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The kinetics of silicon dioxide chemical vapour deposition I: Surface chemical reactions
Surface Technology, 25 (1985) 307 - 313
307
T H E KINETICS OF SILICON DIOXIDE CHEMICAL VAPOUR DEPOSITION I: SURFACE CHEMICAL REACTIONS PIOTR B. GRABIEC Institute o f Electron Technology CEMI. A1. Lotnik6w 32/46, 02-668 Warsaw (Poland)
JAN PRZYLUSKI Institute o f General Chemistry and Inorganic Technology, Warsaw Technical University, ul. Noakowskiego 3,00-663 Warsaw (Poland)
(Received October 10, 1984)
Summary Th e process o f SiO 2 deposition by oxidizing silane with oxygen is widely applied in manufacturing integrated circuits. However, its mechanism and kinetics are n o t fully u n d e r s t o o d . In this paper a general analysis o f chemical vapour deposition o f silicon dioxide is presented. In the first part, it is p o in ted o u t t hat in the case of low t e m p e r a t u r e SiO2 deposition it is necessary to consider all process stages, i.e. diffusion, adsorption and chemical reactions. T h e electron structures of reagents are analysed. As a result o f these studies a mechanism for t h e surface reactions is proposed.
1. I n t r o d u c t i o n Chemical vapour deposition (CVD) processes are widely applied in the s e m i c o n d u c t o r industry t o deposit SiOz, Si3N4, polycrystalline silicon (poly-Si), epitaxial silicon (epi-Si) and o t h e r layers. An extensive review of the literature on the applications of CVD m e t h o d s has been presented by Kern and Schnable [1]. In order t o utilize rationally CVD m e t h o d s it is necessary to u n d e r s t a nd the way in which the process parameters influence the kinetics and mechanisms of the reaction. F r o m consideration o f the kinetics of CVD processes, two regions m a y be distinguished: a kinetic region, wherein the deposition rate is limited (controlled) by surface processes and a diffusive region, wherein the deposition rate is limited (controlled) by diffusive transport of reagents. The t y p e o f cont r ol depends on the conditions in which the process is carried on. Generally, the process is kinetically cont rol l ed at lower temperatures, and diffusion limits the deposition rate at higher temperatures. As a rule it is preferable to p e r f o r m the CVD process at higher temperatures, in the diffusive region. U nde r such conditions the deposition rate and the 0376-4583/85/$3.30
© Elsevier Sequoia/Printed in The Netherlands
308 structure o f the deposited layer are less influenced by the surface properties, which are difficult to control. U n f o r t u n a t e l y , because of t he desired structure of the deposited layer, it is of t e n necessary to perform the deposition at low temperature, in a mixed or even kinetic region. Besides, it is a general t e n d e n c y in microelectronics to lower the tem perat ure of the technological processes. When the process is c ont r ol l ed by surface processes (kinetic region), then for process modelling it is necessary to know the mechanism and the kinetics of the surface processes. An example of such a low temperature CVD process m ay be the deposition of silicon dioxide by reacting silane with oxygen. This process has been applied in s e m i c o n d u c t o r technology for over 20 years. T he dependence of the deposition rate on process parameters has been r e p o r t e d by num e r ous authors [ 2 - 5]. However, the mechanism and the kinetics of CVD of silicon dioxide are n o t fully understood. In recent years the kinetics of CVD processes have been the subject of n u mer o u s studies. In the existing models of CVD it is assumed that growth is controlled by diffusion in the gas phase [6, 7], mass transport and surface reactions [8] or surface reactions and adsorption [3, 9]. These simplifying assumptions result in a limited range of process parameters for which the mo d el is valid. Shaw [10] and Korec [11 - 1 3 ] p o i n t out that there is a possibility of considering all the process stages and then combining various types of growth control in a single model. T he m e t h o d o f CVD modelling proposed by Korec, although very useful in the case of vapour phase silicon epitaxy, shows some difficulties when applied to low t e m p e r a t u r e silicon dioxide deposition. The reasons are as follows. (i) A simplified description of the adsorption (only two e x t r e m e cases are considered, the first in which the surface is saturated with reagents, and the second in which the adsorbed reagent atoms block only a small n u m b e r o f active sites) is used. (ii) It is assumed t hat surface reaction occurs according to the overall equation. As a result o f these simplifications it is possible to obtain a theoretical formula linear with regard to u n k n o w n coefficients, and then to carry o u t c o m p u t a t i o n s with relatively simple mathematical m e t h o d s (linear regression). However, in the case of CVD of silicon dioxide these simplifications seem to be t o o far reaching. In this paper such simplifying assumptions are n o t introduced. T he proposed model o f CVD of silicon dioxide is based on detailed analysis of a hypothetical mechanism of surface reactions. F u r t h e r m o r e , we have a t t e m p t e d to avoid the assumption o f t h e r m o d y n a m i c equilibrium in the system. The m e t h o d of modelling applied in this paper comprises: (i) definition, based on the electron structures of the reagents, of a h y po th etical mechanism f or t he surface chemical reactions, (ii) selection o f characteristic regions, described by reagent concentrations, th at means t hat a real system of distributed parameters is replaced by a model employing lumped parameters,
309 (iii) mathematical description of all fluxes in each characteristic region, based on the mechanism adopted for surface reactions, (iv) solution of the set of equations obtained with respect to deposition rate, (v) computation, based on a set of experimental data, of the u n k n o w n coefficients, (vi) verification of the physical sense of c o m p u t e d coefficients and (vii) verification of the c o n f o r m i t y of the model with an experiment. In the first part of this study, the analysis of the electron structures of the reagents is presented and a mechanism of surface reactions is proposed. In the second part the mathematical model of the process considered will be presented. In a third part, the proposed model will be experimentally verified. The m e t h o d o f modelling described and the model of SiO 2 deposition are presented more extensively in a doctoral thesis [14].
2. The electron structures o f the reagents For temperatures ranging from 513 K to 723 K the overall equation of the reaction considered is [ 15] SiO2 surface SiH4(g) + O2(g) ) SiO2(s ) + 2H2(g ) (1) Therefore, the reagents are oxygen, silane and the surface of SiO2 layer. The silicon dioxide layer deposited by the reaction of silane with oxygen is amorphous [16]. The amorphous silica is built of disordered SiO44- tetrahedrons. The mean distances between the atoms are [16] 0.162 nm for Si-O, 0.305 nm for Si-Si and 0.260 nm for O-O. The density of deposited silicon dioxide is approximately 2.1 g cm -3 [16, 17]. Since the difference in silicon and oxygen electronegativities is large (1.8 for silicon as against 3.5 for oxygen [18]), t h e Si-O bonds have a partly ionic character. At room temperature the silicon bonds at the surface (dangling bonds) are saturated with h y d r o x y l groups. However, at temperatures above 573 K the surface undergoes a dehydration process [ 19] O--H
H--O
I
--Si--
I
I
+
--Si--
I
jO~ ~ --Si--
I
--Si--
I
(2)
Therefore, the silicon dioxide surface consists of positively charged silicon atoms and negatively charged oxygen atoms. The structure of the Sill4 molecule m a y be described by means of molecular orbital (MO) theory, using the linear combination of the atomic orbitals (LCAO) approximation. According to this theory the Sill4 molecule is similar in shape to CH4. It is a tetrahedron structure with a silicon atom in the centre and hydrogen atoms at the corners. The MO comprises four equivalent Si--H bonds. Since the electronegativities of hydrogen and silicon are different, the Si--H bonds are partially polarized. According to Pauling
310 [18], the electronegativity of silicon (1.8 eV) is lower than that of hydrogen (2.1 eV). This means that in contrast to the CH 4 molecule, the hydrogen atoms have some negative charge 5- and the silicon atom has some positive charge 5 +. Such a structure of the Sill 4 molecule suggests that its chemisorption occurs because of the interaction of the negatively charged hydrogen atoms with positively charged surface active centres. Therefore, the electron responsible for the Si--H bond in the Sill4 molecule is transferred into the silicon dioxide surface. This process induces the dissociation of the adsorbed Sill 4 molecule. According to the MO theory, the electron structure of the oxygen molecule, written in generally used notation is [ 20] (ols)2(o*ls)2(O2S)2(O*2S)2(o2p)2(Trzp)4(Trx*2p)l(Try*2p)
1
Successive orbitals are filled with pairs of electrons, but the antibonding orbitals (nx*2p) and (Try*2p) are occupied only by single electrons. The large difference between the silicon and oxygen electronegativities as well as a higher energy of the Si--O bond (807.5 kJ mol 1) than the O--O bond (498 kJ mo1-1) suggest that oxygen chemisorption does not proceed on an oxygen active centre, but on silicon centres. The chemical bond between the 02 molecule and the surface is formed by the transition of an electron from the surface active centre to the antibonding orbital (Trx*2p) or (ny*2p}. In this way the chemical bond between oxygen atoms is weakened and chemisorbed oxygen molecules tend to dissociate.
3. The mechanism o f the surface reactions On the basis of the above considerations as well as the results of Henis e t al. [21] and Tung Yang Yu e t al. [22] a hypothetical mechanism for the
surface reactions, at temperatures above 573 K, is proposed (Fig. 1). It is our working hypothesis that oxygen reactions, i.e. adsorption (step e) followed by dissociation (step f), and silane reactions, i.e. adsorption {step a) followed by dissociation (step b), and dehydration (steps c and g) occur independently. The only dependence between these two chains of reactions is that as a result of silane reactions, new active centres for competitive adsorption of both reagents are created. In addition, since the deposited oxide is amorphous and approximately stoichiometric, the number of negatively charged oxygen active centres is approximately twice as high as the number of positively charged silicon active centres. Integral charges have been assigned to the atoms for simplicity. Since the atoms in SiO 2 are linked with partially polarized covalent bonds, and accepting that the structure of the silicon dioxide surface is defective, we may suppose that the real charges may vary. This simplification does not change the essence of the problem under consideration. The sequence shown in Fig. 1 is only one possible path. In the real process the
311 H
'
'I/
--Si--
IOI
r' H l e-_ H o~iH _
-
J"
.L 1
1°
*_
~
S;
---
OI
'
i{
Sill s
H
ioI, 1
._5~-- ~ :
i~-/
--Si-i )•
,ol "*
÷:(
I01 I z-
~-
o H t
-H 2 I~A
,e *Si--H
,e I01 ell
161e
'
x_/~:
+Ot
He
H le -Si*
1,
÷ .o.~-~..o_ _
.,..e eH H-Si-O~,~Q. ,
_
°t i j•
-
:
'?,'.
-
-
°
Fig. 1. The scheme of surface reactions in silicon dioxide chemical vapour deposition.
situation is mo r e complicated. F o r example, after step b and after step f new centres f o r silane and o x y g e n adsorption have already been created. It should also be n o t e d t ha t according to the results published by T aft [23], the absorbances of the bands at 2230 cm -] and 880 cm -] identified by Steward and Nielsen [24] as characteristic for Si--H bonds, decrease with an increase in tem per a t ur e, and reach values close to zero at a t e m p e r a t u r e a b o u t 573 K. Both observations m e n t i o n e d above, as well as the easy h y d r o g e n separation r e p o r t e d by Tung Yang Yu e t al. [22] and Henis e t al. [21], suggest th at for t e m p e r a t u r e s exceeding 573 K onl y steps a, b, e and f are significant and the remaining steps are negligible. Our hypothesis m a y be regarded as the m o s t probable, m o r e particularly since the apparent activation energy o f SiO2 deposition r e p o r t e d by Strater (335 kJ mo1-1 [15])
312 is equal to the e n e r g y w h i c h is necessary for breaking the H3Si--H b o n d . It s h o u l d be n o t e d t h a t this b o n d is b r o k e n in step b. U n f o r t u n a t e l y , o w i n g t o t h e lack o f sufficiently precise e x p e r i m e n t a l m e t h o d s , it is impossible to verify the p r o p o s e d m e c h a n i s m e x p e r i m e n t a l l y . H o w e v e r , it is possible to c o n s t r u c t a m o d e l o f the CVD o f silicon dioxide, t a k i n g a c c o u n t o f all the i m p o r t a n t process stages, based o n such a m e c h a n i s m . T h e m a t h e m a t i c a l m o d e l o f silicon d i o x i d e CVD, based o n the m e c h a nism f o r surface r e a c t i o n s described, will be p r e s e n t e d in the s e c o n d p a r t o f this s t u d y .
4. C o n c l u s i o n s T h e process of CVD o f silicon d i o x i d e occurs because o f the r e a c t i o n o f silane and o x y g e n o n t h e silicon d i o x i d e surface. T h e silane a n d o x y g e n are c h e m i s o r b e d o n the same positively charged active sites. T h e silane c h e m i s o r p t i o n o c c u r s because o f the i n t e r a c t i o n b e t w e e n h y d r o g e n a t o m s a n d t h e active sites. As a result o f this i n t e r a c t i o n , the H3Si--H b o n d is w e a k e n e d and a dissociation o f c h e m i s o r b e d silane takes place. The o x y g e n c h e m i s o r p t i o n o c c u r s because o f the i n t e r a c t i o n b e t w e e n partially o c c u p i e d a n t i b o n d i n g orbitals and active sites. This process induces dissociation of t h e o x y g e n m o l e c u l e . T h e c o m p l e t e s c h e m e o f the surface reactions, based o n an analysis o f t h e e l e c t r o n s t r u c t u r e s o f the reagents, is s h o w n in Fig. 1. This h y p o t h e t i c a l m e c h a n i s m s h o u l d be verified b y m o d e l l i n g the process. T h e results p r e s e n t e d in this p a p e r c o n f i r m t h a t the a s s u m p t i o n s a d o p t e d b y Baliga and G h a n d h i [ 3 ] are reasonable.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
W. Kern and G. L. Schnable, IEEE Trans. Electron. Devices, 26 (1979) 647. W. Kern, G. L. Schnable and A_ W. Fisher, R C A Rev., 37(1976)3. B. J. Baliga and S. K. Ghandhi, J. Appl. Phys., 44 (1973) 990. W. Kern and R. S. Rosler, J. Vac. Sci. Technol., 14 (1977) 1053. C. Cobianu and C. Pavelescu, J. Electrochem. Soc., 130 (1983) 1888. C. W. Manke and L. F. Donaghey, J. Electrochem. Soc., 124 (1977) 561. J. Subrahmanyam, A. K. Lahiri and K. P. Abraham, J. Electrochem. Soc., 127 (1980) 1394. W. A. P. Claassen and J. Bloem, PhilipsJ. Res., 36 (1981) 124. M. L. Hitchman and J. Kane, J Cryst. Growth, 55 (1981)385. D. W. Shaw, in C. H. L. Goodman (ed.), Crystal Growth Theory and Technique, Vol. 1, Plenum Press, New York, 1974. J. Korec and J. Borkowicz, Electron Technol., I0 (1977) 3. J. Korec, J. Cryst. Growth, 46 (1979) 362. J. Korec, Electron Technol., 11 (1978) 73. P. B. Grabiec, Doctoral Thesis, Warsaw Technical University, Warsaw, 1984. K. Strater, R C A Rev., 29 (1968) 618. N. Nagasima, J. Appl. Phys., 43 (1972) 3378. B. E. Deal, P. J. Fleming and P. L. Castre, J. Electrochem. Soc., 115 (1968) 300.
313 18 L. Pauling, The Nature o f the Chemical Bond, Cornell University Press, Ithaca, N.Y., 1960. 19 L. R. Snyder, Sep. Sci., 1 (1966) 191. 20 W. Kolos, Chemia Kwantowa, PWN, Warsaw, 1975. 21 J. M. S. Henis, G. W. Stewart, M. K. Tripodi and G. P. Gaspar, J. Chem. Phys., 57 (1972) 389. 22 Tung Yang Yu, T. M. H. Cheng, V. Kempter and F. W. Lampe, J. Phys. Chem., 76 (1972) 3321. 23 E. A. Taft, J. Electrochem. Soc., 126 (1979) 1728. 24 W. B. Steward and H. H. Nielsen, Phys. Rev., 47 (1935) 828.
307
T H E KINETICS OF SILICON DIOXIDE CHEMICAL VAPOUR DEPOSITION I: SURFACE CHEMICAL REACTIONS PIOTR B. GRABIEC Institute o f Electron Technology CEMI. A1. Lotnik6w 32/46, 02-668 Warsaw (Poland)
JAN PRZYLUSKI Institute o f General Chemistry and Inorganic Technology, Warsaw Technical University, ul. Noakowskiego 3,00-663 Warsaw (Poland)
(Received October 10, 1984)
Summary Th e process o f SiO 2 deposition by oxidizing silane with oxygen is widely applied in manufacturing integrated circuits. However, its mechanism and kinetics are n o t fully u n d e r s t o o d . In this paper a general analysis o f chemical vapour deposition o f silicon dioxide is presented. In the first part, it is p o in ted o u t t hat in the case of low t e m p e r a t u r e SiO2 deposition it is necessary to consider all process stages, i.e. diffusion, adsorption and chemical reactions. T h e electron structures of reagents are analysed. As a result o f these studies a mechanism for t h e surface reactions is proposed.
1. I n t r o d u c t i o n Chemical vapour deposition (CVD) processes are widely applied in the s e m i c o n d u c t o r industry t o deposit SiOz, Si3N4, polycrystalline silicon (poly-Si), epitaxial silicon (epi-Si) and o t h e r layers. An extensive review of the literature on the applications of CVD m e t h o d s has been presented by Kern and Schnable [1]. In order t o utilize rationally CVD m e t h o d s it is necessary to u n d e r s t a nd the way in which the process parameters influence the kinetics and mechanisms of the reaction. F r o m consideration o f the kinetics of CVD processes, two regions m a y be distinguished: a kinetic region, wherein the deposition rate is limited (controlled) by surface processes and a diffusive region, wherein the deposition rate is limited (controlled) by diffusive transport of reagents. The t y p e o f cont r ol depends on the conditions in which the process is carried on. Generally, the process is kinetically cont rol l ed at lower temperatures, and diffusion limits the deposition rate at higher temperatures. As a rule it is preferable to p e r f o r m the CVD process at higher temperatures, in the diffusive region. U nde r such conditions the deposition rate and the 0376-4583/85/$3.30
© Elsevier Sequoia/Printed in The Netherlands
308 structure o f the deposited layer are less influenced by the surface properties, which are difficult to control. U n f o r t u n a t e l y , because of t he desired structure of the deposited layer, it is of t e n necessary to perform the deposition at low temperature, in a mixed or even kinetic region. Besides, it is a general t e n d e n c y in microelectronics to lower the tem perat ure of the technological processes. When the process is c ont r ol l ed by surface processes (kinetic region), then for process modelling it is necessary to know the mechanism and the kinetics of the surface processes. An example of such a low temperature CVD process m ay be the deposition of silicon dioxide by reacting silane with oxygen. This process has been applied in s e m i c o n d u c t o r technology for over 20 years. T he dependence of the deposition rate on process parameters has been r e p o r t e d by num e r ous authors [ 2 - 5]. However, the mechanism and the kinetics of CVD of silicon dioxide are n o t fully understood. In recent years the kinetics of CVD processes have been the subject of n u mer o u s studies. In the existing models of CVD it is assumed that growth is controlled by diffusion in the gas phase [6, 7], mass transport and surface reactions [8] or surface reactions and adsorption [3, 9]. These simplifying assumptions result in a limited range of process parameters for which the mo d el is valid. Shaw [10] and Korec [11 - 1 3 ] p o i n t out that there is a possibility of considering all the process stages and then combining various types of growth control in a single model. T he m e t h o d o f CVD modelling proposed by Korec, although very useful in the case of vapour phase silicon epitaxy, shows some difficulties when applied to low t e m p e r a t u r e silicon dioxide deposition. The reasons are as follows. (i) A simplified description of the adsorption (only two e x t r e m e cases are considered, the first in which the surface is saturated with reagents, and the second in which the adsorbed reagent atoms block only a small n u m b e r o f active sites) is used. (ii) It is assumed t hat surface reaction occurs according to the overall equation. As a result o f these simplifications it is possible to obtain a theoretical formula linear with regard to u n k n o w n coefficients, and then to carry o u t c o m p u t a t i o n s with relatively simple mathematical m e t h o d s (linear regression). However, in the case of CVD of silicon dioxide these simplifications seem to be t o o far reaching. In this paper such simplifying assumptions are n o t introduced. T he proposed model o f CVD of silicon dioxide is based on detailed analysis of a hypothetical mechanism of surface reactions. F u r t h e r m o r e , we have a t t e m p t e d to avoid the assumption o f t h e r m o d y n a m i c equilibrium in the system. The m e t h o d of modelling applied in this paper comprises: (i) definition, based on the electron structures of the reagents, of a h y po th etical mechanism f or t he surface chemical reactions, (ii) selection o f characteristic regions, described by reagent concentrations, th at means t hat a real system of distributed parameters is replaced by a model employing lumped parameters,
309 (iii) mathematical description of all fluxes in each characteristic region, based on the mechanism adopted for surface reactions, (iv) solution of the set of equations obtained with respect to deposition rate, (v) computation, based on a set of experimental data, of the u n k n o w n coefficients, (vi) verification of the physical sense of c o m p u t e d coefficients and (vii) verification of the c o n f o r m i t y of the model with an experiment. In the first part of this study, the analysis of the electron structures of the reagents is presented and a mechanism of surface reactions is proposed. In the second part the mathematical model of the process considered will be presented. In a third part, the proposed model will be experimentally verified. The m e t h o d o f modelling described and the model of SiO 2 deposition are presented more extensively in a doctoral thesis [14].
2. The electron structures o f the reagents For temperatures ranging from 513 K to 723 K the overall equation of the reaction considered is [ 15] SiO2 surface SiH4(g) + O2(g) ) SiO2(s ) + 2H2(g ) (1) Therefore, the reagents are oxygen, silane and the surface of SiO2 layer. The silicon dioxide layer deposited by the reaction of silane with oxygen is amorphous [16]. The amorphous silica is built of disordered SiO44- tetrahedrons. The mean distances between the atoms are [16] 0.162 nm for Si-O, 0.305 nm for Si-Si and 0.260 nm for O-O. The density of deposited silicon dioxide is approximately 2.1 g cm -3 [16, 17]. Since the difference in silicon and oxygen electronegativities is large (1.8 for silicon as against 3.5 for oxygen [18]), t h e Si-O bonds have a partly ionic character. At room temperature the silicon bonds at the surface (dangling bonds) are saturated with h y d r o x y l groups. However, at temperatures above 573 K the surface undergoes a dehydration process [ 19] O--H
H--O
I
--Si--
I
I
+
--Si--
I
jO~ ~ --Si--
I
--Si--
I
(2)
Therefore, the silicon dioxide surface consists of positively charged silicon atoms and negatively charged oxygen atoms. The structure of the Sill4 molecule m a y be described by means of molecular orbital (MO) theory, using the linear combination of the atomic orbitals (LCAO) approximation. According to this theory the Sill4 molecule is similar in shape to CH4. It is a tetrahedron structure with a silicon atom in the centre and hydrogen atoms at the corners. The MO comprises four equivalent Si--H bonds. Since the electronegativities of hydrogen and silicon are different, the Si--H bonds are partially polarized. According to Pauling
310 [18], the electronegativity of silicon (1.8 eV) is lower than that of hydrogen (2.1 eV). This means that in contrast to the CH 4 molecule, the hydrogen atoms have some negative charge 5- and the silicon atom has some positive charge 5 +. Such a structure of the Sill 4 molecule suggests that its chemisorption occurs because of the interaction of the negatively charged hydrogen atoms with positively charged surface active centres. Therefore, the electron responsible for the Si--H bond in the Sill4 molecule is transferred into the silicon dioxide surface. This process induces the dissociation of the adsorbed Sill 4 molecule. According to the MO theory, the electron structure of the oxygen molecule, written in generally used notation is [ 20] (ols)2(o*ls)2(O2S)2(O*2S)2(o2p)2(Trzp)4(Trx*2p)l(Try*2p)
1
Successive orbitals are filled with pairs of electrons, but the antibonding orbitals (nx*2p) and (Try*2p) are occupied only by single electrons. The large difference between the silicon and oxygen electronegativities as well as a higher energy of the Si--O bond (807.5 kJ mol 1) than the O--O bond (498 kJ mo1-1) suggest that oxygen chemisorption does not proceed on an oxygen active centre, but on silicon centres. The chemical bond between the 02 molecule and the surface is formed by the transition of an electron from the surface active centre to the antibonding orbital (Trx*2p) or (ny*2p}. In this way the chemical bond between oxygen atoms is weakened and chemisorbed oxygen molecules tend to dissociate.
3. The mechanism o f the surface reactions On the basis of the above considerations as well as the results of Henis e t al. [21] and Tung Yang Yu e t al. [22] a hypothetical mechanism for the
surface reactions, at temperatures above 573 K, is proposed (Fig. 1). It is our working hypothesis that oxygen reactions, i.e. adsorption (step e) followed by dissociation (step f), and silane reactions, i.e. adsorption {step a) followed by dissociation (step b), and dehydration (steps c and g) occur independently. The only dependence between these two chains of reactions is that as a result of silane reactions, new active centres for competitive adsorption of both reagents are created. In addition, since the deposited oxide is amorphous and approximately stoichiometric, the number of negatively charged oxygen active centres is approximately twice as high as the number of positively charged silicon active centres. Integral charges have been assigned to the atoms for simplicity. Since the atoms in SiO 2 are linked with partially polarized covalent bonds, and accepting that the structure of the silicon dioxide surface is defective, we may suppose that the real charges may vary. This simplification does not change the essence of the problem under consideration. The sequence shown in Fig. 1 is only one possible path. In the real process the
311 H
'
'I/
--Si--
IOI
r' H l e-_ H o~iH _
-
J"
.L 1
1°
*_
~
S;
---
OI
'
i{
Sill s
H
ioI, 1
._5~-- ~ :
i~-/
--Si-i )•
,ol "*
÷:(
I01 I z-
~-
o H t
-H 2 I~A
,e *Si--H
,e I01 ell
161e
'
x_/~:
+Ot
He
H le -Si*
1,
÷ .o.~-~..o_ _
.,..e eH H-Si-O~,~Q. ,
_
°t i j•
-
:
'?,'.
-
-
°
Fig. 1. The scheme of surface reactions in silicon dioxide chemical vapour deposition.
situation is mo r e complicated. F o r example, after step b and after step f new centres f o r silane and o x y g e n adsorption have already been created. It should also be n o t e d t ha t according to the results published by T aft [23], the absorbances of the bands at 2230 cm -] and 880 cm -] identified by Steward and Nielsen [24] as characteristic for Si--H bonds, decrease with an increase in tem per a t ur e, and reach values close to zero at a t e m p e r a t u r e a b o u t 573 K. Both observations m e n t i o n e d above, as well as the easy h y d r o g e n separation r e p o r t e d by Tung Yang Yu e t al. [22] and Henis e t al. [21], suggest th at for t e m p e r a t u r e s exceeding 573 K onl y steps a, b, e and f are significant and the remaining steps are negligible. Our hypothesis m a y be regarded as the m o s t probable, m o r e particularly since the apparent activation energy o f SiO2 deposition r e p o r t e d by Strater (335 kJ mo1-1 [15])
312 is equal to the e n e r g y w h i c h is necessary for breaking the H3Si--H b o n d . It s h o u l d be n o t e d t h a t this b o n d is b r o k e n in step b. U n f o r t u n a t e l y , o w i n g t o t h e lack o f sufficiently precise e x p e r i m e n t a l m e t h o d s , it is impossible to verify the p r o p o s e d m e c h a n i s m e x p e r i m e n t a l l y . H o w e v e r , it is possible to c o n s t r u c t a m o d e l o f the CVD o f silicon dioxide, t a k i n g a c c o u n t o f all the i m p o r t a n t process stages, based o n such a m e c h a n i s m . T h e m a t h e m a t i c a l m o d e l o f silicon d i o x i d e CVD, based o n the m e c h a nism f o r surface r e a c t i o n s described, will be p r e s e n t e d in the s e c o n d p a r t o f this s t u d y .
4. C o n c l u s i o n s T h e process of CVD o f silicon d i o x i d e occurs because o f the r e a c t i o n o f silane and o x y g e n o n t h e silicon d i o x i d e surface. T h e silane a n d o x y g e n are c h e m i s o r b e d o n the same positively charged active sites. T h e silane c h e m i s o r p t i o n o c c u r s because o f the i n t e r a c t i o n b e t w e e n h y d r o g e n a t o m s a n d t h e active sites. As a result o f this i n t e r a c t i o n , the H3Si--H b o n d is w e a k e n e d and a dissociation o f c h e m i s o r b e d silane takes place. The o x y g e n c h e m i s o r p t i o n o c c u r s because o f the i n t e r a c t i o n b e t w e e n partially o c c u p i e d a n t i b o n d i n g orbitals and active sites. This process induces dissociation of t h e o x y g e n m o l e c u l e . T h e c o m p l e t e s c h e m e o f the surface reactions, based o n an analysis o f t h e e l e c t r o n s t r u c t u r e s o f the reagents, is s h o w n in Fig. 1. This h y p o t h e t i c a l m e c h a n i s m s h o u l d be verified b y m o d e l l i n g the process. T h e results p r e s e n t e d in this p a p e r c o n f i r m t h a t the a s s u m p t i o n s a d o p t e d b y Baliga and G h a n d h i [ 3 ] are reasonable.
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