Solid source MOCVD system

A system for MOCVD fabrication of superconducting and non-superconducting oxide films provides a delivery system for the feeding of metalorganic precursors for multi-component chemical vapor deposition. The delivery system can include multiple cartridges containing tightly packed precursor materials. The contents of each cartridge can be ground at a desired rate and fed together with precursor materials from other cartridges to a vaporization zone and then to a reaction zone within a deposition chamber for thin film deposition.

FIELD OF THE INVENTION 
The present invention relates to a solid-source chemical vapor deposition 
system useful in techniques for fabricating thin films of multicomponent 
oxide materials, such as, e.g., high temperature superconducting oxide 
thin films, by metalorganic chemical vapor deposition (MOCVD). 
BACKGROUND OF THE INVENTION 
The discovery of superconducting oxide materials with critical temperatures 
above the boiling point of liquid nitrogen has stimulated much interest in 
methods for making thin films of these materials. Such films can be 
extremely useful in electronic devices and energy transport systems, and 
many researchers have devoted substantial effort to finding a satisfactory 
method of fabricating these films. 
Chemical vapor deposition has been extensively used for preparation of 
films and coatings in a variety of applications. The advantages of 
chemical vapor deposition include higher quality, faster processing and 
the ability to coat substrates of irregular shapes. Accordingly, much work 
has been focused on attempts to fabricate superconducting oxide films by 
chemical vapor deposition in a feasible manner. In particular, much 
attention has been devoted to the growth of thin films of superconductive 
materials, e.g., YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO) material, by MOCVD. 
Such thin films were first fabricated in 1988, soon after the discovery of 
the superconducting properties of YBCO. 
The primary difficulty with such a MOCVD process arises from the solid 
precursor reagents that are used as the source materials. These 
metalorganic precursors have typically been tetramethylheptanedionate 
(TMHD) powders that are chelates of the yttrium, barium, and copper source 
metals. These precursors tend to decompose at temperatures close to the 
vaporization temperatures. This is particularly true of the barium-TMHD 
precursor, which decomposes almost as fast as it vaporizes. Thus, the 
quantity and volatility of the source materials change continuously and 
non-reproducibly. Therefore, it is important to control precisely the 
exposure of these precursors to elevated temperatures. Ideally this 
exposure should be as short as possible. Yet the chemical vapor deposition 
process requires that the precursor sources be stabilized at the 
vaporization temperatures, which means that this exposure may extend over 
a substantial period of time. High quality YBCO films have been 
fabricated, despite these obstacles, but low deposition rates are 
reported, typically 1 micron per hour (.mu.m/h). In addition, the control 
of the deposition process is very delicate, repeatability is poor, and 
previous processes have not attained commercial feasibility. 
The main disadvantage of MOCVD of YBCO stems from the difficulty of 
reproducibly transporting the metalorganic precursor materials to the 
substrate for deposition. Nearly all YBCO deposition systems that have 
been developed to date differ from one another primarily by their methods 
of precursor material delivery. Naturally, there are a host of other 
factors that have determined the relative success or failure of any of 
these systems, but many researchers have come to recognize that optimal 
precursor material delivery is the key to successful YBCO superconductor 
formation. MOCVD processes presently utilize one of about four types of 
precursor delivery techniques: (1) liquid-source delivery with or without 
aerosol assistance and plasma enhancement; (2) bubbler delivery with or 
without carrier gas; (3) free-flowing powder delivery; and, (4) 
solid-source delivery. 
One present bubbler delivery MOCVD process employs precursor sources in 
separate vaporization chambers. The precursors, which may be solid or 
liquid, are placed into boats or bubblers, and individually heated to 
temperatures at which they develop appreciable vapor pressures. The 
vaporized precursor materials are then transported to a reaction chamber 
by passing a carrier gas over the boats or bubblers, and sweeping the 
various vapors to the reaction chamber where they are mixed and a reaction 
product is deposited on a heated substrate. This technique works well for 
liquid precursors, where the carrier gas can be bubbled up through the 
liquid reservoir to fully saturate it. In this way, the amount of material 
transported to the reaction chamber is precisely controlled by the 
temperature of the precursor reservoir (which must be in thermal 
equilibrium) and by the flow rate of the carrier gas bubbled through the 
reservoir. Elaborate schemes have been developed to ensure complete 
saturation, precise temperature control, and accurate gas flow regulation. 
When this method is used to grow heterostructure films with sharp 
boundaries between layers, the pressures of the various gas streams must 
be balanced to avoid composition excursions when the composition is 
changed. One drawback is that this system relies on vaporization of 
precursor materials to initiate mass flow to downstream processes. 
This technique encounters further problems when it is used for solid 
precursors. It is difficult to ensure complete saturation of the carrier 
gas stream at the elevated temperatures required to raise the solid vapor 
pressure to appreciable values because the surface area of the solid is 
changing continuously due to depletion and grain growth effects. This 
problem may be alleviated by using large excesses of precursor material 
beyond the amount needed for film growth. This can result in a substantial 
waste of precursor materials. 
The use of a conventional bubbler delivery MOCVD method for making 
superconducting oxide films is discussed in the article "Preparation of 
Superconducting-Oxide Films by CVD and Their Properties" by H. Yamane et 
al., published in Journal de Physique, Colloque C5, Supplement au 
n.degree. 5, Tome 50, Mai 1989. This paper describes the parameter control 
required to grow these films and the resulting properties of films made in 
this way. The various problems of the process are discussed. 
The decomposition problem is also addressed by W. J. Lackey et al. in 
"Rapid Chemical Vapor Deposition of Superconducting YBa.sub.2 Cu.sub.3 
O.sub.x ", Applied Physics Letters, vol. 56, no. 12, 19 Mar., 1990, pages 
1175-1177. A powder feed method is described for introducing mixtures of 
finely ground precursor reagents into the carrier gas, which itself is a 
mixture of argon and oxygen. The powder is transported to the reaction 
zone where it vaporizes, reacts, and deposits YBa.sub.2 Cu.sub.3 O.sub.x 
on a hot substrate. This method achieves substantially higher deposition 
rates (from about 200 to 240 .mu.m/h) and improved film quality control in 
comparison with the conventional bubbler delivery method. Improvements 
with respect to the process control problem are also reported. 
Yet, drawbacks to this process remain. For example, powders are generally 
difficult to handle and cannot be dispersed in a dependable and continuous 
particle by particle manner into a gas stream. Also, some powder materials 
tend to agglomerate into larger particles, and delivery lines and chambers 
can be coated with finely divided particles. 
A deposition system for forming superconducting thin films by solid-source 
MOCVD is also described in U.S. Pat. No. 5,447,569 by Hiskes et al. 
wherein a single precursor reagent source, i.e., a mixture of metal 
chelates in powdered form, is packed into a glass tube having a 
longitudinal slot running the length of the tube. The powder mixture 
composition is determined by the desired stoichiometric ratios of the 
deposited film. The tube is longitudinally moved at a controlled rate 
through a high temperature region defined by a sharp temperature gradient, 
such that the powder mixture passing into this region vaporizes at a 
steady state and escapes from the tube through the slot. The vaporized 
precursor mixture is transported by a carrier gas to the substrate where 
the reaction and deposition takes place. Oxygen is introduced into the 
mixture at a controlled rate, and the reaction zone is heated to promote 
the chemical reaction. The reaction may be enhanced further by surrounding 
the zone with coils driven by an ac generator to produce an rf plasma in 
the mixture. This apparatus and method have been used to grow high quality 
superconducting oxide films where the film growth rate and composition can 
be independently controlled. 
Yet, in the process of Hiskes et al. deposition rates are generally rather 
slow, ranging from about 0.3 to about 0.8 .mu.m/h, and continuous-process 
runs cannot be supported. Additionally, composition of the final deposited 
film is controlled only by the pre-mixture of species packed into the 
glass tube or cartridge. Finally, this system relies on vaporization of 
precursor materials to initiate mass flow to downstream processes. 
It is an object of this invention to provide an apparatus and method for 
fabricating multicomponent oxide thin films such as superconducting oxide 
thin films by the MOCVD process using a precursor material delivery system 
capable of dispensing small amounts of source materials at precisely 
controlled rates over long periods of time. 
Another object of this invention is to provide an apparatus and method 
including real-time control over composition. 
Still another object of this invention is to provide an apparatus and 
method including real-time control over composition in which the 
decomposition of the precursor reagents prior to arrival in the reaction 
zone is minimized. 
A further object of this invention is to provide an apparatus and method 
for fabricating superconducting oxide thin films by the MOCVD process in 
which the total surface area of source materials is maximized to increase 
volatility and deposition rates. 
Yet another object of this invention is to provide an apparatus and method 
for fabricating superconducting oxide thin films by the MOCVD process in 
which the source materials are kept in the solid phase until just prior to 
vaporization. 
SUMMARY OF THE INVENTION 
To achieve the foregoing and other objects, and in accordance with the 
purposes of the present invention, as embodied and broadly described 
herein, the present invention provides an improved apparatus and method 
for fabricating metal oxide thin films, e.g., superconducting metal oxide 
thin films, with the MOCVD process. The apparatus includes a delivery 
means for feeding a solid including at least one precursor material into a 
cutting means for generating small diameter particles of said solid, a 
vaporizing means for raising the temperature of said small diameter 
particles above their vaporization temperatures whereby vapors of said 
small diameter particles are generated, a reaction zone wherein said 
vapors undergo chemical reaction and deposit on a substrate within said 
reaction zone, and, a transport means for transporting said small diameter 
particles from said cutting means to said vaporizing means and for 
transporting said vapors of said small diameter particles from said 
vaporizing means to said reaction zone.

DETAILED DESCRIPTION 
The present invention is concerned with an apparatus and method for MOCVD 
processing of multicomponent oxide materials. By "multicomponent oxide 
materials" is meant oxide materials including more than one metal 
component such as found in high temperature superconductor compositions 
such as YBCO, electro-optical materials such as LiNbO.sub.3, KNbO.sub.3, 
Sr.sub.x Ba.sub.1-x Nb.sub.2 O.sub.6, and the like. 
In an embodiment of the present invention for the MOCVD processing of thin 
films of YBCO, precursor powders of yttrium, barium and copper 
beta-diketonates are used as sources for the metallic ions. Initially, the 
precursor powders are packed into hollow stainless steel cartridges using 
a hand press at pressures of about 8000 pounds per square inch (psi) to 
about 12,000 psi whereby the free-flowing powders are transformed into 
packed solids with the basic consistency of chalk. 
Packed cartridges including the precursor powders are loaded into the top 
of a delivery/feeder device. A bottom-most cartridge in the vertical 
stacking of the cartridges is positioned for processing. The packed 
precursor powder within a cartridge is slowly pushed from inside of the 
cartridge into a high speed grinding mechanism whereby submicron powder 
particles can be generated. The powder particles are then swept downstream 
by a carrier gas or mixture of carrier gases into a vaporizer chamber 
whereat the powder particles are relatively instantly sublimed. After 
sublimation the carrier gas or gases carry the sublimed vapors of 
precursor materials into a deposition chamber situated in close proximity 
to the vaporizer for contact with a target substrate. 
Referring now to FIG. 1, the MOCVD reactor apparatus 10 includes a stack of 
packed cartridges 12 within a vertical arm 14 of apparatus 10. The linear 
feedthrough rod 16 is situated within a horizontal arm 18 of apparatus 10 
and rod 16 can be moved horizontally at a controlled rate. Rod 16 serves 
to push the packed precursor powder within bottom-most packed cartridge 12 
into grinding mechanism 20. Carrier gas source 22 provides the carrier gas 
or gases to carry powder particles resulting from grinding of packed 
precursor powder through line 24 into a vaporizer chamber 26. As the 
particles are not subjected to any potential thermal degradation until 
arriving at the vaporizer, the line 24 can be of any arbitrary length. 
From vaporizer chamber 26, carrier gas or gases carry the precursor vapors 
through a short transport line 39 and into a reaction zone within 
deposition chamber 28 including a substrate holder and heater 30. 
Preferably, this transport line is as short as possible to minimize the 
length of time between vaporization and deposition. Additionally, 
transport line 39 may be wrapped with heating tape to keep the walls at a 
desired uniform temperature of, e.g., about 250.degree. C. to 300.degree. 
C. to prevent condensation of metalorganic vapors. Preferably, deposition 
chamber 28 includes outwardly tapered sides 32 and diffusion baffling 34 
to aid in providing the preferred laminar flow of precursor vapors and 
carrier gas or gases. Also shown is an optional gas stoichiometry analyzer 
36, such gas stoichiometry analyzer 36 linked as a feedback controller to 
linear feedthrough rod 16. The drive mechanism for linear feedthrough rod 
16 can be a precision linear feedthrough stepper motor and ballscrew 
configuration 8 controlled by feedthrough controller 9. In this way the 
horizontal position and movement of bottom-most packed cartridge 12 and 
the packed precursor powder within bottom-most packed cartridge 12 is 
controlled. 
Referring to FIGS. 2(a) and 2(b), the configuration of a cartridge 12 used 
in the apparatus of the present invention is shown. FIG. 2(a) shows a side 
view of cartridge 12, while FIG. 2(b) shows an end view of cartridge 12. 
In FIG. 2(a) is seen an indented central portion 40 and protruding end 
portions 42. In FIG. 2(b) is seen an end view of protruding end portion 42 
and aperture or core 46 which can contain pressed precursor powder having 
the proper composition to yield the desired stoichiometry of the deposited 
film. For the case of YBCO films the precursor powders may be chelates of 
yttrium, barium and copper. The powders can be thoroughly mixed, dried, 
and packed under pressure into the packed cartridge 12. If more than one 
MOCVD apparatus is used, individual precursor powders can be separately 
located within different packed cartridges which can then be independently 
controlled and ground. 
Still referring to FIG. 1, oxygen is introduced into the vapor mixture 
through the intake tube and valve 38 at a flow rate which may vary from 
about 100 to about 1000 cm.sup.3 /minute. The gases flow upward through an 
outwardly tapered section with sloped walls 32 and diffusion baffling 34 
to enhance further mixing and laminar flow. 
Additionally, coils connected to a 13.54 MHz generator may be wrapped 
around outwardly tapered section 32 to produce an rf plasma and enhance 
the chemical reactions as the gas mixture arrives at the reaction zone 
within the deposition chamber. 
A target substrate is attached, e.g., with thermally conductive paste, to a 
substrate holder and heater 30 inside the deposition chamber, heated by a 
susceptor. The substrate holder and heater 30 and an attached substrate 
are typically maintained at the necessary temperatures, generally 
temperatures of from about 750.degree. C. to about 800.degree. C. for YBCO 
deposition, which can be measured by a thermocouple. Continuous 
depositions onto moving, elongated substrates are possible if substrate 
holder 30 is replaced by an outlay reel and a takeup reel, with substrate 
material held in tension between them as they rotate, such reels not 
shown. 
The upper end of the deposition chamber is sealed through which an exhaust 
tube extends. The exhaust tube leads to a vacuum pump, which maintains the 
interior of the deposition chamber at a reduced pressure during deposition 
of generally less than about 20 Torr. This pressure can be regulated by a 
manometer, not shown. 
Referring to FIG. 3, a vertical cross-section of a delivery/feeder device 
50 is shown. Vertical arm 14 within delivery/feeder device 50 includes 
upper guide tracks 52 and lower guide tracks 54. The upper portion of 
vertical arm 14 (above the intersection of vertical and horizontal arms, 
such intersection referred to as the reducing cross) serves as a magazine 
for the loading and storage of packed cartridges 12 and the protruding end 
portions 42 align within upper guide tracks 52. The shape of the guide 
tracks closely conforms to the end portions of the packed cartridges 
thereby preventing the cartridges from moving out of position. Within 
vertical arm 14 is a structure 56 providing a supporting ledge that 
precisely positions the bottom-most packed cartridge 12 for processing. 
Structure 56 is situated at the intersection of the vertical arm 14 and 
horizontal arm 18 with the position fixed by an anchor plate attached, 
e.g., by welding, to the inner walls of the arms. The lower portion of 
vertical arm 14 (below the intersection of vertical and horizontal arms) 
serves as a temporary waste receptacle for empty cartridges that have been 
previously processed. The vertical arm 14 can also provide facilities for 
the removal of empty cartridges from the apparatus and the loading of 
packed cartridges into the apparatus without breaking of the vacuum. This 
would allow nearly continuous feeding of precursor materials to the 
remainder of the MOCVD apparatus for long durations without interruption. 
Referring again to FIG. 1, for example, a quick-access door 5, nipple 
section 6 with guide tracks, and a clear bore ball valve 7 attached to the 
upper portion of vertical arm 14 would allow replenishment of the supply 
of packed cartridges without the loss of vacuum. 
A typical reloading procedure could involve closing of the ball valve, 
increasing the nipple pressure to 1 atmosphere (atm), opening the 
quick-access door at the top of the vertical arm, filling the nipple 
section with a load of packed cartridges, closing the quick-access door, 
pumping the nipple down to vacuum conditions, and finally opening the ball 
valve to allow the packed cartridges to fall down the guide tracks of the 
vertical arm. Likewise, a quick-access door 2, nipple section 3 with guide 
tracks, and a clear ball bore valve 4 attached to the lower portion of 
vertical arm 14 would allow a similar procedure to periodically remove 
empty cartridges that have accumulated in the lower portion or nipple of 
the vertical arm. Vacuum pumping devices would be attached to the ball 
valves to pump down nipple portions of the vertical arm as necessary. 
Referring to FIG. 4, a horizontal section through the reducing cross as 
seen from above, structure 56 provides the supporting ledge that precisely 
positions the bottom most packed cartridge 12 for processing. Also seen is 
drop channel 60 which includes lower guide tracks 54. Grinding assembly 62 
is shown with carrier gas inlet 64. Also seen is probe entry tube 67 and 
probe exit tube 66 for precisely guiding the linear motion of linear 
feedthrough rod 16 situated within horizontal arm 18. Linear feedthrough 
rod 16 includes a Teflon.RTM.-coated tipped steel probe. 
The movements of the probe and cartridges over one complete processing 
cycle are shown in the diagrams of FIGS. 5(a)-5(h). As the linear 
feedthrough extends towards the grinding mechanism, the probe is guided by 
a probe entry tube and ultimately pushes the precursor powder mixture out 
of the packed cartridge positioned at the intersection of the two arms. 
The probe is pushed through the cartridge and continues to move slowly 
through the probe exit tube until the entire amount of packed powder has 
been pushed into the grinder and ground into a fine (sub-micron) powder by 
the grinder assembly. Then the linear feedthrough pulls the probe rapidly 
back away from the grinder assembly. Because the Teflon.RTM.-coated tipped 
steel probe makes an interference fit with the inside diameter of the now 
empty cartridge, the cartridge is pulled by the probe tip off of the 
supporting ledge and is positioned over the lower guide tracks when the 
edge of the cartridge bumps into a stop. Although the empty cartridge is 
now hanging over a similarly shaped hole, it does not fall because the 
probe tip is still wedged within the empty core. As the motorized 
feedthrough continues to retreat away from the grinder assembly, the probe 
tip eventually exits the cartridge whereupon the empty cartridge falls 
down lower guide tracks into the lower portion of the vertical arm and 
another packed cartridge immediately drops into the bottom-most processing 
position. 
The linear feedthrough rod 16 including the Teflon.RTM.-coated steel probe 
tip uses only horizontal motion to process the packed precursor powder, 
eject the empty cartridge and load a new packed cartridge into position. 
In theory, the probe can complete all of the required actions by 
cyclically traversing a distance that is just slightly longer than the 
length of the cartridge. 
The cutting means can be a grinder assembly centered around a tiny steel 
cube attached to the end of horizontal arm 14. As the linear feedthrough 
rod 16 pushes the packed precursor powder from the packed cartridge into 
the grinder assembly, the packed powder is ground into fine, sub-micron 
powder particles by a pair of steel blades revolving inside the steel cube 
at approximately 3000 RPM. The blades are secured to the shaft of a high 
speed rotary motion feedthrough by two halves of a cylindrical blade 
holder assembly. The shaft of a brushless DC motor is attached to the 
feedthrough via standard flexible shaft coupler and set screws, while the 
body of the motor is supported by an adjustable L-bracket motor mount. 
Rotational speed is controlled through a potentiometer built into the body 
of a outboard driver box. Powder particles are blown off of the cutting 
blades by subsonic carrier gas entering the cube from a jet nozzle, 
preferably a 0.02 inch diameter jet nozzle assembly, at rates from about 1 
liter per minute (l/min) to about 5 l/min. Particulates are gathered by a 
funneling assembly and are immediately whisked downstream by the carrier 
gas or gases. Space filler plugs are incorporated into the inside faces of 
the blank flanges attached to the steel cube. These plugs can 
significantly reduce the volume of the cube so that powder particles are 
more readily directed towards the funnel assembly and then out of the 
cube. If desired, additional vertical and horizontal tube assemblies can 
be attached to the cube in place of blank flanges. Where all the blank 
flanges are replaced by such assemblies, a single grinder assembly cube 
could process up to four different powder species, each from a separately 
packed cartridge, simultaneously. Mass flow for each of multiple species 
can be regulated independently by adjusting the linear speed of the 
corresponding linear motion feedthrough controllers and linear feedthrough 
rods. 
Alternatively, the cutting means can be a tiny jet of high-velocity gas 
impacting upon the leading edge of the compacted precursor material. 
The style and method of the cutting means is of little concern as long as 
two criteria are met: (1) the cutting method must be capable of dividing 
the compacted precursor material into very small particles at a precisely 
and reliably controlled rate over a long period of time; and, (2) the 
cutting method must sustain a rate of particulate generation that is very 
small, for example, less than 1 gram per hour. 
The heating means can be a vaporizer. One possible embodiment of a 
vaporizer can be a simple device consisting of as few as six parts: 1/4" 
ID coiled tubing, 4" diameter aluminum cylinder and supporting base, 
coiled heater elements, electrical wall plug, vaporizer temperature 
controller, and a thermocouple. Powder-laden carrier gases flow through 
the coiled tubing at the rate of about 1 meter per second (m/s). Within a 
fraction of a second, heater elements sublime the powder particles, 
turning them to vapor. With the optional addition of nitrogen Lewis bases 
to the carrier gas stream, sufficient sublimation of the Y, Ba, and Cu 
.beta.-diketonates should be achieved with vaporizer temperatures no 
higher than 250.degree. C. 
The style and method of vaporization is of little concern as long as two 
criteria are met: (1) the vaporizer must be capable of completely 
vaporizing micron-sized precursor particulates in less than a second; and, 
(2) after sublimation, precursor vapors should spend no more than an 
additional half second residence time in the vaporizer before they are 
transported on and enter the deposition chamber. These time durations are 
approximate, but the general directive is to choose a vaporizer that 
inflicts as little thermal degradation on the precursor materials as 
possible. 
If desired, a feedback control loop can be incorporated into the MOCVD 
system with the addition of a gas analyzer just downstream from the 
vaporizer. This analyzer would be sensitive to changes in precursor vapor 
density and stoichiometry, and could then send correction signals to one 
or more linear feedthrough controllers to speed up or to slow down one or 
more probes to control the release of more or less powder into the gas 
stream. An analyzer feedback control loop is presented in the overview 
schematic of FIG. 1. 
Precursor vapors enter the deposition chamber from the bottom. Laminar flow 
into the chamber is preserved by a gradual increase in chamber diameter, 
e.g., from 14 mm to 200 mm at a 15 degree cone angle. The chamber design 
preferably includes geometric features that tend to thin down boundary 
layers and improve the uniformity of vapor pressures across the entire 
deposition zone near the substrate. Thus, the chamber preferably 
incorporates warm-wall diffusion baffling just upstream from the substrate 
and alignment of the substrate parallel to the flow of incoming gases. 
The carrier gas generally includes inert gases such as helium and argon. 
The addition of nitrogen Lewis bases to the inert carrier gas or gases can 
allow barium .beta.-diketonates to be transported in the vapor phase at 
temperatures as low as 70.degree. C. (at 1 atm) with no decomposition. 
Suitable nitrogen Lewis bases can be selected from N.sub.2 O, NH.sub.3, 
and N(CH.sub.3).sub.3 
Using rf coils to generate a plasma in the reacting gases may have a 
profound effect in improving the quality of the films. Both the 
oxygenation and the organometallic reactions may be enhanced during film 
growth. This may allow the process to proceed at lower temperatures, since 
energy is supplied by the rf field. 
This technique has also been found useful with a variety of substrates and 
other films. It is important that the same ligands be used for all 
components of a given film to avoid ligand exchange rendering some of the 
components nonvolatile. 
The foregoing description is directed toward the growth of a single film of 
a given stoichiometric composition. Multilayer heterostructure films may 
also be grown with this apparatus by filling successive packed cartridges 
12 with different mixtures of different precursor compositions, with each 
packed cartridge 12 having a uniform mixture of given precursor 
composition. As each packed cartridge 12 is processed, the corresponding 
film layer is grown. It will be appreciated that with the present method a 
great variety of multilayer heterostructure films can be grown. 
The present MOCVD apparatus controls precursor mass control with a simple 
mechanical process, i.e., the linear motion of the probe pushing on the 
end of packed precursor powder within a packed cartridge. Mass flow rate 
is increased by increasing probe velocity, or decreased by decreasing 
probe velocity. Additional advantages of the present invention are 
obtained in the method by which the sub-micron particles become 
disassociated from the main body of the precursor powder pack. As the 
probe pushes the packed powder into the blades of a high-speed grinding 
rotor, sub-micron particulates of precursor material are scraped off the 
end of the packed precursor powder. These particles are later vaporized by 
suitable heating and thermal processing is not relied upon for initial 
separation from the packed powder. Particle size can be altered by varying 
the grinder rotor speed. Generally, as smaller particles are preferred, 
the rotor speed is maintained at the highest available speed. As the 
velocity of the probe or probes determines precursor flow rate, the 
present apparatus uses one set of control variables, i.e., probe velocity 
or velocities, to regulate composition and precursor delivery rate 
independently. 
Although the present invention has been described with reference to 
specific details, it is not intended that such details should be regarded 
as limitations upon the scope of the invention, except as and to the 
extent that they are included in the accompanying claims.