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Rate constant and mechanism of the SiH3+SiH3 reaction
4 October 199 I
CHEMICAL PHYSICS LETTERS
Volume 184, number 5,6
Rate constant and mechanism of the SiH3t SiH3 reaction Mitsuo Koshi a, Akira Miyoshi b and Hiroyuki Matsui a a Department ofReaction Chemistry, Umversity ofTokyo. 7-3-l Hongo, Bunkyo-ku, b Division
ojdtmosphene
Environment,
The National
Instilutefor
Tokyo 113, Japan
Enwronmanta(Studies.
Tsukuba-gakuen,
Ibarakl30.5,
Japan
Received 2 May 1991; in final form 29 June 1991
The kinetics of the SiH?-t SiH3 reaction was studied with time-resolved mass spectrometly. Silyl radicals, as well as S&H6 and HCI. produced by ArF laser photolysis of C&/SiH,/He mixtures were detected by using a near-threshold electron-impact ionization technique.The rate constant of ( 1.2iO.4) x 10-‘Ocm’ molecule-’ s-’ for the SiHSt SiH, reaction at T=293 K was derived both from the decay rate of SiH> and the production rate of SiIH6. Steady-state analysis indicated that the fraction of SiH2 formatlon in the SiH3+SiHS reaction was equal to 0.62 + 0.14 at p= 5 Torr.
1. Introduction SiH3 t SiHI-Si2H:, The silyl radical is expected to play a key role in silane plasma chemical-vapor-deposition (CVD) processes [ 1,2]. Since the kinetic lifetime of the silyl radical in typical CVD environments is dominantly controlled by the SiH3 + SiH, recombination reaction, detailed knowledge of this reaction is necessary for modelling the CVD processes. Despite its importance, the rate constant of the SiH3 t SiH? reaction has been measured only by two groups, and the reported values differ from each other by a factor of more than 2. Itabashi et al. [ 31 measured the absolute concentrations of SiH3 produced in a pulsed SiHJ/HZ discharge by using infrared diode laser spectroscopy. Loh et al. [4] also used an infrared diode laser to detect SiH3 produced in ArF laser photolysis ofSiH,Br/He and SiH,/CCl,/He mixtures at 193 nm. Since the SiHj recombination reaction is a biomolecular reaction which cannot be studied under pseudo-first-order conditions, the accuracy of the rate constants obtained in these works depends largely on the accuracy of the method for the determination of the SiH3 absolute concentrations. The recombination of SiH1 initially forms vibrationally excited disilane, S&HE, which is either stabilized or decomposed according to the following reactions [ 5 ] : 442
0009-2614/91/$
(1)
Si2HE+M+SizH6 tM .
@a)
S&H;: +SiHz t SiH4 ,
(2b)
SizHt+SizH1 t Hz .
(2c)
In addition, the direct disproportionation SiH3 + SiH, t SiH, t SiH, ,
reaction, (3)
is also possible. Silylene radical, formed by reactions (2b) and/or (3), can easily insert into SiH4 yielding S&H,: SiH*+SiH,(
tM)+Si,H,(
tM)
.
(4)
The formation of S&H6 often observed in studies of the photolysis of SiH, was explained by this reaction [6-IO]. Since the rate of reaction (4 ) is very fast [ 11,12], the formation of Si2H, is controlled by the formation rate of SiH2. Although a value of k,,/ klb=O. 18 & 0.05 was suggested by Becerra and Walsh [ 51 on the basis of simulations with a complex reaction mechanism, this value is largely affected by the values of k, and k, used in the simulation. In the present study, the SiH, radical generated by ArF laser photolysis of CCl,/SiH,/He mixtures was detected by using time-resolved mass spectrometry and the rate constant was determined. Attempts were 03.50 8 1991 Elsevier Science Publishers B.V. 411 rights reserved.
Volume184,number$6 also
4 October1991
CHEMICALPHYSICSLETTERS
made to detect possible reaction products.
2. Experimental The reaction was studied in SiH,/CCl,/He mixtures which were irradiated by the unfocused output of an ArF laser (Questek 2220) at 193 nm. Gas mixtures were flowed in a Pyrex cell 76 cm in length and with an inner diameter of 1.6 cm. The partial pressures of the reactants and the total pressure were measured with a capacitance manometer (MKS Baratron 122A). Experiments were carried out at room temperature 293 f 2 K) with a typical total pressure of 5 Tort-. The partial pressures of SiH, and CC& ranged from 10 to 22 mTorr and from 7 to 3 7 mTorr, respectively. The contents of the cell were sampled continuously by a quadrupole mass spectrometer (Anelva TE-600s) through a 100 pm pinhole located at the center of the cell. Gases from the pinhole were directly introduced into an electron-impact ionization chamber. The ion signals from a secondary electron multiplier, operated under pulse-counting conditions, were recorded with a gated counter following pulse amplification and discrimination. The time dependence of each individual mass peak was obtained by scanning the delay time of the gate with a fixed gate width of 50 or 100 ps. Signals were averaged over 5000 to 15000 laser shots for each wave form. Mass spectra reflecting laser-induced changes were also obtained by scanning the quadrupole mass filter with an internal trigger signal. The silyl radicals were generated by ArF laser photolysis of Ccl, in SiH, [ 4,131 and were detected by using a near-threshold ionization technique [ 14,15 1, in which the SiH: ion (m/z= 3 I), produced by the ionization of SiH3, was separated from SiH: formed by the dissociative ionization of SiH, by using an electron energy below the threshold of the dissociative process of SiH, and above the threshold for SiH3 ionization. Since the threshold energies for SiH: formation from SiH, and from SiH? are 12.09 and 8.01 eV [ 161, respectively, an ionizer electron energy of 10 eV was used for the detection of SiH: from SiH3. An ionizer electron energy of 14 eV was used for the detection of HCI+ at m/z= 36, and 16 eV for both Si2H6+at mJz=62 and Si2H4+at mjz=60.
In the present experiments. the Cl atoms produced by the laser photolysis are efficiently converted to SiH3 and HCl via the reaction
SiH,+Cl-+SiH,
tHC1.
(5)
Since this reaction is essentially gas kinetic (k=4.4>( 10-‘0cm3molecule-‘s-‘) [ 171 andSiH, is in large excess, the generation rate of SiH3 is much faster than the rates of other SiH3 loss processes. Thus, the amount of SiH3 formed initially is equal to that of HCI produced by reaction (5 ). Typical examples of the time profiles of SiH3 at m/i= 3 1 and HCl at m/z=36 are shown in figs. la and lb. The signal intensity at m/z=36 was calibrated against the concentration of HCl in separate experiments, and then the initial concentrations of SiH, were obtained from the ratio of signal intensities at m/z= 3 1
I
-10
I
I
0
10
Time
20
/ ms
Fig. I. Timedependenceof (a) m/z=31 [SiH: 1, (b) m/z=36 [HCl+], (c) m/z=60 [S&H:], (d) m/z=62 [S&H$], and (e) net signal from S&H,: leO- 1.81,, lsee text), in ArF ( 193 nm) laser photolysis of SiHJCCI,/He mixture; p(SiH,,) = 17 mTorr and p( Ccl,) = 14 mTorr at a total pressure of 5 Torr. The solid curve in (a) is calculated from eq. (7) withp=2.49 and !Q= I60 S-I and that in (d) is calculated from eq. (IO) with b= 2.42 and k6= I~OS-~.
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and 36 measured under identical conditions. The yield of HCl (and, hence, SiH3) from Ccl, in the I93 nm photolysis of SiH,/CCl, mixtures was found to be 0.37% with a typical laser fluence of 12 mJ/ cm’. The accuracy of this calibration procedure is largely limited by the instability of the detection system. The possible maximum error in the determination of SiH3 concentrations was estimated to be & 27%.
3. Results and discussion 3. I. Detection of thereaction products According to the reaction mechanism (l)-(4), SizHb and S&H4 are expected as reaction products and attempts have been made to detect these species. Fig. 2a shows a mass spectrum observed during the photolysis of a SiH4/CC14 mixture with an ionization energy of 16 eV. Since the ionization energy was considerably higher than the threshold energy ( IO.15 eV for Si?H, and 8.9 eV for Si;H4 [ 14]), fragment peaks of Si2H, in the range of m/z=58-62 were observed. The mass-fragment pattern of S&H, is shown in fig. 2b for comparison. It is noted that the ratios
of the mass peak intensities at m/z=58-61 to the intensity at m/z=6.2, R,=I,,/lb,, (n=58-61), for the photolysis products (fig. 2a) are larger than those for Si,H, (fig. 2b). The mass-pattern coefficient, R,,. for Si2H6 at an electron energy of 16 eV was determined to be 1.80? 0.11, whereas the value of RbOwas found to be 2.20 &0.19 for the products of the laser photolysis. This implies that there is some “extra” signal at m/z= 60 for the photolysis product. Silylsilylene is a possible source for this “extra” signal. This is further indicated by the fact that the time profiles of the signals at m/z=60 and 62 are slightly different, as shown in figs. lc and Id. The signal at m/z=60 shows a slow decay after a rapid rise, whereas the signal at m/z= 62 remains almost constant. In fig. le, the difference signal obtained from I,,, by subtracting the contribution of the fragment signal from Si,H,, i.e. Z6,,- 1.816?,is also shown. Although no conclusive assignment of this “extra” signal at m/z=60 can be made, the slow decay of the time profile is possibly caused by the consecutive reactions such as the insertion reaction of HSiSiHX into SiH4. The mass peaks in the range of m/z=63-66 can be assigned to the fragment ions from SiH,Cl, since they disappeared when SiH, was produced by the photolysis of N,O/SiH, mixtures. The rise rate of these signals was found to be much faster than the decay rate of SiHj. The mechanism of SiH,Cl formation is not clear. 3.2. Mechanism and the rate constant ,for the SiHj + SiH, reaction
(a) SiH4/CCl4/He + 193 nm
Since SiH3 is unreactive towards SiH, and CCI, [4], the decay rate of SiHJ is controlled by the recombination reaction ( 1) and the disproportionation reaction (3). The dissociation of Si,H; back to reactants,
1
Si2Hz+SiHI +SiH3 , 1
45
50
,/
l,,,,,,,,t,
55 Mass
L,,,,,
60
65
’
Number
Fig. 2. (a) Product mass spectrum in ArFlaser photolysis of SiH,/ CCI,/He mixture; p(SiH4) = 17 mTorr and p( CCL,) = 16 mTorr at a total pressure of 5 Torr. (b) Mass spectrum of S&H,; p( S&H,) =0.03 mTorr at a total pressure of 5 Torr. Both spectra were taken with 16 eV electron-impact ionization energy. Each spectrum is normalized to the mass peak intensity at m/z= 60.
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4 October 1991
(2d)
is relatively improbable [ 41, since other decomposition channels (2b) and (2~) have large exothermicity, and the excess energy of the Si2Hg formed by reaction ( 1 ) is much higher than the energy barriers of these decomposition channels [ 18,191. The decay rate of SiH7 is independent of the total pressure if reaction (2d) is negligible. In our preliminary experiments, it was found that the decay rates of SiH,
Volume184,number 5,6
CHEMICALPHYSICSLETTERS
4 October1991
at a total pressure of 2 Torr were essentially the same as those at 5 Torr. Moreover, the mass spectrum in the range of nz/z=45-IO showed no significant difference in the mass pattern and the intensities at p= 3, 4 and 5 Torr. These observations indicate that vibrationally excited disilane decomposes irreversibly to SiH2+ SiH, or SizH4+Hz in the present experimental conditions. The pressure-independent decay rates of SiH3 were also found by others [ 3,4,20]. The deactivation of S&H% (reaction (2a) ) can compete with these decomposition reactions at high pressures [ lo], but recent RRKM calculations for the decomposition of vibrationally excited disilane [ 51 indicated that the collisional stabilization was much slower than the decomposition channels. SiH, radicals were also eliminated at the wall of the cell, and this process is governed by the diffusion of SiH3: SiHl -+wall .
(6)
Analytical solution for the time profile of SiH3 is obtained with reactions (I), (3), and (6): (7) P= [2(k, +Mlkl
W310.
(8)
Because of the large correlation between k6 and /I in eq. (7), the determination of these parameters by a direct non-linear least-squares fit to the experimental data introduced a large uncertainty [ 31. Instead, we measured the time constant z ( 1/e time) of each set of data as a function of the initial SiHX concentration, as shown in fig. 3. An expression for the time constant is derived from eq. (7 ):
action (4). This reaction is known to be pressure dependent, and its rate constant is 1.1 x lo-” cm3 molecule- ’ s- ’ at a= 5 Torr [ 111. Since the condition of [SiH,] >> [SiH3] is always valid in the present experiments, the rate of reaction (4) is much faster that the production rate of SiH2 (sum of reactions ( 1) and (3)). With the steady-state assumption for SiHz and Si,Hg, the following equations for the time profile of Si2H6 are derived: [Si2H6]=f[SiH3105
(1+P)[1-w(-Wl ltjI[l-exp(-k6t)]
ln{lt~[l-exp(-k,t)]}
1
k6 ~=ln[(etfl)/(ltfl)]’
Fig. 3. Time constants for SiHj decay (open circles) and S&H6 rise (closed circles) as a function of the initial concentration of SiH,; p( SiH,) = I7 mTorr at a total pressure of 5 Torr. Solid curves are calculated from eq. (8) for the SIH, decay rate and from eqs. ( 10) and ( 1I ) for the Si2H6rise rate.
(9)
Extrapolation of 1/z values in fig. 3 to [ SiH3 ] D= 0
P
>’
[Si,H,l,=1[SiH,l,~~[1-(1/P)ln(1-tP)l,
(10)
(11)
gives kg= 160 s-’ and the parameter p for each set
of data can be determined by using eq. (9). The time profile of SiHJ, calculated by eq. (7) with the values of k, and j3 thus determined, is compared with the experimental time profile in fig. la. The parameter j3 can also be obtained from the production rate of Si2H6. Since reaction (Za) is not important in the present experimental conditions as noted above, S&H6 is exclusively produced by re-
(12) Here, k2=k2,,tkzo k’=k,-tk, and [SizHslm is the concentration of S&H, at t=co. The parameter r represents the yield of SiH2 in the SiH3 t SiH3 reaction. It is noted that the normalized time profile of Si,H6, [ Si,H,] / [ Si2H, ] 8, contains only two parameters, k, and p. With a value of kc= 160 SK’, it 445
Volume 184, number 5,6
4 October 199 1
CHEMICAL PHYSICS LETTERS
is easy to determine the values of p (and, hence, k’) from the time constant ( ( 1 - 1/e) time in this case) of the normalized time profile of Si2H6. The experimental time constants for the rise rate of Si,H, are shown in fig. 3, and the time profile of Si2H6 calculated with eqs. ( 10) and ( 11) is compared with the experimental profile in fig. Id. The excellent agreement demonstrated in figs. la and Id indicates the validity of the reaction mechanism. In addition, the same decay rates of SiH3 were observed in ArF photolysis of N,O/SiH, mixtures in which SiH3 was produced by the following very fast reactions: O(‘D)+SiH,+SiH,+OH,
(13)
OH+SiH+SiH,+H,O.
(14)
The values of p obtained both from the decay rates of SiH, and the rise rates of S&H6 are summarized in fig. 4, in which 1 is plotted against [SiH310. From the slope of this plot, which includes the calibration error in the determination of [SiH310, k’ is determined to be ( 1.16?0.39)~10-lo cm3 molecule-’ s-l. The time constants for the SiH3 decay and the S&H6 rise calculated with this value are compared with the experimental results in fig. 3. The present value of k’ is to be compared with the value obtained by Itabashi et al. (k’ = ( 1.5+ 0.6) x lo-” cm? molecule-’ s-‘) [3] and by Loh et al.
(k’ < (0.61+0.35)x IO-“cm3 molecule-‘s-l) [4]. Although these three values overlap within the combined uncertainties, the rate constant reported by Loh et al. is about one half of the present result. In their procedure for measurement of the absorption cross section, Loh et al. assumed that all the Cl atoms produced by the 193 nm photolysis of Ccl,, were converted to SiH3. On the other hand, the quantum yield for the SiH, production in the photolysis of SiH,/ Ccl, mixtures was estimated to be less than 0.95 on the basis of the present measurements of the absolute concentration of HCl. Since the quantum yield of the Cl atom from Ccl, in the 193 nm photolysis was estimated to be 1.2 [ 41, some of the Cl atoms generated by the photolysis are converted to species other than I-ICI. If this is the case, the concentration of SiH3 was overestimated in the experiments of Lob et al. 3.3. Fraction of SiH2 production in the SiHjSSiHj reaction
Since the absolute concentrations of S&H, formed in the SiH, t SiH, reaction have been obtained by calibrating the signal intensity at m/z= 62 and since the values of k, and/I are already known, the parameter 5 in eq. (11) can be determined. It was found that the yields of Si2H, decreased with increasing concentration of Ccl, at a fixed total pressure. This might be caused by the existence of an additional loss process of SiH2. S&H6is formed by reaction (4)) and, therefore, the loss of SiHz reduces the S&H6 yield. Since the rate of reaction (4) is almost gas kinetic with the concentration of SiH4 in a large excess, the most likely competitive reaction is the reaction of SiHz with Ccl,, SiHz + Ccl, +products
(15)
Inclusion of reaction ( 15) in the mechanism needs modification of eqs. ( 10) and ( 11); parameter 5 in these equations has to be replaced by q given by
[SiH&
/ 1Ol2 molecules
cme3
Fig. 4. Plot of the parameter /I obtained from SiH, decay (open circles) and Si2Hb rise (close circles) as a function of initial concentration of SiH,. The solid line is a hnear least-squares tit.
446
1 --_ v-t
1
1f
kS,[CChl k,[SiH4] [M] >
(16)
The values of )1-’are plotted as a function of [Ccl,] in fig. 5. As expected from eq. ( 16), q-’ depends linearly on [CCL,], and an extrapolation of the data to
Volume 184,number 5,6
CHEMICALPHYSICSLETTERS
4 October I99 I
from the Ministry of Education (No. 02650702). The authors are grateful to Dr. M. Ito (Mitsui-Toatsu Co. Ltd.) for his kind offer of Si2H6. 0
0
References
0 0
/
oI 0
/ 0
B
[CC:,] / “lo14 E
10
12
molecules cm -3
Fig. 5. Plot of 8-l as a function of the CCll concentration. Definition of parameter v is given in eqs. ( 12) and (16). The solid line is a linear least-squaresfit.
[Ccl,] = 0 gives a value of <= 0.62 ? 0.14. This implies that about 60% of SiH, is converted to SiH, t SiH4, and the rest of it is converted to Si,H, t HZ_The rate constant for the additional SiH,
loss process was obtained from the slope of the plot in fig. 5. With the value of k, [ M] = 1.1x lo- lo cm3 molecule-’ s-’ at p= 5 Torr [ 111, the rate constant was estimated to be (7.4+3.0)x lo-” cm3 molecule-’ s- ‘. Although the linear dependence of 9-l
on [CCL,] indicates that this rate constant can be attributed to reaction (15), a more direct experiment is required for conclusive evidence of this reaction.
Acknowledgement This work was partly supported by a Grant-in-Aid
[ 1 ] J.M. Jasinski,B.S.Meyersonand B.A.Scott,Ann. Ret. Phys. Chem. 38 (1987) 109. [2] R.M. Osgood,Ann. Rev. Phys. Chem. 34 (1983) 77. [3] N. ltabashi, K. Kate, N. Nishiwaki, T. Goto, C. Yamada and E. Hirota, Japan. J. Appl. Phys. 28 ( 1989) L325. [41 S.K. Loh,D.B. Beachand J.M. Jasinski,Chem. Phys. Letters 169 (1990) 55. [S] R. Becerra and R. Walsh,.I.Phys. Chem. 91 (1987) 5765. [61 G.G.A. Perkins,E.R.Ausrinand F.W.Lamps, J. Am. Chem. sot. 101 (1979) 1109. [7] P.A. Longewayand F.W. Lamp, J. Phys. Chem. 87 (1983) 354. [8] P.A. Longewayand F.W. Lamp, J. Am. Chem. Sot. 103 (1981) 6813. [9]G. Olbrich, P. Potzinger, B. Reimann and R. Walsh, Organometallics3 ( 1984) 1267. [IO] J. Zavelovich and J.L. Lyman, J. Phys. Chem. 93 (1989) 5740. [I l] J.M. Jasinskiand J.O. Chu, J. Chem. Phys. 88 (1988) 1678. 121J.P.M. Schnutt, P. Gressier, M. Krishnan, G. De Rosny and J. Perrin, Chem. Phys. 84 (1984) 281. 131I.R. Slagle,J.R. Bernhardt and D. Gutman, Chem. Phys. Letters 146 (1988) 180. 141R. Robertson and A. Gallagher, J. Appl. Phys. 59 ( 1986) 3402. 151R. Robertson, D. Hlls, H. Chatham and A. Gallagher,Appl. Phys. Letters 43 (1983) 544. 161J. Berkowitz,J.P. Greene, H. Cho and B. Rustic, J. Chem. Phys. 86 (1987) 1235. 171H. Niki, P.D. Marker, CM. Savageand L.P. Breitenbach, J. Phys. Chem. 89 (1985) 1752. 181M.S. Gordon, TN Truong and E.K. Bonderson, I. Am. Chem. Sot. 108 (1986) 1421. 191P. Ho, M.E. Coltrin, J.S. Binkleyand CF. Melius,J. Phys. Chem. 90 (1986) 3399. 201K. Kamisako, T. Imai and Y. Tarui, Japan. J. Appl. Phys. 27 (1988) 1092.
441
CHEMICAL PHYSICS LETTERS
Volume 184, number 5,6
Rate constant and mechanism of the SiH3t SiH3 reaction Mitsuo Koshi a, Akira Miyoshi b and Hiroyuki Matsui a a Department ofReaction Chemistry, Umversity ofTokyo. 7-3-l Hongo, Bunkyo-ku, b Division
ojdtmosphene
Environment,
The National
Instilutefor
Tokyo 113, Japan
Enwronmanta(Studies.
Tsukuba-gakuen,
Ibarakl30.5,
Japan
Received 2 May 1991; in final form 29 June 1991
The kinetics of the SiH?-t SiH3 reaction was studied with time-resolved mass spectrometly. Silyl radicals, as well as S&H6 and HCI. produced by ArF laser photolysis of C&/SiH,/He mixtures were detected by using a near-threshold electron-impact ionization technique.The rate constant of ( 1.2iO.4) x 10-‘Ocm’ molecule-’ s-’ for the SiHSt SiH, reaction at T=293 K was derived both from the decay rate of SiH> and the production rate of SiIH6. Steady-state analysis indicated that the fraction of SiH2 formatlon in the SiH3+SiHS reaction was equal to 0.62 + 0.14 at p= 5 Torr.
1. Introduction SiH3 t SiHI-Si2H:, The silyl radical is expected to play a key role in silane plasma chemical-vapor-deposition (CVD) processes [ 1,2]. Since the kinetic lifetime of the silyl radical in typical CVD environments is dominantly controlled by the SiH3 + SiH, recombination reaction, detailed knowledge of this reaction is necessary for modelling the CVD processes. Despite its importance, the rate constant of the SiH3 t SiH? reaction has been measured only by two groups, and the reported values differ from each other by a factor of more than 2. Itabashi et al. [ 31 measured the absolute concentrations of SiH3 produced in a pulsed SiHJ/HZ discharge by using infrared diode laser spectroscopy. Loh et al. [4] also used an infrared diode laser to detect SiH3 produced in ArF laser photolysis ofSiH,Br/He and SiH,/CCl,/He mixtures at 193 nm. Since the SiHj recombination reaction is a biomolecular reaction which cannot be studied under pseudo-first-order conditions, the accuracy of the rate constants obtained in these works depends largely on the accuracy of the method for the determination of the SiH3 absolute concentrations. The recombination of SiH1 initially forms vibrationally excited disilane, S&HE, which is either stabilized or decomposed according to the following reactions [ 5 ] : 442
0009-2614/91/$
(1)
Si2HE+M+SizH6 tM .
@a)
S&H;: +SiHz t SiH4 ,
(2b)
SizHt+SizH1 t Hz .
(2c)
In addition, the direct disproportionation SiH3 + SiH, t SiH, t SiH, ,
reaction, (3)
is also possible. Silylene radical, formed by reactions (2b) and/or (3), can easily insert into SiH4 yielding S&H,: SiH*+SiH,(
tM)+Si,H,(
tM)
.
(4)
The formation of S&H6 often observed in studies of the photolysis of SiH, was explained by this reaction [6-IO]. Since the rate of reaction (4 ) is very fast [ 11,12], the formation of Si2H, is controlled by the formation rate of SiH2. Although a value of k,,/ klb=O. 18 & 0.05 was suggested by Becerra and Walsh [ 51 on the basis of simulations with a complex reaction mechanism, this value is largely affected by the values of k, and k, used in the simulation. In the present study, the SiH, radical generated by ArF laser photolysis of CCl,/SiH,/He mixtures was detected by using time-resolved mass spectrometry and the rate constant was determined. Attempts were 03.50 8 1991 Elsevier Science Publishers B.V. 411 rights reserved.
Volume184,number$6 also
4 October1991
CHEMICALPHYSICSLETTERS
made to detect possible reaction products.
2. Experimental The reaction was studied in SiH,/CCl,/He mixtures which were irradiated by the unfocused output of an ArF laser (Questek 2220) at 193 nm. Gas mixtures were flowed in a Pyrex cell 76 cm in length and with an inner diameter of 1.6 cm. The partial pressures of the reactants and the total pressure were measured with a capacitance manometer (MKS Baratron 122A). Experiments were carried out at room temperature 293 f 2 K) with a typical total pressure of 5 Tort-. The partial pressures of SiH, and CC& ranged from 10 to 22 mTorr and from 7 to 3 7 mTorr, respectively. The contents of the cell were sampled continuously by a quadrupole mass spectrometer (Anelva TE-600s) through a 100 pm pinhole located at the center of the cell. Gases from the pinhole were directly introduced into an electron-impact ionization chamber. The ion signals from a secondary electron multiplier, operated under pulse-counting conditions, were recorded with a gated counter following pulse amplification and discrimination. The time dependence of each individual mass peak was obtained by scanning the delay time of the gate with a fixed gate width of 50 or 100 ps. Signals were averaged over 5000 to 15000 laser shots for each wave form. Mass spectra reflecting laser-induced changes were also obtained by scanning the quadrupole mass filter with an internal trigger signal. The silyl radicals were generated by ArF laser photolysis of Ccl, in SiH, [ 4,131 and were detected by using a near-threshold ionization technique [ 14,15 1, in which the SiH: ion (m/z= 3 I), produced by the ionization of SiH3, was separated from SiH: formed by the dissociative ionization of SiH, by using an electron energy below the threshold of the dissociative process of SiH, and above the threshold for SiH3 ionization. Since the threshold energies for SiH: formation from SiH, and from SiH? are 12.09 and 8.01 eV [ 161, respectively, an ionizer electron energy of 10 eV was used for the detection of SiH: from SiH3. An ionizer electron energy of 14 eV was used for the detection of HCI+ at m/z= 36, and 16 eV for both Si2H6+at mJz=62 and Si2H4+at mjz=60.
In the present experiments. the Cl atoms produced by the laser photolysis are efficiently converted to SiH3 and HCl via the reaction
SiH,+Cl-+SiH,
tHC1.
(5)
Since this reaction is essentially gas kinetic (k=4.4>( 10-‘0cm3molecule-‘s-‘) [ 171 andSiH, is in large excess, the generation rate of SiH3 is much faster than the rates of other SiH3 loss processes. Thus, the amount of SiH3 formed initially is equal to that of HCI produced by reaction (5 ). Typical examples of the time profiles of SiH3 at m/i= 3 1 and HCl at m/z=36 are shown in figs. la and lb. The signal intensity at m/z=36 was calibrated against the concentration of HCl in separate experiments, and then the initial concentrations of SiH, were obtained from the ratio of signal intensities at m/z= 3 1
I
-10
I
I
0
10
Time
20
/ ms
Fig. I. Timedependenceof (a) m/z=31 [SiH: 1, (b) m/z=36 [HCl+], (c) m/z=60 [S&H:], (d) m/z=62 [S&H$], and (e) net signal from S&H,: leO- 1.81,, lsee text), in ArF ( 193 nm) laser photolysis of SiHJCCI,/He mixture; p(SiH,,) = 17 mTorr and p( Ccl,) = 14 mTorr at a total pressure of 5 Torr. The solid curve in (a) is calculated from eq. (7) withp=2.49 and !Q= I60 S-I and that in (d) is calculated from eq. (IO) with b= 2.42 and k6= I~OS-~.
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and 36 measured under identical conditions. The yield of HCl (and, hence, SiH3) from Ccl, in the I93 nm photolysis of SiH,/CCl, mixtures was found to be 0.37% with a typical laser fluence of 12 mJ/ cm’. The accuracy of this calibration procedure is largely limited by the instability of the detection system. The possible maximum error in the determination of SiH3 concentrations was estimated to be & 27%.
3. Results and discussion 3. I. Detection of thereaction products According to the reaction mechanism (l)-(4), SizHb and S&H4 are expected as reaction products and attempts have been made to detect these species. Fig. 2a shows a mass spectrum observed during the photolysis of a SiH4/CC14 mixture with an ionization energy of 16 eV. Since the ionization energy was considerably higher than the threshold energy ( IO.15 eV for Si?H, and 8.9 eV for Si;H4 [ 14]), fragment peaks of Si2H, in the range of m/z=58-62 were observed. The mass-fragment pattern of S&H, is shown in fig. 2b for comparison. It is noted that the ratios
of the mass peak intensities at m/z=58-61 to the intensity at m/z=6.2, R,=I,,/lb,, (n=58-61), for the photolysis products (fig. 2a) are larger than those for Si,H, (fig. 2b). The mass-pattern coefficient, R,,. for Si2H6 at an electron energy of 16 eV was determined to be 1.80? 0.11, whereas the value of RbOwas found to be 2.20 &0.19 for the products of the laser photolysis. This implies that there is some “extra” signal at m/z= 60 for the photolysis product. Silylsilylene is a possible source for this “extra” signal. This is further indicated by the fact that the time profiles of the signals at m/z=60 and 62 are slightly different, as shown in figs. lc and Id. The signal at m/z=60 shows a slow decay after a rapid rise, whereas the signal at m/z= 62 remains almost constant. In fig. le, the difference signal obtained from I,,, by subtracting the contribution of the fragment signal from Si,H,, i.e. Z6,,- 1.816?,is also shown. Although no conclusive assignment of this “extra” signal at m/z=60 can be made, the slow decay of the time profile is possibly caused by the consecutive reactions such as the insertion reaction of HSiSiHX into SiH4. The mass peaks in the range of m/z=63-66 can be assigned to the fragment ions from SiH,Cl, since they disappeared when SiH, was produced by the photolysis of N,O/SiH, mixtures. The rise rate of these signals was found to be much faster than the decay rate of SiHj. The mechanism of SiH,Cl formation is not clear. 3.2. Mechanism and the rate constant ,for the SiHj + SiH, reaction
(a) SiH4/CCl4/He + 193 nm
Since SiH3 is unreactive towards SiH, and CCI, [4], the decay rate of SiHJ is controlled by the recombination reaction ( 1) and the disproportionation reaction (3). The dissociation of Si,H; back to reactants,
1
Si2Hz+SiHI +SiH3 , 1
45
50
,/
l,,,,,,,,t,
55 Mass
L,,,,,
60
65
’
Number
Fig. 2. (a) Product mass spectrum in ArFlaser photolysis of SiH,/ CCI,/He mixture; p(SiH4) = 17 mTorr and p( CCL,) = 16 mTorr at a total pressure of 5 Torr. (b) Mass spectrum of S&H,; p( S&H,) =0.03 mTorr at a total pressure of 5 Torr. Both spectra were taken with 16 eV electron-impact ionization energy. Each spectrum is normalized to the mass peak intensity at m/z= 60.
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4 October 1991
(2d)
is relatively improbable [ 41, since other decomposition channels (2b) and (2~) have large exothermicity, and the excess energy of the Si2Hg formed by reaction ( 1 ) is much higher than the energy barriers of these decomposition channels [ 18,191. The decay rate of SiH7 is independent of the total pressure if reaction (2d) is negligible. In our preliminary experiments, it was found that the decay rates of SiH,
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at a total pressure of 2 Torr were essentially the same as those at 5 Torr. Moreover, the mass spectrum in the range of nz/z=45-IO showed no significant difference in the mass pattern and the intensities at p= 3, 4 and 5 Torr. These observations indicate that vibrationally excited disilane decomposes irreversibly to SiH2+ SiH, or SizH4+Hz in the present experimental conditions. The pressure-independent decay rates of SiH3 were also found by others [ 3,4,20]. The deactivation of S&H% (reaction (2a) ) can compete with these decomposition reactions at high pressures [ lo], but recent RRKM calculations for the decomposition of vibrationally excited disilane [ 51 indicated that the collisional stabilization was much slower than the decomposition channels. SiH, radicals were also eliminated at the wall of the cell, and this process is governed by the diffusion of SiH3: SiHl -+wall .
(6)
Analytical solution for the time profile of SiH3 is obtained with reactions (I), (3), and (6): (7) P= [2(k, +Mlkl
W310.
(8)
Because of the large correlation between k6 and /I in eq. (7), the determination of these parameters by a direct non-linear least-squares fit to the experimental data introduced a large uncertainty [ 31. Instead, we measured the time constant z ( 1/e time) of each set of data as a function of the initial SiHX concentration, as shown in fig. 3. An expression for the time constant is derived from eq. (7 ):
action (4). This reaction is known to be pressure dependent, and its rate constant is 1.1 x lo-” cm3 molecule- ’ s- ’ at a= 5 Torr [ 111. Since the condition of [SiH,] >> [SiH3] is always valid in the present experiments, the rate of reaction (4) is much faster that the production rate of SiH2 (sum of reactions ( 1) and (3)). With the steady-state assumption for SiHz and Si,Hg, the following equations for the time profile of Si2H6 are derived: [Si2H6]=f[SiH3105
(1+P)[1-w(-Wl ltjI[l-exp(-k6t)]
ln{lt~[l-exp(-k,t)]}
1
k6 ~=ln[(etfl)/(ltfl)]’
Fig. 3. Time constants for SiHj decay (open circles) and S&H6 rise (closed circles) as a function of the initial concentration of SiH,; p( SiH,) = I7 mTorr at a total pressure of 5 Torr. Solid curves are calculated from eq. (8) for the SIH, decay rate and from eqs. ( 10) and ( 1I ) for the Si2H6rise rate.
(9)
Extrapolation of 1/z values in fig. 3 to [ SiH3 ] D= 0
P
>’
[Si,H,l,=1[SiH,l,~~[1-(1/P)ln(1-tP)l,
(10)
(11)
gives kg= 160 s-’ and the parameter p for each set
of data can be determined by using eq. (9). The time profile of SiHJ, calculated by eq. (7) with the values of k, and j3 thus determined, is compared with the experimental time profile in fig. la. The parameter j3 can also be obtained from the production rate of Si2H6. Since reaction (Za) is not important in the present experimental conditions as noted above, S&H6 is exclusively produced by re-
(12) Here, k2=k2,,tkzo k’=k,-tk, and [SizHslm is the concentration of S&H, at t=co. The parameter r represents the yield of SiH2 in the SiH3 t SiH3 reaction. It is noted that the normalized time profile of Si,H6, [ Si,H,] / [ Si2H, ] 8, contains only two parameters, k, and p. With a value of kc= 160 SK’, it 445
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CHEMICAL PHYSICS LETTERS
is easy to determine the values of p (and, hence, k’) from the time constant ( ( 1 - 1/e) time in this case) of the normalized time profile of Si2H6. The experimental time constants for the rise rate of Si,H, are shown in fig. 3, and the time profile of Si2H6 calculated with eqs. ( 10) and ( 11) is compared with the experimental profile in fig. Id. The excellent agreement demonstrated in figs. la and Id indicates the validity of the reaction mechanism. In addition, the same decay rates of SiH3 were observed in ArF photolysis of N,O/SiH, mixtures in which SiH3 was produced by the following very fast reactions: O(‘D)+SiH,+SiH,+OH,
(13)
OH+SiH+SiH,+H,O.
(14)
The values of p obtained both from the decay rates of SiH, and the rise rates of S&H6 are summarized in fig. 4, in which 1 is plotted against [SiH310. From the slope of this plot, which includes the calibration error in the determination of [SiH310, k’ is determined to be ( 1.16?0.39)~10-lo cm3 molecule-’ s-l. The time constants for the SiH3 decay and the S&H6 rise calculated with this value are compared with the experimental results in fig. 3. The present value of k’ is to be compared with the value obtained by Itabashi et al. (k’ = ( 1.5+ 0.6) x lo-” cm? molecule-’ s-‘) [3] and by Loh et al.
(k’ < (0.61+0.35)x IO-“cm3 molecule-‘s-l) [4]. Although these three values overlap within the combined uncertainties, the rate constant reported by Loh et al. is about one half of the present result. In their procedure for measurement of the absorption cross section, Loh et al. assumed that all the Cl atoms produced by the 193 nm photolysis of Ccl,, were converted to SiH3. On the other hand, the quantum yield for the SiH, production in the photolysis of SiH,/ Ccl, mixtures was estimated to be less than 0.95 on the basis of the present measurements of the absolute concentration of HCl. Since the quantum yield of the Cl atom from Ccl, in the 193 nm photolysis was estimated to be 1.2 [ 41, some of the Cl atoms generated by the photolysis are converted to species other than I-ICI. If this is the case, the concentration of SiH3 was overestimated in the experiments of Lob et al. 3.3. Fraction of SiH2 production in the SiHjSSiHj reaction
Since the absolute concentrations of S&H, formed in the SiH, t SiH, reaction have been obtained by calibrating the signal intensity at m/z= 62 and since the values of k, and/I are already known, the parameter 5 in eq. (11) can be determined. It was found that the yields of Si2H, decreased with increasing concentration of Ccl, at a fixed total pressure. This might be caused by the existence of an additional loss process of SiH2. S&H6is formed by reaction (4)) and, therefore, the loss of SiHz reduces the S&H6 yield. Since the rate of reaction (4) is almost gas kinetic with the concentration of SiH4 in a large excess, the most likely competitive reaction is the reaction of SiHz with Ccl,, SiHz + Ccl, +products
(15)
Inclusion of reaction ( 15) in the mechanism needs modification of eqs. ( 10) and ( 11); parameter 5 in these equations has to be replaced by q given by
[SiH&
/ 1Ol2 molecules
cme3
Fig. 4. Plot of the parameter /I obtained from SiH, decay (open circles) and Si2Hb rise (close circles) as a function of initial concentration of SiH,. The solid line is a hnear least-squares tit.
446
1 --_ v-t
1
1f
kS,[CChl k,[SiH4] [M] >
(16)
The values of )1-’are plotted as a function of [Ccl,] in fig. 5. As expected from eq. ( 16), q-’ depends linearly on [CCL,], and an extrapolation of the data to
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CHEMICALPHYSICSLETTERS
4 October I99 I
from the Ministry of Education (No. 02650702). The authors are grateful to Dr. M. Ito (Mitsui-Toatsu Co. Ltd.) for his kind offer of Si2H6. 0
0
References
0 0
/
oI 0
/ 0
B
[CC:,] / “lo14 E
10
12
molecules cm -3
Fig. 5. Plot of 8-l as a function of the CCll concentration. Definition of parameter v is given in eqs. ( 12) and (16). The solid line is a linear least-squaresfit.
[Ccl,] = 0 gives a value of <= 0.62 ? 0.14. This implies that about 60% of SiH, is converted to SiH, t SiH4, and the rest of it is converted to Si,H, t HZ_The rate constant for the additional SiH,
loss process was obtained from the slope of the plot in fig. 5. With the value of k, [ M] = 1.1x lo- lo cm3 molecule-’ s-’ at p= 5 Torr [ 111, the rate constant was estimated to be (7.4+3.0)x lo-” cm3 molecule-’ s- ‘. Although the linear dependence of 9-l
on [CCL,] indicates that this rate constant can be attributed to reaction (15), a more direct experiment is required for conclusive evidence of this reaction.
Acknowledgement This work was partly supported by a Grant-in-Aid
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