Process for producing Bi- and Pb-containing oxide superconducting wiring films

In an oxide superconducting film wiring, when the line width is reduced, the evaporation of a component during firing becomes so vigorous that it becomes impossible to form a desired single crystal phase, which causes a significant lowering in the properties of the oxide superconducting wiring. This problem can be solved by preventing the evaporation of the evaporable component during the firing. Examples of this include a process wherein plate is placed above the superconductor forming material film wiring pattern on the substrate so as to face each other, the plate comprising a material having no chemical influence on the superconducting wiring, and a pattern of a material containing an evaporable component is arbitrarily formed, a process wherein a pattern having a smaller line width is sandwiched between patterns having a larger line width, and a process wherein the firing atmosphere or the concentration of the evaporable component in the pattern is varied depending upon the line width.

BACKGROUND OF THE INVENTION 
The present invention relates to a superconducting film, and more 
particularly, to a process for producing a film of a bismuth based 
perovskite superconducting material containing lead. 
A superconducting film wiring currently practicable in the art is a Bi-base 
superconducting film wiring. 
Examples of the Bi-base superconducting film wiring realizable in the art 
include that comprising a 10 K phase having a superconducting transition 
temperature, i.e., a critical temperature, Tc, of 10 K at which a normal 
conducting phase is transferred to a superconducting phase, that 
comprising a 80 K phase having a critical temperature, Tc, of 80 K, and 
that comprising a 110 K phase having a critical temperature, Tc, of 110 K, 
but it is known in the art that it is very difficult to prepare a Bi-based 
superconducting film wiring of a 110 K phase. 
In the superconductor field, however, since a high superconducting 
transition temperature greatly benefits the cooling, such as a reduction 
of size of the whole equipment through a simplification of a cooling 
device, a Bi-based superconducting wiring of a 110 K phase is required. 
DESCRIPTION OF THE RELATED ART 
In general, a Bi-based superconducting film has been produced by depositing 
a material film on a substrate by sputtering, vacuum evaporation or the 
like and subjecting the material film to a post-heat treatment to 
synthesize a superconducting film. In this case, the patterning is usually 
conducted either by using a mask in the deposition of the superconducting 
material film or by conducting etching after the deposition. 
A very precise control of the temperature is necessary for producing a 
Bi-based superconducting film comprising a 110 K phase, and the formation 
of a single phase of a 110 K phase without the addition of Pb to the 
composition has been regarded as very difficult. As the present inventors 
have already reported (see "Appl. Phys. Lett., 54", pp. 1362-1364 (1989)), 
however, since Pb vigorously evaporates during sintering, the amount of Pb 
becomes insufficient even in the case of a film having wide pattern width, 
and thus it is not easy to form a Bi-based superconducting film comprising 
a 110 K phased. 
The present inventors have succeeded in forming a Bi-based superconducting 
film having a single phase of a 110 K phase by adding Pb in a considerably 
higher concentration than that in the case of a bulk or the like (see 
"Appl. Phys. Lett., 55", pp. 1252-1254 (1989)). 
In general, in a Bi-based superconducting film, there exists a 
superconducting phase wherein the critical temperature, Tc, varies 
depending upon the difference in the number of Cu-O planes contained in 
the unit cell. At the present time, in the Bi-based superconducting film 
represented by the formula Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x, 
three superconducting compounds are known, i.e., a phase having a Tc of 10 
K wherein n is 1, a phase having a Tc of 80 K wherein n is 2, and a phase 
having a Tc of 110 K wherein n is 3. 
The synthesis of a superconducting film comprising a 110 K phase having the 
highest critical temperature, Tc, is expected from the practical 
viewpoint, but as described above, even in the case of a film having a 
width pattern width, the Pb vigorously evaporates during sintering and 
reached a state such that the amount of Pb becomes insufficient. For this 
reason, a technique wherein a large amount of Pb is added has been 
developed. In this case, however, it has been found that the evaporation 
of Pb becomes more vigorous in the case of a pattern having a small line 
width, so that the proportion of the formation of the 110 K phase is 
decreased. When a larger amount of Pb is added to a pattern having a small 
line width, the superconducting material film partially melts during 
sintering to form a 110 K phase. The partial melting temperature is 
closely related to the proportion of Pb. Specifically, when the proportion 
of Pb is high, the superconducting material film unfavorably melts at a 
low temperature of about 840.degree. C. and the melting becomes vigorous. 
This is liable to cause variations in the composition from place to place, 
and consequently, a Tc phase or a crystal having a small grain diameter is 
often formed, and thus the texture becomes heterogeneous. This causes the 
critical current density, Jc, and the critical temperature, Tc, to be 
lowered, and thus favorable results can not be obtained. 
Accordingly, an object of the present invention is to form a substantially 
single phase of a 110 K phase through the prevention of an evaporation of 
a large amount of an evaporable component such as Pb in an early stage of 
the firing and formation of a heterogeneous texture by the use of a very 
simple technique in the firing of a film having a smaller width pattern of 
a Bi-based superconducting material containing Pb. 
SUMMARY OF THE INVENTION 
To attain the above-described object, the present invention provides a 
process for producing a superconducting film, comprising the steps of: 
forming on a substrate a film wiring pattern of a material capable of 
producing a superconducting material upon being fired; and firing the 
wiring pattern of the superconductor forming material film while 
preventing or compensating for the evaporation of an evaporable component 
(hereinafter referred to as "easily evaporable component"). 
The term "superconductor forming material" is intended to mean a material 
capable of becoming a superconducting material upon being fired and is an 
aspect including a superconducting material per se. 
The specific means for preventing or compensating for the evaporation of 
the easily evaporable component of the superconducting material may be, 
for example, a first, to arrange a plate above a superconductor forming 
material film wiring pattern on a substrate so as to face each other, the 
plate comprising a material having no chemical influence on the 
superconducting wiring; 
second, to form a film of a material containing an easily evaporable 
component (hereinafter referred to as "material containing an easily 
evaporable component") on the surface of the plate into a pattern, 
preferably a pattern corresponding to a superconductor forming material 
film wiring pattern; 
third, to form a film pattern of a material containing an easily evaporable 
component in a larger width than that of the superconductor forming 
material film wiring pattern along and on both sides of the superconductor 
forming material film wiring pattern on the substrate; 
fourth, to place a material containing an easily evaporable component 
within a firing oven for firing a superconductor forming material film 
wiring pattern; 
fifth, to feed a vapor of an easily evaporable component within the firing 
oven for firing the superconductor forming material film wiring pattern; 
sixth, to separate a wiring pattern of a superconductor forming material 
film into a wiring portion having a larger pattern width and a wiring 
portion having a smaller pattern width, and placing the wiring portion 
having a larger pattern width and the wiring portion having a smaller 
pattern width in a different firing atmosphere; or 
seventh, to differentiate the concentration of the easily evaporable 
component in the wiring portion having a larger pattern width from that of 
the wiring portion having a smaller pattern width in the wiring pattern of 
the superconductor forming material film, so that the concentration of the 
easily evaporable component in the wiring portion having a smaller pattern 
width is higher than that in the wiring portion having a larger pattern 
width. 
In a preferred embodiment, the superconducting material is a 
Bi-Pb-Sr-Ca-Cu-O-base perovskite superconducting material, and the easily 
evaporable component is Pb. In this case, the superconductor forming 
material is a Bi-Pb-Sr-Ca-Cu-O-base material comprising Bi, Pb, Sr, Ca and 
Cu in a Bi:Pb:Sr:Ca:Cu molar ratio of preferably (1.9 to 2.1):(1.2 to 
2.2):2:(1.9 to 2.2):(3 to 3.5), more preferably (1.9 to 2.1):(1.5 to 
1.8):2:(1.9 to 2.2):(3 to 3.5). The general composition of the 
Bi-Pb-Sr-Ca-Cu-O-base perovskite superconducting material thus prepared is 
represented by the formula (Bi.sub.1-x Pb.sub.x).sub.2 (Sr.sub.1-y 
Ca.sub.y).sub.4 Cu.sub.3 O.sub.z wherein 0&lt;x&lt;1, 0&lt;y&lt;1 and 0&lt;z. 
In the present invention, a particularly large effect can be attained when 
the wiring width is 1 mm or less, more preferably 0.5 mm, most preferably 
0.3 mm. Although the present invention can be applied to both a thin film 
and a thick film, a larger effect can be attained particularly when the 
film is a thin film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An oxide superconducting film having a critical temperature of 100 K or 
above can be formed by any of the thick film method and the thin film 
method, but when the line width is about 1.0 mm or less, it becomes 
difficult to form the 110 K phase as a single phase and the 80 K phase as 
well is present, so that the critical temperature is rapidly lowered. In 
this case, the material often exhibits no superconducting state even at a 
liquid nitrogen temperature (77 K) The present inventors consider that the 
reason why it becomes difficult to form the 110 K phase when the line 
width is reduced resides in the evaporation of a component having a 
particularly high vapor pressure, among the components constituting the 
superconductor. 
Specifically, with respect to a Bi-Pb-Sr-Ca-Cu-O-based material which the 
present inventors have studied for practical use, a component having a 
particularly high vapor pressure, and having a great influence on the 
properties is PbO, which exhibits the relationship between the temperature 
and the vapor pressure as shown in Table 1. 
TABLE 1 
______________________________________ 
Temperature (.degree.C.) 
Vapor pressure (Torr) 
______________________________________ 
about 680 1 .times. 10.sup.-2 
about 750 1 .times. 10.sup.-1 
about 930 .sup. 1 .times. 10.sup.0 
about 1100 .sup. 1 .times. 10.sup.1 
1470 (b.p.) 760 (1 atm) 
______________________________________ 
Since the temperature at which firing is conducted for crystallization to 
form a superconducting phase is about 850.degree. C., the vapor pressure 
of PbO is so high that an evaporation easily occurs at this stage. 
In particular, the reason why it becomes difficult to form the 110 K phase 
as a single phase in a small pattern width in question is believed to be 
because, since the partial pressure of PbO attributable to the evaporation 
from the periphery including the PbO itself decreases with a reduction in 
the line width, the evaporation of the component is not inhibited and the 
exposed surface area per unit volume becomes large, which causes the 
amount of evaporation of PbO to become large. 
Accordingly, in the present invention, a 110 K phase is formed as a 
substantially single phase even in a small line width, through the 
prevention or compensation for the evaporation of particularly PbO during 
firing, to thereby produce an oxide superconducting film having a high 
critical temperature. 
The first means for preventing the evaporation of an easily evaporable 
component is to spatially limit the firing atmosphere around the wiring 
portion. Specifically, a plate (a substrate) of a material having no 
chemical influence on a superconducting film, for example, MgO, Al.sub.2 
O.sub.3, LaAlO.sub.3, sapphire, SrTiO.sub.3, ZrO.sub.2 (including a 
stabilized or partially stabilized element), LaGaO.sub.3, MgAl.sub.2 
O.sub.4, Y.sub.2 O.sub.3, SiO.sub.2, 2MgO.SiO.sub.2, Si, MgO.SiO.sub.2 or 
a quartz glass is disposed so as to face a substrate having a wiring 
pattern of a superconductor forming material formed thereon. The gap 
between the wiring pattern on the wiring substrate and the counter plate 
(counter substrate) is preferably narrow, and is generally 1 mm or less, 
preferably about 0.5 mm. 
The second means is to form a pattern of a material containing an easily 
evaporable component on the counter substrate. This enables the vapor 
pressure of the easily evaporable component from the superconducting 
wiring pattern limited by the counter substrate to be maintained at a high 
level such that the evaporation of the easily evaporable component from 
the wiring pattern of the superconductor forming material film is further 
prevented. Further, when the counter substrate having this pattern formed 
thereon is heated to a temperature equal to or above the temperature of 
the substrate on which a superconducting wiring pattern is to be formed, 
the evaporation of the easily evaporable component from the counter 
substrate is accelerated. In this case as well, the gap between the wiring 
substrate and the pattern of the counter plate is preferably as narrow as 
possible, i.e., preferably 1 mm or less, more preferably about 0.5 mm. 
The pattern of the material containing an easily evaporable component may 
be formed on the whole surface of the counter substrate. In a preferred 
embodiment, however, preferably the configuration of the pattern 
corresponds to the superconducting wiring pattern fired. This is because, 
in the formation of a multilayer structure etc., preferably that no excess 
evaporated component is deposited on a portion other than the 
superconducting wiring pattern of the substrate. Further, the evaporated 
component might accumulate within the oven, to thus make it impossible to 
control the atmosphere in the oven. In this sense, said first means is 
preferred because no deposit occurs on the counter substrate, and there is 
no pattern of a material containing an easily evaporable component. 
The material containing an easily evaporable component for forming a 
pattern on the counter substrate may be a material comprising the same 
component or composition as that of the superconducting wiring material. 
For example, in the case of a Bi-Pb-Sr-Ca-Cu-O-based superconductor, use 
is made of a Bi-Pb-Sr-Ca-Cu-O-based material. In the 
Bi-Pb-Sr-Ca-Cu-O-based superconductor, the easily evaporable components 
are Bi, Pb, Cu, etc. In this case, Pb is particularly evaporable, and at 
the same time, plays an important role in the formation of a 110 K phase. 
In the case of a thick film having a thickness of 20 .mu.m or more, when a 
printed wiring pattern of a superconductor forming film is fired, if the 
material is maintained for a short time at a temperature above a 
predetermined firing temperature to an extent such that the evaporation of 
PbO does not vigorously occur and the temperature is returned to the 
predetermined firing temperature to conduct firing, the growth of a 
crystal grain is accelerated, and a dense film having no gap in the grain 
boundary can be obtained. 
The third means is that when a wiring pattern of a superconductor forming 
material film is formed on a substrate, a pattern of a material containing 
an easily evaporable component, the line width of the wiring pattern being 
larger than that of the wiring pattern of the superconductor forming 
material film, is formed along and both sides of the wiring pattern of the 
superconductor forming material film. Specifically, when the line width of 
a superconductor wiring pattern is 1 mm or less, particularly 0.5 mm or 
less, it becomes difficult to form a 110 K phase. On both sides of such a 
narrow wiring pattern of a superconductor forming film is provided a 
pattern of a material containing an easily evaporable component in a 
larger line width than that of the wiring pattern of the superconductor 
forming material film, generally 1 mm or more, for example, 2 mm or more, 
further 3 mm or more, while leaving a gap of preferably 1 mm or less, more 
preferably 0.5 mm or less between the narrow wiring pattern of a 
superconductor forming film and the pattern of the material containing an 
easily evaporable component. 
Since the pattern of a material containing an easily evaporable component 
is formed on the same substrate as that of the wiring pattern of the 
superconductor forming material film, it is preferred that an identical 
composition be used for the material containing an easily evaporable 
component and the superconductor forming material, and the film formation 
and the patterning for the material containing an easily evaporable 
component be conducted simultaneously with the film formation and the 
patterning for the superconductor forming material. In this case, since 
both the patterns formed by the firing comprise a superconducting film, a 
device may be made on the superconducting wiring pattern so that a pattern 
having a larger line width (for example, a ground line) is disposed on 
both sides of a pattern having a smaller line width (for example, a signal 
wiring), thus causing the larger wiring pattering provided on both sides 
of the smaller wiring pattern to be left in the final product. Since, 
however, the superconducting wiring is used in applications such as a high 
electron mobility transistor (HEMT) and a circuit wherein use is made of a 
Josephson element, when an excess pattern exists in close vicinity of the 
line at the time of transmission of a radio frequency signal, the excess 
pattern exhibits the same function as that of the earth and lowers the 
quality of a signal. Therefore, such an excess pattern should be removed. 
The excess pattern can be removed by wet etching (with an aqueous solution 
of hydrochloric acid, phosphoric acid or the like) or dry etching (a 
reactive ion etching or the like). 
The fourth means is to feed a vapor of an easily evaporable component into 
a firing oven or to place a material containing an easily evaporable 
component (in the form of a pellet or the like) within a firing oven to 
feed a vapor of the material containing an easily evaporable component 
into the firing atmosphere. 
The fifth means comprises dividing the superconducting wiring pattern into 
a wiring portion having a larger line width and a wiring portion having a 
smaller line width and firing the wiring portion having a larger line 
width in an atmosphere different from that used in the firing of the 
wiring portion having a smaller line width. Since the wiring layer having 
a larger line width and the wiring layer having a smaller line width are 
fired in respective separate atmospheres, it is possible to select 
respective firing atmospheres so that the amounts of evaporation of high 
vapor pressure components in respective wiring layer compositions become 
substantially equal to each other. This enables the compositions of the 
respective wiring layers after the firing to be coincided with each other, 
and predetermined superconducting properties to be simultaneously obtained 
in all wiring layers. 
For example, the wiring portion having a larger line width and the wiring 
portion having a small line width may be separated from each other in an 
identical firing chamber by means of a partitioning member to make the 
atmospheres of the two wiring portions different from each other. The 
following method is particularly convenient. A wiring portion having a 
larger line width is formed on one principal surface of a substrate, and a 
wiring portion having a smaller line width is formed on the other 
principal surface of the substrate. The substrate is held on a through 
hole provided on a member for partitioning the firing chamber into two 
regions. Both sides of the substrate are respectively exposed to the two 
regions on both sides of the partition, and the through hole is 
hermetically sealed by the substrate, thus enabling the atmospheres of the 
two regions to be made different from each other. 
The sixth means is to differentiate the concentration of the easily 
evaporable component in the wiring portion having a larger line width from 
that of the easily evaporable component in the wiring portion having a 
smaller line width, so that the concentration of the easily evaporable 
component of the wiring portion having a smaller line width becomes 
higher. That is, the concentration of the evaporable component is 
previously regulated depending upon the line width or the evaporability. 
In this case, when the superconductor is a Bi-Pb-Sr-Ca-Cu-O-based 
superconductor, the Pb/Bi molar ratio is preferably from 0.6 to 1.0 in any 
of the wiring patterns to be fired. 
An experiment was conducted on how the Pb concentration of a 
Bi-Pb-Sr-Ca-Cu-O-based superconductor wiring pattern changes with the 
wiring width during the firing and the confirmation of the formed phase at 
that time. 
A Bi-Pb-Sr-Ca-Cu-O layer was deposited on a MgO single crystal substrate by 
RF magnetron sputtering and fired to form a Bi.sub.2 Sr.sub.2 Ca.sub.2 
Cu.sub.3 O.sub.x oxide superconductor wiring. In a deposit composition 
comprising Bi, Pb, Sr, Ca and Cu in a molar ratio of 1.0:0.9:1.0:1.0:1.7, 
a whole ground plane layer having a size of 10 mm.times.10 mm and a signal 
layer having a line width of 1 mm were deposited, and firing was conducted 
according to a temperature profile shown in FIG. 1. In FIG. 1, the OdD is 
a temperature profile used in the actual firing. In this case, the 
temperature is raised from room temperature to 805.degree. C., maintained 
at that temperature for 20 min, further raised to 855.degree. C., 
maintained at that temperature for one hour and lowered to room 
temperature. The OaA, ObB and OcC are each a temperature profile wherein 
the firing is discontinued without continuation to the final stage. The 
firing was conducted in the air. After the firing, the film was subjected 
to measurement of an X-ray diffraction and analysis of the composition. 
The X-ray diffraction patterns of samples fired according to the profile 
OabB are shown in FIGS. 2A and 2B. In the drawings, L, LL and P 
respectively represent peaks attributable to a 80 K phase, a 10 K phase 
and Ca.sub.2 PbO.sub.x. As shown in FIG. 2A, in a ground plane layer 
sample having a size of 10 mm.times.10 mm, a strong peak attributable to 
Ca.sub.2 PbO.sub.x is observed, whereas as shown in FIG. 2B, in a signal 
layer sample having a line width of 1 mm, a peak attributable to Ca.sub.2 
PbO.sub.x is very weak. 
The results of the analysis of the composition conducted in each stage of 
the firing with respect to a sample of a ground plane layer and a sample 
of a signal layer are shown in FIG. 3. In both cases, the relative 
concentration of each component is plotted by taking the concentration of 
Sr concentration as 1. The Bi, Cu and Ca concentrations are substantially 
constant from the deposited state through each stage of the firing. On the 
other hand, it is apparent that the Pb concentration decreases with the 
advance of the firing. In particular, in samples fired according to the 
temperature profile OabB corresponding to FIGS. 2A and 2B, the signal 
layer having a smaller width (line width: 1 mm) exhibits a more rapid 
reduction in the Pb concentration than the ground plane layer (10 
mm.times.10 mm) having a larger width signal layer, 
.quadrature..largecircle..DELTA.: ground plane layer). 
FIG. 4 is a quasi binary equilibrium phase diagram of PbO-CaO, and it is 
apparent that when the concentration ratio of PbO to CaO is larger than 
1:2, partial melting occurs at 815.degree. C., and accordingly, it is 
expected that, in more a complicated Bi-Pb-Sr-Ca-Cu-O-based superconductor 
also, the melting occurs at least partially at a temperature equivalent to 
or below that temperature. Therefore, it is substantially certain that 
melting occurs when a large amount of PbO stays within the film in the 
course of the temperature rise from 805.degree. C. to 855.degree. C. 
according to the actual firing temperature profile OabcdD shown in FIG. 1 
and the holding of the temperature at 855.degree. C. 
EXAMPLE 1 
A superconductor forming material was deposited by radio-frequency 
magnetron sputtering on a MgO single crystal substrate covered with a 
metal mask having a linear pattern, to form a superconductor forming 
material film pattern having a line width of 0.5 mm (500 .mu.m) and a 
thickness of 0.8 .mu.m and comprising Bi, Pb, Sr, Ca and Cu in a ratio of 
1.0:0.8:1.0:1.0:1.6. 
Then, the substrate having a superconductor forming material film formed 
thereon was fired in the air at 850.degree. C. for one hour to form a 
Bi-based perovskite superconducting film. 
In the firing, as Example 1, (a) a MgO substrate was covered with a FGA 
(fine grained alumina) plate and the distance between the MgO plate and 
the FGA plate was set to about 1 mm, and as comparative examples, (b) 
firing was conducted in a state such that the distance between the FGA 
plate and the MgO substrate was set to about 5 mm, and (c) no FGA plate 
was used. 
The X-ray diffraction patterns determined on samples (a), (b) and (c) thus 
prepared are respectively shown in FIGS. 5A, 5B and 5C. In these diagrams, 
H and L represent a 110 K phase and an 80 K phase, respectively. 
From FIG. 5A, it is apparent that main peaks in the X-ray diffraction 
pattern of the sample (a) are H, i.e., attributable to the 110 K phase. On 
the other hand, from FIGS. 5B and 5C, it is apparent that main peaks in 
the X-ray diffraction patterns of the samples (b) and (c) are L, i.e., 
attributable to the 80 K phase. 
FIG. 6 is a graph showing a change of electrical resistivity with 
temperature in a superconducting film wiring having a line width of 0.5 mm 
prepared by firing under the same condition as that used in the 
preparation of the sample (a). In FIG. 6, the abscissa represents the 
temperature and the ordinate represents the electrical resistivity value. 
From FIG. 6, it is apparent that the critical temperature is 97 K, i.e., 
substantially the same as that in the case of a superconducting film 
wiring provided with a pattern having a large line width of 1 mm or more. 
The critical current density at the liquid nitrogen temperature was 
2.times.10.sup.3 A/cm.sup.2, i.e., sufficiently large. 
EXAMPLE 1 (EXAMPLE OF THICK FILM) 
Raw material powders of Bi.sub.2 O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3 and 
CuO were prepared and mixed with each other so that the molar ratio of 
Bi:Pb:Sr:Ca:Cu was 0.7:0.3:1:1:1.8. The mixed powder was fired at 
845.degree. C. for 150 hr to prepare a Bi-Pb-Sr-Ca-Cu-O-based oxide 
superconductor. 
The oxide superconductor was coarsely ground in a mortar and then subjected 
to regulation of the grain size in a ball mill. Terpineol was added as a 
viscosity modifier to the powder, and the mixture was kneaded with acetone 
as a solvent. The kneaded product was dried to remove acetone, and benzene 
was mixed with the dried product. The mixture was dried to regulate the 
viscosity to prepare a superconducting paste. 
A plurality of sheets of a magnesia single crystal substrate having a size 
of 15 mm square and a thickness of 0.5 mm were provided, and a line 
pattern having a line width of 0.5 mm and a length of 10 mm was printed by 
screen printing through the use of this paste. 
Then, a firing oven shown in FIG. 7 was provided. Specifically, alumina 
plates 8,9 respectively provided with heaters 5,6 at the back thereof were 
mounted on a frame 7 comprising a quartz glass, and substrates 2,4 were 
placed opposite each other on the ceramic plates 8,9 though the use of a 
magnesia spacer 10. The substrate 2 is a substrate for forming a 
superconducting wiring, and the substrate 4 is a substrate having a 
pattern for preventing the evaporation of PbO from the superconducting 
wiring. Thus, the gap between the wiring pattern 1 and the counter pattern 
3 was kept at 0.5 mm. 
Two sheets of a magnesia substrate having, printed thereon, a conductor 
line having a line width of 0.5 mm, a thickness of 30 .mu.m and a length 
of 10 mm were mounted in a firing oven shown in FIG. 7 and placed opposite 
to each other at a space of 0.5 mm therebetween, and the upper substrate 
was used as a wiring pattern. 
In the air, heaters 5,6 were energized, the two substrates were heated at 
860.degree. C. for 10 min, and the temperature of the upper substrate 2 
was lowered to 840.degree. C. with the temperature of the lower substrate 
4 being kept at 850.degree. C. for 6 hr to conduct firing. 
The resultant conductor line (superconducting phase) of the upper substrate 
2 was subjected to a measurement of the temperature dependence of the 
electrical resistivity thereof. The results are shown in FIG. 8. The 
critical temperature was about 98 K. 
For comparison, the above-described procedure was repeated. Specifically, 
raw material powders of Bi.sub.2 O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3 and 
CuO were prepared and mixed with each other so that the molar ratio of 
Bi:Pb:Sr:Ca:Cu was 0.7:0.3:1:1:1.8. The mixed powder was fired at 
845.degree. C. for 150 hr to prepare a Bi-Pb-Sr-Ca-Cu-O-based oxide 
superconductor. 
The oxide superconductor was coarsely ground in a mortar and then subjected 
to a regulation of the grain size in a ball mill. Terpineol was added as a 
viscosity modifier to the powder, and the mixture was kneaded with acetone 
as a solvent. The kneaded product was dried to remove acetone, and benzene 
was mixed with the dried product. The mixture was dried to regulate the 
viscosity to prepare a superconducting paste. 
A line pattern having a line width of 1 mm was printed by screen printing 
through the use of this paste, and dried. In the air, the printed and 
dried substrate was heated to 860.degree. C. for 10 min, lowered to 
245.degree. C. and fired at that temperature for 6 hr. The temperature 
dependence of the resistivity measured on the line pattern of this sample 
is also given in FIG. 8, which clearly demonstrates the effect of the 
present invention. 
EXAMPLE 3 (EXAMPLE OF THICK FILM) 
The procedure of Example 2 was repeated, except that a line pattern having 
a line width of 0.5 mm and a length of 10 mm was formed on one magnesia 
single crystal substrate and a pattern having a size of 10 mm square was 
formed on another magnesia single crystal substrate by screen printing. 
The magnesia substrate having printed thereon a line pattern having a line 
width of 0.5 mm, a thickness of 30 .mu.m and a length of 10 mm was mounted 
on the upper part of the firing oven, while the magnesia substrate having, 
printed thereon, a solid pattern having a size of 10 mm square and a 
thickness of 30 .mu.m was mounted on the lower part of the firing oven. 
The firing was conducted in the same manner as that of Example 2. The 
resultant conductor line pattern (superconducting phase) was subjected to 
a measurement of the temperature dependence of the resistivity. The 
results are shown in FIG. 8. The critical temperature was about 100 K. 
EXAMPLE 4 
As shown in FIG. 9, a 1 .mu.m-thick thin film of an oxide having a 
composition ratio of Bi:Pb:Sr:Ca:Cu of 1.0:0.8:1.0:1.0:1.6 was formed on a 
magnesia single crystal substrate 13 by radio-frequency magnetron 
sputtering through the use of a metal mask. On both sides of a line 14 
having line widths of 0.5 mm (500 .mu.m) and 1 mm were provided patterns 
15,16 each having a line width of 3 mm while leaving a gap of 0.5 mm (500 
.mu.m) between the line 14 and each pattern. 
This substrate was fired in the air at 855.degree. C. for one hour to form 
a superconducting phase. 
X-ray diffraction patterns measured on the resultant lines respectively 
having line widths of 0.5 mm and 1 mm are shown in FIGS. 10A and 10B. In 
these drawings also, H and L represent a 110 K phase and an 80 K phase, 
respectively. 
As can be seen from these drawings, main peaks in the X-diffraction 
patterns are attributable to the 110 K phase. 
The results of measurement of the temperature dependence of the resistivity 
on a line having a line width of 1 mm are shown in FIG. 11. The critical 
temperature was 107 K, which was exactly the same as that in the case of a 
line having a large line width. 
The critical current density at liquid nitrogen atmosphere was sufficiently 
large and 4.times.10.sup.3 A/cm.sup.2. 
The results where the line width was 0.5 mm and the patterns 15,16 were not 
formed on both sides of the line are as shown as Comparative Example in 
Example 1. Specifically, as shown in FIG. 5C, main peaks were attributable 
to an 80 K phase. The measurement of the temperature dependence of the 
resistivity has revealed that, although the resistivity rapidly decreased 
around 110 K, the temperature at which the resistance became zero (0) was 
as low as 78 K. Further, at liquid nitrogen temperature, the 
superconducting state was broken by a small amount of current. 
EXAMPLE 5 
In this example, firing was conducted through the use of a tubular oven as 
shown in FIG. 12. In the drawing, numeral 21 designates a tubular firing 
chamber, numeral 22 a gas feeding pipe, numeral 23 an evacuation pipe, 
numeral 24 a first heater, numeral 25 a second heater, numeral 26 a third 
heater, numeral 27 a Bi-Pb-Sr-Ca-Cu-O pellet, and numeral 28 a substrate 
having, formed thereon, a wiring pattern 29 of a superconductor forming 
material. This tubular oven is usually called a "three zone tubular oven" 
and has three zones which can be independently subjected to regulation of 
the temperature respectively by three pairs of heaters (the first heater 
24, the second heater 25 and the third heater 26). 
In this firing oven, a magnesia single crystal substrate 28 having the same 
superconductor forming material film pattern 29 [line width: 0.5 mm (500 
.mu.m)] as that prepared in Example 1 was mounted. 
A mixed gas (O.sub.2 +N.sub.2) was fed at a flow rate of 5 liters/min into 
the tubular firing chamber 21 through the gas feed pipe 22, and the 
setting was conducted so that the temperature of the substrate 28 became 
850.degree. C. during firing. An oxide pellet 27 having a diameter of 30 
mm and a height of 2 mm and containing Bi, Pb, Sr, Ca and Cu in a molar 
ratio of 1:1:1:1:1.5 was placed in the first zone corresponding to the 
first heater 24, and the temperature of the first zone was set to 
800.degree. C. 
Under the above setting conditions, the vapor pressure of PbO around the 
substrate 28 was enhanced to about 10.sup.-5 Torr. 
As can be seen from FIG. 13 showing an X-ray diffraction pattern of the 
resultant sample, a superconducting wiring composed mainly of a 110 K 
phase was formed even in a small line width by firing under the above 
conditions. 
EXAMPLE 6 
Bi-Pb-Sr-Ca-Cu-O was subjected to RF magnetron sputtering to deposit a 1 
.mu.m-thick ground plane layer having a size of 10 mm.times.10 mm on one 
principal surface of a MgO substrate. A metal mask was applied to the 
other principal surface of the MgO substrate, and deposition was conducted 
in the same manner as that described above to deposit a 1 .mu.m-thick 
signal layer having a width of 0.5 mm (500 .mu.m). 
Both the above depositions were conducted at a substrate temperature of 
350.degree. C. so as to form a deposit having such a composition that the 
Bi:Pb:Sr:Ca:Cu molar ratio was 1.0:0.6:1.0:1.0:1.6. 
The deposit layers were then fired. The tubular oven used in the firing and 
a substrate holding portion of the oven are respectively shown in FIG. 14A 
and 14B. 
The tubular oven 31 comprises a quartz glass. The inside of the firing 
chamber is partitioned into two regions, that is, upper and lower regions 
(M, N), by means of a partitioning plate 32 comprising a quartz glass, and 
a substrate 33 is held in substantially a central portion of the 
partitioning plate 32. The substrate holding portion has an opening 
passing through the thickness, and the edge of the opening was prepared in 
the form of a ring alumina substrate receiver 37 provided with a flange. 
The outer edge of the substrate 33 was put and held on the flange of the 
substrate receiver 37, and the outer edge of the substrate and the flange 
were brought into close contact with each other to block the opening to 
separate the two regions, i.e., upper and lower regions (M, N), from each 
other. A gas for a firing atmosphere was introduced through a gas flow 
inlet 34 provided at the left end of the tubular oven 31, and evacuation 
was conducted through a gas flow outlet 35 provided at the right end of 
the tubular oven. A Bi-Pb-Sr-Ca-Cu-O pellet 36 having a Bi:Pb:Sr:Ca:Cu 
ratio of 1.0:1.0:1.0:1.0:1.5 was placed upstream of the position holding 
the substrate 33 in the upper region M, and a suitable amount of PbO was 
fed into an atmosphere gas flow from the flow inlet 34. 
The MgO substrate 33 was held in a substrate holding portion of the 
partitioning plate 32 within the tubular oven 31 in such a manner that the 
surface having a signal layer deposited thereon faced upward and the 
surface having a ground plane layer deposited thereon faced downward. A 
mixed gas (N.sub.2 +O.sub.2) (flow ratio N.sub.2 :O.sub.2 =4:1, total flow 
rate=5 liters/min) was fed through the gas flow inlet 34. Firing was 
conducted according to a firing temperature profile OabcdD (805.degree. 
C..times.20 min 805.degree. C..times.1 hr) shown in FIG. 1. 
After firing, the substrate 33 was taken out of the tubular oven 31 and 
subjected to X ray diffraction. The results are shown in FIGS. 15A and 
15B. In the drawings, H and L represents a 110 K phase and an 80 K phase, 
respectively. From the results, it is apparent that the ground plane layer 
(FIG. 15A) and the signal layer (FIG. 15B) in the film after the firing 
each consist essentially of a single phase of the 110 K phase. 
The resultant individual wiring layers were subjected to measurement of a 
change of the electrical resistance with the temperature, and the results 
are shown in FIG. 16. The critical temperature, Tc, of the ground plane 
layer (a) was exactly same as that of the signal layer (b) and 100 K. In 
both wiring layers, the critical current density at liquid nitrogen 
temperature was 4.times.10.sup.3 A/cm.sup.2, i.e., satisfactory from the 
viewpoint of practical use. 
EXAMPLE 7 
A ground plane layer having a size of 10 mm.times.10 mm was deposited on a 
MgO substrate by RF magnetron sputtering. A metal mask was applied to the 
opposite surface of the MgO substrate, and deposition was conducted in the 
same manner as that described above to deposit a signal layer having a 
width of 0.5 mm (500 .mu.m) was deposited. 
Both of the above depositions were conducted at a substrate temperature of 
350.degree. C. so as to form a deposit having such a composition that the 
Bi:Pb:Sr:Ca:Cu ratio was 1.0:0.6:1.0:1.0:1.6 in the case of the ground 
plane layer and 1.0:0.8:1.0:1.0:1.6 in the case of the signal layer. That 
is, the Pb concentration of the signal layer was higher than that of the 
ground plane layer. 
The deposit layers were then fired. The firing was conducted within a 
conventional quartz tubular oven through the use of a mixed gas (N.sub.2 
+O.sub.2) (flow ratio N.sub.2 :O.sub.2 =4:1, total flow rate=5 liters/min) 
as a firing atmosphere according to a firing temperature profile OabcdD 
(805.degree. C..times.20 min+855.degree. C..times.1 hr) shown in FIG. 1. 
After the firing, the substrate was taken out of the tubular oven and 
subjected to X ray diffraction measurement. The results are shown in FIGS. 
17A and 17B. In the drawings, H represents a diffraction peak attributable 
to a 110 K phase. From the results, it is apparent that the ground plane 
layer (FIG. 17A) and the signal layer (FIG. 17B) in the film after the 
firing each consist essentially of a single phase of the 110 K phase. 
The resultant individual wiring layers were subjected to measurement of a 
change of the electrical resistivity with the temperature, and the results 
are shown in FIG. 18. The critical temperature, Tc, of the ground plane 
layer (a) was exactly the same as that of the signal layer (b) and 100 K. 
In both wiring layers, the critical current density at liquid nitrogen 
temperature was 1.times.10.sup.4 A/cm.sup.2, i.e., satisfactory from the 
viewpoint of practical use. 
For comparison, wiring layers was formed in the same manner as that of the 
above Example, except that both the ground plane layer and the signal 
layer had a deposit composition having a Bi:Pb:Sr:Ca:Cu ratio of 
1.0:0.8:1.0:1.0:1.6. The resultant wiring layers were subjected to X-ray 
diffraction. The results of the X-ray diffraction on the ground plane 
layer are shown in FIG. 19. In the drawing, H, L and LL represent 
diffraction peaks attributable to a 110 K phase, an 80 K phase and a 10 K 
phase. It is apparent that the Pb concentration during the deposition of 
the ground plane layer was so high that the 80 K phase and the 10 K phase 
were present as well as the 110 K phase. 
According to the present invention, it is possible to form an oxide 
superconducting wiring having a high critical temperature, particularly a 
Bi-Pb-Sr-Ca-Cu-O based oxide superconducting wiring rich in a 110 K phase 
even when the line width is small, which renders the oxide superconducting 
wiring formed by the present invention useful as a high-temperature 
superconducting wiring used at liquid nitrogen temperature in high 
electron mobility transistors, Josephson elements, etc.