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Analysis and modelling of tetraethoxysilane pyrolysis
Journal of Analytical and Applied Pyrotjsis, 13 (1988) 141-149 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
141
ANALYSIS AND MODELLING OF TETRAETHOXYSILANE PYROLYSIS
B. DELPERIER Laboratoire des Materiaux en Couches Minces, U.A. 445 CNRS, E. N.S.C. T., 118 route de Narbonne, 31077 Toulouse Cedex (France) C. VINANTE Institut du Genie Chimique, Chemin de la Loge, 31078 Toulouse Cedex (France) R. MORANCHO * Laboratoire des Matdriaux en Couches Minces, U.A. 445 CRNS, E.N.S.C.T., 118 route de Narbonne, 31077 Toulouse CPdex (France) (Received February 16th, 1987; accepted April lSth, 1987)
ABSTRACT The thermal behaviour of tetraethoxysilane was investigated in a chemical vapour deposition reactor at atmospheric and reduced total pressure. By-products of the reaction were analysed by gas chromatography. The measured conversion rates were used to build up a model which accounted for both homogeneous and heterogeneous reactions. The homogeneous reaction could be described by the quasi-molecular theory while the heterogeneous reaction followed a Langmuir scheme. Plug flow provided a hydrodynamic description. All these chemical and hydrodynamical assumptions provided a model which permitted a comparison between the relative importance of the two kinds of reactions, and an understanding of the influence of the main experimental parameters: temperature, total pressure and reactant concentration. Pyrolysis; tetraethoxysilane.
INTRODUCIION
The deposition of thin films on solid surfaces by an heterogeneous chemical reaction between gaseous reactants and a hot substrate [chemical vapour deposition (CVD)] is finding increasing applications in industry. For example, silica deposits are used either in the electronics field as insulating material or in the metallurgical industry as corrosion protection devices. 0165-2370/88/$03.50
0 1988 Elsevier Science Publishers B.V.
142 TABLE 1 Experimental Substrate
results for SiO, deposit Experimental
from tetraethoxysilane
conditions
Pressure (mbar)
Temperature
(TEOS) Apparent activation energy (kcal/mol)
Ref.
(“C)
Si
Pt = 0.3 P TEOS = o.2 PO, = 0.05
700- 800
46.5
3
Si
P, =l-3 TTEOS= 0.5-2
650-800
45
4
Steel
P, = 1000 TEOS = 0.3% (v/v)
600-900
52
5
MO
P=lOOO
720-840
Silica can be formed follows: SiH,Cl,
2
from dichlorosilane
+ 2N,O e SiO, + 2HCl+
N,
and nitrogen
protoxyde
[l] as
(900 o C)
The same deposit is obtained at lower temperatures from pyrolysis of alkoxysilanes [2], the most widely used being tetraethoxysilane (TEOS) [3-51 (Table 1). Brown et al. [5] produced protective coatings on steels using TEOS at atmospheric pressure while Adams and Capio [3] and Huppertz and Engl [4] realized thin dielectric silica films in conventional low pressure CVD reactors and gave growth “rules”. These authors 13-51 work in the same temperature range and showed that the limiting step of the solid growth was the surface reaction. As a consequence the growth rate was given by a Langmuir-Hinshelwood equation: R = k0 exp(-WRT)Prnos/(l + Wrnos) (1) where R = growth ‘rate; PTEOS= TEOS partial pressure; k,, k, = rate constants; and E = apparent activation energy. A better knowledge of the physico-chemical mechanisms involved and their kinetics appears to be necessary for a complete description of TEOS pyrolysis. So we have made a chromatographic analysis of the gaseous phase in an attempt to complete the picture given by this previous work. The measured conversion rates cannot be explained by the heterogeneous reaction alone. Both homogeneous and heterogeneous phenomena must be taken into account to describe the complex pyrolysis yield.
APPARATUS
AND ANALYTICAL
PROCEDURE
TEOS thermal decomposition was conducted in a hot wall low pressure CVD laboratory reactor (Fig. 1). Liquid TEOS was contained in a thermo-
143
ROTAMETERS
HOT WALL
REACTOR
GAS CHROYATOGRAPHV
“2
N2 BUBBLER
Fig. 1. Schematic
FURNACE
diagram
of the low pressure
COLD-TRAP
chemical
PUMP
vapour
deposition
apparatus.
stated bubbler at 0” C. Knowledge of the partial vapour pressure and in the reactor enflow-rates :,‘.lowed control of the gaseous composition trance. The reaction was carried out in a quartz tube of 500 mm length with an inside radius (r) of 12 mm. The tube was heated with one zone furnace which gave a flat temperature profile on a reaction zone length (L) of 100 mm. The reduced pressure was obtained with a mechanical pump protected with a liquid nitrogen-cooled trap. The pressure was measured by a mercury manometer located at the exhaust of the reactor. After reaction, the gases either entered a six-way sampling valve which permitted direct analysis by gas chromatography, or were collected in the trap and analysed by mass spectrometry.
RESULTS
AND MODELLING
Qualitative analysis and mechanism approach For the following experimental conditions: temperature (T) = 50043OO”C, total pressure (P) = 0.16-l bar and TEOS molar fraction (x0) = 1 . 10e4-5.5 . 10e4, direct gas chromatography and mass spectrometry analysis showed that the gaseous exhaust contains TEOS, ethylene, ethanal and ethanol. The main by-products were ethylene and ethanol while ethanal was present in a very small concentration in the gas phase. Typical variations of the chromatography areas of TEOS, ethanol and ethylene as a function of temperature are given in Fig. 2. The minimal decomposition
144 14.0
“0 5
0 .^ “0 7 . a4 E
12.0
10.0 8-O
z
6.0
9 6
4.0
0 s 0’
E v
2.0 o-o
500
550
600
Fig. 2. Chromatography pyrolysis temperature.
650
700
areas of TEOS (0),
750
ethylene
800
(A) and ethanol
(A) as a function
of
temperature of TEOS was at about 520 “C and full pyrolysis occurred around 680°C. For this range of temperatures, the ratio of ethanol and ethylene remained essentially constant; above 700 o C ethylene dominated. The amounts of both products decreased quickly and disappeared between 760 and 780 o C. These results were confirmed by mass spectrometry of the products trapped at liquid nitrogen temperature. These observations, i.e. the low molar fraction of TEOS and the nature of the by-products, suggest that the decomposition scheme is an intramolecular elimination. This scheme has often been found in organometallic thermal decomposition experiments [6]. This led us to use Lindeman’s quasi-molecular reaction theory [7] as a first approach to describe the results. Modelling
The global conversion yield of TEOS was obtained by chromatographic measurements. We assumed the sum of two contributions: first, a heterogeneous reaction which took place on the hot surface (27~~~5) and second, a homogeneous reaction which occurred in the bulk phase ( mr2L). So, using a plug flow model, these assumptions lead, for an elementary reaction volume, to the material balance: F TEOS d&EoS
=
R,&27rr
dL) + &,,,,(~TTT~dL)
and for the overall reaction zone: X TEOS
J0
d xTEOS
&(27V)
+ Rhom( VY2)
=-
L FTEOS
(2)
145
rate and mass flow of TEOS, where Xr,, and FTEos are conversion respectively; R betdescribes the solid growth rate of silica deposit according rate of TEOS decomposition; Rhom can to eq. 1; Rhom is the homogeneous be expressed by application of a quasi-molecular reaction scheme [6] following the differents steps: activation
TEOS + N, Ft TEOS*
deactivation
TEOS*
+ TEOS * TEOS + TEOS
K,,
reaction
TEOS*
+ products
K,
Then the general expression
+ N,
K,, K-i K-,
of Rho,,, is given by:
K~(K~[TEos][N~] + ~,pEosl~) R horn
=
K, + K_,[N,] + K_,[TEOS]
which can be simplified assumed: (i) diluted phase:
in many cases according
to the reaction
conditions
K,K,[TEOS] [N21
R horn
= K,
+K-,[N2]
(ii) diluted
R horn
=
K, K, [TEOS] K -1
(iii) diluted
R horn
=
phase and high total pressure:
phase and low total pressure:
K, [TEOS] [N,]
All the gaseous concentrations can be expressed in terms of the conversion rate and the total pressure, which allows integration of eq. 2, to give the general equation: f( XT,,
3 kc,, k,,
c, P, T, q,,
E, K,, Fmos,
K-1,
K,,
L, R) =O
K-2,
K,, (3)
the heterogeneous reaction, expression (k,,k,, E) describe K_ 1,K,, K_ 2, K3) the homogeneous reaction, (P, T, x0, FTEos)the experimental conditions, (L, R) the geometry of the system and Xr,, the
In
this
( K,,
decomposition rate given by measurements (Figs. 3 and 4). For each experimental temperature and conversion rate, eq. 3 is solved with a Newton’s algorithm. Unknown homogeneous kinetic parameters are calculated using successively assumption (iii) and assumption (ii). The low pressure and dilute phase (iii) was obtained using a total pressure of 0.16 bar and a TEOS molar fraction of 1 . 10P4. The corresponding values for case (ii) for case atmospheric pressure and 5 . lop4 TEOS molar fraction.
146 6.0
2 =
3.0
% 2
2.0
,
5 0e
1.0
600
560
600
660
700
760 TEMPERATURE
800 VW
Fig. 3. TEOS chromatography areas as a function of the decomposition temperature with the following conditions of total pressure and entrance TEOS molar fraction: (a) P = 1 bar, x0 = 5.5.10P4; (b) P = 0.16 bar, x0 =1.10P4.
Experimental pressure results resembled the atmospheric ones (Figs. 3a and 4a) with a translation to higher temperature. Case (iii) led to a determination of K, for each temperature. Arrhenius transformation of calculated values showed (Fig. 5) good agreement with a thermally activated phenomenon. Using a similar procedure, K,/K_, was determined in case (ii). Then, a
5 u,
0.80
-
0.60
-
0.40
-
E 5 8
0.00 600
660
600
660
700
760 TEMPERATURE
800 VW
Fig. 4. TEOS experimental conversion rate and calculated heterogeneous conversion (dashed line) against temperature. Conditions: (a, a’) P =1 bar, x0 = 5.5.10P4; (b, b’) P = 0.16 bar, x,=1.1o-4.
147
2. 1.
5 3 2
O-1
-
-2
-
-3
d
-4 I 0.95
1.05
1.00
Fig. 5. Calculated
1.10
1.20
1.15
K, values against reciprocal
10 3,.K-’
temperature.
global kinetic expression was used in order to calculate the decomposition rate under chosen CVD conditions. This total yield must be compared with the heterogeneous decomposition rate and the importance of the homogeneous contribution with respect to the heterogeneous one can be estimated. The calculated heterogeneous conversion rate is represented in Fig. 4 by the dashed lines and shows that the majority of the decomposition occurred in the bulk phase.
E
1.00
d : jj 0.60
-
fI z’ s 0.60
-
0.40
-
0.20
-
0.00
600
600
700
600
900
TEMPERATURE@C)
Fig. 6. Conversion rate modelling of TEOS pyrolysis. Solid lines: (a) P = 1 bar, x,, = 5. 10m4; (b) P = 0.16 bar, x0 =1.10P4; (c) P = 0.01 bar, x,, =lO-*. Dashed lines: heterogeneous conversion (a’), (b’), (c’).
148
The solid lines in Fig. 6 represent the TEOS conversion rate calculated for homogeneous decomposition occurring in a dilute phase for the following conditions of total pressure and TEOS molar fraction: P = 1 bar,
X0 = 5. 1o-4
(curve a)
P = 0.16 bar,
x0 = 1. 10m4
(curve b)
P = 0.01 bar,
x,, = 1 - lo-*
(curve c)
Dashed lines represent the heterogeneous yield with respect to the same conditions (Fig. 6, curves a’, b’ and c’). At both atmospheric and reduced total pressure, TEOS was nearly completely pyrolyzed in the range 650-750” C. These experimental CVD conditions resulted in a powdered deposit which was generally observed at the reactor exhaust. Better results are expected at low pressure (Fig. 6c, c’): between 700 and 800” C, total conversion was lower than 0.1 and consisted mainly of the heterogeneous reaction.
CONCLUSION
TEOS pyrolysis was analyzed by coupling homogeneous and heterogeneous phenomena. The chemical gas phase behaviour was described satisfactorily by the quasi-molecular theory. The model that was built, with plug flow as the hydrodynamical description, allowed prediction of the global conversion rate and the respective contributions of the two kinds of reaction. So it can be used to choose a convenient set of CVD experimental conditions. The widespread use of low pressure reactors is justified because increased diffusivities of gaseous species allow a better deposit uniformity, even on complex shapes [8]. We have pointed out that at reduced pressures the homogeneous reaction is strongly limited and, as a consequence, undesirable powder formation is avoided. The plug flow assumption, helpful in a first approach, is only suitable in simple kinds of geometry. A further improvement can be made in modelling the low pressure CVD reactor using a complete hydrodynamical description [9,10].
REFERENCES 1 B.M. Kemlage, in J.M. Blocher, Jr., G.E. Vuillard and G. Wahl (Editors), Proc. 8th Int. Conf. Chemical Vapor Deposition, Chantilly, Paris, September 15-18, 1981, The Electrochemical Society, Pennington, NJ, p. 418. 2 C.F. Powell, J.H. Oxley and J.M. Blocher, Jr., Vapor Deposition, Wiley, New York, 1966, p. 391. 3 A.C. Adams and D.C. Capio, J. Electrochem. Sot., 126 (1979) 1042.
149 4 H. Huppertz and W.L. Engl, IEEE Trans. Electron Devices, 26 (1979) 658. 5 D.E. Brown, J.T.K. Clark, A.I. Foster, J.J. McCaroll, MS. Richards, M.L. Sims and M.A.M. Swidzinski, in J.M. Blocher, Jr., G.E. Vuillard and G. Wahl (Editors), Proc. 8th Int. Conf. Chemical Vapor Deposition, Chantilly, Paris, September 15-18, 1981, The Electrochemical Society, Pennington, NJ, p. 699. 6 J.E. Taylor and T.S. Milazzo, J. Phys. Chem., 82 (1978) 847. 7 G. Pannetier and P. Souchay, Cinttique Chimique, Masson, Paris, 1964, p. 83. 8 W. Kern and G.L. Schnable, IEEE Trans. Electron Devices, 26 (1979) 647. 9 C. Vinante, J. Bertrand and J.P. Couderc, in R. Porat (Editor), Proc. 6th European Conf. Chemical Vapour Deposition, Jerusalem, April 1987, p. 42. 10 D.W. Hess, K.F. Jensen and T.J. Anderson, Rev. Chem. Eng., 3 (1985) 174.
141
ANALYSIS AND MODELLING OF TETRAETHOXYSILANE PYROLYSIS
B. DELPERIER Laboratoire des Materiaux en Couches Minces, U.A. 445 CNRS, E. N.S.C. T., 118 route de Narbonne, 31077 Toulouse Cedex (France) C. VINANTE Institut du Genie Chimique, Chemin de la Loge, 31078 Toulouse Cedex (France) R. MORANCHO * Laboratoire des Matdriaux en Couches Minces, U.A. 445 CRNS, E.N.S.C.T., 118 route de Narbonne, 31077 Toulouse CPdex (France) (Received February 16th, 1987; accepted April lSth, 1987)
ABSTRACT The thermal behaviour of tetraethoxysilane was investigated in a chemical vapour deposition reactor at atmospheric and reduced total pressure. By-products of the reaction were analysed by gas chromatography. The measured conversion rates were used to build up a model which accounted for both homogeneous and heterogeneous reactions. The homogeneous reaction could be described by the quasi-molecular theory while the heterogeneous reaction followed a Langmuir scheme. Plug flow provided a hydrodynamic description. All these chemical and hydrodynamical assumptions provided a model which permitted a comparison between the relative importance of the two kinds of reactions, and an understanding of the influence of the main experimental parameters: temperature, total pressure and reactant concentration. Pyrolysis; tetraethoxysilane.
INTRODUCIION
The deposition of thin films on solid surfaces by an heterogeneous chemical reaction between gaseous reactants and a hot substrate [chemical vapour deposition (CVD)] is finding increasing applications in industry. For example, silica deposits are used either in the electronics field as insulating material or in the metallurgical industry as corrosion protection devices. 0165-2370/88/$03.50
0 1988 Elsevier Science Publishers B.V.
142 TABLE 1 Experimental Substrate
results for SiO, deposit Experimental
from tetraethoxysilane
conditions
Pressure (mbar)
Temperature
(TEOS) Apparent activation energy (kcal/mol)
Ref.
(“C)
Si
Pt = 0.3 P TEOS = o.2 PO, = 0.05
700- 800
46.5
3
Si
P, =l-3 TTEOS= 0.5-2
650-800
45
4
Steel
P, = 1000 TEOS = 0.3% (v/v)
600-900
52
5
MO
P=lOOO
720-840
Silica can be formed follows: SiH,Cl,
2
from dichlorosilane
+ 2N,O e SiO, + 2HCl+
N,
and nitrogen
protoxyde
[l] as
(900 o C)
The same deposit is obtained at lower temperatures from pyrolysis of alkoxysilanes [2], the most widely used being tetraethoxysilane (TEOS) [3-51 (Table 1). Brown et al. [5] produced protective coatings on steels using TEOS at atmospheric pressure while Adams and Capio [3] and Huppertz and Engl [4] realized thin dielectric silica films in conventional low pressure CVD reactors and gave growth “rules”. These authors 13-51 work in the same temperature range and showed that the limiting step of the solid growth was the surface reaction. As a consequence the growth rate was given by a Langmuir-Hinshelwood equation: R = k0 exp(-WRT)Prnos/(l + Wrnos) (1) where R = growth ‘rate; PTEOS= TEOS partial pressure; k,, k, = rate constants; and E = apparent activation energy. A better knowledge of the physico-chemical mechanisms involved and their kinetics appears to be necessary for a complete description of TEOS pyrolysis. So we have made a chromatographic analysis of the gaseous phase in an attempt to complete the picture given by this previous work. The measured conversion rates cannot be explained by the heterogeneous reaction alone. Both homogeneous and heterogeneous phenomena must be taken into account to describe the complex pyrolysis yield.
APPARATUS
AND ANALYTICAL
PROCEDURE
TEOS thermal decomposition was conducted in a hot wall low pressure CVD laboratory reactor (Fig. 1). Liquid TEOS was contained in a thermo-
143
ROTAMETERS
HOT WALL
REACTOR
GAS CHROYATOGRAPHV
“2
N2 BUBBLER
Fig. 1. Schematic
FURNACE
diagram
of the low pressure
COLD-TRAP
chemical
PUMP
vapour
deposition
apparatus.
stated bubbler at 0” C. Knowledge of the partial vapour pressure and in the reactor enflow-rates :,‘.lowed control of the gaseous composition trance. The reaction was carried out in a quartz tube of 500 mm length with an inside radius (r) of 12 mm. The tube was heated with one zone furnace which gave a flat temperature profile on a reaction zone length (L) of 100 mm. The reduced pressure was obtained with a mechanical pump protected with a liquid nitrogen-cooled trap. The pressure was measured by a mercury manometer located at the exhaust of the reactor. After reaction, the gases either entered a six-way sampling valve which permitted direct analysis by gas chromatography, or were collected in the trap and analysed by mass spectrometry.
RESULTS
AND MODELLING
Qualitative analysis and mechanism approach For the following experimental conditions: temperature (T) = 50043OO”C, total pressure (P) = 0.16-l bar and TEOS molar fraction (x0) = 1 . 10e4-5.5 . 10e4, direct gas chromatography and mass spectrometry analysis showed that the gaseous exhaust contains TEOS, ethylene, ethanal and ethanol. The main by-products were ethylene and ethanol while ethanal was present in a very small concentration in the gas phase. Typical variations of the chromatography areas of TEOS, ethanol and ethylene as a function of temperature are given in Fig. 2. The minimal decomposition
144 14.0
“0 5
0 .^ “0 7 . a4 E
12.0
10.0 8-O
z
6.0
9 6
4.0
0 s 0’
E v
2.0 o-o
500
550
600
Fig. 2. Chromatography pyrolysis temperature.
650
700
areas of TEOS (0),
750
ethylene
800
(A) and ethanol
(A) as a function
of
temperature of TEOS was at about 520 “C and full pyrolysis occurred around 680°C. For this range of temperatures, the ratio of ethanol and ethylene remained essentially constant; above 700 o C ethylene dominated. The amounts of both products decreased quickly and disappeared between 760 and 780 o C. These results were confirmed by mass spectrometry of the products trapped at liquid nitrogen temperature. These observations, i.e. the low molar fraction of TEOS and the nature of the by-products, suggest that the decomposition scheme is an intramolecular elimination. This scheme has often been found in organometallic thermal decomposition experiments [6]. This led us to use Lindeman’s quasi-molecular reaction theory [7] as a first approach to describe the results. Modelling
The global conversion yield of TEOS was obtained by chromatographic measurements. We assumed the sum of two contributions: first, a heterogeneous reaction which took place on the hot surface (27~~~5) and second, a homogeneous reaction which occurred in the bulk phase ( mr2L). So, using a plug flow model, these assumptions lead, for an elementary reaction volume, to the material balance: F TEOS d&EoS
=
R,&27rr
dL) + &,,,,(~TTT~dL)
and for the overall reaction zone: X TEOS
J0
d xTEOS
&(27V)
+ Rhom( VY2)
=-
L FTEOS
(2)
145
rate and mass flow of TEOS, where Xr,, and FTEos are conversion respectively; R betdescribes the solid growth rate of silica deposit according rate of TEOS decomposition; Rhom can to eq. 1; Rhom is the homogeneous be expressed by application of a quasi-molecular reaction scheme [6] following the differents steps: activation
TEOS + N, Ft TEOS*
deactivation
TEOS*
+ TEOS * TEOS + TEOS
K,,
reaction
TEOS*
+ products
K,
Then the general expression
+ N,
K,, K-i K-,
of Rho,,, is given by:
K~(K~[TEos][N~] + ~,pEosl~) R horn
=
K, + K_,[N,] + K_,[TEOS]
which can be simplified assumed: (i) diluted phase:
in many cases according
to the reaction
conditions
K,K,[TEOS] [N21
R horn
= K,
+K-,[N2]
(ii) diluted
R horn
=
K, K, [TEOS] K -1
(iii) diluted
R horn
=
phase and high total pressure:
phase and low total pressure:
K, [TEOS] [N,]
All the gaseous concentrations can be expressed in terms of the conversion rate and the total pressure, which allows integration of eq. 2, to give the general equation: f( XT,,
3 kc,, k,,
c, P, T, q,,
E, K,, Fmos,
K-1,
K,,
L, R) =O
K-2,
K,, (3)
the heterogeneous reaction, expression (k,,k,, E) describe K_ 1,K,, K_ 2, K3) the homogeneous reaction, (P, T, x0, FTEos)the experimental conditions, (L, R) the geometry of the system and Xr,, the
In
this
( K,,
decomposition rate given by measurements (Figs. 3 and 4). For each experimental temperature and conversion rate, eq. 3 is solved with a Newton’s algorithm. Unknown homogeneous kinetic parameters are calculated using successively assumption (iii) and assumption (ii). The low pressure and dilute phase (iii) was obtained using a total pressure of 0.16 bar and a TEOS molar fraction of 1 . 10P4. The corresponding values for case (ii) for case atmospheric pressure and 5 . lop4 TEOS molar fraction.
146 6.0
2 =
3.0
% 2
2.0
,
5 0e
1.0
600
560
600
660
700
760 TEMPERATURE
800 VW
Fig. 3. TEOS chromatography areas as a function of the decomposition temperature with the following conditions of total pressure and entrance TEOS molar fraction: (a) P = 1 bar, x0 = 5.5.10P4; (b) P = 0.16 bar, x0 =1.10P4.
Experimental pressure results resembled the atmospheric ones (Figs. 3a and 4a) with a translation to higher temperature. Case (iii) led to a determination of K, for each temperature. Arrhenius transformation of calculated values showed (Fig. 5) good agreement with a thermally activated phenomenon. Using a similar procedure, K,/K_, was determined in case (ii). Then, a
5 u,
0.80
-
0.60
-
0.40
-
E 5 8
0.00 600
660
600
660
700
760 TEMPERATURE
800 VW
Fig. 4. TEOS experimental conversion rate and calculated heterogeneous conversion (dashed line) against temperature. Conditions: (a, a’) P =1 bar, x0 = 5.5.10P4; (b, b’) P = 0.16 bar, x,=1.1o-4.
147
2. 1.
5 3 2
O-1
-
-2
-
-3
d
-4 I 0.95
1.05
1.00
Fig. 5. Calculated
1.10
1.20
1.15
K, values against reciprocal
10 3,.K-’
temperature.
global kinetic expression was used in order to calculate the decomposition rate under chosen CVD conditions. This total yield must be compared with the heterogeneous decomposition rate and the importance of the homogeneous contribution with respect to the heterogeneous one can be estimated. The calculated heterogeneous conversion rate is represented in Fig. 4 by the dashed lines and shows that the majority of the decomposition occurred in the bulk phase.
E
1.00
d : jj 0.60
-
fI z’ s 0.60
-
0.40
-
0.20
-
0.00
600
600
700
600
900
TEMPERATURE@C)
Fig. 6. Conversion rate modelling of TEOS pyrolysis. Solid lines: (a) P = 1 bar, x,, = 5. 10m4; (b) P = 0.16 bar, x0 =1.10P4; (c) P = 0.01 bar, x,, =lO-*. Dashed lines: heterogeneous conversion (a’), (b’), (c’).
148
The solid lines in Fig. 6 represent the TEOS conversion rate calculated for homogeneous decomposition occurring in a dilute phase for the following conditions of total pressure and TEOS molar fraction: P = 1 bar,
X0 = 5. 1o-4
(curve a)
P = 0.16 bar,
x0 = 1. 10m4
(curve b)
P = 0.01 bar,
x,, = 1 - lo-*
(curve c)
Dashed lines represent the heterogeneous yield with respect to the same conditions (Fig. 6, curves a’, b’ and c’). At both atmospheric and reduced total pressure, TEOS was nearly completely pyrolyzed in the range 650-750” C. These experimental CVD conditions resulted in a powdered deposit which was generally observed at the reactor exhaust. Better results are expected at low pressure (Fig. 6c, c’): between 700 and 800” C, total conversion was lower than 0.1 and consisted mainly of the heterogeneous reaction.
CONCLUSION
TEOS pyrolysis was analyzed by coupling homogeneous and heterogeneous phenomena. The chemical gas phase behaviour was described satisfactorily by the quasi-molecular theory. The model that was built, with plug flow as the hydrodynamical description, allowed prediction of the global conversion rate and the respective contributions of the two kinds of reaction. So it can be used to choose a convenient set of CVD experimental conditions. The widespread use of low pressure reactors is justified because increased diffusivities of gaseous species allow a better deposit uniformity, even on complex shapes [8]. We have pointed out that at reduced pressures the homogeneous reaction is strongly limited and, as a consequence, undesirable powder formation is avoided. The plug flow assumption, helpful in a first approach, is only suitable in simple kinds of geometry. A further improvement can be made in modelling the low pressure CVD reactor using a complete hydrodynamical description [9,10].
REFERENCES 1 B.M. Kemlage, in J.M. Blocher, Jr., G.E. Vuillard and G. Wahl (Editors), Proc. 8th Int. Conf. Chemical Vapor Deposition, Chantilly, Paris, September 15-18, 1981, The Electrochemical Society, Pennington, NJ, p. 418. 2 C.F. Powell, J.H. Oxley and J.M. Blocher, Jr., Vapor Deposition, Wiley, New York, 1966, p. 391. 3 A.C. Adams and D.C. Capio, J. Electrochem. Sot., 126 (1979) 1042.
149 4 H. Huppertz and W.L. Engl, IEEE Trans. Electron Devices, 26 (1979) 658. 5 D.E. Brown, J.T.K. Clark, A.I. Foster, J.J. McCaroll, MS. Richards, M.L. Sims and M.A.M. Swidzinski, in J.M. Blocher, Jr., G.E. Vuillard and G. Wahl (Editors), Proc. 8th Int. Conf. Chemical Vapor Deposition, Chantilly, Paris, September 15-18, 1981, The Electrochemical Society, Pennington, NJ, p. 699. 6 J.E. Taylor and T.S. Milazzo, J. Phys. Chem., 82 (1978) 847. 7 G. Pannetier and P. Souchay, Cinttique Chimique, Masson, Paris, 1964, p. 83. 8 W. Kern and G.L. Schnable, IEEE Trans. Electron Devices, 26 (1979) 647. 9 C. Vinante, J. Bertrand and J.P. Couderc, in R. Porat (Editor), Proc. 6th European Conf. Chemical Vapour Deposition, Jerusalem, April 1987, p. 42. 10 D.W. Hess, K.F. Jensen and T.J. Anderson, Rev. Chem. Eng., 3 (1985) 174.