EUV-Driven Carbonaceous Film Deposition and Its Photo-oxidation on

Article pubs.acs.org/JPCC

EUV-Driven Carbonaceous Film Deposition and Its Photo-oxidation on a TiO2 Film Surface Nadir S. Faradzhev,*,† Monica McEntee,† John T. Yates, Jr.,† Shannon B. Hill,‡ and Thomas B. Lucatorto‡ † ‡

Department of Chemistry, University of Virginia , Charlottesville, VA 22904, United States National Institute of Standards and Technology, Gaithersburg, MD 20899, United States

ABSTRACT: We report the photodeposition of a carbonaceous layer grown on a TiO2 thin film by extreme ultraviolet (EUV)induced chemistry of adsorbed n-tetradecane and the subsequent photo-oxidation of this film. Chemical analysis of the carbonaceous layer indicates that irradiation by 92 eV photons converts a fraction of the 14-carbon-atom alkane into two polycyclic aromatic hydrocarbon (PAH) molecules, anthracene and phenanthrene, both also containing 14 carbon atoms. Under continuing irradiation, EUV-induced dehydrogenation and cross-linking of PAHs and precursor alkane fragments form a complex network of carbon bonds with the inferred structure of an sp2-carbon rich film. Exposure of the carbonaceous film to five oxygencontaining molecules has little or no effect in the dark at 300 K. However, in the presence of EUV photons, the ability of the molecules to etch the carbonaceous film increases significantly. Their relative photo-oxidation activity follows the trend H2O2 ≈ O3 > NO ≈ O2 > H2O. The rate of the photo-oxidation reaction increases with the partial pressure of the oxidizer. Raising the substrate temperature has little effect on the photo-oxidation reaction rate, which is in contrast to the rate of EUV-induced carbonaceous film growth that exhibits a strong temperature dependence. The difference between mechanisms of EUV-induced hydrocarbon decomposition and photo-oxidation of the carbonaceous film is discussed.

1. INTRODUCTION The photoactivated oxidation of organics and carbonaceous films plays an important role in diverse applications such as biomedical technology, waste remediation, self-cleaning window glass, photocatalysis, and semiconductor manufacturing.1,2 The present study has been motivated by the needs of extreme ultraviolet lithography (EUVL) that uses 92 eV photons to produce a pattern on a resist-coated wafer. Photon-induced reactions involving residual hydrocarbons and resist outgassing products in the vacuum system lead to a rapid build-up of a carbonaceous layer on the surface of expensive EUVL optics, reducing the optical transmission and the wafer throughput. To clean the optics, either by occasional cleaning cycles or by continuous cleaning methods, one may envision using EUVactivated cleaning processes involving the addition of small quantities of oxidizer molecules to the vacuum environment. This paper reports the chemical processes involved in the EUVinduced formation of a carbonaceous film and its photooxidative removal by 92 eV (EUV) photons. There have been a few attempts to study the properties of surface-bound carbon produced via EUV-induced decomposition of hydrocarbons.3−7 X-ray reflectivity measurements show that the density of such a layer is nearly half that of graphite5 and that its hydrogen content decreases with EUV dose. Our earlier studies indicate that the films produced by EUV © XXXX American Chemical Society

decomposition of hydrocarbons evolve in the course of irradiation: prolonged EUV illumination leads to compaction of the film.6 High photon doses will apparently convert such a film to a graphitic layer as found in a recent study of the graphitization of a thick amorphous carbon layer upon irradiation by EUV light from a free-electron laser.7 The phenomena of compaction and graphitization will affect the chemical activity of the grown carbonaceous layer and thus the efficiency of the cleaning technique of choice. Hence, a detailed analysis of the EUV-induced chemistry of adsorbed organics is important. Oxygen plasmas and ozone are routinely used to remove organic contamination from silicon wafers. Active oxygen species readily react with adsorbed organic molecules producing volatile species (e.g., carbon dioxide) that desorb from the surface.8,9 Unfortunately, these highly energetic processes may lead to undesirable effects such as an increase of the surface roughness and the accumulation of subsurface oxygen, detected for example for Ru-capped optics.10 Atomic oxygen generated by thermal cracking of O2 has also been shown to react with carbonaceous layers on metal films, Received: September 12, 2013 Revised: September 20, 2013

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dx.doi.org/10.1021/jp4091427 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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2. EXPERIMENTAL SECTION Photo-oxidation experiments are performed on a carbonaceous layer grown by hydrocarbon photodecomposition on the surface of a tri-layer film structure (TiO2/Si/Mo) fabricated on silicon. The structure is terminated by an amorphous 1.5 nm thick TiO2 film and mimics the three top layers of optics designed for EUVL. A carbonaceous film (several nanometers thick and ∼1 mm in diameter) is grown by exposing the sample to the EUV beam at normal incidence at a n-tetradecane (C14H30) partial pressure of ∼10−5 Pa. For chemical analysis, the deposited C was partially dissolved and extracted with a 2 μL droplet of n-heptane and then also with a 2 μL droplet of benzene. The solution was subjected to GC-MS analysis. The GC-MS spectrometer (Thermo Finnigan Voyager with Trace 2000 GC, Rxi-17 column)26 was calibrated against standard naphthalene solutions in n-heptane giving a linear calibration plot. In addition, the GC-MS spectrometer was calibrated against standard solutions of anthracene and phenanthrene in heptane. While analysis of the extracted products is very informative, one must remember that only soluble molecules are observed. Photo-oxidation of the carbonaceous deposit produced by EUV degradation of C14H30 is achieved using several oxygencontaining molecules: NO, H2O, H2O2, O2 and O3. Their mass spectra detected under gas flow conditions prior to EUV exposures are shown in Figure 1. Nearly pure hydrogen

producing a clean surface with no effect on roughness and with an efficiency considerably higher than that provided by atomic hydrogen.11,12 Moreover, atomic oxygen is able to volatilize dense forms of carbon not susceptible to atomic hydrogen;11 an excess of surface oxygen can be removed later by atomic hydrogen that reduces oxide films on Ru efficiently.13 The work described here uses various oxygen-containing molecules, excited by 92 eV radiation, to achieve oxidation. This presumably occurs by means of O atom production and possibly other processes. Ozone (O3), hydrogen peroxide (H2O2), and nitric oxide (NO), having unpaired or lone-pair valence shell electrons, demonstrate strong oxidation properties. The thermally activated surface oxidation mechanism associated with all three oxidizer molecules involves their decomposition to form free radicals containing oxygen followed by oxidation product formation and desorption.14−16 Deitz and Bitner observed an oxidative etching reaction on charcoal induced by ozone.16 Mawhinney and Yates used ozone to thermally oxidize singlewalled carbon nanotubes17 and amorphous carbon.18 Kumar et al.19 found that adding hydrogen peroxide to an aqueous solution of a dye intermediate increased the rate of its degradation under UV light. The rate increased further when H2O2 was used in combination with TiO2 nanoparticles. Davis et al.20 showed that C deposited by e-beam exposure could be removed by e-beam exposure in 3 × 10−5 mbar NO at a rate of approximately 0.2 nm/h. TiO 2 is widely used in solar-driven environmental remediation of organic contamination. The redox reactions at a TiO2 surface are driven by photon-generated excitons21 produced by radiation with energy above the TiO2 bandgap (∼3.0 eV). Charge separation following this process produces electron−hole pairs that activate oxidation or reduction processes at the surface. Because of its photoactivity and favorable optical properties in the EUV region22 as well as the stability of the TiO2/SiO2 interface on Si,23 thin amorphous TiO2 films are an attractive substitute for Ru-capping layers currently used to protect EUVL optics. A TiO2 coating upon irradiation above ∼3.0 eV is expected to result in self-cleanable EUVL optics22 by indirect excitation in TiO2, although the large concentration of defects in nanometer thick TiO2 films may diminish the photocatalytic activity by creating efficient electron−hole recombination processes. In addition to the indirect surface photochemistry involving low-energy secondary electrons caused by electronic excitation in TiO2 (or other substrates), EUV radiation of 92 eV energy is expected to result in chemical bond breaking in adsorbed molecules via direct photoexcitation. The rate of a photooxidation reaction is expected to be significant if the photons are capable of producing long-lived active species from an oxidizer molecule. Earlier studies show that broadband synchrotron radiation can excite O2 to produce etching of sputter-deposited carbon24 and even crystalline diamond,25 materials that are resistant to oxidation under normal conditions. In this investigation, we probe the chemistry of the 92 eV photon-induced decomposition of a long-chain hydrocarbon to produce a highly disordered, nonvolatile carbonaceous layer. We also compare the efficiency of five oxygen-containing molecules able to photo-oxidize the carbonaceous layer.

Figure 1. Mass spectra of NO, H2O, H2O2, O2, and O3 measured at the NIST-EUV exposure facility at the beginning of each experiment. Ve = 70 eV. For clarity, the cracking products corresponding to H2O and O2 are shown as shaded areas.

peroxide is obtained by vacuum distillation of 30 wt % H2O2 using a glass apparatus27 that is directly connected to the irradiation chamber. The distilled peroxide reveals a prominent parent mass spectrometer peak at 34 amu (H2O2+) with other features associated with peroxide decomposition products. High-purity ozone is generated from O2 at 1 atm in a glass vacuum cell by a silent electric discharge technique,28 trapped in Si-gel at 195 K, and transferred line-of-sight to the sample using an apparatus similar to that described elsewhere.29 On the basis of the ratio of the 48 and 32 amu peaks in the O3 mass spectrum (Figure 1), electron impact ionization cross sections,30 and the position of the mass spectrometer with B



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information about the nature of carbon produced under EUV is scarce and controversial. Using laser-generated surface acoustic waves, a carbonaceous layer produced by EUV (hν = 92 eV) decomposition of residual hydrocarbons from a vacuum chamber is inferred to be mechanically soft and polymeric with a high percentage of hydrogen.3 Fitting of the energy loss near edge spectra from the layer formed by EUV decomposition reveals that the film consists mainly of distorted sp2 fullerene and graphitic bonds.4 Studies of small hydrocarbon molecules adsorbed on Si(100) and irradiated with electrons in the energy range 0.4−2.0 keV33 have shown that radiation damage produces an amorphous and hydrogenated polymeric carbonaceous product, containing a high fraction of carbon−carbon double bonds. The X-ray excited valence band (VB) spectrum of the carbonaceous film formed in this work by EUV decomposition of n-tetradecane (EUV-C) reveals a broad feature indicative of a complex network of carbon bonds (Figure 3a). This broad

respect to the sample, we conclude that the purity of ozone in our experiments was >15%. All measurements are made under dynamic pumping conditions assuring that constant gas purity is achieved throughout the experiment. The oxidizing gases are introduced to the exposure chamber using a Pyrex glass directional gas doser. Under our geometric conditions, the measured pressure of the flowing gas in the chamber is a good indication of the incident flux of gas onto the sample because, even with the doser geometry used, most gas arrives at the sample surface from a random reflected flux from the cell walls. Exposures to 92 eV photons during carbonaceous film growth and photo-oxidation experiments are done at the NIST synchrotron ultraviolet facility (SURF III) in an ultrahigh vacuum chamber with a base pressure of ∼7 × 10−9 Pa.6 The peak intensity at the center of the EUV spot corresponds to ∼2 × 1016 92 eV photons cm−2 s−1. The film thickness evolution is monitored in real time using the in situ null-field ellipsometric imaging system (NEIS).31 The system provides an image of the carbonaceous spot with subnanometer thickness sensitivity. We also use XPS to measure the absolute thickness of the resulting carbonaceous film ex situ after each set of experiments and to analyze film composition and the carbon chemical state. Figure 2 shows a

Figure 3. (a) X-ray excited valence band (VB); (b) C 1s phototoelectron spectra for pyrolytic graphite and carbonaceous film (EUV-C) grown by EUV decomposition of n-tetradecane. The oxygen signal observed in (a) arises from surface contamination of the airexposed thin film sample. The C 1s region (b) is expanded to emphasize the π → π* transition (shakeup) in graphite and shows that this graphitic transition is absent in the EUV-carbonaceous films grown here.

Figure 2. Dependence of NEIS signal as a function of XPS C thickness determined ex situ for the carbonaceous film grown using EUV in the presence of tetradecane (C14H30) and for the partially oxidized film exposed to various doses of EUV photons in the presence of NO, H2O, H2O2, and O2 as indicated in the graph. The horizontal axis is crossed at ≈0.2 nm. This thickness corresponds to the carboncontaining layer observed initially on the air-exposed TiO2 surface.

feature is clearly different from the distinct peaks in these VB spectra of n-alkanes34 and diamond35 and is normally observed for hydrogenated amorphous carbon with embedded sp2 clusters,36 condensed polycyclic aromatic hydrocarbons (PAHs),37 plasma-treated fullerene films,38 and graphite.35,39 Analysis of the C 1s region of the EUV-deposited carbon shows a mixed sp2/sp3 hybridization and the absence of the π → π* transition (shakeup satellite) associated with an ordered aromatic network (e.g., graphite) (Figure 3b). The π → π* transition is sensitive to surface disorder: 10 min of Ar+ sputtering eliminated this feature from the C 1s photoelectron spectrum of highly oriented pyrolytic graphite.40 The GC-MS spectra shown in Figure 4a clearly display the presence of both phenanthrene and anthracene products and also show control experiments containing no evidence for phenanthrene or anthracene from extraction on the unirradiated area and from the pure solvent. It is clear that

nearly linear correlation between the spatially integrated ellipsometric thickness (called hereafter the NEIS signal) and the carbon thickness measured by a microprobe XPS system. The XPS instrument is a commercial Kratos Axis Ultra spectrometer.26 The thickness calculations are based on the measured C 1s signal and the effective attenuation length calculated for the inelastic mean free path measured in graphite.32

3. RESULTS 3.1. n-Tetradecane Decomposition and Chemical Analysis of the Carbonaceous Deposit. The basic chemistry of the decomposition of adsorbed n-alkanes under EUV radiation is not well-understood. Moreover, the C



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Figure 4. GC-MS spectra of PAH compounds formed from n-tetradecane irradiation with 92 eV photons. (a) The carbonaceous deposit, measured by NEIS, was extracted sequentially with n-heptane and benzene to specifically dissolve phenanthrene and anthracene in the two extraction steps. (b) Standard GC-MS retention times. Both the retention times and the GC peak shapes agree well for phenanthrene and anthracene. The mass spectra agree with the literature for phenanthrene and anthracene and are very similar for the two molecules. Not shown are GC-MS spectra showing short chain (C14) hydrocarbons also produced by radiation damage of n-C14H30.

Figure 5. Proposed major carbonaceous product reaction pathways deduced from VB and GC-MS spectroscopic measurements done after EUV irradiation of adsorbed n-tetradecane. (A) C−C bond scission and formation of other alkane fragments; (B) H atom abstraction to produce the CC moiety and subsequent cyclization to form PAH molecules. (C) Radical−radical disproportionation to produce the CC moiety which can cyclize as in (B). D



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phenanthrene and anthracene products are observed from the irradiated area. Naphthalene was not observed in the extracts. The calibrated sensitivity of the GC-MS was ∼5 × 1012 molecules in the 2 μL extracts in each case, and control GCMS experiments are shown in Figure 4b in order to compare retention times for both phenanthrene and anthracene standards. The results of the GC-MS analysis lead to the conclusion that polycyclic aromatic hydrocarbon molecules are generated from n-alkane molecules by radiation damage and that the retention of the carbon chain length in this cyclization process is a characteristic feature of the reactions involved. The number of PAH molecules extracted using 2 μL solvent droplets from the EUV-irradiated region is on the order of 1 × 1013 molecules, based on calibration of the GC-MS instrument. On the basis of the size of the carbonaceous deposit, this is a small fraction of the remaining insoluble carbon. The thickness of the carbonaceous spot is little affected by the solvent extraction process as measured by spectroscopic ellipsometry (not shown) performed after extraction. On the basis of our photoelectron spectroscopy data (Figure 3) and earlier reports,41 we speculate that the insoluble part of the carbonaceous deposit is mainly produced by cross-linking and consists of sp2-carbon structures embedded in an sp3-carbon disordered and amorphous network. The hydrogen content, degree of graphitization, and layer compaction depend on the overall photon dose and is inhomogeneous throughout the film.6 The fact that both phenanthrene and anthracene contain the same number of carbon atoms as n-tetradecane indicates that the radiation damage process causing C−H bond scission in ntetradecane is more efficient than C−C bond scission. The cyclization of the carbon skeleton in a normal alkane is postulated to occur as a result of the radiation damage and a subsequent hydrogen abstraction process by atomic H,42 breaking multiple C−H bonds and leading to CC bond formation plus 6-member ring closure as depicted by processes (B) and (C) in Figure 5. Phenanthrene and anthracene are the only two isomers possible for the 14-carbon-fused-ring PAH molecules. 3.2. Oxidation of Carbonaceous Film during EUV Exposure. Following carbonaceous film growth, samples were exposed to oxidizer molecules in the dark and then under EUV illumination. Figure 6 shows the evolution of the NEIS signal as a function of time during such a sequence for NO. In this specific case, the NO gas pressure is ∼10−3 Pa and the sample is kept at room temperature. As indicated above, in this range of thickness, the NEIS signal is proportional to the carbonaceous film thickness (Figure 2). Figure 6 reveals progressive growth of the carbonaceous film during the hydrocarbon exposure period ∼0−6 h at a rate that is nearly constant aside from the slight modulation due to the gradual decay of the synchrotron current, which can be neglected in these experiments. The growth of the carbonaceous film is followed by a period of constant thickness (from 6 to 11 h) during which time the admission of the hydrocarbon gas is stopped, the EUV is blocked, and the deposited carbonaceous layer is exposed only to ∼10−3 Pa of NO. The oxidizer in the dark is observed to be inactive for NO as well as for all the gases studied here. At the 11 h point EUV illumination of the sample is begun again while NO dosing continues, resulting in a monotonic decrease in the NEIS signal. The postexposure XPS analysis indicates that the reduction of the NEIS signal observed in this experiment is due

Figure 6. Carbonaceous film growth and removal via EUV-mediated oxidation using NO. Evolution of NEIS signal as a function of time under three conditions: (1) EUV-induced carbonaceous film growth using C14H30 at ∼10−5 Pa as a precursor; followed by (2) exposure to NO (∼10−3 Pa) without EUV; followed by (3) continued exposure to NO in the presence of EUV light. Sample is at room temperature.

to the photo-oxidation and partial volatilization of the carbonaceous deposit. Ancillary exposures of the deposited carbonaceous layer to EUV in vacuum in the absence of oxidizer gases produce no change in the NEIS signal or XPSmeasured thickness for similar EUV doses. Evolution of the carbonaceous spot profiles during photooxidation reveals a dramatic difference between C-etch kinetics observed for ozone and hydrogen peroxide. Photo-oxidative removal of carbon by H2O2 appears to be more efficient at the edges as compared to that at the center of the spot, whereas O3 data indicate a constant, nearly linear carbon removal rate across the carbonaceous spot. Analysis of this effect and its discussion will be given in our subsequent publication. In Figure 7, the average cleaning rates measured under EUV for molecules normally classified as strong oxidizers (NO, H2O2, and O3) are compared with the cleaning rates detected for H2O and O2. (The later molecules have been discussed as potential additives to the EUVL stepper vacuum environment

Figure 7. Average carbonaceous film removal rates for various oxidizer molecules (NO, H2O, H2O2, O2, and O3) at ∼10−3 Pa in the presence of EUV light (photon flux 1.4 × 1016 cm−2 s−1). Sample is at room temperature. The data are compared with similar exposures for NO and O3 done at ∼10−4 Pa. The cleaning rates are based on C 1s XPS thickness measurements. E



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Figure 8. Evolution of NEIS signal as a function of photon dose and sample temperature during EUV-activated carbonaceous film growth using ntetradecane and during EUV-induced film oxidation using NO, O2, and H2O2 gases at ∼10−3 Pa. The temperatures are indicated in the plots. The lines are shown to guide the eye.

to promote optics self-cleaning during operation.2,43) Specifically, Figure 7 shows that the removal rate of EUV-deposited carbonaceous material is much larger for EUV exposure in H2O2 than it is for EUV exposure in H2O. Similarly, the removal rate for EUV exposure in O3 is larger than the rate in O2. The rate of C removal for NO (∼0.25 nm/h) is comparable to that for O2 and agrees with the C-etch rate observed earlier by Davis et al.20 for nitric oxide under low-energy electron irradiation. Hydrogen peroxide and ozone exhibit the highest etch rates. The etching rate of a EUV-deposited carbonaceous film (2.1 nm/h or ∼5.3 × 1012 atom cm−2 s−1 assuming graphite ρ = 1.8 g/cm3 or N = 9 × 1022 C atom/cm3) measured in this study for H2O2 is comparable to that reported for atomic hydrogen when the substrate is at room temperature.44 Note that hydrogen etching requires a considerably higher pressure of etching gas (0.2 mbar in the paper cited). Figure 7 also illustrates the influence of the partial pressure of the oxidizer. The data for NO and O3 admitted at two pressures (∼10−3 and ∼10−4 Pa) demonstrate that the rates of EUVactivated oxidation reactions scale in a sublinear manner with oxidizer pressure. This is consistent with our earlier studies (e.g., ref 6 and references therein) for carbonaceous film growth under EUV irradiation and is attributed to a quasi-logarithmic pressure scaling of the equilibrium coverage of adsorbed organic precursor molecules due to the distribution of adsorption sites and energies of a nonideal surface. The same effect may influence the equilibrium coverage of the oxidizer molecules. The effect of substrate temperature on the rate of photonactivated oxidation is compared in Figure 8 with the effect of temperature on carbonaceous film growth. The film is formed by EUV-induced deposition of n-tetradecane at ∼10−5 Pa. The plot demonstrates a significant drop of the carbonaceous film growth rate when the temperature is increased from ∼293 K to ∼323 K. This is expected as the rate of carbonization is determined by the equilibrium coverage of adsorbed organic precursor molecules,45 and the equilibrium coverage is determined by the thermal desorption rate of the adsorbed molecules, which can vary significantly with temperature.41 In contrast, increasing the temperature of the carbonaceous sample by 20−30 °C during photo-oxidation and using either

NO or O2 does not appear to have a statistically significant effect on the rate of carbon removal. Hydrogen peroxide exhibits a complicated temperature dependence in etch rate, first decreasing with temperature and then becoming almost constant. To clarify the mechanism of photoactivated oxidation, we performed pre-irradiation experiments consisting of sequential exposures of the carbonaceous film to EUV light and then to oxidizing gas in small, equal time intervals in the absence of 92 eV irradiation. Each interval corresponds to a gas dose of ∼104 molecular impingements per site when we admit an oxidizer and to a photon dose of 2 × 1019 cm−2 (∼2 × 104 photons/ site) when the sample is preirradiated in vacuum by EUV light. In Figure 9, we compare the results of pre-irradiation experiments with our standard exposures (simultaneous

Figure 9. Evolution of NEIS signal as a function of gas dose for (a) NO and (b) H2O2 at ∼10−3 Pa in the standard experiment (simultaneous exposure to EUV light and oxidizer) and in preirradiation experiments (sequential exposure) where EUV exposure occurs only before exposure to the oxidizer molecule. The sample is at room temperature. Lines are drawn to guide the eye. F



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is postulated causing the formation of fused conjugated ring systems resembling PAHs, based on analysis of the UPS spectra. As C−H bonds are broken by radiation damage, and conjugated organic molecules containing multiple CC double bonds (as ...−CHCH−CHCH−..., units of which exist either in straight chains or in cyclized structures) are formed, it is well-known that the presence of the conjugated product molecules lends chemical stability to the carbonaceous film. Conjugation of CC double bonds leads to electronic stabilization in molecules and hence to the persistence of conjugated product molecules under reactive conditions. Additionally, an electronically excited alkane molecule can transfer its energy to a neighbor conjugated molecule and be quenched before subsequent bond-breaking processes occur in the alkane, leading to lowered cross sections for radiation damage in the film and to the enhanced degradation of electronic energy into heat.46 A surprising finding in this work is that a large fraction of the soluble part of the EUV-deposited carbonaceous film consists of 14-carbon atom PAH molecules produced from the 14carbon atom alkane at the radiation damage levels employed here. This must mean that radiation-induced C−H bond scission accompanied by carbon chain cyclization predominates and that 14-C atom cyclized PAH molecules form an end point of stability in the complex sequence of radiation damage processes, starting with a 14-carbon atom normal alkane. 4.2. Photo-oxidation of Carbonaceous Films. We observe photoinduced oxidation and elimination of carbon from the carbonaceous film grown by EUV-decomposition of a linear 14-carbon atom hydrocarbon. The film consists of alkane fragments and three-ring PAHs bonded in an amorphous carbonaceous network. It is expected that active oxidizer molecules will react with the types of molecules produced by EUV degradation of an alkane such as n-C14H30. For example, in the liquid phase, the initial step of ozone reaction with PAHs is the electrophilic addition of ozone to the CC bond in the aromatic ring, formation of ozonides48,49 and scission of the most reactive bonds to yield quinones.48,50 Phenanthrene observed in our study readily reacts with ozone,48 but after the first ozone molecule is added to the aromatic ring the reaction rate substantially decelerates. This is due to the electro-negativeinduced effect of the ozonide group that protects the remaining aromatic nucleus from ozone. The phenanthrene isomer, anthracene, reacts with ozone at two positions of the middle ring consuming three ozone molecules per single PAH. The ozone reaction rates differ by several orders of magnitude for PAHs in the range 1 to 5 aromatic rings with the anthracene exhibiting the highest rate.50 Under UHV conditions, reaction of O3 with amorphous carbon is also reported to produce ozonides which then decompose to yield surface COOH groups.18 The general oxidation mechanism of graphitic surfaces by O2 is expected to be similar to PAHs, and at high temperatures it preferably takes place at defect sites (vacancies and zigzag and arm-chair edge sites51). The elimination of carbon from oxidized surfaces usually occurs at elevated temperatures via decarbonylation (desorption of CO) or decarboxylation (desorption of CO2). Oxidation of PAHs with hydrogen peroxide is also reported to yield quinones.52 The thermal desorption spectra of CO and CO2 measured for H2O2-oxidized colloidal graphite and carbon nanofibers indicate that decarbonylation and decarboxylation

exposure to EUV light and oxidizer). The data shown are NEIS signals plotted as a function of gas dose for NO and H2O2 at ∼10−3 Pa. These experiments show that for NO and H2O2, both classified as strong oxidizing molecules, the combination of EUV light and gas incidence is needed to oxidize the carbonaceous film; pre-irradiation of the carbonaceous film does not prepare it for oxidation in the dark.

4. DISCUSSION 4.1. Polycyclic Aromatic Hydrocarbon Production by EUV Irradiation of n-Tetradecane. The production of complex molecules such as the PAH molecules detected here occurs by a sequence of EUV-induced elementary reaction steps involving the n-tetradecane molecule (Figure 5). The 92 eV radiation employed in this work far exceeds the chemical bond energies present in n-alkanes, namely, C−H (4.3 eV) and C−C (3.7 eV). For ionizing radiation above 10−20 eV, it has been reported that C−H bonds are more readily broken than C−C bonds in linear alkanes.46 Such bond-breaking processes will lead to the formation of atomic hydrogen (H·) and a carbon dangling bond with a missing H atom. A well-known reaction type, involving atomic H attack on a remaining neighbor C−H bond in the alkane, will lead to the formation of H2 and to the production of a CC double bond as shown in Figure 5 (Process B) and in eq 1 below. ...CH 2−CH 2−CH 2−CH 2−CH 2...+hν→ ̇ −CH 2−CH 2... + H·→ → ...CH 2−CH 2−CH → ...CH 2−CH=CH−CH 2−CH 2... + H 2

(1)

The hydrogen-atom extraction process from C−H bonds in hydrocarbon molecules to produce H2 is well-known to be very efficient and to occur with an activation energy of about 0.3 eV.42 Hydrogen atom removal, followed by a second similar process on an adjacent −CH2− will induce CC formation at neighbor molecular sites that have lost H atoms. Thus, a film of irradiated hydrocarbon may be considered as a film in which radiation-produced atomic H is permeating and causing multiple C−H bond scission events throughout the film, subsequently forming CC double bonds throughout. In addition, CC formation may also occur by radical−radical disproportionation as shown in Figure 5(C). The alternate radiation-induced C−C bond-breaking reactions are less likely, and when they happen, because of the trapping of the large radical fragments in the “cages” present in the film surrounding the radical pairs, the C−C bond-breaking event will often be reversed as bond healing occurs under the sterically confined conditions in the cage. In addition to CC bond formation, it is also possible for C-dangling bonds, produced on neighbor hydrocarbon molecules, to couple together, causing intermolecular cross-linking in the film. The branching structures caused by cross-linking are more easily destroyed by radiation than unbranched C−C bonds, so that cross-linking products may be subsequently decomposed into radicals by further irradiation.46 These results are consistent with the studies by Ono and Morikawa47 of radiation damage [hν = 10−150 eV] in a polyethylene thin film, consisting of long chain linear alkane molecules. Here, clear experimental evidence for CC double bond formation is obtained by comparison of the UVphotoelectron spectrum (UPS) of the irradiated polymer to the original polymer and by theoretical calculations. In addition, cross-linking between polymer chains containing double bonds G



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require extra energy and take place well above 300 K.53 It is also noticed that O-groups formed by H2O2-treatment of carbon nanofibers are more thermally stable than those formed by O3treatment.54 Our data indicate that in the ∼10−3 Pa region, no thermal oxidation can be observed near 300 K in the carbonaceous films produced from a radiation-damaged alkane molecule (Figure 6). The reactivity of oxidizer molecules also cannot be induced by pre-exposure of the film to EUV radiation, indicating (Figure 9) that stored free radical sites are not maintained in the irradiated film with a long lifetime. It must therefore be concluded that the oxidizer molecules studied here are themselves activated by EUV radiation. The activation of the oxidizer molecules may also be accompanied by electronic excitation processes in the carbonaceous film which itself has been produced previously by irradiation effects. In the present experiments the rate of photo-reaction is much higher for molecules which are known to be strong oxidizers, e.g., O3 and H2O2. The photo-oxidation rate under irradiation increases sublinearly with the pressure of the oxidizer molecule. Near 300 K, the rate of oxidation under irradiation exhibits a weak dependency on the substrate temperature. This is in contrast to the rate of irradiation-induced carbonaceous film formation which exhibits a significant decrease in rate for increasing temperature due to thermally controlled adsorption lifetimes of the precursor hydrocarbon molecule.45,55,56 The results of this study show that 92 eV radiation effects on alkane molecules leads to scission of both C−H and C−C bonds. Carbon−carbon double bonds are postulated to form in the layer both through C−H bond scission and through H atom abstraction from C−H bonds by atomic hydrogen generated photochemically from the alkane as well as by radical−radical disproportionation. Cyclization of organic residues occurs with the production of fused aromatic-ring hydrocarbon species, mixed with cross-linked nonvolatile material of high molecular weight and containing CC bonds. The alkane molecule may be excited by either direct (photoexcitation) or indirect (secondary electron) excitation on the surface, e.g., by dissociative electron attachment (DEA) processes, but the activation mechanism by 92 eV photons has not been investigated here. For photo-oxidation of the carbonaceous film, the electronic excitation of an oxidizer molecule is required to produce reactive oxidizing fragment molecules. The most thermally active oxidizer molecules (O3 and H2O2) are also most active under photo-oxidation conditions. 4.3. Parameters for Growth of the Carbonaceous Spot by 92 eV Radiation. Table 1 summarizes the experimental parameters employed in this work.

(1) EUV (92 eV) photodecomposition of alkane molecules adsorbed on a TiO2 film surface near 300 K causes multiple C−H bond scission steps which lead to CC bond formation and ultimately to carbon-chain cyclization to form polycyclic aromatic hydrocarbon (PAH) molecules. In this process, C−C bond scission is a minor process compared to C−H bond scission. One multistep process leads to retention of the original carbon chain length as the alkane molecule is converted by a series of complex radiation-induced processes into PAH molecules having the same number of carbon atoms. Continued irradiation leads to further dehydrogenation, cross-linking, and formation of a carbonaceous film built of sp2-bonding units embedded in an amorphous network. (2) Highly active oxidizer molecules such as O3 and H2O2 are efficient photo-oxidizers for removal of the carbonaceous residues formed by radiation damage of normal alkanes. Molecules such as NO, O2, and H2O are less efficient in the photo-oxidation process. Little temperature dependence near room temperature is observed in the rate of photo-oxidation. The formation of a strong bond of the oxidizing molecule to the surface followed by its excitation to cause bond dissociation and desorption of reaction products is inferred as a main mechanism of carbon etch by the strong oxidizer (O3 and H2O2) molecules. (3) The use of active oxidizer molecules such as O3 and H2O2 for the photomediated cleanup of EUV photolithography optics (which are resistant to irreversible oxidation) is demonstrated using EUV processes working at high vacuum with small partial pressures of easily produced oxidizer molecules. Either continuous or stepwise optics treatment seems practical given the measured rates for the EUV-oxidative cleanup process. The C-etch rates measured in this study for ozone and hydrogen peroxide are comparable to those reported for atomic hydrogen produced at much higher H2 pressures.

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* Phone: (301) 975-3736. Fax: (301) 208-6937. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the generous support of Intel Corporation. We also acknowledge partial support of this work by the AES Corporation through The AES Graduate Fellowship in Energy Research to Monica McEntee. We thank Professor Linda Columbus for the use of a nanodrop UV−vis absorption spectrometer in preliminary work leading to this paper. We also thank a reviewer for perceptive comments which have led to an improved paper.

5. SUMMARY OF RESULTS The major results discovered in this work are shown below.

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Table 1. Parameters for Growth of the Carbonaceous Spot of Area = 0.01 cm2 EUV Fluence (92 eV) yield of C atoms in Spot extracted yield of phenanthrene or anthracene fluence of n-tetradecane

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REFERENCES

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5.4 × 1018 photons/0.01 cm2 2 × 1015 C atoms/0.01 cm2 ∼1013 molecules/0.01 cm2 ≈ 1014 C atoms/0.01 cm2 3.0 × 1015 molecules/0.01 cm2 H



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EUV-Driven Carbonaceous Film Deposition and Its Photo-oxidation on

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