J. Phys. Chem. 1993,97, 10198-10203
10198
+ 2
+
Dissociative Charge-Transfer Reactions of N+(jP), N2 ( 2, ), Ar+(2P~/q1/2), and Kr+(2P3,2)with SiFd. Thermochemistry of SiF4+ and SiF3+ Bernice L. Kickel, EUen R. Fisher,? and P. B. Armentrout' Department of Chemistry, University of Utah, Salt Luke City, Utah 841 12 Received: May 24, 1993; In Final Form: July 16, 19936
Guided ion beam mass spectrometry is used to measure cross sections as a function of kinetic energy for interaction of N+(3P), N:(2Z:), Ar+(*P3/2,1/2),and Kr+(2P3/2)with SiF4. Energy dependences of these cross sections and previous results for the reaction of O+(4S) with SiF4 are analyzed to yield thermochemistry for SiF4+ and SiF3+. From the O+and N+ systems, we find that the adiabatic ionization energy of SiF4 is 15.29 f 0.08 eV, consistent with less precise determinations in the literature. The heat of formation of SiFs+ a t 0 K is determined as -29.3 f 0.6 kcal/mol and that for N F as 63.4 f 3.9 kcal/mol.
Introduction Silicon fluoride species are found in many fluorine-based chemicalvapor deposition (CVD) systems,plasma-enhanced CVD (PECVD) systems, and plasma etching systems, including CVD of amorphous hydrogenated/fluorinated silicon (a-Si:H,F) for solar cells,' CVD of silicon nitride for protective coatings? and plasma etching of silicon materials for microelectronic devices.' During etching, highly reactive fluorine radicals and ions present in the plasma bombard the surfaceand volatilize the silicon surface as SiF4 and SiF2.4.5 Deposition of silicon layers involves polymerization by insertion of SiF, species into the surface. The thermochemistry of the SiF, and SiF,+ species and reactions involving these species are thus important to a complete understanding of these plasma processes. To address these issues, we have previously examined the interactions of SiF4 with rare-gas ions (Ar+, Ne+, and He+)? Si+? 02+,and O+ions! In the following paper? we use collisioninduced dissociation and charge-transfer studies to more thoroughly examine the thermochemistry of the SiF, and SiF,+ species. In the course of this work, it became evident that literature informationon the ionization energy (IE) of SiF4 was particularly unreliable. We therefore initiated the present work in order to more carefully establish this IE. In the process, we are also able to refine our value for the heat of formation of SiF3+ obtained previously.6 Both pieces of thermochemicalinformationare then used to advantage in establishing the thermochemistry of SiF, and SiF,+ (x = 1-4) in the following paperag Ionization Energy of SiF4. An accurate measurement of the adiabatic ionization energy of SiF4 is elusive because, like all tetrahedral molecules, the SiF4 molecule undergoes extensive Jahn-Teller distortion upon ionization, such that the FranckCondon factors for ionization at threshold are small. Thevertical IE for SiF4 is 16.46 f 0.04 eV,1° while the adiabatic IE has been reported as 15.71 t 0.3 eV in an electron impact retardingpotential difference (RPD) study;" 15.81 f 0.0212 and 16.1 f 0.113 eV from photoelectron studies; 15.0 t 1.014 and 15.4 t 0.415 eV in electron impact studies. In their photoelectron study, Bull et al. also report an adiabatic IE although there is some confusion aboutthevalue. In thetext, they statethat 'theonsetofionization occurs at 15.92 eV", while in a table they give a value of 15.19 eV as the "threshold for ionization (upper limit of the adiabatic ionization energy)". Data are shown only as low as about 15.6 eV. The uncertainty in this value is not given in this paper, but a subsequent paper16 quotes uncertainties of 0.04 eV. The f
Present address: Department of Chemistry, Colorado State University,
Fort Collins, CO 80523.
Abstract published in Advance ACS Abstracrs, October I , 1993.
0022-365419312097- 10198$04.00/0
compilationof Rosenstocket al." cites the 15.92-eVvalue, while Murphy and Beauchampla cite the 15.19-eV value but attribute it to ref 16 rather than ref 10. Levin and Liaslgalso cite the latter value, incorrectly attributing it to a photoionization study of Murphy and Beauchamp,Is who actuallystudied only SiFX(CF3)~, (x = 0-3). In previous we have cited the 15.81-eV value from Bassett and Lloyd12 and, more recently, the 15.19-eV value from Bull et al.1° Lias et aLzoassign a value for IE(SiF4) of 15.7 eV with no uncertainty listed. Two recent theoretical values are also available from the literature. Ishikawa et al. calculate a non-spin-polarized IE of 15.09 eV and a pin-polarized IE of 14.95eV,21 while Ignacio and Schlegel calculate 15.34 eV and give a best estimate of 15.23 eV.22 An estimate for how severe the Franck-Condon effects may be in the SiF4 system comes from a comparison with CF4. The adiabatic IE for CF4 has never been accuractely measured using photon techniques, and our measurement of theappearanceenergy for CF3+ from CF4, a rigorous upper limit to IE (CFd), is 14.24 f 0.07 eV?3 well below the 15.3 eV onset for ionizationobserved in the photoelectron spectrumZ4and 2 eV below the vertical IE of 16.25 eV.10J4 Thus, it is possible that IE(SiF4) could be as low as 14.5 eV, although the shift between the adiabatic and vertical IEs of SiF4 is not expected to be as severe as in the case of CF4. Overall, it seems likely that 15.8 eV is a reasonable upper limit and 15.0 eV a reasonable lower limit to the true adiabatic IE.
Experimental Methods General Techniques. Complete descriptions of the apparatus and experimentalprocedures are given e l s e w h ~ r e .Briefly, ~~ ions are produced as described below and are focused into a magnetic sector momentum analyzer for mass analysis. They are then decelerated to the desired translational energy and focused into an octopole ion beam guide26 that traps ions in the radial direction. Theoctopolepasses througha staticgascellintowhich theneutral gas is introduced at sufficiently low pressures, 0.024.15 mTorr, that multiple ion-molecule collisions are improbable. Pressure dependent studies verify that the cross sections measured here are due to single ion-molecule interactions. After leaving the octopole, transmitted and product ions are extracted and analyzed in a quadrupole mass filter. Ions are detected by a secondary electron scintillation ion counter using pulse countingtechniques. Raw ion intensities are converted to absolute reaction cross sections as described previ0usly.~5 Charge-transferand dissociativecharge-transferreactions may occur through long-range electron jumps such that little or no forward momentum is transferred to the ionic products.27 In such instances, it is possible that up to 50%of these ions may have Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 10199
Dissociative Charge-Transfer Reactions
no forward velocity in the laboratory and will not drift out of the octopole to thedetector. Such slow product ions which do traverse the octopole may be inefficiently transmitted through the quadrupole mass filter.25 Exothermic or nearly thermoneutral reaction channels, in particular, can be subject to these effects. The cross-section results presented here are averages of several determinations taken at different times over the course of 10 months. In our previous work on the Ar+ system? we chose to display cross sections corresponding to the largest magnitudes observed, such that the cross sections shown here are about half as large as those published before although the energy dependences are the same. On the basis of reproducibility, the uncertainty in the absolute cross sections is estimated as 604, while the relative error is about 20%. Laboratory ion energies (lab) are converted to energies in the center-of-massframe (CM) by using the conversionECM= E ~ I , M / ( m M),where m is the ion mass and M is the neutral molecule mass. The absolute energy and the energy distribution of the ions in the interaction region are measured by using the octopole as a retarding field a n a l y ~ e r .These ~ ~ measurements show that the distribution of ion energies is Gaussian with a typical full width at half-maximum (fwhm) of -0.4 eV (lab) for the ion beams used in these experiments and that the uncertainty in the absolute energy scale is f0.05 eV (lab). The thermal motion of the gas in the reaction cell has a distribution with a fwhm of 0.4 EcM'/~ (eV).*8 Both distributions are taken into account when analyzing the experimental results.25 At very low energies, the slower ions in the kinetic energy distribution of the beam are not transmitted through the octopole, resulting in a narrowing of the ion energy distribution. We take advantageof this effect to access very low interaction energies as described previo~sly.~5.~~ Ion Source. A flow tube (FT) ion source, described in detail previously,30.31 is used for form N+, N2+, Ar+, and Kr+ ions. The FT operates at a pressure of 0.4-0.7 Torr with a helium flow rate of 350CL7000 standard cm3/min. He+ and He* formed by microwave discharge interact with the N2, Ar, or Kr reagent gas to form the desired ions through charge transfer and Penning ionization. The ions then undergo lo5 collisions with the He bath gas in the meter long flow tube that should help cool the ions to their ground states. We have previously verified that N2+ ions produced in this source are in their ground electronic and vibrational ~ t a t e . 3The ~ charge-transfer reaction of N+ with D2, endothermicfor ground stateN+ and exothermicfor excited state N+,29was used to verify that no large amounts of excited states were present, although the results below suggest that there is a residual population of N+ excited electronic states. To form Ar+, we used both the flow tube source and a lowpressure electron-impact (EI) source operating at an electron energy of 20 eV.25 We have recently performed a number of studies comparing the reactivity of Ar+ formed by these two meth0ds.3~These results are consistent with the formation of a statistical distribution of the two spin-orbit components of the 2P state, the J = 3/2 ground state and the J = l / 2 excited state, in the E1 source. Further, these results suggest that the flow tube forms only the J = 3/2 ground state within several percent. The present results are also consistent with this distribution of states, and suggest that there is less than 4% of the beam in the J = l / 2 state, as discussed in detail below. The flow tube source was also used to produce the Kr+ ion beam. We have previously verified that Kr+ ions generated in this source are in their ground electronic state.23 Although no specific diagnosticswere used here to determine the distribution of the 2P3/2 and 2P1/2 states of Kr+, separated by 0.67 eV,34 the threshold data presented here are consistentwith a beam composed exclusively of the 2P3/2 ground state. Thermochemical AMIYS~S. The threshold regions of the experimental reaction cross sections are analyzed by using the empirical model of eq 1. Here EOis the reaction endothermicity
+
-
-
at 0 K, uo is an energy independent scaling factor, and n is an adjustable parameter. This equation takes the internal energy of the SiF4and N2+ reagents into considerationby including the average rotational energy, Emt(SiF4)= 3kT/2 = 0.039 eV and ETOt(N2+)= kT = 0.026 eV at 300 K. Vibrational energy contributions are negligible. The summation is over the distribution of electronic states i having relative populations gl, where Zgi = 1. This term is needed only for the case of Ar+ formed in the E1 source, where two spin-orbit states are present and assumed to have equal reactivities. The resulting model cross section is then convoluted with the ion and neutral translational energy distribution^^^ in order to reproduce the data. The parameters n, UO, and EOare allowed to vary freely to best fit the data as determined by nonlinear least-squares analysis. Errors in threshold values, determined by the variation in EOamong several data sets and the absolute uncertainty in the energy scale, are believed to be reasonable measures of the absolute accuracy of these threshold values. The general form of eq 1 has been derived as a model for translationally driven reactions35 and has been found to be quite useful in describing the shapes of endothermic reaction cross sections and in deriving accurate thermochemistry (within the stated error limits) for a wide range of systems.36J7
Results and Discussion In the following discussion, thermochemicalvalues needed are taken from Table I. O+ SiF4. In a previous study! we examined the reaction of O+(4S) with SiF4. Four reaction products, SiF,+ (x = l a ) , correspondingto charge transfer and dissociative charge transfer are observed. For our present purposes, we are interested in reaction 2, observed to have an apparent threshold of 1-2 eV, but
+
' 0
+ SiF,
-
SiF,'
+0
not analyzed in detail in our previous report. Analysis of the SiF4+cross section with eq 1 gives the optimum parameters listed in Table 11. When the measured threshold of 1.67 f 0.08 eV is combinedwithIE(0) = 13.618 06f0.000 01 eV,o8theadiabatic IE of SiF4 is determined as 15.29 f 0.08 eV. This value is well within the range of values discussed in the Introduction. N+ SiF,. Results for the interaction of N+ with SiF, are shown in Figure 1. The dominant product at all energiesstudied is SiF3+,which must be formed in reaction 4 at the lowest energies, because this process is exothermic by 1.42 f 0.34 eV while formation of SiF3+ N F is endothermic by 1.68 f 0.03 eV. This ion then dissociates to form SiF2+and SiF+ at higher kinetic energies in reactions 5 and 6.
+
+ +
N+
+ SiF,
-
SiF,'
+N
(3)
+
(4)
+F+NF
(5)
SiF3+ NF SiF:
+ 2F + N F SiF+ + 3F + N
SiF+
(6d (6b)
Analysis of the cross section for SiF2+ with eq 1 gives the optimum parameters listed in Table 11. The threshold of 5.02 f 0.16 eV for formation of SiF2+is in agreement with the threshold of 4.85 f 0.35 eV calculated for reaction 5 from the literature thermochemistry in Table I. Given the heat of formation for
Kickel et al.
10200 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
TABLE I: Heats of Formation at 0 I(. species A&, kcal/mol species AfHo, kcal/mol 59.5 (7.9), 63.4 (3.9)b O+ 314.04 NF 151.7 (1.2)c N 112.53 (0.02) SiF+ SiF2+ 96.9 (1.2)c N+ 447.69 (0.10) SiF3+ -29.3 (0.6)b N2+ 359.30 (0.01) SiF4+ -32.1 (1.7)b Ar+ 363.426 Kr+ 322.839 SiF4 -384.7 (0.2) F 18.47 (0.07) a JANAF Tables:8 unless otherwise noted. Uncertainties are in parentheses. b This study. e Following paper in this issue. TABLE II: Optimum Values for Parameters in Eq 1. reaction EO,eV n 00 0.7 (0.1) 1.67 (0.08) 1.3 (0.1) O+ + SiF4 SiF4++ 0 N+ + SiF4 SiF4++ N 0.77 (0.19) 2.9 (1.1) 0.1 (0.1)
--
-
+ + +
5.02 (0.16) SiF2++ [F NF] 0.65 (0.06) SiFs+ F N2 9.85 (0.25) SiF2+ + 2 F + Nz Ar+(EI)*+ SiF4 SiF3+ + F + Ar 0.42 (0.04) Ar+(P,/z)S SiF4 SiF,+ F + Ar 0.43 (0.04) Kr+(2P,p) SiF4 SiS+ + F + Kr 2.23 (0.05) +
N2++ SiF4
--
+
+ +
1.7 (0.1) 1.1 (0.1) 1.6 (0.1) 1.3 (0.1) 1.1 (0.1) 1.4 (0.1)
+
0.3 (0.1) 26.4 (10) 0.6 (0.5) 21.4 (0.7) 22.0 (1.3) 6.6 (0.5)
a Uncertainties are in parentheses. b Ar ions generated in the electron impact (EI) ionization source. Ar ions generated in the flow tube ion source.
ENERGY (eV. Lab)
0.0
10.0
20.0 I
Nt
+
,
SiF,+
!-
u
8
ground state N+(3P) to form ground state SiF4+(2T1),where the state designation indicates the orbital of tetrahedral symmetry from which the electron was removed. The latter increase may be associated with formation of N or SiF4+in an excited electronic state, although this state cannot be identified unequivocally because the threshold energy for its production cannot be measured precisely. Possibilities consistent with an onset of about 4 eV include the *P and 2D states of N, the first two excited states which lie 2.4 and 3.6 eV above the ground state,34or an SiF4+ state lying 18.5 eV above the SiF4 neutral. This could be the 2E state, which has an adiabatic IE of 19.15 eV, or with the 2A1 state, which has a vertical IE of 18.09 eV.10 A likely explanation for the exothermic reactivity observed for process 3 is that it is due to excited electronic states of N+. If these react efficiently, then an excited-state population as small as 0.2% of the ion beam could account for the observed cross section. This portion of the cross section varies with energy approximately as E-1, consistent with a reaction that requires a transition between two electronic surfaces.3g Such a transition is needed for reaction of N+('D,lS), the first two excited states of N+, with SiF4(lAl)to form N(4S) + SiF4+(2T~).In contrast, the reaction of the third excited state, N+(%) + SiF&A1) N(4S) SiF4+(2Tl), can occur in a spin-allowed process. Analysis of the first endothermic feature in the cross section for reaction 3 with eq 1 gives the optimum parameters listed in Table 11. When the threshold of 0.77 f 0.1 9 eV is combined with IE(N), the adiabatic ionization energy of SiF4 is determined as 15.30 f 0.19 eV. This value is in good agreement with the value obtained in the O+ system. We take our best determination of IE(SiF4) to be the average of these two numbers (weighted by their uncertainties),a 15.29 f 0.08 eV. This leads to AtHo(SiF,+) = -32.1 f 1.7 kcal/mol. Nz++ SiF4. Previous work in our laboratory has demonstrated that formation of N2+ in the flow tube source generates electronically and vibrationally cold N;(X221, u = 0) ions.32 Results for the reaction of N2+with SiF4at low kinetic energies are shown in Figure 2. The dominant process observed is charge transfer (reaction 7), although the SiF3N2+ ion is also generated
-
-
+
N:
I
.
I
0.0
,
I
.
10.0
'
"
I
20.0
ENERGY
