J. Phys. Chem. 1995,99, 12863-12874
12863
Effect of Incident Translational Energy and Surface Temperature on the Sticking Probability of F2 and 0 2 on Si(100)-2x1 and Si(lll)-7x7 E. R. Behringer, H. C. Flaum, D. J. D. Sullivan? D. P. Masson, E. J. Lanzendorf, and A. C. Kummel* Department of Chemistry, University of Califomia at San Diego, 9500 Gilman Drive, La Jolla, Califomia 92093-0358 Received: December 22, 1994; In Final Form: May 4, I995@
We have measured the initial (clean surface) reflectance of F2 and 0 2 normally incident on Si(100)-2x 1 and Si(l11)-7x7 as a function of incident kinetic energy Ei at different surface temperatures T,. For 02, the technique of King and Wells yields the initial sticking probability, SO,which increases monotonically with E, on both surfaces for nearly all Ei studied. For F2, the presence of abstractive chemisorption complicates the interpretation of the measurements. We find that the apparent sticking probability of F2 increases monotonically with E,on both Si(100)-2x 1 and Si(ll1)-7x7, consistent with the idea that F2 does not undergo precursormediated chemisorption on these clean surfaces. Using a crude model, we show that the data obtained with F2 on Si( 111)-7 x 7 are consistent with F2 abstractive chemisorption dominating for Ei < 0.1 eV and dissociative chemisorption becoming more probable as Ei is increased. We find that the apparent initial sticking probability of F2 depends linearly on the fluorine coverage, which is consistent with a stepwise chemisorption mechanism. For both F2 and 02, the sticking increases with T, for intermediate incident energies (0.1 eV < Ei 0.3 eV). The increase with T, in the case of F2 is consistent with a surface dynamical effect whereas for 0 2 the increase may be explained by the existence of a negative ion-like intermediate state.
I. Introduction The etching and oxidation of silicon surfaces is extremely important in the production of electronic devices and has therefore stimulated a great deal of effort toward understanding the fundamental mechanisms by which these reactions proceed.‘-3 Such understanding can then be used to design new fabrication techniques to produce electronic devices. The initial step of both the etching and oxidation reactions is adsorption, whether via physisorption or chemisorption. One method by which one can obtain information about the adsorption pathway is by measuring the sticking probability on the clean surface, SO(also referred to here as the initial sticking probability), and its dependence on the incident kinetic energy Ei of the impinging molecule and on the temperature of the surface, T,.4.5 Such measurements have often been used to determine whether the adsorption of molecules on surfaces is mediated by a precursor state or is a direct process (possibly with an energy barrier). Information about the adsorption mechanism is useful for designing “digital” etching processes in which an integral number of atomic layers is removed each etching cycle. Among the most promising agents for a digital etch process are compounds containing halogens. In earlier work, we measured the initial sticking probability of 12, Br2, and Cl2 on Si(100)2 x le6$’We have now extended this work to F2 and 0 2 and have also measured SO of these species on the Si(111)-7x7 surface. Using both F2 and 0 2 allows us to compare the behavior of two species with quite different dissociation energies (1.6 eV for F2, 5.1 eV for 0 2 ) 8 as well as different electron affinity energies (2.96 eV for F2, 0.44 eV for O# while using the two silicon surfaces allows us to determine the role of the substrate structure on the adsorption probability. We find that
’
Present address: Philips Research Laboratories, 81 1 E. Arques Avenue, M/S 65, P.O. Box 3409, San Jose, CA 94088-3409. * Author to whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, August 1, 1995. @
0022-365419512099-12863$09.0010
the initial sticking behaviors of F2 and 0 2 to be qualitatively different, while sticking on the two different surfaces is quantitatively similar for a given molecule within the sensitivity of our measurements. Fluorine in various forms (e.g., CF4) is an important reagent for etching. Engstrom et al. previously performed a thorough experimental study of the adsorption of F2 and F on the Si(100) surface using molecular beam techniques, X-ray photoelectron spectroscopy (XPS), and ion-scattering spectroscopy Using X P S , they found the F2 initial sticking probability, SO,to be 0.46 f 0.02 for Ei in the range 0.07 eV Ei < 0.83 eV and at an incident angle 8i = 75’ measured with respect to the surface normal. Also, they found that the sticking was independent of T, for 120 K < Ts .c 600 K and that the sticking probability, S, depends on fluorine coverage, 0, as S ( 0 ) = (1 - 0 / 0 J 2 for Ts > 300 K. Carter et al. have performed molecular dynamics simulationsI0 of F2 impinging on Si(100)-2x 1 and predicted that the initial sticking probability slowly increases with Ei. In that work, they describe the different chemisorption mechanisms observed in their simulations and compare their theoretical results to the experimental work of Engstrom et aL9 and Li et a l . l l $ i zFor E, < 0.1 eV, they find that the dominant chemisorption mechanism is “abstraction”: the leading F atom of the impinging F2 molecule reacts with a dangling bond of a Si dimer pair; the energy liberated in that reaction goes to the trailing F atom, which is ejected into the vacuum, roughly along the axis of the original Si dangling bond. This abstractive chemisorption mechanism was first observed experimentally by Li et aLI2 Dissociative chemisorption differs only in that the trailing F atom also remains on the surface. As Ei is increased, Carter et a1.I0 found that the probability of dissociative chemisorption increases, mainly because the increased momentum of the trailing atom permits a closer approach to the surface and thus a subsequent reaction with a dangling bond. Since the dangling bonds largely determine the mechanisms and extent of adsorp0 1995 American Chemical Society
12864 J. Phys. Chem., Vol. 99, No. 34, 1995
tion, we expect that the adsorption mechanisms are the same on Si(111)-7x7 as on Si(100)-2xl. Therefore, one would expect single-site adsorption to dominate when Si( 111)-7x 7 is dosed with low translational energy F2 and that neighboringsite adsorption would increase with the incident energy of the impinging F2. A recent scanning tunneling microscopy (STM) studyI3provides direct evidence for the trend predicted by Carter et al. An excellent review by Engel on the adsorption of oxygen on Si( 100)-2x 1 and Si( 111)-7x 7 has recently a ~ p e a r e d .We ~ limit our discussion below to work concerned primarily with measuring the initial sticking probabilities of 0 2 on these two silicon surface^.'^-'^ The studies of 0 2 adsorption on Si(100) by D'Evelyn et al.,I4 Miyake et al.,'8.'9 Yu and Eldridge,20and Memmert and YuI7 have shown that for normal incident energies, E, = Ei cos2 e,, less than 0.08 eV, the adsorption is precursor-mediated while for E, > 0.08 eV the adsorption is direct-activated. The dependence of the initial sticking probability on the surface temperature has consequently been found to vary with normal incident en erg^.'^,'^ In particular, for En < 0.08 eV, the initial sticking probability decreases with increasing T, while, for E, > 0.08 eV, the initial sticking probability increases with T,. Miyake et al.I8 suggested that the increase with T, for large E,, is due to the increased probability of forming 0 2 - which results from the increased abundance of thermally excited electrons. Measurements of 0 2 adsorption on Si(111) produce similar dependences of the initial sticking probability on normal incident energy and surface temperature.l6.l7 To summarize, previous studies of the initial (clean surface) adsorption of 0 2 on Si( 100)-2x 1 and Si( 111)7 x 7 indicate that, for normal incident energies less than approximately 0.08 eV, the adsorption is precursor-mediated while at higher energies it is direct-activated and mediated by a state of unknown character. We have measured the initial (clean surface) reflectance of F2 and 0 2 normally incident on the Si( 100)-2x 1 and Si( 111)7 x 7 surfaces as a function of the incident kinetic energy Ei and for different surface temperatures Ts using the technique of King and Wells. For 0 2 , this technique yields the absolute initial sticking probability SO. For F2, which can abstractively chemisorb, the interpretation of the apparent initial sticking probability is more complicated. To check the FZmeasurements, the scattering of FZwas measured using nonresonant multiphoton ionization together with time-of-flight mass spectroscopy. In section 11, we describe the experimental techniques, and in section I11 we present the data. We discuss the data in section IV and conclude with a summary in section V.
11. Experimental Technique The experiments were performed in a vacuum system that has been described in detail e l ~ e w h e r e . ~ , ~ JThis ' + ~ ~system consists of two major parts: a molecular beam source and an ultrahigh vacuum (UHV) chamber. Information relevant to these experiments is given below, along with a description of the procedures for measuring sticking probabilities. A. Molecular Beam Source. The molecular beam source consists of three differentially pumped chambers: chamber 1 contains a pulsed valve and skimmer; chamber 2 contains a chopper wheel and gate valve; chamber 3 houses a collimating aperture and a beam flag. Typical pressures in chambers 1, 2, and 3 when the molecular beam source is operating are and Torr, respectively. The source gas for the F2 molecular beam consists of F2 seeded in a pure noble gas (He, Ne, or Kr) or in mixtures of the noble gases. Different Fz/pure noble gas mixtures were
Behringer et al. obtained from Spectra Gas: 5%F2 in He, 20%F2 in Ne, and 10%F2 in Kr. These mixtures are used or are mixed with other noble gases to obtain molecular beams of F2 with different translational energies. The source gas is passed through a hydrogen fluoride trap (Matheson) before being admitted to the pulsed valve. The gas is then expanded through the 2.0 mm aperture of the valve (a Teflon poppet was used in all of the measurements presented in this paper), and the resultant beam pulses pass through the skimmer, an open slit of the chopper wheel, and the collimating aperture before entering the main UHV chamber. The axis of the molecular beam source is coincident with the axis of a quadrupole mass spectrometer (see section 1I.B below). Using the quadrupole, the beam pulses can be observed; when also using the chopper wheel, the timeof-arrival of a beam pulse can be measured, and hence the velocity of the molecules constituting the pulse can be determined. Since the rotational temperature of the incident beam is 4 K and the translational temperature is always less than the rotational t e m p e r a t ~ r ethe , ~ ~F2 beam is nearly monoenergetic. The rotational temperature of the incident beam was measured using resonantly enhanced multiphoton ionization (REMPI) together with time-of-flight mass spectroscopy (TOWS), as discussed in section II.C.2. The source gas for the 0 2 molecular beam is composed of pure 0 2 or 0 2 seeded in He or H2. 0 2 was obtained from Matheson (99.98% purity). When seeding with Hz, care must be taken not to achieve a stoichiometric mix of H2 and 0 2 in order to avoid the possibility of obtaining an explosive gas mixture. The highest beam energies were attained by using a heated S i c tube (Carborundum) attached to the end of the pulsed valve. This arrangement is based on a design by Kohn et aLZ4 The semiconducting S i c tube (0.280 in. o.d., 0.100 in. id., 4 in. length, 100-200 51) was connected in series to two 100 W light bulbs to provide current limiting when heating the tube. By heating the S i c tube, we can increase the beam energy by about a factor of 2. Using this technique to attain high beam energies has the advantage that one needs relatively low current to heat the tube (< 10 AC amps) but has the disadvantage that reactions of F:! or 0 2 with the nozzle walls at high temperature introduce impurities into the beam and simultaneously remove the desired incident species. Inserting a thin-walled A1203 tube inside the S i c tube did not alleviate this problem. Another disadvantage of using this technique is that the molecular beam is no longer nearly as monoenergetic. Therefore, when using the S i c tube to generate a molecular beam, the sticking measurement yields a value for the sticking probability that is an average over a range of incident energies. One might either assume that the initial sticking does not vary over the range of energies of the constituent particles or chop the broad pulses to determine the value of the sticking probability for a particular energy. The latter unfortunately reduces the signal so much that the resulting signal-to-noise ratio is usually too low for the data to be meaningful. B. Main Chamber. The main UHV chamber is pumped by a liquid nitrogen-trapped diffusion pump and a liquid nitrogen-trapped Ti sublimation pump. The base pressure of this two-tiered chamber after bakeout is (6-7) x lo-" Torr. The upper tier contains the molecular beam, a quartz beam flag, and a quadrupole mass spectrometer (UTI 1OOC). The lower tier is used for sample characterization and contains a set of optics for Auger electron spectroscopy (AES) and a separate set of reverse-view optics for low-energy electron diffraction (LEED).The lower tier also contains a load lock for sample
F2 and 0 2 on Si( 100)-2 x 1 and Si( 111)-7 x 7
J. Phys. Chem., VoE. 99, No. 34, 1995 12865
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Time (seconds) Figure 1. (a) Schematic diagram of the King and Wells technique for measuring initial sticking probabilities. The partial pressure of the species of interest (illustrated here to be F2) is monitored while (A) the monoenergetic molecular beam is on and blocked by the quartz beam flag, (B) the beam impinges on the clear surface, and (C) the beam is off. The ratio APlP,,,, is the experimentally derived apparent (b) Typical data from a initial sticking probability measurement for F2 incident on Si(100)-2x 1. transfer onto the sample m a n i p ~ l a t o rwhich , ~ ~ has xyz translation and two axes of rotation. C. Sticking and Scattering Measurements. 1. King and Wells Technique. The apparent initial (clean surface) sticking probabilities, of F2 and 0 2 on silicon surfaces are measured using the beam reflection technique originated by King and Wells.26 For 0 2 , (SO)exptis identical to the true initial sticking probability SO,but for F2, which can undergo abstractive chemisorption, the interpretation of is more complicated (as we discuss below). The King and Wells technique, illustrated schematically in Figure 1a, requires monitoring the partial pressure of the beam component of interest, i.e., either
F2 or 02, in three different situations: (A) while the monoenergetic molecular beam impinges on the quartz beam flag (on which F2 and 02 have sticking probabilities of zero), (B) while the beam flag is removed from the beam path and the beam impinges on the clean surface, and (C) while the beam is off. Typical data from the measurement of the initial sticking probability for F2 incident on Si(100)-2x 1 are shown in Figure lb. Here the partial pressure of F2 is monitored beginning at some arbitrary time t = to after the beam has been turned on. The beam flag is then withdrawn from the beam path at t = tl, allowing the molecular beam to impinge on the clean surface. Since some fraction of the incident molecules adsorb, the partial pressure of F2 decreases from PO to P I . The flag is then reinserted into the beam path at t = t2, and the process is immediately repeated (flag withdrawal at t = r3, flag insertion at t = t 4 ) . Finally, the molecular beam is turned off at t = t 5 , which decreases the F2 partial pressure to P5. The experimentally determined, apparent initial sticking probability (SO)expt is given by the ratio (PO - P,)/(Po - P5). We use this nomenclature, since the occurrence of abstractive chemisorption can cause to differ from SO, the true initial sticking probability. The King and Wells method for determining SO, discussed by Rettner et al.,27is valid when the vacuum time constant z is negligible compared to the time between pressure measurements, the pumping speed of the species of interest is stable, and the observed partial pressure changes are due entirely to the direct molecular beam. The data reported here for (SO)expt are averages of at least three measurements with the standard deviations shown as vertical error bars, one standard deviation shown on each side of each data point. To utilize the King and Wells technique, a constant pumping speed of the species of interest must be attained. To achieve this, we take care to run the molecular beam for enough time to stabilize the measured partial pressure of F2. Also, in previous work we have verified that it is possible to measure a value of 1.0 for SO(for C 4 incident on a copper block cooled to 77 K). During the initial stabilization of the pumping speed, the number of fluorine molecules which get to the surface is negligible in comparison to the fluorine molecules incident on the surface when the beam flag is removed from the beam path. We are also careful to frequently check that effusive contributions to the monoenergetic beam are negligible. We note that the presence of abstractive chemisorption can affect the interpretation of the data. For example, if abstractive chemisorption occurred with unit probability and the resultant ejected F atoms do not recombine on the chamber walls, then no FZ molecules would be reflected from the clean surface during the King and Wells measurement and the experimentally determined value of the initial sticking probability, (SO)expt, would be 1.O. This value would also be obtained if dissociative chemisorption occurred with unit probability. In these two extreme cases, however, different amounts of fluorine are left on the surface. We therefore carefully examine different scenarios which are consistent with the values of (SO)exptin section 1V.A. The King and Wells technique may also be used to obtain S ( O ) , the sticking probability as a function of the surface coverage of the impinging Since S(t) is defined as the time rate of change of the surface coverage 0 ( t ) of the incident species divided by the flux F of the incident species, we have
The saturation coverage 0, is defined to be
12866 J. Phys. Chem., Vol. 99, No. 34, 1995
Behringer et al.
0,= FJmS(t') dt' The quantity @(t)/@, therefore does not depend on the value of F. By using a surface technique such as XPS, one can directly measure @(t) and so obtain the coverage dependence of S. With a reflection technique, we instead measure an apparent sticking probability, (S(t))expt. This quantity is obtained from the data by dividing the instantaneous pressure change AP(t) (see Figure la) by the pressure change Ptotal due to the molecular beam. Using the relationship between (S(t))expt and S(t), derived in section IV, and eqs 1 and 2 to calculate (1 @/@,), we can generate a plot of S/So versus (1 - O/@Jd, from which one obtains the "order" d of the adsorption. The dependence of S/So on 0 is typically interpreted with the aid of kinetic models, as we discuss further in section 1V.B. 2. Time-ofFlight Mass Spectrometry. As a qualitative check of the King and Wells measurements, we have also measured the scattering of fluorine molecules using nonresonant multiphoton ionization with 193 nm light together with time-offlight mass spectroscopy (TOFMS). The essential idea of TOFMS is to ionize neutral species of interest and then count the resultant ions in order to obtain a measure of the neutral flux. The flux is then obtained by multiplying the signal by the velocity, since the probability for ionization is proportional to the density of the neutral species. In section II.A, we stated that the rotational temperature of our incident beam was 4 K. In the following, we describe this measurement in detail to illustrate the TOFMS technique (see Figure 2a). The F2 molecules in the incident beam were ionized with 2+1 photon REMPI via the F i n g (Y' = 2) X%,+ (Y" = 0) transition using light of wavelength 207 nm. The 207 nm light was produced by using the second harmonic from an injectionseeded, 10 Hz pulsed Nd:YAG laser (Quantel YG581C) to pump a dye laser with an intracavity etalon (Lambda Physik FL3002E). The dye laser output of 621 nm was doubled using an angleturned KDP crystal (Inrad Autotracker 11), producing 31 1 nm light. This 3 11 nm light is rotated 90" with a polarization rotator (Inrad) and then mixed with the residual visible light in an angleturned P-BaB204 crystal. The resulting 207 nm light is separated from the remaining fundamental and doubled light with dichroic mirrors and is focused into the UHV chamber with a 25 cm lens. The ions produced by the laser pulse are electrostatically accelerated into a drift tube and impinge on a set of microchannel plates (MCPs). The arrival of an energetic ion at the MCPs produces a charge pulse that is converted to a voltage pulse which can be counted with a gated integrator. The rotational distribution is obtained by varying the laser wavelength while firing the laser at a fixed time delay with respect to the opening of the chopper wheel in the molecular beam source. The rotational spectrum of the incident F2 is shown in Figure 2b. The resonant transition described above that was used to characterize the incident beam cannot be used to monitor the scattered molecules, since the resulting signal is below the noise limits of the detection scheme. Instead, an excimer laser (Lambda Physik) producing 193 nm light pulses, operating at 10 Hz, and synchronized to the opening of the chopper wheel in the molecular beam source is used to ionize some of the scattered F2 molecules. As described above, the arrival of an energetic F2+ ion at the MCPs produces a charge pulse which is converted to a voltage pulse and is then counted. By monitoring the pulse count rate as a function of the time delay between the molecular beam pulse and the laser pulse, a timeof-arrival spectrum can be generated. By comparing spectra obtained from the clean and fluorine-saturated surface of Si-
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(100)-2x 1, one can deduce certain qualitative aspects about the F2 adsorption.
111. Results A. Fluorine Data. In Figure 3a, we present a plot of the experimentally determined apparent initial sticking probability, (SO)expt, versus the incident energy, Ei, of the F2 molecules impinging at normal incidence onto n- and p-type doped Si(100)-2x 1 when the surface temperature was Ts = 300 K. The p-type sample had a nominal resistivity between 0.008 and 0.020 Q-cm while the n-type sample had nominal resistivity of 0.005 Q-cm. The data for the n- and p-type samples are essentially the same, indicating that dopant type does not affect the value of (SO)expt for these dopant concentrations. For the p-type sample, at the lowest incident energy, (SO)expt = 0.58, while at the highest incident energy, (SO)expt = 0.92; for the n-type sample, (SO)expt= 0.54 and 0.88 at the lowest and highest incident energies, respectively. A similar dependence on Ei is observed (although not shown, for the sake of clarity; see Figure 3b) when T, is increased from 300 to 900 K; we note that the value of (SO)expt increases with Ts for incident energies 0.05 eV < Ei < 0.40 eV, while for Ei outside this range, (SO)expt is independent of T,. In Figure 3b and c, we present the measurements of (SO)expt versus Ei for F2 impinging normally onto n-type doped Si(100)2 x 1 and Si( 111)-7x 7 at two different surface temperatures, 7'..
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J. Phys. Chem., Vol. 99, No. 34, 1995 12867
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= 300 K and T, = 900 K. (Data from the n-type Si(100)-2x 1 sample from Figure 3a are reproduced in Figure 3b). The n-type Si( 111)-7x 7 sample had a nominal resistivity of 0.005 Dcm.
For Si( 111)-7x7 with T, = 300 K, at the lowest incident energy, (SO)expt= 0.59, while at the highest incident energy, (So)expt = 0.93. A similar dependence of (&)expi on Ei is observed on both silicon surfaces when the surface temperature is increased from 300 to 900 K. Finally, the value of increases with T, in the range 0.05 -= Ei < 0.40 eV; we are the first to observe a variation of with T, for F2 adsorption on the silicon surfaces. In Figure 4a, we show the raw time-of-anival spectra acquired when 0.275 eV F2 is incident on the saturated and clean Si(100)-2x1 surfaces. The spectra show that the scattered intensity from the clean surface is smaller than that from the
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12868 J. Phys. Chem., Vol. 99,No. 34, 1995 3.1 3
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Figure 5. (a) Partial pressure of FZversus time during saturation exposure of the Si(100)-2x 1 surface to normally incident, Ei = 0.089 eV Fz.Here derived from the data in part a. (c) Partial pressure of Fz versus Bi = 0" and T, = 300 K. (b) Normalized sticking probability versus (1 time during saturation exposure of the Si(11 1)-7 x 7 surface to normally incident, Ei = 0.058 eV Fz. Again, Bi = 0" and T, = 300 K. (d) Normalized sticking probability versus (1 - @/O,)" derived from the data in part c. Note that d = 1 best describes the data, which suggests that a single site is required to initiate the chemisorption event. This is consistent with the stepwise chemisorption mechanism suggested in ref 10.
saturated surface. Unfortunately, these data are insufficient to accurately estimate the ratio of the scattered intensities. A feature of note in Figure 4a is the long-time tail in the timeof-arrival spectrum acquired from the saturated surface, which indicates the presence of an extrinsic precursor state, as suggested by Engstrom et ale9 This feature is emphasized in Figure 4b, where the spectra from Figure 4a are normalized to one another. We present typical measurements of the partial pressure of F2 during saturation exposure of Si( 100)-2x 1 and Si( 111)-7x7 to low-energy molecular beams in parts a and c, respectively, of Figure 5. The data from these measurements can be used to obtain the coverage dependence of S, which is shown plotted versus (1 - O/OJdfor d = 1 and 2 in Figure 5b and d. We note that one must take care to achieve saturation when performing these measurements. When using the King and Wells technique with species as reactive as fluorine, the level of signal to noise can make it difficult to determine when saturation is attained. Thus we have obtained data for different lengths of time after the initial exposure (not shown) to ensure that saturation has been attained. For both silicon surfaces, S
= (1 - @/a,) provides a better description of the data than S = (1 - O/O,)*. This is also true for the higher beam energies used in our experiments. B. Oxygen Data. We present the measurements of SOfor 0 2 normally incident on p-type doped Si(100)-2x 1 in Figure 6a (note that we have dropped the subscript expt from SO,since no abstraction occurs for 0 2 adsorption on silicon). The data show that SO is approximately 0.05 at the lowest incident energies and steadily increases with Ei to a value of roughly 0.50 when Ei = 0.62 eV. For Ei > 0.2 eV, the value of SO increases when T,is increased from 300 to 900 K. The Ei dependence of SO is shown in Figure 6b for 0 2 normally incident on n-type doped Si( 111)-7 x7. At the lowest beam energy, SOis 0.05; as E, increases, so does SO,until at Ei = 0.51 eV, SO attains the value 0.55. For the highest energy attained, Ei = 1.2 eV, SO = 0.9. The data for Ei = 0.8 eV appear to have artificially low values, which may be due to the large dispersion in the kinetic energy of the molecules in the beam pulses created with the hot S i c nozzle. That is, using the heated S i c nozzle to attain high translational energies produces beam pulses containing a broader range of translational
F2
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on Si( 100)-2x 1 and Si( 111)-7x7
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J. Phys. Chem., Vol. 99, No. 34, 1995 12869
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Reactions at the Chamber Walls 'rec, wall
6237
0.2
(i) Abstractive Chemisorption
0 0
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Ei, Incident Energy (eV)
Figure 6. Initial sticking probability SOversus incident energy Ei for 0 2 on (a) p-type Si(100)-2x 1 and (b) n-type Si(ll1)-7x7. For parts a and b, 8i = 0' and T, = 300 or 900 K.
energies. Thus the value of SO measured is a weighted average over different beam energies, the weights given by the beam intensity at a given energy. Since the beam pulses slowly decay with time, the lower energies make a significant contribution to the measured value of SO,as has been verified by performing these measurements with the S i c tube at room temperature and chopping these pulses. We note that SO increases when Ts is increased from 300 to 900 K for 0.2 eV < Ei < 0.5 eV. However, for Ei > 0.7 eV, SO remains constant when Ts is increased from 300 to 900 K.
IV. Discussion A. Fluorine Data: Energy Dependence. In Figure 3, we presented measurements of of F 2 on Si( 100)-2x 1 and Si( 111)-7x7 as a function of Ei for different Ts. To illustrate how the presence of abstractive chemisorption affects the interpretation of the data obtained with the King and Wells technique, we develop a crude model of the measurement process in the form of a reaction sequence, as shown in Figure 7a. The sequence begins with an intact F2 molecule incident on a silicon surface, includes reactions with the silicon surface
7
'diss ' d i s s , wall
(ii) Dissociative Chemisorption
(iii) Non-reaction
(v) Abstractive Recombination
'NR, wall
(vi) Atomic Reactions
Figure 7. (a) Reaction sequence for F2. Only the encounter with the sample surface and the first encounter with the vacuum chamber walls are included. The subscripts NR, abstr, diss, abstr rec, and rec refer to nonreactive scattering, abstractive chemisorption, dissociative chemisorption, abstractive recombination, and recombination, respectively. The subscript wall refers to processes that occur at the vacuum chamber walls. Three pathways lead to the presence of F2 after the encounters with the surface and the chamber walls. (b) Schematic diagrams of the different processes described in part a.
and the chamber walls, and ends with either adsorption or detection in the quadrupole mass spectrometer. The different types of reactions included are abstractive chemisorption, dissociative chemisorption, abstractive recombination, and recombination, which are shown schematically in Figure 7b.
Behringer et al.
12870 J. Phys. Chem., Vol. 99, No. 34, 1995 Within this model, we obtain the relationship between the apparent initial sticking probability derived from the experimental data, (So)expt, and the true initial sticking probability SO (see Appendix): (3) where Sabs[r is the probability that F2 undergoes abstractive chemisorption at the clean silicon surface and a is a constant depending on the probabilities of reactions of F2 and F with the vacuum chamber walls. The true initial sticking probability SOis given by
= o*5Sabstr + sdiss
(4)
where SdiSsis the probability that F2 undergoes dissociative chemisorption at the clean silicon surface. An important, related quantity is the total probability of reaction SR,which we define to be
The result expressed in eq 3 shows immediately that if abstractive chemisorption occurs, then the value of the apparent initial sticking probability derived from a King and Wells measurement, (SO)expt, can differ from SO. Conversely, if abstractive chemisorption does not occur, then (SO)expt = SO.The values of (SO)expt and SOare also identical in a few other special situations; if the conditions for these situations are not satisfied, then a reflection measurement will yield a value of (SO)expt which is most likely larger than the true initial sticking probability SO (which can be obtained by a surface technique such as XPS). 1. Si(lll)-7x7. We begin by briefly noting the results of recent experiments which bear on the analysis which follows. Jensen et al. have obtained STM images after dosing Si( 111)7 x 7 with low-energy, normally incident, monoenergetic molecular beams of F2.I3 In the STM images, single-site adsorption indicative of abstractive chemisorption was prevalent at low incident energies. An analysis employing a simple Monte Carlo model indicates that Sabs@/SR is nearly unity for Ei = 0.03 eV F2 on Si( 111)-7x7, demonstrating that abstractive chemisorption dominates at low thermal energies and decreases as the incident energy is increased. Physically, the increase of dissociation is due to the increased momentum of the incoming molecule, which allows the trailing atom to make a closer approach to the surface and find a dangling bond with which to react.I0 To summarize, the STM experiments of Jensen et al. unambiguously show that abstractive chemisorption is the dominant adsorption mechanism at low incident energies (i.e., Sabs@/SR is nearly unity) and that the probability for abstraction decreases with increasing incident energy on Si( 111)-7x7. With regard to the crude model described above, the experiments of Jensen et al. require that the value of S a b s d S R be nearly unity. The only other requirement is that SRalways be greater than (SO)expt.Details about how we use the model to reproduce the data from Figure 3c can be found in the Appendix. However, we note here that, when modeling the data from Si(1 11)-7x7, we assume that the total reaction probability SR increases with incident kinetic energy E,. In Figure 8a and b, the results of the model are shown assuming different values of &,,,,in, the value of SRat the lowest beam energy. Since the values of &s[r are between 0 and 1, these results show that the model can reproduce the data with many different values of SR,,,,,,, However, . since the S T M results require that Sabstr decrease with increasing incident energy, the value of SRmust exceed 0.75, as one can infer from Figure 8a and b (e.g., for
= 0.70, Sabs[r attains a maximum with increasing E,, in contradiction to the STM results. Therefore must be greater than 0.70). We therefore conclude that the data presented in Figure 3c are qualitatively consistent with the STM data of Jensen et al. and that the total reaction probability, SR, is greater than 0.75. 2. Si(l00)-2xl. In this subsection, we discuss the predictions of the model for a variety of assumptions. We will first attempt to reconcile the experimental results given in this paper with those of Engstrom et al.9 and the theoretical results of Carter et al.,’O all obtained on the Si(100)-2xl surface. We will then adopt a “purist experimentalist” approach and discuss the results obtained with the model when the theoretical results of Carter et al. are ignored. Within the latter approach, we will find that the model can reproduce the data for a wide range of input parameters. However, when the results of recent scanning tunneling microscopy experiments on Si( 111)-7x 7 are taken into account,I3the range of input parameters is reduced and we will suggest that abstractive chemisorptionis the major reaction mechanism for incident energies E, < 0.1 eV for F2 adsorption on the clean Si(100)-2x 1 surface. As noted in section I, Engstrom et al. used X P S to measure SOof F2 on Si(lOO)-2xl and found that SO= 0.46 f 0.02, independent of incident energy (E, < 0.83 eV) for F2 beams incident at 8, = 75’ measured from the surface normal (the normal energy for E, = 0.83 is En = E, cos2 8, = 0.056 eV).9 In addition, Carter et al. found that the total reaction probability of F2 with Si(100)-2x1 increases from 0.93 to 0.97 (with an absolute uncertainty as large as 0.10) as the normal energy of the incident molecules is increased from 0.078 to 0.91 eV. Using the results of Carter et al. as a guide,28we can choose a value of SR which is consistent with the value of SO measured by Engstrom et d. Solving eqs 4 and 5 for Sabsv in terms of SR and SO, one obtains Sabstr = 2(& - SO). The upper limit on Sabstr is Set by the condition Sabstr = SR,in which case we obtain Sabstr = 0.92 (and Sdlss = 0). For now, the lower limit for Sabstr is set by the theoretical results for SR;assuming SRas small as 0.82, then Sabstr = 0.72 (and Sdlss = 0.10). Using the results of the preceding paragraph, the data presented in Section IKA, and the crude model described in the Appendix, we obtain values for SabsW as a function of E, for two distinct cases. In the first case, we assume SR = 0.92, independent of E,. If we use Sdiss = 0 and Sabs@ = 0.92 to reproduce the value of (SO)expt at the lowest beam energy ((S0)expt = 0.54 when E, = 0.067 eV), we obtain a = 0.087 (see eq 3). Implicit here is the assumption that the value of (SO)expt measured at the lowest beam energy should be reconciled with the value of SOmeasured by Engstrom et al. although these measurements were performed at slightly different normal energies. Assuming that a is also independent of E,, we use the model to obtain values for Sabs@. In the second case, we assume that SR= 0.82 at low E, and increases to 1.OO at high E, (see Appendix). The results for these two cases are shown in Figure 8c. We now adopt the purist experimentalist approach, in which we disregard the theoretical results of Carter et al. Within this approach, we still require that the apparent initial sticking probability measured at the lowest beam energy be reconciled with the value of SO measured by Engstrom et al. The only other restriction is that SRalways be greater than (S0)expt. Thus SR,mln, the Value Of SRat the lowest beam energy, can now take on a larger range of values than we assumed above. We have tried different values of (which, together with the value of SO= 0.46, sets the value of a),as described in the Appendix. The main result is that the model can reproduce the data for nearly the entire range of allowed SR,,,,,~ values. This is SR,mln
F2 and 0 2
on Si( 100)-2x 1 and Si( 111)-7 x 7
J. Phys. Chem., Vol. 99, No. 34, 1995 12871 F, --> Si(100)-2xl 1.2
1
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Ei (ev)
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EL(ev)
Figure 8. Plot of calculated values of Sabstr versus Ei, obtained from using eq 3 together with the experimental data, and the corresponding assumed values of (a and b) for Si(1 11)-7 x 7 and (c and d) for Si(100)-2 x 1. &,,,,in refers to the value of SRat the lowest beam energy. Note that the solid squares in part c were obtained by setting SRto the constant value of 0.92. In parts a and b, the quantity Sabstr/SR.mln is set to 0.95 for the lowest beam energy of the data set. In parts c and d, the values of Sabstr predicted by Carter et al.IOfrom molecular dynamics simulations are also shown. In parts b and d, the calculated values of Sabslrdo not decrease monotonically with E,, which is inconsistent with the experimental results of Jensen et al.I3 This implies that, within the model, the total reaction probability SRmust be greater than 0.75. Error bars derived from the uncertainties in the data shown in Figure 3 are shown for only one set of data in each plot for the sake of clarity.
demonstrated in Figure 8c and d, which shows that sabstr assumes values between 0 and 1 when the value of is between 0.92 and 0.61. Although nearly any allowed values of SR,min can be used to reproduce the data, the analysis may be pushed further by considering the results of Jensen et al.I3 In particular, we expect that the probability for abstractive chemisorption, Sabstrr for FZ incident on Si( 100)-2x 1 will decrease with increasing incident kinetic energy E,, just as on Si(l11)-7x7. If this expectation is fulfilled, then we require SR> 0.75 at the lowest beam energy, shown by the results of Figure 8c and d. Thus, since we expect the probability for abtraction to decrease with increasing incident energy, we suggest that abstractive chemisorption is the major adsorption mechanism for incident energies Ei < 0.1 eV. We note that one would expect the value of Sabstr/SR to be lower on Si( 100)-2x 1 than on Si( 111)-7x 7 because the dangling bonds are more closely spaced on the Si(100)-2x 1 surface and dissociative chemisorption occurs preferentially on closely spaced pairs of dangling bonds.I3 Thus the recent results of Li et a1.I2 are consistent with this expectation. We note, however, that the results presented here are at variance with those presented by Li et al., a situation which we cannot presently explain. The values of SOwhich result from the above analysis increase with Ei,in seeming constrast to the previous measurements of Engstrom et al. Although one might claim that variations in sample preparation might account for the observed differences, the samples for all of these studies were prepared in a similar
way (Ar+ ion sputtering followed by annealing). Additionally, molecular dynamics simulations on defective Si( 100)-2x 1 surfaces indicate that the value of SOis insensitive to the presence of steps or defect^.^^.^^ The data of Engstrom et al., obtained with XPS, show that SO is independent of incident energy for normal energies of 0.005 < E,, < 0.056 eV. The data presented in this paper were obtained with larger values of E,,. We therefore suggest that the data presented here and the data of Engstrom et al. are complementary, since they were obtained over different ranges of normal energies. 3. Comparison of F2 Adsorption on Si(100)-2x1 and Si(III)-7x 7. It is somewhat surprising that the data for Si(100)2 x 1 and Si( 111)-7x 7 (see Figure 3) are so similar because the number of dangling bonds per unit area on these two surfaces differs by more than a factor of 2 and because one might expect the total reaction probability SR to be correlated with this number, as is the case for 0 2 adsorption." An explanation for the unexpected similarity of the data may arise from an absence of strong molecular alignment during the reaction of the F2 molecule with the dangling bonds and from the large corrugation of the Si(ll1)-7x7 surface. Since the molecules impinging on the surface are randomly oriented, most of the molecules hit the surface in a roughly "side-on" orientation such that the molecular bond axis is approximately parallel to the surface plane. In the absence of orientation during the formation of the Si-F bond, the trailing F atom will be ejected roughly parallel to the surface, Le., along the former F-F bond axis. This implies that the ejected F atom travels a considerable
12872 J. Phys. Chem., Vol. 99, No. 34, 1995 distance while it is still near the surface, which increases the probability that the atom will encounter a dangling bond. In such a scenario, the number of dangling bonds per unit area is unimportant. The preceding argument relies on the assumption that the F2 molecule encounters a dangling bond on the incoming trajectory. However, on the Si( 111)-7x 7 surface, the dangling bonds are oriented normal to the surface plane and are relatively widely separated (the smallest adatom-adatom distance is 6.9 A, and the smallest restatom-adatom distance is about 5 A). It is therefore possible that the incoming F2 (with bond length 1.5 A) will not encounter a dangling bond. (This seems most likely for incoming molecules with bond axes oriented roughly normal to the surface plane.) Because the corrugation of the surface is large, however, a significant component of parallel momentum may be imparted to the unreacted scattered molecule, thereby increasing the probability that the molecule will encounter a dangling bond during the outgoing trajectory. Thus, for F2 incident on Si(ll1)-7x7, it would be likely that an impinging molecule encounters a dangling bond on either the incoming or outgoing trajectory, which would produce the observed similar reactivities of F2 on both Si( 100)-2x 1 and Si( 111)-7x 7. We note that charge transfer is not likely to be important here, since the reactivity is independent of the dopant type, as shown in Figure 3. B. Fluorine Adsorption: Coverage Dependence. We have measured the partial pressure of F2 while exposing initially clean Si(100)-2x 1 and Si(ll1)-7 x7, as shown in Figure 5a and c. These data can be used to obtain an experimentally derived sticking probability (S(t))expt, as described in section IIC.1. Since abstractive chemisorption occurs, (S(t))expt differs from S(t). The relation between the two quantities is
where a is a constant dependent on the rates of different reactions which occur on the vacuum chamber walls (see Appendix). In principle, one must therefore know Sabstr(f) to derive S(t) from the experimental data. However, if d a b s t r ( f ) 0.1 eV. The increase with Ts in the case of F2 is consistent with a surface dynamical effect whereas for 0 2 the increase may be explained by the existence of a negative ion-like intermediate state. Acknowledgment. E.R.B. thanks Lany Carter for useful discussions. We thank the National Science Foundation (Grant NSF-DMR-9307259) and the Air Force Office of Scientific Research (Grant AFOSR-F496209410075) for supporting this research. Appendix In this appendix, we develop the crude model illustrated in Figure 7 to interpret the King and Wells measurements when
J. Phys. Chem., Vol. 99, No. 34, 1995 12873 abstractive chemisorption occurs, e.g., for F2 incident on silicon. In this model, it is assumed that the reactions at the walls of the vacuum chamber occur at constant rates during the entire SOmeasurement. Since the incident F2 molecule either does or does not react with the silicon surface, the sum of the probabilities for reaction and nonreaction is unity: SR SNR= 1. The total probability that the molecule will react with the clean silicon surface, SR,is equal to the sum of the probabilities for abstractive and dissociative chemisorption, i.e.,
+
The initial sticking probability-that is, the number of pairs of F atoms left on the surface divided by the number of pairs of F atoms incident-is
= o*5Sabstr
0'
+ 'diss
(A2)
However, experiments utilizing reflection techniques can yield an apparent value of the initial sticking probability, (SO)expt, which differs from SO. From Figure 7, the number of F2 molecules left in the vacuum after the beam pulse interacts with the silicon surface and once with the chamber walls near the beam source is NSNR'NR.wall
+ N'abstr(Sabstr
rec.wall
+ o.5Srec,wall)
(A3)
where N = the number of F2 molecules constituting the beam pulse, SNR= the probability that F2 scatters intact from the clean silicon surface, S N R = , ~the~ probability ~~ that F2 scatters intact from the chamber walls, Sabstr = the probability that F2 undergoes abstractive chemisorption at the clean silicon surface, Sabstr rec,wall= the probability that an abstracted F atom undergoes abstractive recombination at the chamber wall, and Srec,wall = the probability that an abstracted F atom recombines with another abstracted F atom at the chamber wall. The number of molecules left in the vacuum after an identical beam pulse interacts with the quartz beam flag and once with the chamber walls near the beam source is NSNR,wall, since no F2 molecules adsorb onto the flag. Thus the experimental value of the initial sticking probability, which is the difference between the number of molecules remaining after encountering the flag and chamber walls and the number of molecules remaining after encountering the clean silicon surface and chamber walls divided by the number of molecules remaining after encountering the flag and chamber walls, is ('0)expt
= (NSNR.wall - NSNRSNR.wall NSabstr(Sabstr
rec,wall
-
+ o~5'rec,wall))~N'NR.wall
(A4)
Solving eq A1 for SNRand substituting the result into eq A4 yields ('0)expt
= 'diss + 'abstr(l
-
('abst;
rec,wall
+ o~5'rec,wall~~SNR.wall~ (A5)
Using eq A2, eq A5 can be expressed in terms of the initial sticking probability, SO: ('0)expt
= SO
+ Sabstr(o*5 (Sabstr rec,wall
or more simply as
where
+ o~5Srec,wall)~SNR,wall) (A6)
Behringer et al.
12874 J. Phys. Chem., Vol. 99, No. 34, 1995
Equations A7 and A8 may also be written in terms of SR:
(‘49) where
The quantity b can also be expressed in terms of the pumping speed time constant for F atoms, ZI, and the recombination time for F atoms on the chamber walls, tree. We reiterate that an implicit assumption in this simple model is that the wall reactions occur at constant rates for the duration of the measurement and that, after the first encounter with the wall, no recombination reactions occur. The result expressed in eq A7 (or eq A9) shows immediately that if abstractive chemisorption occurs, then the value of the initial sticking probability derived from a King and Wells measurement, can differ from SO(or SR). Conversely, if abstractive chemisorption does not occur, then (S0)expt = SO. The values of (SO)expt and SOare identical in two other limiting situations: (1) if the chamber walls are completely passivated to reaction with F2, no abstractive recombination occurs, and recombination of abstracted F atoms on the walls is completely efficient, i.e., if SNR.wall = 1, Sabstr rec,wall = 0, and Srec.wall = 1; or (2) if the chamber walls are completely passivated to reaction with F2, abstractive recombination occurs with probability 0.5, and recombination of abstracted F atoms does not occur, i.e., if S N R , W ~ I= I 1, Sabstr rec,walI = 0.5, and Srec,wall = 0. In almost all other situations, a reflection measurement will yield a value of (SO)expt which is most likely larger than SO. To obtain the results shown in Figure 8, we assumed that all FZincident on the chamber wall near the beam source scattered intact and also that abstracted atoms did not recombine with another abstracted atom on the chamber wall. That is, we assumed that S N R = . ~1 and ~ ~Srec,wall ~ = 0. Although we cannot experimentally verify our choices of the values of SNR,wall and Srec,wall, these values are at least plausible because (1) a large probability of nonreactive scattering would be expected for F2 incident on chamber walls near the beam source which are nearly saturated with fluorine and (2) a small probability of recombination is consistent with the fact that the scattered intensity is distributed over such a large area that the probability for two abstracted F atoms to encounter one another on the chamber wall is very small. When trying to reproduce the data from the Si(100)-2x1 surface, we may let SR assume values between roughly 0.54 and 0.92 at the lowest beam energy. We define SR,min to be the value of SR at the lowest beam energy. We assume, for simplicity, that the total reaction probability is given by SR(E,) = SR,min + (1.0 - SR,min)(l.O - exp(-4(Ei - &“/(6nax E m m ) ) ) , where E m i n (Emax) is the lowest (highest) beam energy of a given data set (Le., for Si( 100)-2x 1 or Si( 111)-7x 7). Since we will impose the requirement that the data obtained at the lowest incident energies be reconciled with the value SO= 0.46 f 0.02 measured by Engstrom et al., we accordingly adjust the quantity b. For example, assuming SR = 0.92 implies a =
0.087, this value of a corresponds to Sabstr rec,wall = 0.413 (see eq A8). We note that if the value of b is nearly zero (OS), then trec >> ti (tree
