Note: Descriptions are shown in the official language in which they were submitted.
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~C~GROUND O~ THE INVEN~ION
The discovery of high Tc superconductors like Y-Ba-Cu-O,
Bi-Sr-Ca-Cu-O and Tl-Ca-Ba-Cu-O with resistance transition tem-
peratures above the boiling point of liquid nitrogen (about 77 K~
has attracted the world wide attention. The high Tc superconduc-
tors have many potential applications in physics, medical
sciences, computers and in most of the engineering areas. For
some of the applications, superconductors in a bulk form are
required (for example in power transmission and magnetic levita-
tion). For applications in electronic areas like in electronic
devices and circuits, superconductor materials in a thin film
form are needed (thickness about 1 micron). Thin superconductor
films are needed for interconnects, high frequency transmission
lines and for the formation of activP devices like transistors
and ~unctions. Deposition of the thin films is usually carried
out in a vacuum chamber in order to achieve good crystalline
quality. There are several vacuum methods which can be used for
such film deposition: sputtering from a single target, sputtering
from multiple targets, electron beam co-evaporation, and electron
beam sequential evaporation. For all of these methods, except the
sputtering from a single target, a precise thickness control or
deposition rate control is needed in order to achieve the re-
quired film composition for the high Tc superconductivity. The
required precise thickness control or deposition rate control
often makes the vacuum processes to be complex and expensive. For
the high Tc film deposition using the single target sputtering
method in a conventional vacuum system, the film composition is
often not uniform even over a relatively small substrate area of
about 1 cm2. ~rom the above statement, it is clear that for the
meth~ds involving more than one source (or target), very precise
control over the deposition rate of the elements is needed. For
the case of sequential electron beam evaporation, a precise
control of thickness of individual layer is required. The precise
control of the composition (atomic ratio of Y:Ba:Cu:O, for
example, for the Y-Ba-Cu-O compound) is needed in order to
achieve the crystalline phase for the high Tc superconductivity.
In the present invention, we describe an improved vacuum method
which will allow one to deposit the high Tc superconductor thin
films with controlled stoichiometry without the need of
deposition rate controllers and thickness controllers. The
composition of the final films is controlled simply by
controlling the amounts of source materials introduced into the
boats during the deposition. The thickness of the films is
controlled by the total weight of the source materials and the
distance between substrates and boats containing the source
materials.
During the vacuum deposition of the high Tc superconductor
thin films, it is also beneficial to avoid the presence of one or
more of the constituent elements. For example, it is desirable to
avoid Tl in the vacuum system during the deposition of the Tl-Ca-
Ba-Cu-O films tTl is highly toxic and may contaminate the vacuum
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system and thus the environment)~ In the present invention, a
method is also introduced to deposit films without containing
initial Tl. The required amounts of Tl for the formation of the
superconducting phase are introduced into the films by carrying
out a subsequent heat treatment.
For the electronic and power applications, it is also
required to deposit the high Tc superconductor films on objects
with an arbitrary or irregular shape. For such objects with the
arbitrary or irregular shape, it is often difficult to deposit
films with the conventional vacuum methods. In the second part of
the invention, we describe a paint on method for this purpose.
This paint on method does not require a vacuum system and can be
conveniently used to prepare thick films of the high Tc
superconductors (thickness more than 10 microns).
OBJECTS AND STATEMENT
An object of the present invention is to provide an improved
vacuum evaporation method to deposit thin films of the high Tc
superconductors with controlled composition and thus the
superconducting properties.
Another object of the invention is to provide a method to
incorporate selected elements (or oxides) in the films to achieve
superconductivity during a post deposition heat treatment.
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Yet another object of the invention is to teach a non vacuum
method for the preparation of thick films of the high Tc
superconductors on regular or irregular substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a chamber arrangement with two boats to achieve the
controlled deposition of the multi element Tl-Ca-Ba-Cu-O films.
Fig. 2 shows a chamber arrangement with four boats to achieve the
controlled deposition of the multi element Tl-Ca-Ba-Cu-O films.
Fig. 3 shows a chamber arrangement with one boat to achieve the
controlled deposition of the multi element Tl-Ca-Ba-Cu-O films.
Fig. 4 shows a chamber arrangement with three boats to achieve to
the controlled deposition of the multi element Tl-Ca-Ba-Cu-O
films.
Fig. 5 shows the cross sectional view of the as-deposited multi
layer films using the two boat arrangement shown in Fig. 1.
Fig. 6 shows the cross sectional view of the as-deposited multi
layer films using the four boat arrangement shown in Fig. 2.
Fig. 7 shows the furnace arrangement for the heat treatment in an
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environment containing 2 and Tl or T102.
Fig. 8 illustrates the non vacuum paint on method to produce
superconducting thick films on substrates wlth an arbitrary or
irregular shape.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention relates to the production of high Tc
superconductor thin films and thick films. For the thin
superconducting films, Figs. 1, 2, 3 and 4 illustrate several
vacuum chamber arrangements to achieve sequential deposition of
the films. The source materials, either elements (Tl, Ca, Ba, Cu
for example) or oxides (T102, CaO, BaO and CuO), are weighted and
put in the boats. One or more boats are adopted to achieve the
sequential deposition. In Fig. 1, a system with two boats is
shown where Tl, Ca, Ba are put in a Ta boat (1) and Cu in a MO
boat (2). The films are deposited on supportive substrates (3).
The substrates are mounted on a substrate holder (4). The
deposition is carried out in a vacuum environment enclosed by a
vacuum chamber (5) and pumped by a pumping unit. Substrate
heating may be applied during the multi layer film deposition
process. Fig. 2 shows a system with four separate boats which can
be heated separately to achieve the sequential multi-layer. Boat
(2) is for Cu, (6) for Ba, (7) for Ca and (8) for Tl. A single
boat system is shown in Fig. 3 where the boat (9) is used to
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evaporate Tl, Ca, Ba and Cu. A system with 3 boats [(10), (11)
and (12)] is shown in Fig. 4. The actual elemental sequence of
the multilayer film is determined by the sequence of boat heating
during the evaporation for the four boat arrangement. For the two
boat arrangement, it is determined by the relative melting points
of the elements. In all cases, the element Cu is evaporated last
in order to cover and to protect the more active elements like Ba
and Ca. The deposition is repeated several times on the same set
of substrates to obtain the desired thickness of about 1 micron
(the actual thickness is determined by the total amounts
evaporated). The cross sectional views of the as-deposited films
using the two-hoat arrangement is shown in Fig. 5 and is shown in
Fig. 6 for the ones evaporated using the four boat arrangement in
Fig. 2. As stated before, for the films deposited using the two-
boat arrangement, the elemental sequence is determined by the
boat heating sequence and the elemental melting points in one
boat (1). Therefore, if the process described in Example 1 is
carried out, then multi-layer films with the cross section shown
in Fig. 5 will be produced. In Fig. 5, the Tl~Ca+Ba layers (13)
are not completely uniform due to the difference of the
individual vapollr pressure. The material is rich in Tl near the
substrate (3) and is rich in Ca away from the substrate (3). For
the four boat arrangement, the elemental sequence is determined
purely by the heating sequence of the elements. For example, one
can first evaporate a Cu layer (14) followed by a Tl layer (15),
a Ba layer (16) and a Ca layer (17). This sequence is repeated to
obtain the proper thickness of about 1 micron. It is also noted
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that the actual deposition sequence for Tl, Ca and Ba is not
important and may be varied during the experiments. For this
method, it is therefore clear that the film stoichiometry is
controlled simply by controlling the weights of the materials
loaded into the boats during each evaporation. During the
evaporation, a fixed amount (or fixed pressure) of oxygen may be
allowed in the deposition chamber. After each evaporation, new
materials are loaded into the boats. The loading of the source
materials is achieved by opening the chamber and then adding the
materials directly into the boat or by adding materials with a
jig located inside the vacuum chamber without breaking the
vacuum. It is important to note that the geometry of the boats
should be similar in order to achieve uniform deposition.
After the multi layer film deposition, a high temperature
treatment is made on the films in a quartz chamber ((18), Fig~ 7)
containing 2 This high temperature treatment is needed for
interdiffusion of the elements and thus the formation of the
compounds. The presence of 2 in the chamber is needed in order
to supply sufficient oxygen to the films (19). It is also noted
that a quartz or alumina container (20) with Tl or TlO2 (21) in
it is placed inside the heating chamber near the films for the
Tl-Ca-Ba-Cu-O compound. During the treatment process, a Tl or
TlO2 vapour pressure will be maintained in order to avoid the
loss of Tl during the process. After the high temperature
treatment, a second treatment at a lower temperature in the same
chamber is carried out. The second treatment is needed to adjust
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the content of oxygen and thallium in the final thin films.
For the production of high Tc superconductor thin films
containing volatile and toxic elements like Tl, it is beneficial
not to include it inside the vacuum chamber during the deposition
to minimize the environmental contamination. The film preparation
is carried out by first depositing Ca-Ba-Cu-O layers on
supportive substrates. After the deposition, the films are
treated in the chamber containing both oxygen and thallium as
shown in Fig. 7. Longer treatment time should be allowed in order
to let sufficient amounts of Tl or T102 to diffuse into the films
to form the superconducting compounds.
Alternate method to prepare the Ca-Ba-Cu-O films is
sputtering. In this method, a target containing Ca, Ba, Cu and O
is prepared by mixing and pressing weighted amounts of CaC03,
BaC03 and CuO. The pressed material is heated at about 900C for
2 hours and then re-ground to form new powder. The new powder is
pressed and heated again to forrn the final target. The Ca-Ba-Cu-O
films are sputt:ered on substrates. After the sputtering, the
films are heat treated in the furnace containing both oxygen and
thallium. Both the oxygen and thallium will diffuse into the
films during the treatment process to form the compounds. This
process again allows one to obtain high Tc superconductor films.
In many electrical and electronic applications, it is
required to form superconductor films on objects with an
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irregular shape. For such objects, the conventional vacuum
deposition methods may not be applicable for the film deposition.
In the present invention, a simple paint on method is described.
In this method, oxide powder of the source materials (TlO2, CaO,
BaO and CuO for example) are prepared and weighted. The materials
are then mixed with a liquid to form a paste. The process is
illustrated in Fig~ 8. The paste (22) is applied using a brush
(23) or other tool onto the surface of the substrate (24). After
the paste application, the film is heated at a relatively low
temperature to remove the liquid in the film. The film is then
heated in an environment containing oxyyen and/or other elements
like thallium. The compounds are formed during the heat treatment
process. The thickness of the thick films is about 10 microns or
more.
EXAMPLE 1
Elemental Tl, Ca, ~a and Cu are first weighted to the
desired atomic ratio, for example Tl:Ca:Ba:Cu = 2:2:2:3 or
Tl:Ca:Ba:Cu = 2:2:2:4. The actual amounts are: Tl = 614 mg, Ca =
120.4 mg, Ba = 412.62 mg and Cu = 286.35 mg. Tl, Ca and Ba are
divided equally into four parts for four separate evaporations
while Cu is divided into five parts for five evaporations. The
additional Cu layer is intended to serve as the "buffer" layer to
the substrate. The evaporation process is described as follows.
One part of Cu is introduced into a Mo boat in the vacuum system.
Substrates of ZrO2, MgO or ZrO2 coated Si substrates are then
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mounted on a substrate holder and loaded into the vacuum system
as shown in Fig. 1. The surfaces of the substrates to be
deposited should face the source boats. The vacuum chamber is
assembled and evacuated by the pumping unit. After the pressure
has reached a value below about 10-5 torr, power is applied to
the Mo boat to evaporate the first Cu layer. After the first Cu
layer evaporation, the chamber is open and one part of Cu is
introduced into the same Mo boat and one part of Tl, Ca, and Ba
is introduced into a Ta boat (length of the heating zone about 5
cm). The Ta boat should be located near the Mo boat to achieve
uniform deposition of all the elements. After proper evacuation
of the chamber to a pressure below about 10~5 torr, the Ta boat
is first heated to evaporate sequentially Tl, Ba and Ca. Care
should be taken to heat the Ta boat gradually in order to
minimize the reaction between elements in the Ta boat. The
reaction often results in heat which will produce vigorous and
non-uniform evaporation. After all three elements have
evaporated, the power to the Ta boat is turned off and the power
to the Mo boat is turned on to evaporate Cu. The second Cu layer
will cover the combined Tl, Ca and Ba layer on the substrates and
on the bell jar and parts inside the vacuum jar. Therefore, the
active elemental layers are protected and the system
contamination is minimized. Once the second Cu has been
evaporated completely, the system is allowed to cool for about 10
minutes before opening the chamber to re-load the source
materials. Same amounts of Tl, Ca, Ba are introduced into the Ta
boat and Cu to the Mo boat for the second evaporation. The above
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evaporation is repeated for four times. It is noted again that
after each of the evaporation, the film is covered with Cu. This
top copper layer will minimize unwanted oxidation during the
chamber opening operation. In addition, environmental
contamination by other active elements like Tl, Ba and Ca also
will be minimized.
After the sequential evaporation, the films are removed from
the evaporation unit and stored in an evacuated container or in a
container with inert atmosphere. Samples are then selected for
heat treatment in a horizontal resistively heated furnace with a
quartz chamber. The furnace is first set at 830C. The samples
and a quartz boat with a piece of Tl in it are put in the cool
zone of the furnace tube. Oxygen valve is now open to allow
oxygen to flow and to purge the furnace tube. After this, the
quartz tube with the samples and Tl piece are pushed in two steps
to the central zone of the furnace and treated for about 5
minutes. After this short treatment, the quartz tube with the
samples is pulled to the cool zone and the furnace temperature
reduced to 730C. A new Tl piece is put in the quartz boat and
the samples are treated at the low temperature for a period of 1-
3 hours. Finally the samples are removed from the furnace and
cooled. The superconducting thin films are now formed on the
substrates. It is noted that during the above described heat
treatment processes, the quartz boat with the Tl piece is placed
inside the quartz reaction tube in the up stream position. This
position is beneficial in maintaining a steady Tl or TlOX vapour
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pressure in the sample area.
EXAMPLE 2
In this example, we would like to describe the preparation
of Tl-Ca-Ba-Cu-O superconducting films from materials without
initial Tl. The system used is shown in Fig. 1 where one boat
(Mo) is used for Cu and the other (Ta) for Ca and Ba. Pure
elemental materials with the following amounts are prepared: Ca =
120.4 mg, Ba = 412.62 mg and Cu = 286.35 mg. The atomic ratio is
: Ca:Ba;Cu = 2:2:3 or 2:2:4. Again Ca and Ba are divided equally
into four parts and Cu is divided into five parts. One part of Cu
in loaded into the Mo boat and the substrates also mounted on the
substrate holder. After evacuation to the desired pressure, Cu is
evaporated on the substrates. The chamber is now open and one
part of Cu is loaded into the Mo boat and Ca and Ba are loaded
into the Ta boat. After evacuation, the power to the Ta boat is
turned on to deposit Ba and then Ca. After this deposition, Cu is
evaporated to cover the entire surface. The above sequential
evaporation is repeated for four times to give a total thickness
of about 1 micron. It is noted that the film is always covered
with Cu after each evaporation and therefore the active elements
are protected by the Cu layer. After the evaporation, the films
are first treated at 830C in the quartz chamber described in
EXAMPLE 1 for about 5 minutes. A second treatment at about 730C
is then carried out with a piece of Tl placed in the quartz boat
located in the up stream posikion for a period of about 5-10
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hours. During the low temperature treatment, it is beneficial to
add one or two new pieces of Tl into the quartz boat to maintain
the steady Tl or TlOX vapour pressure. After the complete
treatment, Tl atoms diffuse into the films to form the
superconducting compounds.
EXAMPLE 3
This example describes the formation of Tl-Ca-Ba-Cu-O films
from rf sputtered Ca-Ba-Cu-O layer samples. High purity powder of
CaC03, BaCO3, and CuO are weighted to the following atomic ratio:
Ca:Ba:Cu = 2:2:3. Total weight of the materials is about 40 gm.
The powder is thoroughly mixed and then pressed to form disks
with a diameter of 2.5 cm and a thickness of 0.5 cm. The disks
are then put into a box furnace set at 900C for a two-hour heat
treatment~ After the treatment in room atmosphere, the disks are
cooled and then re-ground to form new powder. The new powder is
pressed again in a jig to form a disk with a diameter of 5 cm and
a thickness of about 2 mm. After a second heat treatment at 900C
for 2 hours, the disk is cooled and then used as the sputtering
target. Deposition of Ca-Ba-Cu-O films is carried out in a second
vacuum unit with a rf sputtering gun. The distance between the
target and the substrates (ZrO2, MgO or ZrO2 coated Si) is about
3 cm and the samples are mounted in position about 2.5 cm from
the normal projection of the target center on a sample holder.
The sample holder is water cooled during the deposition. The
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incident rf power (frequency 13.5 MHz) is set at about 60 watts
and an argon + 5~ oxygen mixture is used as the sputtering gas.
The sputtering is carried out for a period of about 5 hours to
obtain films with a thickness of about 1 micron~ Smooth and
amorphous films are usually obtained using the above process.
After the deposition, the Ca-Ba-Cu-O films are treated in the
quartz chamber described in EXAMPLE 2 to diffuse Tl and 2-
Superconducting thin films finally can be obtained from the
sputtered Ca-Ba-Cu-O films without containing the Tl.
While the invention has been described with reference to Tl-
Ca-Ba-Cu-O high Tc superconducting compounds, preparation of
other high Tc superconductors like Bi-Sr-Ca-Cu-O and Y-Ba-Cu-O
may well be achieved by the same methods. Therefore, the present
invention should not be considered to limit to only one type of
high Tc superconductors. In addition, the heat treatment times
and the temperature required to formed the high Tc compounds many
change with the treatment system. The amounts of source materials
required for the sequential evaporation also will vary with the
thickness requirement and the distance between the boat and the
substrates. The sequential evaporation can be achieved by using a
single long boat, two boats, three boats or even four boats (as
shown in Figs. 1-4). Finally, it is worthwhile to point out that
the present multi-layer sequential evaporation method and the
paint-on method are very simple for the production of the high Tc
thin films.
