5532
J. Phys. Chem. 1995,99, 5532-5539
Effect of Incident Translational Energy on the Saturation Coverage of and Si(lll)-7x7
F2
on Si(100)-2x1
E. R. Behringer, H. C. Flaum, and A. C. Kummel" Department of Chemistry, University of Califomia at San Diego, 9500 Gilman Drive, La Jolla, Califomia 92093-0358 Received: October 17, 1994; In Final Form: January 18, 1995@
We have studied the dependence of the saturation coverage of fluorine on the Si( 100)-2 x 1 and Si( 111)-7 x 7 surfaces upon F2 incident energy by measuring the desorption yields of SiF2 and SiF4 as a function of Fz exposure generated by normally incident, nearly monoenergetic molecular beams. When the F2 molecules have incident translational energy Ei = 0.060 eV, the desorption yields rapidly approach limiting values with increasing exposure. However, when Ei = 0.275 eV, the SiF4 desorption yields on both surfaces slowly approach limiting values which are increased by nearly a factor of 4 over that for Ei= 0.060 eV. These observations indicate that although there is no barrier to the dissociative adsorption of F2 onto the Si dangling bonds of the clean surfaces, there exists a barrier, between 0.060 and 0.275 eV, to the insertion of fluorine into the most vulnerable Si-Si backbonds on the saturated silicon surfaces.
I. Introduction Molecular beam studies of adsorption on single-crystal surfaces have shown that the incident translational energy E, of the adsorbing molecules plays an important role in determining the adsorption mechanism.'x2 For example, for small values of E,, adsorption via a physisorbed precursor state may prevail, while for larger values of E, the incident molecules may directly chemisorb, perhaps even dissociatively. Because of the different natures of the mechanisms, the adsorbate structures can depend on the incident energies of the molecules used to dose the surface. Such a dependence on E, has recently been demonstrated for the C12lSi(111)-7x 7 ~ y s t e m thus ; ~ it may be possible to control the adsorbate structures by varying E,. One may also control the saturation adsorbate coverage by varying E, as has been demonstrated previously for adsorption on metals at elevated temperature^.^ Essentially, different reaction pathways become accessible as E, is increased. For a given value of E,, the accessible pathways will be followed until the surface becomes passivated due to the presence of reaction products, resulting in a saturation coverage of the incident species. If E, is increased to make another reaction possible, the saturation coverage may be increased. Certain systems which possess negligible barriers may never attain a saturation coverage but rather a steady-state coverage. This is especially true of technologically relevant systems for which the goal is the removal of substrate material, Le., etching. For example, fluorine can etch silicon surfaces spontaneously at 300 K if delivered to the surface as either F atoms or XeF2 (not F z ) . ~For such atomic fluorine dosing, it is found that adsorption on silicon is not limited to a monolayer and that a complex fluorosilyl layer develops as the fluorine exposure is increased.6 The corresponding physical picture derived from other energy calculations,'O-l and molecular dynamics simulation^'^^^^ is that F atoms impinging on the clean surface swiftly saturate the available dangling bonds and that the large exothermicity of Si-F bond formation produces local heating of the silicon lattice which eventually results in the formation of vacancy defects. These defects apparently permit etching reactions to c o n t i n ~ e . ~Other . ~ energy calculations suggest the existence of other bonding geometries which may @
Abstract published in Advance ACS Abstracts, March 15, 1995.
0022-365419512099-5532$09.0010
play a role in the etching p r o c e ~ s . ' ~ - 'Earlier ~ molecular dynamics studies of 3.0 eV F atoms incident on Si(100)-2xl have shown etching via the formation of SiF4 and Si2F6,173'8 in agreement with experiment. If the silicon surface is dosed by a fluorine-containing molecular species in which the F atoms are more strongly bound, then it is possible that a barrier could exist which would not only inhibit spontaneous etching but would also allow control of the saturation coverage with incident energy. In this paper, we demonstrate this phenomenon using thermal desorption measurements of the reaction products which result when nearly monoenergetic molecular beams of F2 impinge on the Si( 100)2 x 1 and Si(111)-7x7 surface^.'^.^^ Formation of a saturation coverage on these surfaces may be useful for developing "digital" etching processes in which an integral number of atomic layers are removed per etch c y ~ l e . ~ 'For - ~ ~example, attaining a saturation coverage of reaction products is important for maintaining uniformity of the etching rate across a large area during digital etching. A previous, exhaustive experimental study of the adsorption of fluorine (F2 and F) on the Si(100) surface was performed by Engstrom et a1.26 In that study, molecular beams with E, in the range 0.07 < E, < 0.83 eV were directed at Si(100) with an incident angle 0, = 75" measured with respect to the surface normal. The largest incident energy corresponds to a "normal" energy (E, = E , cos2 0,)of less than 0.06 eV. They determined that the adsorption of FZ must be dissociative and that the thermal desorption products resulting from exposure to beams containing either F2 or both F2 and F were SiF2 and SiF4, with SiF2 being the major reaction product. They also found that the SiF4 yield increases monotonically as the initial exposure is increased. Carter et al. performed molecular dynamics simulations of F2 impinging on the Si( 100)-2x 1 surface27and predicted that the initial sticking probability should slowly increase with E,. For E, < 0.1 eV, they found 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, and the energy liberated in that reaction goes to the trailing F atom which is then ejected into the vacuum. This mechanism was first observed experimentally by Ceyer et aL2* Dissociative 0 1995 American Chemical Society
Saturation Coverage of F2 on Si( 100)-2x 1 and Si( 111)-7x7 chemisorption differs only in that the trailing F atom also remains on the surface. As Ei is increased, Carter et al. found that the probability of dissociative chemisorption increases. Scanning tunneling microscope (STM) images providing evidence for this have recently been obtained.29 Finally, Carter et al. also found that the probabilities for abstraction and dissociative chemisorption are quite insensitive to the presence of steps and defects on the Si(100)-2x 1 ~ u r f a c e . ~ ~ - ~ ' Some results of an earlier, unpublished study by Ceyer et al. have appeared recently in a review article.20 In that study, thermal desorption measurements were used to determine the coverage of SiF2 and SiF4 on Si(100)-2xl as a function of incident energy. An activation barrier of 0.16 eV for increasing SiF4 formation was measured, and, using helium atom scattering, it was determined that this threshold was associated with insertion of fluorine into the Si-Si dimer bonds and Si-Si backbonds. The SiF4 yield also scaled with normal energy, indicating that the barrier is along the surface normal. The results that we present in this paper are consistent with these earlier findings and extend the study to the Si( 111)-7x 7 surface. We have measured the desorption yields of SiF2 and SiF4 resulting from exposure of the Si( 100)-2x 1 and Si( 111)-7x 7 surfaces to nearly monoenergetic molecular beams of F2 of different translational energies. Upon increasing the incident kinetic energies of the impinging F2 molecules from 0.060 to 0.275 eV, the limiting value of the SiF4 desorption yield is increased. This indicates the existence of a barrier, between 0.060 and 0.275 eV, to the insertion of fluorine into the Si-Si bonds on the saturated surfaces. In section II, we describe the experimental techniques for measuring these yields and in section I11 present the data. We discuss the data in section IV and conclude with a summary in section V. The work presented here is a part of a larger study of fluorine adsorption on silicon
surface^.^^^^^ 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 . ~ This ~ - ~ ~system consists of two major parts: a molecular beam source and an ultrahigh-vacuum (UHV) chamber. Details which are relevant to the experiments described in this paper are given below, along with a description of the procedures for sample preparation and for measuring the desorption yields. 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, chopper motor, and gate valve, and chamber 3 houses a collimating aperture. The source gas for the F2 molecular beam is composed of F2 seeded in either He or Ar and is expanded through a pulsed nozzle valve with a 2.0 mm aperture. The different gas mixtures were obtained from Spectra Gas: 5% F2 in He and 20% F2 in Ar. The 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) and hence the velocity and kinetic energy of the molecules constituting the pulse can be determined. The beam is verified to be nearly monoenergetic (i.e., has a large speed ratio, defined as the range of particle velocities divided by the mean velocity of the particles constituting the beam pulse) since the F2 beam has a rotational temperature of 4 K as measured using resonantly enhanced multiphoton i ~ n i z a t i o n . ~ ~ B. Main Chamber. The main UHV chamber has two tiers and has a base pressure (after bakeout) of (6-7) x lo-" Torr.
J. Phys. Chem., Vol. 99, No. 15, 1995 5533 The upper tier contains the molecular beam, a quartz beam flag, and a quadrupole mass spectrometer (UTI lOOC). 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). C. Sample Preparation. Just before being placed into the main chamber, the n-type silicon samples (Virginia Semiconductor, nominally 0.005 SZ cm) were prepared by etching in a hot acid bath of H2S04:H202 (1:2 ratio) for 15 min followed by 15 s in a solution of HF and HPLC water (1:25 ratio) and finished by a 15 s immersion in HPLC water. The samples were cleaned by sputtering with 2 keV Ar+ ions at normal incidence followed by annealing to a sample temperature Ts = 1100 K, measured with an infrared thermometer and assuming an emissivity of 0.7 for the silicon sample. The samples were radiatively heated by passing altemating current through two thoriated iridium filaments which are 0.006 in. thick and 0.125 in. wide and approximately 0.75 in. long. This method of sample heating results in uniform temperatures across the central portion of the sample as verified with the infrared thermometer. The sampes were clean to within the sensitivity of AES (C:Si < 1 5 0 and 0:Si < 150) and the LEED pattems characteristic of the different reconstructions were sharp and bright. As in previous studies by other researcher^,^' we found that AES is at best difficult to use to monitor surface fluorine since the impinging electron beam rapidly desorbs fluorine from the surface. D. Desorption Measurements. We have measured the desorption yields of SiF2 and SiF4 as a function of F2 exposure generated with the molecular beam source (section 1I.A) using the following procedure. A clean, annealed sample was exposed to a pulsed, normally incident, nearly monoenergetic molecular beam of F2. During the exposure, the partial pressure of F2 in the main UHV chamber was monitored using the quadrupole to provide a measure of the Fz flux incident on the sample. Afterward, the sample was rotated to face the quadrupole mass spectrometer and then radiatively heated to thermally desorb the reaction products. For all of the desorption spectra shown in this paper, the altemating current through the thoriated iridium heating filaments was manually increased from 0 to 12 A over a time interval of 6-7 s beginning at time to = 10 s; the current was manually decreased from 12 to 0 A at time tf = 70 s. While the sample was heated, the ion currents due to d q = 66 (SiF2+) and d q = 85 (SiF3+) were simultaneously measured as a function of time with the quadrupole, which was operated with an emission current of 2.63 mA and an electron energy of 105 eV. After acquiring the desorption spectra, the sample was then rotated away from the quadrupole and annealed at Ts = 1100 K for 1 min before attempting a different FZ exposure. Winters and Coburns have found that SiF2+ is the major cracking product of SiF2, constituting 0.6-0.7 of the SiF2 cracking products, and that SiF4 is cracked with nearly unit efficiency to make SiF3+. Thus, by monitoring the SiF2+ and SiF3+ ion currents, we obtain a measure of the SiF2 and SiF4 evolved from the sample surface. Spot checks of the ion current for mlq = 47 (SF+) produced spectra identical to that for m/q = 66 (SiFI+), consistent with the previously observed cracking of SiF2.5 A typical pair of desorption spectra are shown in Figure la. The features of the desorption spectra which are due to desorption from the Si surface are labeled (arbitrarily) with italicized letters. Comparison of the two spectra suggests that the smaller peaks in the SiF2 spectrum occurring at t = 20 and t = 25 s are due to the cracking of SiF4. After completing the desorption measurements, we vented
5534 J. Phys. Chem., Vol. 99, No. 15, 1995
Behringer et al.
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Figure 1. (a) Ion current versus time spectra for SiF2+ and SiF3+ obtained after exposing the Si( 111)-7x 7 surface, at 325 K, to a normally incident beam of 0.060 eV Fz. The initial dose was 8.51 ML, where 1 ML corresponds to 3.0 x 10l4atoms/cm2. The heating current was increased from 0 to 12 ac amps beginning at ro = 10 s and was decreased from 12 to 0 ac amps beginning at rf = 70 s. The features of the spectra arising from desorption from the silicon surface are labeled by italicized letters. (b) The surface temperature T, versus time t profile used in the desorption measurements. Triangles indicate measurements, and the solid line serves as a guide to the eye. (c) Same spectra as in (a) with the abscissa converted to temperature using the data shown in (b).
the UHV chamber and used ceramic cement (Omega CC hightemperature cement) to affix a chromel-alumel thermocouple, fabricated from wire of diameter 0.005 in., to the front of the sample. After pumping the chamber to obtain high vacuum, we then measured the sample temperature T, versus time t while heating the sample in the same way as for the desorption measurements. The accuracy of these measurements is no better than f 5 0 K since the recorded values depended on the placement of the thermocouple, thermal hysteresis, etc. The data are shown in Figure lb; the solid line is intended as a guide to the eye. The profile of Ts versus t is clearly nonlinear; we therefore make no attempt to derive kinetic information from the desorption spectra. The data shown in Figure l b permit the conversion of ion current versus time spectra to ion current versus Ts. Converted spectra corresponding to those in Figure l a are shown in Figure IC. The major peak in the SiF2 spectrum occurs at approximately 720 K; this is to be compared to the value of 800 K obtained by Engstrom et a1.26 Since T, is a sensitive function o f t (note that the heating rate at t = 25 s is approximately 20 Ws),and since we are only concerned with qualitative trends here, we present the desorption spectra as ion current versus time. When trying to reproduce an individual spectrum exactly, time shifts of the desorption peaks of up to 2 s were obtained. The desorption yield is defined to be the area underneath the ion current versus time spectrum over the time interval during which the sample is being heated (10 s < t < 70 s). The desorption yields of SiF2 are corrected to take the cracking of SiF4 into account. On the basis of spectra obtained for long exposures, we estimate the cracking fraction of SiF4 to SiF2 to be 0.11. We have not attempted to correct the yields for the relative efficiencies of SiF2 and SiF4 detection. However, if we assume that the pumping speed for SiF2 is 2 orders of magnitude greater than that for SiF4, as determined by Engstrom et al., then we would conclude that the SiF4 yield is only a few percent of the SiF2 yield. We have no experimental justification for making such an assumption. Finally, we note that SiF2 is known to react on the chamber walls to produce SiF438and that the SiF4 desorption spectra (see below) indeed display a very small peak that tracks the major peak in the SiFz spectra. We have not attempted to correct the SiF4 yields for this effect. The product of the average partial pressure of F2 during exposure of the sample to the molecular beam and the time of exposure is proportional to the dose. We choose to normalize this measure of the dose in the following way. First, for a given flux, we determine the time at which the sticking probability of F2 decreases to one-tenth of its initial (clean surface) value. On the basis of other measurement^,^^ we assume that the sticking coefficient S depends linearly on the coverage. The time dependence is then given by S(t) = So(1 - exp(-S&/ Osat)),where So is the initial sticking coefficient, F is the flux of F2 incident on the surface, and Os, is the saturation coverage of fluorine. We have also independently measured SO for F2 on the Si(100)-2x 1 and Si( 111)-7x 7 surfaces and found that for E, = 0.060 eV, SO = 0.58 while for E, = 0.275 eV, SO = 0.78.32 The assumption of a linear dependence of S(t) on coverage and knowledge of SO allow one to assign a dose, in units of saturated monolayers (for Si(100)-2x 1, a saturated monolayer corresponds to one fluorine atom per dangling bond, or 6.8 x lOI4 atoms/cm2), to the time at which S(t) = 0.1So: for E, = 0.060 eV, this dose is 3.97 ML; for E, = 0.275 eV,2.95 ML. These values (which change by less than 10% when assuming S to have a second-order dependence on coverage) serve to normalize the dose measurements.
J. Phys. Chem., Vol. 99, No. 15, 1995 5535
Saturation Coverage of F2 on Si( 100)-2 x 1 and Si( 111)-7x 7
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Figure 2. (a) Selected desorption spectra of SiF2 from Si(100)-2x 1 resulting from different initial exposures to a normally incident FZ molecular beam with E, = 0.060 eV. T, = 325 K during dosing. Here 1 ML corresponds to 6.8 x 1014 atoms/cm2. The spectra are offset by arbitrary amounts for clarity. (b) Same as (a) but for SiF4. (c) Selected desorption spectra of SiF2 from Si(100) resulting from different initial exposures to a F2 molecular beam with Ei = 0.275 eV. The spectra are offset by arbitrary amounts for clarity. (d) Same as (c) but for SiF4.
The reproducibility of the main features of the spectra was checked. We found that the magnitude and the width of the peaks in the spectra depended on the exposure history of the sample, and we took care to expose the sample in such a way as to obtain spectra that would result from exposing a freshly sputtered and annealed surface. The history dependence displayed by the desorption spectra are indicative of structural changes at the surface, but we do not have direct observations of pitting, roughening, etc.; these effects are best studied with a STM. 111. Results
In Figure 2, we present desorption spectra of SiF2 and SiF4 measured after different exposures of n-type Si( 100)-2x 1 to nearly monoenergetic beams of F2. We arbitrarily choose to label those features of the spectra which are due to desorption from this surface with plain Greek letters for Ei = 0.060 eV (Figures 2a and 2b) and primed Greek letters for E, = 0.275
eV (Figure 2c,d). As shown in Figure 2a, a single peak a appears in the desorption spectra of SiF2. This peak shifts to increasingly later times as the dose is increased; these shifts are probably largely an artifact of the method of data acquisit i ~ n .Two ~ ~ peaks, p and y , are observed in the desorption spectra of SiF4, as shown in Figure 2b. A peak occurring at later times in Figure 2b shifts to later times with increasing dose, closely tracking the shifts of peak a in the corresponding SiF2 spectra (cf. Figure 2a). This peak (and the corresponding peaks seen in Figures 2d and 3b,d) in the SiF4 desorption spectra is therefore probably due to the combination reaction of SiF2 on the walls of the UHV ~ h a m b e r . ~The ~ . peaks ~ ~ a'$', and y', seen in the desorption spectra of SiF2 and SiF4 obtained with Ei = 0.275 eV, shown in Figure 2c,d, correspond to the peaks a,/3, and y obtained with Ei = 0.060 eV. In Figure 2c, an additional peak appears at early times in the SiF2 spectra for high exposures; this is probably due to cracking of SiF4 by the quadrupole. We note a striking difference between the SiF4
5536 J. Phys. Chem., Vol. 99, No. 15, 1995 5.5
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Time (seconds) Time (seconds) Figure 3. (a) Selected desorption spectra of SiF2 from Si(111)-7x7 resulting from different initial exposures to a normally incident F2 molecular beam with E, = 0.060 eV. T, = 325 K during dosing. Here 1 ML corresponds to 3.0 x lOI4 atoms/cm*. The spectra are offset by arbitrary amounts for clarity. (b) Same as (a) but for SiF4. (c) Selected desorption spectra of SiF2 from Si(ll1) resulting from different initial exposures to a F2 molecular beam with E, = 0.275 eV. The spectra are offset by arbitrary amounts for clarity. (d) Same as (a) but for SiF4. desorption spectra shown in Figure 2b,d: for Ei = 0.275 eV, peak y' grows increasingly dominant as the exposure increases, in sharp contrast (note the scale change on the ion current axis) to the behavior of the corresponding peak y seen for Ei = 0.060 eV . The desorption spectra obtained from n-type Si( 111)-7x7, shown in Figure 3, are similar in structure but somewhat different in detail from those obtained from Si( 100)-2x 1. We arbitrarily choose to label the features of the spectra which are due to desorption from this surface with italicized Roman letters for Ei = 0.060 eV (Figure 3a,b) and primed italicized letters for E, = 0.275 eV (Figure 3c,d). For Ei = 0.060 eV, a single SiF2 desorption peak a is present for short exposures as shown in Figure 3a. As the exposure increases, this peak shifts to somewhat later times and a second peak, probably due to the cracking of SiF4 (cf. Figure 3b), appears at earlier times. In Figure 3b, the corresponding desorption spectra for SiF4 show the emergence of two peaks, b and c, at early times, as the exposure is increased. Note that peak c emerges before peak
b. The desorption spectra obtained from Si( 111)-7x 7 when E, = 0.275 eV are shown in Figure 3c,d. They possess the same peak structure as the spectra obtained when E, = 0.060 eV and the peaks a',b', and c' correspond to a, b, and c, seen in Figure 3a,b. Here we note a significant difference between the SiF4 desorption spectra shown in Figure 3b,d: for E; = 0.275 eV, peaks b' and c' grow more rapidly with exposure than the corresponding peaks b and c seen for Ei = 0.060 eV. Using the data in Figures 2 and 3 and data from other measurements performed with different exposures, we generated the plots of desorption yield versus dose shown in Figure 4. Yields from Si(100)-2x1 are shown in Figures 4a and 4b while yields from Si( 111)-7x 7 are shown in Figure 4c,d. The SiF2 yields obtained from the Si( 100)-2x 1 surface, plotted in Figure 4a, approach similar limiting values for either Ei = 0.060 or 0.275 eV. In contrast, the corresponding SiF4 yields significantly differ from one another. In particular, the yield of SiF4 when Ei = 0.275 eV slowly approaches a limiting value which is nearly a factor of 4 greater than when Ei = 0.060 eV. The
Saturation Coverage of F2 on Si( 100)-2x 1 and Si( 111)-7x 7
J. Phys. Chem., Vol. 99, No. 15, 1995 5537
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Dose (ML) Dose (ML) Figure 4. (a) Uncorrected desorption yields of SiF2 from Si(100)-2x1 versus dose for E, = 0.060 and 0.275 eV. Here 1 ML corresponds to 6.8 x l O I 4 atoms/cm2. (b) Same as (a) but for SiF4. (c) Uncorrected desorption yields of SiF2 from Si(l11)-7x7 versus dose for E, = 0.060 and 0.275 eV. Here 1 ML corresponds to 3.0 x lOI4 atomslcm2. (d) Same as (c) but for SiF4. Solid and dashed lines are intended as guides to the eye.
increase in SiF4 yield is mainly due to the increasing yield from peak y' shown in Figure 2d. Note that the data in Figure 4a,b indicate that the fluorii.ated surface is essentially passivated to further exposure to F2 molecules with kinetic energy 0.060 eV. Similar dependences of the SiF2 and SiF4 yields are obtained for the Si( 111)-7 x 7 surface, as shown in Figure 4c,d. For this surface, the increase in the limiting value of the SiF4 yield as E; is increased from 0.060 to 0.275 eV (see Figure 4d) is mainly due to significant increases in both peaks b' and c', as can be seen in Figure 3d.
IV. Discussion From the measurements presented in the previous section, we can draw three conclusions: a saturation coverage of fluorine can be obtained by exposing silicon surfaces to a nearly monoenergetic beam of F2 with incident translational energy Ei = 0.060 eV; this saturation coverage can be increased by increasing the incident energy of the impinging F2 molecules to 0.275 eV; more higb'y fluorinated species are produced during desorption when dosing with a F2 beam with E; = 0.275 eV. Specifically, by increasing Ei from 0.060 to 0.275 eV, the limiting value of the SiF4 desorption yield increases by nearly
a factor of 4 on the Si(100)-2x 1 surface and by nearly a factor of 3 on the Si( 111)-7x 7 surface. These increases may be due to the ability of the more energetic F2 molecules to overcome a barrier to attack Si-Si back-bonds, resulting in the production of more highly fluorinated surface species. These measurements indicate that the barrier to insertion of fluorine into the most vulnerable Si-Si back-bond on the saturated surface is between 0.060 and 0.275 eV, consistent with the results of Ceyer et al.I9 The barrier to fluorine atom insertion into Si-Si back-bonds may be similar to the insertion of oxygen into Si-Si back-bonds during the formation of Si02.40 The observation of peak a seen in Figure 2a (and peak a' in Figure 2c) is qualitatively consistent with the observations of Engstrom et who determined that this feature of the desorption spectra obeys zero-order kinetics for initial coverages between 1.O and 1.3 ML. Mechanistically, the finding of zeroorder kinetics probably corresponds to an equilibrium between SiF and SiF2 species on the surface during decomposition, with the (dominant) SiF species maintaining the surface concentration of S ~ F Z A. ~similar ~ feature has been previously seen in the desorption of Sic12 and SiBr2 from Si(100).42-u The two peaks /3 and y (and the corresponding peaks p' and
Behringer et al.
5538 J. Phys. Chem., Vol. 99,No. 15, 1995
y’) in the SiF4 spectra obtained from Si(100)-2x1, shown in Figure 2b (Figure 2d) are also qualitatively consistent with the observations of Engstrom et al., which suggested that these peaks obey second-order kinetics.26 That these peaks occur at earlier times (lower temperatures) than peak a (or a’)merely indicates that the binding energy of SiF4 to the silicon surface is less than that of SiF2. The existence of peaks /3 and y may be explained by two different recombination reactions, e.g., SiF(a) SiF3(a) Si(a) SiF4(g) or 2SiFz(a) Si(a) SiF4(g). Alternatively, peaks /3 and y may be due to the decomposition of two different disilicone species terminated by SiF3: =SiF-SiF3 Si(a) SiF4(g) or -SiFz-SiS SiF(a) SiF4(g), as suggested by Engstrom et al.26 It is not possible to assign specific mechanisms to the different peaks with the data presented here since no information can be obtained about the dominant surface species after the dosing (although molecular dynamics simulations suggest that SiF and SiF2 p r e d ~ m i n a t e ~ ~ ) with the present experimental apparatus. We propose the following sequence for the adsorption of F2 on the Si( 100)-2x 1 surface based on the available experimental data’9,26and the molecular dynamics simulations of Carter et alez7 The FZ approaches the clean surface and reacts with a dangling bond on a dimer. The leading F atom in the FZ molecule forms a strong bond with the Si atom in the dimer while the trailing F atom is either ejected into the vacuum (abstraction) or eventually reacts with some other Si atom (dissociative chemisorption). For E, = 0.060 eV, this occurs until all of the dimer dangling bonds are saturated with F; more highly fluorinated species (most likely SiF2) may be simultaneously produced by breaking Si-Si dimer bonds. Coverages of less than 1.5 ML are expected due to the rapid increase of repulsions between F atoms belonging to different SiF, groups for higher coverages.” For E, = 0.275 eV, however, insertion of F into the Si-Si back-bonds occurs more readily, which increases the number of more highly fluorinated species and hence the saturation coverage (see Figure 4b). The increased production of SiF4 during desorption upon increasing the F2 incident energy from 0.060 to 0.275 eV is consistent with dimer bond breaking and no reaction of FZ with Si-Si back-bonds for 0.060 eV F2, i.e., consistent with different reactivities of 0.060 eV F2 with Si-Si dimer bonds and Si-Si back-bonds. The attainment of saturation coverage with F2 dosing is in contrast to the steady-state etching which occurs when dosing with XeF2 or F atoms. In the latter case, etching probably occurs because there is little or no energy cost, respectively, to free an F atom to attack Si-Si back-bonds. The desorption spectra for Si(111)-7x7 show the same qualitative structure as the desorption spectra for Si( 100)-2x 1 although the details are different, Le., the times at which the peaks occur in the SiF4 spectra, and the relative magnitudes of the peaks. One significant difference is the magnitude of peak b’ relative to peak c’ in the desorption spectra obtained with E, = 0.275 eV, shown in Figure 3d. The magnitudes of peak b’ and c’ are similar, in contrast to the spectra obtained from Si( 100)-2x 1 in which the peak y’ dominates (see Figure 2d). Also, peaks c’ and c are larger than peaks b’ and b, respectively (see Figure 3b,d), which is not the case for the corresponding peaks in the spectra obtained from Si(100)-2xl (see Figure 2b,d). Since the magnitudes of the peaks in the desorption spectra are measures of the rates of formation of SiF2 or SiF4, these results show that (1) the rates of the processes corresponding to peaks b and c are similarly enhanced when E, is increased to 0.275 eV and (2) the rate of the process corresponding to peaks c‘ and c is higher than that corresponding to
-
+
-
+
-
+
-
+
+
peaks b‘ and b, regardless of the incident energy of the F2 molecules used to dose the surface. We suggest that the different behavior observed on the Si( 111)-7x 7 surface is due to its relatively open structure. The larger spacing between adsorbate groups reduces the repulsions that arise between F atoms belonging to different SiF, groups; in turn, this lessens the tendency for short-range order that may be present for the Si(100)-2x 1 surface. The presence of shortrange order may produce alternating SiF and SiF2 groups on each dimer or perhaps analogous to the 3 x 1 structure seen for h y d r ~ g e nand ~ ~ bromine47 .~~ adsorption on Si( 100)-2x 1. This might result in the preference of certain processes over others as the level of fluorination is increased (Figure 2b,d), while its absence would produce no preference (Figure 3b,d). Further experiments are needed to determine the mechanisms responsible for the different features of the spectra shown in Figures 2 and 3.
V. Summary We have studied the dependence of the saturation coverage of fluorine upon F2 incident energy by measuring the desorption yields of SiF2 and SiF4 on the Si( 100)-2x 1 and Si( 111)-7x 7 surfaces as a function of F:! exposure generated by normally incident, nearly monoenergetic molecular beams at two different incident energies. For Ei = 0.060 eV, the desorption yields on both surfaces rapidly tend to limiting values, while for Ei = 0.275 eV, the SiF4 desorption yield slowly approaches a limiting value increased by approximately a factor of 4. These data indicate that the increase in the SiF4 desorption yield is due to overcoming a barrier, between 0.060 and 0.275 eV, to the insertion of fluorine into the most vulnerable Si-Si back-bonds on the saturated surfaces. The data obtained with the Si( 100)2 x 1 surface are consistent with different reactivities of 0.060 eV F2 with Si-Si dimer bonds and Si-Si back-bonds. Different recombination processes for SiF4 formation are enhanced on the two surfaces for E, = 0.275 eV, which we attribute to the structural differences between Si(100)-2x 1 and Si( 111)-7x 7. The above result that the attack of Si-Si back-bonds by F:! is activated on both surfaces implies that 300 K thermal F2 is a useful reagent for digital etching, in contrast to XeFz or fluorine atoms which spontaneously break Si-Si back-bonds.
Acknowledgment. We thank John Jensen for help during these experiments. This work was supported by the National Science Foundation (NSF-DMR-9307259) and the Air Force Office of Scientific Research (AFOSR-F496209410075). References and Notes (1) Barker, J. A.; Auerbach, D. Surf. Sci. Rep. 1985, 4 , 1. (2) D’Evelyn, M. P.; Madix, R. J. Surf. Sci. Rep. 1984, 3, 413. (3) Yan, C.; Jensen, J. A.; Kummel, A. C. Phys. Rev. Leu. 1994, 72, 4017. (4) Pfnur, H. E.; Rettner, C. T.; Lee, J.; Madix, R. J.; Auerbach, D. J. J . Chem. Phys. 1986, 85, 7452. (5) Winters, H. F.; Cobum, J. Surf. Sci. Rep. 1992, 14, 161. (6) Lo, C. W.; Shuh, D. K.; Chakarian, V.; Durbin, T. D.; Varekamp, P. R.; Yarmoff, J. A. Phys. Rev. B 1993, 47, 15648. (7) McFeely, F. R.; Mora, J. F.; Shinn, N. D.; Landgren, G.; Himpsel, F. J. Phys. Rev. B 1984, 30, 764. (8) Yarmoff, J. A.; Joyce, S. A. Phys. Rev. B 1989, 40, 3143. (9) Lo, C. W.; Shuh, D. K.; Yarmoff, J. A. J . Vac. Sci. Technol. A 1993, 11, 2054. (10) Wu, C. J.; Carter, E. A. J . Am. Chem. SOC.1991, 113, 9061. (11) Wu, C. J.; Carter, E. A. Phys. Rev. B 1992, 45,9065. (12) Weakliem, P. C.; Wu, C. J.; Carter, E. A. Phys. Rev. Left. 1992, 69, 200. (13) Weakliem, P. C.; Carter, E. A. J . Chem. Phys. 1993, 98, 737. (14) Craig, B. I.; Smith, P. V. Surf. Sei. 1990, 239, 36. (15) Craig, B. I.; Smith, P. V. Surf. Sci. 1992, 262, 235. (16) Radny, M. W.; Smith, P. V. Surf. Sei. 1994, 301, 97.
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Saturation Coverage of F2 on Si(100)-2x 1 and Si(ll1)-7x7 (17) Schoolcraft, T. A.; Garrison, B. J. J. Vac. Sci. Technol. A 1990,8, 3496. (18) Schoolcraft, T. A,; Garrison, B. J. J . Am. Chem. SOC. 1991, 113, 822 1. (19) A similar study by Ceyer et al. on the Si(100)-2x1 surface has been performed previously, results of which have appeared in the review article by Yu and DeLouise (see ref 20). (20) Yu, M. L.; DeLouise, L. A. Sutf. Sci. Rep. 1994, 19, 285. (21) Bourne, 0. L.; Hart, D.; Rayner, D. M.; Hackett, P. A. J. Vac. Sci. Technol. B 1993, 11, 556. (22) Meguro, T.; Hamagaki, M.; Modaressi, S.; Hara, T.; Aoyagi, Y.; Ishii, M.; Yamamoto, Y. Appl. Phys. Lett. 1990, 56, 1552. (23) Meguro, T.; Ishii, M.; Kodama, H.; Hamagaki, M.; Hara, T.; Yamamoto, Y.; Aoyagi, Y. Jpn. J. Appl. Phys. 1990, 29, 2216. (24) Sakaue, H.; Iseda, S.; Asami, K.; Yamamoto, J.; Hirose, M.; Horike, Y. Jpn. J. Appl. Phys. 1990, 29, 2648. (25) Honke, Y.; Tanaka, T.; Nakano, M.; Iseda, S.; Sakaue, H.; Nagata, A.; Shindo, H.; Mayazaki, S.; Hirose, M. J. Vac. Sci. Technol. A 1989, 8, 1844. (26) Engstrom, J. R.; Nelson, M. M.; Engel, T. Surf. Sci. 1989, 215, 431. (27) Carter, L. E.; Khodabandeh, S.; Weakliem, P. C.; Carter, E. A. J. Phys. Chem. 1994, 100, 2277. (28) Yang, J. J.; Pullman, D. P.; Li, Y. L.; Gosalvez, D.; Tsekouras, A. A,; Ceyer, S. T. Abstr. Pap.-Am. Chem. SOC. 1993, 205, C2. (29) Jensen, J. A,; Yan, C.; Kummel, A. C. Science 1995, 267, 493. (30) Carter, L. E.; Carter, E. A. J. Vac. Sci. Technol. A 1994, 12,2235. (31) Carter, L. E.; Carter, E. A. Surf: Sci. 1995, 323, 39. (32) Behringer, E. R.; Flaum, H. C.; Masson, D. P.; Lanzendorf, E. J.; Kummel, A. C., private communication. (33) Sullivan, D. J. D.; Flaum, H. C.; Kummel, A. C. J . Phys. Chem. 1993, 97, 12051.
(34) Flaum, H. C.; Sullivan, D. J. D.; Kummel, A. C. J. Phys. Chem. 1994, 98, 1719. (35) Flaum, H. C.; Sullivan, D. J. D.; Kummel, A. C. J . Chem. Phys. 1994, 100, 1634. (36) Sullivan, D. J. D.; Kummel, A. C. Rev. Sci. Instrum. 1992, 63, 4285. (37) Bozack, M. J.; Dresser, M. J.; Choyke, W. J.; Taylor, P. A,; Yates, J. T. Surf. Sci. 1987, 184, L332. (38) Winters, H. F.; Houle, F. A. J. Appl. Phys. 1983, 54, 1218. (39) Typically, the desorption spectra were acquired in order of increasing exposure time (dose). Thus the sample had an increasing amount of time to cool between desorption spectra. This may lead to an increased amount of time to attain the higher temperatures, which would result in the observation of a shifting high-temperature desorption peak as the exposure time increased. However, Engstrom et al. (see ref 26) also observed small shifts to higher surface temperatures with increasing exposure. (40) Engel, T. Surf. Sci. Rep. 1993, 14, 91: (41) Yates, J. T. In: SolidState Physics: Surfaces; Park, R. L., Lagally, M. G., Eds.; Academic Press: Orlando, 1985; p 433. (42) Szabo, A.; Farrall, P. D.; Engel, T. Surf. Sci. 1994, 312, 284. (43) Jackman, R. B.; Ebert, H.; Foord, J. S. Surf. Sci. 1986, 176, 183. (44) Jackman, R. B.; Price, R. J.; Foord, J. S. Appl. Surf. Sci. 1989,36, 296. (45) Boland, J. J. Phys. Rev. Left. 1990, 65. 3325 (46) Boland, J. J. Sui$ Sci. 1992, 261, 17. (47) Chander, M.; Li, Y. 2.;Rioux, D.; Weaver, J. H. Phys. Rev. Lett. 1993, 71, 4154. JP942783E
