J. Phys. Chem. 1992, 96, 10856-10862
10856
Some Mechanlstic Problems in the Kinetic Modeling of Monosilane Pyrolysis Rosa Becerra Instituto de Quimica-Fisica "Rocasolano",C.S.I.C., CISerrano, 1 1 9, 28006 Madrid, Spain
and Robin Walsb Department of Chemistry, University of Reading, Whiteknights, P.O. Box 224, Reading RG6 2AD, U.K. (Received: May 6, 1992; In Final Form: July 7, 1992)
The original static system SiH4pyrolysis product-time evolution data of Pumell and Walsh have been modeled with a mechanism involving 15 gas-phase elementary steps together with five alternative solid deposition processes. The gas-phase steps incorporate the most recent experimental and theoretical information concerning the kinetics and thermodynamics of the transient intermediates, SiH2,SiH3SiH,and H2Si=SiH2,and the isomers of Si2H2and Si3H6. Although the observed kinetic behavior can be fitted with reactive transients H2SiSiH2,Si2H2,or Si3H6as the species leading to solid deposition, it is only possible with unrealistically large wall termination (or polymerization) rate constants. Trisilane, Si3H8,offers a better alternative solid sink process with realistic rate constants but leads to a prediction of kinetic wall effects which are not observed. In the overall process the observed rate acceleration and transition between 3 / 2 order and first-order kinetics is accounted for by the reaction Si$, SiH3SiH + H2
-
whose rate is enhanced by chemically activated Si2H6produced via SiH2 insertion into SiH4. This finding is independent of mechanisms of solid deposition.
stages but also the region of rate acceleration and growth of deposited s o l i d ~ . ~ J -The ~ ~ most elaborate of these, by White, Espino-Rios, Rogers, Ring, and O'Nea15 (WERRO), postulated a significant role for the Si2H4isomers, silylsilylene, SiH3SiH, SiH4(g) Si(s) + 2H2(g) and disilene, H2Si=SiH2. The latter species was proposed as the key intermediate leading to polymerization and wall deposition. Its importance as a source of epitaxial silicon and amorphous The WERRO mechanism also proposed a heterogeneous initiation silicon hydride has stimulated enormous interest in the mechanism which switched on in the middle stage of reaction to supplement of this process.'*2However despite over 50 years of investigation, the homogeneous reaction, at the point where solid material deour understanding of this reaction is far from complete. position begins. This is justified by the argument that the reactor Silane pyrolysis has been studied in s t a t i P and flow sy~tems,'~ surface changes during the course of the reaction from pure silicon in the shock tubel0,lI and by the laser-powered homogeneous to an (SiH2)xwall coating (as found experimentally3). The pyrolysis method.I2 The initial step has been the source of conmechanistic fit of WERRO imposed constraints on the magnitudes siderable controversy,s'2 as has the role of heterogeneous processes of certain rate constants such as that for the isomerization reaction in pyrolysis s t ~ d i e s . ~ The J ~ -onset ~ ~ of SiH4decomposition occurs SiH3SiH H2Si=SiH2 (4) at temperatures of ca. 370 OC, and it is now generally agreed that at moderate pressures (>40 Torr) the reaction is initiated, as for which no kinetic data existed at the timeas originally pro@ by Purnell and Walsh13 (PW) some years ago, There have been a number of developments which make a by the homogeneous elementary step reexamination of this kinetic model worthwhile. Firstly, direct time-resolved measurements of rate constants for the SiH2.rp SiH4 SiH2 + H2 (1) actions have become available.24-B secondly, high quality ab mho calculations of the enthalpies of formation,*34 and, in some cases, There is some information on the conditions (generally low barriers to rearrangement of some of the key species have been pressures and low temperatures) which favor heterogeneous incarried o ~ t . ~Thirdly, ~ 3 ~a ~recognition that several of the steps itiations and reacti0n.l~9~~ It is not the purpose of the present paper are either unimolecular dissociation or bimolecular association to model the reaction under such conditions, but it is important reactions and are subject to characteristic pressure dependences, to realizt that both homogeneous and heterogeneous reactions may which require RRKM (or other) theoretical treatment to calculate occur under the conditions of chemical vapor deposition (with their rate constants at a given set of reaction conditions. Ever heated substrates and nonuniform gas temperatures). since our earlier experimental involvement in this chemical system3 In the early stages of the pyrolysis (1.0 x 0.012 Si2H2(w) 2Si(w) H2 Si(H)2Si H2SiSi 1.0 x H2SiSi Si(H)$i 9.0 x H2SiSi SiH4 Si3H6 1.3 X 1.5 x Si3H6 Si2H5SiH 1.8 x Si2H5SiH Si3H6
I
200.0
time/sec
Figure 1. Experimental product time evolution curves for principal products of SiH4 pyrolysis (154 Torr). (a) H2 (O), Si2H6(O), Si3H8(A) and (b) Si(s) (B), HAS) ( 0 ) .
yields were calculated as a function time over the range of experimental pyrolysis times. The mechanism and its rate constants were chosen as described in the next section. Whilst most rate constants were fixed, a few were adjusted to produce reasonable fits to the product-time evolution curves. The experimental data employed for the fitting was that from the original PW static system study3 in which the products monitored were Hz, Siz&, and Si3H8(in the gas phase) and silicon and hydrogen (in the solid deposited material), designated Si(s) and Hz(s). In the original study, experiments were carried out at four pressures (range, 38-154 Torr) at each of five temperatures (range, 652-703 K),with individual product-time runs taken to ca.25% conversion. However for simplicity we have limited the modeling Cxcrcise presented here to runs at one temperature (703.3 K) and two pressures (38 and 154 Torr). The main objective is to fit the pressure and time dependencies since these place the more demanding requirements on the mechanism. The temperature dependence, in the original study, is determined largely by k, and is already known5 to be consistent with the most recent studies of the initial The data to be fitted is illustrated by the product-time evolution results shown in Figure la,b. This figure illustrates the main features of the reaction. In particular modeling has to reproduce the initial kinetics (3/2 order in SiH4),
0.10 0.025
+
23. 24. 25. 26.
0 0
38 Torr
Mechanism A 16. H2Si=SiH2 Si2H4(w) 5.0 x 104 17. Si2H4(w) 2Si(w) + 2H2 0.035
Mechanism C 21. Si3H8 Si&(W) + H2 0.10 22. Si,&(W) 3si(W) 3H2 0.039
. ' ,
rate 154 Torr
--- + -+- --
+
lo3 106 io6
109 107 1010
3.8 x 109 1.1 x 105 1.0 x 104 0.020 4.0 x lo3 >1.0 x 106 0.0050 2.5 x 105 2.3 x 109 1.3 X 7.5 x i o 6 9.0 x 109
For selection of rate constants, see text. Units: second-order rate constants, cm3 molecule-' s-l; first-order rate constants, s-I.
the region of rate acceleration (leading to maximum rates which are first order in SiH4), the steady-state pressures of SizHs and Si3Hs, and the Si@), and H2(s) growth characteristics. Reaction Mechanism. In this paper we have attempted to keep the mechanism as simple as possible whilst allowing the possibility of exploring a number of different options. We have divided the mechanism into two parts. The first part, shown in Table I, contains the gaseous product generating steps, whilst the second part, in Table 11, contains five possible routes (sink processes) leading to solid formation. Unlike the WERRO mechanism: this mechanism does not include a surface initiation step. We note that in the original PW experimental study,3 tests were carried out showing that product formation (including the solid deposition) was independent of reaction surface area-to-volume ratio, A/ V. This result argues against a single rate determining step involving the surface. It is, of course, possible to have both surface initiation and termination steps, whose rates compensate one another.
Becerra and Walsh
10858 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
The mechanism of Table I is fairly straightforward and follows TABLE 111: High-Pressure Arrhenius Parameters of Important Reaction steps* from the discussion in the Introduction. Initiation via step 1 creates SiH2 which reacts sequentially to form disilane, trisilane, and log Ea, kJ tetrasilane via the known insertion processes (steps 2, 3, and 4). step reaction ( A l s d l mol-' ref Although not observed, tetrasilane is almost certainly formed 5. Si2H6 SiH4 + SiH, 15.75 218.4 18 Si3H8 SiH4 + SiH3SiH 15.41 7. 214.1 20 at a low steady-state level and is important enough to influence 8. Si,H8 SizH6+ SiH, 15.70 222.6 20 the modeling. However for simplifcation purposes we have treated Si,H,, SiH4 + Si2H5SiH 15.5 9. 208.8 20 Si4HI0as the normal isomer and neglected any contributions from 10. Si4HIo Si2H6+ SiH,SiH 15.5 219.2 20 i-Si4HI0.Likewise we have ignored higher silanes. Steps 5-1 1 11. Si4HI0 Si3Hs + SiH, 15.6 223.8 20 represent the known or expected decomposition reactions for 12.9 22.2 35 14. SiH3SiH H,Si=SiH, disilane,I7J8t r i ~ i l a n e , l and ~ > ~tetrasilane.20 ~ These are experiHzSi=SiHz SiH3SiH 13.4 70.3 35 15. Si2H5SiH c-Si,H, 11.5 8 see text 23. mentally established thermal decomposition processes except step c-Si,H, Si2H5SiH 13.5 96 see text 24. 6 which is discussad in detail later. Amongst the products of these SizHJSiH 13.4 33. Si3H, 70.3 see text steps are the substituted silylenes, SiH3SiH (steps 6, 7, and 10) 12.9 22.2 see text 34. Si2H5 Si3H6 and Si2HSSiH(step 9). In the early stages of reaction the most H2Si=SiH2 Si(H),Si + H, 14.0 136 see text 21. likely bimolecular reactions of these are their insertions into SiH4, H,SiSi 13.5 30. Si(H),Si 74 see text 31. H,SiSi Si(H,)Si 13.5 21 see text steps 12 and 13, reforming Si3H8and Si4HI0,respectively. For reactions 14 and 15, the reversible isomerization of SiH3SiH, no "See text for choice of particular Arrhenius parameters. direct experimental rate data exists. However there is strong evidence from our earlier modeling of H atom/SiH4 system35 is one of the main sources of data on which the theory has been supported by theoretical calculations for these p r o c e s s e ~ . ~It' * ~ ~ based. All that is necessary in the present study is to adjust kl appears unlikely" that there are direct pathways to H$iSiH2 from to give optimal fit. For Si2H6,Si3H8,and Si4HI0the most recent Si2H6(or higher silanes). Such a pathway was assumed earlier.5 thermal decomposition studies'8-20have been used as the basis for Although other gaseous mechanistic steps were considered in the values for ksr k,, k8, kg, klo,and k l l in Table I. The limiting previous modeling e ~ e r c i s e s , ~we ~ ~do~ not - * ~consider ~ ~ ~ them here high-pressure Arrhenius parameters for these rate constants are either because they are marginal or there is no experimental shown in Table 111. Of these steps, the only one not at the evidence for their involvement. high-pressure limit is k5. We have used existing estimates, from The mechanisms of Table I1 show the five processes we conRRKM calculations based on measured pressure dependences,18 sidered for solid deposition. All species deposited at the wall are to obtain k5/ks" = 0.78 (at 154 Torr) = 0.51 (at 38 Torr). This labeled (w) and all are assumed to undergo partial dehydrogeassumes SiH4 has the same collision efficiency as propane. Under nation to give Si(w) and gaseous H2. The molecular designation, static pyrolysis conditions primary H2 formation from Si2H6,step e.g., Si2H4(w),merely serves to identify the solid progenitor. In 6, is negligible (15%)." On the other hand, H2 formation occurs fact it represents the solid polymer (SiH2), as indeed do Si3H6(w) under shock tube This reaction may not be a simple and Si4H8(w).Si2H2(w)represents the polymer (SiH),. The solid thermal process, and so we have allowed k6 to float and become products are calculated as follows. Si(s) is given by combining an adjustable parameter for fitting. the atomic Si contributions from the undecomposed polymer and (ii) Insertion Reactions of SiH2, SiHJiH, and Si2H$iH. Earlier modeling s t ~ d i e s ~ .used ~ ' - estimates ~~ for these rate conSi(w). H2(s) is simply the H2 combined in the undecomposed polymer. Mechanism A assumes solid deposition via unimolecular stants. Since then direct measurements of k2 and k3 at room migration of H2SiSiH2to the wall. Mechanism B postulates a temperature have been p ~ b l i s h e d . ~More ~ - ~ ~recently we have carried out our own s t u d i e ~of~ reactions ~ - ~ ~ 2, 3, and 4 over the gaseous dimerization of H2SiSiH2prior to fast migration of Si4H8 to the wall. This is the simplest representation of gaseous rather temperature range 295-593 K. These investigations show small than wall-controlled polymerization. Mechanism C considers decreases in the magnitudes of k2, k3, and k4 with temperature. termination via Si3H8. This mechanism, suggested by Ring and However the Arrhenius plots are nonlinear, and furthermore k2 O'Nea1,36 is based on the fact that higher silanes are known to is pressure dependent.28 The figures given in Table I represent be more reactive than BH4 at silicon surfaces'*I6and is supported short extrapolations of our data to 703 K with RRKM assisted by gaseous material balance deficiencies in Si3H8p y r o l y ~ i s . ~ ~ ~adjustments ~~ from the high-pressure limit for k2 (high pressure Mechanism D introduces a new terminating species c-Si3H6, limit 1.3 X 1O-Iocm3molecule-' s-I). It should be noted that the cyclotrisilane, formed via intramolecular cyclization from Si2extent of fall off, k2/k2" = 0.77 (154 Torr) = 0.62 (38 Torr), is HSSiH. Other Si3H6isomers such as H3SiSi(H)=SiH2, silylexactly the same as for the reverse step 5. Even with what are disilene (the silicon analogue of propene), could also participate, close to the experimental values for k2, k3, and k4 it proved a but c-Si3H6was chosen as representing probably the most stable disappointment to us that good fits were not possible (see later), Si3H6isomer34and therefore the one likely to develop the highest and so the value for k3 was adjusted from the experimental one steady-state concentration and the best possibility of possessing (3.8 x 10-lo cm3 molecule-l s-l). In the absence of experimental a sufficient flux to the wall. Mechanism E is based on the posdata the values for kI2and k13 were set at the high-pressure limit sibility of decomposition of H2SiSiH2to the Si2H2isomers by H2 of that for k2. It is thought that SiH3- and Si,HS-substituents elimination. These processes are the most speculativebeing based should, if anything, activate ~ilylenes?~ but no allowance was made largely on theoretical calculations of stabilities of these ~ p a c i e s . ~ ~ for this since k2 is close to the collisional limit. The cyclic form of Si(H)2Si,recently observed spectros~opically~~ (iii) Interconversionof Sifl, Isomers. No direct experimental is proposed as the initial product of H2 elimination. Being the data exists for kI4and kI5. Step 14 is known to be a fast reaction most stable form this is proposed as the solid product precursor. and from our analysis of the H atom/SiH4 system3swe have Allowance, however, has to be made for a gas-phase feed back previously estimated Arrhenius parameters for kI4. Some support mechanism involving isomerization (reversible) to H2SiSi, silylfor our estimate of E,(14) = 22 8 kJ mol-' comes from a idene, another Si2H2isomer which can react via insertion with competitive study of SiH3SiH isomerization and trapping (by SiH4 to yield Si3H6,silyldisilene. The Si3H6couples back into butadiene) carried out by Gaspar et al.43 They obtained an the mechanism via reversible isomerization to Si2HSSiH. activation energy difference of 42 f 8 kJ mol-' for these two Mechanism E is clearly the most speculative, although still based processes. We have found, in direct kinetic studies, that silylene largely on processes with reasonable mechanistic precedent. reactions with ~ - s y s t e m shave ~ ~ *negative ~~ activation energies, which can be as high as -20 kJ mol-'. Thus the Competitive study3 Rate Constant Selection. ( i ) Monosilane and Higher Silane implies E,( 14) is less than 42 kJ mol-' by possibly as much as Decompositions. For SiH4,step 1 is a pressure dependent uni20 kJ mol-'. We have also arguedgsthat E,(15) 1 70 kJ mol-'. molecular reaction. Accurate Arrhenius parameters are available With the aid of theoretical estimates of vibration frequencies3' and RRKM calculations have been carried out by others to esand transition-state theory we estimated The Arrhenius timate the pressure dependen~e.ll.~q~~ In practice the PW study3
-----+
*
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10859
Kinetic Modeling of Monosilane Pyrolysis
TABLE IV: Values for k6 Required To Fit the Modeling with Different Deposition Mechanisms pressure, Torr 154 38
a
mechanism A 0.040 0.050
B
C
D
E
0.040 0.042
0.030 0.037
0.038 0.040
0.042 0.055
parameters are included in Table 111. We have chosen the minimum value for E,( 1 5), because theoretical calculation^^^-^^ suggest an enthalpy difference, (AH0~(SiH3SiH) AHi‘(H2SiSiH2)) in the range 33-50 kJ mol-’. This is, if anything, slightly lower than our activation energy difference &( 15) - Ea(14).The Arrhenius parameters for k14 and k15 are not very precise, but we feel that they are the best currently available. Ring and O”ea136 have chosen similar values. Another complication is that step 14 and 15 are in the pressure dependent, fall-off region and 90 require RRKM calculation of their rate constants. Using our previous model35we have estimated kI4/kl4*is ca. (at 154 Torr) and ca. 2.5 X lo-’ (at 38 Torr). This is the basis of the values of kipquoted in Table 1. k15values were calculated on the assumption that ratio kI5/kI4equals k15m/k14m, Le., the equilibrium cqnstant derived from our previous work.35 (io) Deposition and Wall Rate Constants. Our approach in this paper is not to estimate or precalculate values for these, as others have but rather to derive values for them by optimization of the fitting procedure. Thus many of the rate constants of Table 11, (for steps 16-18, 20-22, 25, 26, 28, and 29) are fitted. They are discussed in the next section. kI9was artificially set at 1.O X 1Olo s-l in order to ensure that step 18 was rate determining leading to an irreversible sink process for H2SiSiH2. (o) Reactions of Si3H6Isomers. No experimental data exists for these species. Rate constants for steps 23 and 24 involving c-Si3H6were calculated from the estimated Arrhenius parame111, with allowance for unimolecular fall-off t e r ~shown ~ - ~in Table ~ effects of k/k” = 0.1 (154 Torr) and 0.05 (38 Torr). These in turn depend on a theoretically-based estimate of AHfo(c-Si3H6) of ca. 266 kJ mol-’ (a compromise value between those of refs 34, 48,and 49) and an additivity estimate of AHfo(Si2H5SiH) = 345 kJ mol-’ (based on AHy(SiH3SiH) = 312 kJ mol-’ with an increment of +42 kJ mol-’ for SiH3-for-H replacement). Rate constants for step 33 and 34 involving silyldisilylene were calculated from the estimated Arrhenius p a r a m e t e r ~shown ~~,~~ in Table 111, with allowance for unimolecular fall-off effects of k/k“ = 0.1 (154Torr) and 0.05 (38 Torr). These in turn depend on a theoretically based value34of M:(H3SiSi(H)SiHz) = 306 kJ mol-’ and AHfo(Si2H5SiH)estimated above. Activation energies were assumed to be the same as for the steps 14 and 15 involving the Si2H4analogues. (vi) Reactions of Si2H2Isomers. No experimental data exists for these species. Rate constants can (as for Si3H6)be estimated by the methods of thermochemical kinetic^?^,^^ Arrhenius parameters so derived are shown in Table I11 for steps 27,30, and 3 1. AHfovalues are required for Si(H)2Si and H2SiSi and these are taken from the calculations of Ho and M e l i ~ Activation .~~ energies are based on the assumptions that step 31 is similar to 14 and that reaction of Si(H)2Si with H2 (reverse of step 27) has no barrier. Fall-off effects are assumed (based on analogous reactions for which RRKM calculations have been carried out of k/k” = (154Torr), 2.5 X (38 Torr) for steps 30 and 31. Comparison of Model Predictions with Experiment and Optimization of the Fits. The gaseous mechanisms of Table I were tested with each of the five solid deposition mechanisms of Table I1 in turn to investigate whether a fit could be achieved. With any of the deposition mechanisms A-C, reasonable fits could be obtained, with the listed rate constants. The only adjustments required to obtain these fits were k6 and the deposition and solid decomposition rate constants, listed in Table 11. The values for k6 associated with each deposition process are listed in Table 1V. Examples of the quality of the fit are shown in Figures 2a,b and 3a,b, for mechanism A. Similar qualities of fit were obtained for
1
50’0
10.0
HI
d i / h Si,H.
0.0
SilH.
0.0
50.0
100.0
150.0
200.0
time/sec
25’0 20.0
1
4
0.0
Si(.)
50:O
100.0
150.0
200.0
time/sec
Figure 2. Optimal Model fits with solid deposition mechanism A (SiH4 initial pressure, 154 Torr).
mechanisms B and C. As well as general fitting for all products we tested the value of the quotient ([si#6]2/[siH4] [Si3H&,,, (=MA), examined by Ring and ONea1.3‘j Experimentally3thls quantity has the value 0.20 f 0.01 after ca. 150 s. In the fits described here this had values in the range 0.14 f 0.03 after 180 s. The underestimate was associated with slight overestimates of [Si3H8]. This could have been rectified by modification of k2 and k3. However we felt this was not justified.50 Interestingly the modeling value underestimates the equilibrium value of MA (=k2k8/k3k7) by ca. a factor of 2,suggesting this quotient does not reach equilibrium, in contrast to the findings of Ring and O”eal.36 In spite of this we feel the fitting, as judged by product-time evolution curves, is fairly good. We have tried to carry out crude sensitivity tests on these mechanisms, but because of the difficulties with the rate constants (see Discussion) we restricted our investigations to a few steps only using deposition mechanism A. In view of the similarity of response found during fitting with mechanisms A-C, these findings should be general. Two findings were noteworthy. First the modeling was sensitive to the magnitudes of kI4and k15,but nor to the ratio kI4/kl5. Each rate constant could be changed by a factor of lo2(increase or decrease) but provided kI4/kl5was the same, the modeling showed no change. This indicates that the
10860 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
a
Becerra and Walsh
1o.u
50.0 0.04
/
40.0
/
7.5
L
30.0
?
2
i
5.0
0
$
20.0
////
2.5 10.0
////
0.00 -
Siltis
0.0 0.0
50:O
100.0
150.0
200.0
time/sec
b
_
50.0
100.0
150.0
200.0
time/sec
Si(o) P
3.0 i i
0
2.0
O l' L 0.0
0.0
y-
00
Figure 4. The effect of variation of k6 on H2yields (with solid deposition mechanism A). Values of k 6 / d are indicated for each curve.
5.0 -
4.0 -
t a
0.0_
50:O
100.0
150.0
200.0
time/sec
Figure 3. Optimal model fits with solid deposition mechanism A (SiH., initial pressure, 38 Torr).
Si2H4isomers are in equilibrium. Increase of kl4 or decrease of kls, within a small range could be compensated by decrease of the appropriate sink constant (kI6 for mechanism A). This is understandable, since these changes cause an increase in the equilibrium concentration of H2Si=SiH2, the deposition species in mechanism A. Secondly the modeling was quite sensitive to the magnitude of k6 A test was carried out in which k6 was varied between zero and 0.04 (at 154 Torr). Figure 4 shows the yields of H2 obtained in these runs. The acceleration in H, formation rate with k6 is clearly visible. Examination of the different H2 sources reveals that the origin of this rate increase is the secondary formation of H2 (steps 6 and 17) and not the primary H2via step 1. Marked increases in the other secondary products (Si3H8and Si(s)) were also observed. This confirms the importance of step 6 in the transition between the initial and secondary stages of the pyrolysis. The sensitivity of the H2 and other yields to k6 was independent of chosen deposition mechanism A, B,or C. This indicates that this feature of the mechanism is probably correctly accounted for in spite of difficulties over the deposition processes (see Discussion). Fitting to mechanisms D and E was more problematical. It was not possible to fit the data to D by variation of k6 and the deposition rate constants, k25and k26, alone. Either gaseous
products were overestimated or solid products were underestimated. Tolerable fits could be approached if k3 and k4 were and 5.8 X cm3 molecule-l s-', increased to 3.6 X respectively. This would require our experimental insertion rate data29for (4) rather than (3) to be in error (see previous section). Changes in kz3 and k24 could improve the fit (increase in k23, decrease in k24), but, since these have no independent basis, we did not use them as fitting parameters. Mechanism E was even more difficult to fit to the data. With only adjustment of k6 and the deposition constants, kz8and kZ9, no satisfactory fit was possible. Gaseous products were overestimated and solids underestimated. Variation of kZ8between 0 and 1O1O s-l did modify the yields somewhat but did not alter the overall situation. We concluded that the rate of Si(H)+3i formation in step 27 was too slow and became rate limiting. However even if k27was increased (by ca. a factor of XlO), the fits were unsatisfactory unless kz8was also greater than ca. IO6 cl,thus preventing the recycling of the gaseous species Si(H)2Si through its isomer H2SiSi and the molecule Si3H6(via steps 30-34). We conclude that unless rate constants are adjusted arbitrarily this mechanism cannot provide a satisfactory fit.
Discussion The Surface Deposition Processes. The present exercise was undertaken to throw more light on the SiH4pyrolysis mechanism. It would be nice to report a definitive outcome. Unfortunately we are unable to do this. The key difficulty lies in the failure to find a fully satisfactory deposition process. Good fits can be obtained with mechanism A-C, but examination of the rate constants required indicate that none of them is reasonable. Mechanism A is unsatisfactory (a) because values of k16 of.ca. lo4 s-l are unreasonably fast for a surface controlled reaction, implying an almost unit collision efficiency of H2Si=SiH2with the walls1and (b) because kl6 should be pressure dependent if it is diffusion-limited, in which case it should be faster at lower pressures. Mechanism B is unsatisfactory because it requires values of k18of ca. 10" cm3 molecule-I s-I for the dimerization of H2Si==!3iH2, which are ca.104 higher than collision frequency! Since dimerization rates of Hsi=SiH2 are unreasonable, gaseous particle formation cannot be initiated in this way. Thus the claimed gaseous particle formationI6 must occur via other processes. Mechanism C, is more reasonable with a wall rate constant, kZlrof 0.10 s-l, representing ca. 18% of the total Si3HBdecomposition, broadly in line with o b s e r ~ a t i o n . ~ ~The * ~ ~problem * ~ * is that it is hard to see why this constant, involving the wall, should lead to kinetics which are independent of vessel A / V ratio as observed.' However apart from this difficulty it remains the most
Kinetic Modeling of Monosilane Pyrolysis plausible of the mechanisms suggested. Mechanism D, although initially attractive because of involvement of the most stable of the Si3H6isomers, fails for the same reason as mechanism A, because of the unreasonably high value of kzs,the surface deposition rate constant. Mechanism E does not fit the data, as already discussad, without arbitrary changes in the rate constants used. The general finding here is that, unless there are serious errors in their estimated rate constants, none of the intermediates, H2Si=SiH2, Si(H),Si, and c-Si3H6,offer satisfactory routes to solid deposition in this system. The underlying reason for this is that their theoretically estimated stabilities do not lead to sufficient stationary-state concentrations to permit a reasonable flux to the vtssel surface. Only the higher silanes themselves, such as Si3H8,seem to offer a plausible deposition route, in support of earlier suggestions16and in agreement with Ring and O”eal.36 However they run up against the difficulty, which any surface deposition process poses, that the rates are independent of A / V’, an awkward fact also true for Si2H6I7and Si3H838pyrolyses. General Comments Concerning the Mechanism. In view of difficulties of the deposition mechanism it seems reasonable to enquire into the reliability of the gaseous mechanism. The choice of steps and selection of rate constants has been largely discussed. The elementary steps 1-1 1, except 6,have all been subject to individual study although there is probably some room for improvement in the pressure and temperature dependenciesof some of them. The collision efficiency of SiH4, assumed to be unity, may not be 50 for the pressure dependent processes (of which there are several). The fitted rate constantsfor the initiation, k l ,showed the expected squareroot pressure dependence, consistent with the 3 / 2 order originally obtained.’ The fitted rate constants for k6 appeared to be relatively independent of the choice of deposition mechanism, with a slight increase at lower pressures. Comparison with ks showed that k6/kslay in the range 0.10-0.15 at 154 Torr and 0.17-0.25at 38 Torr. These are higher than observedI7 (10.05) for the static pyrolysis. Step 6 is thought to have a threshold slightly higher in energy than step 5, with the activation energy in the range 23 1-242 kJ mo1.20,35*42*52 The explanation for the observed ratios of k6/ksmay lie in the fact that the Si2H6 produced in step 2 is chemically activated. The pressure dependence of step 2 demonstrates that energized Si2H6molecules initially formed are not all stabilized. With a broad distribution of energies at 703 K, a fraction of the steady-state population of SizH6will lie above the threshold for step 6. This can lead to the enhancement of H2 formation via step 6. At lower pressures, stabilizationof the energized Si2& will be less, leading to a higher fractional loss via step 6. Very recently Moffatt et alas2have carried out quantitative RRKM calculations which support this point. Because of the importance of step 6 to the mechanism we plan to verify these. As shown in the modeling, step 6 is the key to the transition from initial to intermediate stage of the pyrolysis. The acceleration can be attributed to the combination of steps 6,8, and 12,as suggested by us earlier,I4 which constitute a chain cycle leading to e n h a n d formation of SiH2from SiH4as Si2& builds up. The chemical activation explanation accounts for increased effectiveness of this cycle at lower pressures, which causes the apparent change in the overall kinetics to a first-order dependence in SiH4in this stage of the pyrolysis. Although we have limited our modeling to a single temperature, we would expect that k6/k5would be only slightly, it at all, temperature dependent. Thus the activation energy of the intermediate stage of pyrolysis should not differ substantially from that of the initial stage, in accordance with experiment.3 The most uncertain part of the gaseous mechanism concerns the reactions of SiH’SiH, H z S i 5 i H z ,and SiZH5SiHfor which no direct experimental values exist. Direct studies of these species as well as being of intrinsic interest would be invaluable here. The rate constants for these and the reactions involving other transient species (Si3H6and Si2H2isomers) are almost entirely reliant on theory. If, for instance the stability of H$3i=SM2 is greater than ab initio calculations suggest, then k15will be lower and a higher
The Journal of Physical Chemistry, Vol. 96, No. 26, I992 10861 stationary concentration of H2Si=SiH2 would be obtained. This in turn would reduce the requirement for such unreasonable rate constants (kl6, k I 8 )for the deposition mechanisms A and B. It is also possible that nonequilibrium effects (leading to different pressure dependencies) on steps 14 and 15 would alter the values of k14 and k l s used here, but they would have to be fairly substantial to make H2Si=SiH2 a sink species. We judge this unlikely. Comparison with Previous Modeling Studies. Apart from differences mentioned already (surface initiation) the main difference with the WERRO mechanisms is in the treatment of the SizH4isomers. Without the benefits of more recent theoretical and modeling information, they proposed direct formation of H2Si=SiHz and also much slower interconversion of SiH’SiH and H2Si=SiH2 (i.e., lower values of k14 and k l S ) .As we have shown, this can lead to solid deposition via H$i=SiH2 without the necessity of invoking unreasonable deposition rate constants. The new mechanism of Ring and ONeaP6 essentially recognizes and updates this aspect of their earlier m~deling.~ There remains a difference over the cause of the acceleration stage. Ring and ONeaP6 (and WERROS) attribute this to the secondary surface initiation rather than the chain initiated by step 6. WERR05 rejected silylsilylenechain mechanisms, since they (apparently) required cycling via H2Si=SiH2 and would have had the wrong temperature and pressure effects. This difficulty is apparently obviated in the present work since although HzSiSiH2 is involved in the mechanism, it is not directly involved in the chain cycle of steps 6, 8, and 12. Detailed comparison with the modeling of Coltrin et al.23was not attempted since their experimental conditions are significantly different (flow system, rotating disc CVD reactor). Their mechanism is substantially more complex, involving more intermediates. However for the key species S M 2and the Si2& isomers, the mechanism is based on the same thermochemistry and similar kinetics. The model calculates decomposition rates (although it is not compared with experiment). It uses experimental surface reaction probabilities (for SiH4 and Si2H6)and assumes a unit sticking probability for all transient species. Of note is the finding that for 0.1% SiH4 in He, the principal depositing species are H2SiSiH2,Si2Hz(most stable isomer), and Siz between 900 and 1300 K. However until some kind of chemical species analysis is developed for such systems, there will always be too many uncertain variables. We note a report of the detection (by LMR) of S M 3radicals in a high-temperature (ca. lo00 K) flow pyrolysis of SiHQS3These are attributed to a heterogeneous reaction.53 SM, is not thought to be involved in the mechanism under the lower temperature, higher pressure conditions modeled in this paper.596s12-14 Acknowledgment. We thank the Anglo-Spanish Research Collaboration Fund (Acciones Integradas) for financial support. We also thank Ed O’Neal and Morey Ring for an advanced copy of their paper, Roger Grev and Harry Moffatt for preprints, Carlos Canosa for helpful information, and Joe Jasinski for a translation of ref 53. Registry No. SiH4, 7803-62-5.
References and Notes (1) Jasinski, J. M.; Gates, S. M.Acc. Chem. Res. 1991, 24, 9. (2) Other silicon or silicon hydride deposition methods, such as plasma
enhanced CVD or laser initiated CVD, are not considered in this paper. (3) Purnell, J. H.; Walsh, R. Proc. Roy. Soc. London, A 1966,293, 543. (4) Neudorfl, P.;Jodhan, A.; Strausz, 0. P.J. Phys. Chem. 1980,84,338. ( 5 ) White, R. T.; EspineRios, R. L.; Rogers, D. S.; Ring, M.A.; O’Neal, H. E. In?. J. Chem. Kine?. 1985, 17, 1029. (6) Erwin, J. W.; Ring, M.A.; ONeal, H. E. In?. J. Chem. Kine?. 1985, 17, 1067. (7) Ring, M. A.; Puentes, M. J.; ONeal, H. E. J. Am. Chem. SOC.1970, 92, 4845. (8) Haas, C.H.; Ring, M.A. Inorg. Chem. 1975, 14, 2253. (9) Robertson, R.; Hils, D.; Gallagher, A. Chem. Phys. Le??.1984, 103, 397. (10) Newman, C. G.; Ring, M.A.; ONeal, H. E. J. Am. Chem. Soc. 1978, loo,5945. (11) Newman, C. G.; ONeal, H. E.; Ring, M. A.; Lcska, F.; Shipley, N. In?. J . Chem. Kine?. 1979, 11, 1167.
10862
J . Phys. Chem. 1992, 96, 10862-10868
(12)Jasinski, J. M.; Estes, R. D. Chem. Phys. Leo. 1985,117,495. (13)ONeal, H.E.; Ring, M. A. Chem. Phys. Lett. 1984, 107,442. (14)Purnell, J. H.;Walsh, R. Chem. Phys. Lett. 1984,110,330. (15)Robertson, R.; Gallagher, A. J . Chem. Phys. 1986,85,3623. (16)Scott, B. A.; Estes, R. D.; Jasinski, J. M. J. Chem. Phys. 1988,89, 2544. (17) Bowrey, M.; Pumell, J. H. P m . Roy. Soc. London, A 1971,321,341. (18) Martin, J. G.; Ring, M. A.; ONeal,H. E. Inr. J . Chem. Kinet. 1987, 19,715. (19)Vanderweilen, A. J.; Ring, M. A.; ONeal,H. E. J . Am. Chem. Soc. 1975,97,993. (20) Martin, J. G.; ONeal, H. E.; Ring, M. A. Int. J . Chem. Kiner. 1990, 22,613. (21)Coltrin, M. E.; Kee, R. J.; Miller, J. A. J. Electrochem. SOC.1984, 131,425. (22)Coltrin, M. E.; Kee, R. J.; Miller, J. A. J . Electrochem. SOC.1986, 133, 1206. (23)Coltrin, M. E.; Kee, R. J.; Evans, G. H. J . Electrochem. SOC.1989, 136,819. (24) Inoue, G.; Suzuki, M. Chem. Phys. Lett. 1985,122, 361. (25)Jasinski, J. M.; Chu, J. 0. J . Chem. Phys. 1988,88, 1678. (26) Baggott, J. E.;Frey, H. M.; Lightfoot, P. D.; Walsh, R.; Watts, I. M. J. Chem. SOC.,Faraday Trans. 1990,86,27. (27)Dietrich, T. R.; Chiussi, S.;Marek, M.; Roth, A,; Comes, F. J. J . Phys. Chem. 1991, 95,9302. (28) &cerra, R.; Frey. H. M.; Mason, B. P.; Walsh, R.; Gordon, M. S. J . Am. Chem. Soc. 1992,114, 2751. (29)Becerra, R.; Frey, H. M.; Mason, B. P.; Walsh, R., unpublished results. (30) Ho, P.; Coltrin, M. E.; Binkley, J. S.;Melius, C. F. J . Phys. Chem. 1986,90,3399. (31)Gordon, M. S.;Truong, T. N.; Bonderson, E. K. J . Am. Chem. Soc. 1986,108, 1421. (32)Ho,P.; Melius, C. F. J . Phys. Chem. 1990,94,5120.
(33)Boatz, J. A.; Gordon, M. S . J. Phys. Chem. 1990,94,7331. (34)Sax, A. F.; Kalcher, J. J . Phys. Chem. 1991,95, 1768. (35) Becerra, R.; Walsh, R. J . Phys. Chem. 1987,91,5765. (36) Ring, M. A.; ONeal,H. E. J. Phys. Chem. 1992,%, preceding paper in this issue. (37)Braun, W.; Herron, J. T.; Kahaner, D. K. Inr. J . Chem. Kinet. 1988, 20,51. (38)Maddern, P., Ph.D. Thesis, University of Swansea, UK, 1974. (39)Bogey, M.; Bolvin, H.; Demuynck, C.; Destombes, J. L. Phys. Reu. Lett. 1991, 413. (40)Roenijk, K. F.; Jensen, K. F.; Carr, R. W. J . Phys. Chem. 1987,91, 5726. (41) Moffatt, H. K.; Jensen, K. F.; Carr, R. W. J . Phys. Chem. 1991,95, 145. (42) Dzarnoski, J.; Rickborn, S.F.; ONeal, H. E.; Ring, M. A. Organometallics 1982, I, 1217. (43)Gaspar, P. P.; Boo, B-H.; Svoboda, D. L. J . Phys. Chem. 1987,91, 501 I. .... (44) Blitz, M. A.; Frey, H. M.; Walsh, R., unpublished results. (45)Al-Rubaiey, N.; Mason, B. P.; Walsh, R., unpublished results. (46)Estimates were based on the methods of thermochemical kinetics (A factors)" and an assumed intramolecular insertion activation energy. (47)Benson, S.W. Thermochemical Kinerics; Wiley: New York, 1976. (48) Boatz, J. A.; Gordon, M. S.J . Phys. Chem. 1989,93,3025. (49)Horner, D. A.; Grev, R. S.;Schaeffer 111, H. F. J . Am. Chem. Soc. 1992,114,2093. (50)In preliminary studies, we used a higher (e~perimental~~) value for k3,but this was lowered because Si,Hs yields were always overestimated and MA values were too low. (51)See, for example: Keyser, L. F. J. Phys. Chem. 1984, 88, 4750. (52)Moffatt, H.K.; Jensen, K. F.; Carr, R. W. J . Phys. Chem. 1992,96, 7695. (53)Krasnoperov, L. N.; Nosov, V. V.; Baklanov, A. V.; Panfilov, V. N. Khim. Fir. 1988,7, 528.
Flash Photolysls-Time-Resolved UV Spectroscopy of the CF&FHO, Self-Reaction M. Matti Maricq* and Joseph J. Szente Research Laboratory, Ford Motor Company, P.O.Box 2053, Drop 3083, Dearborn, Michigan 481 21 (Received: July 14, 1992; In Final Form: September 2, 1992)
The self-reaction of CF,CFH02 has been studied via time-resolved ultraviolet spectroscopy over the temperature range 21 1-372 K. The absorption spectrum of CF3CFHO2extends from 190 to 275 nm with a maximum cross section of (5.2 0.3) X cm2molecule-' at 213 nm. The UV absorbance of the reaction mixture decreases and shifts to the blue as the reaction progresses. This is consistent with the CF3CFH02self-reaction producing CF3CFH0,the alkoxy radical then decomposing to yield CF3,which adds molecular oxygen to form CF30,. The CF3CFH02self-reaction has a negative temperature dependence with rate constant given by kl = (7.8 1.3) X 10-13e(aSfa)/Tcm3s-I. The rate of alkoxy radical dissociation, at 230 Torr of total pressure, is kza = (3.7 f 0.7) X 107e-(z2wfi50)~T s-I. The rate constants for CF302reaction with CF3CFHOzand itself are determined to be k8 = (8 3) X and kg = (1.8 0.5) X cm3 S-I, respectively, at 297 K.
* *
I. Introduction Over the past few years, an increasing wealth of data has been collected supporting the postulate that chlorofluorocarbon(CFC) compounds are transported into the stratosphere where they are photolyzed to produce atomic chlorine and thereby contribute to the catalytic destruction of stratospheric ozone.' The environmental ramifications of this have led to international agreements to phase out and eventually eliminate the manufacture and use of these compounds.2 The principal strategy in finding CFC replacements has been to examine chlorine- and fluorine-substituted methane, ethane, and propane species which retain at least one hydrogen atom, namely, the hydrochlorofluorocarbon (HCFC) compounds. The reason for this strategy is that, while retaining many of the useful physical properties of the CFC's, the HCFC compounds, by virtue of their hydrogen atoms, are significantly more reactive than CFC's toward OH radicals and are removed before they reach the ~tratosphere.~ However, comparativelylittle is currently known about the tropospheric chemistry of the HCFC molecules. The very reactivity which makes these compounds safe Author to whom correspondence should be addressed.
*
with respect to the stratospheremay potentially penvironmental problems in the troposphere. HFC-134a (CF3CFH2) is the designated replacement for CFC-12 as an automobile refrigerant. Because it is unreactive with oxidizers such as oxygen and ozone and because its has very small UV absorption cross sections at wavelengths penetrating into the troposphere: the principal removal mechanism of HFC-134a is expected to be via CF3CFH2 + OH CF3CFH + HzO Recent measurements of the W absorption spectrum of CF3CFH2 and its reaction rate constant with OH indicate that HFC-134a has an expected tropospheric lifetime of approximately 17 ~ears.39~ The hydrofluorocarbon radical produced by the above hydrogen-abstraction reaction will, in the presence of >10 Torr of O2 and 50 Torr of total pressure, add molecular oxygen on a microsecond time scaleS CF3CFH + 0 2 + M CFSCFH02 + M to form the corresponding peroxy radical. Peroxy radicals are relatively stable in the troposphere. They are unreactive toward the major components of the atmosphere +
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0022-365419212096-lO862$03.00/0 0 1992 American Chemical Society
