CVD thin film compounds

A stream of gaseous source compounds and their ranges of relative ratios useful in the deposition of thin film ferroelectric materials by CVD are disclosed. The stream of gaseous source compounds are used in combination with a CVD reactor flushed with an inert gas and maintained at a predetermined internal pressure and, a substrate disposed within the CVD reactor and maintained at a predetermined temperature. The steam of gaseous source compounds include a Zr source compound, a Ti source compound, a Pb source compound, an oxidizing agent compound, and an inert gas, as well as their ranges of relative ratios to deposit lead-zirconate-titanate, related ferroelectrics, specifically including lead-lanthanum-zirconium-titanate.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention relates generally to chemical vapor deposition (CVD) 
of thin films and, more particularly, to specific source compounds useful 
in the CVD of ferroelectric thin films. 
2. The Prior Art 
Lead zirconate titanate (PZT) ferroelectric thin films and their related 
alloys, such as ferroelectric PLZT thin films (i.e., less than ten microns 
and preferably about 0.15 micron to about one micron) find widespread 
applications in capacitors, specifically high energy density capacitors 
(requiring a dielectric possessing a high dielectric constant), for fast, 
nonvolatile integrated digital memories, for pyroelectric infrared 
sensors, for optical switches with nonlinear optical properties, for 
piezoelectric transducers and for surface acoustic wave devices. Despite a 
heavy demand, utilization of such ferroelectric thin films has been 
hampered by the non-existence of a reliable, large scale manufacturing 
process for economically producing device-quality ferroelectric thin 
films. To be sure, this was not for a lack of trying. 
Beginning in the 1970's and continuing to the present, a lot of 
investigative effort has been directed at depositing ferroelectric thin 
films by sputtering methods, see M. Ishida et al., "Preparation and 
properties of ferroelectric PLZT thin films by rf sputtering," Journal of 
Applied Physics, Vol 48, No. 3, March 1977, pp. 951-953, and R. Takayama 
et al., "Preparation of epitaxial Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 thin 
films and their crystallographic, pyroelectric, and ferroelectric 
properties," J. Appl. Physics 65(4), 15 Feb. 1989, pp. 1666-1670. A 
further effort involving the preparation of ferroelectric thin films has 
focused on sol gel processing, see G. Yi et al., "Preparation of 
Pb(Zr,Ti)O.sub.3 thin films by sol gel processing: Electrical, optical, 
and electro-optic properties," J. Appl. Physics 64(5), 1 Sept. 1988, pp. 
2717-2724. Both of these processes suffer from: limitations in achieving 
uniformity and reproducibility (including mixed phases of microscopically 
varying compositions) in the thin films produced, relatively low 
production rates, difficulties encountered by incompatibility of 
substrates and, inability to adapt the process to scale-up production. 
Additionally, the thin films produced have exhibited unacceptable levels 
of impurity concentrations, i.e., in excess of 100 parts per million 
atomic concentration (ppma) of contaminants. 
With the advent of chemical vapor deposition (CVD) and metal-organic 
chemical vapor deposition (MOCVD), several workers in the field have begun 
to use such techniques with several materials and with various degrees of 
success. As known, CVD offers fast deposition rates, CVD reactors can be 
scaled up to the deposition on large-area substrates and film uniformity 
is excellent with the CVD process. Most of this effort has, however, 
focused on the CVD or MOCVD deposition of ferroelectric lead-titanate 
(PbTiO.sub.3) thin films. See S. Yoon et al., "Preparation and Deposition 
Mechanism of Ferroelectric PbTiO.sub.3 Thin Films by Chemical Vapor 
Deposition," J. Electrochem. Soc. (Dec. 1988), pp. 3137-3140; B. S. Kwak 
et al; "Metalorganic chemical vapor deposition of PbTiO.sub.3 thin films," 
Appl. Phys. Lett. 53(18) 31 Oct. 1988, pp. 1702-1704; and S. L. Swartz et 
al., "Characterization of MOCVD PbTiO.sub.3 Thin Films," presented at the 
Zurich ISAF Conference, Sept, 1988. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to overcome the above 
disadvantages by providing ferroelectric thin film materials by chemical 
vapor deposition having improved electrical properties and greater 
uniformity and providing specific source compounds and their relative 
ratios useful in the deposition of lead-zirconate-titanate and related 
ferroelectrics, specifically including lead-lanthanum-zirconate-titanate. 
More specifically, it is an object of the present invention to provide, in 
combination with a CVD reactor maintained at a predetermined pressure and 
flushed with an inert gas, and a substrate within the reactor and 
maintained at a predetermined temperature, a stream of gases useful in the 
deposition of thin film ferroelectric materials and introduced into the 
reactor at certain relative ratios. Preferably, the thin film 
ferroelectric material is lead zirconate titanate [Pb(Zr,Ti)O.sub.3 =PZT] 
or lead-lanthanum zirconate titanate [(Pb,La)(Zr,Ti)O.sub.3 =PLZT]. 
Preferably, the inert gas is argon, helium or nitrogen and the stream of 
gases comprises a zirconium source compound consisting of one of 
tetrakisdiethylamino zirconium, trifluoroacetylacetonate zirconium or 
acetylacetonate zirconium; a titanium source compound consisting of one of 
tetrakisdimethylamino titanium, tetrakisdi-ethylamino titanium, or 
titanium isopropoxide; a lead source compound consisting of tetraethyl 
lead or z-ethylhexanoate; and an oxidizing agent consisting of one of 
nitrous oxide, oxygen, water vapor, n-propanol, or i-butanol. The PZT 
material may be doped with (a) barium, calcium, lanthanum or strontium in 
lieu of lead, or (b) iron and niobium as substitutes for titanium and 
zirconium. 
Other and further objects of the present invention will in part be obvious 
and will in part appear hereinafter. 
The invention accordingly comprises, in combination with a CVD reactor and 
a substrate provided therein, the stream of gaseous source compounds and 
their respective ratios introduced therein useful in the deposition of 
thin film ferroelectric materials of the present disclosure, the scope of 
which will be indicated in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In general, the present invention relates, in combination with a CVD and/or 
MOCVD reactor containing an inert gas and a heated substrate, to a stream 
of gaseous source compounds and their ranges of relative ratios and 
introduced into the CVD reactor in the deposition of high purity thin 
films of doped PZT and PLZT materials. The dopants can include barium, 
calcium, lanthanum and strontium in lieu of lead, or iron and niobium as 
substituted for titanium and zirconium. As known, ferroelectric PZT and 
PLZT thin films (i.e., less than ten microns and preferably about 0.15 
micron to about one micron) find widespread applications in capacitors, 
specifically high energy density capacitors (requiring a dielectric 
possessing a high dielectric constant), for fast, nonvolatile integrated 
digital memories, for pyroelectric infrared sensors, for optical switches 
with nonlinear optical properties, for piezoelectric transducers and for 
surface acoustic wave devices. Despite a heavy demand, utilization of such 
ferroelectric thin films has been hampered by the non-existence of a 
reliable, large scale manufacturing process for economically producing 
device-quality ferroelectric thin films. 
The particular CVD apparatus used can be either a vertical design reactor 
illustrated in FIG. 1, a horizontal design reactor illustrated in FIG. 2, 
or a belt furnace type reactor illustrated in FIG. 3, or any like CVD 
reactor. One specific preferred CVD reactor useful in practicing the 
invention and similar to the one illustrated in FIG. 1 herein is more 
fully described in U.S. Pat. No. 4,596,208, Wolfson et al., CVD REACTION 
CHAMBER, granted Jun. 24, 1986 and assigned to a common assignee, Spire 
Corporation of Bedford, Mass., the disclosure of which is incorporated 
herein by reference. 
As known, MOCVD generally refers to the deposition of multiconstituent 
films, whether epitaxial, polycrystalline or amorphous, employing any of 
several metalorganic compounds for the source of one or more of the 
constituents. In all cases, a vapor-phase reaction takes place in a 
reactor having a heated susceptor and a thereon disposed substrate. In the 
reactor, the organometallic compounds are pyrolized by the heat of the 
susceptor and the substrate to form the respective atomic or molecular 
forms of the constituents, which constituents recombine to form the 
semiconductor films on the substrate. 
The CVD reactor of said U.S. Pat. No. 4,596,208 features a reaction chamber 
for CVD comprising a vertical reaction tube having a tapered top provided 
with a concentric gas inlet, a bottom provided with a gas outlet, a base 
plate for supporting the reaction tube, and a tapered susceptor 
operatively supported within the reaction tube in close proximity to its 
tapered top and defining an angle therebetween. A vertical reactor of this 
type operates in a non-laminar, turbulent mode, precluding abrupt changes 
in gas compositions. The CVD reactor further includes a gas-mixing system 
preferably including a fully automated gas manifold, a power supply and a 
heat source, and a microprocessor. The gas-mixing system preferably 
includes a plurality of temperature controlled bubbles containing the 
various metalorganic source compounds required to deposit the desired thin 
film ferroelectric materials on the heated substrates within the CVD 
reactor chamber. The operation of the gas-mixing system is controlled and 
monitored on a panel. The panel displays, among others, a flow chart 
featuring lines and lights to indicate the admission of one or more of the 
source compounds, plus an inert gas into a mixing manifold prior to 
reaching the reactor tube. The gas-mixing system preferably is designed to 
allow the growth of thin layers with abrupt transitions between the 
layers. The combination power supply and heat source preferably includes 
the devices needed to power the radio frequency (RF) coils so as to 
inductively heat the susceptor in the reaction chamber. Susceptor 
temperature preferably is controlled by thermocouple feedback to the 
source. The CVD reactor also preferably includes a vacuum system to 
achieve the required vacuum within the reaction chamber. 
Thus, the selected particular CVD reactor provides, in all cases, means for 
controlled heating of a substrate upon which the PZT film is deposited, 
controlled means of delivering vapors of metalorganic compounds to the 
deposition reactor which contain lead, titanium, zirconium, and other 
possible dopants, and controlled means of adding oxidants and possibly 
inert gases to the deposition reactor. 
The pressure in the deposition reactor may be near atmospheric pressure for 
all configurations shown, or it may be in the range 0.01 to 100 torr for 
the configurations shown in FIGS. 1 and 2 only. In low pressure operation, 
a plasma may be induced in the gaseous environment in the deposition 
reactor. 
For a plasma ion deposition process of large-grain, thin semiconductor 
films directly on low-cost amorphous substrates, see U.S. Pat. No. 
4,443,488, Little et al., granted Apr. 17, 1984, also to the common 
assignee herein, Spire Corporation of Bedford, Mass., the disclosure of 
which is incorporated herein by reference. 
Plasma ion deposition of ceramics comprises ionizing a metalorganic gaseous 
compound and oxygen, extracting high energy ions containing metal atoms 
and excited oxygen ions thereof, depositing the ions on a substrate held 
at a controlled temperature so as to permit the ions to react on the 
surface enhancing the grain growth of the ceramic crystals. Preferably, 
the extracted ions possess high surface mobilities. The ionization is 
effected in a chamber by an electron-supported plasma, the plasma being a 
large volume, low pressure, high temperature plasma. 
Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 (PZT) thin films can be deposited in a CVD 
reactor at atmospheric pressure in an inert atmosphere of either argon, 
helium or nitrogen with the addition of: 
______________________________________ 
tetrakisdiethylamino zirconium 
0.1 to 1000 ppma 
tetrakisdimethylamino titanium 
0.1 to 1000 ppma 
tetraethyl lead 0.1 to 1000 ppma 
nitrous oxide 0.1 to 1000 ppma 
______________________________________ 
The abbreviation ppma signifies parts per million atomic concentration. 
EXAMPLE I 
A thin film ferroelectric PZT material has been depicted in a CVD reactor, 
with the following deposition parameters: 
tetraethyl-lead, held at 17.8.degree. C., vapor pressure about 0.2 mm, 
carrier flow 63 sccm; 
tetrakisdimethyl titanium held at 46.5.degree. C., vapor pressure about 
0.05 mm, carrier flow 50 sccm; 
tetrakisdiethyl zirconium, held at 110.degree. C., vapor pressure about 0.1 
mm, carrier flow 90 sccm; 
1%N.sub.2 O in Ar, flow at 20 sccm; and 
additional argon flow at 4800 sccm, with argon being used as a carrier gas; 
and at atmospheric pressure (i.e., about 760 mm; 760 torr) with CVD 
reactor concentrations being at about: 
Pb 3.3 ppma; 
Ti 0.65 ppma; 
Zr 2.4 ppma; 
nitrous oxide 39.8 ppma; and 
balance argon; 
which has resulted in a ferroelectric thin film having an approximate 
composition of Pb(Zr.sub.0.85,Ti.sub.0.15)O.sub.3. 
The composition of the resultant thin film can be varied by changing the 
relative flow rates of the carrier gas. For constant flow of the lead 
compound, any pseudo-binary between pure lead-titanate and pure 
lead-zirconate can be deposited for Ti concentrations between zero and 4 
ppma; zirconium concentrations between zero and 3 ppma. The concentration 
of the elements in the final film do not vary linearly with the 
concentrations of the elements in the gas phase. 
The composition of the resultant thin film, therefore, can vary as follows: 
EQU Pb(Zr.sub.x, Ti.sub.1-x)O.sub.3 
with x ranging from about 0.1 to about 0.9; and more specifically from 
about 0.45 to about 0.65. 
Metalorganic sources for dopants for PZT include: 
La(C.sub.11 H.sub.19 O.sub.2).sub.3 Tris (2,2,6,6 - 
tetramethyl-3,5-heptanedionato) lanthanum; 
Sr(C.sub.11 H.sub.19 O.sub.2).sub.3 Tris (2,2,6,6 - 
tetramethyl-3,5-heptanedionato) strontium; 
Nd(C.sub.11 H.sub.19 O.sub.2).sub.3 Tris (2,2,6,6 - 
tetramethyl-3,5-heptanedionato) neodymium; also known as (TMHD); and 
Nd(C.sub.5 H.sub.4 O.sub.2 F.sub.3).sub.3 Neodymium 
trifluoroacetylacetonate. 
Thus it has been shown and described, in combination with a CVD reactor 
flushed with an inert gas and maintained at a predetermined internal 
pressure and a substrate disposed within the reactor and maintained at a 
predetermined temperature, a stream of gaseous source compounds and their 
ranges of relative ratios useful in the deposition of thin film PZT and/or 
PLZT ferroelectric materials by CVD and/or MOCVD, which combination and 
compounds satisfy the objects and advantages set forth above. 
Since certain changes may be made in the present disclosure without 
departing from the scope of the present invention, it is intended that all 
matter described in the foregoing specification or shown in the 
accompanying drawings, be interpreted in an illustrative and not in a 
limiting sense.