Chemical vapor deposition process where an alkaline earth metal organic precursor material is volatilized in the presence of an amine or ammonia and deposited onto a substrate

A method is disclosed for the volatilization and transport of an alkaline earth metal precursor. The presence of an amine or ammonia significantly increases transport of the voltalized alkaline earth metal precursor as compared to transport under the same conditions but without the amine or ammonia.

BACKGROUND OF THE INVENTION 
Alkaline earth metals have applications as components of ferroelectric, 
ferromagnetic and super-conducting materials. Deposition of these metals 
has often been difficult, however, because many of the precursor 
compounds, particularly compounds of barium, strontium and calcium, 
typically have low volatilities or significant tendencies to decompose at 
volatilization temperatures. 
.beta.-diketonate compounds, including those of alkaline earth metals, have 
been reported to be sufficiently volatile for such depositions, 
particularly when substituent groups of the .beta.-diketonate are 
fluorocarbons. These compounds, however, are generally still not 
sufficiently stable for most chemical vapor deposition processes. Also, 
the fluorinated species often cause fluorine to be deposited with the 
alkaline earth metal thereby diminishing the purity and performance of 
deposited films. 
A need exists, therefore for new methods and apparatus for depositing 
alkaline earth metal compounds which overcome or minimize the 
aforementioned problems. 
SUMMARY OF THE INVENTION 
The present invention relates to a new method and apparatus for 
volatilizing and transporting alkaline earth metal compounds. 
A method for volatilizing and transporting an alkaline earth metal organic 
precursor includes contacting the alkaline earth metal organic precursor 
with an amine or ammonia during volatilization. Transport of the 
volatilized precursor is thereby significantly increased compared to 
transport under the same conditions but without contact of the amine or 
ammonia. 
A preferred embodiment of the method of the present invention is a chemical 
vapor deposition (CVD) process in which an alkaline earth metal-containing 
material is deposited onto a substrate. A carrier gas is directed to 
contact an alkaline earth metal organic precursor and then directed to the 
substrate. The precursor is volatilized in the presence of an amine or 
ammonia to significantly increase transport of the volatilized precursor 
to the substrate compared to transport under the same conditions but 
without contact of the amine or ammonia. The substrate is heated to a 
temperature sufficient to cause the volatilized precursor to decompose 
within the CVD chamber resulting in deposition of the alkaline earth 
metal-containing material on the substrate. 
Apparatus for chemical vapor deposition of an alkaline earth 
metal-containing material onto a substrate includes means for directing a 
carrier gas to contact an alkaline earth metal organic precursor and then 
directing it to the substrate. Means for volatilizing the precursor in the 
presence of an amine or ammonia are provided which significantly increases 
transport of the volatilized precursor compared to transport under the 
same conditions but without contact of the amine or ammonia. Means for 
heating the substrate in a CVD chamber to a temperature sufficient to 
cause the volatilized precursor to decompose and to deposit the alkaline 
earth metal-containing material onto the substrate are also provided. 
This invention has many advantages. In general, the amine or ammonia 
significantly increases transport of the volatilized alkaline earth metal 
precursors compared to transport under the same conditions but without 
contact of the amine or ammonia. It also provides a convenient technique 
for volatilizing and transporting alkaline earth metal precursors which 
otherwise could not be volatilized, such as barium acetacetonate, or which 
were difficult to volatilize. Increased transport of volatilized alkaline 
earth metal precursors increases the deposition rate of alkaline earth 
metal-containing materials which can be obtained by chemical vapor 
deposition. Deposition of alkaline earth metal-containing materials can be 
performed continuously over longer periods of time, thereby enabling 
results which are more easily reproduced. Further, contaminating atoms, 
such as fluorine, can be avoided because non-fluorinated precursors or 
compounds can be volatilized and transported by the method and apparatus 
of this invention.

DETAILED DESCRIPTION OF THE INVENTION 
The features and other details of the method and apparatus of the invention 
will now be more particularly described with reference to the accompanying 
drawing and pointed out in the claims. It will be understood that the 
particular embodiments of the invention are shown by way of illustration 
and not as limitations of the invention. The principal features of this 
invention can be employed in various embodiments without departing from 
the scope of the invention. 
In a preferred embodiment of the present invention, CVD system 10, 
illustrated in the FIGURE, employs a carrier gas originating from a 
carrier gas source 12, which can be a cylinder of compressed gas. The 
carrier gas can be a reactive gas, such as air, which provides oxygen to 
react with a volatilized precursor or an inert gas such as helium, 
nitrogen or argon. The carrier gas is passed through conduit 14, 
comprising a suitable tubing of glass, ceramic, stainless steel or other 
metal, polymer such as Teflon.RTM. polytetrafluoroethylene, rubber etc., 
into transport agent bubbler 16, which includes transport agent vessel 18 
and cover 20 on transport agent vessel 18. Transport agent vessel 18 can 
be constructed of a suitable material such as glass or steel. Conduit 14 
extends through cover 20 into liquid transport agent 22 within transport 
vessel 18. The carrier gas is bubbled through liquid transport agent 22 in 
transport agent vessel 18 to thereby volatilize transport agent 22. 
Transport agent bubbler 16 can be heated by a heater, not shown, such as 
an electrical resistance heater, an oil bath, or a medium recirculating 
through a jacket at transport agent vessel 18 to increase the rate of 
volatilization of the transport agent. 
The carrier gas and volatilized transport agent are transported from 
transport agent bubbler 16 through conduit 24 to precursor bubbler 26. 
Conduit 24 is comprised of a tubing material such as is suitable for 
conduit 14. Precursor bubbler 26 includes precursor vessel 28 and cover 30 
on precursor vessel 28. Precursor bubbler 26 can be constructed of a 
suitable material such as glass or steel. Conduit 24 extends through cover 
30 into close proximity with precursor 32 within precursor bubbler 26. The 
carrier gas and transport agent can be bubbled through precursor 32 in 
percursor bubbler 26 when precursor 32 is in liquid form. Alternatively, 
the carrier gas and transport agent can be directed from conduit 24 to 
contact precursor 32 contained within a suitable vessel when precursor 32 
is a solid. The carrier gas can be passed over the surface of a solid, or 
if the solid is granular, through a granular bed. Precursor 32 is thereby 
exposed to the transport agent and volatilized. Heater 34, at precursor 
vessel 28, can be used to heat precursor 32 and thereby increase the rate 
of volatilization of precursor 32 by the volatilized transport agent. 
Heater 34 can be, for example, an electrical resistance heater, an oil 
bath, or a recirculating medium through a jacket. CVD system 10 can also 
be disposed in an oven, not shown, to heat transport agent bubbler 16 and 
precursor bubbler 26. Volatilization of transport agent 22 and precursor 
32 can be controlled by the oven temperature and by cooling transport 
agent bubbler 16 and precursor bubbler 26 by a cooling medium, such as 
water, which can be flowed around either, or both of the bubblers. Under 
some circumstances, such as under high vacuum and/or in the presence of 
the most volatile transport agents such as ammonia or trimethylamine, 
precursor 32 can be volatilized without the need for any carrier gas. 
The volatilized precursor and volatilized transport agent are transported 
by the carrier gas from precursor bubbler 26 through conduit 36 to CVD 
chamber 38. Conduit 36 can be formed of a suitable tubing, such as is used 
for conduit 14 and conduit 24. Substrate 40 is disposed within CVD chamber 
38. CVD chamber 38 can be constructed of a suitable material, such as 
glass, quartz or stainless steel. Heater 42 is proximate to substrate 40 
for heating substrate 40 and is disposed outside of CVD chamber 38. Heater 
42 can be, for example, a radiant heater, such as wherein heater 42 
comprises a block 44 of metal, such as nickel, which is heated by an 
electrically resistant coil 46. Heater 42 can also be, for example, an 
infrared heater, not shown, or an inductive coil, also not shown, wrapped 
about CVD chamber 38. 
The volatilized alkaline earth metal precursor is transported to CVD 
chamber 38 containing substrate 40, which is heated to a temperature 
sufficient to cause decomposition of the precursor in CVD chamber 38 
thereby resulting in deposition of film 48 of an alkaline earth 
metal-containing material on substrate 40. The alkaline earth metal 
containing material may or may not be a chemical compound. Carrier gas, 
transport agent and remaining volatilized earth metal precursor are 
exhausted from chemical vapor deposition chamber 38 through conduit 50 of 
suitable tubing, such as that used for conduits 14, 24 and 36. 
Alkaline earth metal precursor 32 comprises any alkaline earth metal 
compound which can be volatilized and transport of which is significantly 
increased when precursor 32 is in contact with the transport agent 
compared to transport under the same conditions but without contact of the 
transport agent. The reason that contact between the transport agent and 
the alkaline earth metal precursor results in better volatility and 
transport is not fully understood. A theory which satisfactorily explains 
these phenomena has not yet been developed. It is believed, however, that 
in any event, at least in some cases, the contact of amines or ammonia 
with the alkaline earth metal precursor causes increased volatility of the 
precursor and/or a decreased tendency of the volatilized precursor to 
decompose prematurely. 
In a preferred embodiment of the present invention, the alkaline earth 
metal is selected from the group comprising calcium, barium and strontium. 
The alkaline earth metal can be included in an organic compound. The 
preferred organic compounds are .beta.-diketonates having the following 
formula: 
##STR1## 
M is the alkaline earth metal. R.sub.1,2,3,4,5,6 are individually selected 
from hydrogen, alkyl, aryl, alkoxy, including substituted alkyl, aryl and 
alkoxy groups. An example of a suitable .beta.-diketonate for use with the 
present invention includes barium dipivaloylmethane (Ba(dpm).sub.2), 
wherein R.sub.1,3,4,6 are tertiary butyl groups and R.sub.2,5 are hydrogen 
groups. Another example of a suitable .beta.-diketonate is barium 
hexafluoroacetylacetonate (Ba(hfa).sub.2), wherein R.sub.1,3,4,6 are 
fluorinated methyl groups and R.sub.2,5 are hydrogen groups. A third 
example of a suitable .beta.-diketonate is barium acetylacetonate where 
R.sub.1,3,4,6 are methyl groups and R.sub.2,5 are hydrogen groups. Other 
.beta.-diketonates suitable for use in the present invention are described 
in "Comprehensive Coordination Chemistry," 3.1 (Eds. G. Wilkinson, R. D. 
Gillard, J. A. McCleverty) Pergamon Press. Oxford. (1987), the teachings 
of which are hereby incorporated by reference. Other suitable metal 
organic for use in the precursor include alkoxy ethers, such as 
methyldiethyleneglycol, MeOC.sub.2 H.sub.4 OC.sub.2 H.sub.4 OH. 
The alkaline earth metal organic precursor decomposes within the CVD 
chamber to deposit an alkaline earth metal compound. Examples of alkaline 
earth metal compounds deposited are alkaline earth metal oxides, which can 
be deposited by decomposition of Ba(dpm).sub.2 onto substrate 40, and 
alkaline earth metal fluorides, which can be deposited by decomposition of 
Ba(hfa).sub.2 onto substrate 40. 
Transport agent 22 significantly increases transport of the volatilized 
precursor as compared to transport under the same conditions but without 
contact of the transport agent. Suitable transport agents include amines 
which are volatile and do not decompose under conditions at which the 
precursor decomposes. Preferred amines are those that are the most 
volatile because they are the easiest to volatilize for contact with 
precursor 32. Examples of a suitable amines include: monodentates, such as 
the trialkylamines, trimethylamine and triethylamine; and multidentates, 
such as ethylenediamine and N,N,N,N-tetramethylethylenediamine. Other 
suitable amines include: primary amines, such as ethylenediamine; 
secondary amines, such as dimethylamine; and tertiary amines, such as 
trialkylamines, trimethylamine, pyridine and triethylamine. Also, 
transport agent 22 can operate additionally as the carrier gas. For 
example, ammonia or trimethylamine can be used to transport the precursor 
without introduction of an additional gas. Further, mixtures of ammonia 
and amines can be used to form transport agent 22 for volatilization of 
precursor 32 in precursor bubbler 26. 
The preferred transport agent for use in the present invention is ammonia 
because the combination of ammonia and precursor typically is less 
flammable than combination of amines with suitable precursors volatilized 
by the method of the present invention. Also, ammonia is typically gaseous 
at atmospheric pressure and, therefore, a carrier gas is generally not 
needed to bubble ammonia through precursor 32 in precursor bubbler 26. 
Further, ammonia is often less expensive than are suitable amines. 
Substrate 40 is comprised of a material on which an alkaline earth 
metal-containing material can be deposited. Examples of such substrate 
materials include silicon, glass boron, metals and any other sufficiently 
refractory material. The volatilized precursor decomposes within CVD 
chamber 38 and thereby deposits on substrate 40 to form film 48. The 
presence of carbon in film 48 of alkaline earth metal compound can be 
reduced by introducing an oxidant into CVD chamber 38 during deposition of 
the alkaline earth metal compound. 
Although it is customary to employ heat to volatilize precursors and 
transport agents, other techniques can sometimes be employed. One example 
is the use of vacuum in the apparatus employed. 
Other methods, such as spin coating techniques, can be used to deposit 
alkaline earth metal compounds by the method of the present invention. In 
a typical spin coating technique, a solution of transport agent and 
precursor is formed which is deposited onto the center of a substrate on a 
spinning wheel. The substrate on the wheel is heated to decompose the 
precursor and to dissipate the transport agent. Centrifugal force imparted 
to the solution by the spinning wheel distributes the solution, whereby 
the precursor decomposes and is deposited onto the substrate. 
The method of the present invention is useful for deposition of alkaline 
earth metal compounds onto substrates by chemical vapor deposition or by 
other processes. The present invention can be used to form ferromagnetic 
materials, such as barium ferrite, and ferroelectric materials, such as 
barium titanate. The present invention is particularly suitable for 
depositing oxides with a high superconducting transition temperature, such 
as yttrium barium copper oxide (YBa.sub.2 Cu.sub.3 O.sub.7-x, wherein x is 
between 0 or 1) and bismuth strontium calcium copper oxide (Bi.sub.2 
Sr.sub.2 CaCu.sub.2 O.sub.9-x). It is also suitable for forming barium 
fluoride glass, for example, such as is used for optical windows of 
heat-seeking missiles. 
The method of volatilizing and transporting alkaline earth metal precursors 
by the method of the present invention is also suitable for purification 
of alkaline earth metal compounds, such as by selective distillation. A 
typical selective distillation method using the method of the present 
invention includes heating an impure alkaline earth metal precursor to 
volatilize relatively low boiling impurities which include, for example, 
silicon and aluminum compounds. The material is then heated in the 
presence of ammonia to contact the alkaline earth metal compound with the 
ammonia to thereby volatilize and transport the alkaline earth metal 
precursor from the solution. Substantially non-volatile impurities remain 
following volatilization of the alkaline earth metal precursors. 
Another application of the present invention includes volatilizing alkaline 
earth metal precursors for introduction to separation media, such as that 
done in gas chromatography. The invention thus facilitates chromatographic 
separation of alkaline earth metal compounds. 
EXAMPLE I 
Barium oxide was deposited by chemical vapor deposition using apparatus as 
shown in the FIGURE. A carrier gas comprising dry air from a cylinder and 
at atmospheric pressure was conducted through a transport agent bubbler 
and a precursor bubbler at a flow rate of about 5.0 liters per minute. The 
transport agent bubbler was glass and had a volume of about 60 cubic 
centimeters contained liquid triethlyamine at a temperature of about 
25.degree. C. The precursor bubbler was constructed of glass and had a 
volume of about 20 cubic centimeters. The precursor bubbler contained 
relatively pure Ba(dpm).sub.2. The chemical vapor deposition chamber was 
constructed of glass and had dimensions of 1 cm.times.40 cm.times.8 cm. 
All conduits were constructed of stainless steel. The carrier gas was 
directed from the precursor agent bubbler to the chemical vapor deposition 
chamber wherein a silicon substrate (P doped [1,1,0]) having a surface 
area of about 10 cm.sup.2 was disposed. The substrate was heated to a 
temperature of about 500.degree. C. by a heater comprising a block of 
nickel and an electrically resistant coil. A blue film formed on the 
substrate having a thickness of about 300 nanometers during a deposition 
period of about 60 minutes. X-ray photoelectron spectroscopy tests 
indicated that the film was comprised of substantially pure barium oxide, 
having no detectable amounts of carbon or nitrogen present. 
EXAMPLE II 
The chemical vapor deposition of Example I was repeated wherein 
tetramethylethylene diamine was substituted as the transport agent for 
triethylamine. Equivalent deposition of barium oxide resulted. 
EXAMPLE III 
The chemical vapor deposition of Example I was repeated with no transport 
agent present. Only a trace of film was deposited on the substrate. 
EXAMPLE IV 
Example II was repeated using barium acetylacetonate as the alkaline earth 
metal organic precursor. R.sub.1,3,4,6 of the general .beta.-diketonate 
formula, described supra, were methyl groups and R.sub.2,5 were hydrogen 
groups. An equivalent barium oxide film on the substrate to that obtained 
for Examples I and II was deposited. 
EXAMPLE V 
The chemical vapor deposition of Example IV was repeated without the 
presence of a transport agent. No film was detected on the silicon 
substrate following the deposition period. 
EXAMPLE VI 
Ba(dpm).sub.2 was sublimed from the precursor bubbler using trimethylamine 
as the transport agent and carrier gas. The precursor bubbler was at a 
temperature of approximately 100.degree. C. and the precursor sublimed at 
a rate of about 0.1% per minute. 
EQUIVALENTS 
Those skilled in the art will recognize or be able to ascertain using no 
more than routine experimentation, many equivalents to the specific 
embodiments of the invention described specifically herein. Such 
equivalents are intended to be encompassed in the scope of the following 
claims.