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Kinetics and mechanisms in CVD of boron
171
Journal of Ciystal Growth 94 (1989) 171—181 North-Holland, Amsterdam
KINETICS AND MECHANISMS IN CVD OF BORON Ulf JANSSON, Mats BOMAN and Jan-Otto CARLSSON Thin Film and Surface Chemistry Group, Department of Chemistry, University of Uppsala, Box 531, S-751 21 Uppsala, Sweden Received 14 May 1988; manuscript received in final form 31 August 1988
The influence of BC1
3, H2 and HCI on the deposition rate of boron has been investigated in a region controlled by chemical kinetics. The separate contributions of the various gases on the deposition rate was determined by using helium as as diluent gas. From the results, the following empirical rate equation was derived: r = Ap(BCI3)p”(H2)— Bp(HCI), with n = 0 and n = 0.5 for low and high partial pressures of BCI3, respectively. The influence of the substrate material and gas purity on the kinetics in boron CVD was also demonstrated. It was concluded that impurities in the BCI3 gas affected the deposition rates as well as the phase composition of the boron coatings. Moreover, it was found that the kinetics in boron CVD can be affected by the substrate material used. No single rate-determining step could be identified from the extensive general kinetics data set. Analysis of the data showed that the boron deposition may take place along the several alternative reaction paths which cannot be separated.
1. Introduction Boron can be deposited by chemical vapor deposition (CVD) from a reaction gas mixture of boron trichioride and hydrogen. The kinetics of this CVD process has been studied by several authors [1—4]and various rate-determining steps have been proposed to explain the results. Gruber considered the rate-determining step to be a surface reaction between atomically adsorbed hydrogen and some adsorbed boron-carrying molecule [1]. The hydrogen reduction of adsorbed dichloroborane or adsorbe& boron dichloride was proposed as a rate-limiting step by Carlton et al. [2]. Mehalso and Diefendorf stated that the deposition rate was limited partly by the desorption rate of hydrogen chloride and partly by the diffusion of hydrogen chloride from the surface of the deposit [3]. In a recent publication, Tanaka et al. also proposed that desorption of hydrogen chloride was a rate-limiting step in boron CVD [4]. The aim of this work was to investigate the kinetics in CVD of boron in a region controlled by chemical kinetics. Efforts were made to verify that the deposition process was controlled by chemical kinetics at vapor compositions used in this work. The influence of substrate material and
gas purity on the deposition process was also demonstrated.
2. Experimental procedure A low pressure CVD system with a cold-water reactor was used in this investigation. Details of the CVD system have been published elsewhere [5]. The leak rate of air into the system was less than i0~ Torr 1 s~. This leak rate adds less than 0.1 ppm of impurities to the reaction gas mixture at the gas flows generally used. Thin foils the molybdenum (purity 99.99%) and titanium (purity 99.9%), mounted parallel to the gas stream, were used as substrate materials. By virtue of the dimensions of the foil (1 mm broad and 100 ~sm thick), surface kinetics control was easily obtained. The substrates were resistively heated and the temperature was measured with a micro-optical pyrometer. Corrections for glass absorption and emissivity (c = 0.71) were applied. Prior to the deposition experiments, the substrates were cleaned by ultrasonic washing in pure ethanol. Finally, in order to remove oxides from the surface, the substrates were heat-treated in hydrogen for three minutes at 1400 K.
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
172
U. Jansson et al.
/ Kinetics and mechanisms on
Hydrogen (purity 99.9997%), helium (purity 99.9996%), boron trichloride (purities 99.99% (electronic grade) and 99.98%) and hydrogen chloride (purity 99.99%) were used as gases. The influence of BCI5 gas quality on the deposition process was demonstrated in two ways: Comparative deposition experiments with three different BC13 gases from two manufacturers. The claimed purity of the gases were: gas I 99.98% and gas II and III 99.99%. The main impurities in the BCI3 gases, as specified by the manufacturers, were COC12 (< 50 ppm) and Cl, (<40 ppm). Addition of small amounts of phosgene, COd7, which is a major impurity in BC13 gas. The phosgene was added as a COCI 2/He gas mixture (100 ppm COd, in He (purity 99.9996%)) This procedure allowed a controlled addition of very low phosgene concentrations. During the deposition experiments, boron atoms diffused into the substrate with the subse—
C VD of boron
3. Influence of impurities on the kinetics in boron
CVD There exist several sources of impurities which may affect the kinetics in a CVD process. The influence of impurities originating from the substrate material and the BCI3 gas is examined below.
=
=
—
quent formation of molybdenum horides. The loss of boron atoms from the coating ceased after a
deposition time of a few minutes. To obtain accurate growth-rate data, the substrates were precoated with a 12 ~rm thick boron coating from which virtually no further loss of boron occurred to the substrate, The deposition rates were obtained from thickness measurements on scanning electron micrographs of polished cross-sections of the samples. The deposition rate at a given set of experimental parameters was calculated from 1—S individual deposition experiments. Two phases of boron (amorphous and a-rhombohedral boron) were obtained in the deposition experiments. Literature values for the density of these two phases differ by less than 5% [6]. No correction for phase composition, however, was made in the deposition rate calculations. The accuracy of the calculated deposition rates was estimated to be better than 0.15 ~sm/min. Phase analysis of the boron coatings was performed by X-ray powder diffraction, using a Guinier—Hagg type focusing camera with Cu Kcx1 radiation. Optical microscopy and scanning elec-
3.1. Influence of the substrate material
The influence of the substrate material on the kinetics in boron CVD was demonstrated by using three different substrate materials: titanium, molybdenum and a-rhombohedral boron. With titanium as a substrate, different phase composition and deposition rates were obtained than with the other substrate materials (table 1). In a previous investigation it was shown that the boron films deposited on the titanium substrate contained small amounts (<1000 ppm) of titanium [7]. No molybdenum was detected in the coatings deposited on molybdenum substrates. The titanium was probably introduced into the coating by diffusion in the solid state. The presence of titanium in the boron coating stabilized the growth of amorphous boron and increased the stability range of this phase. The titanium-containing amorphous phase was deposited with a lower growth rate than the crystalline boron coatings obtained on the other substrate materials. Details of the influence of titanium as well as molybdenum on boron CVD has been studied in greater detail elsewhere [7]. The results show that the substrate
Table 1 Influence of substrate material on CVD of boron: molar ratio H2/BCI3 = 20: deposition time = 7 mm Substrate Temperature Deposition rate Phase (K) (~sm/min) Mo 1273 3.1 n-B Mo 1373 6.5 a-B B 1273 a-B B 1373 6 a-B
tron microscopy were used for the morphological
Ti
examination.
_____________________________________________
Ti
1273
1373
2.4
3.2
Amorphous Amorphous
U. Jansson et at
/ Kinetics and mechanisms on CVD
p(BCI3) (Pa)
Deposition rate (~m/min)
Phase
I I
170 670
2.5 2.6
a-B Orthorhombic boron carbide
II II
170 670
2.5 3.3
a-B a-B
III III
170 670
1.8 2.8
a-B a-B
material may influence the growth anu xinetics in boron CVD. The choice of substrate material is therefore important in a kinetics investigation of this kind, 3.2. Influence of the BC!3 gas purity
The influence of gas purity on the deposition process was demonstrated by using three different, commercially available BC13 gases. The results (summarized in table 2) showed that the deposition rates were dependent on the BC13 gas used in the experiments. In general, the lowest deposition rates were obtained with BC13 gas III (claimed purity 99.99%) while the highest rates were observed for BC13 gas II (from the same manufacturer as as III claimed urity 99.99%). The largest variations in the deposition rate were observed at high BC13 partial pressures. The different BC13 gases did not influence the reaction order with respect to the various reactants except for high partial pressures of BCI3 in the vapor. For some vapor compositions, the phase cornposition of the coating was affected by the BC13 gas used. With BC13 gases II and III, a-rhombohedral boron was the only phase deposited at a total pressure of 3.33 kPa and a temperature of 1300 K. For high concentrations of BC13 gas I in the vapor, an orthorhombic phase was obtained. The X-ray diffraction pattern of this phase was identical to the pattern from an orthorhombtc boron carbide previously observed in CVD of boron carbides from a reaction mixture of BC13,
173
7.
Table 2 Influence of gas purity on the growth rate and phase composition of the boron coatings; temperature = 1300 K; p(H 2) = 1.2 kPa Gas No.
of boron
_______
Fig. 1. Morphology of boron film without addition of COCI, in the vapor. Temperature 1400 K. Total pressure 6.67 kPa. Vapor composition 7 vol% BCI3, 71 vol% H2, 21 vol% He.
H2 and CH4 [8]. This observation suggests that the CVD of boron is affected by a carbon-containing compound in the BC13 gas. Phosgene (COC12) is a major impurity in many BC13 gases. The maximal concentration of COC12 (given by the manufacturer) in the gases used in this study was 50 ppm. The reaction gas mixtures used in the experiments contained 3—23 vol% BC1 This means that the maximum concentration of COCI2 in the vapor was about 10 ppm. The influence of COC12 on boron CVD was investigated by adding small amounts of this compound to a reaction gas mixture containing BC13 gas II.
.
20 .
4.
~
Fig. 2. Morphology of boron film with added COCI~ in the vapor. Deposition temperature 1400 K. Total pressure 6.7 kPa. Vapor composition, 7 vol% BCI1. 71 vol% H2, 21 vol% He. 20 ppm COd2.
174
U. Jansson eta!.
/ Kinetics and
mechanisms on CVD of boron
No significant change in deposition rate was observed for COC12 concentrations up to 25 ppm. However, for some deposition conditions, the addition of phosgene influenced the phase composition as well as the morphology. Figs. 1 and 2 show the morphology of two coatings deposited with 20 ppm added COC12 and without any COC12 added, respectively. The boron coating prepared without COC12 exhibited a mixed morphology of crystalline boron in a matrix of nodules, typical of amorphous boron. The coating prepared with 20 ppm COd2 exhibited a much more crystalline morphology with only a few amorphous nodules. The results show that the observed variations in the deposition rate obtained with the different BC13 gases cannot be due to the specified concentrations of phosgene in the gas.
08
—
05
06
~ 04
04
02
2 0.2
Bc
-~
He
02
04
06
08
H2
1
Fig. 3. Ternary deposition diagram the vapor com~H2”~H2~ showing ~He positions used in the kinetic investigation.
4. Results and discussion of the kinetics investigation The vertices of the triangle BC1 The results in the previous section showed that the deposition rate of boron is influenced by the purity of the BC13 gas as well as by the substrate material. With the different BCI3 gases used, however, the same relative change in the deposition rate was obtained upon increasing the BC13 concentration (except for very high BC13 concentrations). For this reason, it was assumed that a kinetics investigation could give valuable information about the kinetics and mechanisms in CVD of boron. For the kinetics investigation, one of the purest gases (gas III, electronic grade) and substrates of a-rhombohedral boron were used. In the kinetics investigation, the respective influences of boron trichloride, hydrogen and hydrogen chloride on the deposition rate were studied. Helium was added as an inert gas to study the separate influence of the reactants on the deposition rate at a constant linear gas flow velocity, At least three gases (BC13, H2 and He) were used in this investigation. The experimental strategy is summarized in a ternary deposition diagram in which all compositions of a BC13/H2/ He vapor can be represented (fig. 3).
3—H2—He represent reaction gas mixtures with 100% BC13, H2 and He, respectively. The sides of the triangle represent the binary reaction gas mixture He—BC13, H2—BC13 and He—H2. Vapor compositions lying on a line parallel to a triangle side represent pseudo-binary gas mixtures in which the concentration of the third gas is constant. The influence of BC13 on the deposition rate was investigated in the composition ranges C—D and E—G, respectively, while the influence of hydrogen was investigated in the ranges A—B and D—G, respectively. The hydrogen chloride dependence was investigated for the vapor composition B. 4.1. Determination of the type of control
In a kinetics investigation, it is important to know if the CVD process is controlled by thermodynamics, by mass transport in the vapor or by chemical kinetics. In this study, this was done by measuring the apparent activation energy (AAE). The vapor compositions used are indicated in fig. 3 and labeled A, B, D, E and G, respectively. High AAE values were obtained at 1300 K for all investigated vapor compositions except for E.
U Jansson et a!.
/ Kinetics and mechanisms on
The AAE value at composition D was determined at 140 kJ/mol (fig. 4). This value is in good agreement with previously reported AAE values (see, e.g., refs. [1,2,4,7,9]. The AAE values for the compositions A, B and G were in the same range (fig. 5). Accurate calculations of the AAE values at these compositions were not possible, however, because of the few data points for each vapor composition. The high AAE values at 1300 K obtained for the vapor compositions A, B, D and G suggest that the process for these vapor compositions is controlled by chemical kinetics. Accordingly, the deposition process must be controlled by chemical kinetics for all compositions lying on the lines or in the ranges A—B, B—D, D—G and F—G, provided that the reaction mechanism is the same in the whole composition range. Low AAE values were observed for higher deposition temperatures and low partial pressures of BC13 in the vapor (compositions A, B and E). This indicates that the process for these experimental conditions is controlled by mass transport of BC!3. For the vapor
1400
Temperature/K 1300
CVD of boron
1400 I
175
Temperature/K 1300 1200 ~ ~,aporcoripositionG - ‘ B - ,, 4 E
2
.
E
.“
,i~
~_
1
~
‘~1~ ~
—
.
,~
\~~\ I \
‘~ —
—
— —
— =
‘~)~
\ \ 0 .
__________________________________ 7.0
7.5
8.0 -1
Fig. 5. Influence of temperature on the deposition rate of boron at the vapor compositions A, B, E and G. Total pressure 3.3 kPa.
1200
composition E, mass transport control is attained at the deposition temperature used in this kinetics investigation (1300 K). —
4.2. Influence of vapor composition
140 kJ/mol
2
N 1
‘N
-
TN~ 0
7.0
7.5
1
8.0
10~/T/ K~ Fig. 4. Influence of temperature on the deposition rate of boron at vapor composition D. Total pressure 3.3 kPa.
The influence of hydrogen, boron trichloride and hydrogen chloride on the deposition rate is shown in figs. 6, 7 and 8, respectively. For all deposition conditions, polycrystalline a-rhombohedral boron was grown. The deposition rate was independent of the partial pressure of hydrogen in the vapor composition range A—B, while a dependence was observed in the range D—G (fig. 6). The deposition rate of boron increased with increasing partial pressure of BC13 (fig. 7). A stronger influence was observed for the composition reaction gas mixtures range withoutC—D dilute(i.e., gas) BC13/H2 than for the range E-.G (i.e., a BC1 3/H2 gas mixture strongly diluted with He). Finally, for the vapor
176
U Jansson et at
/ Kinetics and
mechanisms on CVD of boron
E -7---
3.
I
VT
2
11
50
100
150
0
Portiat pressure Rd/Pa D1~
Fig. 8. Influence of hydrogen chlonde on the deposition rate of boron. Temperature 1300 K. Total pressure 3.3 kPa
1 Partial
I
composition B, an almost linear decrease in de-
2
position rate was observed with increasing HC1 H
pressure
concentrations in the vapor (fig. 8).
2/kPa
Fig. 6. Influence on hydrogen on the deposition rate of boron. Temperature 1300 K. Total pressure 3.3 kPa.
__________________________________ ~ composition range 4
c-D
~ composition range E-G
~
1’ 3
o
2
______________________________________ 100
300 Partial
pressure
500 BCt3/Po
700
Fig. 7. Influence of boron trichloride on the deposition rate of boron. Temperature 1300 K. Total pressure 3.3 kPa
The results of the kinetics investigation can be used to derive apparent reaction orders with respect to BC!3, H2 and HC1. The results suggest a reaction order of 1 with respect to HCI. The most plausible reaction order with respect to 2 is 0 for lower BC13 concentrations (fig. 6, lower curve). For higher BC!3 concentrations (fig. 6, upper curve), the data points can be fitted to a squareroot dependence (i.e., a reaction order of 1/2). It was more difficult to find an unambiguous reaction order with respect to BC!3. Fig. 9 shows the relationship between deposition rate and BC13 partial pressure in the composition range E—G. The results in the previous section indicated that the deposition process for composition E was rate-limited by mass transport in the vapor, while the process was controlled by chemical kinetics in the composition range E—G. This means that a transition between the two types of control must occur somewhere in between the vapor compositions E and F. Consequently, the curve in fig. 9 is composed of two regions; a region controlled by mass transport (dashed line) and a region controlled by chemical kinetics (solid line). The form of the entire curve suggest a fractional reaction order with respect to BC13. In a recent publication, Tanaka et al. reported a reaction order of 1/3 [4] and the curve in fig. 9 can actually be
U Jansson eta!.
/ Kinetics and
mechanisms on CVD of boron
177
The results of the kinetic investigation can be summarized in the empirical rate equation: r A p(BC1 3) p”(H2) Bp(HC1), (1)
3 E
=
—
sures nand 0n for0.5 with lowforboron higher trichloride partial pressures. partial pres-
2
=
=
C
2
U) ° 0. U)
1
o
B
____________
I
100
300
5. Mechanistic discussion 5.1. Some reaction steps in boron CVD
I
500
The most important step in the modeling for a
Partial pressure BCI3/Pa Fig. 9. Influence of boron trichloride on the deposition rate of boron in the vapor composition range E—G. Temperature 1300 K. Total pressure 3.3 kPa.
CVD process controlled by chemical kinetics is to write/define a relevant set of elementary reaction steps. In general, a large number of elementary reactions are possible in a CVD process. In reality, however, only a few of these may be of impor-
fitted to a cube-root dependence (i.e., a reaction order of 1/3). This reaction order can also be obtained in this investigation by including regions controlled by mass transport as well as the chemical kinetics. For the region controlled by the chemical kinetics, however, a reaction order of 1 with respect to BC!3 is obtained. This reaction order was confirmed by plotting 2(H the deposition rate as a function of p(BCl3)p~ 2)(fig. 10). All experimental results (except those with added HC1) are included in the plot. Attempts to fit the experimental data to a reaction order than other than 1 with respect to BC!3 failed.
tance. A list a plausible elementary reactions, which not is complete is given below (asterisk denotes molecules adsorbed on the surface, (s) a solid phase): BC13(g) + * BCl~’, (R.1) —‘
H2(g) + 2* HC1(g) + HBC12(g) + *
BC12(g) + BCI + H ~‘
— —~ *
(R.2) (R.3) (R.4)
BCl~’, BC1 + HC1
(R.S) (R.6)
~,
—~
—~
*
—‘
*
2H*, HC1 HBC1~’,
~‘
BC!~~+H*_~BCl*+HCl*, BC1* + H* B(s) + HC1* +
(R.7) (R.8)
~
(R.9)
—~
*,
*,
HBCl~’+ _SBC1*+HC1*,
(R.10)
HBC1~’—e BC1* + HC1(g), BCl~’+ H2(g) BC1* + 2 HC1(g),
(Rh) (R.12)
*
B
22
BCI~+ H2(g)
a
a1
_________________________________________ 10
3~PBcI 20 ~H 1/2 3/2 10 2~~°
30
40
1”2(H Fig. 10. Deposition rate of boron versus p(BC13)p
2).
—*
B(s)
+
2 HC1(g)
BC13(g)
+
H*
BC12(g)
BC13(g)
+
H2(g)
—e
+
+
HC1*,
HBC12(g)
+
*,
(R.13) (R.14)
HC1(g). (R.15)
Other chemical reactions which may be of importance are, for example, the formation of HBC1 and H2BCI. It should also be noted that some of the listed elementary reactions may be incorrectly
/ Kinetics and mechanisms on
U Jansson eta!.
178
written. For example, the adsorption of BC13 might require more than the single adsorption site assumed in (R.1). Such incorrectly written elementary reactions may of course affect the modeling, 5.2. Mechanistic modeling of boron CVD The elementary reactions above describe a flow of atoms and molecules in the process. exam2) of For adsorbing ple, thetrichioride net flow, molecules J1 (in mol/s. m by (R.1) and boron is given may be expressed as: /
J 1
=
k~p(BC12) ~9(v)
K_1 e~BCl3)
—
(2)
where k1 and k_1 are the rate constants for the reaction to the right and to the left, respectively, e(v) and e(BC13) denote the surface concentration of vacant adsorption sites and adsorbed BC13, respectively. The reaction gas mixture used in this study contains three elements (B, H, and Cl). The net flows of the elements in the process are difficult to survey from the different reactions given above, The use of reaction networks, however, facilitates the visualization of these networks. A reaction network describing the flow of boron between various boron carrying molecules is presented in fig. 11. This network is constructed from the ele_______
BCI~
I
________
rHBCI2~ HBCI
________
I
______ ~
i5
Gd3
6
[f I ~l2
___________
I
BCI
e Fig. 11. Reaction network for CVD of boron.
CVD of boron
mentary reactions (R.1)—(R.15). The direction of the arrows indicates a positive net flow of boron according to the elementary reactions. This basic network is the starting point for further modeling of the process. As will be shown later, this network can be modified by additional assumptions of the mechanism. It is frequently assumed that the surface concentrations of the various adsorbed during molecules surface intermediates are constant the and deposition process (steady-state approximation). The flows of atoms and molecules in the individual reaction steps are then fixed by their stoichiome.
try. The steady-state approximation yields a number of relationships between the various flows and the deposition rate, r. For example (see fig. 11): r j +j (3 =
8 =
13’
J6 + J12,
j, + j4 + j5
=
j + .j 8 13
(4) (5)
The individual flows, .J1 —J15, can easily be derived from rate equations of the same type as eq. (2). Together, they form a complex system of coupled rate equations which must be solved explicitly to provide an exact solution. This means that a large number of rate constants and surface concentrations must be calculated. The limited kinetics data usually available does not permit a complete solution of the equation system and approximations have to be used. The most common approximation is to assume the existence of a rate-determining step (RDS). The rate of the RDS is assumed to be so low compared to the other reaction steps that the deposition rate is completely determined by the rate of the RDS. A major problem associated with this approximation is the large number of models which can be proposed, since every elementary reaction (in principle) may be a RDS. The number of possible mechanisms is also increased by the fact that the reaction steps preceeding and following the RDS can be varied. From all these possible models, rate-expressions relating the deposition rate to the vapor composition can be derived. Unfortunately, many of these rate expression are of the same form. This means that the experimental data can usually be fitted to a number of possible models.
U Jansson eta!.
/ Kinetics and
BCI
compared with the reactions to the left, that these reaction steps are irreversible. The deposition process may now follow four possible alternatives (see fig. 12):
~ct2 ~Jg ______________________
Jil
179
actions are denoted in the network by two bars crossing the reaction arrows. For the model in fig. 12, we have assumed that the reactions to the right in (R.7), (R.9) and (R.13) are so strongly favored,
3
____________
mechanisms on CVD of boron
BCI il3
Fig. 12. Reaction network for CVD of boron with (R.6) as a RDS (denoted in the network with a circle).
(i)
(R.6)
(ii) (iii)
(R.6) (R.6)
—.*
—e —e
(R.13), (R.7) (R.9)
—e
(R.8),
~—e(R.10)/(R.11) —e
(R.8),
(iv) a combination of (i)—(iii). The reaction alternative followed is dependent Using the elementary reaction steps (R.1)— (RJ5), several rate equations of the same form can be derived and fitted to the experimental data. Hence, no single rate-determining step can be identified. For this reason, a detailed modeling of the process was considered to be of limited value, The difficulties of finding an unambiguous model for CVD of boron are illustrated in the following section. 5.3. A possible reaction mechanism in CVD of boron On the basis of the experimental data and the discussion above, one tentative mechanistic model for CVD of boron is presented as a network in fig. 12. This network has been constructed by assuming that the hydrogen reduction of adsorbed BC!3 (reaction (R.6)) is the RDS. As can be seen, some reaction steps have been neglected, compared with the basic network in fig. 10. This is a consequence of the assumption that (R.6) is rate-determining. The following condition must then be fulfilled: the flow of boron atoms through all pathways parallel to (R.6) must be very small, compared with the flow through the RDS. If not, the reaction will follow an alternative pathway and (R.6) cannot be rate-determining. The parallel flows j4, .)‘5 and J12 can therefore be neglected. Moreover, we can also make further assumptions in the mechanistic modelling. For example, it can be assumed that some of the reaction steps (R.1)—(R.15) are irreversible (i.e., they occur only in one direction). Such irreversible elementary re-
on the magnitude of the rate constants for the reaction steps. The network in fig. 12 is based on the assumption that the CVD process occurs according to alternative (iv). It can be shown (see appendix) that irrespective of the reaction pathway followed (alternative (i)—(iv)), the deposition rate is given by the rate expression: 1”2(H p(BC13) p 2) e(v) (6) r= ~ +p(HC1) f(p(H2), e(v))’ where f( p (H2), t9(v)) is a function dependent on the hydrogen partial pressure and the concentration of vacant adsorption sites. At a first glance, eq. (6) seems to differ from the empirical rate equation derived in the previous section (eq. (1)). All experimental data obtained in this study may, however, be fitted to eq. (7). This means that four different mechanisms (alternative (i)—(iv)), explaining the experimental results can be constructed. The procedure sketched above can be repeated with other rate determining steps, leading to a large number of models which can all be fitted to the kinetics data. The number of models can only be reduced by using complementary techniques to investigate homogeneous reactions in the vapor as well as reactions on the boron surface.
6. Concluding remarks CVD of boron from a reaction gas mixture of BC13 and H2 is affected by the purity of the BC!3
180
U. Jansson eta!.
/ Kinetics and
gas and in some cases by the purity of the substrate material. In this investigation, two of the purest commercially available qualities of BC13 were used. Nevertheless, different results were obtamed with the different gases. This indicates that even a small concentration of some impurity in the BC13 gas affects the deposition process. Experiments with small amounts of phosgene (an impurity in ~ gas) added to the vapor did not affect the deposition rate but affected the phase content and the morphology. It is also concluded that some substrate material such as titanium may affect the growth behavior of boron. Hence, prior to a kinetics investigation, the influence of the substrate materialand on the deposition processsuch should be investigated substrate materials as
mechanisms on CVD of boron
Appendix Assume that CVD of boron occurs according to the reaction network in fig. 12. If (R.6) is the RDS, the deposition rate is proportional to the net flow (or net rate) of this step: r=J6=k6 e(BC13) e(H) —
k6 e (BC12) e ( HC1).
(A. 1)
e(BC13), ~(H) and ~9(HCl) can be calculated by assuming a quasi-equilibrium for (R.1)—(R.3): K1p(BC13)O(v) e(BC!3), (A.2) 1 /2 e (v) e (H), (A .3) (K2 K p (H2)) 3p(HC1)e(v) e(HC1), (A.4) =
=
=
titanium should be avoided. In the kinetics investigation, the type of control of the deposition process was determined in a large vapor composition range. Regions of chemical kinetics control as well as of mass transport control were identified. Results indicating mass transport control were obtained for low partial pressures of BC!3. The separate influence of BC13, H2 and HC1 (with He as a diluent gas) on the deposition rate was studied. The experimental data suggested a reaction order of 1 and 0/0.5 with respect to BC13 and H2, respectively. An almost linear decrease in deposition rate was observed with increasing partial pressures of HC1. A large number of possible elementary reaction steps may occur in CVD of boron. In the absence of more detailed knowledge about these reactions, a large number of mechanisms can be constructed to which the kinetics data can be fitted. A detailed modelling of the deposition process is therefore not considered to be meaningful from the kinetics data. The number of possible models can be reduced by using complementary techniques (e.g., mass-spectrometric analysis of the vapor, in situ surface analysis, thermal desorption studies etc.) to investigate the process.
where K, k/k The concentration of adsorbed BC!2, O(BC12), is more difficult to calculate. 1(BC12) is dependent on the reaction pathway followed after the RDS. Four alternative pathways exist (see fig. 12): (i) (R6) —e (R.13), =
(ii) (iii)
(R.6) (R6)
-,.
— —~
(R.7) (R.9)
—*
—e
(R.8), (R.10)/(R.11)
—e
(R.8),
(iv) a combination of (i)—(iii). Assume that the steady-state approximation is valid in CVD of boron. The formation rate of adsorbed BC!2 (reaction (R.6)) must then be identical to the rate at which this molecule reacts. The rate of the step(s) following (R.6) must be identical to the net rate of (R.6). This means that: for reaction alternative (i): r k130(BC13)p(H2); for reaction alternative (ii): =
r
=
k7e(BC12)e(H);
(A.5) (A.6)
for reaction alternative (iii):
r=k9e(BCI2)O(H);
(A.7)
Acknowledgment
and finally for reaction alternative (iv):
Financial support for this project by the Swedish Natural Science Research Council is gratefully acknowledged.
r
=
[k13 p(H2)
+
(k7
+
k9) e(H)} o(BC12) (A.8)
U. Jansson eta!.
/ Kinetics and mechanisms on
The rate expressions for the different possible reaction alternative may now be calculated by combining eqs. (A.1)—(A.4) and one of eqs. (A.5)—(A.8). For example with alternative (i) r
=
A p(BC13) p”2(H2) e2(v) Bp(HC1) 0(v) r p(H 2) —
(A.9)
=
181
References [1] P.E. Gruber, in: Proc. 2nd Intern. Conf. on CVD, Eds. J.M. Blocher and J.C. Winters (Electrochem. Soc., 1970) p. 209. [2] HE. Carlton, J.H. Oxley, E.H. Hall and J.M. Blocher, Jr., in: Proc. 2nd Intern. Conf. on CVD, Eds. J.M. Blocher and J.C. Winters (Electrochem. Soc., 1970) p. 25. [3] R.M. Mehalso and R.J. Diefendorf, in: Proc. 5th Intern. Conf. on CVD, Eds. J.M. Hintermann and L.E. Hall (Electrochem. Soc., 1975) p. 84. Conf. N. on CVD, Eds. G.W. Cullen Blocher, [4] Intern. H. Tanaka, Nakanishi and E. Kato,andin:J.M. Proc. 10th
This gives: r
CVD of boron
1”2(H 2(v) B p(HC1) A1 + p(BC!3) p 0(v)/p(H 2) 0 2)
(A.10)
Jr. (Electrochem. Soc., Pennington, NJ, 1987) p. 155. [5] J.O. Carlsson and M. Boman, in: Proc. 8th Intern. Conf. on
Irrespective of the reaction alternative (i)—(iv) which actually occurs, the deposition rate can be written on the general form: 2(H 2(v) r Ap(BC13) pi/ (A.11) 2) 0 1 + Bp(HC1) f(p(H =
2),(v))
‘
where f(p(H2), 0(v)) is a function dependent on the partial pressure of hydrogen and the concentration of vacant adsorption sites.
Vacuum Metallurgy, Linz, 1985, p. 257. [6] Gmelin Handbook of Inorganic Chemistry, Boron Supplement, Vol. 2 (Springer, Berlin, 1981). [7] U. Jansson, M. Boman, L. Markert, J.O. Carlsson and i.E. to be published. [8] Greene, U. Jansson, JO. Carlsson, B. Stndh, S. Soderberg and M. Olsson, to be published. [9] L. Vandenbulcke and G. Vuillard, J. Electrochem. Soc. 124 (1977) 1937.
Journal of Ciystal Growth 94 (1989) 171—181 North-Holland, Amsterdam
KINETICS AND MECHANISMS IN CVD OF BORON Ulf JANSSON, Mats BOMAN and Jan-Otto CARLSSON Thin Film and Surface Chemistry Group, Department of Chemistry, University of Uppsala, Box 531, S-751 21 Uppsala, Sweden Received 14 May 1988; manuscript received in final form 31 August 1988
The influence of BC1
3, H2 and HCI on the deposition rate of boron has been investigated in a region controlled by chemical kinetics. The separate contributions of the various gases on the deposition rate was determined by using helium as as diluent gas. From the results, the following empirical rate equation was derived: r = Ap(BCI3)p”(H2)— Bp(HCI), with n = 0 and n = 0.5 for low and high partial pressures of BCI3, respectively. The influence of the substrate material and gas purity on the kinetics in boron CVD was also demonstrated. It was concluded that impurities in the BCI3 gas affected the deposition rates as well as the phase composition of the boron coatings. Moreover, it was found that the kinetics in boron CVD can be affected by the substrate material used. No single rate-determining step could be identified from the extensive general kinetics data set. Analysis of the data showed that the boron deposition may take place along the several alternative reaction paths which cannot be separated.
1. Introduction Boron can be deposited by chemical vapor deposition (CVD) from a reaction gas mixture of boron trichioride and hydrogen. The kinetics of this CVD process has been studied by several authors [1—4]and various rate-determining steps have been proposed to explain the results. Gruber considered the rate-determining step to be a surface reaction between atomically adsorbed hydrogen and some adsorbed boron-carrying molecule [1]. The hydrogen reduction of adsorbed dichloroborane or adsorbe& boron dichloride was proposed as a rate-limiting step by Carlton et al. [2]. Mehalso and Diefendorf stated that the deposition rate was limited partly by the desorption rate of hydrogen chloride and partly by the diffusion of hydrogen chloride from the surface of the deposit [3]. In a recent publication, Tanaka et al. also proposed that desorption of hydrogen chloride was a rate-limiting step in boron CVD [4]. The aim of this work was to investigate the kinetics in CVD of boron in a region controlled by chemical kinetics. Efforts were made to verify that the deposition process was controlled by chemical kinetics at vapor compositions used in this work. The influence of substrate material and
gas purity on the deposition process was also demonstrated.
2. Experimental procedure A low pressure CVD system with a cold-water reactor was used in this investigation. Details of the CVD system have been published elsewhere [5]. The leak rate of air into the system was less than i0~ Torr 1 s~. This leak rate adds less than 0.1 ppm of impurities to the reaction gas mixture at the gas flows generally used. Thin foils the molybdenum (purity 99.99%) and titanium (purity 99.9%), mounted parallel to the gas stream, were used as substrate materials. By virtue of the dimensions of the foil (1 mm broad and 100 ~sm thick), surface kinetics control was easily obtained. The substrates were resistively heated and the temperature was measured with a micro-optical pyrometer. Corrections for glass absorption and emissivity (c = 0.71) were applied. Prior to the deposition experiments, the substrates were cleaned by ultrasonic washing in pure ethanol. Finally, in order to remove oxides from the surface, the substrates were heat-treated in hydrogen for three minutes at 1400 K.
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
172
U. Jansson et al.
/ Kinetics and mechanisms on
Hydrogen (purity 99.9997%), helium (purity 99.9996%), boron trichloride (purities 99.99% (electronic grade) and 99.98%) and hydrogen chloride (purity 99.99%) were used as gases. The influence of BCI5 gas quality on the deposition process was demonstrated in two ways: Comparative deposition experiments with three different BC13 gases from two manufacturers. The claimed purity of the gases were: gas I 99.98% and gas II and III 99.99%. The main impurities in the BCI3 gases, as specified by the manufacturers, were COC12 (< 50 ppm) and Cl, (<40 ppm). Addition of small amounts of phosgene, COd7, which is a major impurity in BC13 gas. The phosgene was added as a COCI 2/He gas mixture (100 ppm COd, in He (purity 99.9996%)) This procedure allowed a controlled addition of very low phosgene concentrations. During the deposition experiments, boron atoms diffused into the substrate with the subse—
C VD of boron
3. Influence of impurities on the kinetics in boron
CVD There exist several sources of impurities which may affect the kinetics in a CVD process. The influence of impurities originating from the substrate material and the BCI3 gas is examined below.
=
=
—
quent formation of molybdenum horides. The loss of boron atoms from the coating ceased after a
deposition time of a few minutes. To obtain accurate growth-rate data, the substrates were precoated with a 12 ~rm thick boron coating from which virtually no further loss of boron occurred to the substrate, The deposition rates were obtained from thickness measurements on scanning electron micrographs of polished cross-sections of the samples. The deposition rate at a given set of experimental parameters was calculated from 1—S individual deposition experiments. Two phases of boron (amorphous and a-rhombohedral boron) were obtained in the deposition experiments. Literature values for the density of these two phases differ by less than 5% [6]. No correction for phase composition, however, was made in the deposition rate calculations. The accuracy of the calculated deposition rates was estimated to be better than 0.15 ~sm/min. Phase analysis of the boron coatings was performed by X-ray powder diffraction, using a Guinier—Hagg type focusing camera with Cu Kcx1 radiation. Optical microscopy and scanning elec-
3.1. Influence of the substrate material
The influence of the substrate material on the kinetics in boron CVD was demonstrated by using three different substrate materials: titanium, molybdenum and a-rhombohedral boron. With titanium as a substrate, different phase composition and deposition rates were obtained than with the other substrate materials (table 1). In a previous investigation it was shown that the boron films deposited on the titanium substrate contained small amounts (<1000 ppm) of titanium [7]. No molybdenum was detected in the coatings deposited on molybdenum substrates. The titanium was probably introduced into the coating by diffusion in the solid state. The presence of titanium in the boron coating stabilized the growth of amorphous boron and increased the stability range of this phase. The titanium-containing amorphous phase was deposited with a lower growth rate than the crystalline boron coatings obtained on the other substrate materials. Details of the influence of titanium as well as molybdenum on boron CVD has been studied in greater detail elsewhere [7]. The results show that the substrate
Table 1 Influence of substrate material on CVD of boron: molar ratio H2/BCI3 = 20: deposition time = 7 mm Substrate Temperature Deposition rate Phase (K) (~sm/min) Mo 1273 3.1 n-B Mo 1373 6.5 a-B B 1273 a-B B 1373 6 a-B
tron microscopy were used for the morphological
Ti
examination.
_____________________________________________
Ti
1273
1373
2.4
3.2
Amorphous Amorphous
U. Jansson et at
/ Kinetics and mechanisms on CVD
p(BCI3) (Pa)
Deposition rate (~m/min)
Phase
I I
170 670
2.5 2.6
a-B Orthorhombic boron carbide
II II
170 670
2.5 3.3
a-B a-B
III III
170 670
1.8 2.8
a-B a-B
material may influence the growth anu xinetics in boron CVD. The choice of substrate material is therefore important in a kinetics investigation of this kind, 3.2. Influence of the BC!3 gas purity
The influence of gas purity on the deposition process was demonstrated by using three different, commercially available BC13 gases. The results (summarized in table 2) showed that the deposition rates were dependent on the BC13 gas used in the experiments. In general, the lowest deposition rates were obtained with BC13 gas III (claimed purity 99.99%) while the highest rates were observed for BC13 gas II (from the same manufacturer as as III claimed urity 99.99%). The largest variations in the deposition rate were observed at high BC13 partial pressures. The different BC13 gases did not influence the reaction order with respect to the various reactants except for high partial pressures of BCI3 in the vapor. For some vapor compositions, the phase cornposition of the coating was affected by the BC13 gas used. With BC13 gases II and III, a-rhombohedral boron was the only phase deposited at a total pressure of 3.33 kPa and a temperature of 1300 K. For high concentrations of BC13 gas I in the vapor, an orthorhombic phase was obtained. The X-ray diffraction pattern of this phase was identical to the pattern from an orthorhombtc boron carbide previously observed in CVD of boron carbides from a reaction mixture of BC13,
173
7.
Table 2 Influence of gas purity on the growth rate and phase composition of the boron coatings; temperature = 1300 K; p(H 2) = 1.2 kPa Gas No.
of boron
_______
Fig. 1. Morphology of boron film without addition of COCI, in the vapor. Temperature 1400 K. Total pressure 6.67 kPa. Vapor composition 7 vol% BCI3, 71 vol% H2, 21 vol% He.
H2 and CH4 [8]. This observation suggests that the CVD of boron is affected by a carbon-containing compound in the BC13 gas. Phosgene (COC12) is a major impurity in many BC13 gases. The maximal concentration of COC12 (given by the manufacturer) in the gases used in this study was 50 ppm. The reaction gas mixtures used in the experiments contained 3—23 vol% BC1 This means that the maximum concentration of COCI2 in the vapor was about 10 ppm. The influence of COC12 on boron CVD was investigated by adding small amounts of this compound to a reaction gas mixture containing BC13 gas II.
.
20 .
4.
~
Fig. 2. Morphology of boron film with added COCI~ in the vapor. Deposition temperature 1400 K. Total pressure 6.7 kPa. Vapor composition, 7 vol% BCI1. 71 vol% H2, 21 vol% He. 20 ppm COd2.
174
U. Jansson eta!.
/ Kinetics and
mechanisms on CVD of boron
No significant change in deposition rate was observed for COC12 concentrations up to 25 ppm. However, for some deposition conditions, the addition of phosgene influenced the phase composition as well as the morphology. Figs. 1 and 2 show the morphology of two coatings deposited with 20 ppm added COC12 and without any COC12 added, respectively. The boron coating prepared without COC12 exhibited a mixed morphology of crystalline boron in a matrix of nodules, typical of amorphous boron. The coating prepared with 20 ppm COd2 exhibited a much more crystalline morphology with only a few amorphous nodules. The results show that the observed variations in the deposition rate obtained with the different BC13 gases cannot be due to the specified concentrations of phosgene in the gas.
08
—
05
06
~ 04
04
02
2 0.2
Bc
-~
He
02
04
06
08
H2
1
Fig. 3. Ternary deposition diagram the vapor com~H2”~H2~ showing ~He positions used in the kinetic investigation.
4. Results and discussion of the kinetics investigation The vertices of the triangle BC1 The results in the previous section showed that the deposition rate of boron is influenced by the purity of the BC13 gas as well as by the substrate material. With the different BCI3 gases used, however, the same relative change in the deposition rate was obtained upon increasing the BC13 concentration (except for very high BC13 concentrations). For this reason, it was assumed that a kinetics investigation could give valuable information about the kinetics and mechanisms in CVD of boron. For the kinetics investigation, one of the purest gases (gas III, electronic grade) and substrates of a-rhombohedral boron were used. In the kinetics investigation, the respective influences of boron trichloride, hydrogen and hydrogen chloride on the deposition rate were studied. Helium was added as an inert gas to study the separate influence of the reactants on the deposition rate at a constant linear gas flow velocity, At least three gases (BC13, H2 and He) were used in this investigation. The experimental strategy is summarized in a ternary deposition diagram in which all compositions of a BC13/H2/ He vapor can be represented (fig. 3).
3—H2—He represent reaction gas mixtures with 100% BC13, H2 and He, respectively. The sides of the triangle represent the binary reaction gas mixture He—BC13, H2—BC13 and He—H2. Vapor compositions lying on a line parallel to a triangle side represent pseudo-binary gas mixtures in which the concentration of the third gas is constant. The influence of BC13 on the deposition rate was investigated in the composition ranges C—D and E—G, respectively, while the influence of hydrogen was investigated in the ranges A—B and D—G, respectively. The hydrogen chloride dependence was investigated for the vapor composition B. 4.1. Determination of the type of control
In a kinetics investigation, it is important to know if the CVD process is controlled by thermodynamics, by mass transport in the vapor or by chemical kinetics. In this study, this was done by measuring the apparent activation energy (AAE). The vapor compositions used are indicated in fig. 3 and labeled A, B, D, E and G, respectively. High AAE values were obtained at 1300 K for all investigated vapor compositions except for E.
U Jansson et a!.
/ Kinetics and mechanisms on
The AAE value at composition D was determined at 140 kJ/mol (fig. 4). This value is in good agreement with previously reported AAE values (see, e.g., refs. [1,2,4,7,9]. The AAE values for the compositions A, B and G were in the same range (fig. 5). Accurate calculations of the AAE values at these compositions were not possible, however, because of the few data points for each vapor composition. The high AAE values at 1300 K obtained for the vapor compositions A, B, D and G suggest that the process for these vapor compositions is controlled by chemical kinetics. Accordingly, the deposition process must be controlled by chemical kinetics for all compositions lying on the lines or in the ranges A—B, B—D, D—G and F—G, provided that the reaction mechanism is the same in the whole composition range. Low AAE values were observed for higher deposition temperatures and low partial pressures of BC13 in the vapor (compositions A, B and E). This indicates that the process for these experimental conditions is controlled by mass transport of BC!3. For the vapor
1400
Temperature/K 1300
CVD of boron
1400 I
175
Temperature/K 1300 1200 ~ ~,aporcoripositionG - ‘ B - ,, 4 E
2
.
E
.“
,i~
~_
1
~
‘~1~ ~
—
.
,~
\~~\ I \
‘~ —
—
— —
— =
‘~)~
\ \ 0 .
__________________________________ 7.0
7.5
8.0 -1
Fig. 5. Influence of temperature on the deposition rate of boron at the vapor compositions A, B, E and G. Total pressure 3.3 kPa.
1200
composition E, mass transport control is attained at the deposition temperature used in this kinetics investigation (1300 K). —
4.2. Influence of vapor composition
140 kJ/mol
2
N 1
‘N
-
TN~ 0
7.0
7.5
1
8.0
10~/T/ K~ Fig. 4. Influence of temperature on the deposition rate of boron at vapor composition D. Total pressure 3.3 kPa.
The influence of hydrogen, boron trichloride and hydrogen chloride on the deposition rate is shown in figs. 6, 7 and 8, respectively. For all deposition conditions, polycrystalline a-rhombohedral boron was grown. The deposition rate was independent of the partial pressure of hydrogen in the vapor composition range A—B, while a dependence was observed in the range D—G (fig. 6). The deposition rate of boron increased with increasing partial pressure of BC13 (fig. 7). A stronger influence was observed for the composition reaction gas mixtures range withoutC—D dilute(i.e., gas) BC13/H2 than for the range E-.G (i.e., a BC1 3/H2 gas mixture strongly diluted with He). Finally, for the vapor
176
U Jansson et at
/ Kinetics and
mechanisms on CVD of boron
E -7---
3.
I
VT
2
11
50
100
150
0
Portiat pressure Rd/Pa D1~
Fig. 8. Influence of hydrogen chlonde on the deposition rate of boron. Temperature 1300 K. Total pressure 3.3 kPa
1 Partial
I
composition B, an almost linear decrease in de-
2
position rate was observed with increasing HC1 H
pressure
concentrations in the vapor (fig. 8).
2/kPa
Fig. 6. Influence on hydrogen on the deposition rate of boron. Temperature 1300 K. Total pressure 3.3 kPa.
__________________________________ ~ composition range 4
c-D
~ composition range E-G
~
1’ 3
o
2
______________________________________ 100
300 Partial
pressure
500 BCt3/Po
700
Fig. 7. Influence of boron trichloride on the deposition rate of boron. Temperature 1300 K. Total pressure 3.3 kPa
The results of the kinetics investigation can be used to derive apparent reaction orders with respect to BC!3, H2 and HC1. The results suggest a reaction order of 1 with respect to HCI. The most plausible reaction order with respect to 2 is 0 for lower BC13 concentrations (fig. 6, lower curve). For higher BC!3 concentrations (fig. 6, upper curve), the data points can be fitted to a squareroot dependence (i.e., a reaction order of 1/2). It was more difficult to find an unambiguous reaction order with respect to BC!3. Fig. 9 shows the relationship between deposition rate and BC13 partial pressure in the composition range E—G. The results in the previous section indicated that the deposition process for composition E was rate-limited by mass transport in the vapor, while the process was controlled by chemical kinetics in the composition range E—G. This means that a transition between the two types of control must occur somewhere in between the vapor compositions E and F. Consequently, the curve in fig. 9 is composed of two regions; a region controlled by mass transport (dashed line) and a region controlled by chemical kinetics (solid line). The form of the entire curve suggest a fractional reaction order with respect to BC13. In a recent publication, Tanaka et al. reported a reaction order of 1/3 [4] and the curve in fig. 9 can actually be
U Jansson eta!.
/ Kinetics and
mechanisms on CVD of boron
177
The results of the kinetic investigation can be summarized in the empirical rate equation: r A p(BC1 3) p”(H2) Bp(HC1), (1)
3 E
=
—
sures nand 0n for0.5 with lowforboron higher trichloride partial pressures. partial pres-
2
=
=
C
2
U) ° 0. U)
1
o
B
____________
I
100
300
5. Mechanistic discussion 5.1. Some reaction steps in boron CVD
I
500
The most important step in the modeling for a
Partial pressure BCI3/Pa Fig. 9. Influence of boron trichloride on the deposition rate of boron in the vapor composition range E—G. Temperature 1300 K. Total pressure 3.3 kPa.
CVD process controlled by chemical kinetics is to write/define a relevant set of elementary reaction steps. In general, a large number of elementary reactions are possible in a CVD process. In reality, however, only a few of these may be of impor-
fitted to a cube-root dependence (i.e., a reaction order of 1/3). This reaction order can also be obtained in this investigation by including regions controlled by mass transport as well as the chemical kinetics. For the region controlled by the chemical kinetics, however, a reaction order of 1 with respect to BC!3 is obtained. This reaction order was confirmed by plotting 2(H the deposition rate as a function of p(BCl3)p~ 2)(fig. 10). All experimental results (except those with added HC1) are included in the plot. Attempts to fit the experimental data to a reaction order than other than 1 with respect to BC!3 failed.
tance. A list a plausible elementary reactions, which not is complete is given below (asterisk denotes molecules adsorbed on the surface, (s) a solid phase): BC13(g) + * BCl~’, (R.1) —‘
H2(g) + 2* HC1(g) + HBC12(g) + *
BC12(g) + BCI + H ~‘
— —~ *
(R.2) (R.3) (R.4)
BCl~’, BC1 + HC1
(R.S) (R.6)
~,
—~
—~
*
—‘
*
2H*, HC1 HBC1~’,
~‘
BC!~~+H*_~BCl*+HCl*, BC1* + H* B(s) + HC1* +
(R.7) (R.8)
~
(R.9)
—~
*,
*,
HBCl~’+ _SBC1*+HC1*,
(R.10)
HBC1~’—e BC1* + HC1(g), BCl~’+ H2(g) BC1* + 2 HC1(g),
(Rh) (R.12)
*
B
22
BCI~+ H2(g)
a
a1
_________________________________________ 10
3~PBcI 20 ~H 1/2 3/2 10 2~~°
30
40
1”2(H Fig. 10. Deposition rate of boron versus p(BC13)p
2).
—*
B(s)
+
2 HC1(g)
BC13(g)
+
H*
BC12(g)
BC13(g)
+
H2(g)
—e
+
+
HC1*,
HBC12(g)
+
*,
(R.13) (R.14)
HC1(g). (R.15)
Other chemical reactions which may be of importance are, for example, the formation of HBC1 and H2BCI. It should also be noted that some of the listed elementary reactions may be incorrectly
/ Kinetics and mechanisms on
U Jansson eta!.
178
written. For example, the adsorption of BC13 might require more than the single adsorption site assumed in (R.1). Such incorrectly written elementary reactions may of course affect the modeling, 5.2. Mechanistic modeling of boron CVD The elementary reactions above describe a flow of atoms and molecules in the process. exam2) of For adsorbing ple, thetrichioride net flow, molecules J1 (in mol/s. m by (R.1) and boron is given may be expressed as: /
J 1
=
k~p(BC12) ~9(v)
K_1 e~BCl3)
—
(2)
where k1 and k_1 are the rate constants for the reaction to the right and to the left, respectively, e(v) and e(BC13) denote the surface concentration of vacant adsorption sites and adsorbed BC13, respectively. The reaction gas mixture used in this study contains three elements (B, H, and Cl). The net flows of the elements in the process are difficult to survey from the different reactions given above, The use of reaction networks, however, facilitates the visualization of these networks. A reaction network describing the flow of boron between various boron carrying molecules is presented in fig. 11. This network is constructed from the ele_______
BCI~
I
________
rHBCI2~ HBCI
________
I
______ ~
i5
Gd3
6
[f I ~l2
___________
I
BCI
e Fig. 11. Reaction network for CVD of boron.
CVD of boron
mentary reactions (R.1)—(R.15). The direction of the arrows indicates a positive net flow of boron according to the elementary reactions. This basic network is the starting point for further modeling of the process. As will be shown later, this network can be modified by additional assumptions of the mechanism. It is frequently assumed that the surface concentrations of the various adsorbed during molecules surface intermediates are constant the and deposition process (steady-state approximation). The flows of atoms and molecules in the individual reaction steps are then fixed by their stoichiome.
try. The steady-state approximation yields a number of relationships between the various flows and the deposition rate, r. For example (see fig. 11): r j +j (3 =
8 =
13’
J6 + J12,
j, + j4 + j5
=
j + .j 8 13
(4) (5)
The individual flows, .J1 —J15, can easily be derived from rate equations of the same type as eq. (2). Together, they form a complex system of coupled rate equations which must be solved explicitly to provide an exact solution. This means that a large number of rate constants and surface concentrations must be calculated. The limited kinetics data usually available does not permit a complete solution of the equation system and approximations have to be used. The most common approximation is to assume the existence of a rate-determining step (RDS). The rate of the RDS is assumed to be so low compared to the other reaction steps that the deposition rate is completely determined by the rate of the RDS. A major problem associated with this approximation is the large number of models which can be proposed, since every elementary reaction (in principle) may be a RDS. The number of possible mechanisms is also increased by the fact that the reaction steps preceeding and following the RDS can be varied. From all these possible models, rate-expressions relating the deposition rate to the vapor composition can be derived. Unfortunately, many of these rate expression are of the same form. This means that the experimental data can usually be fitted to a number of possible models.
U Jansson eta!.
/ Kinetics and
BCI
compared with the reactions to the left, that these reaction steps are irreversible. The deposition process may now follow four possible alternatives (see fig. 12):
~ct2 ~Jg ______________________
Jil
179
actions are denoted in the network by two bars crossing the reaction arrows. For the model in fig. 12, we have assumed that the reactions to the right in (R.7), (R.9) and (R.13) are so strongly favored,
3
____________
mechanisms on CVD of boron
BCI il3
Fig. 12. Reaction network for CVD of boron with (R.6) as a RDS (denoted in the network with a circle).
(i)
(R.6)
(ii) (iii)
(R.6) (R.6)
—.*
—e —e
(R.13), (R.7) (R.9)
—e
(R.8),
~—e(R.10)/(R.11) —e
(R.8),
(iv) a combination of (i)—(iii). The reaction alternative followed is dependent Using the elementary reaction steps (R.1)— (RJ5), several rate equations of the same form can be derived and fitted to the experimental data. Hence, no single rate-determining step can be identified. For this reason, a detailed modeling of the process was considered to be of limited value, The difficulties of finding an unambiguous model for CVD of boron are illustrated in the following section. 5.3. A possible reaction mechanism in CVD of boron On the basis of the experimental data and the discussion above, one tentative mechanistic model for CVD of boron is presented as a network in fig. 12. This network has been constructed by assuming that the hydrogen reduction of adsorbed BC!3 (reaction (R.6)) is the RDS. As can be seen, some reaction steps have been neglected, compared with the basic network in fig. 10. This is a consequence of the assumption that (R.6) is rate-determining. The following condition must then be fulfilled: the flow of boron atoms through all pathways parallel to (R.6) must be very small, compared with the flow through the RDS. If not, the reaction will follow an alternative pathway and (R.6) cannot be rate-determining. The parallel flows j4, .)‘5 and J12 can therefore be neglected. Moreover, we can also make further assumptions in the mechanistic modelling. For example, it can be assumed that some of the reaction steps (R.1)—(R.15) are irreversible (i.e., they occur only in one direction). Such irreversible elementary re-
on the magnitude of the rate constants for the reaction steps. The network in fig. 12 is based on the assumption that the CVD process occurs according to alternative (iv). It can be shown (see appendix) that irrespective of the reaction pathway followed (alternative (i)—(iv)), the deposition rate is given by the rate expression: 1”2(H p(BC13) p 2) e(v) (6) r= ~ +p(HC1) f(p(H2), e(v))’ where f( p (H2), t9(v)) is a function dependent on the hydrogen partial pressure and the concentration of vacant adsorption sites. At a first glance, eq. (6) seems to differ from the empirical rate equation derived in the previous section (eq. (1)). All experimental data obtained in this study may, however, be fitted to eq. (7). This means that four different mechanisms (alternative (i)—(iv)), explaining the experimental results can be constructed. The procedure sketched above can be repeated with other rate determining steps, leading to a large number of models which can all be fitted to the kinetics data. The number of models can only be reduced by using complementary techniques to investigate homogeneous reactions in the vapor as well as reactions on the boron surface.
6. Concluding remarks CVD of boron from a reaction gas mixture of BC13 and H2 is affected by the purity of the BC!3
180
U. Jansson eta!.
/ Kinetics and
gas and in some cases by the purity of the substrate material. In this investigation, two of the purest commercially available qualities of BC13 were used. Nevertheless, different results were obtamed with the different gases. This indicates that even a small concentration of some impurity in the BC13 gas affects the deposition process. Experiments with small amounts of phosgene (an impurity in ~ gas) added to the vapor did not affect the deposition rate but affected the phase content and the morphology. It is also concluded that some substrate material such as titanium may affect the growth behavior of boron. Hence, prior to a kinetics investigation, the influence of the substrate materialand on the deposition processsuch should be investigated substrate materials as
mechanisms on CVD of boron
Appendix Assume that CVD of boron occurs according to the reaction network in fig. 12. If (R.6) is the RDS, the deposition rate is proportional to the net flow (or net rate) of this step: r=J6=k6 e(BC13) e(H) —
k6 e (BC12) e ( HC1).
(A. 1)
e(BC13), ~(H) and ~9(HCl) can be calculated by assuming a quasi-equilibrium for (R.1)—(R.3): K1p(BC13)O(v) e(BC!3), (A.2) 1 /2 e (v) e (H), (A .3) (K2 K p (H2)) 3p(HC1)e(v) e(HC1), (A.4) =
=
=
titanium should be avoided. In the kinetics investigation, the type of control of the deposition process was determined in a large vapor composition range. Regions of chemical kinetics control as well as of mass transport control were identified. Results indicating mass transport control were obtained for low partial pressures of BC!3. The separate influence of BC13, H2 and HC1 (with He as a diluent gas) on the deposition rate was studied. The experimental data suggested a reaction order of 1 and 0/0.5 with respect to BC13 and H2, respectively. An almost linear decrease in deposition rate was observed with increasing partial pressures of HC1. A large number of possible elementary reaction steps may occur in CVD of boron. In the absence of more detailed knowledge about these reactions, a large number of mechanisms can be constructed to which the kinetics data can be fitted. A detailed modelling of the deposition process is therefore not considered to be meaningful from the kinetics data. The number of possible models can be reduced by using complementary techniques (e.g., mass-spectrometric analysis of the vapor, in situ surface analysis, thermal desorption studies etc.) to investigate the process.
where K, k/k The concentration of adsorbed BC!2, O(BC12), is more difficult to calculate. 1(BC12) is dependent on the reaction pathway followed after the RDS. Four alternative pathways exist (see fig. 12): (i) (R6) —e (R.13), =
(ii) (iii)
(R.6) (R6)
-,.
— —~
(R.7) (R.9)
—*
—e
(R.8), (R.10)/(R.11)
—e
(R.8),
(iv) a combination of (i)—(iii). Assume that the steady-state approximation is valid in CVD of boron. The formation rate of adsorbed BC!2 (reaction (R.6)) must then be identical to the rate at which this molecule reacts. The rate of the step(s) following (R.6) must be identical to the net rate of (R.6). This means that: for reaction alternative (i): r k130(BC13)p(H2); for reaction alternative (ii): =
r
=
k7e(BC12)e(H);
(A.5) (A.6)
for reaction alternative (iii):
r=k9e(BCI2)O(H);
(A.7)
Acknowledgment
and finally for reaction alternative (iv):
Financial support for this project by the Swedish Natural Science Research Council is gratefully acknowledged.
r
=
[k13 p(H2)
+
(k7
+
k9) e(H)} o(BC12) (A.8)
U. Jansson eta!.
/ Kinetics and mechanisms on
The rate expressions for the different possible reaction alternative may now be calculated by combining eqs. (A.1)—(A.4) and one of eqs. (A.5)—(A.8). For example with alternative (i) r
=
A p(BC13) p”2(H2) e2(v) Bp(HC1) 0(v) r p(H 2) —
(A.9)
=
181
References [1] P.E. Gruber, in: Proc. 2nd Intern. Conf. on CVD, Eds. J.M. Blocher and J.C. Winters (Electrochem. Soc., 1970) p. 209. [2] HE. Carlton, J.H. Oxley, E.H. Hall and J.M. Blocher, Jr., in: Proc. 2nd Intern. Conf. on CVD, Eds. J.M. Blocher and J.C. Winters (Electrochem. Soc., 1970) p. 25. [3] R.M. Mehalso and R.J. Diefendorf, in: Proc. 5th Intern. Conf. on CVD, Eds. J.M. Hintermann and L.E. Hall (Electrochem. Soc., 1975) p. 84. Conf. N. on CVD, Eds. G.W. Cullen Blocher, [4] Intern. H. Tanaka, Nakanishi and E. Kato,andin:J.M. Proc. 10th
This gives: r
CVD of boron
1”2(H 2(v) B p(HC1) A1 + p(BC!3) p 0(v)/p(H 2) 0 2)
(A.10)
Jr. (Electrochem. Soc., Pennington, NJ, 1987) p. 155. [5] J.O. Carlsson and M. Boman, in: Proc. 8th Intern. Conf. on
Irrespective of the reaction alternative (i)—(iv) which actually occurs, the deposition rate can be written on the general form: 2(H 2(v) r Ap(BC13) pi/ (A.11) 2) 0 1 + Bp(HC1) f(p(H =
2),(v))
‘
where f(p(H2), 0(v)) is a function dependent on the partial pressure of hydrogen and the concentration of vacant adsorption sites.
Vacuum Metallurgy, Linz, 1985, p. 257. [6] Gmelin Handbook of Inorganic Chemistry, Boron Supplement, Vol. 2 (Springer, Berlin, 1981). [7] U. Jansson, M. Boman, L. Markert, J.O. Carlsson and i.E. to be published. [8] Greene, U. Jansson, JO. Carlsson, B. Stndh, S. Soderberg and M. Olsson, to be published. [9] L. Vandenbulcke and G. Vuillard, J. Electrochem. Soc. 124 (1977) 1937.