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chemiluminescence study of the reaction O+Si2H6 at room temperature
Volume 205, number 6
CHEMICALPHYSICSLETTERS
23 April 1993
A discharge-flow/chemiluminescence study of the reaction 0 + Si2H6at room temperature Craig A. Taylor and Paul Marshall Departmentof Chemistryj UniversityofNorth Texas, P.O.Box 5068, Denton,TX 76203.5068,USA Received 30 September 1992;in final form 8 December 1992
Discharge-Bow kinetic studies using NO1 chemiluminescence detection of 0 yielded a rate constant for OtSizHs of (6.Ok 1.0) x lo-r2 cm3 s-’ at 295 K, where the 95% confidence interval includes both precision and accuracy. Insertion and abstraction mechanisms are discussed, and on the assumption that H-atom abstraction dominates, a likely activation energy is about 8 kJ mol-‘. This is an upper limit if insertion intotheSi-Sibondissignificant.
1, Introduction
by an electropositive SiHs group, rather than electronegative CHs groups.
Disilane has been employed for chemical vapor deposition (CVD) of silicon oxide [I], and the reaction 0 + S&H6-products
(1)
is a likely initial step in systems where atomic oxygen is present, e.g. plasma-enhanced CVD or photochemical vapor deposition from Si2H6/N20s mixtures [ 21. Modeling of CVD mechanisms requires kinetic information about the individual elementary reactions, which has motivated this first characterization of the rate constantk, for reaction ( 1). The results are also compared with recent measurements of the kinetics of the reactions of atomic oxygen with SiH., [ 3-61 and methyl-substituted silanes [5,7], which indicate that increasing methylation increases the room temperature rate constant. This is perhaps surprising given that methyl substitution is thought either to leave the Si-H bond dissociation enthalpy D 298 essentially unchanged [ 81 or to raise Dzga slightly [ 9 1. Horie et al. [ 5 ] have rationalized this reactivity trend by relating the room temperature rate constants to the ionization potential of the silane, which reflects the ease of electron transfer to the attacking oxygen atom. The present results permit a test of this correlation where H in SiH* is substituted
2. Experimental technique We employed the discharge-flowtechnique as described in our previous study [ 61 of OtSiH,+OHtSiH,.
(2)
Briefly, 0( 2p) ‘P, atoms were generated by a microwave discharge through a dilution of NrO in Ar, or through impurities in a large flow of 99.998%Ar, and were detected by O/NO chemiluminescence at a wavelengthof about 5 10 nm. Absolute [0 ] were determined via the 0 t NOz titration reaction and pseudo-first-order condition [ 0] Q: [S&H,] were maintained. A new, narrower Pyrex flow tube was constructed with an inner diameter of 2.2 cm, to yield higher mean gas velocities v than in the previous apparatus [ 6 1, and thus to permit use of shorter reaction times t = i/v, where I is the distance from the O-atom injector to the observation window. In each experiment the distance I and [NO] were held constant while [SizHb] was varied, and therefore any diffusional loss of 0 to the reaction tube walls kdipr was constant. Typically u=20 m s-l which leads to a negligiblepressure drop along the reaction zone of about 4 Pa according to the Poiseuille equation. As
COO9-2614/93/S06.00 8 1993 Elsevier Science Publishers B.V. AU rights reserved.
493
CHEMICALPHYSICSLETTERS
Volume 205, number 6 2.40
1.60 F
1.20
=o.uo r 0.40 0.00 -.dO 20
40
[Si,H,l
60
/
80
’100
1012 cm-’
Fig. 1.Plotof In [versus disilaneconcentrationto obtaink, from the slope.
23 April 1993
reaction (8 ) . k9 was assumed to equal the measured SiH3tN0 rate constant [ 131, although the larger number of degrees of freedom in SizHSNOas compared to SiH3N0 mean that this kg is an underestimate, and therefore that the effect of the secondary chemistry in table 2 is overestimated. The effects of other reactions not considered here will tend to oppose this error. The resulting k, values are typically up to about 100/6smaller than kr,,,bs(table 1) , showing that secondary chemistry is minor. The mean k, is 6.04~ lo-l2 cm3 s-* with a statistical error of aE0.22 x lo-” cm3 s-l. The 2aprecision limits together with a f lOohallowancefor any unrecognizedsources of systematic error *l yield 95% confidence limits of + 1.Ox lo- ‘* cm3 s-‘.
discussed earlier [ 61, the measured chemiluminescence intensity I is given by
4. Discussion
I=AWOl [Oh ewI-(%+wo [ArlWI
Reaction ( 1) is approximately 20 times faster than 0 t SiH4,reaction (2 ) , at 295 K. There is evidence that the dominant channel for (2) is H-atom abstraction [5,14-l 6 ] so this is a plausible channel for ( 1) as well. Recent measurements in our laboratory [7] yielded k,= 1.2x 10-lOexp( - 14.4 kJ mol-‘/ RT) cm3 s- *, thus a reasonable estimate of A for reaction ( 1) based on a similarly loose transition state is 1.5x lo-lo cm3 s-r. This value is supported by a preliminary transition state theory analysis based on ab initio HF/6-31G’ data for an abstraction pathway. For comparison, over the temperature range 400-600 K Arrhenius fits to the abstraction reactions 0+CH4 and O+CzHs [ 161, which also proceed via loose transition states [ 171, yield similar preexponential A factors of (1-2)X lo-lo cm3 s-’ [ 161, even though the C-H bond is less polar than the Si-H bond. Combination of the present measurement with the assumedA factor for reaction ( 1) yields an activation energy E. of about 8 kJ mol-‘, and provides a possible extrapolation of kl to elevated temperatures. The E, is 6 kJ mol-’ below that for reaction (2), which is reasonable in view of D29s(Si2HS-H)lying approximately 17 kJ mol-’ below Dzg8(SiH3-H) [ 81. Horie et al. [ 51 proposed
+kl,obs[Si2H6l+kdiff)tj
>
(3)
where A is an arbitrary constant. An example of a plot of In I versus [ Si2H6] is shown in fig. 1 and has a slope of -kl,obst, where kl,obsis the observed or effective value of k,.
3. Results Thirty-one kl,obsmeasurements at 295t3 K are listed in table 1. 1 was varied by a factor of 2, the pressure P by a factor of 4 and [0] o by a factor of 5. These kl,& values were corrected for possible secondary reactions by means of computer simulations with the ACUCHEMprogram [ lo], according to the scheme given in table 2, and for axial diffusion [ 111, to yield k,. Reaction (4) was found to be an unimportant route for O-atom loss because [OH] is quickly rcmoved, mainly by Si2H6. SizHs could potentially consume 0 in the fast reaction (8 ), for whose rate constant we have assumed a nearly gas-kinetic value by analogy with 0tCH3 [ 121. At the same time SizHs will recombine with the NO (added to detect the 0 atoms) via reaction (9 ) . This removal of SizHs eliminatesmuch of the potential O-atom loss through
494
WIComparison of results for reaction (2) obtained by the present technique with those from other methods suggestssuch errors
are probablysmall [6].
Volume 205, number 6
CHEMICAL PHYSICS LETTERS
23 April 1993
Table 1 Summary of 0+ Si2H6 measurements at 295 5 3 K I (cm)
v (ems-I)
P &Pa)
NOI (IO’* cmm3)
WI (1014cm-J)
[Si&lmu (IO”cm-“)
[CID (1O1'cm-))
k,,.+,, ( 10~‘*cm3s-I)
k, (10-‘2cm3s-‘)
7.5 7.5 7.5 7.5 7.5 7.5 7.5 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
2390 2210 2130 2050 2oso 2020 1870 2460 2160 2240 2160 1950 1890 2120 1970 1900 1980 2oso 1950 1860 2160 2010 2000 2020 1950 2040 1970 1920 1890 1920 1880
0.25 0.43 0.52 0.67 0.76 0.83 0.93 0.28 0.29 0.35 0.37 0.41 0.56 0.61 0.67 0.71 0.75 0.85 0.94 0.98 0.29 0.32 0.40 0.41 0.47 0.67 0.67 0.71 0.88 0.93 0.94
0.9 1.3 1.7 1.9 2.1 2.2 2.6 0 0.8 0.6 0 1.2 1.8 0 1.0 2.6 0 0 0 3.7 0.8 0.6 1.0 0.8 0 1.9 1.2 0 2.1 3.1 0
3.7 3.8 3.8 3.9 3.6 3.5 3.6 4.3 4.8 3.9 4.0 4.4 4.2 5.3 4.5 5.5 5.0 5.0 4.9 5.3 4.8 3.9 4.8 3.6 4.4 4.4 3.6 4.3 3.5 4.0 4.2
7.9 7.9 9.1 9.1 7.2 6.9 8.3 5.2 8.0 4.3 3.7 7.6 8.5 6.1 5.2 9.4 8.9 6.7 6.6 9.7 4.9 9.6 5.1 4.4 3.6 4.9 5.1 5.0 5.7 4.3 3.6
7.6 6.7 7.9 4.9 4.5 3.7 3.9 2.2 4.3 4.0 1.5 4.9 6.8 1.6 2.1 4.1 3.5 1.5 2.2 4.7 2.6 1.7 3.6 3.5 2.2 6.4. 3.4 2.3 3.9 6.0 2.6
9.14 8.01 7.11 6.50 7.08 6.65 5.74 8.74 6.45 8.63 7.53 5.41 4.86 5.51 6.80 4.13 4.20 4.92 5.16 4.29 6.05 4.05 6.10 7.30 6.18 7.47 6.86 6.19 5.93 7.44 7.57
8.10 7.26 6.44 6.10 6.64 6.35 5.48 8.29 6.02 7.88 7.26 5.04 4.52 5.41 6.62 4.02 4.09 4.88 5.07 4.18 5.73 3.96 5.72 6.72 5.90 6.81 6.55 6.02 5.66 6.97 7.43
10.0 10.0 10.0 10.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 IS.0 15.0
Table 2 Reaction scheme for secondary chemistry in 0+Si2H6 Reaction 0 t S&H+OH tSi2Hs 0+OH-+02+H 0H+SixHs-+H20+SilHI OH+NOtAr+HONOtAr OH+OH+OtH1O 0tSi2H,-+proclucts Si2HrtNOtAr-rSixHsNOtAr
(1) (4) (5) (6) (7) (8) (9)
k(298K) (cm’s_I)
Ref.
6.7x10-‘* 3.3x lo-” 1.4x10-” 1.2x10-1°C) 1.8x10-‘* 1.4x10-‘0 1.3x10-‘2c)
this work Or 1121 [31 b, [121 1121 see text [13]b’
r1 Initial experimental estimate. bJ Value for analogous SiH, reaction employed. ‘) Third-order rate constant employed: effective second-order rate constant at 0.67 kPa quoted.
495
Volume 205, number 6
CHEMICALPHYSICSLETTERS
that the room temperature rate constants for H-atom abstraction by 0 from substituted silanes are correlated to the ionization potential (IP); because the IPs for SizH6and ( CHa) sSiH are similar [ 18] a similar rate constant per Si-H bond of 3.1x lo-l2 cm3 s-’ (51 mightbeexpected,i.e.k,~2X10-11cm3s-1. This is about a factor of 3 larger than measured here, which may indicate uncertainty in the correlation or that it cannot be extended beyond methyl substitution. However, other reaction channels apart from abstraction are possible, which makes the proposed E, an upper limit. For example, Hoffmeyer et al. have studied the reaction 0+Si2(CH3)6+products
(10)
at room temperature and obtained klD= 1.3x lo-l3 cm3 s-r [ 191, and the mechanism they deduced involves O-atom insertion into the Si-Si bond followed by fragmentation of excited siloxane. k,, is about 50 times smaller than k,. If both reactions proceed via the same mechanism and have similar A factors, this implies E,,, xE,,rO-9 k.I mol-‘. This difference in E, would be consistent with D,,,( H,SiSiH3)&29s((CH3)3Si-Si(CH3)3)-17 kJ mol-’ 191. It may be possibleto distinguish between insertion
and abstraction through consideration of the Arrhenius parameters, since formation of a singlet intermediate adduct [ 191 is nonadiabatic with respect to spin and therefore may have a reduced A factor. These parameters are presently unavailable and will be the subject of a future study. Experimental detection of the possible OH product of reaction (1) is also desirable.
5. Conclusions The room temperature rate constant for 0 + Si2Hs has been determined to be (6+1)x1O-12 cm3 s-l. On the assumption that H-atom abstraction dominates, a likely activation energy is about 8 kJ mol-‘. This is an upper limit if other channels, such as insertion into the Si-Si bond, are also important.
496
23 April 1993
Acknowledgement
This work was supported by Texas Instruments, Inc., the Robert A. Welch Foundation (Grant B1174) and the UNT Organized Research Fund.
References [ 1 ] C.J. Girmta, J.D. Chapple-Sokol and R.G. Gordon, J. Electrochem. Sot. 137 (1990) 3237. [2] Y.K. Bhatnagar and W.I. Milne, Thin Solid Films 168 (1989) 345. [ 31R. Atkinson and J.N. Pitts Jr., Intern. J. Chem. Kinetics 10 (1978) 1151. [4] T.G. Mkryan, E.N. Sarkisyan and S.A. Arutyunyan, Arm. Khim.Zh.34 (1981)3. [ 510. Horie, R. Taege, B. Reimann, N.L.Arthur and P. Potzinger, J. Phys. Chem. 95 (1991) 4393. [6] C.A. Taylor, L. Ding and P. Marshall, Intern. J. Chem. Kinetics, in press. [ 71L. Ding, C.A. Taylor and P. Ma&all, Experimental and theoreticalstudies of atomic H, 0 and halogenreactionswith silanes, abstract E30, in: 12 International Symposium on Gas Kinetics, University of Reading July 1992. [S] R. Walsh, in: The chemistry of organic silicon compounds, eds. S. Patai and Z. Rappoport (Wiley, New York, 1989) ch. 5. [9] L. Ding and P. Marshall, J. Am. Chem. Sot. 114 ( 1992) 5754. [lo] W. Braun, LT. Herron and D.K. Rahaner, Intern. J. Chem. Kinetics 20 (1988) 51. [ 111A. Fontijn and W. Felder, in: Reactive intermediates in tbe gas phase, ed. D.W. Setser (Academic Press, New York, 1979) ch. 2. [ 121R. Atkinson, D.L. Baulch,R.A. Cox, R.F. HampaonJr., J.A. KerrandJ.Troe.,J.Phys.Chem.Ref.Data18(1989)881. [ 131K. Sugawara,T. Nakanaga, H. Take0 and C. Matsumura, Chem. Phys. Letters 157 (1989) 309. [ 141B.S. Agrawallaand D.W. Setser, J. Chem. Phys. 86 (1987) 5421. [ 151C.R. Park, G.D. White and J.R. Wiesenfeld, J. Phys. Chem. 92 (1988) 152. [ 161J.T. Herron, I. Phys. Chem. Ref. Data 17 (1988) 967. [ 171N. Cohen and K.R. Westberg,Intern. J. Chem. Kinetics 18 (1986) 99. [ 181S.G. Lias, J.E. Bartmeas, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, Gas-phase ion and neutral thermochemistry; J. Phys. Chem. Ref. Data 17 (1988) (Suppl. No. 1) [ 191H. Hoffmeyer,P. Potzinger and B. Reimann, J. Phys. them. 89 (1985) 4829.
CHEMICALPHYSICSLETTERS
23 April 1993
A discharge-flow/chemiluminescence study of the reaction 0 + Si2H6at room temperature Craig A. Taylor and Paul Marshall Departmentof Chemistryj UniversityofNorth Texas, P.O.Box 5068, Denton,TX 76203.5068,USA Received 30 September 1992;in final form 8 December 1992
Discharge-Bow kinetic studies using NO1 chemiluminescence detection of 0 yielded a rate constant for OtSizHs of (6.Ok 1.0) x lo-r2 cm3 s-’ at 295 K, where the 95% confidence interval includes both precision and accuracy. Insertion and abstraction mechanisms are discussed, and on the assumption that H-atom abstraction dominates, a likely activation energy is about 8 kJ mol-‘. This is an upper limit if insertion intotheSi-Sibondissignificant.
1, Introduction
by an electropositive SiHs group, rather than electronegative CHs groups.
Disilane has been employed for chemical vapor deposition (CVD) of silicon oxide [I], and the reaction 0 + S&H6-products
(1)
is a likely initial step in systems where atomic oxygen is present, e.g. plasma-enhanced CVD or photochemical vapor deposition from Si2H6/N20s mixtures [ 21. Modeling of CVD mechanisms requires kinetic information about the individual elementary reactions, which has motivated this first characterization of the rate constantk, for reaction ( 1). The results are also compared with recent measurements of the kinetics of the reactions of atomic oxygen with SiH., [ 3-61 and methyl-substituted silanes [5,7], which indicate that increasing methylation increases the room temperature rate constant. This is perhaps surprising given that methyl substitution is thought either to leave the Si-H bond dissociation enthalpy D 298 essentially unchanged [ 81 or to raise Dzga slightly [ 9 1. Horie et al. [ 5 ] have rationalized this reactivity trend by relating the room temperature rate constants to the ionization potential of the silane, which reflects the ease of electron transfer to the attacking oxygen atom. The present results permit a test of this correlation where H in SiH* is substituted
2. Experimental technique We employed the discharge-flowtechnique as described in our previous study [ 61 of OtSiH,+OHtSiH,.
(2)
Briefly, 0( 2p) ‘P, atoms were generated by a microwave discharge through a dilution of NrO in Ar, or through impurities in a large flow of 99.998%Ar, and were detected by O/NO chemiluminescence at a wavelengthof about 5 10 nm. Absolute [0 ] were determined via the 0 t NOz titration reaction and pseudo-first-order condition [ 0] Q: [S&H,] were maintained. A new, narrower Pyrex flow tube was constructed with an inner diameter of 2.2 cm, to yield higher mean gas velocities v than in the previous apparatus [ 6 1, and thus to permit use of shorter reaction times t = i/v, where I is the distance from the O-atom injector to the observation window. In each experiment the distance I and [NO] were held constant while [SizHb] was varied, and therefore any diffusional loss of 0 to the reaction tube walls kdipr was constant. Typically u=20 m s-l which leads to a negligiblepressure drop along the reaction zone of about 4 Pa according to the Poiseuille equation. As
COO9-2614/93/S06.00 8 1993 Elsevier Science Publishers B.V. AU rights reserved.
493
CHEMICALPHYSICSLETTERS
Volume 205, number 6 2.40
1.60 F
1.20
=o.uo r 0.40 0.00 -.dO 20
40
[Si,H,l
60
/
80
’100
1012 cm-’
Fig. 1.Plotof In [versus disilaneconcentrationto obtaink, from the slope.
23 April 1993
reaction (8 ) . k9 was assumed to equal the measured SiH3tN0 rate constant [ 131, although the larger number of degrees of freedom in SizHSNOas compared to SiH3N0 mean that this kg is an underestimate, and therefore that the effect of the secondary chemistry in table 2 is overestimated. The effects of other reactions not considered here will tend to oppose this error. The resulting k, values are typically up to about 100/6smaller than kr,,,bs(table 1) , showing that secondary chemistry is minor. The mean k, is 6.04~ lo-l2 cm3 s-* with a statistical error of aE0.22 x lo-” cm3 s-l. The 2aprecision limits together with a f lOohallowancefor any unrecognizedsources of systematic error *l yield 95% confidence limits of + 1.Ox lo- ‘* cm3 s-‘.
discussed earlier [ 61, the measured chemiluminescence intensity I is given by
4. Discussion
I=AWOl [Oh ewI-(%+wo [ArlWI
Reaction ( 1) is approximately 20 times faster than 0 t SiH4,reaction (2 ) , at 295 K. There is evidence that the dominant channel for (2) is H-atom abstraction [5,14-l 6 ] so this is a plausible channel for ( 1) as well. Recent measurements in our laboratory [7] yielded k,= 1.2x 10-lOexp( - 14.4 kJ mol-‘/ RT) cm3 s- *, thus a reasonable estimate of A for reaction ( 1) based on a similarly loose transition state is 1.5x lo-lo cm3 s-r. This value is supported by a preliminary transition state theory analysis based on ab initio HF/6-31G’ data for an abstraction pathway. For comparison, over the temperature range 400-600 K Arrhenius fits to the abstraction reactions 0+CH4 and O+CzHs [ 161, which also proceed via loose transition states [ 171, yield similar preexponential A factors of (1-2)X lo-lo cm3 s-’ [ 161, even though the C-H bond is less polar than the Si-H bond. Combination of the present measurement with the assumedA factor for reaction ( 1) yields an activation energy E. of about 8 kJ mol-‘, and provides a possible extrapolation of kl to elevated temperatures. The E, is 6 kJ mol-’ below that for reaction (2), which is reasonable in view of D29s(Si2HS-H)lying approximately 17 kJ mol-’ below Dzg8(SiH3-H) [ 81. Horie et al. [ 51 proposed
+kl,obs[Si2H6l+kdiff)tj
>
(3)
where A is an arbitrary constant. An example of a plot of In I versus [ Si2H6] is shown in fig. 1 and has a slope of -kl,obst, where kl,obsis the observed or effective value of k,.
3. Results Thirty-one kl,obsmeasurements at 295t3 K are listed in table 1. 1 was varied by a factor of 2, the pressure P by a factor of 4 and [0] o by a factor of 5. These kl,& values were corrected for possible secondary reactions by means of computer simulations with the ACUCHEMprogram [ lo], according to the scheme given in table 2, and for axial diffusion [ 111, to yield k,. Reaction (4) was found to be an unimportant route for O-atom loss because [OH] is quickly rcmoved, mainly by Si2H6. SizHs could potentially consume 0 in the fast reaction (8 ), for whose rate constant we have assumed a nearly gas-kinetic value by analogy with 0tCH3 [ 121. At the same time SizHs will recombine with the NO (added to detect the 0 atoms) via reaction (9 ) . This removal of SizHs eliminatesmuch of the potential O-atom loss through
494
WIComparison of results for reaction (2) obtained by the present technique with those from other methods suggestssuch errors
are probablysmall [6].
Volume 205, number 6
CHEMICAL PHYSICS LETTERS
23 April 1993
Table 1 Summary of 0+ Si2H6 measurements at 295 5 3 K I (cm)
v (ems-I)
P &Pa)
NOI (IO’* cmm3)
WI (1014cm-J)
[Si&lmu (IO”cm-“)
[CID (1O1'cm-))
k,,.+,, ( 10~‘*cm3s-I)
k, (10-‘2cm3s-‘)
7.5 7.5 7.5 7.5 7.5 7.5 7.5 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
2390 2210 2130 2050 2oso 2020 1870 2460 2160 2240 2160 1950 1890 2120 1970 1900 1980 2oso 1950 1860 2160 2010 2000 2020 1950 2040 1970 1920 1890 1920 1880
0.25 0.43 0.52 0.67 0.76 0.83 0.93 0.28 0.29 0.35 0.37 0.41 0.56 0.61 0.67 0.71 0.75 0.85 0.94 0.98 0.29 0.32 0.40 0.41 0.47 0.67 0.67 0.71 0.88 0.93 0.94
0.9 1.3 1.7 1.9 2.1 2.2 2.6 0 0.8 0.6 0 1.2 1.8 0 1.0 2.6 0 0 0 3.7 0.8 0.6 1.0 0.8 0 1.9 1.2 0 2.1 3.1 0
3.7 3.8 3.8 3.9 3.6 3.5 3.6 4.3 4.8 3.9 4.0 4.4 4.2 5.3 4.5 5.5 5.0 5.0 4.9 5.3 4.8 3.9 4.8 3.6 4.4 4.4 3.6 4.3 3.5 4.0 4.2
7.9 7.9 9.1 9.1 7.2 6.9 8.3 5.2 8.0 4.3 3.7 7.6 8.5 6.1 5.2 9.4 8.9 6.7 6.6 9.7 4.9 9.6 5.1 4.4 3.6 4.9 5.1 5.0 5.7 4.3 3.6
7.6 6.7 7.9 4.9 4.5 3.7 3.9 2.2 4.3 4.0 1.5 4.9 6.8 1.6 2.1 4.1 3.5 1.5 2.2 4.7 2.6 1.7 3.6 3.5 2.2 6.4. 3.4 2.3 3.9 6.0 2.6
9.14 8.01 7.11 6.50 7.08 6.65 5.74 8.74 6.45 8.63 7.53 5.41 4.86 5.51 6.80 4.13 4.20 4.92 5.16 4.29 6.05 4.05 6.10 7.30 6.18 7.47 6.86 6.19 5.93 7.44 7.57
8.10 7.26 6.44 6.10 6.64 6.35 5.48 8.29 6.02 7.88 7.26 5.04 4.52 5.41 6.62 4.02 4.09 4.88 5.07 4.18 5.73 3.96 5.72 6.72 5.90 6.81 6.55 6.02 5.66 6.97 7.43
10.0 10.0 10.0 10.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 IS.0 15.0
Table 2 Reaction scheme for secondary chemistry in 0+Si2H6 Reaction 0 t S&H+OH tSi2Hs 0+OH-+02+H 0H+SixHs-+H20+SilHI OH+NOtAr+HONOtAr OH+OH+OtH1O 0tSi2H,-+proclucts Si2HrtNOtAr-rSixHsNOtAr
(1) (4) (5) (6) (7) (8) (9)
k(298K) (cm’s_I)
Ref.
6.7x10-‘* 3.3x lo-” 1.4x10-” 1.2x10-1°C) 1.8x10-‘* 1.4x10-‘0 1.3x10-‘2c)
this work Or 1121 [31 b, [121 1121 see text [13]b’
r1 Initial experimental estimate. bJ Value for analogous SiH, reaction employed. ‘) Third-order rate constant employed: effective second-order rate constant at 0.67 kPa quoted.
495
Volume 205, number 6
CHEMICALPHYSICSLETTERS
that the room temperature rate constants for H-atom abstraction by 0 from substituted silanes are correlated to the ionization potential (IP); because the IPs for SizH6and ( CHa) sSiH are similar [ 18] a similar rate constant per Si-H bond of 3.1x lo-l2 cm3 s-’ (51 mightbeexpected,i.e.k,~2X10-11cm3s-1. This is about a factor of 3 larger than measured here, which may indicate uncertainty in the correlation or that it cannot be extended beyond methyl substitution. However, other reaction channels apart from abstraction are possible, which makes the proposed E, an upper limit. For example, Hoffmeyer et al. have studied the reaction 0+Si2(CH3)6+products
(10)
at room temperature and obtained klD= 1.3x lo-l3 cm3 s-r [ 191, and the mechanism they deduced involves O-atom insertion into the Si-Si bond followed by fragmentation of excited siloxane. k,, is about 50 times smaller than k,. If both reactions proceed via the same mechanism and have similar A factors, this implies E,,, xE,,rO-9 k.I mol-‘. This difference in E, would be consistent with D,,,( H,SiSiH3)&29s((CH3)3Si-Si(CH3)3)-17 kJ mol-’ 191. It may be possibleto distinguish between insertion
and abstraction through consideration of the Arrhenius parameters, since formation of a singlet intermediate adduct [ 191 is nonadiabatic with respect to spin and therefore may have a reduced A factor. These parameters are presently unavailable and will be the subject of a future study. Experimental detection of the possible OH product of reaction (1) is also desirable.
5. Conclusions The room temperature rate constant for 0 + Si2Hs has been determined to be (6+1)x1O-12 cm3 s-l. On the assumption that H-atom abstraction dominates, a likely activation energy is about 8 kJ mol-‘. This is an upper limit if other channels, such as insertion into the Si-Si bond, are also important.
496
23 April 1993
Acknowledgement
This work was supported by Texas Instruments, Inc., the Robert A. Welch Foundation (Grant B1174) and the UNT Organized Research Fund.
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