U.S. patent application number 10/344169 was filed with the patent office on 2004-07-08 for fabrication of high curret coated high temperature superconducting tapes.
Invention is credited to Selvamanickam, Venkat.
Application Number | 20040131773 10/344169 |
Document ID | / |
Family ID | 22835370 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040131773 |
Kind Code |
A1 |
Selvamanickam, Venkat |
July 8, 2004 |
Fabrication of high curret coated high temperature superconducting
tapes
Abstract
The fabrication of high current coated high temperature
superconducting tapes utilizing vaporized rare earth, barium and
copper tetramethyl heptanedionates is disclosed.
Inventors: |
Selvamanickam, Venkat;
(Wynantskill, NY) |
Correspondence
Address: |
KATTEN MUCHIN ZAVIS ROSENMAN
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Family ID: |
22835370 |
Appl. No.: |
10/344169 |
Filed: |
July 23, 2003 |
PCT Filed: |
August 3, 2001 |
PCT NO: |
PCT/US01/41529 |
Current U.S.
Class: |
427/248.1 |
Current CPC
Class: |
H01L 39/143 20130101;
Y10T 29/49014 20150115; C23C 16/408 20130101; H01L 39/2441
20130101; C23C 16/4481 20130101 |
Class at
Publication: |
427/248.1 |
International
Class: |
C23C 016/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 7, 2000 |
US |
60223175 |
Claims
1. A process for the production of HTS conductors consisting of a
metallic substrate and a thin film coating of rare
earth-barium-copper oxides and capable of conducting currents
greater than 100 A/cm width and current densities of greater than 1
MA/cm.sup.2 at 77 K, self field comprising a] separately dissolving
a rare earth, barium and copper tetramethyl heptanedionates in a
suitable solvent; b] combining the solutions from step a]; c]
vaporizing the mixed solution of step b] by flash vaporization in a
first vessel; d} placing the substrate to be coated in the second
vessel; e] heating the substrate; f) injecting the vaporized mixed
solution through a showerhead into the second vessel where the
vaporized coating components contact and are deposited on the
substrate surface; and g] slowly cooling the coated substrate.
2. The process of claim 1 wherein the molarity of the precursor
solution in step b] is approximately 0.03 M/l.
3. The process of claim 1 wherein the ratio of the rare earth
compound is dissolved in a solvent containing 2 parts by weight
tetrahydrofuran and 1 part by weight isopropanol; the barium
compound is dissolved in a solvent containing about 3.5 parts by
weight tetrahydrofuran and 1 part by weight isopropanol and the
copper compound is dissolved in tetrahydrofuran.
4. The process of claim 1 wherein the ratio of the rare earth, Ba
and Cu compounds is in the range of form about 1:1.65:3 to about
1:2.65:3.
5. The process of claim 1 wherein the ratio of the rare earth, Ba
and Cu compounds is in the range of form about 1:2:3 to about
1:2.3:3.
6. The process of claim 1 wherein the ratio of the rare earth, Ba
and Cu compounds is about 1:2.15:3.
7. The process of claim 1 wherein the temperature in the first
vessel is in the range of from about 180.degree. C. to about
300.degree. C.
8. The process of claim 1 wherein the temperature in the first
vessel is in the range of from about 210.degree. C. to about
270.degree. C.
9. The process of claim 1 wherein the temperature in the first
vessel is in the range of from about 230.degree. C. to about
240.degree. C.
10. The process of claim 1 wherein the temperature of the
showerhead is maintained in the range of from about 210.degree. C.
to about 270.degree. C.
11. The process of claim 1 wherein the temperature of the
showerhead is maintained in the range of from about 230.degree. C.
to about 240.degree. C.
12. The process of claim 1 wherein the pressure in the second
vessel is maintained in the range of from about 1 to about 5
Torr.
13. The process of claim 1 wherein the pressure in the second
vessel is maintained in the range of from about 1.5 to about 2.5
Torr.
14. The process of claim 1 wherein the pressure in the second
vessel is maintained at about 2.0 Torr.
15. The process of claim 1 wherein the oxygen partial pressure in
the second vessel is maintained in the range of from about 0.3 to
about 1 Torr.
16. The process of claim 1 wherein the oxygen partial pressure in
the second vessel is maintained in the range of from about 0.35 to
about 0.8 Torr.
17. The process of claim 1 wherein the oxygen partial pressure in
the second vessel is maintained at about 0.55 Torr.
18. The process of claim 1 wherein the distance between the
showerhead and the substrate is in the range of from about 15 to
about 30 mm.
19. The process of claim 1 wherein the rare earth is yttrium.
20. A process for the production of HTS conductors consisting of a
metallic substrate and a thin film coating of yttrium-barium-copper
oxides and capable of conducting currents greater than 100 A/cm
width and current densities of greater than 1 MA/cm.sup.2 at 77 K,
self field comprising a] separately dissolving yttrium, barium and
copper tetramethyl heptanedionates in suitable solvents; b]
combining the solutions from step a]; c] introducing the combined
solutions to a vaporizer at a flow rate of about 0.25 ml/min; d]
vaporizing the mixed solution of step b] by flash vaporization in a
first vessel; e] placing the substrate to be coated in the second
vessel; f] heating the substrate to a temperature in the range of
from about 700.degree. C. to about 850.degree. C.; g) injecting the
vaporized mixed solution through a showerhead into the second
vessel where the vaporized coating components contact and are
deposited on the substrate surface; and h] slowly cooling the
coated substrate.
21. An HTS conductor capable of conducting currents greater than
100 A/cm width and current densities of greater than 1 MA/cm.sup.2
at 77 K, self field comprising a metallic substrate and a thin film
coating comprising tare earth-barium-copper oxides.
22. An HTS conductor produced by the process of claim 1 and capable
of conducting currents greater than 100 A/cm width and current
densities of greater than 1 MA/cm.sup.2 at 77 K, self field
comprising a metallic substrate and a thin film coating comprising
rare earth-barium-copper oxides.
23. A vaporizer suitable for use in preparing HTS conductors by
MOCVD processes comprising a cylindrical body, an inlet means for a
liquid to be vaporized mounted on one end of the vaporizer, an
annular gas inlet means mounted on one end of the vaporizer and
surrounding the liquid inlet means, and an outlet means for
transferring the vaporized liquid and gas to a deposition
reactor.
24. A deposition reactor suitable for use in preparing HTS
conductors by MOCVD processes comprising a cylindrical body, a
delivery means for delivering vaporized deposition composition to
an inlet means for a vaporized deposition composition, an inlet
means for oxygen rich gas, a heating means, a cooling means, a
substrate holding means for holding a substrate to be coated by the
vaporized deposition composition, a means for inserting and
removing the substrate to be coated, and an outlet means for waste
deposition composition wherein the inlet means for the vaporized
deposition composition is a showerhead constructed with numerous
fine openings, the total cross sectional area of which are less
than the cross sectional area of the vaporized deposition
composition delivery means, the perforations have a minimum length
of about 1 mm, the cooling means is a cooling coil welded to the
showerhead and the heating means is a heating coil surrounding the
showerhead.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to an apparatus and method
for the production of high temperature superconducting materials.
More specifically, the invention relates to the preparation of high
current and high current density HTS materials prepared by MOCVD
deposition of dissimilar precursor materials.
[0003] 2. Description of the Related Art
[0004] High temperature superconducting (HTS) materials have
immense potential for use in electric power, electronics, and
medical industries. Presently, the HTS material that is
manufactured in considerable quantities by industry is based on
(Bi,Pb)SrCaCuO (BSCCO) superconductor. This material and the
process used to manufacture it have proven to be expensive.
Furthermore, the properties of this material degrade quickly in the
presence of magnetic fields that are generated in a number of
electric power devices.
[0005] ReBaCuO (Re=rare earth) superconductor is being developed as
a potential alternative to the BSCCO superconductor. When
fabricated in the form of a thin film coated on a metal substrate
(HTS coated conductor), this material exhibits superior current
carrying capability compared to BSCCO.
[0006] Several deposition processes are being developed to
fabricate HTS Coated conductor. In general thin film deposition
techniques can be classified into two major categories: (1)
physical vapor deposition (PVD) and (2) chemical processes (see
"The Materials Science of Thin Films", Milton Ohring, Academic
Press, 1992; S. L. Swartz, IEEE Transactions on Electrical
Insulation, 25(5), 1990, 935; S. B. Krupanidhi, J. Vac. Sci.
Technol. A, 10(4), 1992, 1569).
[0007] Most of the approaches are based on physical vapor
deposition techniques where a source of HTS material is vaporized
by means of (1) ablation with a high power laser, (2) evaporation
using an electron-beam source, or (3) sputtering using high energy
argon ions. However, these techniques are limited in several ways.
First, they are all limited by line of sight i.e. the vapors can
coat the substrate only where they can `see` the substrate, which
means that the coated area is small. This limits the throughput of
coated tape. Second, the composition of the coated film is limited
to the composition of the material being vaporized. Third, the
source material has to be maintained under vacuum causing refill to
be difficult and in turns poses a problem for long-length
manufacturing. Fourth, the source material has to be formed into a
monolith, which adds cost to the process. Fifth, a high vacuum is
needed which increases cost of capital equipment.
[0008] The chemical processes can further be divided into two
subgroups i.e., chemical vapor deposition and wet chemical
processes including sol-gel and metalorganic decomposition MOD.
[0009] The sol-gel and MOD processes for deposition of thin films
are popular because of their simplicity. Additionally, they provide
molecular homogeneity, high throughput, excellent compositional
control and low capital cost since no vacuum is required.
[0010] The limitations of wet chemical processes are thickness
control, multiple steps for film formation (deposition, bakeout,
& heat treatment as a minimum), need for repeat these multiple
steps multiple times to build thick films, carbon residue
incorporation in the films, difficulties in epitaxial growth in
thick films, and evolution of harmful byproducts such as HF if
fluorinated precursors are used.
[0011] A wide variety of source materials have been employed to
form thin films, layers and coatings on substrates. These source
materials include reagents and precursor materials of widely
varying types, and in various physical states. Vapor deposition has
been used widely as a technique to achieve highly uniform thickness
layers of a conformal character on the substrate. In vapor phase
deposition, the source material may be of initially solid form that
is sublimed or melted and vaporized to provide a desirable vapor
phase source reagent. Alternatively, the reagent may be of normally
liquid state, which is vaporized, or the reagent may be in the
vapor phase in the first instance.
[0012] These reagents may be used in mixture with one another in a
multicomponent fluid, which is utilized to deposit a corresponding
multicomponent or heterogeneous film material. Such advanced thin
film materials are increasingly important in the manufacture of
high-power electrical devices, microelectronic devices and in the
emerging field of before reaching the reactor deposition chamber.
When compounds are desired to be deposited, obtaining optimal
properties requires close control of stoichiometry that can be
achieved if the reagent can be delivered into the reactor in a
controllable fashion. In this respect the reagents must not be so
chemically stable that they are non-reactive in the deposition
chamber.
[0013] Desirable CVD reagents therefore are fairly reactive and
volatile. Unfortunately, for many of the materials described above,
volatile reagents do not exist. Many potentially highly useful
refractory materials have in common that one or more of their
components are elements, i.e., the Group II metals barium, calcium,
or strontium, or the early transition metals zirconium or hafnium,
for which no or few volatile compounds well suited for CVD are
known. In many cases, the source reagents are solids whose
sublimation temperature may be very close to the decomposition
temperature, in which case the reagent may begin to decompose in
the lines before reaching the reactor, and it therefore is very
difficult to control the stoichiometry of the deposited films from
such decomposition-susceptible reagents.
[0014] When the film being deposited by CVD is a multicomponent
substance rather than a pure element, such as barium titanate, lead
zirconate titanate (PZT), lead lanthanum titanate (PLT), or the
oxide superconductors, controlling the stoichiometry of the film is
critical to obtaining the desired film properties. In the
deposition of such materials, which may form films with a wide
range of stoichiometries, the controlled delivery of known
proportions of the source reagents into the CVD reactor chamber is
essential.
[0015] In other cases, the CVD, reagents are liquids, but their
delivery into the CVD reactor in the vapor phase has proven
difficult because of problems of premature decomposition or
stoichiometry control. Examples include the deposition of tantalum
oxide from the liquid source tantalum ethoxide and the deposition
of titanium nitride from bis(dialkylamide)titanium reagents.
[0016] In the liquid delivery approach, the liquid or solid
precursor is typically dissolved in a solvent, and the solution is
stored, e.g., at ambient temperature and pressure. When the
deposition process is to be run, the solution is transported to a
high temperature vaporization zone within the CVD system, where the
precursor is flash vaporized (along with the solvent), and the
gas-phase precursor then is transported to the deposition zone,
such as a chemical vapor deposition reactor, containing a substrate
on which deposition of the desired component(s) from the
vapor-phase precursor composition takes place.
[0017] The liquid delivery technique has been found to be useful
for deposition of multicomponent oxide thin films such as
(Ba,Sr)TiO.sub.3, SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT), (Pb, La)
TiO.sub.3 (PLT) and Pb(Zr, Ti)O.sub.3 (PZT) for example. In CVD
processes developed for these and other compounds, it is desirable
to dissolve all the precursors in a single solution, and vaporize
them simultaneously, following which the vaporized precursor
composition containing the respective components is transported to
the deposition chamber, as described above.
[0018] Liquid delivery systems of varying types are known in the
art, and for example are disclosed in U.S. Pat. No. 5,204,314
issued Apr. 20, 1993 to Peter S. Kirlin et al. and U.S. Pat. No.
5,536,323 issued Jul. 16, 1996 to Peter S. Kirlin et al., which
describe heated foraminous vaporization structures such as
microporous disk elements. In use, liquid source reagent
compositions are flowed onto the foraminous vaporization structure
for flash vaporization. Vapor thereby is produced for transport to
the deposition reactor. The liquid delivery systems of these
patents provide high efficiency generation of vapor from which
films may be grown on substrates. Liquid delivery systems of such
type are usefully employed for generation of multicomponent vapors
from corresponding liquid reagent solutions containing one or more
precursors as solutes, or alternatively from liquid reagent
suspensions containing one or more precursors as suspended
species.
[0019] The simplicity of such liquid delivery approach has been
fortuitous, because each component in this system of metalorganic
precursors can be treated identically in the respective
solution-forming, vaporization and transport steps of the process.
Thus, in such a compatible system of multiple, well-behaved
precursors, (i) the precursors can be dissolved in the same solvent
with high solubility, (ii) the precursors maintain their identity
in the single solution, without deleterious chemical reactions with
the solvent or net ligand exchange with each other, (iii) the
precursors can be efficiently vaporized under the same temperature
flow, pressure and ambient (carrier) gas conditions, and (iv) the
CVD deposition process can be performed using a fixed ratio of the
CVD precursors in the solution, distinct advantage since the
relative proportions of the respective components cannot be easily
quickly changed.
[0020] CD using metalorganic precursors (MOCVD) is used for
fabrication of films of various materials including HTS. However,
MOCVD has yet to be shown to be a viable approach to achieve high
current and high current density with HTS Coated conductors because
suitable MOCVD apparatus and process has not been developed.
[0021] Therefore, an object of this invention is to provide an
MOCVD Process and MOCVD System to produce HTS Coated Conductors
suitable for use in high current and high current density
environments.
[0022] Other objects and advantages of the invention will be more
fully apparent from the ensuing disclosure and appended claims.
SUMMARY OF THE INVENTION
[0023] The present invention relates to the use of a metal organic
chemical vapor deposition (MOCVD) process for fabrication of coated
HTS conductors. In a MOCVD process, chemical precursors of the
constituent elements are vaporized at a low temperature and the
vapors are deposited on a heated substrate to form the HTS film.
The equipment can be designed for the vapors to flow over a large
area i.e. a long-length of tape can be coated instantaneously or a
number of tapes can be coated in parallel, both of which result in
high throughput. The precursors of individual elements of the HTS
material can be mixed in an infinite number of combinations.
Therefore, the composition of the HTS film can be tailored to that
preferred to achieve specific performance parameters. The
precursors are maintained outside the deposition chamber under
ambient conditions. Therefore, refill of precursors is very simple
which is important for long length manufacturing. The precursors
need not be formed into any shape, which does not add extra cost.
MOCVD is performed at a pressure much higher than that used for PVD
techniques and so, cost of capital equipment is relatively low.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic drawing of the vaporizer showing the
orifice and heating elements.
[0025] FIG. 2 is a schematic drawing of the deposition reactor
showing the showerhead, the oxygen injector and the heating and
cooling coils.
[0026] FIG. 3 is a chart showing the current-voltage curve from a
high-current YBC on metal substrate prepared utilizing the
inventive process.
[0027] FIG. 4 is a chart showing the performance of present
generation HTS materials and of YBC and of on a metal substrate
prepared utilizing the inventive process, in magnetic fields of
interest for electric-power applications.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The selection of the precursors is the most critical step
for successful deposition of complex oxide films. The ideal
precursors for MOCVD have to meet the requirements of high vapor
pressure at low vaporization temperatures, low decomposition
temperatures, large "window" between vaporization and decomposition
temperatures, no contamination from organic constituents of the
precursors, stability under ambient conditions and nontoxicity. In
the present invention, tetramethyl heptanedionates have been used
as precursors to grow the complex layered structure films.
[0029] A liquid precursor is preferred to achieve high performance
in HTS coated conductors fabricated by MOCVD. Advantages of liquid
precursors over solid precursors include a single point of
temperature control for vaporization, longer period of stability
and easy refill, both of which are critical for long-length tape
manufacturing, and higher precursor feed rates, which is important
for high rates of deposition. Precursors that are especially useful
in the innovative process are tetramethyl heptanedionate compounds
of yttrium (or other rare earth such as Sm, Nd, Yb, Eu, Gd, Dy, Ho,
Er), barium and copper. The Ba compound is preferably adducted with
a compound such as phenanthroline to assure long-term stability.
Each compound is individually dissolved in solvents such as
tetrahydrofuran and isopropanol. Both THF and isopropanol are
required to dissolve the Y and Ba compounds whereas THF alone is
sufficient to dissolve the Cu compound. When the precursors have
dissolved, the individual solutions are mixed together.
[0030] Tetramethyl heptanedionates of yttrium, barium [adducted
with phenanthroline] and copper are obtained from InOrgatech,
Mildenhall, Suffolk, U.K. The Y compound is dissolved in solvent
containing 2 parts of THF and 1 part isopropanol. The Ba compound
is dissolved is a solvent containing 3.5 parts THF and 1 part
isopropanol. The Cu compound is dissolved only in THF.
[0031] When the individual precursors are dissolved they are mixed
together in a ratio determined by the desired composition of the
thin film. It is one of the particular benefits of the process that
the film characteristics can be varied over a wide range by
modifying the ratio of the precursors in the combination. The
molarity of overall solution is in the range of from about 0.003
M/l to about 0.03 M/l.
[0032] The ratio of the rare earth, Ba and Cu compounds, as
determined by the elemental ratio in the precursor solution should
be between 1 to 1.65-2.65 to 3, preferably 1 to 2-2.3 to 3, and
most preferably 1 to 2.15 to 3. The three solutions are combined by
low shear mixing to form a uniform solution.
[0033] The liquid precursor solution is then pumped at a constant
rate of from about 0.1 to about 10 ml/min into the vaporizer using
a low-flow rate, high-pressure pump such as a HPLC (High Pressure
Liquid Chromatography) pump. The pump should be capable of delivery
low flow rates with a high accuracy and without pulsation. The
wetted components of the pump should not react with the precursor
solution. The rate of introduction will of course depend on the
size of the vaporizer and deposition reactor. We have found that a
rate of approximately 0.25 ml/min is suitable for deposition rate
of about 2 microns/hour.
[0034] An important component of the MOCVD system is the vaporizer.
The vaporizer used to flash evaporate a liquid precursor has to be
maintained at a steady temperature. More importantly, the vaporizer
should be maintained clog-free. Several designs of vaporizer
including commercially available vaporizers such as MKS Model
#DLI25BS99 have failed for use with the liquid precursors of the
invention because of vaporizer clogging by the Ba precursor. It is
also important that the vaporizer be designed to avoid streaming of
the precursor solution, which is caused by suction of the solution
by the vacuum in the system. Further, the vaporizer and has to be
designed to avoid deposits of the precursor.
[0035] Referring to FIG. 1, the vaporizer 10 is a cylindrical
vessel constructed of non-reactive materials. Mounted at one end of
the cylinder is an inlet means for the introduction of precursor
solution to the vaporizer. The precursor is preferably delivered to
the vaporizer through small bore tubing 20. The tubing is
maintained wet using solvent when the vaporizer is not in operation
to avoid clogging. Heating means 30 are placed around the exterior
of the vaporizer body to maintain a high and even temperature
distribution.
[0036] The precursor entry tubing ends in an orifice tip 40 with a
diameter in the range of from about 10% to about 50% of the
diameter of the delivery tubing. This narrow opening builds
pressure and prevents suction of the precursor by the vacuum in the
vaporizer. A typical orifice opening is from about 0.001 to about
0.005 inch inside diameter.
[0037] The delivery tubing is surrounded by an annular carrier gas
delivery means 50, typically in the form of annular tubing
surrounding the precursor delivery means 20. The carrier gas,
typically argon, enters the vaporizer through delivery means 50 and
is used to push the precursor exiting from nozzle 40 downward away
from the orifice and into the body of the vaporizer 10.
[0038] The region of the vaporizer above the orifice is minimized
to prevent precursor deposits.
[0039] The solution of mixed precursors is rapidly heated in the
vaporizer to a temperature in the range of from about 180 to about
300.degree. C., preferably from about 210 to about 270.degree. C.,
and most preferably from about 230 to about 240.degree. C. and is
pumped to a vaporizer. It enters the vaporizer through tubing
swaged to a nozzle of about 0.004" ID orifice where the solution is
flash vaporized. The pressure in the feed tubing is in the range of
from about 0 to about 15 psi, preferably from about 0 to about 10,
and most preferably from about 0 to about 5 psi. The pressure in
the vaporizer is in the range of from about 1 to about 15 Torr,
preferably from about 1 to about 10 Torr, and most preferably from
about 2 to about 5 Torr.
[0040] A tubing of very small diameter, shaped into a small orifice
at the tip to build enough pressure to avoid sucking of precursor
by reactor vacuum is preferred (too small a diameter will end up in
clogging, too large a diameter will prevent pressure buildup). The
tip of the tubing has to be placed in a high temperature zone in
the vaporizer. If it is at a low temperature, the precursors will
not vaporize instantaneously. The region above the tip of the
delivery tubing should be minimized and should be maintained at a
high temperature, both for avoiding formation of precursor
deposits. The outlet for the argon `push gas` should be placed
around the orifice to push the vaporized precursors downward. The
tubing should be cleaned with solvent such as THF at high pressure
after the deposition run to avoid clogging. The tubing should be
maintained wet prior to ramping the vaporizer to set temperature.
The tip of the tubing should be maintained approximately at
230.degree. to 240.degree. C. The cross-section of the delivery
tube should not be decreased beyond the vaporizer so as to avoid
deposits of the precursor.
[0041] In a preferred embodiment, the orifice is sized to inject
approximately 0.25 ml/min of precursor into the vaporizer. In
another preferred embodiment an annular inert gas injector used to
force the precursor vapors downward away from the orifice. In
general, it is useful to supply the inert gas to the vaporizer at a
rate of about 500 to 4000 ml/min. Argon is the preferred inert gas
but nitrogen or other inert gas may be used.
[0042] The vaporizer is connected to the deposition reactor through
small-bore tubing. The minimum diameter of the tubing is 0.25"; the
maximum is 5". Preferably, the diameter of the tubing is in the
range of from about 1 to about 1.5".
[0043] Referring to FIG. 2, vaporized precursors are received from
vaporizer 10 [not shown] through delivery tubing 60 along with
introduced oxygen. The vaporized precursors and oxygen are
dispersed into the deposition reactor [not shown] by means of
showerhead 70. The vaporized precursors are maintained at an
appropriate temperature prior to entry into the showerhead 70 by
heating means 80 placed around the delivery tubing. The temperature
of the vaporized precursors is maintained at an appropriate
temperature as it passes through the showerhead 70 by heating means
90 and cooling means 80.
[0044] A high temperature valve with a direct line of sight and
uniform bore is used between vaporizer and reactor. The path of the
precursor vapor should not be obstructed prior to injection into
the reactor showerhead. Oxygen must be introduced close to the
precursor injection point and is preferably uniformly directed over
the entire showerhead. A showerhead in the form of a disc with
small perforated holes is a suitable way to achieve such an uniform
flow. If it is introduced non-uniformly, the film growth will be
non-uniform when depositing on large areas. The entire delivery
line from the vaporizer to showerhead should be maintained between
230.degree. and 270.degree. C.
[0045] The precursor vapors are uniformly injected over the entire
substrate area using a showerhead. The showerhead is preferably
constructed with numerous fine openings; the total cross sectional
area of the openings should be less than the cross sectional area
of the precursor delivery tubing. The perforations should have a
certain length to them so as to direct the flow downward. The
showerhead should be maintained in a temperature range of
230.degree. to 270.degree. C. prior to film deposition. A shutter
should be used between the showerhead and the substrate heater
prior to film deposition so as to prevent any film formation at a
lower temperature while the substrate heater is ramped up to
deposition temperature. The showerhead may not reach the preferred
temperature range mentioned above with the shutter in place. A
suitable way to achieve the desired showerhead temperature is to
install a heater around the showerhead itself. However, after the
shutter is opened the showerhead may overheat due to exposure to
the heat from the substrate heater. To avoid this problem, it is
desirable to install a cooling coil around the showerhead. To
facilitate cooling, the showerhead should be constructed of a
single block of metal the cooling coil welded to it and the heating
coil placed around the cooling coil.
[0046] The vaporized precursors are delivered from the vaporizer to
the deposition chamber through a showerhead. The showerhead is
preferably constructed with numerous fine openings, the total cross
sectional area of which should be less than the cross sectional
area of the precursor delivery tubing. The perforations have a
certain length to them so as to direct the flow downward. The
length should be sufficient to direct the flow onto the substrate
but not so long as to prevent adequate spread. Lengths of from
about 1 to about 10 mm have been found useful. A cooling coil
welded to the showerhead and a heating coil surrounding the
showerhead to maintain constant temperature (230.degree. C. to
270.degree. C.) at all times. Prior to film deposition, when the
shutter is closed, the heating coil heats up the showerhead.
[0047] During deposition, when the shutter is open and the
showerhead is exposed to the substrate heater, the cooling coil
cools the showerhead.
[0048] To facilitate cooling, the showerhead should be constructed
of a single block of metal and the cooling coil be welded to it and
the heating coil placed around the cooling coil.
[0049] A high temperature valve with a direct line of sight and
uniform bore should be used between vaporizer and reactor. The path
of the precursor vapor should not be obstructed prior to injecting
to the showerhead.
[0050] Oxygen must be introduced to the reactor along with the
precursors, preferably uniformly directed over the entire
showerhead. A ring with small perforated holes is a suitable way to
achieve such an uniform flow. If introduced nonuniformly, the film
growth will be nonuniform when depositing on large areas. The
entire delivery line from the vaporizer to showerhead should be
maintained between 230.degree. C. and 270.degree. C.
[0051] The substrates to be coated are heated to about 700.degree.
to 850.degree. C., preferably about 750.degree. to 800.degree. C.
Lower reactor pressure in the lower portion of the acceptable
range, oxygen partial pressure in the lower portion of the
acceptable range and substrate temperature in the higher portion of
the acceptable range are found to favor formation of high quality
HTS films on commercial substrates.
[0052] The substrate may be any metallic substrate conventionally
used as a substrate for HTS films. Stainless steel or nickel alloy
are particularly well suited. The substrate heater should be placed
at a distance of about 15 to 30 mm from the showerhead. If it is
placed too close, the showerhead will overheat due to the heat from
the heater. Also, the precursor may decompose and result in
deposition of particles on the substrate. If it is kept too far,
the thickness of the film will be reduced and the precursors may
condense into their individual constituents.
[0053] The MOCVD reactor is preferably of cold-wall type i.e. the
walls of the reactor are not heated. The pressures in the reactor
and the by-pass line are maintained constant using throttle valves
that are automatically controlled by pressure readings using
capacitance manometers.
[0054] At reactor start-up, and until the reactor reaches optimum
operating conditions, the vaporized mixture of precursors is
shunted to a by-pass line maintained at the same pressure as that
in the MOCVD reactor. After the substrates are heated to deposition
temperature, the vapor flow is switched to the reactor input line.
This procedure is accomplished using two high temperature
valves.
[0055] A pressure of 1 to 5 Torr, preferably 1 to 2.5 Torr, most
preferably 1.6 Torr is maintained in the reactor (as well as in the
by-pass line). The low reactor pressure in the presence of high gas
flow rates is preferably achieved using a roots blower-vacuum pump
system.
[0056] Oxygen is flowed into the reactor to achieve a partial
pressure of about 0.3 to 1 Torr, preferably 0.35 to 0.8 Torr, most
preferably about 0.55 Torr.
[0057] After deposition, the films are cooled at a controlled rate
in a high partial pressure of oxygen (100 to 760 Torr) to room
temperature. In general, it will require from about 30 to about 60
minutes to complete the cooling.
[0058] Using the process and system design described above,
critical currents of almost 100 A (130 A/cm width) and current
density of 1.3 MA/cm.sup.2 have been achieved for the first time in
HTS coated conductor. FIG. 3 demonstrates the high performance
results achieved in a YBC coated conductor fabricated by the
claimed process.
[0059] FIG. 4 demonstrates that high performance can be achieved in
HTS YBC coated conductor fabricated utilizing the inventive
process, in high magnetic fields at 75 K and 64 K. The performance
of the coated conductor far exceeds that of present generation
BSCCO-based HTS tape technology and meets the requirements of
various electric power devices.
[0060] Having described the invention, the following example is
provided to illustrate a specific application of the invention
including the best mode now known to perform the invention. This
specific example is not intended to limit the scope of the
invention described in this application.
EXAMPLE
[0061] Tetramethyl heptanedionates of yttrium, barium [adducted
with phenanthroline] and copper are obtained from Inorgatech,
Mildenhall, Suffolk, U.K. The Y compound is dissolved in solvent
containing 2 parts of THF and 1 part isopropanol. The Ba compound
is dissolved is a solvent containing 3.5 parts THF and 1 part
isopropanol. The Cu compound is dissolved only in THF. The molarity
of overall solution is 0.03 M/l.
[0062] The liquid components are combined at room temperature in an
ultrasound mixer in the ratio of 1 part Y composition, 2.15 parts
Ba composition and 3 parts Cu composition. The mixed liquid
precursor is pumped at a rate of 0.25 ml/min through a HPLC pump.
From the pump, the precursor is pumped through 0.0625" OD, 0.010"
ID stainless steel tubing at a pressure of 5 psi to the
vaporizer.
[0063] The mixed component precursor enters the vaporizer through
the nozzle that is made from a 0.0625" OD, 0.01" ID tubing swaged
to an ID of 0.004" at tip. The tip is placed in a vaporizer that is
maintained at 240.degree. C. using a band heater and heating
jackets. The orifice is surrounded by an annular argon gas
injector. The argon is supplied to the vaporizer at the rate of
1000 ml/min.
[0064] The vaporized precursors are transferred to the reactor
through 1.5"-diameter stainless steel tubing. The composition is
maintained at a temperature of 240.degree. C. while in the delivery
tubing.
[0065] The reactor, a cold wall type, is maintained at a pressure
of 1.6 Torr. The composition is introduced into the reactor through
a showerhead. Heating and cooling coils to maintain the temperature
in the range of about 235.degree. C. surround the showerhead. In
this example the showerhead is comprised of two 2.75" con-flat
flanges bolted together. The top flange is connected to the tubing
from the vaporizer. The bottom flange has perforations disposed
uniformly on the surface. The plate has a thickness of about 6 mm,
and the perforations are about 0.02 to 0.03" in diameter. The
length of the perforation is about 6 mm, sufficient to direct the
flow downward. The showerhead can deliver precursors over a
diameter of 2.5 cm over the substrate.
[0066] Oxygen is introduced into the showerhead at a flow rate of
500 ccm.
[0067] The substrate, comprising of an Inconel 625 metal tape, is
introduced into the reactor and heated, utilizing a substrate
heater, to a temperature of 775.degree. C.
[0068] The vaporized precursor entering the reactor through the
showerhead contacts the substrate and is deposited as a thin
film.
[0069] The resulting coated substrate is cooled to ambient
temperature in an oxygen rich atmosphere over a period of 1
hour.
[0070] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that, within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described.
* * * * *