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Retarding Effect of HCl on Rate of Chemical Vapor Deposition of
TiN from TiCl4-H2-N2
E. Zimmermann, T. Wang, Ch.-Ho Yu, and D. Neuschützz
Lehrstuhl für Theoretische Hüttenkunde, Rheinisch-Westfälische Technische Hochschule Aachen, D-52056 Aachen, Germany
The deposition rate of TiN was measured in a hot-wall chemical vapor deposition reactor using TiCl4-H2-N2(-HCl) gas mixtures
at 1 bar total pressure and 950-1470 K. With respect to HCl, the reaction order was found to be 23. From the experimental results,
a reaction model was derived that assumes (Reaction I) TiCl4(g) 1 1/2H2 r TiCl3(g) 1 HCl to be the rate-determining reaction in
the homogeneous gas phase, and (Reaction III) Ti(ad) 1 1/2N2 r TiN(s) to be the slow heterogeneous step. The heterogeneous
reaction preceding Reaction III, which may be described as (Reaction II) TiCl3(g) 1 3/2H2 5 Ti(ad) 1 3HCl, was assumed to be
sufficiently fast to attain equilibrium. Reaction II accounts for the strongly retarding effect of HCl on the rate of TiN deposition.
By proper adaptation of the rate constants and the activation energies of Reactions I and III, the numerical simulation led to deposition rates in good agreement with the measured values.
© 2000 The Electrochemical Society. S0013-4651(99)08-016-7. All rights reserved.
Manuscript submitted August 5, 1999; revised manuscript received February 21, 2000.
Chemical vapor deposition (CVD) frequently is applied to deposit titanium nitride on cutting tools (for its erosion and corrosion
resistance)1 and on a variety of parts (for its decorative color).
The kinetics of TiN deposition from the gas phase containing
TiCl4, H2, and N2 have been investigated for more than 20 years. In
early papers,2,3 the deposition rate was proportional to the partial
pressure of TiCl4 in the feed gas, but later, a rate maximum was reported with increasing p(TiCl4); i.e., at higher p(TiCl4) beyond the
maximum, the TiN deposition rate decreased again.4-8 Since partial
pressures of reactive species are likely to change inside the hot-wall
reactor due to chemical reactions, their values at the sample may be
different from those in the feed gas. Therefore, the feed gas values
are designated p°(i). The reaction order with respect to N2 was usually 0.5.2,4,8,9 When the gas flow rate was varied, the deposition rate
increased with the flow rate and then reached a plateau, indicating
that, above a critical gas flow, mass transport in the gas phase had no
noticeable influence on the deposition rate of TiN.3,4,10 From the
temperature dependence, several authors also concluded that chemical reactions, rather than gas diffusion, were rate determining since,
at least below 1273 K, the apparent activation energies were higher
than 100 kJ/mol.3,4,8-10 Only one investigation4 reports on the retarding influence of HCl added to the feed gases. Later papers neither
extended these investigations nor took the HCl effect into consideration when discussing possible reaction mechanisms.
Thus, the present work was carried out to determine experimentally the retarding influence of HCl, either added to the precursor gas
mixture or formed in the reactor before the gases reach the sample.
Using the measured deposition rates, a reaction model then was
developed and the deposition rates simulated to validate the model.
In a preceding investigation,11 we had determined similarly the influence of HCl on the rate of SiC deposition from methyltrichlorosilane/hydrogen mixtures.
Experimental
The deposition rate of TiN was measured in a hot-wall reactor at
a total pressure of 1 bar in the temperature range of 950 to 1470 K.
The mass increase of the substrate, which was a 10 3 50 3 1 mm
alumina platelet evenly coated with TiN, was continuously recorded
using a magnetic levitation balance. The TiN coating excluded
nucleation problems from the investigation of deposition kinetics.
The feed gases and their partial pressure ranges were TiCl4 (0.2313.6 hPa), H2 (33-500 hPa), N2 (6.7-500 hPa), HCl (0-21 hPa),
and Ar (balance). Most of the measurements were carried out at an
H2/N2 ratio of 1, at 1193 K, and using a gas flow rate of 2.5 slm
(standard liters per minute). Figure 1 shows the experimental setup
with reactor, furnace, thermobalance, and gas supply, as previously
z
E-mail: [email protected]
used in kinetic investigations on the deposition of SiC,11,12 pyrocarbon,13 and hexagonal BN.14 The reactor tube had 28 mm inside diam
and was composed of 78% Al2O3 and 22% SiO2. The substrate was
connected to the magnetic coupling device by means of a thin alumina rod, which was kept free of TiN deposit by flushing a small
argon stream downward through a thin tube surrounding the rod. The
substrate was placed into the tube furnace (the tube length was
700 mm, the heating zone length from room temperature to the temperature maximum was 250 mm) so that its center was in the temperature maximum at typical flow rates and gas compositions. The
temperature variations along the substrate axis were within 65 K.
The yellow-gold-colored deposit was analyzed with X-ray diffraction and electron probe microanalysis for its structure and its composition. Only cubic TiN was identified with a molar Ti/N ratio of
about 51/49. Discoloration of the deposit was not noticed at modified test conditions. Since the partial pressure of N2 was always high,
the nitrogen content of the TiN deposited was in all cases at or near
the maximum.
Results
When the deposition rate of TiN was measured from TiCl4-H2-N2
mixtures initially free of HCl as a function of gas flow rate and temperature, results were obtained as shown in Fig. 2. At temperatures
up to 1223 K, the rates approach nearly constant values with increasing flow rates, while at 1323 K the rate neither originates at zero
Figure 1. Experimental setup with hot-wall reactor, thermobalance, and gas
supply system.
Journal of The Electrochemical Society, 147 (6) 2206-2209 (2000)
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Figure 2. Measured deposition rate as a function of the gas flow rate at different temperatures; H2/N2 ratio 5 1.
flow rate nor becomes constant at higher flow rates. Most further
measurements were carried out at 1193 K, and in all cases the gas
flow rate was held constant at 2.5 slm.
In Fig. 3, the same results are plotted vs. the reciprocal temperature with flow rates as the parameter. The influence of the flow rate
becomes more pronounced at higher temperatures. The reason for
this is that the increasing deposition rate leads to a noticeable depletion of the gas phase with respect to the main reactant (TiCl4) while
it flows from the cold inlet to the hot substrate. When the flow rate
is increased, the residence time of the reactant gases before reaching
the substrate is shortened, and the depletion reduced. Figures 2 and
3 show clearly that under the conditions chosen here, the deposition
rates, as measured at the substrate hanging in the maximum temperature zone of the hot-wall reactor, are in the range where depletion
of reactants cannot be neglected. This implies that the formation of
HCl through the reactions occurring in the hot tube must be taken
into consideration at all times.
Figure 4 shows the influence of HCl additions to the feed gas
mixture on the TiN deposition rate at 1193 K and different TiCl4 partial pressures in the feed gas. At initial HCl pressures of >6 hPa, the
apparent reaction order with respect to HCl was 23 (dashed line in
Fig. 4). At lower p°(HCl), the retarding influence of HCl added to
the feed gas became smaller, because the total HCl partial pressure
at the substrate was increasingly determined by the HCl formed
in situ. The curves in Fig. 4 are results of a mathematical simulation
described later.
Figure 4. Deposition rate as a function of HCl partial pressure in the feed gas
at different TiCl4 feed gas pressures; H2/N2 ratio 5 1. Symbols: measured;
curves: simulated.
From the results presented in Fig. 2-4 it is obvious that the deposition rates of TiN measured in a hot-wall reactor cannot be correlated directly to the feed gas partial pressures p°(i). The reactions
taking place before the gas reaches the substrate at the temperature
maximum of the furnace lead to noticeable depletions of reactants
and a rise in the concentration of the product gas HCl, both strongly
influencing the actual deposition rate at the substrate. A tentative
reaction model and rate equations for the deposition of TiN were
derived from the results of Fig. 2-4 and taken as the basis for the
numerical simulation of local deposition rates to be calculated as a
function of feed gas partial pressures, temperature, and flow rate.
The model then was validated by comparison of the simulated rates
with the experimental results.
Reaction Model and Rate Equations
The most striking dependencies of the deposition rate as a function of feed gas partial pressure have been the rate maximum with
varying p°(TiCl4) and the strong retardation of the deposition with
rising HCl content. Both observations have been reported previously.3-7 However, the magnitude of the negative reaction order of HCl,
nHCl 5 23, was measured here for the first time. Any reaction model
must be able to reproduce these two phenomena with sufficient
accuracy.
A number of models were conceived, tested, and found incompatible with these experimental observations. It became clear that a
model with only one rate-determining reaction step would not
describe the observed dependencies. The reaction model would have
to consist of two subsequent rate-determining steps, one in the homogeneous gas phase and the other in the adsorbed state on the surface of the reactor walls and the substrate.
The reaction model finally selected contains as the first step the
decomposition of TiCl4 in the presence of hydrogen in the gas phase.
As was shown by Teyssandier and Allendorf15 by kinetic simulations, TiCl3 1 HCl are the predominant decomposition products of
TiCl4 in the presence of hydrogen, the approach of equilibrium being
fast (10 ms) at 1500 K and slow (102-104 s) at 1000 K. We may conclude that in our case [temperatures around 1200 K, p°(TiCl4) 1001000 Pa], the TiCl3 formation in the gas phase (Reaction I), is still
slow. Its rate jI is assumed to be given by Eq. 2
Reaction I
Figure 3. Measured deposition rate as a function of temperature for different
gas flow rates; H2/N2 ratio 5 1.
TiCl4(g) 1 1/2H2 r TiCl3(g) 1 HCl
[1]
jI 5 d[TiCl3]/dt 5 kI[TiCl4][H2]1/2
[2]
where the brackets denote concentrations and kI is a temperaturedependent rate constant.
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Reaction I takes place in the hot-wall reactor as soon as the temperature of the gas phase is sufficiently high. It leads to a depletion
of TiCl4, while the effect on hydrogen is small because of the relative abundance of H2. Reaction I produces equal amounts of TiCl3
and HCl.
A fraction of the TiCl3(g) formed is assumed to adsorb on the
reactor wall and the substrate surface, establishing an adsorption
equilibrium. The model implies that the adsorbed species involved
are present in dilute concentrations, i.e., far from saturation, so that
their concentrations are proportional to the respective partial pressures in the gas phase. Due to the presence of hydrogen, the adsorbed TiCl3(ad) is then, stepwise, reduced to finally form elemental
Ti(ad). These reactions are assumed to be relatively fast and therefore to attain equilibrium
Reaction II
TiCl3(g) 1 3/2H2 5 Ti(ad) 1 3HCl
KII 5
[Ti(ad)][HCl]3[TiCl3(g)]21[H2]23/2
[3]
[4]
The adsorbed elemental Ti finally reacts with nitrogen to form solid
TiN (Reaction III)
Reaction III
Ti(ad) 1 1/2N2 r TiN(s)
[5]
This reaction is taken to be the rate-determining step on the surface.
Its rate jIII is
jIII 5 d[TiN]/dt 5 kIII[Ti(ad)][N2]1/2
[6]
When [Ti(ad)] is replaced by kII, the rate equation becomes
jIII 5 k9III[TiCl3(g)][H2]3/2[N2]1/2[HCl]23
[7]
where k9III is another temperature-dependent rate constant.
Gaseous TiCl4 also adsorbs at the substrate and is eventually
reduced to elemental Ti and nitrided to form TiN. However, a parallel reaction path based on TiCl4(ad) cannot be split into two consecutive rate-determining steps and is not suited to reproduce both the
rate maximum and the HCl dependence. Therefore, it is concluded
that the contribution of such a parallel reaction path to the overall
deposition rate of TiN can only be relatively small under the conditions of our experiments.
The reaction model and the rate equations given above were used
to simulate local TiN deposition rates and to reproduce the experimentally determined rates as a function of feed gas partial pressures,
temperatures, and flow rates.
For the numerical simulation, the rate constants ki in Eq. 2 and 7
were assumed to consist of a pre-exponential factor ki8 and an exponential term containing the temperature dependence
ki 5 ki8 exp(2Ai/RT)
TiCl3(g) concentration just determined. Finally, a mass balance was
made with respect to remaining TiCl4, TiCl3, H2, N2, and HCl, which
were then transferred to the adjacent upper cell, where these concentrations represented the starting values for the next calculations. When
the cell at the temperature maximum was reached, the deposition rate
of TiN on the wall (which then also means: on the substrate) was calculated as a function of the real local concentrations of the gas species
involved. Since these real local concentrations are unambiguously related to definite sets of feed gas partial pressures, the simulated deposition rates on the substrate could be plotted vs. the feed gas partial
pressures and thus be compared directly to the experimental values.
Discussion
In Fig. 4-8, measured deposition rates as functions of feed gas partial pressures po(TiCl4), po(HCl), po(N2), and po(H2) and of temperature are compared with the results of the mathematical simulations.
In Fig. 4, the deposition rates determined experimentally at 1193
K as a function of HCl partial pressures in the feed gas are compared
to simulated results obtained on the basis of the rate equations, Eq. 2
and 7. The general agreement is good. At po(HCl) $ 1000 Pa, the
rate is strongly retarded by HCl, the reaction order with respect to
HCl reaching a value of nHCl 5 23. At lower HCl concentrations in
the feed gas, the curves become flatter and finally horizontal below
p°(HCl) 5 100 Pa. The reason for this is that the actual HCl partial
pressure at the substrate is the sum of the HCl formed while the gas
flows to the substrate plus the HCl contained in the feed gas. Above
po(HCl) 5 1000 Pa, the contribution of the newly formed HCl
becomes negligible, while below po(HCl) 5 100 Pa, the HCl in the
feed gas can be neglected.
In Fig. 5, the influence of the TiCl4 partial pressure in the feed
gas on the deposition rate of TiN is shown at 1193 K and at different HCl feed gas concentrations. As reported in the literature,4,5,10
the rates increase with po(TiCl4), pass through a maximum, and decrease again. Rising po(HCl) shifts the curves to lower rates and the
rate maxima to higher po(TiCl4). The calculated rate curves coincide
relatively well with the experimental results. At low po(TiCl4) and
low po(HCl), the measured rate of TiN deposition is nearly proportional to p°(TiCl4). With increasing feed gas concentration of TiCl4,
the rising deposition rate leads to a depletion in TiCl4 and to an
increase in HCl. Both effects eventually slow the deposition rate and
finally combine to decrease the measured rate.
In Fig. 6 and 7, the influence of the hydrogen and the nitrogen partial pressures in the feed gas on the deposition rate is shown at constant
temperature and different TiCl4 concentrations in the feed gas. These
po(TiCl4) values were selected to have two measurements each on the
[8]
with ki8 and Ai as adaptable parameters. Optimum agreement with the
experimental results was achieved when the activation energies were
assumed to be AI 5 150 and AIII 5 340 kJ/mol.
In the simulation, laminar vertical gas flow was assumed in the
hot-wall reactor with fast horizontal mass and heat transfer. A parabolic temperature profile along the furnace axis was used. The substrate was placed at the maximum of this temperature profile. The
tube reactor was divided into about 150 disk-shaped cells with constant inner tube diameter and variable height to accommodate in
each cell the same number of gas molecules at constant pressure and
changing temperature. The mean of the residence time of the flowing gas was selected as the reaction time. The lowest cell was placed
at 600 K, the highest cell at the temperature maximum.
The calculations started with the lowest cell taking the feed gas
concentrations as input values. From the gas flow rate and the temperature, the residence time of the gas in the cell was calculated, and
with the rate equation for Reaction I, the TiCl3 concentration in that
cell at the end of the corresponding residence time. For the rate jIII of
the subsequent heterogeneous reactions, the area of the tube reactor
wall belonging to this cell was taken as the reaction surface. The concentration [TiCl3(g)] in Eq. 7 was taken to be proportional to the
Figure 5. Deposition rate as a function of TiCl4 partial pressure in the feed
gas at different HCl feed gas pressures; H2/N2 ratio 5 1. Symbols: measured;
curves: simulated.
Journal of The Electrochemical Society, 147 (6) 2206-2209 (2000)
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Figure 6. Deposition rate vs. feed gas hydrogen partial pressure at different
feed gas TiCl4 pressures (Ar is balance). Symbols: measured; curves:
simulated.
Figure 8. Deposition rate vs. reciprocal temperature at different feed gas
TiCl4 pressures; H2/N2 ratio 5 1. Symbols: measured; curves: simulated.
equations with the numerical values selected are a useful approximation of the deposition process of TiN from TiCl4-H2-N2-HCl gas
mixtures at 1 bar total pressure in a hot-wall CVD reactor.
Conclusions
The experiments confirmed that HCl has a strongly retarding
influence on the rate of TiN deposition from TiCl4-N2-H2 gas mixtures in the temperature range of 950 to 1470 K. The reaction model
developed as a basis for the mathematical simulation of local deposition rates contains this influence in the form of a negative reaction
order, nHCl 5 23. With an optimized set of four parameters, kI, k9III,
AI, the AIII, simulations delivered deposition rates that were in good
agreement with measured results as a function of feed gas partial
pressures and temperature. Therefore, it is concluded that the model
chosen here serves as a realistic basis for further and more detailed
investigations into the reaction mechanisms involved and for the
mathematical simulation of local deposition rates of TiN on substrates of more complex geometries.
Figure 7. Deposition rate vs. feed gas nitrogen partial pressure at different
feed gas TiCl4 pressures (Ar is balance). Symbols: measured; curves: simulated.
left and the right side of the rate maximum in Fig. 5. The results of both
measurements and calculations show that, at low po(TiCl4), the deposition rates increased only little with po(N2) and po(H2), while at high
po(TiCl4), the increases were more pronounced. The general agreement between experiments and simulations appears fairly acceptable.
Finally, Fig. 8 shows the deposition rate as a function of temperature at po(HCl) 5 0 and three different TiCl4 concentrations in the
feed gas. These three feed gas TiCl4 values were selected to represent widely different conditions with respect to the rate maximum of
Fig. 5, upper curve: p°(TiCl4) 5 66 Pa is situated far to the left of the
rate maximum, 446 Pa is at the maximum, and 950 Pa is somewhat
to the right of it. With rising temperature, the rates first display normal behavior with a typical Arrhenius rate increase, but above a certain temperature, the measured rates drop below the straight lines
and even decrease below values at lower temperatures. This effect is
caused by the depletion of the gas phase with respect to TiCl4 (and
TiCl3) at the substrate. With lower p°(TiCl4), the effect of depletion
appears earlier, i.e., at lower temperatures. In fact, the precursor may
be completely consumed by the time the gas reaches the substrate
when the feed gas concentration and the flow rate are low and the
temperature in the reactor tube is sufficiently high.
The simulated rate curves in Fig. 8 are in good agreement with
the measured rates. This supports the idea that the model and the rate
Acknowledgment
The authors gratefully acknowledge financial support by the
Deutsche Forschungsgemeinschaft (DFG) within the Collaborative
Research Center 289 “Thixoforming.” Dedicated to Professor Günter Marx, TU Chemnitz, on the occasion of his 60th birthday.
Rheinisch-Westfälische Technische Hochschule Aachen assisted in meeting the publication costs of this article.
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