Chlorine-Activated Diamond ChemicalVapor Deposition
Chenyu Pan, C. Judith Chu, John L. Margrave,* and Robert H. Hauge*
Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77251
ABSTRACT
A novel method of producing atomic hydrogen and the active carbon species necessary for diamond chemical vapor
deposition (CVD) has been demonstrated. This method starts with the generation of atomic chlorine from the thermal
dissociatidn of molecular chlorine in a resistively heated graphite furnace at temperatures from 1300-1500~
Atomic
hydrogen and the carbon precursors are subsequently produced through rapid hydrogen abstraction reactions of atomic
chlorine with molecular hydrogen and hydrocarbons at the point where they mix. It was found that the quality of the
diamond deposits depends on both substrate temperatures and H2/C12 mole ratios. Substrate temperatures are found to be
-150 lower than for a hydrogen/hydrocarbon hot filament system for similar growth rates.
Numerous C V D methods have been developed and employed in the synthesis of polycrystalline diamond. 1-7It is
generally accepted that the C V D of diamond requires high
energy to activate carbon species and to generate atomic
hydrogen. Currently, there are five major C V D diamond
processes, namely, hot filament, combustion, microwave
plasma, R F plasma, and plasma arc deposition. All of these
processes involve gas temperatures greater than 2000~ In
hot filament C V D one has to consider the possible incorporation of the filament metal into diamond. Also, in the hot
filament, and especially the plasma activation methods,
one is likely to create active nitrogen species which are then
responsible for nitrogen incorporation in diamond. Nitrogen impurities are k n o w n to affect electrical, thermal, and
optical properties of diamond. 8 In this paper, w e present a
novel method which allows generation of atomic hydrogen
and thus C V D diamond at lower gas process temperatures.
This is accomplished by firstgenerating atomic chlorine by
thermally dissociating molecular chlorine and subsequently mixing the atomic chlorine with molecular hydrogen downstream. Atomic chlorine is k n o w n to rapidly react
with molecular hydrogen 9 so that the ratio of atomic hydrogen to atomic chlorine is quickly established by the ratio of
H2 to HC1 through the following steady-state equilibrium,
i.e.,Cl + H2 +~ HCI + H.
D u e to the fact that the CI-Cl bond is m u c h weaker than
the H - H bond (243 vs. 436 kJ/mel), atomic chlorine is m u c h
more easily generated than atomic hydrogen by thermal
dissociation from their respective precursors. This is readily seen from Fig. i which shows that at 17 Torr 8 5 % dissociation of Cl2 is achieved at 1500~ while 2900~ is required
for similar dissociation of H2. At 5 Torr, the percentage of
dissociation of Cl2 and H2 is increased to 9 5 % at the respective temperatures. Graphite is an excellent container for
chlorine dissociation since graphite is thermodynamically
inert to chlorine because CCI4 completely decomposes to
graphite and atomic chlorine at these temperatures.
A simple compact C V D reactor containing a graphite furnace has been built, which allows for separate introduction
of hydrogen and chlorine with subsequent mixing of the
heated gases in the front of the furnace. Diamond deposits
on platinum substrates were obtained over a variety of substrate temperatures and H2/CI2 mole ratios. Parameters
controlling the quality of diamond deposits were investigated with the quality of the diamond deposition characterized by R a m a n spectroscopy.
Experimental Details
The chlorine-activated CVD (CA-CVD) reactor is 12 in.
long and 4.5 in. in diam. As shown in Fig. 2, it consists of a
water-cooled graphite furnace, sample holder, and doublewall glass chamber. The low pressure of the system is provided by a rotary pump and Roots blower combination in
which a fluorocarbon pump fluid is used to handle chlorine-containing gases. The combined pumping system is
capable of pumping at 180 cfm. The connections between
the pump and the reactor are made with 1 and 2 in. diam.
* Electrochemical Society Active Member.
3246
PVC tubing. A trap containing sodium hydroxide solution
is installed at the pump exhaust to remove HCI and Cl2.
The graphite furnace, a key component of the CA-CVD
reactor, has been designed and constructed to generate an
atomic chlorine flux. Because of its excellent resistance to
a chlorine environment, especially at elevated temperatures, high density graphite has been used in the construction of the graphite furnace. The heated cell of the furnace
is a tube with a 3/8 in. outer diam. and 1/32 in. wall thickness and 23/8 in. in length. The chlorine exits from a 1/16 in.
center hole. The heated cell fits snugly over another
graphite tube which is screwed into a water-cooled copper
electrode. A 3/16 in. Teflon tube runs from this graphite
tube through the copper electrode. The Teflon tube
prevents direct contact with and hence corrosion of the
copper electrode by the CI2 gas. Inside the heated graphite
cell, there is a 5/32 in. diam. graphite rod insert which
provides additional surface heating of the C12 as it flows
through the cell. At the upstream end of the graphite red,
four holes act as C12 inlets so that Cl2 gas entering the cell
is directed toward a hot cell wall before flowing along the
space between the cell wall and the center rod. Temperatures measured by an optical pyrometer sighted on the
front face of the heated cell and the graphite rod were
found to be same. The radiation shield is furnished with
three concentric thin wall graphite tubes. The innermost
tube, which is connected to the outer copper electrode,
functions as an electrical current return route for the
heated graphite cell. In the space between the outer and the
two inner graphite radiation shields, mixtures of H2 and
carbon species such as CH 4 flow downstream
and are
forced by a graphite cap, which fits over the outer radiation
shield, to turn toward the center of the furnace where they
mix with the atomic chlorine gas exiting the heated cell
orifice. The opening of the graphite cap is 1/8" in diam. The
r
0.4
rr
/2
0.2
~
5(18
~
~
I000
J
i500
j
E/"r
~.
2800
~
~
2500
3080
Clz at 5 T~
H2 at I7 T o r t
H2 at 5 Torr
35fl8
400g
Temperature CC)
Fig. 1. Equilibrium calculations of tho dissociation of molecular
chlorine and hydrogen at pressures of 5 and 17 Torn"
a The Equil Package-Thermoehemica] Inlormation and Equilibrium Calculations, KSG Associates, Inc., 1990.
J. Electrochem. Soc., Vol. 141, No. 11, November 1994 9 The Electrochemical Society, Inc.
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J. Electrochem. Soc., Vol. 141, No. 11, November 1994 9 The Electrochemical Society, Inc.
3247
Fig. 2. Schematic diagram of
the chlorine-activated CVD diamond reactor, a: Heated graphite
cell; b: graphite rod; c: graphite
radiation shields; d: graphite
cap; e: water-cooled copper electrodes; f: Teflon tube; g." Teflon
plug; h: graphite electrodes; i:
water-cooled copper leads; j: water-cooled double-wall glass
chamber.
entire furnace assembly is mounted on a Teflon plug which
fits into the 3 in. inner diam. of the glass chamber.
A platinum substrate is mounted about 5 mm away from
the graphite cap on graphite electrodes which are connected to a pair of water-cooled copper leads. The Cu leads
are wrapped with a heat shrinkable tubing to prevent corrosion from chlorine species. They are fixed on a Teflon
flange which also contains a 1 in. diam. centered window
for in situ growth studies with a Fizeau fringe monitor. The
graphite electrodes allow the substrate to be heated to the
desired temperatures. The substrate temperature is measured by a Pt vs. Pt-13% Rh thermocouple spot-welded to
the middle of the Pt substrate.
Results
The performance of the furnace with respect to dissociating Cl2 and the subsequent generation of atomic hydrogen
were qualitatively investigated with the use of a i0 ~ m
diameter Pt vs. Pt-13% R h thermocouple. Temperature
measurements with thermocouples offer a simple w a y to
qualitatively and sometimes quantitatively determine gas
phase H atom concentrations) ~ After radiant and gas
conduction heating, which are usually relatively small, are
taken into account, the remaining temperature rise can be
primarily attributed to atom-atom recombination on the
thermocouple surface.
The graphite furnace was resistively heated under vacu u m to approximately 1500~ which typically required
700 W. The operation temperature range of the furnace is
typically 1300-1500~ where the dissociation of C12 varies
from approximately 5 0 % to 8 5 % at 17 Torr pressure. CI~
gas was then introduced into the heated graphite cell.Conductive heat transfer due to the introduction of the Cl2 at a
system pressure of 6 Torr was found to be negligible w h e n
compared with the temperature rise due to atom-atom re1200
]10O
1000
G
?...
900
Y
800
700
6O0
combination. Figure 3 shows the thermocouple temperature rise as the amount of molecular chlorine is increased.
Initially one sees a rapid rise in temperature with chlorine.
However, at higher chlorine flow rates, the temperature
appears to become independent of chlorine flow rate. The
initial rise clearly results from chlorine atom recombination and the magnitude of the temperature rise indicates
appreciable dissociation of molecular chlorine. The leveling effect at higher chlorine flow rates, we suspect, is due to
a surface saturation or boundary layer effect which puts an
upper limit on the number of chlorine atoms which can
undergo recombination on the limited surface area of the
thermocouple.
Currently we believe the efficiency of
atomic chlorine production is the same for all flow rates.
This is supported by an analogue measurement, also shown
in Fig. 3, where a large excess of molecular hydrogen is
mixed with the atomic chlorine stream. In this case atomic
chlorine is quantitatively converted to atomic hydrogen.
The thermocouple temperature rise shows a linear dependence on the chlorine flow rate, which indicates that increased atomic hydrogen production is occurring at the
higher chlorine flow rates. This would not be the case if the
production of the atomic chlorine is saturating at the
higher C12 flow rates.
Addition of small amounts of HCI to a hot filament CVD
diamond process has been shown to lower substrate temperatures for equivalent deposition rates.1~ It is therefore of
interest to study both the effects of different H2/CIz mole
ratios and substrate temperatures on the quality of diamond deposits. Detailed carbon concentration dependence
studies will be presented in a subsequent paper. Typical
diamond deposition conditions are shown in Table I. A
SPEX Raman spectrometer with 500 ~m slit width was
employed to study the quality of the depositions. Excitation was provided by means of the 488 nm line of an argon
ion laser with an output power in the range of i00 to
500 mW.
Two series of experiments were carried out. In a series of
H2/C12 ratio studies, the substrate temperature, H2 and CH4
flow rates were kept at 760~ 1140 and 1.0 seem, respectively. Only the CI2 flow rate was varied to obtain different
ratios of H2/C12. All the depositions were run for 2 h. The
Raman spectra of deposits at different H2/C12 ratios are
shown in Fig. 4. The diamond peak at 1332 cm -1 in each
series was brought to the same peak height for simplicity of
comparison. As shown in Fig. 4, the quality of the diamond
deposit decreases as the ratio of H2/C12 decreases from 30 to
CI 2 o n l y
C~ 2 m~x~d wt~h 114~ ~
500
400
~
0
'
Table I. Typical deposition conditions.
R2
'
lO
20
30
40
50
60
C l 2 F l o w Rate ~ s c c m )
Fig. 3. Measurements of thermocouple temperature rise with respect to CI~ flow rates. The CI2 was passed through the graphite
furnace heated at -1500~
In CI2 only and CI2 mixed with
1140 sccm H2 runs, the system pressures were mostly at 5 and
17 Torr, respectively.
Hydrogen gas flow
CH 4 gas flow
CI2 gas flow
Substrate temperature
Furnace temperature
Substrate to furnace distance
Chamber predssure
Substrate material
i000-2000 sccm
1.0 sccm
50-100 sccm
450-950~
1300-1500~
-5 mm
15-20 Tort
Pt
aH2 and CH4 are mixed.
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J. Electrochem. Soc., Vol. 141, No. 11, November 1994 9 The Electrochemical Society, Inc.
3248
7
i
i
i
i
,
i
~"
H2JCI2 ffi 30
850oc.
I
.~-
1250
1350
1450
1550
1650
Frequency Shift (cm -I)
Fig. 4. Raman spectra of deposits at various H2/CI2 mole ratios;
H2= 1140 sccm, CH4 = 1.0 sccm, T~b = 760~ pressure ~ 17 Torr.
15 until the diamond peak is replaced by the two amorphous carbon bands at the ratio of 10. This indicates that
there is a m i n i m u m ratio of I-IJC12 required to grow diamond in the CA-CVD process. However, it is likely that this
m i n i m u m ratio will be dependent on the substrate temperature with lower ratios possible at lower substrate temperatures. Recent studies have demonstrated substantial diamond growth at a substrate temperature of 470~ with a
higher chlorine to hydrogen ratio. At the substrate temperature of 720~ it appears that the highest quality diamond
films are grown at H2/Cl~ ratios equal to or larger than 30.
The drop in diamond quality with a decreasing H2/CI2
ratio is surprising in thai the amount of atomic hydrogen
produced is expected to increase with increased flow of
chlorine. Howevel, at the same time the concentration of
atomic chlorine is also expected to increase at a rate faster
than that of atomic hydrogen. At low chlorine concentrations, the atomic hydrogen concentration increases approximately linearly with the chlorine flow while the
atomic chlorine concentration increases approximately as
the square of the chlorine flow rate.
The increased concentration of atomic chlorine will lead
to an increased concentration of methyl chloride and thus
the monochloromethyl
radical (CH2CI). Additional evidence for the importance of the CH2CI radical is inferred
from the fact that diamond growth rates for the CA-CVD
reactor are similar to those for hot filament reactors even
though the methane concentration (<0.1%) is approximately one order of magnitude smaller. This is supported
by previous studies which show that this radical is a very
effective carbon source for diamond growth.
We thus speculate that loss of diamond quality with low
HJCI2 ratios is due to a rapid deposition of carbon via the
monochloromethyl
radical at higher chlorine contents.
This situation is similar to the case when methane is present at high concentrations in other CVD diamond reactors.
In a series of substrate temperature studies, the H2, Cl2,
and CH4 flow rates were held at 2000, 66.6, and 1.0 sccm,
respectively. Figure 5 shows the evolution of the Raman
spectra of diamond deposits at elevated substrate temperatures. As was the case for I.I2/CI2 ratios, substrate temperatures also have a significant effect on diamond quality. The
diamond quality for substrate temperatures from 550 to
850~ appears similar with respect to the presence of amorphous carbon but an underlying fluorescence is seen to decrease as the substrate temperature increases. However, it
is not known whether this fluorescence is diamond related
or results from the substrate. The presence of amorphous
carbon at 950~ with chlorine present is in sharp contrast
to diamond CVD without the presence of chlorine. Clearly
the presence of chlorine has lowered the temperatures for
both diamond growth and the deposition of amorphous
carbon.
6SOoc
1250
1350
1450
1550
1650
Frequency Shift (cm "~)
Fig. 5. Raman spectra of deposits at various substrate temperatures; H2 = 2 0 0 0 sccm, H2/CI2 = 30, CH4 = 1.0 sccm, pressure 17 Torr.
The appearance of amorphous carbon at a 950~ which
is lower than found for other hydrogen-based CVD processes may be caused by the presence of the active
monochloromethyl radical and/or chloromethane (CH3C1).
At 950~ it is possible that these species undergo surface
thermal pyrolysis which enhances carbon deposition.
Initial growth rate measurements indicate that diamond
growth rates in the chlorine-activated diamond CVD process are as large as or larger than the microns per hour
typically found in hot filament reactors. This is supported
by the fact that diamond films grown in 1 h have visible
thickness under a low magnification microscope. More
quantitative measurements of growth rates are underway.
A scanning electron microscope (SEM) picture of a diamond film deposited at H2/C12 = 30 and T~ub = 750~ is
shown in Fig. 6. The po]ycrystalline diamond crystals are
clearly seen to possess both square and triangular facets.
The polycrystalline diamond film appears white to transparent and continuous to the eye except for a small portion
which peeled off due to the mismatch of the thermal expansions of diamond and the platinum substrate.
The lower gas process temperatures required for CACVD reduce the power requirements of the reactor and,
more importantly, lessen the possibility of nitrogen incorporation into the diamond as the result of the presence of
small amounts of dinitrogen invariably present in reactant
gases. As shown in Fig. 7, thermodynamic equilibrium calculations predict that small amounts of N2 are largely converted to HCN in typical gas mixtures of H2 and CH4. The
chemistry of HCN is likely to be closely analogous to C~H2,
Fig. 6. Scanning electron microscope (SEM) picture of diamond film
deposited on Pt at H2/CI2 = 30, T, ub = 750~ and 17 Torr for 2 h.
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J. Electrochem. Soc., Vol 141, No. 11, November 1994 9 The Electrochemical Society, Inc.
has the advantage of not venting any chlorocarbons which
may be formed in the process. Similar gas handling requirements currently exist in the deposition of silicon from
the reduction of SIC14 with H2. As a result, commercially
available HC1 gas scrubbers have been developed.
Finally it is not currently known to what degree various
substrates will experience chlorine etching processes in the
presence of hot 2-10% HCI/H~ mixtures. If substrate
etching is observed, however, it is expected that a thin carbide, oxide, or nitride layer would selwe as an initial protective layer.
1.0 10`5
9
-o--
N HI
/
cN
~
3249
\
Conclusions
0.0
. . . .
500
~
. . . . .
1000
i
. . . .
1500
L
2000
J.
2500
3000
.
3500
.
.
4000
Temperature (~
Fig. 7. Equilibrium calculations of male fractions of nilmgen species
in 0.1% of N2 in H2 and CH4(H2/CH4 = 100) at 20 Torr.
which suggests it is the primary chemical route leading to
nitrogen incorporation into diamond when grown by current CVD processes. While the existence of HCN is highly
favored by high gas processing temperatures, the mechanism which leads to HCN formation likely depends on initial
dissociation of N2 to atomic nitrogen. This dissociation occurs readily in plasma based diamond CVD. Production of
HCN in hot filament systems, however, likely involves surface chemical mechanisms on the hot filament which lead
to N2 dissociation. This is evidenced by a study which has
shown that the presence of N2 in a hot flament reactor seriously degrades diamond morphology.~6 In our CA-CVD diamond growth reactor preliminary studies in large excesses
of N2 have shown no chemical effects due to N2.
Discussion
This research has clearly demonstrated a new and potentially useful diamond CVD process. Its useful characteristics involve lower gas processing temperatures (13001500~
lower substrate temperatures (500-900~
and
carbon-only heating elements. These characteristics suggest that a chlorine-activated CVD process (CA-CVD) may
be the best choice for production of electronic grade diamond. Because of the all-carbon nature of the furnace and
the low gas process temperatures involved, it appears that
this technique can more effectively avoid the incorporation
of metal and nitrogen impurities into diamond, and thus it
should be useful in the production of electronic grade
diamond.
The growth of diamond at lower substrate temperatures
typically lessens the strain induced by thermal expansion
differences between diamond and the substrate. The lower
temperatures may also permit the direct coating of lower
melting ceramics, glasses, and metals. The ability to utilize
graphite heating elements to indirectly dissociate Ha removes the possibility of metal contamination which may
occur with hot filaments or dc arc electrodes.
Efforts directed at scaling the CA-CVD
process to large
area growth must deal with questions of heater design, and
fluid dynamics. In addition, questions related to the costs
of chemicals and reactors as well as the cleanup and handling of corrosive chemicals will be important to the commercial production of diamond by a CA-CVD
process.
We currently believe that scaleup can be achieved
through a design which utilizes large area resistively
heated graphite tubes, similar in concept to linear or distributed low pressure flames. The cost of chlorine is relatively low since industrial production of elemental chlorine
is a major chemical industry.
For large scale reactors it would seem most efficient to
recycle the hydrogen since the ratio of hydrogen to chlorine
is likely to be in the 10-50 range depending on the application. In this ease the removal of the 2 to 10% HC1 must be
carried out in the recycling process. Recycling the gas also
We have demonstrated substantial low temperature diamond deposition by a novel chlorine-activated CVD process (CA-CVD). Atomic chlorine and subsequently atomic
hydrogen were generated by dissociating molecular chlorine in a graphite furnace near 1500~ and subsequently
mixing the atomic chlorine with molecular hydrogen. Carbon precursors were also generated from the exchange reaction with atomic chlorine. It is suggested that the allgraphite furnace and the lower process temperatures
involved favor the formation of diamond deposits free of
metals and nitrogen. The quality of the diamond deposition
was found to be dependent on both Hz/C]2 ratios and substrate temperatures. Polycrystalline diamond films were
deposited over a variety of the H2/CI2 ratios and substrate
temperatures. The growth rates were estimated to be on the
order of a few microns per hour.
Acknowledgments
The authors acknowledge the financial support of the
Office of Naval Research (Grant No.: N00014-92-J-1701),
the National Science Foundation, and the Robert A. Welch
Foundation.
Manuscript submitted Oct. 11, 1993; revised manuscript
received June 21, 1994.
Rice University assisted in meeting the publication costs
of this article.
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