Method of fabricating a superconducting junction using cubic YBa.sub.2 Cu.sub.3 O .sub.x thin film as a barrier layer

The present invention forms a superconducting junction using a cubic YBa.sub.2 Cu.sub.3 Ox thin film as a barrier layer. The present invention forms a first YBCO superconducting thin film, a SrTiO.sub.3 insulating layer thin film on the substrate, etches a side of them in the form of inclination, subsequently integrates a non-superconducting cubic YBCO barrier thin film, a second YBCO superconducting thin film, a SrTiO.sub.3 protecting layer thin film in series on the whole surface of the substrate, etches an opposite side of the etched part of the SrTiO.sub.3 insulating layer thin film in the form of inclination, fabricates a superconducting junction by forming a metal electrode to said aperture after forming apertures which expose said first YBCO superconducting thin film, the second YBCO superconducting thin film, fabricates a superconducting junction upon forming the metallic electrode to the apertures, and deposits a cubic YBa.sub.2 Cu.sub.3 Ox barrier thin film at a temperature of 600-650.degree. C. and a depositing velocity of 6.5-12.2 nm/s.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of fabricating a superconducting 
junction using an oxide superconducting thin film, and more particularly, 
to a method of fabricating a superconducting junction by integrating a 
first oxide superconducting thin film, a non-superconducting barrier thin 
film, and a second oxide superconducting thin film in series on an oxide 
single crystal substrate. 
2. Description of the Prior Art 
Generally, there must be fabricated a superconducting junction in order to 
apply an oxide superconducting thin film for an electronic device. There 
show various kinds of superconducting junction structure so far, the 
simplest one is a tunnel type Josephson junction. 
The tunnel type Josephson junction is composed of a three layer thin film 
structure according as a non-superconducting barrier layer is inserted in 
the form of sandwich between a pair of superconducting thin films which 
performs a superconducting electrode function. Typically, such a tunnel 
type Josephson junction is fabricated by the process of depositing a first 
superconducting thin film, a non-superconducting barrier thin film, and a 
second superconducting thin film successively on the substrate. 
Conventionally, there are performed a process of depositing a first YBCO 
superconducting thin film and a second YBCO superconducting thin film, a 
process of fabricating a non-superconducting barrier thin film using a 
YBCO superconducting thin film and a metal or oxide thin film whose the 
composition is different from the YBCO thin film in order to fabricate a 
tunnel type Josephson junction. 
It is difficult to fabricate a good quality tunnel type Josephson junction 
because the tunnel type Josephson junction fabricated as above generates a 
stress by means of the difference of a lattice constant and a thermal 
expansion coefficient between a superconducting thin film and a 
non-superconducting barrier thin film. Accordingly, there is necessary a 
process using a barrier thin film having the composition equal to both a 
first superconducting thin film and a second superconducting thin film in 
order to improve the performance of the tunnel type Josephson junction. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process of 
fabricating a tunnel type superconducting junction composed of a first 
YBCO superconducting thin film, a SrTiO.sub.3 insulating layer thin film, 
a non-superconducting cubic YBCO barrier thin film, a second YBCO 
superconducting thin film, and a SrTiO.sub.3 protecting layer thin film 
upon a second YBCO superconducting thin film in order to protect the 
degradation of superconducting properites of second YBCO thin film during 
ion million process. 
In order to accomplish the above object, the present invention provides a 
method of fabricating a superconducting junction comprising steps of: 
forming a first YBCO superconducting thin film and insulating layer thin 
film on an oxide single crystal substrate; forming a first photoresist 
pattern on the insulating layer thin film, removing the insulating layer 
thin film and first superconducting thin film exposed upon etching them in 
the form of inclination; forming a non-superconducting cubic YBCO barrier 
thin film, a second YBCO superconducting thin film and protecting layer 
thin film on a whole surface of the substrate; forming a second 
photoresist pattern which exposes the opposite side of the part etched in 
the form of inclination on the protecting layer thin film, etching the 
protecting layer thin film and second YBCO superconducting thin film and 
non-superconducting cubic YBCO barrier thin film exposed in series in the 
form of inclination; and depositing the thin film in a deposition rate of 
6.5-12.2 nm/s by a pulse laser deposition method, forming the 
non-superconducting cubic YBCO barrier thin film at a temperature of 
600-650.degree. C. 
In one aspect of the present invention, the present invention provides a 
method of fabricating a superconducting junction comprising steps of: 
forming a first YBCO superconducting thin film and insulating layer thin 
film on an oxide single crystal substrate; forming a first photoresist 
pattern on the insulating layer thin film, removing the insulating layer 
thin film and first superconducting thin film exposed upon etching them in 
the form of inclination; forming a non-superconducting cubic YBCO barrier 
thin film, a second YBCO superconducting thin film and protecting layer 
thin film on a whole surface of the substrate; forming a second 
photoresist pattern which exposes the opposite side of the part etched in 
the form of inclination on the protecting layer thin film, etching the 
protecting layer thin film and second YBCO superconducting thin film and 
non-superconducting cubic YBCO barrier thin film exposed in series in the 
form of inclination; and depositing the thin film at a temperature of 
600-650.degree. C., forming the non-superconducting cubic YBCO barrier 
thin film in a deposition rate of 6.5-12.2 nm/s.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the embodiment of the present invention will be explained with 
referring to the attached draws. 
FIGS. 1a through 1j show a process sectional view of fabricating a tunnel 
type superconducting junction according to the present invention. 
As shown in FIGS. 1a through 1j, a process of fabricating a tunnel type 
superconducting junction of the present invention comprises a process of 
depositing a first YBCO superconducting thin film, a SrTiO.sub.3 
insulating thin film, a non-superconducting cubic YBCO barrier thin film, 
a second YBCO superconducting thin film, and a SrTiO.sub.3 protecting 
layer thin film on a substrate in series. 
The first characteristic of the present invention resides in a process of 
depositing a non-superconducting cubic YBCO barrier thin film in a 
deposition rate of 12.2 nm/s at a substrate temperature in the range of 
600-650.degree. C. In the case that the substrate temperature is lower 
than 600.degree. C., the YBCO barrier thin film grows in amorphous, in the 
case of higher than 650.degree. C., a non-superconducting barrier thin 
film doesn't grow by growing in the superconducting thin film having a 
c-axial oriented orthorhomic crystal structure. 
The second characteristic of the present invention resides in a process of 
depositing a non-superconducting barrier thin film in a deposition rate of 
the range of 6.5-12.2 nm/s at a substrate temperature of 650.degree. C. In 
the case that the deposition rate is lower than 6.5 nm/s, the c-axial 
oriented orthorhomic YBCO superconducting barrier thin film grows, a cubic 
YBCO barrier thin film doesn't grow. 
Embodiment 1 
Hereinafter, there will be explained in detail with respect to a process of 
fabricating a tunnel type superconducting junction composed of a first 
YBCO superconducting thin film, a SrTiO.sub.3 insulating layer thin film, 
a non-superconducting cubic YBCO barrier thin film, a second YBCO 
superconducting thin film, and a SrTiO.sub.3 protecting layer thin film 
according to embodiment 1 of the present invention. 
First, referring to FIG. 1a, there is deposited a first YBCO 
superconducting thin film 2 in the thickness of 250 nm under the 
depositing condition of depositing oxygen pressure of 100 mTorr, a 
distance between a substrate and a target of 4.2 cm, a deposition rate of 
12.2 nm/s, a pulse laser repeating ratio 100 Hz, a pulse laser energy 
density of 1 J/cm.sup.2, and a depositing temperature of 750-800.degree. 
C. on a SrTiO.sub.3 100 or a single crystal substrate 1 by the pulse laser 
deposition method. 
Thereafter, as shown in FIG. 1b, there is deposited a SrTiO.sub.3 
insulating layer thin film 3 in the thickness of 20 nm under the 
depositing condition of a pressure 100 mTorr of depositing oxygen, a 
distance between a substrate and a target of 4.2 cm, a deposition rate of 
0.6 nm/s, a pulse laser repeation rate of 5 Hz, a pulse laser energy 
density of 1 J/cm.sup.2, and a depositing temperature of 750.degree. C. on 
a first YBCO superconducting thin film 1. Subsequently, after a 
photoresist is applied on the SrTiO.sub.3 insulating layer thin film 3, 
there is formed a first photoresist pattern 4 which exposes a part region 
of the SrTiO.sub.3 insulating layer thin film 3 in a prescribed width by 
patterning it. 
Next, as shown in FIG. 1c, there are etched the SrTiO.sub.3 insulating 
layer thin film 3 and first YBCO superconducting thin film 2 exposed on 
the ion beam using the photoresist pattern 4 as an etching mask in the 
form of inclination, thereby removing the photoresist pattern 4. 
As shown in FIG. 1d, there is deposited a non-superconducting cubic YBCO 
thin film 5 in the thickness of 12.2-24.4 nm under the depositing 
condition of a pressure of 100 mTorr of depositing oxygen, a distance 
between a substrate and a target of 4.2 cm, a deposition rate of 12.2 
nm/s, a pulse laser repeation rate 100 Hz, a pulse laser energy density of 
1 J/cm.sup.2, and a depositing temperature of 600-650.degree. C. on an 
entire surface of the substrate. 
At this time, as shown in FIG. 2a, a crystal structure of a YBCO thin film 
3 is analyzed in a X-ray reflection pattern upon changing the depositing 
temperature at a deposition rate of 12. 2 nm/s, as the result, to show 
only a reflection ray in a X-ray reflection pattern(00L) (L=2,3,4,5,6,7) 
of the YBCO thin film deposited at a temperature of 750.degree. C. 
This shows that the YBCO thin film grew in an orthorhomic crystal 
structure, and a c-axis of an orthorhomic crystal structure grows in the 
thin film oriented vertically on the substrate surface. 
However, as shown in FIG. 2b, there exists only a reflection pattern (h00) 
(h=1,2) of the YBCO thin film deposited at the depositing temperature of 
650.degree. C. This shows that the crystal structure of the YBCO thin film 
is an orthorhomic, a-axis crystal axis grew in the thin film or the cubic 
thin film vertically grown on the substrate. 
The lattice constant value of the YBCO thin film deposited at the 
depositing temperature of 650.degree. C. and the deposition rate of 12.2 
nm/s is calculated in 0.389 nm, which is large compared to 0.382 nm, of 
the lattice constant of an a-axis oriented orthrhomic crystal. 
Accordingly, the YBCO thin film grown in the depositing velocity of 12.2 
nm/s grew in the cubic thin film, the cubic thin film growth was confirmed 
by the Raman analysis and the fine structure analysis using a high 
resolution transmission penetrating electron microscope. 
Next, as shown FIG. 1e, there is deposited a second YBCO superconducting 
thin film 6 in the thickness of 250 nm under the depositing condition of a 
pressure of 100 mTorr of depositing oxygen, a distance between a substrate 
and a target of 4.2 cm, a deposition rate of 12.2 nm/s, a pulse laser 
repeation rate 100 Hz, a pulse laser energy density of 1 J/cm.sup.2, and a 
depositing temperature of 750.degree. C. on a non-superconducting cubic 
YBCO barrier thin film 5, and there is deposited a SrTiO.sub.3 protecting 
layer thin film 7 in the thickness of 20 nm under the depositing condition 
of a pressure 100 mTorr of depositing oxygen, a distance between a 
substrate and a target of 4.2 cm, a deposition rate of 0.6 nm/s, a pulse 
laser repeation rate of 5 Hz, a pulse laser energy density of 1 
J/cm.sup.2, a depositing temperature of 700.degree. C. on the second YBCO 
superconducting thin film 6. 
Thereafter, as shown in FIG. 1f, after a photoresist is spreaded on the 
SrTiO.sub.3 protecting layer thin film 7, a second photoresist pattern 8 
is formed patterning it so that the opposite side of the side removed of 
the first photoresist pattern 4 can be removed. 
As shown in FIG. 1g, after there in series are etched the SrTiO.sub.3 
protecting layer thin film 7, the second YBCO superconducting thin film 6, 
and the non-superconducting cubic YBCO thin film 5 exposed in the form of 
inclination using the second photoresist pattern 8 as the etched mask on 
the ion beam, the second photoresist pattern 8 is removed. 
Next, as shown in FIG. 1h, after the photoresist is spreaded on the entire 
surface of the substrate, there is formed a third photoresist pattern 9 
which exposes a prescribed portion of the SrTiO.sub.3 insulating layer 3 
and the SrTiO.sub.3 protecting layer thin film 7 by patterning it. 
Subsequently, as shown in FIG. 1i, there are removed the SrTiO.sub.3 
insulating layer 3 and the SrTiO.sub.3 protecting layer thin film 7 
exposed using the third photoresist pattern 9 as the etching mask by use 
of HF solution of 1%, therefore, to form apertures 10, 11 which expose 
electrode forming regions of the first YBCO superconducting thin film 2 
and second YBCO superconducting thin film 6. 
Next, as shown in FIG. 1j, there is deposited a gold thin film in the 
thickness of 300 nm under the depositing condition of a pressure of 100 
mTorr of depositing oxygen, a distance between a substrate and a target of 
4.2 cm, a deposition rate of 0.6 nm/s, a pulse laser repeation rate 5 Hz, 
a pulse laser energy density of 2 J/cm.sup.2, and a depositing temperature 
of 25.degree. C. on the third photoresist pattern 9 and in the apertures. 
The substrate is soaked in an acetone solution, the gold thin film formed 
thereon is removed with the third photoresist pattern 9 and the metallic 
electrodes 12, 13 are formed, therefore, to fabricate a tunnel type 
superconducting junction. 
Embodiment 2 
The characteristic of the second embodiment according to the present 
invention resides in a process of depositing a non-superconducting cubic 
YBCO barrier thin film by use of a deposition rate of higher than 6.5 nm/s 
at a temperature of 650.degree. C. 
The method of fabricating a tunnel type superconducting junction according 
to the second embodiment is proceeded according to the process sequence 
equal to the first embodiment, the process condition depositing each thin 
film is different. 
Accordingly, with referring to FIGS. 1a through 1j, the second embodiment 
of the present invention will be explained as follows. 
First, referring to FIG. 1a, there is deposited a first YBCO 
superconducting thin film 2 in the thickness of 250 nm under the 
depositing condition of 100 mTorr of depositing oxygen pressure of 100 
mTorr, a distance between a substrate and a target of 4.2 cm, a deposition 
rate of 0.1 nm/s, a pulse laser repeation rate 1 Hz, a pulse laser energy 
density of 1 J/cm.sup.2, and a depositing temperature of 650.degree. C. on 
a SrTiO.sub.3 100 or LaSrGaO.sub.4 100 single crystal substrate 1 by the 
pulse laser deposition method. 
Thereafter, as shown in FIG. 1b, there is deposited a SrTiO.sub.3 
insulating layer thin film 3 in the thickness of 20 nm under the 
depositing condition of a pressure 100 mTorr of depositing oxygen, a 
distance between a substrate and a target of 4.2 cm, a deposition rate of 
0.6 nm/s, a pulse laser repeation rate of 5 Hz, a pulse laser energy 
density of 1 J/cm.sup.2, and a depositing temperature of 700.degree. C. on 
a first YBCO superconducting thin film 1. Subsequently, after a 
photoresist is applied on the SrTiO.sub.3 insulating layer thin film 3, 
there is formed a first photoresist pattern 4 which exposes a part region 
of the SrTiO.sub.3 insulating layer thin film 3 in a prescribed width by 
pattering it. 
Next, as shown in FIG. 1c, there are etched the SrTiO.sub.3 insulating 
layer thin film 3 and first YBCO superconducting thin film 2 exposed by 
use of the photoresist pattern 4 as an etched mask on the ion beam in the 
form of inclination, thereby removing the photoresist pattern 4. 
As shown in FIG. 1d, there is deposited a non-superconducting cubic YBCO 
thin film 5 in the thickness of 12.2-24.4 nm under the depositing 
condition of a pressure of 100 mTorr of depositing oxygen, a distance 
between a substrate and a target of 4.2 cm, a deposition rate of 12.2 
nm/s, a pulse laser repeation rate 100 Hz, a pulse laser energy density of 
1 J/cm.sup.2, and a depositing temperature of 650.degree. C. on an entire 
surface of the substrate. 
At this time, as shown in FIG. 3a, a crystal structure of YBCO thin film 5 
is analyzed in a X-ray reflection pattern upon changing the deposition 
rate at a depositing temperature of 650.degree. C., as the result, as 
shown in FIG. 3a, to show only a reflection ray in a X-ray reflection 
pattern(00L) (L=2,3,4,5,6,7) of the YBCO thin film deposited at a 
deposition rate of 0.1 nm/s. 
This shows that the YBCO thin film grew in an orthorhomic crystal 
structure, and a c-axis of an orthorhomic crystal structure grew in the 
thin film oriented vertically on the substrate surface. 
However, as shown in FIG. 3b, there exists only the X-ray reflection 
pattern (h00)(h=1,2) of the YBCO thin film deposited at the depositing 
velocity of 12.2 nm/s. This shows that the crystal structure of the YBCO 
thin film is an orthrhomic, an a-axis crystal axis grew in the thin film 
or cubic thin film grown vertically on the substrate surface. 
The lattice constant value of the YBCO thin film deposited at the 
deposition rate of 12.2 nm/s is calculated in 0.389 nm, which is large 
compared to 0.382 nm of the lattice constant of an a-axis oriented 
orthrhomic crystal. 
Accordingly, the YBCO thin film grown in the deposition rate of 12.2 nm/s 
grew in the cubic thin film, the cubic thin film growth was confirmed by 
the Raman analysis and the fine structure analysis using a high magnifying 
penetrating electronic microscope. 
Next, as shown FIG. 1e, there is deposited a second YBCO superconducting 
thin film 6 in the thickness of 250 nm under the depositing condition of a 
pressure of 100 mTorr of depositing oxygen, a distance between a substrate 
and a target of 4.2 cm, a deposition rate of 0.1 nm/s, a pulse laser 
repeating ratio 1 Hz, a pulse laser energy density of 1 J/cm.sup.2, and a 
depositing temperature of 650.degree. C. on a non-superconducting cubic 
YBCO barrier thin film 5, and there is deposited a SrTiO.sub.3 protecting 
layer thin film 7 in the thickness of 20 nm under the depositing condition 
of a pressure 100 mTorr of depositing oxygen, a distance between a 
substrate and a target of 4.2 cm, a deposition rate of 0.6 nm/s, a pulse 
laser repeation rate of 5 Hz, a pulse laser energy density of 1 
J/cm.sup.2, a depositing temperature of 700.degree. C. on the second YBCO 
superconducting thin film 6. 
Thereafter, as shown in FIG. 1f, after a photoresist is spreaded on the 
SrTiO.sub.3 protecting layer thin film 7, a second photoresist pattern 8 
is formed patterning it so that the opposite side of the side removed of 
the first photoresist pattern 4 can be removed. 
As shown in FIG. 1g, after there in series are etched the SrTiO.sub.3 
protecting layer thin film 7, the second YBCO superconducting thin film 6, 
and the non-superconducting cubic YBCO thin film 5 exposed in the form of 
inclination using the second photoresist pattern 8 as the etching mask on 
the ion beam, the second photoresist pattern 8 is removed. 
Next, as shown in FIG. 1h, after the photoresist is spreaded on the entire 
surface of the substrate, there is formed a third photoresist pattern 9 
which exposes a prescribed portion of the SrTiO.sub.3 insulating layer 3 
and the SrTiO.sub.3 protecting layer thin film 7 by patterning it. 
Subsequently, as shown in FIG. 1i, there are removed the SrTiO.sub.3 
insulating layer 3 and the SrTiO.sub.3 protecting layer thin film 7 
exposed using the third photoresist pattern 9 as the etching mask by use 
of HF solution of 1%, therefore, to form apertures 10, 11 which expose 
electrode forming regions of the first YBCO superconducting thin film 2 
and second YBCO superconducting thin film 6. 
Next, as shown in FIG. 1j, there is deposited a gold thin film in the 
thickness of 300 nm under the depositing condition of a pressure of 100 
mTorr of depositing oxygen, a distance between a substrate and a target of 
4.2 cm, a deposition rate of 0.6 nm/s, a pulse laser repeation rate of 5 
Hz, a pulse laser energy density of 2 J/cm.sup.2, and a depositing 
temperature of 25.degree. C. on the third photoresist pattern 9 and in the 
apertures. The substrate is soaked in an acetone solution, the gold thin 
film formed thereon is removed with the third photoresist pattern 9 and 
the metallic electrodes 12, 13 are formed, therefore, to fabricate a 
tunnel type Josephson junction. 
According to the fabricating method of the present invention, since there 
can be deposited a non-superconducting barrier material using a cubic YBCO 
material, it is to solve the stress problem which occurs due to the 
difference between the lattice constant and thermal expansion coefficient, 
thereby being able to fabricate a good quality tunnel type Josephon 
junction. 
As described above, although the present invention has been described in 
detail with reference to illustrative embodiments, the invention is not 
limited thereto and various modifications and changes may be effected by 
one skilled in the art within the scope of the invention.