Plasma jet system

A plasma jet CVD system includes gas injectors and a stand-off ring. The gas injectors have outlet holes preferably flared to approach the expansion angle of the injected jet, thereby keeping the holes substantially free from entrained atomic hydrogen. The injectors are arranged counter-rotational to the swirl of the primary jet, providing a more uniform mixture of hydrocarbons and atomic hydrogen. The stand-off ring provides vents for cooler gases to enter the nozzle, thereby decreasing the overall temperature of the injectors and decreasing the temperature gradient experienced by the injectors, thereby preventing injector cracking. In addition the vents reduce shear, thereby increasing jet velocity and increasing the deposition rate for the coating. In addition, a new method of injector design permits optimal mixing characteristics to be obtained across various recipes whereby the ratio of the mass flux of the primary flow of the jet to the mass flux of the injected flow from the downstream injectors is kept constant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates broadly to a chemical vapor deposition system. More 
particularly, this invention relates to the injectors and the relationship 
between the injectors and the nozzle in a plasma jet chemical vapor 
deposition system for producing diamond films. 
2. State of the Art 
The utility for high quality thin diamond films for various applications is 
well known. Superior physical, chemical, and electrical properties make 
diamond films desirable for many mechanical, thermal, optical and 
electronic applications. For example, diamond has the highest 
room-temperature thermal conductivity of any material, a high electric 
field breakdown (.about.10.sup.7 V/cm), and an air stable negative 
electron affinity. These properties make possible high power, high 
frequency transistors and cold cathodes, which cannot be made with any 
semiconductor other than diamond. 
One method for producing thin diamond films is by using a chemical vapor 
deposition (hereinafter `CVD`) system. In CVD systems, a mixture of 
hydrogen and a gaseous hydrocarbon, such as methane, is activated and 
contacted with a substrate to produce a diamond film on the substrate. The 
hydrogen gas is disassociated into atomic hydrogen, which is then reacted 
with the hydrocarbon to form condensable carbon radicals including 
elemental carbon. The carbon radicals are then deposited on a substrate to 
form a diamond film. Some CVD methods use a hot filament, typically at 
temperatures up to 2000.degree. C., to provide the thermal activation 
temperatures necessary to bring about the conversion described above. 
Other CVD methods use a plasma jet. 
In some plasma jet methods, hydrogen gas is introduced into a plasma torch 
which produces a hydrogen plasma jet by means of a direct current arc 
(hereinafter "DC arc"), or an alternating current arc ("AC arc"), or 
microwave energy. The plasma torch is hot enough to reduce gases to their 
elemental form. However, the energy level of the plasma jet has a tendency 
to fluctuate. One method of stabilizing the energy level of the plasma is 
to utilize a vortex design in the CVD system. Tangential injection of the 
hydrogen gas into the arc processor may be used to impart the vortex to 
the hydrogen, in gaseous and atomic form. 
The vortex design results in a controlled swirl of plasma. Hydrogen gas is 
introduced into the primary jet and some of the hydrogen gas is thereby 
disassociated into monatomic hydrogen. The hydrogen (in both elemental and 
molecular states), swirling according to the swirl of the plasma, is 
forced through a downstream injector system which introduces jets of 
hydrocarbon needed to react with the elemental hydrogen to form diamond 
films. 
Referring to FIG. 1, a prior art DC arc plasma jet system 10 is shown. The 
assembly includes a hydrogen gas inlet 12, a cathode 14, a cylindrical 
chamber 16 having cylindrical walls 17, an anode 18, downstream injectant 
ports 20a, 20b, a gas injection disc 22 having a plurality of 
radially-positioned injectors 24a-24h (shown only with respect to 24a and 
24b for purposes of clarity), and a nozzle 26 directed toward the 
substrate. The hydrogen gas enters the hydrogen gas inlet 12 and is heated 
to a partial plasma state by an arc across the cathode 14 and the anode 
18. The arc is controlled by solenoids (not shown) surrounding the 
chamber. The tangential injection of the hydrogen contains the plasma and 
imparts the vortex swirl to the plasma. Downstream, hydrocarbon injectant 
and carrier hydrogen gas enter through the injectant ports 20a, 20b into 
the gas injection disc 22, and out of the injectors 24a, 24b where the 
injectant mixes and reacts with the hydrogen plasma, resulting in a 
mixture of molecular hydrogen, atomic hydrogen and carbon radicals which 
exits through the nozzle 26. 
FIG. 2 illustrates the gas injection disc 22 provided with radially aligned 
injectors 24a-24h. Each injector is aligned along a radius formed from the 
periphery of the ring to the center of the ring. Referring to FIG. 3, with 
reference to one injector 24a, the injector is a substantially cylindrical 
bore having three portions: a relatively large diameter inlet hole 30, a 
tapered frustoconical portion 32, and a relatively small diameter outlet 
hole 34. 
There are several known problems associated with state of the art plasma 
jet systems. For example, various challenges and problems have been 
encountered with the hydrocarbon injectors. With reference to FIG. 3, a 
first shortcoming of the prior art injectors is that when using high 
enthalpy, high energy rate recipes, the injector outlet holes clog. One 
potential cause of the clogging is that the expanding injected jets 
entrain atomic hydrogen from the primary jet, bringing atomic hydrogen 
into the injectors and forming diamond or diamond-like carbon deposits at 
the outlet hole. 
Another problem associated with the injectors of a plasma jet system is 
that when using a vortex stabilized arc engine, the vortex has a 
detrimental effect on downstream mixing of the injected hydrocarbon gas. 
The outer swirl of the vortex consists mainly of molecular hydrogen in 
equilibrium between the centrifugal force of the swirl and the resultant 
static pressure gradient throughout the jet. The atomic hydrogen, with 
half the mass of molecular hydrogen, is drawn towards the center of the 
swirling jet. Radially injected hydrocarbon, being many times heavier than 
molecular hydrogen, is forced to the outermost portion of the swirling 
jet, resulting in a non-uniform mixture of hydrogen and hydrocarbons. 
Consequently, a diamond film produced from a non-uniform mixture may have 
a slow growth rate and poor quality. 
An additional problem with the use of high enthalpy, high energy rate 
recipes is that the injectors are subject to excessive heating and are 
subject to high thermal gradients caused by non-uniform cooling of the 
injectors at shutdown. The excessive heating and thermal gradient cause 
the injectors to crack and may further contribute to injector hole 
clogging. Referring back to FIGS. 1 and 2, two possible causes for the 
excessive heating of the injectors 24a-24h are direct impingement of 
recirculated arc gas on the bottom face 22a of the gas injection disc 22, 
and conduction from the nozzle 26 which is heated by the recirculated gas. 
Furthermore, the mixing of downstream injected hydrocarbon gas with a 
primary flow of swirling hydrogen is a key consideration affecting film 
growth rate. The flow of hydrocarbon injectant out of the injector is 
often optimized for mixing with a given flow of hydrogen only by trial and 
error. However, once a specific injector has been found to give optimum 
results for a given primary flow of hydrogen it would be desirable to be 
able to design a hydrocarbon injector which provides the same level of 
mixing at a different flow rate of injectant through the injector without 
entering into a new trial and error process. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a plasma jet 
injection system having an improved injection system for coating diamond 
films on a substrate. 
It is another object of the invention to provide an injection system which 
has injectors having a reduced likelihood of clogging. 
It is a further object of the invention to provide an injection system 
which has injectors having a reduced likelihood of cracking. 
It is an additional object of the invention to provide an injection system 
which has uniform mixing of primary jet hydrogen and downstream jet 
hydrocarbons. 
It is also an object of the invention to provide a method for designing 
injectors permitting a predetermined flow of injectant. 
In accord with these objects which will be discussed in detail below, a 
plasma jet CVD system includes a hydrogen gas inlet into a containment 
chamber, a cathode at the upper portion of the chamber, an anode with a 
constricted diameter relative to the chamber and located at the lower 
portion of the chamber, a gas injection disc having injectors in 
non-radial alignment, a vented stand-off ring, and a nozzle directed 
toward the substrate. The hydrogen gas enters the hydrogen gas inlet and 
is heated into a plasma jet by an arc across the cathode and the anode. A 
tangential injection of the hydrogen gas into the containment chamber 
contains the plasma and imparts the vortex swirl to the plasma. An 
injection port introduces hydrocarbon injectant into the injectors of the 
gas injection disc. According to one aspect of the invention, the 
injectors are arranged such that they inject hydrocarbon 
counter-rotational to the vortex swirl. This novel arrangement provides a 
uniform mixture of hydrocarbon with the hydrogen, resulting in a reactant 
mixture of atomic hydrogen and carbon radicals. According to another 
aspect of the invention, each injector has an outlet hole preferably 
having a flared exit, which keep the holes substantially free from 
entrained atomic hydrogen. The reactant mixture flows through the 
stand-off ring and out of the nozzle, where the reactant mixture is 
deposited onto a substrate located in the fluid path of the nozzle. The 
stand-off ring effectively creates vents between the gas injection disc 
and the nozzle, connecting the nozzle to the gas injection disc by four 
small posts arranged at 90.degree. around the ring. Ambient gas is 
permitted to enter through the vents. The stand-off ring allows the 
flowing reactant mixture to draw cooler gases into the nozzle and thereby 
decreases the formation of recirculation cells at the bottom face of the 
injector. This prevents hot arc gas from impinging on the injector, and 
thereby reduces the injector temperature. 
In addition, a new method of injector design permits optimal mixing 
characteristics to be obtained across various hydrogen/hydrocarbon 
injectant recipes whereby the ratio of the mass flux of the primary flow 
of the hydrogen jet to the mass flux of the injected flow of hydrocarbon 
from the downstream injectors is kept constant. 
Additional objects and advantages of the invention will become apparent to 
those skilled in the art upon reference to the detailed description taken 
in conjunction with the provided figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 4, a plasma jet system 100 for a plasma jet CVD system 
includes a hydrogen gas inlet 112 for the passage of hydrogen gas into a 
containment chamber 116 having an inner wall 117. A cathode 114 is located 
at the upper portion of the chamber 116. An anode 118 having a constricted 
diameter relative to the chamber is located at the bottom portion of the 
chamber. The hydrogen gas enters the hydrogen gas inlet 112 and is heated 
into a plasma jet by an arc across the cathode 114 and the anode 118. The 
arc is controlled by a solenoid (not shown) surrounding the containment 
chamber. The tangential injection of the hydrogen gas and the applied 
magnetic field contain the plasma and impart a vortex to the plasma. 
Adjacent the anode 118 is a gas injection disc assembly 119, through which 
hydrocarbon and carrier hydrogen gas is injected into the plasma swirl, 
forming a reactant mixture of hydrocarbon and hydrogen. The reactant 
mixture flows through a stand-off ring 126 and out through a nozzle 128, 
where a reactant mixture of hydrocarbon and hydrogen is directed toward a 
substrate 199 located in the path of the nozzle. 
Referring to FIGS. 5 and 5A, the gas injection disc assembly 119 includes a 
gas injection disc 120 and a gas injection disc mounting ring 124. The gas 
injection disc 120 has a throat 121 and a plurality of injectors 122a-122h 
organized in an arrangement which is counter-rotational to the swirl of 
the jet, as explained below. Two injection ports 125a, 125b in the gas 
injection disc mounting ring 124 introduce hydrocarbon injectant into the 
injectors 122a-122h, and the injectant is thereby sprayed 
counter-rotationally to the swirl of the hydrogen jet. 
Referring to FIGS. 6, 6A, and 7, the longitudinal axis of each injector 
122a-122h is angled off-axis from the radius which runs from an inlet 
132a-132h of the injector to the center axis of the gas injection disc 
120. The direction of the angular offset is such that the injectors 
122a-122h will stream hydrocarbon injectant at an angle opposing the swirl 
of the primary jet (indicated by the arrow in FIG. 6), reducing the vortex 
and providing a more uniform reactant mixture of atomic hydrogen and 
hydrocarbon. As shown with respect to injector 122h, the offset is 
preferably 30.degree. counter-rotational to the swirl. Compared to 
radial-type injectors, this design significantly increases the amount of 
injectant reaching the center of the jet compared to prior art radial 
injectors. Consequently, at the same flow rates, diamond growth rates have 
been found to increase by 25% compared to radial-type injectors. In 
addition, as seen in FIG. 6A, the injectors 122a-h are also preferably 
angled toward a bottom surface 127 of the gas injector disc 120 by 
5.degree. from a line normal to the center axis of the gas injector disc 
120. 
Turning to FIG. 8, in a second embodiment (with numbers increased by 100 
referring to like parts of the first embodiment) the injectors 222a-h may 
be kept in radial arrangement with only the outlet holes 236a-h 
counter-rotationally angled off-axis. 
Referring to FIG. 9, in third embodiment (with numbers increased by 200 
referring to like parts of the first embodiment) the injectors 322a-h may 
be organized alternatingly in radial 322a, 322c, 322e, 322g and 
counter-rotational 322b, 322d, 322f, 322h arrangements. 
While the first embodiment of the invention having all injectors arranged 
to stream injectant counter-rotational to the swirl of the vortex is most 
preferable, it will be appreciated that each alternate embodiment offers 
an increase in the uniformity of mixing over the prior art and 
consequently provides better results than prior art systems. 
Turning now to FIG. 10, in accord with another aspect of the invention, 
each injector 122a-122h (shown with respect to one injector 122a) is a 
substantially cylindrical bore preferably including three portions: a 
relatively large diameter inlet hole 132a, a frustoconical portion 134a, 
and a relatively smaller diameter outlet hole 136a having a flared exit 
138a. The flared exit 138a is flared to match the expansion angle of the 
jet of hydrocarbon exiting the injector 122a. As a result, the boundary 
layer between the injected jet and the exit hole 138a is reduced and 
migration of atomic hydrogen into the outlet hole 136a is reduced. A flare 
of 15.degree. is preferable, based on the observed expansion angle of the 
injected jet and also having been demonstrated effective through 
experiment. However, other angles above 0.degree. and up to and including 
45.degree. will also provide improved performance over the prior art. 
Additionally, as the prior art injectors show evidence of clogging at the 
outlet hole, enlarging the outlet hole increases the useful lifetime of 
the injector because much more buildup would have to occur before the 
nominal outlet hole size would be reduced. 
The injectors 122a-122h (222a-222h, 322a-322h) can have an outlet hole 
having a diameter determined according to a new method of design. Once a 
first injector has been found by trial and error to supply a flow of 
hydrocarbon injectant into a primary jet of hydrogen such that an optimal 
level of mixing of hydrocarbon and hydrogen results, a new injector can be 
designed for a different recipe of hydrocarbon and hydrogen while 
maintaining the optimal level of mixing even though a different flow of 
hydrocarbon injectant is used. It has been discovered that if the new 
injector is designed to keep the mass flux ratio the same as that with the 
first injector, the new injector will provide the same optimal mixing 
level as that provided by the first injector. For instance, if for a given 
primary jet flow Q.sub.p an optimal level of mixing has been attained for 
a downstream injectant flow Q.sub.s when using an injector with an outlet 
hole having a diameter D.sub.1, an optimal mixing level will also be 
obtained for a new injectant flow Q.sub.sn at the same primary jet flow 
Q.sub.p if the outlet hole diameter is changed such that the new diameter 
D.sub.2 is determined by D.sub.2 =D.sub.1 (Q.sub.sn /Q.sub.s).sup.1/2. 
Utilizing the new injector design method, it will be appreciated that once 
an optimally performing injector has been obtained it will be a simple 
operation to design an injector for an alternate downstream injectant flow 
by changing only the dimension of the outlet hole diameter. 
Turning to FIGS. 11 and 11A, a stand-off ring 126 is provided. The 
stand-off ring is a high temperature alloy ring, preferably made of TZM Mo 
alloy, which includes a planar surface 148 on a first side and four posts 
152a-152d offset by 90.degree. around the ring on a second side. Referring 
back to FIG. 4, the standoff ring 126 is coupled to the gas injection disc 
120 and the nozzle 128 such that the planar surface 148 abuts the gas 
injection disc 120 and the posts 152a-152d are seated against the nozzle 
128. The spaces between the posts 152a-152d act as vents 154a-154d, and 
the flow of reactant mixture exiting the nozzle draws cooler ambient gas 
through the vents into the nozzle. 
It will be appreciated that the stand-off ring 126 serves three cooling 
purposes. First, the stand-off ring prevents direct conduction of heat 
from the nozzle 128 to the injectors 122a-h. Second, the vents 154a-154d 
allow cooler gas to be drawn into the injector/nozzle area, cooling the 
injectors 122a-h and creating a layer of cool gas over the inside wall 158 
of the nozzle 128. Third, the stand-off ring 126 decouples the gas 
injection disc 120, which has a relatively small surface area, from the 
nozzle 128, which has a relatively large surface area, so that upon 
cooldown the injectors 122a-h are subject to a decreased temperature 
gradient. Stand-off rings from 1/16 inch to 5/8 inch have been found to 
result in a gas injection disc temperature drop of approximately 
200.degree. C. when used with high heat hydrogen/hydrocarbon recipes. 
It will also be appreciated that the stand-off ring 126 increases the 
deposition rate of the diamond film. The hot gas leaving the throat 121 of 
the gas injection disc 120 and entering into the larger volume of the 
nozzle 128 creates a low pressure region at the top of the nozzle (the 
Venturi effect). The low pressure region normally draws cooler gas in from 
the bottom of the nozzle. The cool gas being drawn up into the nozzle 
creates a shear region around the hot jet, causing it to spread. By 
introducing vents, the cool gas can now be drawn from the top of the 
nozzle and the amount of shear is reduced. As a result, the jet does not 
expand as much and therefore travels with greater velocity until it 
interacts with the boundary layer above the substrate. Because of the 
increased velocity, the boundary layer thickness is reduced, which results 
in an increased deposition rate. 
There have been described and illustrated herein a plasma jet CVD system 
having several novel design elements and also a novel method of designing 
a hydrocarbon injector for an arc processor. While particular embodiments 
of the invention have been described, it is not intended that the 
invention be limited thereto, as it is intended that the invention be as 
broad in scope as the art will allow and that the specification be read 
likewise. Thus, while eight injectors have been disclosed with respect to 
the gas injection disc, it will be appreciated that fewer or more 
injectors may be used as well. Furthermore, while a 30.degree. angle is 
preferable for the counter-rotational angle for the injectors, it will be 
understood that other angles more than 0.degree. and less than 90.degree. 
can be used. Also, while the stand-off ring has been disclosed to have a 
height of from 1/16 inch to 5/8 inch, it will be appreciated that a 
stand-off ring having another width may also be used. In addition, while 
several embodiments of the gas injection disc have been disclosed, wherein 
each embodiment shows an alternate organization of the injectors, it will 
be appreciated that a gas injection disc may be used which incorporates a 
combination of the illustrated arrangements. Moreover, it will be 
appreciated that while the invention has been disclosed with respect to a 
DC arc plasma jet system incorporating several novel features, it will be 
recognized that another type of plasma jet system, i.e., radio frequency 
or microwave, may utilize the novel features disclosed herein. 
Furthermore, a plasma jet system may be designed which selectively 
incorporates any one or more of the novel features. It will therefore be 
appreciated by those skilled in the art that yet other modifications could 
be made to the provided invention without deviating from its spirit and 
scope as so claimed.