mu.m in a center 
portion of the substrate. Namely. the recessed device region has a 
depth-to-length ratio of 1:3. But, the depth-to-length ratio of the 
recessed device region is not limited to 1:3. The recessed device region 
20 is surrounded by masking wall portions 21 and 22 formed at opposite 
ends of the device region 20, respectively. In this embodiment, this 
processing was effected in such a way that the Pr.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-y layer was slightly left on a bottom surface of the device region 
20 and in the following step, an oxide superconducting thin film was 
deposited on the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y thin film. As a 
patterning process of the above mentioned mask layer 2, a RIE process or 
another method can be also used in place of the ion milling process using 
the Ar gas. 
After the above mentioned processing, the substrate was maintained under 
ultra-high vacuum of about 1.times.10.sup.-9 Torr at a temperature of 
about 350.degree. to 400.degree. C. for an hour so as to clean up the 
surface of the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y thin film of the 
device region 20. Thereafter, as shown in FIG. 1D, a c-axis oriented 
Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, oxide superconducting thin film 31 
was deposited by a sputtering process from an oblique upper direction at 
angle .theta. of 30.degree. to the deposition surface of the substrate 1. 
The conditions for the deposition process were as follows: 
______________________________________ 
Substrate Temperature 
700.degree. C. 
Sputtering gases 
Ar 9 sccm (90%) 
O.sub.2 1 sccm (10%) 
Pressure 5 .times. 10.sup.-2 Torr 
Thickness of film 250 nm 
______________________________________ 
In this process, a c-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, 
oxide superconducting thin film 31 grows, on an upper surface and a side 
surface of the masking wall portion 21 which are not in shadow of the 
masking wall portion 22, on an upper surface of the masking wall portion 
22 and on a portion of the device region 20. 
Then, as shown in FIG. 1E, an a-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x oxide superconducting thin film was deposited by a sputtering 
process from an oblique upper direction at angle .theta. of -30.degree. to 
the deposition surface of the substrate 1. The conditions for the 
deposition process were as follows: 
______________________________________ 
Substrate Temperature 
640.degree. C. 
Sputtering gases 
Ar 9 sccm (90%) 
O.sub.2 1 sccm (10%) 
Pressure 5 .times. 10.sup.-2 Torr 
Thickness of film 250 nm 
______________________________________ 
In this process, the a-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x 
oxide superconducting thin film 32 grows, on the upper surface and a side 
surface of the masking wall portion 22 which are not in shadow of the 
masking wall portion 21, on the upper surface of the masking wall portion 
21 and on a portion of the device region 20. In a region in the 
neighborhood of the center of the device region 20, the a-axis oriented 
Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconducting thin film 32 and 
the c-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide 
superconducting thin film 31 were stacked so that a crystal grain boundary 
33 is formed. 
In the above mentioned embodiment, the substrate 1 was continuously 
subjected to the processing in a deposition chamber where a vacuum was 
maintained, without being taken out from the deposition chamber, from the 
start of the deposition of the c-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x oxide superconducting thin film 31 to the end of the deposition 
of the a-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide 
superconducting thin film 32. 
When the artificial grain boundary type Josephson device manufactured as 
mentioned above was cooled by means of liquid nitrogen and a microwave was 
applied, the AC Josephson effect could be observed. 
As explained above, according to the present invention, it is possible to 
easily manufacture an artificial grain boundary type Josephson junction 
device composed of an oxide superconductor, without using either the 
etching or the patterning of the oxide superconductor. In accordance with 
the method of the present invention, it is possible to continuously 
deposit a c-axis oriented oxide superconducting thin film and an a-axis 
oriented oxide superconducting thin film, while maintaining the vacuum, so 
as to form a Josephson junction. Therefore, it is possible to manufacture 
a Josephson junction device which can be applied to various sensors and 
circuit devices, since a desired performance is exactly realized. 
The invention has thus been shown and described with reference to the 
specific embodiments. However, it should be noted that the present 
invention is in no way limited to the details of the illustrated 
structures but changes and modifications may be made within the scope of 
the appended claims.