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On the role of carrier gas in the deposition kinetics of SiO2 films produced by low temperature chemical vapour deposition
Thin Solid Films, 146 (1987) 183-189 PREPARATION AND CHARACTERIZATION
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ON T H E R O L E O F C A R R I E R GAS IN T H E D E P O S I T I O N K I N E T I C S O F SiO 2 FILMS P R O D U C E D BY LOW T E M P E R A T U R E C H E M I C A L VAPOUR DEPOSITION C. COBIANU AND C. PAVELESCU Microelectronica, Str. Erou lancu Nicolae 34B, R-72996, Bucharest (Romania) E. SEGAL Chair of Physical Chemistry and Electrochemical Technology, Faculty of Chemical Engineering, Polytechnic Institute of Bucharest, Bulevardul Republicii 13, Bucharest (Romania) (Received January 27, 1986; ac~cepted July 2, 1986)
In this paper the results of experiments on the deposition rate as a function of the total gas flow rate and of the ratio of 0 2 to Sill4 under various experimental conditions and also the calculated plots of Sill 4 partial pressure and mixing time (the time available to the reactant gases to come in contact) v s . the total gas flow rate are presented. These data are interpreted in terms of the deposition mechanism and the homogeneous gas phase reaction as follows. (1) At low values of the total gas flow rate, where higher Sill4 partial pressures and longer mixing times of reactant gases are obtained, a mass transport control (in terms of input gas flow rate limitation) is demonstrated. A homogeneous gas phase reaction is characteristic of this deposition region and is determined by the Sill4 partial pressure and/or mixing time. (2) At medium values of the total gas flow rate the mass transport control ceases to determine the deposition kinetics, which is considered to be diffusion or kinetically limited, while the intensity of the homogeneous gas phase reaction is strongly decreased. (3) At high values of the total gas flow rate where low input Sill4 partial pressures and low mixing times of reactant gases are obtained it is shown that the surface kinetics control the deposition rate.
1. INTRODUCTION Earlier papers on the preparation of S i O 2 films by low temperature chemical vapour deposition (LTCVD) from Sill4 oxidation have shown that a controlled surface reaction can be readily obtained by diluting the Sill4 with an inert gas prior to reacting with 0 2 1.2. Later kinetic studies a-s have investigated the effect of most of the experimental process variables on the film by using different types of reactors. However, there is no detailed quantitative analysis of the role of the carrier gas in the deposition of LTCVD SiO2 films at atmospheric pressure. Based on the calculated plots of Sill4 partial pressure and the mixing time of gases as a function of the total gas flow rate, in conjunction with the dependence of the deposition rate on 0040-6090/87/$3.50
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different process variables, an insight into the theoretical mechanism of L T C V D SiO2 film deposition is intended. In this paper, the effect of the carrier gas on the deposition kinetics (in terms of the reaction rate control and the extent of the homogeneous gas phase reaction) is studied by means of various experiments that will be described later. 2. EXPERIMENTAL PROCEDURE The L T C V D SiO2 films were deposited onto cleaned and polished silicon wafers from diluted Sill 4 (5% Sill 4 in argon) and 0 2 (both of them diluted subsequently in N2) in a nozzle-type reactor 6. The film thickness was measured with a Tencor Instruments stylus type profilometer. The deposition temperature was k e p t constant at 400°C and measured with a surface thermometer, Technoterm 9500. During these experiments the Sill4 flow rate was varied from 15 to 90 standard cm 3 m i n - 1 while the total gas flow rate was increased from 1 to 16.7 standard l m i n -1. The effect of the carrier gas on the Sill4 oxidation rate in this open flow system was studied by increasing proportionally the flow rates of N2 and the reactant gases so as to keep constant the partial pressure, or by increasing the N2 flow rate while the flow rates of the reactants were kept constant. In other experiments the 0 2 flow rate was varied with the N 2 flow rate as a parameter in the range 5-16 standard 1 m i n - 1 for a constant Sill4 flow rate of 30 standard cm 3 m i n - 1. 3. RESULTS We will discuss the effect of the carrier gas on the Sill, oxidation reaction in terms of the SiO 2 deposition rate, the extent of the homogeneous gas phase reaction and the reaction rate control based on the following experimental results. Figure l(a) presents the dependence of the deposition rate and the partial pressure of Sill4 as a function of the total gas flow rate when the S i l l , and 0 2 flow rates are kept constant (flow rate of Sill4, 30 standard cm 3 m i n - 1; ratio of 0 2 to Sill,, 12.5) and the deposition temperature is held constant at 400 °C. The deposition rate presents an increase-maximum-decrease type of dependence while the partial pressure of S i l l , shows a hyperbolic variation when the total gas flow rate is increased. When the total gas flow rate is varied from 1 to 3 standard 1min - 1 a fast increase in the deposition rate is obtained after which it levels off for the total gas flow rate ranging from 3 to about 5 standard 1m i n - 1. For higher values of the total gas flow rate the deposition rate begins to decrease, reaching half of its m a x i m u m value at a total gas flow rate of 16.7 standard 1 m i n - x. In Fig. l(b) the dependence of the deposition rate and the mixing time of reactant gases are presented as a function of the total gas flow rate when the flow rates of all gases are increased proportionally as follows: N 2 :Oz:SiH * = 150:15:1 with the S i l l , flow rate increasing from 15 to 90 standard cm 3 min-1. The partial pressures of the gases are Psin4 = 5 x 10- 3 atm, Po2 = 8 x 10- 2 atm, PN2 = 0.87 atm and PAr = 4.47 × 10 -2 atm. The mixing time is defined as the time available for the
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reactant gases to come into contact and is calculated by dividing the small space between the bottom of the reaction gas head and the deposition susceptor by the linear velocity of the gases. (The last parameter is obtained by dividing the total gas flow rate (in standard centimetre cubed per minute) by the distribution area (in centimetre squared) as in ref. 6). In this case, when the flow rates of all gases are increased proportionally (Fig. l(b)), at low values of the total gas flow rate the deposition rate increases rapidly. For higher values of the total gas flow rate the deposition rate increases slowly or even saturates (for total gas flow rates higher than 14 standard litre per minute)• Similarly to the results of the partial pressure of Sill 4 in Fig. l(a), the mixing time is a function of the total gas flow rate.
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Fig. 1. (a) T h e d e p e n d e n c e o f t h e d e p o s i t i o n r a t e a n d p a r t i a l p r e s s u r e o f S i H 4 o n t h e t o t a l g a s f l o w r a t e a t constant 02 and Sill4 flow rates: Sill4, 30 cm 3 m i n - 1; ratio of O2 of SiH~, 12.5; deposition temperature, 400 °C. (b) The dependence of the deposition rate and mixing time of the reactant gases on the total gas flow rate at constant partial pressure of the gases (N2 :O2: Sill4 = 150:15:1). Deposition temperature, 400 °C; Sill4 flow rate in the range 15-90 cm a min - 1
In Fig. 2, the dependence of the deposition rate on the 0 2 flow rate with the total gas flow rate as a parameter is presented. The Sill 4 flow rate is kept constant at 30 standard c m 3 m i n - 1 while the deposition temperature is 400 °C. For the range 5-16 standard I m i n - 1 of the total gas flow rate, the deposition rate shows the same type of dependence (increase-maximum-decrease) on the 0 2 flow rate, even if the m a x i m u m deposition rate is very sensitive to the total gas flow rate (as a parameter), in agreement with the results presented in Fig. l(a). In the decreasing region of the dependence of the deposition rate on the ratio o f O 2 t o Sill4, the effect of the carrier gas on the deposition rate decreases.
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Fig. 2. The dependence of the deposition rate on the 0 2 flow rate at an Sill4 flow rate of 30 cm3 min- ~,a deposition temperature of 400 °C and the total gas flow rate F as a parameter as follows: curve 1, F = 16 standard I min- 1; curve 2, F = 11 standard I min- 1; curve 3, F = 5 standard 1 min - i. 4. DISCUSSION The experimental results presented earlier will be discussed as a function of the total gas flow rate with special emphasis on the types of rate control of the deposition reaction and the extent of the h o m o g e n e o u s gas phase reaction.
4.1. Low values of the total gas flow rate At low N 2 flow rates a fast increase in the deposition rate is obtained for an increase in the N 2 flow rate, in spite of a decrease in the Sill 4 partial pressure (Fig. l(a)). The strong sensitivity of the deposition rate to the total gas flow rate m a y indicate a mass transport control (in terms of input rate limitation) of the surface reaction 9. However, the h o m o g e n e o u s gas phase reaction has presented the highest rate in this deposition region. (The experimental observation of this fact is possible using a continuous nozzle-type reactor1°.) In fact, the higher Sill4 partial pressures and mixing times associated with this region can be directly correlated with a higher development of the h o m o g e n e o u s gas phase reaction, in agreement with the experimental findings of other workers 11-x 3. Similar to the results presented in Fig. l(a), the deposition rate data from Fig. l(b) show that at low values of the total gas flow rate (with partial pressure kept constant) the deposition rate increases rapidly with the total gas flow rate while the mixing time has the largest values. Therefore, we can consider that there is e n o u g h time for the reactant gases to equilibrate with the deposition surface and thus an input gas rate control (or t h e r m o d y n a m i c control) can be assumed 9 to determine the
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Sill4 oxidation reaction. In fact, in the deposition region of low values of the total gas flow rate, the increased rate of the homogeneous gas phase reaction is obtained for a constant Psin, of 0.005 atm (Fig. l(b)) which is much lower than the range of Psm, in the same deposition region from Fig. l(a) (0.02-0.03 atm). Therefore, the extent of the homogeneous gas phase reaction is determined either by Ps~ri,(Fig. l(b)) or the mixing time (Fig. l(a)). The combined effect of both of them can be easily understood. The concentrations of vitreous silicate particulates (0.3-0.5 ~ n in diameter), dispersed in a bell-jar-type reactor and measured using a laser light scattering technique, increased with a decrease in the total gas flow rate with constant reactant concentrations 13. This is in agreement with our qualitative observations on the homogeneous gas phase reaction corresponding to experimental conditions presented in Fig. l(b). A coagulation model for the formation of large SiO2 particulates from small particulates is presented in ref. 13. A similar process of cluster formation takes place in the case of the homogeneous gas phase reaction for polycrystalline silicon deposition at higher partial pressures of silane in low pressure (LP) CVD systems. This phemomenon has been explained by a homogeneous gas phase polymerization reaction in which a monomer reacts with an oligomer and so on, finally producing a large silicon cluster 11. 4.2. Medium values of the total gas flow rate The non-dependence of the deposition rate on the flow conditions (Fig. l(a)) supports the fact that in this deposition region the mass transport control ceases to determine the deposition kinetics. In a similar manner, the results from Fig. l(b), obtained at constant partial pressures of the gases involved, show that at medium values of the total gas flow rate the deposition rate increases more slowly. This is consistent with a change in the type of rate control (to a diffusionally or kinetically limited regime) as is suggested in ref. 9. However, in agreement with the above results, the intensity of,the homogeneous gas phase reaction decreased strongly in this deposition region. 4.3. High values of the total gas flow rate This deposition region can be considered to begin at total gas flow rates where the deposition rate decreases (Fig. l(a)) or slows its increase (Fig. l(b)) with an increase in the total gas flow rate. Figures l(a) and l(b) show that this region can be characterized by low partial pressures of Sill4 and low mixing times of the reactant gases. These two process variables, combined with higher ratios of 02 to Sill 4 (12.5 and 16) associated with those experiments (Figs. l(a) and l(b)) correlate with a strong inhibition of the homogeneous gas phase reaction in agreement with other results 6'13. However, when values of the total gas flow rate were higher, the homogeneous gas phase reaction was present again at the lower ratios of O2 to Sill4 without affecting the uniformity of the deposited layers, unlike the results of silicon deposition where the homogeneous gas phase polymerization was associated with film non-uniformities on the same wafer and from wafer to wafer 11. The effect of the ratio of 02 to Sill, on the film quality was demonstrated by dielectric breakdown
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experiments. It was shown that at the lowest ratios of 0 2 t o Sill 4 used for the deposition the quality of the LTCVD SiO2 films is poor (in terms of low dielectric breakdown) owing to a higher number of SiO2 particulates incorporated in the structure of the layer during deposition 14. In the range of high values of total gas flow rates an important step in the determination of the type of rate control and deposition mechanism can be obtained by analysing the combined effect of the carrier gas and the oxygen flow rate with the Sill, flow rate kept constant (Fig. 2). For the range 5-161 m i n - ~ of the carrier gas used the maximum deposition rate as a function of the ratio of 0 2 to Sill4 strongly decreases with the increase in the N2 flow rate. This result is consistent with the falling off region of the deposition rate presented in Fig. l(a). Taking into account that the N 2 can be physically adsorbed on silica surfaces up to high temperatures t5 (about 1000 °C) an inhibition of the deposition rate by N 2 adsorption on the reaction surface has been earlier proposed 7. In the region where the deposition rate decreases with the increase in the ratio of O2 to Sill 4 (Fig. 2) the effect of the carrier gas on the deposition rate decreases. This fact can be explained by an increased adsorption of 0 2 with respect to N2 on the same surface and thus a rate control by 0 2 adsorption can be considered. Moreover, this type of dependence was obtained for all types of reactors 1'4'5'7 independent of their geometries. All these data suggest that for higher values of the carrier gas flow rate the surface kinetics control the deposition rate. The atomistic model associated with the kinetic control of the deposition rate, which is based on the experimental dependence of the deposition rate on the 0 2 and Sill4 flow rates and the efficiency of the Sill 4 oxidation reaction in chemical vapour deposition of SiO2 films at low temperatures t6, was presented earlier 7 in terms of a bimolecular surface reaction theory that explained the experimental dependence of the deposition rate on temperature, 02, Sill, and N2 flow rates (at high values). 5. CONCLUSIONS
The carrier gas has a strong effect on the deposition kinetics of LTCVD S i O 2 films based on the reaction between diluted Sill 4 and 02. At low values of the total gas flow rate where higher Sill, partial pressures and mixing times of the reactant gases are obtained a mass transport control of the surface reaction is demonstrated. A strong homogeneous gas phase reaction is characteristic of this deposition region. A correlation between the coagulation model for the formation of large SiO2 particulates 13 and the formation of clusters in the case of the homogeneous gas phase reaction for polycrystaUine silicon deposition at higher partial pressure of Sill, 11 is presented in this paper. At medium values of the total gas flow rate, the mass transport control ceases to determine the deposition kinetics while the intensity of the homogeneous gas phase reaction is strongly decreased. At high values of the total gas flow rate, where low input partial pressures of Sill4 and low mixing times of the reactant gases are obtained, it is shown that the surface kinetics control the deposition rate. In the range of high values of the total gas flow rate, the maximum deposition rate as a function of the ratio of 0 2 to Sill 4
ROLE OF CARRIER GAS IN DEPOSITION KINETICS OF Si O 2 FILMS
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decreases with the increase in the N 2 flow rate as a result of i n h i b i t i o n of d e p o s i t i o n by physical a d s o r p t i o n o f N 2 on the r e a c t i o n surface. At high values of the ratio of 0 2 to S i l l , , the effect of the carrier gas on the d e p o s i t i o n rate decreases o w i n g to an increased a d s o r p t i o n of 0 2 with respect to N 2 on the same surface an d thus a rate c o n t r o l by 0 2 a d s o r p t i o n on the surface c a n be considered. T h e a t o m i s t i c m o d e l associated with the kinetic c o n t r o l of the d e p o s i t i o n rate is presented in terms of b i m o l e c u l a r surface reactions. ACKNOWLEDGMENT T h e a u t h o r s wish to t h a n k Mrs. D o r i n a C o b i a n u for the d r a w i n g of the graphics of this paper. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
N. Goldsmith and W. Kern, RCA Rev., 28 (1967) 153. W. Kern and A. W. Fisher, RCA Rev., 31 (1970) 715. K. Strater, RCA Rev., 29 (1968) 618. B.J. Baliga and S. K. Ghandi, ./7.Appl. Phys., 44 (1973) 990. E.A. Taft, J. Electrochem. Sot., 126 (1979) 1728. C. Cobianu and C. Pavelescu, Rev. Roum. Chim., 28 (1983) 57. C. Cobianu and C. Pavelescu, J. Electrochem. Soc., 130 (1983) 1888. M.K. Lee, C. Y. Lu and C. T. Shih, J. Electrochem. Soc., 130 (1983) 2249. D.W. Show, in C. H. Goodman (ed.), Crystal Growth Theory and Techniques, Vol. 1, Plenum, London, 1974, p. 1. W. Kern, SolidState Technol., 18 (1975) 25. C.H.J. van den Brekel and L. J. M. Bollen, J. Crystal Growth, 54 (1981) 310-322. A. Shitani, K. Suda, M. Suzuki, M. Maki and K. Takami, J. Electrochem. Soe., 124 (1977) 1771. A. Shintani, K. Suda and M. Maki, d. Electrochem. Sot., 127 (1980) 426. C. Cobianu and C. Pavelescu, J. Eleetrochem. Soe., 132 (1985) 97C. E. Ries, Jr., in P. H. Emmett (ed.), Catalysis, Vol. 1, Reinhold, New York, 1954, p. 13. C. Cobianu and C. Pavelescu, Thin Solid Films, 102 (1983) 361.
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ON T H E R O L E O F C A R R I E R GAS IN T H E D E P O S I T I O N K I N E T I C S O F SiO 2 FILMS P R O D U C E D BY LOW T E M P E R A T U R E C H E M I C A L VAPOUR DEPOSITION C. COBIANU AND C. PAVELESCU Microelectronica, Str. Erou lancu Nicolae 34B, R-72996, Bucharest (Romania) E. SEGAL Chair of Physical Chemistry and Electrochemical Technology, Faculty of Chemical Engineering, Polytechnic Institute of Bucharest, Bulevardul Republicii 13, Bucharest (Romania) (Received January 27, 1986; ac~cepted July 2, 1986)
In this paper the results of experiments on the deposition rate as a function of the total gas flow rate and of the ratio of 0 2 to Sill4 under various experimental conditions and also the calculated plots of Sill 4 partial pressure and mixing time (the time available to the reactant gases to come in contact) v s . the total gas flow rate are presented. These data are interpreted in terms of the deposition mechanism and the homogeneous gas phase reaction as follows. (1) At low values of the total gas flow rate, where higher Sill4 partial pressures and longer mixing times of reactant gases are obtained, a mass transport control (in terms of input gas flow rate limitation) is demonstrated. A homogeneous gas phase reaction is characteristic of this deposition region and is determined by the Sill4 partial pressure and/or mixing time. (2) At medium values of the total gas flow rate the mass transport control ceases to determine the deposition kinetics, which is considered to be diffusion or kinetically limited, while the intensity of the homogeneous gas phase reaction is strongly decreased. (3) At high values of the total gas flow rate where low input Sill4 partial pressures and low mixing times of reactant gases are obtained it is shown that the surface kinetics control the deposition rate.
1. INTRODUCTION Earlier papers on the preparation of S i O 2 films by low temperature chemical vapour deposition (LTCVD) from Sill4 oxidation have shown that a controlled surface reaction can be readily obtained by diluting the Sill4 with an inert gas prior to reacting with 0 2 1.2. Later kinetic studies a-s have investigated the effect of most of the experimental process variables on the film by using different types of reactors. However, there is no detailed quantitative analysis of the role of the carrier gas in the deposition of LTCVD SiO2 films at atmospheric pressure. Based on the calculated plots of Sill4 partial pressure and the mixing time of gases as a function of the total gas flow rate, in conjunction with the dependence of the deposition rate on 0040-6090/87/$3.50
© ElsevierSequoia/Printedin The Netherlands
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c . COBIANU, C. PAVELESCU, E. SEGAL
different process variables, an insight into the theoretical mechanism of L T C V D SiO2 film deposition is intended. In this paper, the effect of the carrier gas on the deposition kinetics (in terms of the reaction rate control and the extent of the homogeneous gas phase reaction) is studied by means of various experiments that will be described later. 2. EXPERIMENTAL PROCEDURE The L T C V D SiO2 films were deposited onto cleaned and polished silicon wafers from diluted Sill 4 (5% Sill 4 in argon) and 0 2 (both of them diluted subsequently in N2) in a nozzle-type reactor 6. The film thickness was measured with a Tencor Instruments stylus type profilometer. The deposition temperature was k e p t constant at 400°C and measured with a surface thermometer, Technoterm 9500. During these experiments the Sill4 flow rate was varied from 15 to 90 standard cm 3 m i n - 1 while the total gas flow rate was increased from 1 to 16.7 standard l m i n -1. The effect of the carrier gas on the Sill4 oxidation rate in this open flow system was studied by increasing proportionally the flow rates of N2 and the reactant gases so as to keep constant the partial pressure, or by increasing the N2 flow rate while the flow rates of the reactants were kept constant. In other experiments the 0 2 flow rate was varied with the N 2 flow rate as a parameter in the range 5-16 standard 1 m i n - 1 for a constant Sill4 flow rate of 30 standard cm 3 m i n - 1. 3. RESULTS We will discuss the effect of the carrier gas on the Sill, oxidation reaction in terms of the SiO 2 deposition rate, the extent of the homogeneous gas phase reaction and the reaction rate control based on the following experimental results. Figure l(a) presents the dependence of the deposition rate and the partial pressure of Sill4 as a function of the total gas flow rate when the S i l l , and 0 2 flow rates are kept constant (flow rate of Sill4, 30 standard cm 3 m i n - 1; ratio of 0 2 to Sill,, 12.5) and the deposition temperature is held constant at 400 °C. The deposition rate presents an increase-maximum-decrease type of dependence while the partial pressure of S i l l , shows a hyperbolic variation when the total gas flow rate is increased. When the total gas flow rate is varied from 1 to 3 standard 1min - 1 a fast increase in the deposition rate is obtained after which it levels off for the total gas flow rate ranging from 3 to about 5 standard 1m i n - 1. For higher values of the total gas flow rate the deposition rate begins to decrease, reaching half of its m a x i m u m value at a total gas flow rate of 16.7 standard 1 m i n - x. In Fig. l(b) the dependence of the deposition rate and the mixing time of reactant gases are presented as a function of the total gas flow rate when the flow rates of all gases are increased proportionally as follows: N 2 :Oz:SiH * = 150:15:1 with the S i l l , flow rate increasing from 15 to 90 standard cm 3 min-1. The partial pressures of the gases are Psin4 = 5 x 10- 3 atm, Po2 = 8 x 10- 2 atm, PN2 = 0.87 atm and PAr = 4.47 × 10 -2 atm. The mixing time is defined as the time available for the
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reactant gases to come into contact and is calculated by dividing the small space between the bottom of the reaction gas head and the deposition susceptor by the linear velocity of the gases. (The last parameter is obtained by dividing the total gas flow rate (in standard centimetre cubed per minute) by the distribution area (in centimetre squared) as in ref. 6). In this case, when the flow rates of all gases are increased proportionally (Fig. l(b)), at low values of the total gas flow rate the deposition rate increases rapidly. For higher values of the total gas flow rate the deposition rate increases slowly or even saturates (for total gas flow rates higher than 14 standard litre per minute)• Similarly to the results of the partial pressure of Sill 4 in Fig. l(a), the mixing time is a function of the total gas flow rate.
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Fig. 1. (a) T h e d e p e n d e n c e o f t h e d e p o s i t i o n r a t e a n d p a r t i a l p r e s s u r e o f S i H 4 o n t h e t o t a l g a s f l o w r a t e a t constant 02 and Sill4 flow rates: Sill4, 30 cm 3 m i n - 1; ratio of O2 of SiH~, 12.5; deposition temperature, 400 °C. (b) The dependence of the deposition rate and mixing time of the reactant gases on the total gas flow rate at constant partial pressure of the gases (N2 :O2: Sill4 = 150:15:1). Deposition temperature, 400 °C; Sill4 flow rate in the range 15-90 cm a min - 1
In Fig. 2, the dependence of the deposition rate on the 0 2 flow rate with the total gas flow rate as a parameter is presented. The Sill 4 flow rate is kept constant at 30 standard c m 3 m i n - 1 while the deposition temperature is 400 °C. For the range 5-16 standard I m i n - 1 of the total gas flow rate, the deposition rate shows the same type of dependence (increase-maximum-decrease) on the 0 2 flow rate, even if the m a x i m u m deposition rate is very sensitive to the total gas flow rate (as a parameter), in agreement with the results presented in Fig. l(a). In the decreasing region of the dependence of the deposition rate on the ratio o f O 2 t o Sill4, the effect of the carrier gas on the deposition rate decreases.
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Fig. 2. The dependence of the deposition rate on the 0 2 flow rate at an Sill4 flow rate of 30 cm3 min- ~,a deposition temperature of 400 °C and the total gas flow rate F as a parameter as follows: curve 1, F = 16 standard I min- 1; curve 2, F = 11 standard I min- 1; curve 3, F = 5 standard 1 min - i. 4. DISCUSSION The experimental results presented earlier will be discussed as a function of the total gas flow rate with special emphasis on the types of rate control of the deposition reaction and the extent of the h o m o g e n e o u s gas phase reaction.
4.1. Low values of the total gas flow rate At low N 2 flow rates a fast increase in the deposition rate is obtained for an increase in the N 2 flow rate, in spite of a decrease in the Sill 4 partial pressure (Fig. l(a)). The strong sensitivity of the deposition rate to the total gas flow rate m a y indicate a mass transport control (in terms of input rate limitation) of the surface reaction 9. However, the h o m o g e n e o u s gas phase reaction has presented the highest rate in this deposition region. (The experimental observation of this fact is possible using a continuous nozzle-type reactor1°.) In fact, the higher Sill4 partial pressures and mixing times associated with this region can be directly correlated with a higher development of the h o m o g e n e o u s gas phase reaction, in agreement with the experimental findings of other workers 11-x 3. Similar to the results presented in Fig. l(a), the deposition rate data from Fig. l(b) show that at low values of the total gas flow rate (with partial pressure kept constant) the deposition rate increases rapidly with the total gas flow rate while the mixing time has the largest values. Therefore, we can consider that there is e n o u g h time for the reactant gases to equilibrate with the deposition surface and thus an input gas rate control (or t h e r m o d y n a m i c control) can be assumed 9 to determine the
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Sill4 oxidation reaction. In fact, in the deposition region of low values of the total gas flow rate, the increased rate of the homogeneous gas phase reaction is obtained for a constant Psin, of 0.005 atm (Fig. l(b)) which is much lower than the range of Psm, in the same deposition region from Fig. l(a) (0.02-0.03 atm). Therefore, the extent of the homogeneous gas phase reaction is determined either by Ps~ri,(Fig. l(b)) or the mixing time (Fig. l(a)). The combined effect of both of them can be easily understood. The concentrations of vitreous silicate particulates (0.3-0.5 ~ n in diameter), dispersed in a bell-jar-type reactor and measured using a laser light scattering technique, increased with a decrease in the total gas flow rate with constant reactant concentrations 13. This is in agreement with our qualitative observations on the homogeneous gas phase reaction corresponding to experimental conditions presented in Fig. l(b). A coagulation model for the formation of large SiO2 particulates from small particulates is presented in ref. 13. A similar process of cluster formation takes place in the case of the homogeneous gas phase reaction for polycrystalline silicon deposition at higher partial pressures of silane in low pressure (LP) CVD systems. This phemomenon has been explained by a homogeneous gas phase polymerization reaction in which a monomer reacts with an oligomer and so on, finally producing a large silicon cluster 11. 4.2. Medium values of the total gas flow rate The non-dependence of the deposition rate on the flow conditions (Fig. l(a)) supports the fact that in this deposition region the mass transport control ceases to determine the deposition kinetics. In a similar manner, the results from Fig. l(b), obtained at constant partial pressures of the gases involved, show that at medium values of the total gas flow rate the deposition rate increases more slowly. This is consistent with a change in the type of rate control (to a diffusionally or kinetically limited regime) as is suggested in ref. 9. However, in agreement with the above results, the intensity of,the homogeneous gas phase reaction decreased strongly in this deposition region. 4.3. High values of the total gas flow rate This deposition region can be considered to begin at total gas flow rates where the deposition rate decreases (Fig. l(a)) or slows its increase (Fig. l(b)) with an increase in the total gas flow rate. Figures l(a) and l(b) show that this region can be characterized by low partial pressures of Sill4 and low mixing times of the reactant gases. These two process variables, combined with higher ratios of 02 to Sill 4 (12.5 and 16) associated with those experiments (Figs. l(a) and l(b)) correlate with a strong inhibition of the homogeneous gas phase reaction in agreement with other results 6'13. However, when values of the total gas flow rate were higher, the homogeneous gas phase reaction was present again at the lower ratios of O2 to Sill4 without affecting the uniformity of the deposited layers, unlike the results of silicon deposition where the homogeneous gas phase polymerization was associated with film non-uniformities on the same wafer and from wafer to wafer 11. The effect of the ratio of 02 to Sill, on the film quality was demonstrated by dielectric breakdown
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experiments. It was shown that at the lowest ratios of 0 2 t o Sill 4 used for the deposition the quality of the LTCVD SiO2 films is poor (in terms of low dielectric breakdown) owing to a higher number of SiO2 particulates incorporated in the structure of the layer during deposition 14. In the range of high values of total gas flow rates an important step in the determination of the type of rate control and deposition mechanism can be obtained by analysing the combined effect of the carrier gas and the oxygen flow rate with the Sill, flow rate kept constant (Fig. 2). For the range 5-161 m i n - ~ of the carrier gas used the maximum deposition rate as a function of the ratio of 0 2 to Sill4 strongly decreases with the increase in the N2 flow rate. This result is consistent with the falling off region of the deposition rate presented in Fig. l(a). Taking into account that the N 2 can be physically adsorbed on silica surfaces up to high temperatures t5 (about 1000 °C) an inhibition of the deposition rate by N 2 adsorption on the reaction surface has been earlier proposed 7. In the region where the deposition rate decreases with the increase in the ratio of O2 to Sill 4 (Fig. 2) the effect of the carrier gas on the deposition rate decreases. This fact can be explained by an increased adsorption of 0 2 with respect to N2 on the same surface and thus a rate control by 0 2 adsorption can be considered. Moreover, this type of dependence was obtained for all types of reactors 1'4'5'7 independent of their geometries. All these data suggest that for higher values of the carrier gas flow rate the surface kinetics control the deposition rate. The atomistic model associated with the kinetic control of the deposition rate, which is based on the experimental dependence of the deposition rate on the 0 2 and Sill4 flow rates and the efficiency of the Sill 4 oxidation reaction in chemical vapour deposition of SiO2 films at low temperatures t6, was presented earlier 7 in terms of a bimolecular surface reaction theory that explained the experimental dependence of the deposition rate on temperature, 02, Sill, and N2 flow rates (at high values). 5. CONCLUSIONS
The carrier gas has a strong effect on the deposition kinetics of LTCVD S i O 2 films based on the reaction between diluted Sill 4 and 02. At low values of the total gas flow rate where higher Sill, partial pressures and mixing times of the reactant gases are obtained a mass transport control of the surface reaction is demonstrated. A strong homogeneous gas phase reaction is characteristic of this deposition region. A correlation between the coagulation model for the formation of large SiO2 particulates 13 and the formation of clusters in the case of the homogeneous gas phase reaction for polycrystaUine silicon deposition at higher partial pressure of Sill, 11 is presented in this paper. At medium values of the total gas flow rate, the mass transport control ceases to determine the deposition kinetics while the intensity of the homogeneous gas phase reaction is strongly decreased. At high values of the total gas flow rate, where low input partial pressures of Sill4 and low mixing times of the reactant gases are obtained, it is shown that the surface kinetics control the deposition rate. In the range of high values of the total gas flow rate, the maximum deposition rate as a function of the ratio of 0 2 to Sill 4
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decreases with the increase in the N 2 flow rate as a result of i n h i b i t i o n of d e p o s i t i o n by physical a d s o r p t i o n o f N 2 on the r e a c t i o n surface. At high values of the ratio of 0 2 to S i l l , , the effect of the carrier gas on the d e p o s i t i o n rate decreases o w i n g to an increased a d s o r p t i o n of 0 2 with respect to N 2 on the same surface an d thus a rate c o n t r o l by 0 2 a d s o r p t i o n on the surface c a n be considered. T h e a t o m i s t i c m o d e l associated with the kinetic c o n t r o l of the d e p o s i t i o n rate is presented in terms of b i m o l e c u l a r surface reactions. ACKNOWLEDGMENT T h e a u t h o r s wish to t h a n k Mrs. D o r i n a C o b i a n u for the d r a w i n g of the graphics of this paper. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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