1600
I n d . E n g . C h e m . Res. 1988, 27, 1600-1606
Redistribution Reaction of Trichlorosilane in a Fixed-Bed Reactor K. Y. Li* and C. D. Huang Chemical Engineering Department, Lamar University, Beaumont, Texas 77710
The redistribution reaction of trichlorosilane (SiHClJ to dichlorosilane (SiHC12)in a fixed-bed reactor was studied by using six different types of ion-exchange resin catalyst. Effects of temperature, pressure, particle size, reactor length, and flow rate were examined. It has been found that the best condition for the conversion of trichlorosilane is a t 80 " C and 80 psig and using DOWEX MWA-1 as the catalyst. Experimental data indicated that mass-transfer resistances are not important. The kinetic data analysis suggested that this reaction is second order with respect t o trichlorosilane for the forward reaction and is second order overall to dichlorosilane and silicon tetrachloride for the backward reaction. The activation energy is 8.62 kcal/mol for the forward reaction and is 7.08 kcal/mol for the backward reaction. A mechanism with the formation of intermediates, RMe2NHSiC13 and RMe2NHSi2HC1,, was proposed t o explain this redistribution reaction. One of the most important materials for the modern semiconductor industry is single-crystal silicon. Ultrapure single-crystal silicon ingot is obtained from polycrystal silicon through crystal growth. Union Carbide developed a silane process (Taylor, 1987) to produce polycrystal silicon by using chemical vapor deposition (CVD) of silane. This process first converts silicon tetrachloride (STC) to trichlorosilane (TCS) via a hydrogenation reaction. Trichlorosilane is converted to dichlorosilane (DCS) and then to silane through redistribution reactions. Finally, via a pyrolysis reaction, silane is decomposed to polycrystal silicon. Hemlock (1982) claimed that a faster deposition rate could be obtained when dichlorosilane, instead of trichlorosilane, was used in the CVD reactor. Dichlorosilane is obtained from a redistribution reaction of trichlorosilane. Chlorosilanes (silane, dichlorosilane, trichlorosilane, and silicon tetrachloride) have been widely used in epitaxial growth processes to produce integrated circuits. It has been reported (Liaw et al., 1984) that chlorosilanes with higher chlorine content have more serious pattern shift problems but less unwanted reactor wall deposition during high-temperature (900-1350 "C) epitaxial processes. Overall, dichlorosilane was found to be the best material for epitaxial growth of integrated circuits. This study concentrates on the redistribution reaction of trichlorosilane to dichlorosilane in a catalytic fixed-bed reactor. The effects of temperature, pressure, catalyst particle size, flow rate, and reactor length will be examined. The reaction controlling step will be determined, and a kinetic model will be proposed to estimate the rate constants. Finally, a mechanism will be discussed.
Experimental Section Chemicals and Catalysts. All the trichlorosilane used in this study is ACS grade (99.9% purity) with no further purification. The catalysts are ion-exchange resins manufactured from Dow Chemical U S A . The catalysts were heated overnight at a temperature of 80 "C and vacuumed to 20 pmHg to drive the moisture out. Their types and functional groups are listed in Table I. Experimental Setup. The experimental setup consists of a storage cylinder, a flow meter, a pressure gauge, a reactor, an automatic sampling valve, a gas chromatograph, and a scrubbing system, as shown in Figure 1. The whole experimental setup is connected to a vacuum pump and a nitrogen cylinder for the purpose of purging and pressure control. All the material used here is stainless steel.
* Author
to whom correspondence should be addressed.
The TCS storage cylinder has a capacity of 1750 mL which is enough for two experimental runs. A Matheson 602 flow meter is used for the measurement of TCS flow rate. The reactor is a 1/4-in.outside diameter tubing with lengths ranging from 1.625 to 11.75 in. The tubing is packed with catalyst and held by stainless steel screen on both ends. An electrical strip heater is used to heat the reactor. The temperature of the reactor is set by a voltage regulator and monitored by a thermocouple. The variation of the temperature along the reactor has been found to be insignificant. This is because a preheater has been used and a very small heating flux is utilized for the reactor. A six-port on-line sampling valve (Valco E4C6P) with a sampling loop volume of 23.2 pL is used to inject the product sample into the gas chromatograph (Varian 1700). Inside the gas chromatograph, a l/s-in. by 20-ft column packed with 30% DC-200 on Chromosorb PAW/DMCS 500 is used for products analysis. The oven temperature is kept a t 90 "C, the injection port is a t 250 OC, and the detector is a t 259 "C. The chromatograms of silane, dichlorosilane, trichlorosilane, and silicon tetrachloride are identified and are shown on Figure 2. Before the measurement of the experimental data is started, several blank tests are run by passing pure trichlorosilane through a heated empty tube. No conversion of trichlorosilane can be detected in the blank tests. Procedure. Before starting the experiment, the whole system is vacuumed to 20 pmHg and flashed several times with nitrogen to keep moisture out of the system. When the system is ready, trichlorosilane is sucked from a glass bottle into the storage cylinder. During this process, the trichlorosilane bottle is enclosed in a plastic bag filled with nitrogen to prevent a possible reaction between TCS and moisture in the air. To start the experiment, the reactor temperature is heated to a desired constant and then the trichlorosilane liquid is forced to flow through the reactor. The flow rate of the solution is adjusted and monitored by the flow meter. When the flow rate of trichlorosilane solution reached a steady state, the sampling valve is switched to the inject position, and the product sample is carried by helium to the gas chromatograph for analysis.
Results and Discussion Six kinds of catalyst have been tested over a temperature range from 30 to 100 "C and pressure range from 30 to 100 psig. The highest DCS conversions together with their temperature and pressure conditions are listed in Table 11. Results indicate that the DOWEX MWA-1 catalyst gives the highest DCS conversion percent, 10.4%.
0888-5885/88/2627-1600$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1601 Table I. ProDerts of DOWEX Resin Catalyst
item MWA-1 MSC-1 SBR WGR-2 TG-550A TG-650C-H
tvDe macroporous weak base macroDorous cation strong basic anion weakly basic anion
activitv grow tert-amine sulfonic acid trimethylbenzene-ammonium epoxy amine
mesh size 20-50 mesh 20-50 mesh 20-50 mesh 20-40 mesh 550 pm 650 pm
c1H+
Table 11. Highest DCS Conversion with DOWEX Catalyst temp, pressure, catalyst "C psig DCS conversn, % MWA-1 80 80 10.4 MSC-1 100 80 3.95 SBR 60 60 6.22 WGR-2 70 40 1.5 TG-550A a a a TG-650C-H a a a
l2
moisture contents 50-60 % 44-50% 43-48% 50% 46-54% 46-51 %
I '1
T l
Dissolved in TCS. Pressure
4 1
4
~~
thermal stability, "C 100 150 100 93.3 100 150
,
,
I
40
50
-
8Opsig
1
1
60 70 Reaction t e m p e r a t u r e , O C
I
90
80
Figure 3. Temperature effect of DCS conversion at equilibrium.
I
~
1 ~
a
12
' I ~
~
A:Storage Cyllnder &TCS Sample CiFlvvmeLer D'Strlp !'eafer
E:ReacLor
1:Flark
F:Ssmpling Valve G:Pressure Gauge I! P l a s f l c B~~
J:Dryer K:Scrubber L:Recorder
1-
T
L.-...J
Figure 1. Experimental flow diagram of TCS redistribution reaction.
: ZOft x 1 / 8 1 " C o l u m n p a c k i n g : 30X DC-200 o n
co1umii
C h r o m o r o r b P A W I D H C S 500 Ccrpcrarure uf
: inlectian
port 2 5 0 O C
9O0C 259OC
column defector IIC
IeC
L"
r
0
1
2
3
4
5
6
Reten:,""
I
I
40
50
I
I
60
70
I
!
80
90
Reaction p r e s s u r e , psig
Figure 4. Pressure effect of DCS conversion at equilibrium.
TCD
Carrler gas. He, flow rate
41
i
55ml/min
7 flne, m ~ n u t e s
Figure 2. Chromatograms of chlorosilanes.
Therefore, DOWEX MWA-1 is used for the reaction model study. DOWEX MWA-1 is a macroporous weakly basic resin of tert-amine functionality attached to a polymetric styrene-divinylbenzene matrix. The equilibrium conversions of DCS a t different temperatures and pressures are shown in Figures 3 and 4, respectively. These equilibrium conversions were obtained from a sample solution staying in the reactor a t constant temperature for more than 24 h. The wide standard deviations, ranging from 7% to 1770, may be due to the sampling method which is designed particularly for a
continuous-flow experiment. The old sample solution in the sampling valve and lines needs to be flushed by the new sample solution. This is hard to accomplish when the flow rate is very small, i.e., long retention time, or when there is no flow at all, i.e., a t equilibrium. It is noticed that the equilibrium conversion is a function of the pressure as can be seen from Figure 4. This pressure effect on the equilibrium conversion in a liquid phase cannot be explained reasonably. However, the same pressure effect was reported by Union Carbide (1979). Continuous-flow reaction results shown as DCS conversion percent versus the retention time are plotted in Figure 5 for a pressure of 80 psig and in Figure 6 for a temperature of 80 "C. The retention time is calculated as the reactor void volume divided by the trichlorosilane flow rate. It may be seen from these two figures that the temperature affects both the equilibrium conversion and conversion rate, while the pressure affects the equilibrium conversion only. In order to test the importance of mass transfer, three different sized catalysts were used for the continuous-flow
1602 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 12---I1
~~
-
~
I
I
i
2
10
+
Y
1
+
+
I
-
Temperature
Pressure
=
80°C
BOpsig
2 8 e 5 Lo
x
4 0
3 2
+ mesh 1 3 5
I
0
n i n
0
n
6
I
mesh 150 I
4
6
Retention time. minutes
Reteniio" cime, minutes
Figure 5. Temperature effect of DCS conversion rate at 80 psig.
11
I
meeh 120
j
I
Figure 7. Particle size effect of DCS conversion rate at 80 "C and 80 psig. I? 11
1
-
Temperature
Pressure
80°C aopsig F
c L- 1 . 6 2 5 i n c h e s
+ L-
3
inches
L- 7
inches
s L-11.75
inches
0
o
i 0
-
,
----
2
ReLention Lime. mindtee
7 -
3
6 0
reaction. The results are shown in Figure 7 . I t can be seen that the DCS conversion depends on retention time only, and not catalyst size. This phenomenon implies that both the external and the internal mass-transfer resistances are not important for this redistribution reaction. The scattering data points a t higher retention time again indicate that the sampling method may not be suitable for equilibrium data measurement. The effect of external mass-transfer resistance may be verified by further experiment using different reactor lengths as indicated in Figure 8. The DCS conversion percents depend on the retention time but not the flow rate. For instance, at a retention time of 0.8 min, the DCS conversions are about 8.5% for three different flow rates, as may be visualized from Figure 8. These results again indicate that the external mass-transfer process is not important during this redistribution reaction process. Kinetic Analysis. During the redistribution reaction of TCS, the main reaction may be considered as the one which forms dichlorosilane and silicon tetrachloride. The product, dichlorosilane, may redistribute to monochlorosilane or silane. However, these amounts are very small (Union Carbide, 1979) and are neglected during this kinetic analysis. Regardless of the complexity of the mechanism, the overall redistribution reaction may be expressed as
5SiH2C12+ SiC1,
2SiHC13
kf
4
6
Retention time. minutes
Figure 6. Pressure effect of DCS conversion rate at 80 "C.
(1)
where the mole fractions of 2SiHC13, SiH2C12,and SiC14 are respectively at t = 0, 1, 0, and 0; at t = t , 1 - 2X, X,
Figure 8. Reactor length effect of DCS conversion rate at 80 "C and 80 psig.
and X; and at equilibrium, 1- 2X,, X,, and X,, where X and X,are the conversion fractions of dichlorosilane during the reaction and at equilibrium, respectively. If the reaction is considered as second order in the forward reaction as well as in the backward reaction, the rate equation may be written as d[CDcs]/dt = kf[SiHCl3I2- kb[SiH2C12][SiC14](2) or d X / d t = k;(l - 2X)2 - k{X2
(3)
where k j = kf[SiHC13],,k{ = kb[SiHC1,],, and [SiHC1310 is the initial concentration of TCS which is 8.850 mol/L for pure TCS. The above differential equation may be integrated using the initial boundary condition, X = 0 a t t = 0, to obtain (1- 4Xe)X + x, 2(1 - 2X,) kjt (4) In x,-x x e This integrated rate equation is a straight line in a semilogarithmicplot. The forward reaction rate constant, kd, can be calculated from the slope of the straight line. The backward reaction rate constant, kb', then can be computed from (5)
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1603
pressure
-
T e m p e r a t u r e = 80°C Pressure = 8Opsig
sopsig
I / S l o p e = kfCo*2(1-2xe)/xe
k f = 2 . 6 1 ~ 1 0 - ~l i t e r l g m o l e - s e c
-
kb
1 . 5 1 ~ 1 0 - l~i t e r / g m a l e - s e c
Reactor EReactor OReactor OReactor
A
I 1
Figure 9. Plot of second-order rate equation at different temperatures.
-
TemDerature
Slope
-
I
I
4
5
inches inches inches inches 1 6
R e t e n t i o n time, m i n u t e s
Retention time, minutes
1OOC !L
I 3
I 2
-
length 1.625 length I 3 length 7 l e n g t h -11.75
Figure 12. Plot of second-order rate equation at different reactor lengths.
80°C
kfCo*2(l-2x,)/xe
k f = 2 . 9 6 ~ 1 0 - ~l i t e r / g mole-sec kb
-
1 . 7 2 ~ 1 0 - ~l i t e r / g m o l e - s e c
.
il
-
opressure = 4opsig pressure 6Opsig OPressure = sopsig A Pressure 9opsig
I
I
1
I
1
I 2
I
I
3
4
1 5
I 6
R e t e n t i o n time, m i n u t e s
--
Temperature Pressure
I
80°C 8oPSiE
4 kf kb
-
-
c
kfCo*2(1-2xe)/x,
3.52x10-'
l i t e r i s mole-sec
0
I
1
I 2
I
I
3
4
-I 5
1
2.8
I
2.9
.62 - EK f /cRa l / g m o l e
\\
!
I
3.0
3.1
3.2x1d
Temperature c o o r d i n a t e , l/'K
2 . 0 4 x 1 0 - ~ l i t e r / g mole-aec
A C a t a l y s t size G C a t a l y s t size O c a t a l y s t size
ri
C r 2.7
Slope
1
10-4
5 ~ 1 0 - ~
L
-
\Elo!e8=
Figure 10. Plot of second-orderrate equation at different pressures.
mesh #20 meeh 135 mesh 150
6
~ e t e n t i o nt i m e . m i n u t e s
Figure 11. Plot of second-order rate equation at different particle sizes.
The plot of eq 4 using experimental data from Figure 5 is shown on Figure 9. A good straight line fit at temperatures of 50 and 60 "C indicates that the second-order
Figure 13. Arrhenius plot of forward reaction rate constant and backward reaction rate constant.
reaction is a reasonable assumption. The high-temperature data do not fit a straight line quite as well. A first-order rate equation was checked, and the fit of the experimental data was not better. A close examination of eq 4 reveals that the value of the logarithmic function approaches infinity as the value of X approaches X,. Therefore, a small error in the experimental value of conversion, X,results in a large deviation from a straight line on the semilogarithmic plot. In light of this, data points close to X,should be weighed lightly or discarded during the linear regression. A linear regression of eq 4 using experimental data from Figures 6 and 7 indicates that the data points should be disregarded when the retention time is greater than 1.5 min. Results are shown on Figures 10-12. The average values of kf and kb a t 80 "C are 3.01 f 0.38 X lo4 L/(mol.s)
1604 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988
and 1.86 f 0.25 X L/(mol.s) (Huang, 1987), respectively. Following the same method, the reaction rate constants at 50, 60, and 90 "C can be obtained. A plot of reaction rate constants versus 1 / T is shown on Figure 13. It can be seen that the Arrhenius equation is followed quite well for both forward and backward reaction rate constants. The activation energy calculated is 8.62 kcal/mol for the forward reaction and is 7.08 kcal/mol for the backward reaction. The low activation energies are probably due to the fact that the redistribution reaction is only a rearrangement of the silicon-hydrogen and silicon-chlorine bonds. Mass Transfer with Chemical Reaction. Since the distribution reaction of trichlorosilane to dichlorosilane occurred only inside the catalyst, the reactant, trichlorosilane, must be transported from the bulk liquid to the catalyst. This process is called external mass transfer. The transport process of trichlorosilane inside the catalyst is usually described by a molecular diffusion accompanied by chemical reactions. The overall conversion rate of trichlorosilane to dichlorosilane could be controlled by one or any combination of the above processes. The external mass-transfer coefficient is usually a function of fluid velocity, catalyst particle size, and transport properties such as diffusivity and kinematic viscosity. The internal diffusion process depends on the molecular diffusivity, porosity, and catalyst particle size, while the chemical reaction is affected mainly by temperature. An experiment is usually designed to determine which one is the controlling step for this heterogeneous reaction system. For example, the importance of external mass transfer may be tested by changing the fluid velocity or the catalyst particle size, while the influence of internal mass transfer may be examined by using different catalyst particle sizes. The test results are shown in Figures 7 and 8 and have been discussed in an earlier section. The film conversion factor and the Thiele modulus will be calculated as shown below to verify the controlling step. The film conversion factor is used to indicate the relative importance of the mass-transfer resistance in the film (Levenspiel, 1972). This factor is defined as
M = k;D/kC2
m=
(-rA),bC2/(DeCAb)
(10)
where (-rA),bs represents the observed disappearing rate of the reactant, L is the average pore length, De is the effective diffusivity of the reactant in the catalyst, and CA, is the reactant concentration in the bulk liquid. Taking kf from Figure 9, L = one-sixth of the particle diameter = 6.95 x IO-, cm, and De = lo4 cm2/s (Satterfield and Sherwood, 1963), the values of the Thiele modulus are calculated as 0.07 for 50 "C and 0.257 for 90 "C. These small values indicate again that the chemical reaction is the controlling step. Mechanism As trichlorosilane contacts the tert-amine catalyst, an ionic amine-chlorosilane may be formed due to the weak base of the amine catalyst. The reaction step was proposed (Union Carbide, 1979) as RMe2N + SiHCl,
ki k-1
[RMe2NHSiC13]
(11)
The above reaction step was supported by the verification of a similar intermediate when trimethylamine and trichlorosilane are reacted (Ring et al., 1971). This ionic intermediate may react with a second trichlorosilane t o form another intermediate, [RMe2NHSi2HC16],which, with a rearrangement of hydrogen and chlorine, decomposes to form dichlorosilane and silicon tetrachloride. The possible reaction steps may be expressed as [RMe2NHSiCl3]+ SiHCl,
k2 k-2
[RMe2NHSi2HC16](12)
and k3
[RMe2NHSi2HC16]rRMe2N + SiHzClz + SiC14 k-3
(13)
(6)
where k, is the external mass-transfer coefficient and D is the diffusivity of trichlorosilane in the liquid film. The external mass-transfer coefficient in a fixed-bed catalytic reactor may be calculated from the following semiempirical equation (Sherwood et al., 1975): k , = 1.17VRe-0.415S~-2/3 for 20 < Re
mass-transfer resistance between the bulk liquid and the catalyst can be neglected. The relative importance of internal mass-transfer compared with chemical reaction may be evaluated from the Thiele modulus, which can be expressed as (Levenspiel, 1972)
One possible structure of the second intermediate, [RMe2NHSi2HC&], is a nucleophilic attack of chlorine to the silicon atom as shown below (Union Carbide, 1979): Me \
\
< 2500 (7)
where Re = Vd,/v and
H' It is also possible to form another structure: Me
Sc = v/D Here V is the superficial velocity, d, is the catalyst diameter, and v is the kinematic viscosity of the solution. cm (mesh no. 35), v = 2.1 X cm2/s, If d, = 5.6 X D =2X cm2/s, and V = 2.0 cm/s (highest velocity) cm/s. are used, the value of k, is calculated as 2.02 X When the values of k,, D, and k{ (from Figure 9) are substituted in eq 6, the values of film conversion factor for 50 "C and 1.33 X for are obtained as 0.37 X 90 "C. These small values indicate that the reaction is much slower than the external mass transfer. Hence, the
/ /
R-N--Me
H'---SICI~
