Method for the rapid deposition with low vapor pressure reactants by chemical vapor deposition

A method for applying coatings to substrates using chemical vapor deposition with low vapor pressure reagents is disclosed which comprises the steps of: (a) placing a substrate in a furnace means; (b) directly introducing powder reagents by a powder feeder means into said furnace means; and (c) vaporizing and reacting said reagents within said furnace means resulting in the deposition from the vapor phase of a coating on said substrate, wherein said coating can be an oxide superconductor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the rapid deposition of coatings on substrates by 
chemical vapor deposition using low vapor pressure reactants and, more 
specifically, to the use of powder feeding of the low vapor pressure 
reactants into the chemical vapor deposition furnace. 
2. Prior Art 
Current techniques in chemical vapor deposition ("CVD") which use low vapor 
pressure reagents introduce the reactant material too slowly into the 
reactor, which reduces the coating deposition rate. Common solutions to 
this problem of reduced coating deposition rate include: running the 
vaporizer or reactor at reduced pressures, which increases the transport 
rate of the reagent into the reactor; running the reactor with a high 
carrier gas flow rate, which tends to dilute the reactant gas stream since 
saturation may not be achieved; or heating the solid or liquid reactants 
to a high temperature, which increases the vapor pressure above the 
reactant. 
Current coating techniques usually do not include the powder feeding or 
liquid feeding of low vapor pressure reactants to increase the deposition 
rate. One introduction technique consists of a process where reagents 
which are dissolved to form a liquid solution are then sprayed into a 
furnace to form a powder of superconducting oxides. One coating technique 
consists of a process where substrates are coated with superconducting 
oxides by spraying the substrates with or dipping the substrates in liquid 
reactants. These, however, are not CVD processes and do not result in an 
atom-by-atom build-up of the coating. These coatings, therefore, do not 
possess the high degree of crystalline perfection found in CVD coatings. 
Previous CVD techniques have used an auger to feed ZrCl.sub.4 powder into a 
CVD furnace as an alternative to the use of a vaporizer. See Hollabough, 
C. M., L. A. Haham, R. D. Reiswig, R. W. White, and P. Wagner, Chemical 
Vapor Deposition Of ZrC Made By Reactions Of ZrCl.sub.4 With CH.sub.4 And 
With C.sub.3 H.sub.6, 35, (9), NUCL. TECH., 527-35 (1977). This approach 
used powder feeding of ZrCl.sub.4 as a reagent to CVD furnaces instead of 
using a vaporizer. However, the powder feeding method was used in place of 
a vaporizer because, depending on the CVD furnace design, it is sometimes 
difficult to keep the portion of the reagent gas line connecting a 
vaporizer with a CVD furnace at a sufficiently high temperature. This 
approach was used not because the vapor pressure of the reactant, 
ZrCl.sub.4, is low or because heating of ZrCl.sub.4 renders it unstable, 
but because a relatively high temperature is required to develop the 
desired vapor pressure and maintaining the high temperature throughout the 
gas lines going to the furnace often is difficult. 
Newkirt et al. U.S. Pat. No. 4,202,931 dicloses the use of a powder feeder 
to prepare a Nb.sub.3 Ge low temperature superconductor. However, use of a 
powder feeder was known prior to this patent, as disclosed in Newkirk et 
al. U.S. Pat. No. 4,054,686. See also the paper by Hollabough et al. 
referenced above. Newkirk et al. U.S. Pat. No. 4,202,931, disclosed the 
use of a powder feeder to improve the control or accuracy of NbCl.sub.5 
dispensing. 
None of the prior art techniques have been applied to the powder feeding of 
the new generation of high temperature superconducting oxides, into CVD 
furnaces, nor have they been designed to achieve the high deposition rates 
desired for superconductors and other materials. The prior art techniques 
are suitable for the deposition of Nb and Zr compounds, as high deposition 
rates for these compounds can be achieved without powder feeding, for 
example, by vaporization of NbCl.sub.5 and ZrCl.sub.4. Vaporization cannot 
yield the high deposition rates for oxide superconductors, because oxide 
superconductor reagents have very low vapor pressures and cannot be 
practically (i.e., rapidly) introduced into a CVD furnace using 
vaporization techniques. 
The vapor pressure data of FIG. 7 permits clarification of this point. The 
graph is a compilation of reagent vapor pressures obtained from the 
literature. Curves 1-8 are for yttrium, barium, and copper reagents used 
for the CVD of YBa.sub.2 Cu.sub.3 O.sub.x which are considered low vapor 
pressure reagents. Curves 9-14 are for other reagents which commonly are 
used for CVD of other materials which are considered high vapor pressure 
reagents. Two points are of interest; first, the vapor pressures of the 
yttrium, barium, and copper reagents are very low compared to the typical 
CVD reagents; second, the large discrepancy in vapor pressure of yttrium 
tetramethylhepatanedionate from the four literature sources (curves 2, 5, 
7, and 8) is indicative of the difficulty in developing a controllable, 
repeatable CVD process which relies on sublimation of the beta-diketonates 
in vaporizers. Our experience, and that of others, has been that the vapor 
pressure, and therefore flowrate of reagent from a vaporizer, varies with 
time and from lot-to-lot of reagent. The strong variation of vapor 
pressure with temperature, even for a given source, is evident from FIG. 
7. This necessitates very close control of vaporizer temperature if a 
controllable flowrate of reagent is to be achieved. Three such vaporizers 
are involved in the conventional CVD of YBa.sub.2 Cu.sub.3 O.sub.x. 
Curves 10 and 11 in FIG. 7 are for ZrCl.sub.4 and NbCl.sub.5. These 
reagents have sufficiently high vapor pressures, e.g. 100 torr, within the 
temperature range over which they are stable and thus can be supplied by 
use of either vaporizers or by powder feeding. However, FIG. 7 shows that 
the vapor pressures of the yttrium, barium, and copper reagents (Curves 
1-8) do not exceed about 0.1 torr. If these latter reagents are heated to 
higher temperatures than those shown in FIG. 7 in an attempt to obtain 
higher vapor pressures, these reagents decompose rapidly to form 
non-volatile compounds. 
Thus, when the reagents ZrCl.sub.4 and NbCl.sub.5 were introduced via 
powder feeding in the prior art, it was merely at the discretion of the 
investigator as a matter of convenience and as an attempt to improve the 
accuracy of reagent flowrate, rather than for the purpose of increasing 
deposition rate. As ZrCl.sub.4 is not a superconductor reagent, it does 
not have a relevant T.sub.c, and the T.sub.c of Nb.sub.3 Ge is about 20K, 
which is much lower than the T.sub.c of about 84K of the superconductor 
materials used in this invention. However, in the case of the very low 
vapor pressure reagents, the use of powder feeding is mandatory in order 
to achieve high deposition rates, and also maintains the control over 
reagent flowrate (i.e. process repeatability) much more so than in the 
case of high vapor pressure reagents such as ZrCl.sub.4 and NbCl.sub.5 
where vapor pressures can be controlled easily and are highly 
reproducible. 
Chemical vapor deposition is used extensively for the commercial 
preparation of numerous electronic, optical, tribological, and chemically 
protective coatings. Compared to other coating processes, CVD is often 
faster, yields higher quality, more adherent films, and can be used to 
coat multiple, irregularly shaped substrates. Films of the high T.sub.c 
oxide superconductor YBaYBa.sub.2 Cu.sub.3 O.sub.x have been prepared by 
CVD. Typical reagents used in CVD processes for the oxide superconductors 
have consisted of metal complexes of various beta-diketonate ligands, 
which are solids at room temperature and which slowly sublime when heated 
to about 100.degree.-300.degree. C.; the vapor is then swept into the CVD 
system by a carrier gas. Without exception, however, the very low vapor 
pressure of the Y, Ba, and Cu precursor reagents has restricted deposition 
rates to low values, e.g. 1 micron per hour. Furthermore, the vapor 
pressures of the reagents are strongly dependent on temperature and are 
subject to change as a result of thermal-environmental induced degradation 
during sublimation. Hence, process control and repeatability are unusually 
poor. 
BRIEF SUMMARY OF THE INVENTION 
In this invention, a mixture of powdered reagents is fed directly into the 
coating furnace. This results in a faster, more controllable and 
potentially more economical CVD process. The higher deposition rate also 
permits the coating of substrates, e.g. metals, which would be damaged by 
prolonged exposure to a high temperature oxygen environment. This 
invention yields YBa.sub.2 Cu.sub.3 O.sub.x coatings on MgO crystals which 
are superconducting at 80-84K with preferred orientation of the crystals 
of the 123 phase. Superconducting wire also has been prepared by 
deposition of YBa.sub.2 Cu.sub.3 O.sub.x onto alumina (Al.sub.2 O.sub.3) 
fibers. 
This invention relates to a method and apparatus for introducing low vapor 
pressure solid or liquid reactants into a CVD furnace. CVD currently is 
being used to deposit a variety of coatings, including the new 
superconducting material such as, YBa.sub.2 Cu.sub.3 O.sub.x, bismith 
compounds such as Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, and 
thalium compounds such as Tl.sub.2 (Ca,Ba).sub.2 CuO.sub.6, onto 
appropriate substrates. In conventional CVD processing, gases are 
typically used to transport the reagents into the furnace, where the 
reagents combine to form the desired coating. When liquid and solid 
reactants are used in conventional CVD processing, a carrier gas is 
bubbled or blown over the liquid or solid reactants to transport reagent 
vapor into the furnace. The materials then react in the furnace to deposit 
the coating. 
The method and apparatus of the present invention is the first use of 
powder feeding of yttrium, barium or copper reagents to deposit the oxide 
superconductors via CVD. This invention also is useful for depositing 
nonsuperconductors such as yttria (Y.sub.2 O.sub.3), magnesia (MgO), 
barium titanate (BaTiO.sub.3) and rare earth oxides, carbides, nitrides or 
borides using low vapor pressure reagents. The method and apparatus of the 
present invention achieves a faster, more reliable processing technique 
that successfully deposits coating materials, such as the superconductor 
YBa.sub.2 Cu.sub.3 O.sub.x, on substrates. Additionally, the use of the 
powder feeding approach specifically for the purpose of avoiding problems 
such as low deposition rate and the poor repeatability associated with the 
use of very low vapor pressure CVD reagents has not been used for 
deposition of oxide superconductors prior to the development of the method 
and apparatus of this invention. 
A related but alternative method of feeding reagents to the CVD furnace 
involves the atomization of liquids containing dissolved reagents. The 
atomization process is similar to powder feeding in that it allows the 
introduction of low vapor pressure reagents, such as those used for 
depositing the oxide superconductors, into CVD furnaces. Atomization has 
many of the same advantages of powder feeding such as application to low 
volatility reagents, high deposition rates, high controllability and high 
homogeneity. Additionally, the use of the atomization process for the 
purpose of avoiding problems associated with very low vapor pressure CVD 
reagents has not been used for the deposition of oxide superconductors 
prior to this invention. The process of the present invention should be 
amenable to the coating of the inside surface of cavities for microwave 
applications or for the deposition of various materials for 
microelectronic applications. 
Accordingly, it is an object of the present invention to provide a method 
and apparatus for the rapid deposition of coatings by chemical vapor 
deposition using low vapor pressure or thermally unstable reactants. 
It is also an object of the present invention to provide a rapid coating 
method and apparatus which has an increased deposition rate for oxide 
superconductors or other materials where one or more of the constituents 
has only low vapor pressure reagents or thermally unstable reagents. 
It is a further object of the present invention to provide a rapid coating 
method and apparatus which has an increased deposition rate for oxide 
superconductors or other materials where use of low vapor pressure 
reagents is preferred or necessary if a high vapor pressure reagent is 
available but its cost is prohibitively high. 
It is another object of the present invention to provide a rapid coating 
method and apparatus which has improved reproducibility. 
It is a further object of the present invention to provide a rapid coating 
method and apparatus which requires simpler equipment needs and less 
operator skills. 
It is yet another object of the present invention to produce a rapid 
coating method and apparatus that utilizes low cost reagents. 
It is still another object of the present invention to provide a rapid 
coating method and apparatus which is simpler, more economical, and more 
reliable than previous methods. 
It is another object of the present invention to produce coated wires, 
tapes, cavities and electronic devices using the rapid coating method and 
apparatus of the present invention. 
These and other objects will become apparent to those skilled in the art 
when taken in conjunction with the following drawings and the detailed 
description of the method and apparatus contemplated by the present 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
1. Theoretical 
The apparatus and method of the present invention is unique in several 
respects. First, the reagents are fed into the reactor using a powder 
feeder or atomization means, which results in a more reliable, repeatable 
deposition of the coating material onto the substrate. Second, the powder 
feeding approach or atomization of liquids containing dissolved reagents 
also is used for the purpose of avoiding problems associated with the use 
of very low vapor pressure CVD reagents, such as low deposition rate and 
poor repeatability. Third, the reagents being used to deposit the coating 
materials, such as the oxide superconductors including YBa.sub.2 Cu.sub.3 
O.sub.x and others that are unstable at temperatures required for their 
vaporization. 
Suitable substrates include conventional substrates used in superconductor 
applications, such as single crystal magnesium oxide, single crystal 
aluminum oxide, single crystal strontium titanate, single crystal and 
polycrystalline zirconia (ZrO.sub.2), planar polycrystalline aluminum 
oxide, and aluminum oxide or other fibers. Additionally, ribbons or tapes 
of Al.sub.2 O.sub.3, or other appropriate ceramics or metals, are also 
suitable substrates. Suitable reagents include the family of 
beta-diketonate complexes. Commonly used beta-diketonate reagents for 
superconductors include Y(tmhd).sub.3, Ba(tmhd).sub.2 and Cu(tmhd).sub.2 
where tmhd denotes 2,2,6,6-tetramethyl-3,5-heptanedionate. Other reagents 
such as complexes of fluorinated beta-diketonates, acetylacetonate, 
acetates, and halides can be used. 
Materials other than YBa.sub.2 Cu.sub.3 O.sub.x which might be coated by 
the method include other oxide superconductors, such as, Bi.sub.2 Sr.sub.2 
Ca.sub.2 Cu.sub.3 O.sub.10 and Tl.sub.2 (Ca,Ba).sub.2 CuO.sub.6, where one 
or more of the cations is supplied by the new approach of powder feeding 
or atomization utilized in this invention. Additionally, 
nonsuperconductors such as yttria (Y.sub.2 O.sub.3), magnesia (MgO), 
barium titanate (BaTiO.sub.3), ferrites and rare earth oxides, carbides, 
nitrides or borides can be deposited using the powder feeding or 
atomization process with low vapor pressure reagents. 
As mentioned earlier in the Background of the Invention discussing Prior 
Art, the vapor pressure data graphed in FIG. 7 is a compilation of reagent 
vapor pressures obtained from the literature. Curves 1-8 are for yttrium, 
barium and copper reagents, which are considered low vapor pressure 
reagents, while curves 9-14 are for other reagents, which are considered 
high vapor pressure reagents. There is a distinct break in the graph 
occurring at a vapor pressure of about 0.2 mm Hg, with curves 1-8 having a 
vapor pressure of less than about 0.1 torr and curves 9-14 having a vapor 
pressure of more than about 0.1 torr. It is evident from FIG. 7 that those 
reagents having a vapor pressure of less than about 0.1 torr are those 
reagents commonly known to those skilled in the art as "low vapor 
pressure" reagents. Therefore, the term "low vapor pressure" when applied 
to reagents used in CVD generally applies to reagents having a vapor 
pressure at less than about 0.1 torr. Two other points about this process 
deserve mentioning at this juncture. First, as mentioned previously, 
although the low vapor pressure reagents are relatively unstable at 
certain conditions, the instant process allows for the deposition of these 
reagents despite this instability. Second, no heating supply is needed for 
the reagent supply system, allowing the reagent supply system to operate 
at ambient temperature, thus eliminating as unnecessary heating sources, 
lines and multiple vaporizers. Elimination of vaporizers avoids the prior 
art problem of reagent decomposition, that is, aging. 
The method and apparatus of the present invention is a unique modification 
of typical CVD processes which utilize low vapor pressure and/or unstable 
reagents. The method and apparatus of the present invention provide for a 
more reliable transport of the reagents into the CVD furnace for reaction 
and a significantly increased rate at which the reagent can be supplied 
and, therefore, an increased rate of coating deposition on the substrate. 
The shorter processing times and higher efficiency and uniformity achieved 
enhance practical coating technology, specifically in the area of 
superconductivity, and increase the economic feasibility of various 
coating processes such as the deposition of Y.sub.2 O.sub.3, MgO, 
BaTiO.sub.3, SrTiO.sub.3, LaAlO.sub.3, and LaGaO.sub.3, among others. 
The method and apparatus of the present invention allow a significant 
reduction in the quantity of reagent needed to deposit a given amount of 
coating since material remaining in the vaporizer is often damaged when 
the vaporizer approach of the prior art is used. The quantity of reagent 
is a very significant factor considering the cost of the reagents required 
to deposit the new ceramic superconductors. Furthermore, the method and 
apparatus of the present invention permit the use of cheaper reagents, 
such as acetylacetonates, acetates, oxylates, or metal halides, which are 
less expensive compounds. 
2. Apparatus 
In the deposition of several superconducting compounds as coatings onto 
substrates, many of the reactants have extremely low vapor pressures. Even 
by combining high temperature, reduced pressure, and high carrier gas flow 
rate, the deposition rates are extremely low, on the order of about one 
micron per hour. Furthermore, conventional low vapor pressure deposition 
processes are extremely nonreproducible because of variation of the vapor 
pressure with time due to the thermal/environmental degradation, or 
because of variations in the extent of saturation in the vaporizer. The 
use of a powder feeder or atomization of liquid to introduce the reactants 
into the CVD furnace provides a faster, more reliable processing technique 
to deposit the superconducting material onto a substrate. Deposition rates 
of 1 micron per minute have been routinely achieved; in some cases, 
deposition rates as high as 4 microns per minute have been achieved. 
Two schematics of the apparatus are shown in FIGS. 1 and 2. FIG. 1 is a 
schematic for a horizontal furnace arrangement, which is useful for the 
preparation of normal substrates. FIG. 2 is a schematic for a vertical 
furnace arrangement, which is useful for the preparation of ceramic wire 
or tape. These two modes are the most useful in carrying out the process 
contemplated although each arrangement can be used for each of the above 
preparations as well as other preparations. FIG. 3 is a cutaway view of 
the vibratory powder feeder 12 used in the apparatus. 
For example purposes only, the invention will be described below using the 
horizontal furnace arrangement of FIG. 1 to achieve the deposition of 
YBa.sub.2 Cu.sub.3 O.sub.x onto several substrates. The example reagents 
introduced to the CVD furnace via powder feeding to form the YBa.sub.2 
Cu.sub.3 O.sub.x coating are Y(tmhd).sub.3, Ba(tmhd).sub.2 and 
Cu(tmhd).sub.2. 
The deposition of YBa.sub.2 Cu.sub.3 O.sub.x thin films is accomplished in 
a low pressure, hot walled CVD reactor as shown in FIG. 1. The reagents 
are the beta-diketonate complexes Y(tmhd).sub.3, Ba(tmhd).sub.2 and 
Cu(tmhd).sub.2. The yttrium and copper complexes are prepared by minor 
modifications of conventional procedures and are recrystallized from 
hexane before use. The barium complex is prepared by reaction of 
2,2,6,6,-tetramethyl-3,5-heptanedione with barium metal at 100.degree. C., 
followed by dissolution in toluene and precipitation with acetonitrile at 
room temperature. The solid reagents are premixed and ground in air prior 
to deposition using a Spex model 8000 Mixer/Mill; the particles were 
subsequently screened below 44 microns. 
A modified vibratory feeder such as a Syntron EB-051 is used to feed the 
powder reagent mixture slowly into the CVD furnace. The modifications to 
the powder feeder allow for a more uniform powder feed rate over the 
duration of the coating run. The powder is pneumatically transported into 
the furnace using argon (99.999% purity) where it vaporizes and reacts. 
This results in deposition of the mixed metal oxide onto the substrate. 
Oxygen (99.998%), which was pretreated with Ascarite and Drierite to 
reduce CO.sub.2 and H.sub.2 O impurities, is added to the inlet stream 
near the furnace. The flow rates for argon and oxygen are controlled using 
MKS mass flow controllers. The temperature is monitored using an Inconel 
sheathed Type K thermocouple. Typical deposition conditions are in the 
range summarized in Table I. 
Immediately following deposition (without cooling), the system is 
backfilled with argon or oxygen to atmospheric pressure, and the samples 
are annealed at 900.degree. C. for 30 minutes in 1 atm O.sub.2 flowing at 
1 liter/min. Alternatively, samples of equal quality have been obtained by 
omitting the annealing step. The furnace is then cooled in 2 h below 
300.degree. C. also while flowing O.sub.2. Coatings are deposited on both 
planar and fiber substrates. Planar substrates are single crystals of MgO, 
SrTiO.sub.3, and stabilized ZrO.sub.2 and polycrystalline Al.sub.2 
O.sub.3, stabilized ZrO.sub.2, and Ag. Fibers were Sumitomo, Nicalon, 
Nextel, Ag, and Saphikon (single crystal Al.sub.2 O.sub.3). 
______________________________________ 
Range* Preferred 
______________________________________ 
Total Pressure (torr) 
10-760 20 
Deposition Temperature (.degree.C.) 
500-970 900 
Argon Flowrate (1/min) 
0-5 5 
Oxygen Flowrate (1/min) 
1-5 1 
Deposition Time (min) 
5-30 30 
Input Reagent Mass Ratio 
1:(1.25-2.01): 
1:1.38:1.51 
Y(tmhd).sub.3 :Ba(tmhd).sub.2 : 
(1.51-2.56) 
Cu(tmhd).sub.2 
Total Reagent Mass (g) 
2-10 1.75 
______________________________________ 
*The present invention is operative outside of this range 
Argon or oxygen is used as a carrier gas 16 and may be supplied from any 
conventional source such as refillable tanks 72. The flow rate of the 
carrier gas 16 is controlled by a conventional mass flow controller 74. 
The carrier gas 16 is introduced to the vibratory feeder 12 through input 
line 76. Input line 76 splits into two lines, an ambient line 78 and an 
carrier stream line 80. Ambient line 78 introduces the carrier gas 16 into 
the main vacuum cavity 82 of the vibratory feeder 12. 
The vibratory powder feeder 12 is enclosed in a stainless steel chamber 14 
to ensure a vacuum seal. Flowing carrier gas 16 is used as a pneumatic 
transport to assist in carrying the solid powder 18, which is vibrated 
around a track 20 at a uniform rate inside the bowl 22, through the lines 
24 and into the furnace 50. 
When the vibratory feeder 12 is in operation, the powder 18 is vibrated 
around a track 20 inside the vibratory feeder bowl 22. The powder 18 is 
forced from the track 20 and is entrained in the carrier gas 16 flowing 
down the tube 30 located in the center of the bowl 22. The powder 18 
entrained in the ambient carrier gas 16 merges with the carrier gas 16 
contained in the carrier stream 28 forming a carrier gas powder output 
stream 40 which is carried along output line 24 to the furnace 50. 
The rotary bowl powder feeder is a Syntron EB-051 parts feeder from FMC 
with several modifications, although rotary bowl powder feeders, including 
nonvibratory feeders, from other manufacturers can be used, if the 
appropriate modifications are made. These modifications include a tee 26 
which has been attached at the exit lip of the bowl 22. The tee 26 causes 
the powder 18 to fall into carrier stream 28. Carrier gas 16 is also used 
to assist in carrying the powder 18 down through tube 30 into carrier 
stream 28. A second modification to the rotary bowl 22 is that two spots 
along the powder ramp have been restricted to about one sixteenth (1/16) 
of an inch. This modification allows the rotary bowl 22 to disperse powder 
at a more uniform rate. The reactant powder 18 is ground to below about 
forty-four (44) microns in diameter, although the exact diameter of the 
powder is not critical, using a mixer/mill prior to introduction to the 
vibratory feeder 12. 
The substrate 52 to be coated is retained in a substrate holder 54 in the 
furnace 50. The furnace is heated to a temperature to between about 
500.degree. and about 970.degree. C. which is held constant during the 
deposition process. The carrier gas-reagent powder stream 40 enters the 
furnace 50 and is deposited on the substrate 52. When argon is used as the 
carrier gas 16, a stream of flowing oxygen 17 also is brought to the 
inside of the furnace. The thickness of the coating deposited on the 
substrate is controlled by the temperature and pressure of the furnace and 
the amount of reagent introduced into the furnace 50. The powder feed rate 
as well as the duration of the run may be altered in order to vary the 
final coating thickness. 
Alternatively, an atomizer can be used in place of the powder feeder. FIG. 
8 shows this alternate embodiment. The atomizer 112 is placed in the 
carrier gas 16 line at the same point where the powder feeder 12 would be 
placed. Liquid reagents or reagents dissolved in a solvent are fed to the 
atomizer 112 through feed line 114. Carrier gas 16 flows through the 
atomizer 112 and carries the reagents downstream. As the dissolved 
reagents flow through the atomizer, the solvent evaporates and reagents 
particles are produced, resulting in the introduction of a powder to the 
furnace 50 similar to the powder introduced to the furnace 50 using the 
powder feeder apparatus. For solid reagents dissolved in a solvent, solid 
particles are formed. For liquid reagents or reagents dissolved in a 
solvent, liquid particles are formed. The carrier gas-atomized reagent 
stream, along with the evaporated solvent, 140 enters the furnace 50 and 
is deposited on the substrate 52. When argon is used as the carrier gas 
16, a stream of flowing oxygen 17 also is brought to the inside of the 
furnace. The thickness of the coating deposited on the substrate is 
controlled by the temperature and pressure of the furnace and the amount 
of reagent introduced into the furnace 50. The feed rate as well as the 
duration of the run may be altered in order to vary the final coating 
thickness. 
Excess and/or decomposed reagent, carrier gas and oxygen leave the furnace 
through exit line 58 and enter a scrubber 60. The exhaust gas is scrubbed 
to remove any acidic and condensable gases and particulates. The excess 
exhaust gases are pumped through pump 66 and exhausted into the 
atmosphere. 
The carrier gas-powder mixture 40 is introduced into the furnace 50 where 
the powdered reagents vaporize and subsequently react at the substrate 
material surface similar to other CVD processes. This process is 
repeatable, reliable, and provides a faster deposition rate compared to 
approaches which rely on transport of vapor from low vapor pressure and/or 
unstable reagents into the CVD furnace. 
The powder feeding technique of the method and apparatus of the present 
invention can be used for any low vapor pressure reactant compounds used 
in CVD processes. Specific systems where the process does apply include 
superconducting compounds, such as YBa.sub.2 Cu.sub.3 O.sub.x and the 
Bi-Sr-Ca-Cu-0 and Tl-Ca-Ba-Cu-O compounds. One or more or all of the 
cations may be supplied by the powder feeding or atomization process to 
form superconductors which contain elements for which only low vapor 
pressure, unstable, or expensive reagents exist. Alternatively, this 
invention may be used to deposit single or double oxides or other 
compounds containing the troublesome cations. 
In addition to a vibratory feeder, the powder feeder can be any type of 
auger or other powder feeder type or pump in which powder or liquid can be 
fed uniformly into a CVD furnace. A liquid feed system, which atomizes a 
solution containing dissolved or slurried reagents, that is, makes the 
liquid into droplets which dry to yield small powder particles of the 
reagent can be used. Using such an atomizer system, the low vapor pressure 
compounds are dissolved in a liquid, such as carbon tetrachloride or 
hexane, and introduced into a gas stream through a nozzle. This process 
produces very fine powder particles which are then carried by the gas 
stream into the furnace. Alternatively, the atomization and subsequent 
powder formation could occur inside the furnace. 
For those reagents which are liquids at room or a moderately elevated 
temperature (approximately 16.degree.-100 .degree. C.), the atomization 
method is suitable. The liquid droplets introduced to the CVD system 
through atomization vaporize to form gaseous reagents inside the furnace. 
3. Results 
The unique reagent feed system results in deposition rates over one order 
of magnitude greater than those achieved previously using reagent 
sublimation techniques (e.g. 200 microns/h vs 1-10 microns/h). A 
deposition rate of 240 microns/h has been achieved. The coatings appear 
smooth, black, uniform and adherent to MgO. Most coatings on planar 
substrates show extensive preferred orientation, with the c-axis of the 
YBa.sub.2 Cu.sub.3 O.sub.x coating perpendicular to the substrate surface. 
A typical diffraction pattern of such a coating is shown in FIG. 5. The 
principal impurity phases observed in the XRD analysis were cupric oxide, 
barium cuprate, and the 211 phase. Occasionally, barium carbonate or 
yttrium oxide is observed. The quantity of the impurity phases correlates 
relatively well with the chemical analysis of the coatings. For example, 
if the EDX analysis of the sample show the coating to be deficient in 
yttrium, the XRD analysis often reveals the presence of barium cuprate. On 
the other hand, if the coating is barium or copper deficient, the CuO or 
211 phases will usually be observed in the XRD pattern. 
Samples which were processed using the powder feed system of the method and 
apparatus of the present invention had coating growth rates of about four 
(4) microns per minute and exhibited superconductivity. Coating a 
substrate with YBa.sub.2 Cu.sub.3 O.sub.x used an initial charge of 5.0 
grams of powder to the feeder having a reagent ratio of 1:1.38:1.51 of 
Y(tmhd).sub.3 :Ba(tmhd).sub.2 :Cu(tmhd).sub.2. This powder was fed into 
the horizontal reactor using five (5) liters per minute of oxygen flow at 
furnace conditions of 40 torr and 900.degree. C. An analysis of the 
coating by x-ray diffraction, energy dispersive spectroscopy, and 
resistance versus temperature measurements was completed on dozens of 
samples. 
Typical results are presented in FIGS. 4-6. The results show successful 
deposition for several samples, including good repeatability. 
Profilometry, weight gain and scanning electron microscopy of fracture 
cross sections were used to determine coating thickness and therefore 
determine the deposition rate. 
FIG. 4 shows that the coating achieved by this invention contains Y, Ba, 
and Cu in the approximate mole ratio 1:2:3. Coatings like this are 
slightly deficient in Y and are thought to be preferred, although coatings 
of various compositions may be prepared by deliberately varying the amount 
of Y(tmhd).sub.3, Ba(tmhd).sub.2, and Cu(tmhd).sub.2 in the feed powder. 
The x-ray diffraction pattern of FIG. 5 is typical of the YBa.sub.2 
Cu.sub.3 O.sub.x coating prepared using the present invention. These x-ray 
diffraction patterns, by virtue of the unusually intense {00.rho.} 
diffraction peaks, show the coating to be preferentially oriented with the 
basal plane parallel to the substrate surface. 
FIG. 6 shows nine different samples of YBa.sub.2 Cu.sub.3 O.sub.x, all of 
which were deposited using the powder feeding method. The difference in 
the initial and preferred conditions referred to in FIG. 6 is different 
ratios of Y(tmhd.sub.3), Ba(tmhd.sub.2), and Cu(tmhd.sub.2) in the feed 
powder. The T.sub.c values resulting are outstanding in that they 
significantly exceed the value of 66K, which is the maximum value for 
chemically vapor deposited YBa.sub.2 Cu.sub.3 O.sub.x on an MgO substrate 
previously reported. 
The method and apparatus of the present invention has also resulted in a 
Tc(R=0) of 82K for a YBa.sub.2 Cu.sub.3 O.sub.x coating on Saphikon fiber. 
Saphikon is single crystal Al.sub.2 O.sub.3 in the form of a 125 micron 
diameter fiber. This superconducting coating on a Saphikon fiber results 
in a flexible superconducting wire. Fiber tows of various compositions, 
such as polycrystalline Al.sub.2 O.sub.3, and ribbons or tapes of Al.sub.2 
O.sub.3 or ZrO.sub.2 may also be coated. 
The process has the distinct advantage of higher reagent introduction 
rates, and therefore higher coating rates, than the conventional approach 
in which the reagents are vaporized in individual external vaporizers 
prior to introduction into the CVD furnace. 
The invention is not intended to be limited to the examples given above. It 
is obvious that those skilled in the art may make modifications to the 
method or the apparatus or both, without departing from the spirit or 
scope of the invention which is defined by the subjoined claims.