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J. Phys. Chem. C 2007, 111, 10988-10992
Oxidation and Reduction of Ultrathin Nanocrystalline Ru Films on Silicon: Model System for Ru-Capped Extreme Ultraviolet Lithography Optics Y. B. He,†,§ A. Goriachko,† C. Korte,† A. Farkas,† G. Mellau,† P. Dudin,‡ L. Gregoratti,‡ A. Barinov,‡ M. Kiskinova,‡ A. Stierle,§ N. Kasper,§ S. Bajt,| and H. Over*,† Department of Physical Chemistry, Justus-Liebig-UniVersity, Heinrich-Buff-Ring, D-35392 Giessen, Germany, Sincrotrone Trieste, Area Science Park, I-34012 BasoVizza-Trieste, Italy, Max-Planck Institut fu¨r Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany, and Lawrence LiVermore National Laboratory, 7000 East AVenue, LiVermore, California 94550 ReceiVed: February 16, 2007; In Final Form: May 11, 2007
Ultrathin ruthenium films are promising capping layers protecting extreme ultraviolet lithography optics against carbon growth and oxidation. The structure and reactivity of 2, 5, and 7 nm thick nanocrystalline Ru films on Si (serving as a model system for Ru capping layers) were studied by multiple techniques including scanning electron microscopy, X-ray diffraction, X-ray reflectivity, and X-ray photoelectron spectroscopy. The structural analysis indicated dense and flat Ru films, consisting of preferentially (0001)-oriented grains. High resolution core-level Ru 3d5/2 and O 1s spectroscopy studies have revealed that these Ru films exposed to O2 ambient are resistant to oxidation up to ∼470 K similar to the oxidation of single-crystalline Ru(0001) surfaces. The reduction of the oxide in the H2 ambient is highly effective and proceeds already below 370 K.
Introduction Extreme ultraviolet lithography (EUVL) is the leading candidate to be used in semiconductor manufacture in the next decade.1 One of the major challenges for EUVL is the lifetime of the imaging optics, usually consisting of alternating layers of Si and Mo that form reflective multilayer coatings. The combination of exposure to high-intensity EUV radiation (hν ) 92 eV) and the presence of hydrocarbons and water in the residual gas leads to fast degradation of the optical reflectivity due to oxidation and growth of carbon layers. One solution toward improving the optics lifetime is the use of protective capping layers. Ruthenium-based capping layers of 2-3 nm thickness have been promising for the protection of the Si/Mo multilayer systems. Although they also undergo oxidation and C contamination in a typical EUV tool environment, the degradation of the optical reflectivity is significantly slowed down.1 The stringent specification in commercial EUVL, reflectivity loss of 1% over 30 000 h,2 requires however precise knowledge about the reactivity of the Ru capping layers on the atomic scale. The oxidation of Ru to RuO2 is one of the primary reasons for the optical degradation of Ru-protected EUVL mirrors,1 and it proceeds already at room temperature, the microscopic mechanism of which is, however, not well understood.3 For comparison, single-crystalline Ru(0001) starts to oxidize only above 500 K.4,5 From an intuitive notion one could expect that the low onset temperature for surface oxidation of Ru capping layers is related to the nanometer size of the Ru grains and the presence of numerous grain boundaries. In the present study we report structural and chemical characterizations of ultrathin Ru nanofilms with thicknesses of * To whom correspondence should be addressed. Fax: +49-641-9934559. E-mail:
[email protected]. URL: http:// www.chemie.uni-giessen.de/home/Over. † Justus-Liebig-University. ‡ Sincrotrone Trieste. § Max-Planck Institut fu ¨ r Metallforschung. | Lawrence Livermore National Laboratory.
2, 5, and 7 nm deposited on superpolished Si(100) wafers, serving as model systems for the Ru-capped EUV mirrors. The investigations tackle in detail the reactivity of Ru films in O2 gas ambient, the formation and subsequent reduction of the ruthenium oxide with H2. We found that the threshold oxidation temperature of the Ru nanofilms is 470 K, far above room temperature and similar to the case of single crystal Ru(0001). Therefore, the presence of grain boundaries and nanometer Ru grains does obviously not facilitate the initial oxidation process. The reduction of the oxide film with H2 occurs already at 370 K, which may be of practical use for cleaning the Ru capping layers under EUVL operating conditions. Experimental Details The ultrathin Ru capping layers were deposited onto superpolished Si(100) wafers using the dc-magnetron sputtering setup (at the Lawrence Livermore National Laboratory) as described in ref 6. All three samples consisted of three layers deposited on the top of the Si(100) wafer: a 4 nm Si layer, ∼0.5 nm of Mo, and a Ru film (2, 5, or 7 nm thick). The deposition chamber with four sputtering sources, mounted 90° apart, on the bottom of the chamber, was used. The substrates, facing down, were spun during the deposition. These substrates were attached to a platter, which rotates over the sputtering sources with precisely controlled velocity. The platter velocity and the velocity profile determine the exact layer thickness and the thickness uniformity across the substrate. The typical base pressure at the start of the deposition was 2 × 10-7 mbar, and the deposition was performed at 10-3 mbar using ultra-high-purity Ar gas. The smoothness of the Ru films deposited on the Si substrates was verified by X-ray reflectivity (cf. Figure 1) and scanning electron microscopy (SEM) experiments. The crystallographic structure of the films was studied by X-ray diffraction (XRD) using a Siemens diffractometer with Cu KR (λ ) 0.15418 nm) radiation. The morphology and
10.1021/jp071339b CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007
Oxidation and Reduction of Ru Films on Si(100)
J. Phys. Chem. C, Vol. 111, No. 29, 2007 10989 the temperature stepwise to 520 K. The reduction of oxidized Ru films by hydrogen was carried out at p(H2) ) 5 × 10-5 mbar (for 30 min at each temperature) starting from 370 K. The evolution of the surface chemical state after each oxidation or reduction cycle was monitored by measuring the O 1s and Ru 3d5/2 core-level spectra, which have known features, used as fingerprints of the Ru oxidation state. The core-level spectra were measured with a photon energy of 656 eV with spectral resolutions of 400 and 200 meV for the O 1s and Ru 3d5/2 core levels, respectively. The kinetic energies of the O 1s and Ru 3d5/2 photoelectrons are 122 and 372 eV. Using the universal curve for the electron mean free path, the effective escape depths are estimated to be 0.6 and 1.0 nm for O 1s and Ru 3d, respectively. Results and Discussion
Figure 1. XRR study of the 2, 5, and 7 nm thick Ru capping layers on a buffer system on Si(100). On the left, the measured and fitted XRR curves are shown: the hollow circular and triangle symbols represent the measured data, while the solid and dashed lines denote the fitting results for the as-received and H2-reduced films, respectively. On the right, the 2δ profiles obtained by the fit are displayed for the as-received (solid curve) and H2-reduced (dashed line) samples; the insets are zooms into the near surface region. The optical parameter 2δ is proportional to the total electron density of the material; for details see ref 9.
microstructure of the films were characterized using highresolution scanning electron microscopy (LEO Gemini 982). The thickness and surface roughness of the films were determined by X-ray reflectivity (XRR) measurements, performed at the MPI-MF beamline at Ångstrøm Quelle Karlsruhe (ANKA)7 with an X-ray wavelength of λ ) 0.137 nm. The measurements were conducted for both the as-received and H2reduced samples (p(H2) ) 1 mbar at room temperature for a few hours) in a UHV compatible high-pressure chamber. The oxidation and reduction of the ultrathin Ru films were studied by X-ray photoelectron spectroscopy (XPS); the highresolution core-level spectroscopy measurements were performed at the ESCA Microscopy beamline at Elettra.8 The C contamination layers on the ultrathin Ru films due to the air exposure before the transfer to the UHV station for photoemission studies were removed by mild Ar ion sputtering (U ) 0.5 kV, p(Ar) ) 10-5 mbar, for 1 min at room temperature). The loss of Ru during sputtering was estimated by XRR to be less than 0.5 nm for a sputtering cycle. The cleanliness of the samples was verified by the XPS survey spectra, which showed only the Ru-related spectral features. The temperature-dependent oxidation experiments were carried out by exposing the films at each temperature to molecular oxygen (p(O2) ) 5 × 10-4 mbar) for 30 min; we started at room temperature and increased
After cleaning of the Ru films by H2 reduction, the thicknesses as measured via XRR are 2.2, 5.0, and 7.1 nm with surface root-mean-square (rms) roughnesses of 0.15, 0.19, and 0.26 nm, respectively. In Figure 1, the XRR data of the as-received and H2-reduced films are compared along with the corresponding fits and electron density profiles. The fits were performed using the Parratt formalism modified for interfacial roughness, taking reflected and transmitted amplitudes at each interface into account.9,10 For the as-received samples the profile was composed of five layers on top of the Si(100) wafer substrate: a “contamination” layer (adsorbed OH groups, CO, etc.), the Ru cap layer, an ultrathin Mo layer, an amorphous Si layer, and the natural SiO2 layer. The diffusion barrier should prevent the diffusion of hydrogen and oxygen from the gas atmosphere to the Si substrate (in EUVL practice, a Si-Mo multilayer stack). For the H2-reduced films, the topmost layer was removed and only the thickness and roughness of the Ru cap layer were allowed to deviate from the parameter values obtained from the fit of the as-received samples. In all three cases a slight increase in Ru thickness was observed after reduction. The contamination layer forms due to long-term exposure to humid air, and it can be, however, effectively wiped away by exposure to relatively high H2 pressure (100 mbar) at room temperature. For all samples we can follow the reflectivity over 9 orders of magnitude, indicating a flat surface. Despite the overall small changes of the electron density profile by the H2 reduction, a pronounced increase of the reflectivity can be observed at higher reflection angles. This effect is most pronounced in the 2 nm Ru film and can lead at some reflection angles to a dramatic increase of the reflectivity (e.g., about 3 orders of magnitude at 9°). Figure 2a shows the X-ray diffraction θ/2θ scans of the 2, 5, and 7 nm thick Ru films. In the whole scan range of 20-90°, the spectra exhibit only one Ru-related peak: Ru(0002). Together with the azimuthal XRD scan (not shown here), it is evident that the deposited Ru films are textured: Most of the Ru grains are preferentially oriented with the Ru(0001) crystal plane parallel to the substrate surface, while the in-plane orientation of these grains is random. This finding is consistent with a recent transmission electron microscopy (TEM) study of a 2.3 nm thick Ru capping layer.11 Using the Scherrer formula, the vertical grain size can be estimated from the fwhm of the Ru(0002) Bragg reflection. We deduce grain dimensions of 2.2, 4.8, and 7.1 nm along the surface normal, respectively, which correlate nicely with the Ru film thicknesses determined by XRR. In Figure 2b, the in-plane grazing-incidence XRD spectra of all three samples are depicted. Applying the Scherrer formula to the fwhm of the Ru(101h0) peaks, the average lateral
10990 J. Phys. Chem. C, Vol. 111, No. 29, 2007
Figure 2. XRD characterization of the 2, 5, and 7 nm thick Ru films supported on Si(100). (a) Out-of-plane θ/2θ scans: for all samples only the Ru(0002) peak was detected, giving evidence for a (0001) texture of the films. (b) In-plane φ/γ scans through the Ru(101h0) peak in grazing incidence geometry (incident angle Ri ) 0.45°): from the fwhm of the peak the lateral grain sizes of these films were estimated to be 5.3, 8.4, and 8.6 nm, respectively. The bottom panels show the applied scan geometries.
Figure 3. The oxidation of the 7 nm Ru film was studied with Ru 3d5/2 (left panel) and O 1s (right panel) core-level spectroscopy as a function of temperature. The oxygen exposure was in all cases 7 × 105 langmuirs.
grain sizes of the 2, 5, and 7 nm thick Ru films are determined to be 5.3, 8.4, and 8.6 nm, respectively. The increase of the in-plane grain size for the thicker Ru films is consistent with corresponding SEM images (not shown). In the following, we present a detailed study on the reactivity of the ultrathin Ru capping layers. There are a few precautions to be taken when the sample temperature is increased during the oxidation and reduction experiments. Beyond 550 K Si and Ru start to form silicide at the interface.12 Furthermore, during the heating Si segregates gradually toward the Ru surface where it is irreversibly oxidized.13 To minimize the influence of Si segregation on the oxidation and reduction behavior of the Ru capping layer, we focused on the 7 nm thick Ru film in our X-ray photoelectron spectroscopy study. We monitored the oxidation state of the 7 nm thick Ru layer as a function of the oxidation temperatures following the evolution of the O 1s and Ru 3d5/2 spectral regions (Figure 3). The energy positions of the Ru 3d5/2 and O 1s spectral
He et al.
Figure 4. The reduction of the oxidized 7 nm Ru film was studied with Ru 3d5/2 (left panel) and O 1s (right panel) core-level spectroscopy as a function of the sample temperature. The hydrogen exposure was in all cases 7 × 104 langmuirs.
components measured for the single-crystal Ru(0001) surface, relevant to the different Ru oxidation states,5,14,15 adsorbed species containing O,16,17 and silicon oxides,18 are summarized in Table 1. They serve as a general guide for the case of polycrystalline Ru films. In particular, the numbers for the O adsorption states are related to a well-defined coordination of the Ru atoms on a well-ordered Ru(0001) surface with oxygen adatoms, conditions not necessarily encountered with a surface consisting of nanograins and grain boundaries. However, our recent studies on oxidation of polycrystalline Ru showed almost identical Ru 3d and O 1s components for the transient oxide and RuO2 states.19 Figure 3 shows the evolution of the Ru 3d5/2 and O 1s spectra upon exposure of a clean 7 nm Ru film to 7 × 105 langmuirs (1 langmuir ) 1.33 × 10-6 mbar‚s) of molecular oxygen at different temperatures. The clean 7 nm Ru film shows only a single Ru 3d5/2 peak at 280.1 eV. The relative intensity and energy position of a second component at 279.75 eV, reflecting the emission of the surface Ru atoms as observed on the well-defined Ru(0001) single crystal, are very sensitive to the long-range order of the surface and the presence of adsorbed species, the latter inducing a shift of the surface core-level component toward the bulk component energy position. When the nanocrystalline samples stay longer (∼2 h) in the chamber, as in the present case, some CO, H2O, and H2 inevitably adsorb from the residual gas at room temperature so that we are not able to resolve the surface component in the spectra. The COrelated O 1s in the binding energy range 530.8-531.8 eV may account for the weak O 1s emission observed in Figure 3, whereas the C 1s spectral region overlaps with the strong Ru 3d3/2 emission and can hardly be resolved. After O2 exposure at 445 K for 30 min a second component at 280.6 eV appears in the Ru 3d5/2 spectrum, which can be assigned to chemisorbed O and/or a transient oxide (RuxOy) state (cf. Table 1). The main O 1s peak appears at 530.0 eV. The second component at 531.4 eV can tentatively be assigned to particular bonding configurations of oxygen at the grain boundaries. The successive exposures to 7 × 105 langmuirs of O2 at 470, 495, and 520 K lead to gradual growth and a shift of the O-related Ru 3d5/2 component toward the RuO2 position, which is accompanied by attenuation of the metallic Ru 3d5/2 component, screened by the growing oxide film. Comparison with the corresponding O 1s spectra reveals a clear signature of nucleation and growth of a stoichometric RuO2 film at 495 K,
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J. Phys. Chem. C, Vol. 111, No. 29, 2007 10991
TABLE 1: Summary of the Energy Positions of Ru 3d5/2 and O 1s Spectral Components Measured for the Single-Crystal Ru(0001) Surface, Relevant to the Different Ru Oxidation States, Adsorbed Species Containing O, and Silicon Oxide chemical state Ru(0001) (Ru3d5/2) Ru(0001) + oxygen (Ru3d5/2) RuO2 (Ru3d5/2) Ru(0001) + oxygen (O1s) RuO2(O1s) O1s O1s O1s
binding energy ((0.05 eV)
component identity
280.1 279.75 280.1, 280.5, 281.0 280.5-280.6 280.75 280.45 283 530.0 529.5 528.7 530.8-531.8 531.0-531.7 532.5-533.2
Ru bulk Ru surface Ru-0.25, 0.50, and 1.0 monolayer of adsorbed oxygen (Oad) Ru transient surface oxide (TSO) Ru bulk Ru cus satellite Oad and TSO O bulk “bridge” O CO OH silicon oxide
characterized by the feature growing at 529.6 eV. It grows at the expense of the 530.0 eV component and becomes dominant in the spectrum at 520 K. The other intense component at 531.3 eV can be tentatively attributed to overlapping contributions of oxygen at grain boundaries, CO and/or OH adsorption, the latter formed by dissociative adsorption of H2 from residual gas on the oxide surface.17 The “adsorption” origin of this peak is supported by the fact that it grows when the samples stay at room temperature in the residual gas ambient (see the asoxidized O 1s spectrum in Figure 4). Oxidation at T > 470 K also favors some segregation of Si toward the surface, as evidenced in the survey XPS spectra. The O 1s feature at 532.5 eV results from the SiOx formed under oxidation conditions.18 However, in the Ru 3d spectra of the oxidized ultrathin Ru films the broad satellite at ∼283 eV, a distinct feature of the Ru 3d spectrum of the well-ordered RuO2(110) phase grown on Ru(0001),15b was not observed. It is reasonable to assume that the formed oxide film is highly disordered, which accounts also for the observed fast chemical reduction at low temperature (see the discussion below). Summarizing, the oxidation experiments indicate clearly that a thin disordered RuO2 film nucleates and starts to grow between 470 and 495 K. Similar behavior was observed for the 2 and 5 nm thick Ru films. The spectra in Figure 4 show the evolution of the oxidized film exposed to the reducing ambient. The line shape of the initial as-oxidized O 1s spectrum in Figure 4 is different from that of the freshly oxidized film, because the component assigned to adsorbed CO and OH species grew during the preparation period of the reduction experiments when the samples were kept at room temperature. The results in Figure 4 demonstrate that already after the first exposure to H2 at 370 K the oxide film is almost completely reduced. In the resulting Ru 3d5/2 spectrum the metallic Ru component is dominant, and after the next reduction steps at 400, 450, and 500 K, the spectra are identical to that of the clean metallic Ru film (cf. Figure 3). This very efficient reduction is confirmed by the evolution of the O 1s feature. The components at ∼530.0, 531.2, and 532.5 eV remaining after reduction should be assigned to adsorbed O, CO, and SiOx, respectively. At 500 K the SiOx feature, unaffected by the reduction cycles, dominates the spectrum. The traces of adsorbed oxygen can be attributed to a residue of strongly bonded oxygen on the Ru film surface and at the grain boundaries. Concluding Remarks In conclusion, the onset temperature for oxidation of the 7 nm Ru film was found to be between 475 and 490 K. This
ref 14 14, 15a 5 15a 15a 15b 5 15a 15a 16 17 18
threshold temperature is compatible with that (500 K) found for oxidation of the single-crystal Ru(0001).4,5 Surprisingly, the nanometer size character of the Ru film does not facilitate the initial oxidation process. Therefore, the explanation of the observed room-temperature oxidation of Ru nanofilms under EUVL conditions requires some additional processes to be invoked. Detailed modeling studies of the oxidation of Ru surfaces under EUV radiation have recently been reported.20 Consistent with recent detailed surface science studies,3 the oxidation of Ru under EUVL conditions is driven by EUV photon-assisted water fragmentation (direct photon or secondary electron induced fragmentation), thereby producing chemisorbed atomic oxygen on the Ru capping layer surface. The transformation from this adsorption stage toward RuO2 is at room temperature, however, puzzling since without EUV light higher temperatures above 470 K are required for the oxidation (this study). We can only speculate that EUV light may even promote the transformation of chemisorbed O into the oxide, for instance, by a photochemical process. Here further experiments are required. An important outcome of our study for the EUV lithography is that the thin RuO2 film formed on the Ru capping layer can be very effectively removed by a simple in situ procedure, the exposure to molecular hydrogen at slightly elevated temperature (below 370 K). The residual O on the Ru capping layer is only in adsorption form with coverage in the submonolayer range. Compared with the recently suggested cleaning procedure using atomic hydrogen exposure at 323 K,21 the present procedure can be applied in situ without disruption of the EUVL process. In fact, a viable approach is introduction of molecular hydrogen into the EUVL chamber (a typical partial pressure of 10-7 mbar should be sufficient) to decrease the oxidation rate of the Ru capping layer. This active mitigation scheme may even prevent the oxidation of the Ru protective layers. From studies of singlecrystalline RuO2(110) films, we know that efficient H2 reduction needs a temperature of at least 420 K to release the produced water from the surface by desorption.4 For the Ru nanofilms the reduction temperature by H2 is significantly lower, suggesting that here the (nano)size matters. Acknowledgment. We acknowledge partial financial support from the European Union under Contract No. NMP3-CT-2003505670 (NANO2). Part of the work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405ENG-48.
10992 J. Phys. Chem. C, Vol. 111, No. 29, 2007 References and Notes (1) Bajt, S.; Alameda, J. B.; Barbee, T. W., Jr.; Clift, W. M.; Folta, J. A.; Kaufman, B.; Spiller, E. A. Opt. Eng. 2002, 41, 1797. (2) Meiling, H.; Banine, V.; Harned, N.; Blum, B.; Ku¨rz, P.; Meijer, H. Proc. SPIE 2005, 5751, 90. (3) Madey, T. E.; Faradzhev, S.; Yakshinskiy, B. Y.; Edwards, N. V. Appl. Surf. Sci. 2006, 253, 1691 and references therein. (4) He, Y. B.; Knapp, M.; Lundgren, E.; Over, H. J. Phys. Chem. B 2005, 109, 21825. (5) Blume, R.; Niehus, H.; Conrad, H.; Bo¨ttcher, A.; Aballe, L.; Gregoriatti, L.; Barinov, A.; Kiskinova, M. J. Phys. Chem. B 2005, 109, 14052. (6) Montcalm, C.; Bajt, S.; Mirkarimi, P. B.; Spiller, E.; Weber, F. J.; Folta, J. A. Proc. SPIE 1998, 3331, 42. (7) Stierle, A.; Steinha¨user, A.; Kasper, N.; Weigel, R.; Dosch, H. ReV. Sci. Instrum. 2004, 75, 5302. (8) Kiskinova, M.; Marsi, M.; Di Fabrizio, E.; Gentili, M. Surf. ReV. Lett. 1999, 6, 265. http://www.elettra.trieste.it/experiments/beamlines/esca/ index.html. (9) Parratt, L. G. Phys. ReV. 1954, 95, 359. (10) Nevot, L.; Croce, P. ReV. Phys. Appl. 1980, 15, 761. (11) Bajt, S.; Dai, Z. R.; Nelson, E. J.; Wall, M. A.; Alameda, J. B.; Nguyen, N. Q.; Baker, S. L.; Robinson, J. C.; Taylor, J. S.; Aquila, A.; Edwards, N. V. J. Microlithogr., Microfabr., Microsyst. 2006, 5, 023004.
He et al. (12) Arunagiri, T. N.; Zhang, Y.; Chyan, O.; El-Bouanani, M.; Kim, M. J.; Chen, K. H.; Wu, C. T.; Chen, L. C. Appl. Phys. Lett. 2005, 86, 083104 and references therein. (13) Gas, P.; D’Huerle, F. In Silicide-Fundamentals and Application; Miglio, L., D’Huerle, F., Ed.; Word Scientific: Singapore, 1999; p 34. (14) Lizzit, S.; Baraldi, A.; Groso, A.; Reuter, K.; Ganduglia-Pirovano, M. V.; Stampfl, C.; Scheffler, M.; Stichler, M.; Keller, C.; Wurth, W.; Menzel, D. Phys. ReV. B 2001, 63, 205419. (15) (a) Over, H.; Seitsonen, A. P.; Lundgren, E.; Wiklund, M.; Andersen, J. N. Chem. Phys. Lett. 2001, 342, 467. (b) Over, H.; Seitsonen, A. P.; Lundgren, E.; Smedh, M.; Andersen, J. N. Surf. Sci. 2002, 504, L196. (16) Schiffer, A.; Jacob, P.; Menzel, D. Surf. Sci. 1997, 389, 116. (17) (a) Knapp, M.; Crihan, D.; Resta, A.; Lundgren, E.; Andersen, J. N.; Seitsonen, A. P.; Schmid, M.; Varga, P.; Over, H. J. Phys. Chem. B 2006, 110, 14007, (b) Gladys, M. J.; Mikkelsen, A.; Andersen, J. N.; Held, G. Chem. Phys. Lett. 2005, 414, 311. (18) Himpsel, F. J.; McFeely, F. R.; Taleb-Ibrahimi, A.; Yarnoff, J. A.; Hollinger, G. Phys. ReV. B 1988, 38, 6084. (19) Blume, R.; Ha¨vecker, M.; Zafeiratos, S.; Teschner, D.; KnopGericke, A.; Schlo¨gl, R.; Lizzit, S.; Dudin, P.; Barinov, A.; Kiskinova, M. To be published. (20) Hollenshead, J.; Klebanoff, L. J. Vac. Sci. Technol., B 2006, 24, 118. (21) Nishiyama, I.; Oizumi, H.; Motai, K.; Izumi, A.; Ueno, T.; Akiyama, H.; Namiki, A. J. Vac. Sci. Technol., B 2005, 23, 3129.
