Note: Descriptions are shown in the official language in which they were submitted.
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RD-18331
S~ AT~ ~QQhI~G ME~
This invention relates to the chemical vapor depo-
S sition of diamond, and mor~ particularly to an apparatus foruse in such deposition.
Various methods are known for the synthetic pro-
duction of diamond. In particular, the deposition of diamond
coatings on substrates to produce cutting and abrasive tools
is known.
One class of methods developed in recent years for
synthetic diamond deposition consists of the chemical vapor
deposition (hereinafter sometimes "CVD") methods. For a
general summary of various diamond deposition methods
~ncluding CVD methods, reference is made to ChemL~al &
En~ eering Ne~ QL, 24-39 (May l5, 1989).
In the CVD methods, a mix~:uxe of hydrogen and a
hydrocarbon gas such as methane is thermally activated and
passed lnto contact with a substrate. The hydrogen gas i5
converted to atomic hydrogen which reacts with the hydro-
carbon to form elemental carbon, which deposits on the
substrate in the form of diamond. Many of the CVD diamond
coating methods, hereinafter referred to as "filament"
~5 methodsr employ one or more resistance heating units includ-
ing heated wires or filaments, typically a~ temperatures of
at least 2000 C, to provide the high activation temperatures
at which these conversions take place.
Various problems have been encountered in filament
methods of CVD diamond deposition, and they h~ve inhibited to
a considerable extent the usefulness thereof on a commercial
scale. For example, it is difficul~ to create conditions
under which the deposition rate of diamond is high enough to
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be commercially feasible. Numerous methods employing a
horizontal configuration of the substrate(s) and filaments,
with helically wound filaments, have been disclosed, but for
the most part the deposition rate afforded thereby is low.
Also, the high substrate temperature usually
produced is incompatible with a high rate of diamond deposi-
tion. At the filament temperatures which produce atomic
hydrogen in the necessary proportions, the substrate reaches
a temperature higher than about 1000-C. Optimum substrate
temperatures for diamond deposition are in the range of 900-
1000 C, and thus the substrate temperatures generated are too
high for rapid deposition.
Finally, numerous problems with the filaments have
been observecl. Substantial expansion thereof occurs at the
aforementioned high temperatures; this is, in part, thermal
expansion but to a greater extent, in the case of the
tungsten filaments frequently employed, is the result of
carburization, forming tungsten carbide with concomitant
expansion to the extent of about 20%. If such expansion is
uncontrolled, filament breakage and/or contact between the
filaments and substrate can occur, either of which will
terminate diamond deposltion.
The damage resulting from carburization is
particularly severe when a helical filament configuration ls
employed. Carburi7ation is accompanied by crackiny of the
tungsten carbide, often in a spiral configuration which can
cause deformation of a helically wound filament in unpredict-
able directions. The deposition operation is thus usually
prematurely aborted, much short of the 30-40 day period
normally required for the deposition of a diamond film of
0.5-1.0 mm. in thickness.
The present invention is based on several
discoveries of conditions which promote optimum diamond
deposition on substrates. In the first place, it has been
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found that nucleation and growth of diamond is maximized when
the reactor configuration includes two substantially parallel
substrates on opposite sides of a plurality of filaments,
rather than substrates horizontally configured, or vertically
in a square or cylindrical array or one substrate between
banks of filaments. In the second place, long filament life
is attained with linear filaments curving in a single plane
to accommodate thermal expansion and expans~on caused by
carburization. In the third place, employment of at least
one heat sink to regulate substrate temperature permits
control of the deposition reaction to optimize diamond growth
rates.
The invention provides an apparatus for improved
production of CVD diamond coatings by the filament method.
The features of the invention permit closer control of fila-
ment configuration than has previously been possible, as well
as control of substrate temperature for maximization of
deposition rate. One result is an increase in filament life,
which enablés the deposition process to be continued until a
coating of substantial thickness is produced.
Accordingly, the invention is directed to apparatus
for deposition of diamond on substrates by chemical vapor
deposition, comprising:
a closed reaction chamber having at least one gas
inlet and at least one exhaust means, said chamber being
capable of being maintained at a pressure below atmospheric;
support means for supporting said substrates in ;
said chamber parallel to each other and spaced apart to
permit gas flow between said substrates;
resistance heating means comprising a plurality of
vertically extending linear, electrically conductive
filaments prestressed to curve in a single plane
substantially equidistant from and parallel to said
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substrates, each of said filaments being fixedly secured at
each end to a pair of fixed electrodes; and
substrate cooling means situated adjacent one of
said substrates on the opposite side from said filaments.
The invention will be described in detail with
reference to the drawings, in which:
FIGURE l is a schematic side view of an
illustrative apparatus according to the present invention;
FIGURE 2 is a cutaway view of said apparatus on the
line 2-2 of FIGURE l; and
FIGURE 3 is a further cutaway view on the line 3-3
of FIGURE 2.
Referring now to the drawings, there are depicted
the interior Eeatures of a CVD diamond deposition unit
lS according to the present invention. All of said features are
enclosed in a reaction chamber (not shown) which is air-tight
and thus capable of being maintained at reduced pressure and
is fitted with a suitable gas inlet and an exhaust port. All
portions of the apparatus which are present in the reaction
cham~er are constructed of suitable heat-resistant materials,
as necessary to withstand filament temperatures on the order
of about 2000~C and substrate temperatures up to about
lOOO C. Quartz is an illustrative non-conductive heat-
resistant material suitable for this purpose.
The features of the apparatus and associated
articles which are shown in the drawings include a pair of
substrates 1, which are normally planar although they may be
gently curved. Any substrate material suitable for diamond
deposition thereon may be employed; examples of such
materials are boron, boron nitride, platinum, graphite,
molybdenum, copper, aluminum nitride, sllver~ iron, nickel,
silicon, alumina and silica, as well as combinations thereof.
Metallic molybdenum substrates are particularly suitable
under many conditions and are often preferred. Supports 2
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serve as support means for holding substrates l in position
parallel to each other and at a suitable spacing for
deposition to take place.
The apparatus also contains resistance heating
means compxising two electrodes and a number of vertically
extending linear, electrically conductive filaments or wires
(hereinafter generically designated "filaments"~, and
otherwise being of conventional design and circuitry. The
material of which said filaments are comprised is not
critical, any material known in the art as suitable for this
purpose being acceptable. Illustrative materials are
metallic tungsten, tantalum, molybdenum and rhenium; because
of its relatively low cost and particular suitability,
tungsten is often preferred. Filament diameters of about
0.2-l.0 mm. are typical, with about 0.8 mm. frequently being
preferred.
The filaments are located between said substrates,
parallel to and substantially equidistant therefrom.
Distances from filaments to substrates are generally on the
order of 5~10 mm.
In the drawings, fixed electrode 4 is grounded and
is fixedly attached to a number of said filaments, one of
which is desi~nated 5. Since a plurality of filaments and
associated structure are present, reference thereto
hereinater and in the drawings will be to only one; it
should be understood that the total number thereof is not
critical to the invention.
Insulator 6 separates electrode 4 and its base from
conducting element 7. The latter is conductively connected
to a second fixed electrode 9, fixedly attached to the other
end of filaments.
Each of filaments 5 is prestressed to curve in a
single plane substantially equidistant from and parallel to
substrates l. Prestressing of all filaments may be
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conveniently achieved by use of a suitable jig of
conventional construction, configured to hold bcth electrodes
and the attached filaments with prestressing being
accomplished by pegs. Once the filaments are suitably
S prestressed, the entire assembly may be removed from the jlg
and mounted in the CVD apparatus.
It is highly desirable to maintain substrates 1 at
temperatures in the range of about 900-1000-C, since within
this rang~ minimum reaction occurs between the hydrogen pre-
sent in the gas mixture and the elemental carbon formed fromthe hydrocarbon therein; thus, said elemental carbon remains
available to deposit as diamond at a high growth rate on the
substrate. Absent any provisions for independently
controlling substrate temp~rature, said tem~erature
frequently exceeds 1000 C and the diamond growth rate
decreases substantially.
According to the invention, the desired temperature
control is achieved by substrate cooling means comprising at
least one and preferably two heat sinks 18. Each heat sink
is typically made of metallic copper and cooled by attached
serpentine tubing 21 (also usually oE copper) fitted with
cooling water inlet and outlet 19 and 20, rPspectively. The
distance of heat sink 18 from substrate 1 is adjusted by a
conventional screw mechanism control:Led by crank 22, and said
distance and the fLow rate of water through the tubing are
adjusted, either manually or automatically via suitable
sensors, to maintain the substrate within the desired
temperature range.
In operation, the reaction chamber of the apparatus
of this invention is maintained at a pressure up to about 760
torr, typically on the order of 10 torr. A ~lxture of hydro-
gen and a hydrocarbon, most often methane and generally
present in an amount up to about 2% by weight based on total
gases, is passed into the chamber and a current is passed
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through the electrodes and filaments to heat the filaments to
a temperature of at least about 2000~C. With the substrate
configuration employed, gas diffusion between the substrates
and in contact with the filaments promotes excellent
nucleation and growth of diamond particles.
The heat sink(s) is maintained at a distance from
the substrate and water passage through the tubing associated
therewith is maintained at a rate to provide a substrate
temperature in the range of about 900-lOOO~C, most often
about 950 C. At such temperatures, diamond growth rate
approaches its highest value.
During the CYD operation, filaments 5 undergo
thermal expansion and expansion due to carburization. By
reason of their prestressed condition, however, such
expansion merely ca~ses them to bend further in the direction
of prestressing. Thus, they are not subject to failure as a
result of contacting each other or the substrates. Using
this combination of elements, it is possible to grow diamond
films with thicknesses up to about l mm., or even greater on
occasion, within a time span of 30-90 days without filament
breakage or other untoward events occurring.
