Soft x-ray photoreactions of CF3Cl adsorbed on Si(111)-7x7 studied by continuous-time photon-stimulated desorption spectroscopy near F(1s) edge

Soft x-ray photoreactions of CF

Cl adsorbed on Si

_{„111…-7Ã7 studied}

by continuous-time photon-stimulated desorption spectroscopy

near F

_{„1s… edge}

C.-R. Wen,a兲 C.-Y. Jang, L.-C. Chou, J. Chen, Y.-H. Wu, S.-C. Chang, W.-C. Tsai, C.-C. Liu, S.-K. Wang, and Y. Shai

Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan

共Received 9 May 2007; accepted 21 July 2007; published online 18 September 2007兲

The continuous-time core-level photon-stimulated desorption共PSD兲 spectroscopy was employed to monitor the monochromatic soft x-ray-induced reactions of CF3Cl adsorbed on Si共111兲-7⫻7 near the F共1s兲 edge 共681–704 eV兲. Sequential F+_{PSD spectra were measured as a function of photon} exposure at the CF3Cl-covered surface 共dose=0.3⫻1015molecules/ cm2, ⬃0.75 ML兲. The F+ PSD and total electron yield 共TEY兲 spectra of molecular solid CF3Cl near the F共1s兲 edge were also measured. Both F+ _{PSD and TEY spectra show two features at the energy positions of} 690.2 and 692.6 eV, and are attributed to the excitations of F共1s兲 to 11a1关共C–Cl兲*_{兴 and} 共8e+12a1兲关共C–F兲*_{兴 antibonding orbitals, respectively. Following Auger decay, two holes are} created in the F共2p兲 lone pair and/or C–F bonding orbitals forming the 2h1e final state which leads to the F+_{desorption. This PSD mechanism, which is responsible for the F}+_{PSD of solid CF}

3Cl, is employed to interpret the first F+_{PSD spectrum in the sequential F}+_{PSD spectra. The variation of} spectrum shapes in the sequential F+ _{PSD spectra indicates the dissipation of adsorbed CF}

3Cl molecules and the formation of surface SiF species as a function of photon exposure. From the sequential F+_{PSD spectra the photolysis cross section of the adsorbed CF}

3Cl molecules by photons with varying energy 共681–704 eV兲 is determined to be ⬃1.0⫻10−17_cm2_{. © 2007 American} Institute of Physics.关DOI:10.1063/1.2772257兴

I. INTRODUCTION

The monochromatic soft x-ray-induced reactions of mol-ecules adsorbed on solid surfaces has become a subject of considerable interest. It is well known that the excitations of the valence-level or core-level electrons by the soft x-ray photons can cause the dissociation of adsorbed species. From the viewpoint of applications, photochemical processing via core-electron excitation is promising for the fabrication of future fine-structure microelectronic devices.1–3 Because high-intensity soft x-ray synchrotron radiation 共SR兲 can be used to cause site-specific chemical reactions on semicon-ductor surfaces by core-electron excitations of adsorbates or substrates, and its short wavelength nature may allow control of the photon-induced surface reactions to be done with ex-tremely high spatial resolution, soft x-ray SR is considered to be a suitable photon source.4–6 Several experiments on SR-stimulated processes7–14 on semiconductor surfaces have been performed and shown that the SR-stimulated processing is a future prospective technology for low-temperature fabri-cation of semiconductor devices. Therefore, understanding the basic mechanisms responsible for the photochemical re-actions of adsorbates on a semiconductor surface has become a very important research work.15–22

The site-specific chemical bond scission of the adsorbate

or condensed layer systems on the solid surfaces will result in the selective ionic dissociation or desorption. The des-orbed ion yield versus incident photon energy can be mea-sured, and a photon-stimulated desorption共PSD兲 spectrum is obtained. In a PSD spectrum the spectral threshold and shape can provide the information on the basic excitation initiating the dissociation and desorption processes. It is generally as-sumed that the photon flux density is so small that only neg-ligible beam damage of the adsorbate is caused by PSD dur-ing the time of measurdur-ing a PSD spectrum. Therefore, the PSD spectrum is reproducible in further repeated PSD scans. However, for high-intensity soft x rays, especially produced by third-generation synchrotron radiation sources, and/or the molecules with high photolysis cross sections—for example, some fluorine-containing molecules—the decay of the adsor-bate concentration by PSD itself is not negligible. As a re-sult, a dramatic change in a series of PSD spectra, which are measured one by one via repeating the incident photon en-ergy scan, will be observed. This series of PSD spectra could be called continuous-time PSD spectra, and the method to obtain these spectra was called continuous-time PSD spec-troscopy in our previous work 共Ref.22兲. Since a PSD

spec-trum can provide information on the local bonding and elec-tronic structure of the surface, continuous-time PSD spectroscopy can be employed to monitor the variation of the surface chemical bonding structure—the disappearance of a specific state and the formation of a new bonding structure during irradiation of incident photons.

The continuous-time core-level PSD spectroscopy was

a兲_{Author to whom correspondence should be addressed. Also at Institute of}

Electro-Optical Science and Engineering, National Cheng Kung Univer-sity, Tainan 70101, Taiwan. Electronic mail: [email protected]

used in our previous experiments22 to study the photon-induced dissociation and desorption of CF3Cl adsorbed on Si共111兲-7⫻7 near the Si共2p兲 edge 共98–110 eV兲. It has been found that a sequential F+_{PSD spectra show the variation of} their shapes with photon exposure, indicating that the ad-sorbed CF3Cl molecules are dissociated and the surface spe-cies SiF and SiF3 are formed. The dissociation of adsorbed CF3Cl is mainly due to dissociative electron attachment 共DA兲 and dipolar dissociation 共DD兲 induced by photoelec-trons emitted from the silicon substrate. The primary aim of the present work was to employ the continuous-time core-level PSD spectroscopy to study the photon-induced disso-ciation and desorption of CF3Cl/ Si共111兲-7⫻7 system near the F共1s兲 edge 共681–704 eV兲. It was expected that site-specific scission of C–F bond by direct excitation of F共1s兲 core electron will play a significant role.

II. EXPERIMENT

The experiments were performed at the National Syn-chrotron Radiation Research Center共NSRRC兲, Hsinchu, Tai-wan using a wide-range spherical grating monochromator 共WR-SGM, bending magnet beamline兲. The experiments were performed in an ultrahigh-vacuum system 共base pres-sure⬍2⫻10−10Torr兲 equipped with a spherical sector elec-tron energy analyzer共VG CLAM2兲 for synchrotron radiation photoemission spectroscopy 共PES兲. The system is also equipped with an argon gun 共VG AG5000兲 for sputtering, a quadrupole mass spectrometer 共Balzers QMG421兲 for positive- and negative-ion PSDs, and low-energy electron diffraction 共LEED兲 optics 共VG RVL900兲. A variable-temperature sample holder, connected to a closed-cycle re-frigerated cryostat for cooling a sample down to 30 K and equipped with wires for passing current directly through the sample for heating it up to 1100 ° C, was used for sample cleaning and for CF₃Cl adsorption measurements.

The Si共111兲 crystal 共p type, 9.6 ⍀ cm兲 surface was cleaned by cycles of argon-ion bombardment 共800 eV兲 fol-lowed by direct resistive heating to 1100 ° C. The surface structure was checked by LEED, and the cleanliness was checked by PES and evidenced by the characteristic surface states in the valence region.23Chlorotrifluoromethane共purity 艌99.9%兲 was obtained from a commercial source and used without further purification. Gas exposure was made by dos-ing the clean Si共111兲-7⫻7 surface with CF3Cl gas from a gas-dosing system, composed of a miniature cross reservoir, a leak valve, and a stainless-steel tubing with a microchannel-plate doser head. It is equipped with two capacitance-pressure gauges 共MKS-Baratron兲. The gas flux from the dosing system was calibrated by standard volumet-ric technique. We estimate the dose error to be⬃20%. Dur-ing the dosDur-ing of the gas and the PSD experiments, the sample was kept at a temperature of 30 K.

Desorbing positive ions were detected by a Balzers pulse-counting quadrupole mass spectrometer 共QMS兲 共Balz-ers model QMG421 with off-axis secondary electron multi-plier兲, which was positioned normal to the surface, and the incidence angle of the photon beam is 45° from the surface. The sample surface was located⬃3 cm from the entrance of

the QMS. During the measurement of positive ions, the ion-izer filament was turned off. A portion of positive ions des-orbed by photons impinging on the surface are focused by ion lenses at the entrance of the mass spectrometer. The an-gular acceptance of the QMS is ⬃10°.

Two measurement methods of the PSD technique were employed. The first共method 1兲 consisted of opening the pho-ton mask and taking a series of PSD positive-ion mass spec-tra in time during the continual irradiation of incident pho-tons until little visual difference was observed in the two most recent spectra. The second共method 2兲 fixed the mass of the quadrupole mass spectrometer at F+ 共mass=19兲 and monitored the ion signal as a function of incident photon energy near the F共1s兲 edge 共681–704 eV兲. In order to reduce the uncertainty in the photon energy caused by a backlash of the grating movement, the photon energy was varied in one direction from low to high energy for each PSD measure-ment.

In general, the second method is employed to study the positive-ion yields as a function of incident photon energy, assuming that there is no photon-induced change in the chemical state of the overlayer when the PSD spectrum is taken during varying-energy photon irradiation. However, due to the high photolysis cross section of CF₃Cl adsorbed on the Si共111兲-7⫻7 surface, in the present study we in-tended to use the second method to examine the photon-induced changes in the chemical states of adsorbates by ob-serving the variation of the spectrum shape in the series of F+ PSD spectra, which were measured one by one via repeating the incident photon energy scan. For each PSD spectrum, after the photon mask was opened, the F+_{-ion yield was} mea-sured during varying the energy of incident photons from 681 to 704 eV. When a PSD spectrum was finished, the pho-ton mask was closed and the grating of the monochromator was moved back to a low-energy position for the next PSD scan. The photon exposure for each PSD scan is calculated by integrating the photon flux over various incident photon energies, and the accumulated photon exposure for each spectrum in a series of F+ _{PSD spectra is the summation of} the photon exposures of its present and previous PSD scans. A 800 line/ mm grating and 30␮m slits were used, giving an energy resolution of ⬃0.1 eV for the working energy range of 681– 704 eV.

The total electron yield共TEY兲 spectrum from molecular solid CF3Cl was measured by a dual microchannel plate de-tector operated at 1600 V dc. The collected and amplified current was measured with an electrometer共Keithley 6512兲. The analog dc output voltage from the electrometer, corre-sponding to the measured current, was converted into fre-quency by a voltage controlled oscillator共VCO兲. The pulses from the VCO were counted by a counter and recorded by a computer.

III. RESULTS AND DISCUSSION

In the present study we will focus solely on submono-layer CF3Cl coverage and molecular solid CF3Cl. A detailed study of the photolysis of multilayer films of CF3Cl is be-yond the scope of this work. In the case of submonolayer

coverage, the surface was prepared by exposure of a clean Si共111兲-7⫻7 surface to 0.3⫻1015_{molecules/ cm}2_{of CF}

3Cl at 30 K. This produces a surface with submonolayer cover-age of CF₃Cl共⬃0.75 ML兲.23

In order to investigate the photon-induced dissociation/ desorption of the adsorbed CF₃Cl molecules, the PSD mass spectrum technique 共method 1兲 was employed to study the positive ion desorption yields, and a series of PSD positive-ion mass spectra was measured. Figure 1 shows the first, middle, and last sequential PSD positive-ion mass spectra in time for the dose of 0.3⫻1015_{molecules/ cm}2_{, which were} measured in the mass range of 0 – 106 amu and at an incident photon energy of 704.0 eV. The photon of 704.0 eV was employed, because the energy of this photon is just higher than the F共1s兲 binding energy. These PSD mass spectra in-dicate that F+_{is the only desorbed ion, and the intensity of F}+ ion yield increases with increasing photon exposure. The se-quential PSD positive-ion mass spectra in time obtained from the same surface using other incident photon energies near the F共1s兲 edge have also been measured 共not shown兲. These series of PSD positive-ion mass spectra also show that F+_{is the only desorbed ion, and that the intensity of F}+_ion yield increases with increasing photon exposure.

In order to gain insight into the formation of the fluori-nation states of the bonding surface Si atom via the photon-induced dissociation of adsorbed CF3Cl molecules on a Si共111兲-7⫻7 surface using monochromatic soft x-ray SR near the F共1s兲 edge, continuous-time core-level PSD

spec-troscopy共method 2兲 was employed to study the F+ desorp-tion yields. A series of F+_{PSD spectra was measured. Figure}

2 shows the series of F+_{PSD spectra of CF}

3Cl adsorbed on Si共111兲-7⫻7 at 30 K for various photon exposures using 681– 704 eV photons. The CF₃Cl dose of the surface is 0.3 ⫻1015_{molecules/ cm}2_{共⬃0.75 ML兲. The total photon} expo-sure for each spectrum is given in units of 1016_{photons/ cm}2 and shown on the right of each curve. Another series of F+ PSD spectra which extends the series of Fig. 2 is shown in Fig. 3. As mentioned earlier, each F+ _{PSD spectrum was} measured one by one via repeating the incident photon en-ergy scan, the variation of the PSD spectrum shape is due to the damage of the adsorbed CF3Cl molecules.

The first F+_{PSD spectrum关Fig.2共a兲兴 indicates two} reso-nances at 690.2 and 692.6 eV, while the last F+ _PSD spec-trum 关Fig. 3共f兲兴 shows two resonances at 687.0 and 689.6 eV. In order to interpret the first F+_{PSD spectrum, the} F+_{PSD spectrum from molecular solid CF}

3Cl was measured and shown in Fig.4共a兲, which also depicts two resonances at 690.2 and 692.6 eV. We would like to emphasize that the F+ PSD spectrum of molecular solid CF3Cl was obtained by using relatively low photon flux light. In order to minimize FIG. 1. Comparison of the first, middle, and last PSD positive-ion mass

spectra in a series of 45 for CF3Cl adsorbed on a Si共111兲-7⫻7 surface at

30 K as a function of photon exposure using 704.0 eV photons. The CF3Cl

dose of the surface is 0.3⫻1015_{molecules/ cm}2_{共⬃0.75 ML兲.}

FIG. 2. Continuous-time F+ _{PSD spectra of CF}

3Cl adsorbed on a

Si共111兲-7⫻7 surface at 30 K as a function of photon exposure using 681– 704 eV photons. The CF3Cl dose of the surface is 0.3

⫻1015_{molecules/ cm}2 _{共⬃0.75 ML兲. The total photon exposure for each}

spectrum is given in units of 1016_{photons/ cm}2_{and shown on the right of}

the figure. The 690.2 and 692.6 eV features in spectrum 共a兲 are due to excitations of F共1s兲 core electron of adsorbed CF3Cl molecule to

11a1关共C–Cl兲*兴 and 共8e+12a1兲关共C–F兲*兴 antibonding orbitals, respectively.

The peak at the energy position of 687.0 eV in spectrum共b兲 is attributed to a transition from the F共1s兲 core level of surface SiF species to an unoccu-pied state whose symmetry is perpendicular to the surface.

the possible beam damage we reduced the photon flux den-sity by adjusting the refocusing mirror of the beamline to defocus the beam. Furthermore, in order to make sure that there is no significant beam damage of the surface in this measurement, we took a series of five scans in time. No significant change was observed in this series of F+ PSD spectra.

It is well known that the positive-ion core-level PSD spectra of molecular solids are roughly similar to the TEY spectra. The total electron yield spectrum is the yield of pho-toelectrons of all kinetic energies from the solid as a function of incident photon energy. For core-level excitation, TEY spectrum has been shown to be proportional to photoabsorp-tion spectrum.24 Measurement of the TEY spectrum from solid CF3Cl was carried out in the energy region near the F共1s兲 ionization threshold and plotted in Fig. 4共b兲. In this energy region the spectral features correspond to discrete resonances which are due to the excitations of a F共1s兲 core-level electron to unoccupied orbitals.

To the best of our knowledge, the only F共1s兲 gas-phase photoabsorption spectrum was measured by Zhang et al.25

They measured the electron energy loss spectroscopy 共EELS兲 spectrum, then converted the EELS spectrum to a relative photoabsorption spectrum by a so-called Bethe-Born conversion procedure,26,27 and obtained the absolute differ-ential oscillator strength 共cross section兲 spectrum by single point normalization to a known absolute optical value. The gas-phase photoabsorption spectrum is shown in Fig. 4共c兲. Zhang et al. assigned the transition F共1s兲→11a1关共C–Cl兲*_兴 to the low-energy feature of the spectrum, and the transition F共1s兲→共8e+12a1兲关共C–F兲*_{兴 to the high-energy feature.} Since the TEY spectrum is proportional to the photoabsorp-tion spectrum of molecular solid, we compare our TEY spec-trum with the gas-phase photoabsorption specspec-trum to obtain the orbital character in this energy region. Positions of the features in the TEY spectrum are indicated along with those of the gas-phase photoabsorption spectrum in Fig. 4. The F共1s兲 photoionization threshold of gas-phase CF3Cl is also shown in the figure. Because the features at 690.2 and 692.6 eV in our TEY spectrum correspond to those at 690.51 and 692.60 eV in the gas-phase photo-absorption spectrum,25 the photoexcitations leading to the features at 690.2 and 692.6 eV in our TEY spectrum can be assigned to the transitions of F共1s兲→11a1关共C–Cl兲*_{兴 and} F共1s兲→共8e+12a1兲关共C–F兲*_{兴, respectively.}

FIG. 3. Continuous-time F+_{PSD spectra共which extends the series of Fig.2兲}

of CF3Cl adsorbed on a Si共111兲-7⫻7 surface at 30 K as a function of

photon exposure using 681– 704 eV photons.共a兲 is the same as Fig.2共f兲, and共b兲 to 共f兲 are F+_{PSD spectra for further photon exposures. The peaks at}

the energy positions of 687.0 and 689.6 eV in spectrum共f兲 are attributed to a transition from the F共1s兲 core level of surface SiF species to an unoccu-pied state whose symmetry is perpendicular to the surface and a transition from the F共1s兲 core level to a final state which is localized completely on the F atom, respectively.

FIG. 4.共a兲 F+_{PSD spectrum near the F共1s兲 edge from solid CF}

3Cl.共b兲 Total

electron yield 共TEY兲 spectrum from solid CF3Cl.共c兲 EELS spectrum of

gas-phase CF3Cl共see Ref.25兲. 共d兲 F+ion yield curve of gas-phase CF3Cl

共see Ref.32兲. The vertical lines show the energy positions of transitions to the 11a1 and 共8e+12a1兲 unoccupied molecular orbitals. The vertical line

with hatching indicates the F共1s兲 ionization threshold of gas-phase CF3Cl

It is well known that the electronic transition from a core level to an unoccupied bound orbital is usually observed as resonance near the core-level ionization threshold in the pho-toabsorption spectra. The bound orbital can be an antibond-ing valence orbital, a Rydberg orbital, or an orbital with mixed valence-Rydberg character.28 The assignments of these bound terminating orbitals are somewhat frustrating because of their close term values. Robin28,29has pointed out that the photoabsorption of molecules in the condensed phase forms a yardstick to judge the valence/Rydberg char-acter and the local charchar-acter of resonances. The Rydberg orbitals extend beyond the ligand field of the molecule and will be drastically modified upon solidification. As a result, the feature due to the excitation from a core level to a Ryd-berg orbital, observed in the gas-phase photoabsorption spec-trum, will be quenched or disappear in the solid. On the contrary, due to their local character, the antibonding valence orbitals will be much less affected in the solid. Therefore, the excitation from a core-level to these orbitals will persist upon solidification. Since the low- and high-energy features of the gas-phase photoabsorption spectrum关Fig.4共c兲兴 are persistent

in our TEY spectrum of solid CF₃Cl, the unoccupied bound orbitals to which the F共1s兲 electron was excited should have the valence character. This result supports the assignment of 11a1关共C–Cl兲*_{兴 and 共8e+12a1兲关共C–F兲}*_{兴 to the low- and} high-energy features suggested by Zhang et al.25

Now, we compare the F+_{PSD spectrum of solid CF} 3Cl 关Fig.4共a兲兴 with the TEY spectrum 关Fig.4共b兲兴. Both spectra

exhibit two features at the energy positions of 690.2 and 692.6 eV. The main difference is that in the F+ PSD spec-trum the intensity of the low-energy feature is higher than that of the high-energy feature, while in the TEY spectrum the intensity of the low-energy feature is lower than that of the high-energy feature. As discussed previously, the low-energy feature in the TEY spectrum corresponds to the exci-tation of F共1s兲 to an antibonding orbital F共1s兲 →11a1关共C–Cl兲*_{兴, and the high-energy feature is attributed} to F共1s兲→共8e+12a1兲关共C–F兲*_{兴 excitation. The close} resem-blance of the F+PSD and TEY spectra indicates that excita-tions to the 11a1关共C–Cl兲*兴 and 共8e+12a1兲关共C–F兲*兴 orbitals can be assigned to the low- and high-energy features, respec-tively, in the F+ PSD spectrum, and these excitations even-tually result in dissociation. The most possible dissociation mechanism is an Auger decay of the F共1s兲 core hole fol-lowed by a Coulomb explosion of the excited molecular ion. In studies of Auger electron spectra of gas-phase CF₃Cl,30 it was observed that the Auger decay from F共1s兲 hole state produces mostly a doubly charged molecular ion having holes in F共2p兲 lone pair and/or C–F bonding orbitals. This molecular ion is expected to decompose to CF2Cl+and F+.31Thus, the most possible mechanism responsible for the observed 690.2 and 692.6 eV features in the F+PSD spec-trum of solid CF₃Cl is the excitations of F共1s兲 to 11a1关共C–Cl兲*兴 and 共8e+12a1兲关共C–F兲*兴 antibonding orbitals, respectively. Following Auger decay, two holes are created in F共2p兲 lone pair and/or C–F bonding orbitals forming a 2-hole, 1-共excited兲 electron 共2h1e兲 final state. This 2h1e state will cause F+ion desorption following the decomposi-tion of the molecular ion.

On the other hand, the production of F+ _{ion yield near} the F共1s兲 edge in gas-phase CF3Cl has been studied by Su-zuki et al.32 using synchrotron radiation in the 680– 750 eV photon energy range. For comparison, the gas-phase F+ion yield curve32 is shown in Fig. 4共d兲. We find that the gas-phase F+ ion yield curve 关Fig. 4共d兲兴 is similar to the gas-phase and solid-gas-phase photoabsorption spectra 关Figs. 4共c兲 and4共b兲兴. This finding indicates that the photoabsorption re-sults in the dissociation of CF3Cl and the production of F+ ions. However, comparing the F+ _{PSD spectrum of solid} CF3Cl关Fig.4共a兲兴 with the gas-phase F+ion yield curve关Fig.

4共d兲兴, we find that in the F+_{PSD spectrum of solid CF} 3Cl the intensity of the low-energy feature is higher than that of the high-energy feature, on the contrary the intensity of the low-energy feature is lower than that of the high-low-energy feature in the gas-phase F+_{ion yield curve. The difference of these two} spectra in the relative intensities of the low- and high-energy features can be understood by considering the following mechanisms which are effective in the solid, but not in the gas.

共1兲 In order to desorb from the surface, a dissociated ion should have enough kinetic energy to overcome the sur-face potential. If the kinetic-energy distribution of ions is small enough that it overlaps the minimum energy which is necessary to overcome the surface potential, the kinetic-energy distribution will be asymmetric and shows a cutoff. As a result, the number of the desorbed ions will be less than that of the dissociated ions. 共2兲 On the surface of a molecular solid, a dissociated ion

will have some probability of capturing electrons near the surface and becoming neutralized before it escapes from the surface, and the low-kinetic-energy ions will have greater probability than the high-kinetic-energy ions to capture electrons near the surface and become neutralized.

The 2h1e predissociative state of the high-energy fea-ture, in the gas-phase F+ _{ion yield curve and our F}+ _PSD spectrum of solid CF₃Cl, has two holes created in the F共2p兲 lone pair and/or C–F bonding orbitals and one excited elec-tron localized at the C–F bond, while the 2h1e state of the low-energy feature has its excited electron localized at the C–Cl bond. Since for the high-energy feature the two holes and one excited electron are all localized at the C–F bond, it is expected that the kinetic energies of the created F+_{ions of} this feature is lower than that of the low-energy feature in which the excited electron is localized at the C–Cl bond. Therefore, although there is a higher possibility for F+ pro-duction in the high-energy feature, as shown in the gas-phase F+_{ion yield curve 关Fig.4共d兲兴, because of the higher} photo-absorption cross section, the F+ _{ions created in the} high-energy feature are less possible to desorb from the surface because the ions have relatively low kinetic energies to over-come the surface potential and have a higher probability of neutralization. As a result, in our F+ _{PSD spectrum of solid} CF3Cl关Fig.4共a兲兴 the intensity of the high-energy feature is lower than that of the low-energy feature.

We now turn to the continuous-time F+PSD spectra of CF3Cl adsorbed on Si共111兲-7⫻7 as a function of photon

exposure. As mentioned earlier, the first F+ _{PSD spectrum} 关Fig.2共a兲兴 shows two features at 690.2 and 692.6 eV.

Com-paring this spectrum with the F+ _{PSD spectrum of solid} CF₃Cl关Fig.4共a兲兴, we find a strong resemblance of these two

spectra. Both spectra exhibit two features at the energy po-sitions of 690.2 and 692.6 eV. As discussed previously, the feature at 690.2 eV in the F+ PSD spectrum of solid CF3Cl corresponds to the excitation of F共1s兲 to an antibonding or-bitals F共1s兲→11a1关共C–Cl兲*_{兴, and the feature at 692.6 eV is} attributed to F共1s兲→共8e+12a1兲关共C–F兲*_{兴 excitation. The} close resemblance of these two PSD spectra indicates that the same excitation and dissociation mechanisms are also responsible for the features at 690.2 and 692.6 eV in the first F+ _{PSD spectrum 关Fig. 2共a兲兴 of the submonolayer} CF3Cl-covered surface.

In addition to the features at 690.2 and 692.6 eV关shown in Fig.2共a兲兴, a feature at 687.0 eV is seen in the second F+ PSD spectrum关Fig.2共b兲兴. This 687.0 eV feature increases its

intensity with photon exposure, as shown in Figs.2共b兲–2共f兲

and Figs.3共a兲–3共f兲. Because the F+ PSD spectrum with the highest photon exposure关Fig.3共f兲兴 has the largest 687.0 eV feature, it will be discussed first.

In general, regardless of the photolysis pathway共s兲 in-volved, the studies of the retained photochemically generated fluorosilyl products are, themselves, of considerable interest. It is well known that F+_{PSD spectrum near the Si共2p兲 edge} can be employed to identify different SiFx 共x=1–4兲

species.34–36 In order to identify the possible fluorosilyl spe-cies retained on the surface at the near completion of pho-tolysis, we have obtained the F+ _{PSD spectrum near the} Si共2p兲 edge, following the measurement of the sequential F+ PSD spectra near the F共1s兲 edge. The obtained F+_PSD spec-trum共not shown兲 indicates a structure with two peaks at the energy positions of 101.1 and 101.7 eV. This structure re-sults typically from the desorption process which is initiated by the excitation of 2p_3/2and 2p_1/2core levels of the bonding Si atom in SiF species.20,34Therefore, we concluded that the fluorosilyl product created on the surface is SiF. Thus, the F+ PSD spectrum near the F共1s兲 edge at the near completion of photolysis 关Fig. 3共f兲兴 共exposure ⬃39.8⫻106_{photons/ cm}2 to 681– 704 eV photons兲 should be due to the excitation of surface SiF species.

Yarmoff and Joyce have studied the F+ _{PSD near the} F共1s兲 edge from a clean Si共111兲-7⫻7 surface which was exposed to 50 L of XeF₂ at room temperature and subse-quently annealed to 300 ° C.35 The annealed surface was shown to be terminated solely by SiF groups.34For compari-son, the F+_{PSD spectrum obtained from surface SiF species} by Yarmoff and Joyce is included in Fig.5共b兲,35and our last F+_{PSD spectrum关Fig.3共f兲兴 is shown in Fig. 5共a兲.} Compar-ing Fig.5共a兲with5共b兲, we find a strong resemblance of these two PSD spectra. Both spectra exhibit similar shape of de-sorption curves. The small difference is that we observed two narrow peaks at the energy positions of 687.0 and 689.6 eV in our spectrum, while their curve shows two broader peaks at ⬃686 and ⬃690 eV. Yarmoff and Joyce assigned the peaks at⬃686 eV to a transition from the F共1s兲 core level to an unoccupied state whose symmetry is perpendicular to the surface共i.e., along the Si–F bond direction兲, and the peak at

⬃690 eV to a transition from the F共1s兲 core level to a final state which is localized completely on the F atom.35 They believed that the Auger decays following these excitations will result in the bond breaking of Si–F and the desorption of F+ _{ions. Although there are small differences in the peak} positions and the peak width in these two PSD spectra, the great resemblance indicates that the same desorption pro-cesses are also responsible for the features at 687.0 and 689.6 eV in our spectrum.

Since the PSD spectrum near the core level is a convo-lution of the initial core level and the local density of final states, the two narrow features observed in our F+_PSD spec-trum indicate that there is less structure in the local density of final states at our surface than the annealed 50 L XeF₂/ Si共111兲 surface. The difference of the structures in the local density of final states is possibly due to the different number densities of SiF species of these two systems. The number density of SiF species of the annealed 50 L XeF2/ Si共111兲 surface is greater than that of the photolyzed submonolayer CF3Cl/ Si共111兲 surface.

In conclusion, the F+ _{PSD spectrum 关Fig. 3共f兲 or 5共a兲兴} indicates that the surface fluorosilyl product formed by the photolysis of the submonolayer CF3Cl-covered surface with 681– 704-eV photons is SiF. The observation of the 687.0 eV FIG. 5. 共a兲 F+ _{PSD spectrum near the F共1s兲 edge for a CF}

3Cl/ Si共111兲

surface 共gas dose=0.3⫻1015_{molecules/ cm}2_{兲 at high exposure to}

681– 704 eV photons共39.8⫻1016_{photons/ cm}2_{兲 and near completion of}

re-action. Curve共a兲 is the same as Fig.3共f兲.共b兲 F+_{PSD spectrum of a Si共111兲}

surface after exposure to 50 L of XeF2and annealing to 300 ° C共see Ref. 35兲. The polarization direction of the incident light is p polarization for both 共a兲 and 共b兲.

feature in Fig. 2共b兲 indicates detectable damage of the ad-sorbed CF3Cl molecules and production of the surface SiF species even during taking the second F+ _{PSD spectrum.}

We would like to point out that the F+ _{PSD spectrum} does not change its spectrum shape and intensity in the last ten PSD spectra of the sequential F+_{PSD spectra共not shown} in Fig.3兲. This fact indicates that in comparison to the

pho-tolysis cross section of CF3Cl, the photolysis cross section of SiF is very small and the beam damage of SiF species is negligible in our F+PSD experiments.

It is interesting to roughly estimate the photolysis cross section of the adsorbed CF3Cl from the sequential F+ PSD spectra 关Figs. 2 and 3兴. Since the peak at 687.0 eV is

attributed to F+desorption from the SiF species via the ex-citation of the F共1s兲 core level to an unoccupied state, the intensity of this feature共obtained by subtracting the intensity just before the edge from the intensity at the 687.0 eV peak兲 should be proportional to the density of SiF on the surface. Therefore, we can obtain the relative density of SiF versus photon exposure from Figs. 2共a兲–2共f兲 and Figs. 3共a兲–3共f兲. These data points共not shown兲 can be fitted with a function, which has the character of an exponential rise to maximum, a共1−e−␴␾兲, where ␴is the decay constant and␾is the pho-ton exposure. If we simply assume that the photolysis of CF₃Cl produces F, and that the F reacts with the Si surface to form SiF species; then, the photolysis cross section of CF₃Cl adsorbed on Si共111兲-7⫻7 by photons with varying energy 共681–704 eV兲 can be extracted from the decay constant and found to be⬃1.0⫻10−17cm2.

IV. CONCLUSIONS

Continuous-time core-level PSD spectroscopy can be used for an investigation of soft x-ray-induced reactions of adsorbate with the surface in core-electron-stimulated sur-face photochemistry. In the present study, monochromatic synchrotron radiation was employed as a soft x-ray light source to induce the reactions of CF₃Cl adsorbed on Si共111兲-7⫻7, and also as a probe in the positive-ion PSD technique for monitoring the formation and identifying the characteristics of the produced fluorination states of bonding surface Si atom.

The sequential F+ _{PSD spectra of submonolayer CF} 3Cl adsorbed on a Si共111兲-7⫻7 surface as a function of photon exposure using 681– 704 eV photons were measured, and the F+_{PSD and TEY spectra from molecular solid CF}

3Cl were also obtained. The features at the energy positions of 690.2 and 692.6 eV in the F+_{PSD and TEY spectra of solid CF}

3Cl are interpreted as arising from excitations of F共1s兲 core level to 11a1关共C–Cl兲*_{兴 and 共8e+12a1兲关共C–F兲}*_{兴 antibonding} orbit-als, respectively. These excited states decay by an Auger pro-cess into 2h1e final states which are responsible for the F+ desorption. The sequential F+_{PSD spectra show the variation} of their shapes with photon exposure and indicate the dam-age of adsorbed CF3Cl molecules and the formation of sur-face SiF species. From the continuous-time F+_{PSD spectra,}

the photolysis cross section of the adsorbed CF3Cl by pho-tons with varying energy 共681–704 eV兲 is determined to be ⬃1.0⫻10−17_cm2_.

ACKNOWLEDGMENTS

This work was performed at the National Synchrotron Radiation Research Center共NSRRC兲, Hsinchu, Taiwan. We would like to thank all members at NSRRC for their techni-cal support. Financial support by the National Science Coun-cil of the Republic of China through Project No. NSC 93-2112-M-006-023 is gratefully acknowledged.

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