Method for manufacturing an oxide superconductor thin film and a target for use in the method

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for manufacturing an oxide 
superconductor thin film excellent in surface morphology. 
2. Description of the Related Art 
Since the discovery of oxide superconductors with high critical 
temperatures, researches on methods of manufacturing oxide thin films have 
greatly advanced. Hereinafter, the critical temperature will be referred 
to as "Tc". Of such methods, the most widely used is a method comprising 
the steps of applying a laser beam to a target made of an oxide material 
to evaporate the oxide material, and then depositing and accumulating the 
evaporated material on a substrate. In this method, since the thin film is 
formed in an atmosphere with a high oxygen partial pressure, oxygen can be 
incorporated sufficiently into the oxide thin film. Further, this method 
can easily form a thin film whose composition similar or substantially 
same to that of the target. Therefore, the formed film can be used as a 
superconductor film as it is without any further treatment. 
However, in this method, a laser beam applied to a target at a high 
radiation energy density, possibly deteriorates the target. In addition, 
the thin film formed by this method have numerous particles of 
approximately 1 .mu.m in diameter on its surface, unlike thin films made 
by other methods such as a sputter method and a vacuum evaporation method. 
These particles markedly lower the surface morphology. In this case, when 
a plurality of films of poor surface morphology are laid one upon another 
to form a multi layered film or a laminated type junction, the interface 
between the layers becomes a non-homogeneous condition. Due to the 
non-homogeneous condition, it is difficult to accurately evaluate the 
properties of each of the formed thin films. Heretofore, many attempts 
have been made with a view toward reducing the particle generation. 
In an attempt, a thin film was formed by a conventional method under such a 
condition as to reduce the density of particles being generated. However, 
the superconductivity of thus-formed film was found to have deteriorated. 
Heretofore, no methods for forming a thin film having both of an excellent 
morphology (a smooth surface) and superconductivity, have yet been 
proposed. 
SUMMARY OF THE INVENTION 
The present inventors have investigated and researched the causes for 
particles being generated in the conventional method. It is found that the 
cause was present in a target. To be more specific, a conventional target 
manufactured by sintering raw powder material of fine grains has numerous 
void spaces (the apparent density of 90% or less) and extremely fine 
crystal particles as small as approximately 10.mu. in diameter. Therefore, 
when a laser beam is applied to a conventional target, not only crystal 
surfaces but also the inner void spaces and grain boundaries are 
irradiated. As a result of irradiation, the temperature of the target 
abruptly increases, and then, the target melts. However, due to a laser 
beam applied to the inner void spaces and the grain boundaries, fine 
powder is scattered away from the target before and after the melting of 
the target. The present inventors assumed the scattered powder to be a 
cause of particles. 
On the basis of their findings, the present inventors have conducted 
intensive and extensive studies in an effort to solve the problem of 
particles being generated in a conventional method. As a result, they have 
succeeded in increasing the apparent density of a target to avoid the 
radiation to the inner void spaces, and in increasing the diameter of a 
crystal grain to larger than that of a laser beam, without lowering 
mechanical strength of the target. Thus have been made the present 
invention. 
It is an object of the present invention to provide a method for 
manufacturing an oxide superconductor thin film having an excellent 
morphology (a smooth surface), by reducing the density of particles 
without decreasing superconductivity, and also to provide a target for use 
in the method. 
In order to achieve the above-mentioned object, the method for 
manufacturing an oxide superconductor thin film of the present invention, 
comprises the steps of: 
preparing a target having an apparent density of 95% or more and 
substantially composed of an oxide material represented by 
Y.sub.1.+-..alpha. Ba.sub.2.+-..beta. Cu.sub.3.+-..gamma. O.sub.7-.delta. 
(.alpha..ltoreq.0.8, .beta..ltoreq.0.4, .gamma..ltoreq.0.4, 
-2.ltoreq..delta..ltoreq.1); and 
forming a thin film substantially composed of Y.sub.1.+-..alpha. 
Ba.sub.2.+-..beta. Cu.sub.3.+-..gamma. O.sub.7-.delta. 
(.alpha..ltoreq.0.8, .beta..ltoreq.0.4, .gamma..ltoreq.0.4, 
-2.ltoreq..delta..ltoreq.1) on a substrate by applying a laser beam to the 
target. 
The reason for limiting .alpha., .beta., .gamma., and .delta. to the 
above-mentioned composition range is that a thin film within this range 
can provide properties of an oxide superconductor. Further, in the present 
invention, when a Y.sub.2 BaCuO.sub.5 phase is diffused among crystal 
grains in order to strengthen the target, the precipitating particles are 
desired to have a diameter of 1.mu. or less and contained in an amount of 
40% or less in the crystal grains. 
The aforementioned target can be manufactured according to a method for 
manufacturing bulk as described in, for example, Jpn. Pat. Appln. KOKAI 
Publication No. 4-119968, which is a priority document of U.S. Pat. Ser. 
No. 07/606,207 and the continuation application thereof, Ser. No. 
08/073,656. By way of example, one of the methods in the above-mentioned 
applications will be briefly described below. According to this method 
(hereinafter the method is referred to as "MPMG method"), bulk is obtained 
as follows: 
raw material powder for a Y-Ba-Cu-O series oxide superconductor or a 
material produced by a general sintering method is heated to high 
temperatures to obtain a partial liquid phase; 
the heated material is solidified by cooling; 
the solidified material is pulverized and mixed to allow the texture 
diffused homogeneously; 
the pulverized and mixed powder is molded in a predetermined form; and 
the molded body is heated to develop a partial liquid phase therein 
resulting in a superconductor body. 
Bulk obtained by the MPMG method has an apparent density of 95% or more and 
crystal grains of 1 mm or more in diameter. Further, a Y.sub.2 BaCuO.sub.5 
phase of 1 .mu.m or less in diameter can be diffused in an amount of 40% 
or less in the crystal grains. Thus-obtained bulk has a mechanical 
strength of 1.6 to 2.1 MPam.sup.1/2 in terms of the fracture toughness Kc 
and 7 GPa or more in terms of the Vickers hardness Hv by virtue of the 
Y.sub.2 BaCuO.sub.5 phase diffused among the crystal grains. 
Further, the present invention provides a method for manufacturing an oxide 
superconductor thin film and a target for use in the method, comprising 
the steps of: 
preparing a target having a single crystalline oxide material composed of 
Y.sub.1.+-..alpha. Ba.sub.2.+-..beta. Cu.sub.3.+-..gamma. O.sub.7-.delta. 
(.alpha..ltoreq.0.8, .beta..ltoreq.0.4, .gamma..ltoreq.0.4, 
-2.ltoreq..delta..ltoreq.1); and 
forming a thin film substantially composed of Y.sub.1.+-..alpha. 
Ba.sub.2.+-..beta. Cu.sub.3.+-..gamma. O.sub.7-.delta. 
(.alpha..ltoreq.0.8, .beta..ltoreq.0.4, .gamma..ltoreq.0.4, 
-2.ltoreq..delta..ltoreq.1) on a substrate by applying a laser beam to the 
target. 
This target, which is usually formed by the well-known Czochralski method, 
such as rotation upward drawing process and upward drawing process, is a 
single crystal having an apparent density of 100%. 
In the present invention, to the target made of thus-obtained bulk, a 
pulsed laser beam is applied to scatter the oxide material away from the 
target and the material is deposited through evaporation on a substrate. 
The preferable partial oxygen pressure of an atmosphere used for the laser 
radiation is 10 to 40 Pa. Preferable pulsed laser beam to be used for 
radiation is a KrF excimer, an ArF excimer, or a YAG laser. The preferable 
optical amount of a laser beam falls within the range from 3J/cm.sup.2 to 
8J/cm.sup.2. The preferable beam diameter falls within the range from 
0.5.times.0.5 mm to 4.0 mm.times.4.0 mm. The effect from reducing the 
diameter of a laser beam and a radiation area to smaller than that of a 
crystal grain is substantially equivalent to that from applying a laser 
beam to a single crystalline bulk (a target). To be more specific, since 
void spaces and grain boundaries are not irradiated, the formed films are 
free from an adverse influence from the radiation applied to void spaces 
and grain boundaries. As a substrate, MgO, SrTiO.sub.3, LaAlO.sub.3, or 
the like is used. The thickness of thin film deposited on the substrate 
through evaporation is preferred to be 50 nm to 1 .mu.m. This thin film 
possess superconductivity as it is. 
According to the method for manufacturing an oxide thin film of the present 
invention using the above-explained pulsed laser, by virtue of a single 
crystalline target, or a target having a high apparent density, preferably 
having crystal grains which is large enough to function substantially 
equivalent to a single crystal against a laser beam, a thin film can be 
obtained having an excellent surface morphology without particles on the 
surface. Consequently, a multi-layered type tunnel junction, a laminated 
film, and the like having an excellent superconductivity can be easily 
manufactured. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Example 1 
A target was produced according to the MPMG method. This target was 
composed of 60 mol % Y.sub.1.+-..alpha. Ba.sub.2.+-..beta. 
Cu.sub.3.+-..gamma. O.sub.7-.delta. (.alpha.=0.05, .beta.=0.1, 
.gamma.=0.05, and .delta.=0.05) and 40 mol % Y.sub.2 BaCuO.sub.5. The 
target had an apparent density of 95% and crystal grains of at least 1 mm 
in diameter. Among the crystal grains, a Y.sub.2 BaCuO.sub.5 phase of 1 
.mu.m or less in diameter was diffused in an amount of 40%. To this 
target, a Kr-F excimer laser (.lambda.=248 nm) having a laser beam amount 
of 5J/cm.sup.2 and a beam diameter of 1.times.2 mm was applied at a ratio 
of approximately 5 Hz, thereby forming a thin film of 200 nm in thickness 
on a MgO (100) substrate. The oxygen pressure was set to 40 Pa and the 
substrate temperature was set to 750 .degree. C. Under the same condition 
as above, a thin film was manufactured of a conventional target produced 
according to the hot-press method for comparison. The composition of the 
conventional target was Y.sub.1.+-..alpha. Ba.sub.2.+-..beta. 
Cu.sub.3.+-..gamma. O.sub.7-.delta.. An apparent density of the 
conventional target was 90%, and the diameter of crystal grains was at 
most 0.1 mm. 
The surfaces of the thin films made of the targets of Example 1 and of the 
comparative sample were observed under an optical microscope. Particles on 
the thin film of Example 1 were 10.sup.6 per cm.sup.2, which was 1/10 the 
order of numbers of that of the conventional thin film. 
The thin film obtained in Example 1 was checked for the dependence of 
inherent resistance on temperature and the dependence of critical current 
density on temperature. The results are shown in FIGS. 1 and 2. It is 
apparent from the result that the thin film of the Example 1 acquires a 
good surface condition and sufficient superconductivity. 
Example 2 
A target was prepared according to the MPMG method. The composition of the 
target was Y.sub.1.+-..alpha. Ba.sub.2.+-..beta. Cu.sub.3.+-..delta. 
O.sub.7-.delta., in which .alpha.=0.05, .beta.=0.1, .gamma.=0.05, and 
.delta.=0.05. The target had an apparent density of 95%, and crystal 
grains of at least 1 mm in diameter. Among crystal grains of the target, a 
Y.sub.2 BaCuO.sub.5 phase of 1.mu. in an average diameter was diffused. A 
thin film was manufactured by using the target in the same manner as in 
Example 1. 
When observed under an optical microscope, the surface of the film of the 
Example 2 had particles in the number of 1/10 the order of numbers of that 
of the conventional film shown in Example 1. Fine crystal grains oriented 
along the a and b-axes were observed to partially grow besides a region of 
crystal grains oriented along the c-axis. The dependence of inherent 
resistance on temperature and the dependence of critical current density 
on temperature of the thin film of Example 2 were equivalent to those of 
the thin film of Example 1. It was found that both properties of the thin 
film demonstrated good superconductivity. 
Example 3 
Thin films were formed on the substrate by varying the composition (a molar 
ratio) of a target, a crystal grain diameter, and an apparent density. Tc 
and the number of particles of the thin films were determined. The results 
are shown in Table 1. 
Using targets outside the scope of the present invention, thin film was 
formed and the physical properties thereof were determined in the same 
manner as above. The results are also shown in columns of a comparative 
sample and a conventional sample of Table 1. The target of the comparative 
sample had the same composition as that of No. 3-1 of Table 1, an apparent 
density of 90%, and a crystal grain diameter of 0.1 mm. The target of the 
conventional sample had a composition of Y.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-.delta., an apparent density of 90%, and a crystal grain diameter 
of 0.1 mm. Thin films were formed under the radiation condition as 
follows: a laser beam wavelength of 248 nm, a radiation power of 
5J/cm.sup.2, a beam diameter of either 1.0 mm.times.1.0 mm or 1.5 
mm.times.2.5 mm, a radiation area of either 5 mm.times.5 mm or 50 
mm.times.50 mm. Since a laser has a beam diameter of 1.0 mm.times.1.0 mm 
and a radiation area of 5 mm.times.5 mm as compared to 5 mm of a crystal 
grain diameter of the target in sample Nos. 3-2 and 3-6, the target 
substantially works as a single crystal at the time of radiation. 
It is apparent from Table 1 that the number of particles can be reduced by 
setting an apparent density to 95% or more, particularly by adjusting a 
crystal grain diameter to 1 mm or more. Furthermore, by increasing a 
crystal grain diameter much larger than a beam diameter, the number of 
particles can be significantly reduced, for a laser beam is substantially 
applied to a single crystal. 
Example 4 
A target having composition of Y.sub.1.+-..alpha. Ba.sub.2.+-..beta. 
Cu.sub.3.+-..gamma. O.sub.7-.delta. (.alpha.=0.05, .beta.=0.1, 
.gamma.=0.05, and .delta.=0.05) and its apparent density of 100% was 
prepared according to Czochralski method. A thin film of 20 nm in 
thickness was formed on MgO (100) substrate by applying a Kr-F excimer 
laser (.lambda.=248 nm) having a beam amount of 5J/cm.sup.2 and a beam 
diameter of 1.times.2 mm to the target at a rate of 10 Hz. The oxygen 
pressure was 20 Pa and the substrate temperature was 750.degree. C. 
When observed under an optical microscope, the surface of the film of the 
Example 4 had particles in 1/10 the order of number of that of the thin 
film formed of the conventional target shown in Example 1. The thin film 
obtained in Example 4 had the same dependence of inherent resistance on 
temperature and the same dependence of critical current density on 
temperature as those of Example 1. It was found that the obtained thin 
films had a good surface morphology and sufficient superconductivity. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and illustrated examples shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents. 
TABLE 1 
__________________________________________________________________________ 
Target 
Appa- 
Crystal 
Condition for evaporation 
Composition 
rent 
grain Beam Radiation 
Result 
No. (Molar %) 
density 
size .lambda. 
Power 
(mm) area (mm) 
Tc Particles 
__________________________________________________________________________ 
Sam- 
3-1 
Y.sub.1.8 Ba.sub.2.4 Ca.sub.3.4 O.sub.7-.delta. 
98% 5 mm 248 nm 
5 J/cm.sup.2 
1.5 .times. 2.5 
50 .times. 50 
84K 
2.2 .times. 10.sup.6 
/cm.sup.2 
ple 
Sam- 
3-2 
Y.sub.1.8 Ba.sub.2.4 Ca.sub.3.4 O.sub.7-.delta. 
98% 5 mm 248 nm 
5 J/cm.sup.2 
1.0 .times. 1.0 
5 .times. 5 
86K 
2.1 .times. 10.sup.6 
/cm.sup.2 
ple 
Sam- 
3-3 
Y.sub.1.0 Ba.sub.2.0 Cu.sub.3.0 O.sub.7-.delta. 
98% 2-3 
mm 248 nm 
5 J/cm.sup.2 
1.5 .times. 2.5 
50 .times. 50 
84K 
2.3 .times. 10.sup.6 
/cm.sup.2 
ple 
Sam- 
3-4 
Y.sub.1.8 Ba.sub.2.4 Cu.sub. 3.0 O.sub.7-.delta. 
98% 1 mm 248 nm 
5 J/cm.sup.2 
1.5 .times. 2.5 
50 .times. 50 
86K 
2.5 .times. 10.sup.6 
/cm.sup.2 
ple 
Sam- 
3-5 
Y.sub.1.8 Ba.sub.2.4 Cu.sub.3.0 O.sub.7-.delta. 
95% 5 mm 248 nm 
5 J/cm.sup.2 
1.5 .times. 2.5 
50 .times. 50 
86K 
2.6 .times. 10.sup.6 
/cm.sup.2 
ple 
Sam- 
3-6 
Y.sub.1.8 Ba.sub.2.4 Cu.sub.3.0 O.sub.7-.delta. 
95% 5 mm 248 nm 
5 J/cm.sup.2 
1.0 .times. 1.0 
5 .times. 5 
86K 
2.4 .times. 10.sup.6 
/cm.sup.2 
ple 
Sam- 
3-7 
Y.sub.1.0 Ba.sub.2.0 Cu.sub.3.0 O.sub.7-.delta. 
95% 2-3 
mm 248 nm 
5 J/cm.sup.2 
1.5 .times. 2.5 
50 .times. 50 
86K 
2.6 .times. 10.sup.6 
/cm.sup.2 
ple 
Sam- 
3-8 
Y.sub.1.8 Ba.sub.2.4 Ca.sub.3.4 O.sub.7-.delta. 
95% 1 mm 248 nm 
5 J/cm.sup.2 
1.5 .times. 2.5 
50 .times. 50 
86F 
3.0 .times. 10.sup.6 
/cm.sup.2 
ple 
Compara- 
Y.sub.1.8 Ba.sub.2.4 Ca.sub.3.4 O.sub.7-.delta. 
90% 0.1 
mm 248 nm 
5 J/cm.sup.2 
1.5.2.5 
50 .times. 50 
86K 
2.5 .times. 10.sup.7 
/cm.sup.2 
tive 
Conven- 
Y.sub.1.0 Ba.sub.2.0 Cu.sub.3.0 O.sub.7-.delta. 
90% 0.1 
mm 248 nm 
5 J/cm2 
1.5.2.5 
50.50 86K 
1.8 .times. 10.sup.7 
/cm.sup.2 
tional 
sample 
__________________________________________________________________________