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J. Phys. Chem. 1993,97, 3930-3931
Vapor-Phase Homogeneous Nucleation of Silicon Tetrachloride: Revised Results
TABLE I: Ex rimental Measurables at the Onset of Nucleation for Elicon Tetrachloride exvt no. To (K) TI (K) P,(Torr) PIIPO
T.Wimpfheimer and M.A. Chowdhury’
1
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024
2 3 4 5
6 7h 8h 9h
M. S . El-Shall Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284 Received: September 18, 1992; In Final Form: January 4, 1993
I Oh
308.4 309.8 312.1 315.1 315.4 318.3 313.5 315.0 309.0 307.2
1 I25 1 I35 1155 1 I90 1 I95
232.2 235.6 236.7 238.8 242.3 245.0 235.9 238.1 231.7 229.0
3.1 2.96 2.75 2.52 2.51 2.29 2.29 2.20 2.61 2.78
1220 1014 1035 97 1 962
TO,the temperature of the bottom plate; TI,the temperature of the top plate; P,,the total pressure, the pressure ratio defined as the ratio of P,to PO,the equilibrium vapor pressure of the liquid at the temperature TO. Data obtained at the Virginia Commonwealth University using an a
Introduction Silicon tetrachloride is important in silicon chemistry pertaining to the chemical vapor deposition phenomenon.I4 This generated interest in the study of nucleation in Sic14 vapor. M. S.El-Shall investigated nucleation in SiC14vapor using an upward thermal diffusion cloud ~ h a m b e r .Recently, ~ we repeated these experiments as a part of ongoing research on the development of a molecular theory of nucleation for which the CC14-SiC14 system has been chosen as a model binary system.6 A recent calibration of the thermocouple meter used in ref 5 revealed that significant corrections to the meter were necessary. We therefore report the revised results here.
electric field of 200 V across the cloud chamber plates.
Experimental Section The experimental setup and procedure were almost identical to those used in ref 5. Special care was taken in the measurement of temperature, and the thermocouple meter was calibrated prior to the experiments using an ASTM thermometer traceable to NIST. Care was also taken to ensure that P,/Po > 2.5, where P,is the total pressure of the vapor and the carrier gas and PO is the equilibrium vapor pressure of Sic14 a t TO,the bottom plate temperature. Katz et al.’ found this condition to be essential for the correct determination of critical supersaturation. A portion of the data was obtained at the Virginia Commonwealth university where the effects of ions on Sic14 nucleation were investigated by the usual procedure of applying an electric field between the chamber plates. An electric field of 200 V was applied while gathering these data.
M = 4.0026
TABLE I1 a Silicon Tetrachloride
M = 169.9, D(SiCl4-He) = 0.2121,8s = 0.75,* ct = 0.3 [9] log P, = 7.931-1656.580/T [lo]
+
C, = 13.5229+ 3.910(10-2)T-4.5339(10-5)~ l.8081(10-n)P [ I l l X = 2.2889(10-5) - 1.7332(10-’)T+ 7.1219(10-’0)T2-
7.1055(10-13)P [ I l l 7 = 11.379(10-6)T’5 / ( T + 266.94) [ I l l
u = 22.404-4.246(10-’)(T- 273.15) - 1.629(10-’)(T-273.15)’l.320( T - 273.15)’ [ 121 d = 1.5211 -2.0687(lO-’)(T-273.15) [I31
Helium
Results and Discussion Table I shows the results of the measurements on SiCI4vapor. Following the conventional format the table lists the plate temperatures and total pressure in the cloud chamber at which nucleation was observed to occur at a rate of 1-5 drops/(cm3 s). These temperature-pressure data were used along with the thermophysical constants of silicon tetrachloride, helium, and their mixture (listed in Table 11) for solving the heat and mass flux equations. The conventional film correction was applied for determining the precise temperature of the liquid film at the top plateeS Solutions of the heat and mass flux equations gave both the supersaturation of Sic14 and the vapor temperature in the cloud chamber as a function of reduced height. Supersaturation is then plotted against the corresponding temperature. The plots (solid lines) obtained from our temperature-pressure data (from Table I) are shown in Figure 1. The envelope of these plots represents the experimental critical supersaturation of SiC14 in its dependence on temperature. Critical supersaturation of SiCI4 calculated from the classical theory of nucleation is also plotted in Figure 1 as a dotted curve. The theoretical and experimental values lie within 20% of one another. This is usually regarded as satisfactory agreement in the nucleation literature. Our results 0022-365419312097-3930$04.00/0
X = -5.851496(10-5)
+
2.684775(10-‘)T- 6.997783(10-9)T2 1.073445( IO-’‘)T’ - 6.0148( IO-”)P[ 1 I ] 7 ” 1.4083(10-5)T’5/(T+ 70.22) [ I l l
+
r? M , themolecularweight (ing/mol);D, the binarydiffusioncoefficient at 0 OC and 1 atm (in mol cm-’ SKI); s, the exponent of its temperature dependence; a,the thermal diffusion factor and equations as a function of temperature (in K) for P,,the equilibrium vapor pressure (in Torr); C,, the constant pressure specific heat (in cal mol-’ K-I); A, the thermal conductivity (in cal cm-’ K-I s-l); 7,the viscosity of the vapor (in poise); u, the surface tension of the liquid (in dyn cm-I); and d, the density of the liquid (in g cm-9.
100
230 0
,--.
240 o
250 0
260 0
270 0
21 0
TEMPERATURE (K)
Figure 1. Plot ofthecriticalsupersaturationofsilicon tetrachlorideversus temperature at theonset of homogeneous nucleation. Classical nucleation theory (dotted line) is compared with the envelopes of the experimental curves (solid lines using data from this work and dashed lines using data from ref 5 ) .
also indicate that ions have no significant effect on homogeneous nucleation of Sic14 vapor. Experiments 1, 2, 6,and 7 of Table 1 of ref 5 can be directly compared with our data on the basis of TObeing in the same 0 1993 American Chemical Society
Comments range, namely, 307.2-3 18.3 K. We did not use higher values of Toin our experiments because then in order to satisfy the ‘P,/Po > 2.5” condition the cloud chamber would need to be pressurized to over 1300 Torr, which we considered to be unsafe. It should be noted that T I of experiment 2 of ref 5 should read 232.1 K instead of 223.1 K.I4 In the cases of the above-mentioned data points the temperature gradients between the top and bottom plates are greater than those in our experiments. In general, for the same values TOthe temperature gradients shown in ref 5 are approximately 7 K greater than our temperature gradients. This is attributed to the differences in the calibrations of the thermocouple meter used in ref 5 and in the present work. Without the new calibration of the thermocouple meter in our experiments registered the same temperatures that appear in Table 1 of ref 5 for the onset of nucleation. Consequently, the Sic14 supersaturations in the experiments described in ref 5 are greater compared to our experiments. These supersaturation versus temperature plots are also shown in Figure 1 as dashed curves. The experimental critical supersaturationsindicated by thedashed curves generally differ from the theoretical values by approximately 40%. Thus, careful observation of Figure 1 reveals that the envelope of the dashed curves is in poorer agreement with theory. Theauthorofref 5 used thesametheory ofnucleationemployed in the present work. However, the surface tension of Sic14 that we used was determined in recent experiments12 and is slightly different from that used in ref 5 . This caused slight differences between our theoretical critical supersaturations and those used in ref 5 . However, the disagreement between the theory and the results of ref 5 escaped notice mainly because of a hidden error in the computer program used in ref 5 for solving the heat and mass flux equations which led to the impression of good agreement (see Figure 1 of ref 5 ) between theory and experiment in the work reported in that paper.
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3931
We believe that thedata of the present workexhibited in Table I represent the correct onset conditions for the homogeneous nucleation of SiC14vapor. As Figure 1 shows, these data lead to measured values of critical supersaturation in reasonable agreement with classical theory. This conclusion, which was also drawn in ref 5 is supported by the corresponding states theory discussed in detail in ref 5 .
Acknowledgment. This research was carried out with the support of N S F Grant CHE90-22215. References and Notes ( I ) Perrin, J.; Delaforse, E. J . Phys. 1980, 0 1 3 , 759. (2) Kampas, F. J.; Griffith, R. W. J . Appl. Phys. 1981, 52, 1285. (3) Knights, J. C.; Schmitt, J. P. M.; Perrin, J.; Guelachvili, G . J . Chem. Phys. 1982, 76, 3414. (4) Nishizawa, J.; Saito, M. In Proceedings of the Eighth International
Conference on Chemical Vapor Deposition; The Electrochemical Society: Pennington, NJ, 1981; p 317. ( 5 ) El-Shall, M. S. Chem. Phys. Lett. 1988, 143, 381. (6) Wimpfheimer,T.; Chowdhury, M. A,;Reiss, H . J . Phys. Chem. 1993, 97, 716. (7) Katz, J. L.; Scoppa 11, C. J.; Kumar, N.; Mirabel, P. J . Chem. Phys. 1915, 62, 448. (8) Fuller, E. N.; Ensley, K.; Giddings, J. C. J . Phys. Chem. 1969, 73, 3679. (9) Grew, K. E.; Ibbs, T. L. Thermal Dijfusion in Gases; Cambridge University Press: Cambridge, 1952; p 128. (IO) Weast, R. C. Handbook of Chemistry and Physics; 53rd ed.; CRC Press: Boca Raton, FL, 1973.
( I I ) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. Properties of Gases and Liquids; McGraw-Hill: New York, 1987. (12) Wimpfheimer,T.; Chowdhury, M. A,; Reiss, H.J. Phys. Chem. 1992, 96, 9906. ( I 3) Gmelins Handbuch der Anorganischen Chemie, 8th ed.; System No. 15, Part B; Springer: Berlin, 1959. (14) El-Shall, M. S. Private communication.
