Hydrogen-Induced Chemical Erosion of a-C:H Thin Films: Product

Amorphous hydrogenated carbon (a-C:H) films in the thickness range 1−20 nm were deposited at 300 K on a Pt(111) single crystal by means of the ion-b...
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6194

J. Phys. Chem. B 2001, 105, 6194-6201

Hydrogen-Induced Chemical Erosion of a-C:H Thin Films: Product Distribution and Temperature Dependence Thomas Zecho,† Birgit D. Brandner,† Ju1 rgen Biener,*,‡ and Ju1 rgen Ku1 ppers†,‡ Experimentalphysik III, UniVersita¨ t Bayreuth, 95440 Bayreuth, Germany, and Max-Planck-Institut fu¨ r Plasmaphysik (EURATOM Association), 85748 Garching, Germany ReceiVed: January 4, 2001; In Final Form: April 2, 2001

Amorphous hydrogenated carbon (a-C:H) films in the thickness range 1-20 nm were deposited at 300 K on a Pt(111) single crystal by means of the ion-beam deposition (IBD) method. The product distribution as well as the temperature and thickness dependence of the hydrogen-atom-induced chemical erosion was investigated by mass spectrometry. Using an atom flux of ∼1016 H cm-2 s-1, the maximum of the erosion of a-C:H was observed at ∼750 K, with a yield of ∼0.01 C/H. At higher temperatures, the erosion rate decreases due to the thermally activated dehydrogenation of a-C:H. The main products of the hydrogen-atom-induced erosion reaction are C1 (methane), C2 (ethene, ethane), and C3 (propene, propane) hydrocarbon species, which contribute to ∼98% of the observed erosion rate. Higher hydrocarbon species, C4 to C8, were detected as minority species.

1. Introduction Amorphous hydrogenated carbon (a-C:H) films are utilized as wear and corrosion-resistant coatings on optical and electronic components, such as lenses and magnetic hard disk drives.1 They are also used as protective coatings on plasma-facing components in experimental fusion devices in order to reduce the concentration of metal impurities in the plasma.2,3 Hard a-C:H films are usually deposited by means of the plasma-enhanced chemical vapor deposition (PECVD) technique, and consist of an amorphous carbon network of small graphitic domains embedded in a matrix of 4-fold coordinated carbon, and typically exhibit a hydrogen content between 10 and 60%.4-6 Their electronic and mechanical properties are controlled by the bonding configuration of the carbon atoms,5 which in turn is determined by the hydrogen/carbon surface chemistry during the deposition.6-8 Hydrogen atoms promote the diamond-like coordination (sp3) of carbon, assist in the growth of a-C:H by creating reactive adsorption sites for hydrocarbon precursors, and etch a-C:H via the formation of volatile hydrocarbons. The hydrogen-atom-induced chemical erosion affects many relevant applications such as the use of carbon-based first-wall components in future fusion devices,9,10 or the low-pressure synthesis of diamond via the CVD technique which relies on the preferential erosion of graphitic co-deposits.7 The chemical erosion of 100-200 nm thick a-C:H films by thermal hydrogen atoms as a function of temperature, from 300 to 750 K, was previously investigated by Vietzke et al.11-13 The maximum of the erosion yield, ∼0.01-0.1 C/H, depending on the film properties, has been observed at ∼750 K. The CH3 radical was found to be the main product of the chemical erosion, but the formation of higher hydrocarbons, C2 to C4 species, has also been observed. Previously, we investigated the growth and the thermal stability of ultrathin (e2 nm) a-C:H films by means of Auger * Corresponding author. Tel: +49 921-553809. Fax: +49 921-553802. E-mail: [email protected]. † Universita ¨ t Bayreuth. ‡ Max-Planck-Institut fu ¨ r Plasmaphysik (EURATOM Association).

electron spectroscopy (AES), electronic and vibrational electron energy loss spectroscopy (EELS, HREELS), and thermal desorption spectroscopy (TDS).14-18 The films were deposited from 160 eV ethane ions at 300 K on a Pt(111) single-crystal substrate using the ion-beam deposition (IBD) technique. The structural properties and thermal stability of such films resemble those of micrometer thick a-C:H films deposited by plasmabased techniques.18 The films were found to grow in a twodimensional fashion and exhibit a H/C ratio of ∼0.4. The carbon network of such films contains approximately equal parts of graphitic (sp2) and tetrahedrally coordinated (sp3) carbon sites, and a minority of sp hybridized carbon. HREELS spectra revealed the presence of sp3-CHx (x ) 1, 2, 3), graphitic sp2CH, and sp-CH groups at the surface. The films are stable up to 600 K, and start to decompose at higher temperatures. Molecular hydrogen is the main decomposition product, accounting for ∼90% of the hydrogen bound to the carbon network. In addition, the formation of various hydrocarbon species, C1 to C6, has been observed. Around 900 K, sp3-CHx (x ) 1, 2, 3) groups have been observed to decompose leading to a graphitization of the carbon network; sp2-CH groups decompose around 1150 K, accompanied by the diffusion of carbon into the Pt crystal. We also investigated the interaction of hydrogen atoms with a-C:H and identified three basic reaction types: (1) hydrogenation of graphitic carbon; H(g) + sp2-C f sp3-CH,19,20 (2) hydrogen abstraction from sp3-CHx (x ) 1, 2, 3) groups; H(g) + sp3-CHx f *CHx-1 + H2(g) (* denotes a radical site),21 and (3) the hydrogen-induced chemical erosion of a-C:H via formation of volatile hydrocarbons.22,23 The erosion reaction was shown to be thermally activated, and the maximum was observed at 600 K, with a yield of C/H ∼0.01 using a hydrogen flux of ∼1013 H cm-2 s-1. On the basis of the identified hydrogen/carbon reactions, a reaction mechanism of the hydrogeninduced chemical erosion of carbon was proposed which explains the formation of methyl radicals observed by Vietzke et al.11-13 According to this model, the hydrogen-induced erosion of a-C:H films proceeds via cycles of hydrogen

10.1021/jp010013e CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001

H-Induced Chemical Erosion of a-C:H Thin Films abstraction and hydrogenation reactions via radical intermediates; above 400 K, the radical intermediates start to decompose via scission of C-C network bonds with an activation energy of ∼155 kJ/mol, thereby releasing methyl radicals. However, our experimental setup did not allow the direct detection of the reaction products, and AES was the only means to monitor the erosion via the decreasing film thickness. This restricted the investigation to very thin a-C:H films, e2 nm, due to the small mean free paths of the Auger electrons. In the present work, we studied the product distribution of the hydrogen-induced chemical erosion of a-C:H by mass spectrometry. The direct detection of volatile erosion products, in combination with a new atom source delivering a higher flux of ∼1016 H cm-2 s-1, allowed us to extend our investigation toward the erosion of thick a-C:H films, ranging from 1 to 20 nm, and to explore the homogeneity and surface roughness of the a-C:H films. Additionally, the erosion of a-C:H films served as a test case to develop a new experimental method which provides a rapid survey of complex etch reactions. 2. Experimental Section The present study was performed in a UHV system with a base pressure of 5 × 10-11 Torr described previously.24 In short, a hydrogen atom source is installed into a small, differentially pumped vacuum system (source chamber), which also houses a quadrupole mass spectrometer (QMS). The source chamber is connected through a small aperture of 8 mm diameter to the main chamber. The aperture can be closed with a mechanical shutter, thereby allowing to maintain a pressure gradient of three orders between the source chamber and the main chamber. During the erosion experiments the sample was placed just in front of the aperture and the exposure to hydrogen atoms was controlled by means of the mechanical shutter. The hydrogen atoms are generated in a resistively heated tungsten tube. The flux of hydrogen atoms was determined from both the total flux and the angular distribution of the hydrogen atoms following the approach of Schwarz-Selinger et al.,25 making use of the measured rate and spatial variation of the erosion of a-C:H. In the present study an atom flux of ∼1016 H cm-2 s-1 was achieved using a hydrogen flow of 0.45 sccm and a tungsten tube temperature of 2200 K, as determined by pyrometric measurements and a W-W/Re thermocouple attached to the tube front end. The a-C:H films were deposited on a Pt(111) single crystal at 300 K using the IBD technique. The ion gun operated at an ion energy of 180 eV in an ethane (C2H6) ambient of 5 × 10-5 Torr. The resulting a-C:H films were free of oxygen and platinum contaminations as checked with AES. The growth of the a-C:H films was monitored by means of the ion current measured during deposition, and confirmed by the integration of the QMS signals of hydrocarbon species measured during the complete erosion of a-C:H films. The substrate was mounted via two tungsten wires to a precision manipulator, and the temperature was measured by means of a chromel-alumel thermocouple attached to the Pt crystal. The use of a metallic substrate facilitates the temperature control, and sample temperatures between 80 and 1400 K were readily achieved via cryocooling and resistive heating, respectively. Prior to each experiment, the Pt surface was cleaned following standard procedures.26 The product distribution of the H-impact-induced chemical erosion of a-C:H was investigated by means of a multiplexed mass spectrometer which was optically linked to a PC. The setup allows us to monitor up to 80 masses quasi-simultaneously

J. Phys. Chem. B, Vol. 105, No. 26, 2001 6195 during H exposure. The hydrocarbon signals measured during the erosion of a-C:H are proportional to the rates of formation of these species since the QMS is differentially pumped. The a-C:H films were exposed to hydrogen atoms either at constant temperatures (stationary erosion, SE) or while slowly increasing the temperature at a constant heating rate (temperatureprogrammed erosion, TPE). The samples were first positioned in front of the aperture of the source chamber with the atom source working and the shutter closed, and then the hydrogen exposure was started by opening of the shutter at t ) 0 s. To differentiate between the H-atom-induced erosion and the thermally activated decomposition of a-C:H, duplicate films were subjected to a linear temperature ramp without simultaneous hydrogen exposure, while monitoring the release of hydrogen and hydrocarbons with the mass spectrometer (temperature-programmed desorption, TPD). For the purpose of a quantitative analysis of the measured erosion signals, the fragmentation patterns and the sensitivity of the QMS as well as the pumping speed of a number of hydrocarbon molecules were determined in separate experiments, and were generally found to be in good agreement with tabulated data.27 3. Results Our previous investigations14-18 regarding the structure and thermal stability of a-C:H were performed on films with a thickness of less than 2 nm. To verify that the results of these studies can be transferred to thicker films, a 16 nm thick a-C:H film was subjected to a linear temperature ramp while monitoring the evolution of hydrogen. The hydrogen TPD spectrum shown in Figure 1 is in excellent agreement with those measured previously at thinner films: The evolution of hydrogen starts around 750 K, exhibits a maximum at ∼900 K, and a shoulder around 1150 K. Previously, we demonstrated with HREELS that the hydrogen evolution around 900 K is due to the thermally activated decomposition of sp3-CHx groups via hydrogen splitoff, sp3-CHx f sp2-CHx-1 + 1/2H2 (x ) 1-3), and that the hydrogen evolution around 1150 K originates from the decomposition of sp2-CH groups. Additionally, HREELS spectra were collected from a duplicate film after annealing at 700 and 1000 K, respectively (Figure 1, insets). The CH stretch vibrations in the region around 3000 cm-1 allow us to determine the state of hybridization of CH groups at the surface:15-17,28-32 tetrahedrally coordinated sp3CHx groups (x ) 1-3) exhibit CH stretch modes around 2920 cm-1, whereas graphitic sp2-CH groups show a characteristic mode at 3057 cm-1. After annealing at 700 K, the spectrum exhibits two loss features at 2925 and 3065 cm-1 (Figure 1, left inset), indicating the presence of both sp3-CHx and sp2-CH groups, whereas the spectrum measured after annealing at 1000 K (Figure 1, right inset) is characteristic of a graphitic, sp2 hybridized surface structure. Accordingly, both TPD and HREELS results confirm the thermal stability ranges of sp3CHx and sp2-CH groups determined previously. Between 750 and 900 K, sp3-CHx groups decompose via the release of hydrogen, thus being transformed to sp2-CH groups, which further decompose around 1150 K. The arrows in Figure 1 indicate the stability regimes of the sp2 and sp3 species. To obtain a survey of both the temperature dependence and the product distribution of the hydrogen-induced chemical erosion of a-C:H, a 16 nm thick a-C:H film was exposed to hydrogen atoms while increasing the temperature from 300 to 1300 K at a constant heating rate of 0.5 K/s. The formation of volatile products was simultaneously monitored by mass spectrometry. Consequently, the TPE technique provides tem-

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Figure 1. Thermal decomposition of a 16 nm thick a-C:H film monitored in the hydrogen channel (2 amu). The insets display HREELS spectra of a 16 nm thick a-C:H film measured in the CH stretch mode region at room temperature after annealing at 700 K (left) and 1000 K (right), respectively.

perature- and product-resolved erosion rates. The thickness of the film, 16 nm, guaranteed a constant supply of carbon atoms as the film was only partially eroded on the time scale of the experiment, as discussed below. The most abundant fragments, representative of C1 to C6 hydrocarbon species, are displayed in Figure 2 (solid curves); in addition, C7 and C8 hydrocarbons were detected as minority species. Even without a quantitative analysis of the data it is evident from the scaling of the signals that C1 and C2 hydrocarbons are the main products of the erosion reaction. The signal intensity of higher hydrocarbon species was at least one order of magnitude smaller. The products were formed in a broad temperature range, from 400 to 1000 K. Both C1 and C2 hydrocarbon species exhibit a maximum around 800 K, whereas the maximum of the formation of C3 to C5 hydrocarbons was observed around 650 K with a shoulder around 800 K. The temperature dependence of the benzene (C6) signal resembles that of the C1/C2 species with a broad maximum around 800 K. The hydrocarbon species shown in Figure 2 originate from either the hydrogen-atom-induced erosion or the thermally activated decomposition of a-C:H. Previous studies14,18 revealed the formation of various volatile hydrocarbon species during the thermal decomposition of a-C:H. The main product of the thermal decomposition of a-C:H is molecular hydrogen, but, in addition, 10% of the hydrogen atoms were detected as hydrocarbon products containing ∼0.5% of the carbon atoms of the film. To distinguish between these two processes, a duplicate film was prepared, and heated to 1300 K without exposing the

Zecho et al.

Figure 2. Temperature dependence and product distribution of the hydrogen-induced chemical erosion of a 16 nm thick a-C:H film using an atom flux of ∼1016 H cm-2 s-1 (solid curves). Only the main fragments corresponding to C1-C6 hydrocarbons are displayed. For comparison, the thermal decomposition spectrum of a 16 nm thick a-C:H film is shown (dashed curves). The heating rate was 0.5 K/s.

surface to hydrogen atoms (Figure 2, dashed curves). The signals recorded during the thermal decomposition of a-C:H exhibit maxima shifted by approximately 100 K to higher temperatures. The change of the temperature dependence clearly demonstrates that the presence of a flux of hydrogen atoms impinging on the a-C:H film surface opens a new reaction channel toward the formation of hydrocarbons. Additionally, a comparison of the signal intensities of both data sets reveals that in the presence of a flux of hydrogen atoms the products were overwhelmingly generated through hydrogen-induced reactions; only the CH4 signal shows a non-negligible contribution from the thermal decomposition of the film. Therefore, the signal pattern displayed in Figure 2 reflects the product distribution of chemical erosion of a-C:H, and only the methane signal (16 amu) may be influenced by the thermal decomposition of the a-C:H film. Knowing the mass spectrometer sensitivities, fragmentation patterns, and pumping speeds of at least the most important products allows us to calculate the product distribution from the measured signal pattern in the temperature range 400 K to 1000 K. The result of this calculation is shown in Figure 3. The main products are C1 (methane), C2 (ethene, ethane), and C3 hydrocarbons (propene, propane), but higher hydrocarbon species, mostly unsaturated, contribute up to 2% of the observed products. The hydrogeninduced chemical erosion of a-C:H is obviously dominated by the formation of C2 hydrocarbons, approximately 50% of the eroded carbon atoms were detected in the C2 channel. The

H-Induced Chemical Erosion of a-C:H Thin Films

Figure 3. Product distribution of the hydrogen-induced chemical erosion and the thermal decomposition (inset) of 16 nm thick a-C:H films, respectively, deduced from the experiments shown in Figure 2.

product distribution of the hydrogen-induced chemical erosion of a-C:H differs considerably from that observed during the thermal decomposition of a-C:H (Figure 3, inset). This provides further evidence for a new reaction mechanism in the presence of hydrogen atoms. The data shown in Figure 2 can only be interpreted in terms of temperature dependence of the chemical erosion of a-C:H if the film assumes the equilibrium structure during heating and hydrogen exposure. Thus, it would be informative to perform the experiment in a cyclic way, i.e., using a reversed temperature ramp. Unfortunately, the films are not stable above 750 K; the thermally activated release of hydrogen (Figure 1) and hydrocarbons (Figure 2) is accompanied by the “graphitization” of a-C:H, and a restoration of a sp3-dominated surface during hydrogen exposure is not possible because of ongoing graphite formation at temperatures above 800 K. In addition, the diffusion of carbon into the Pt substrate is non-negligible above 1000 K. Thus, the equilibrium erosion rates were determined in separate experiments: identical, 3.8 nm thick a-C:H films were prepared at 300 K, heated to a preselected temperature between 500 and 1000 K, and subsequently exposed to atomic hydrogen while maintaining the temperature. Simultaneously, the formation of erosion products, C1 to C8 hydrocarbon species, was monitored with the mass spectrometer. The signals of the main products, represented by the 16 amu (C1, methane), 28 amu (C2, ethene/ethane), and 41 amu (C3, propene/propane) channels, are displayed in Figure 4. These species contribute up to 98% of the observed erosion yield. At the opening of the shutter at t ) 0 s, admission of H to the surface started. With the exception of the measurements performed at 900 and 1000 K, all signals exhibit some common features: (1) a steplike increase of the intensity immediately after starting the H flux, (2) an interval of constant or slowly increasing erosion, and (3) decreasing signals for prolonged hydrogen exposures. In addition, the methane signal (16 amu) exhibits a spike-like feature within the first few seconds of hydrogen exposure. The decrease of the erosion signals observed for prolonged hydrogen exposures at temperatures between 500 and 800 K is a consequence of the depletion of the films through the erosion reaction since

J. Phys. Chem. B, Vol. 105, No. 26, 2001 6197

Figure 4. QMS signals, representative for methane (16 amu), ethene/ ethane (28 amu), and propene/propane (41 amu) products, monitored during the erosion of 3.8 nm thick a-C:H films at the indicated temperature. The a-C:H films were prepared at 300 K, and annealed at the indicated temperature prior to the hydrogen exposure. The sequence of the temperatures reflects the alignment of the spectra.

AES spectra, collected after the hydrogen exposure, revealed that the a-C:H films were completely removed by the erosion (not shown). Constant erosion rates were not observed in the experiments performed at 900 and 1000 K (Figure 4), most pronounced in the C1 channel (16 amu) at 900 K. The diffusion of carbon into the Pt bulk, which has been observed at temperatures above 800 K,18 is still a slow process on the time scale of the experiment, and is thus not responsible for the almost linear decrease of the 16 amu signal in the time slot between 0 and 500 s. This transient signal therefore indicates that the reactivity of a-C:H toward the hydrogen-induced erosion at 900 K seems to decrease during hydrogen exposure through some kind of surface modification. Around 900 K, the thermally activated decomposition of a-C:H leads to a transition from a sp3dominated carbon network toward a sp2-dominated structure (Figure 1). However, this effect was allowed for by annealing the films at the erosion temperature for approximately 5 min before starting the hydrogen exposure. This procedure guaranteed that the films reached an equilibrium structure prior to the erosion measurement. The observed decrease of the reactivity thus indicates a hydrogen-induced surface modification, probably a hydrogen induced “graphitization” of the a-C:H film surface. The signal intensities in the interval of constant erosion can be interpreted as equilibrium rates at a given temperature. Figure 5 compares the equilibrium erosion rates (SE yield) with those monitored during the TPE experiment (Figure 2). Only data points obtained below 900 K were included; at or above 900 K, constant erosion rates were not observed (Figure 4). The different scales used to display the SE and the TPE yields reflect the reproducibility of the experiments. Both experiments exhibit a maximum total erosion yield of ∼0.01 C/H, and show a close correspondence up to 750 K. However, above 750 K, the SE yield starts to decrease, whereas the TPE yield shows a

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Figure 5. Comparison of TPE and SE yields of a-C:H obtained from the experiments shown in Figure 2 and Figure 4, respectively. Shown are the partial yields of methane (16 amu), ethene/ethane (28 amu), and propene/propane (41 amu) as well as the total erosion yields.

maximum at 800 K. The difference in the high temperature region suggests that the film structure was not completely equilibrated during the TPE experiment. The erosion rates might be sensitive to the thickness of the films, e.g., via inhomogeneous growth of a-C:H. The equilibrium rates were determined from 3.8 nm thick a-C:H films, whereas the temperature-programmed erosion experiment, shown in Figure 2, was performed with a 16 nm thick a-C:H film. To investigate the influence of the film thickness, 0.3 nm to 7.5 nm thick a-C:H films were prepared at 300 K, and exposed to hydrogen atoms at 800 K while monitoring the formation of volatile products (Figure 6). A temperature of 800 K was selected as the maximum erosion yields of C1 and C2 hydrocarbons were observed at this temperature. After opening the shutter at t ) 0 s, an immediate, steplike increase of the product signals was observed. If the thickness of the film exceeded 1 nm, the steplike increase of the signals at t ) 0 s was followed by a period of constant (C1 and C2 species) or slowly increasing (Cg3Hy species) erosion rates; finally, the signals started to decrease due to the depletion of the film. Thinner films exhibit only the steplike increase at t ) 0 s, followed by a rapid decay of the signals. The methane signal (16 amu) exhibits an additional, spike-like feature immediately after opening the shutter. The slow, but steady increase of the 41 amu signal is a typical feature of all signals related to higher hydrocarbon products, Cg3Hy, and is caused by the low pumping speed for these species. The data confirm that the decay of the erosion signals observed for prolonged hydrogen exposures is caused by the depletion of the a-C:H films. The steady-state signals observed in case of thicker films, g1.8 nm, reveal that the erosion rates do not depend on the film thickness, and thus prove the homogeneity of the film structure. The time scale of the experiment demonstrates that the 16 nm thick a-C:H film used to perform the TPE experiment (Figure 2) was only partially eroded in the course of the measurement. The steplike increase of the erosion signals at t ) 0 s suggests the presence of hydrocarbon precursor species, e.g., -CH3, on the surface of a-C:H prior to hydrogen exposure, and the step-

Zecho et al.

Figure 6. Thickness dependence of the erosion. The a-C:H films, 0.3 nm to 7.5 nm thick, were deposited at 300 K, and exposed to hydrogen atoms at 800 K while monitoring the formation of volatile products. Displayed are the signals representative for methane (16 amu), ethene/ ethane (28 amu), and propene/propane (41 amu). The inset shows the step-height of the erosion signal after opening the shutter at t ) 0 s versus the film thickness.

height of the product signals should thus reflect the concentration of these species. A plot of the step-height versus the film thickness is shown in the inset of Figure 6. Up to a film thickness of ∼1 nm, the step-height increases indicating an increasing surface concentration of precursor groups. Thicker films exhibit a constant step-height. This corroborates that 3.8 nm thick a-C:H films can be used to measure the erosion rate of bulk a-C:H. The methane signals shown in Figures 4 and 6 exhibit a spikelike feature immediately after opening the shutter, which rapidly decays within ∼20 s. This feature suggests that the concentration of precursor groups of the methane formation, e.g., -CH3(ad), decreases in the course of hydrogen exposure at 800 K. This effect was further investigated by means of HREELS: a 3.8 nm thick a-C:H film was deposited at 300 K, annealed at 800 K, and subsequently exposed to hydrogen atoms at 800 K while monitoring the evolution of volatile products; at various times the hydrogen exposure was interrupted by closing the shutter, and HREELS spectra were recorded at room temperature (Figure 7). The spectra exhibit loss features in the CH stretch region around 3000 cm-1, and in the fingerprint region, 500 cm-1 to 1500 cm-1, due to CC stretch and CH deformation modes. Prior to the hydrogen exposure, the CH stretch region reveals the presence of both sp3-CH (VCH ∼ 2925 cm-1) and sp2-CH groups (VCH ∼ 3065 cm-1). Within the first few seconds of hydrogen exposure, i.e., in the time-slot of the spike-like methane feature, the sp2-CH related intensity increases, accompanied by a minor increase of the sp3-CH related signal. Spectra collected within the period of constant erosion, i.e., after an accumulated hydrogen exposure of 15 and 140 s, respectively, show a decrease of the sp3-CH related intensity, but no further changes in the sp2-CH related intensity. For the sake of clarity, difference spectra were included in Figure 7. HREELS thus provides evidence that the spike-like methane signals observed

H-Induced Chemical Erosion of a-C:H Thin Films

Figure 7. HREELS spectra of a 3.8 nm thick a-C:H film, deposited at 300 K and annealed at 800 K, prior to and after being exposed to an increasing fluence of hydrogen atoms at 800 K. The inset displays the evolution of methane during the hydrogen exposure at 800 K.

immediately after opening of the shutter are related to the existence of a nonequilibrium concentration of sp3-CHx groups (x ) 1-3) at the surface. The methane signal monitored during the experiment reveals that, once formed, sp3-CHx groups are stable at 800 K in the absence of hydrogen atoms as the kinetics of methane evolution was not affected by the breaks (Figure 7, inset): The methane rate obtained after removal of the “H-off” periods is identical to that shown in Figure 4. 4. Discussion The present investigation reveals that erosion of a-C:H films through impact of thermal hydrogen atoms leads to the formation of C1-C8 hydrocarbons with C1 and C2 species as the dominating products. As illustrated by Figure 4, the temperature dependence of the stationary etching process is affected in two ways; below 900 K the etching rate increases with increasing temperatures as expected from an activated process; at or above 900 K the structural stability of the sp2-dominated network slows down the etch reaction. The maximum of the hydrogen-induced chemical erosion of a-C:H was observed around 750 K, and C1 (methane), C2 (ethene, ethane), and C3 (propene, propane) hydrocarbons were identified as the main products. In a previous study22 we observed the maximum of the erosion around 600 K, however, the hydrogen flux used in that work was considerably lower, ∼1013 H cm-2 s-1 versus ∼1016 H cm-2 s-1. The shift of the erosion maximum toward higher temperature with increasing hydrogen flux is consistent with a previously published model of the erosion mechanism,22 and has been predicted by Wittmann et al.23 who performed a model calculation of the hydrogen induced erosion based on experimentally determined kinetic parameters. The calculation predicted a shift of the peak temperature, from 700 K toward 900 K, with increasing hydrogen atom flux, from 1014 H cm-2 s-1 to 1018 H cm-2 s-1. Using 100 eV H+ ions instead of thermal hydrogen atoms, a

J. Phys. Chem. B, Vol. 105, No. 26, 2001 6199 shift of the peak temperature, from ∼780 K toward 950 K, with increasing ion flux, from ∼1014 H+ cm-2 s-1 to ∼1018 H+ cm-2 s-1, has been reported by Roth et al.33 The occurrence of an erosion maximum (Figure 5), and its flux dependence, is a consequence of two competing reactionss hydrogenation and thermally activated dehydrogenation. A prerequisite to observe steady-state erosion is the continuous formation of polymeric, sp3 hybridized precursor groups, e.g., -CH3. This requires the incessant hydrogenation of sp2 hybridized, unsaturated units of the carbon network via radical intermediates: CdC + H(g) f H-C-C*; the asterisk denotes a radical intermediate, i.e., a dangling bond. Depending on both the temperature and the hydrogen atom flux, the radical intermediate either decomposes via a thermally activated C-H bond scission toward a “graphitic” unit, H-C-C* f CdC + H(g), or is stabilized by a second hydrogenation, H-C-C* + H(g) f H-C-C-H, thereby creating a stable, sp3 hybridized CHx precursor group. The balance between these two reactions is flux-dependent. The higher the flux of hydrogen atoms, the higher the temperature up to which the hydrogenation can compensate for the loss of hydrogen via the thermally activated dehydrogenation of radical intermediates. Using a hydrogen flux of ∼1016 H cm-2 s-1, the transition between a sp3- and a sp2dominated surface structure is observed around 750 K. Additionally, at temperatures around 900 K, the thermally activated dehydrogenation of sp3-CH groups, sp3-CHx f sp2-CHx-1 + 1/2H2 (x ) 1-3), opens up a new loss channel for precursor groups (Figure 1). The balance between these reactions explains the observed shift of the erosion maximum toward higher temperatures with increasing flux of hydrogen atoms. The erosion was shown to be dominated by the formation of C1 and C2 hydrocarbons, but, in addition, higher hydrocarbon products were detected as minority species, and these species exhibit an erosion maximum around 650 K (Figure 2). Previously, we attributed the release of hydrocarbons from the carbon network to a thermally activated cleavage of a C-C bond neighboring a radical site driven by the simultaneous formation of a “graphitic” CdC double bond: *C-C-X f CdC + X*; X* denotes a hydrocarbon radical, and CdC is a structural unit of the carbon network.22 The erosion, i.e., the cleavage of the C-X bond, exhibits an unusually low activation barrier, ∼150 kJ/mol compared to a typical C-C bond strength of ∼350 kJ/ mol.34 The low activation barrier can be attributed to the energy gain by the simultaneous formation of a graphitic CdC bond. This mechanism predicts the generation of radicals, consistent with the results of Vietzke et al.11-13 who identified the methyl radical as the main product of the hydrogen-induced erosion of a-C:H. However, our experimental setup did not allow the direct, collision-free detection of products released from the a-C:H film surface, and methyl radicals were thus not detected; to a great extent, the observed methane probably originates from the recombination of methyl radicals and hydrogen on the walls of the source chamber. The analysis of the QMS signals revealed the formation of unsaturated hydrocarbon species, Cg2Hy, which contain a CdC double bond. This is consistent with the results of Vietzke et al.12 According to the existing erosion model, the generation of the radicals proceeds via a thermally activated C-C bond cleavage in the vicinity of a radical site, accompanied by the formation of a CdC double bond. The formation of unsaturated products can thus be envisioned by changing the role of product and carbon network, i.e., the radical center stays on the surface and the erosion product contains a CdC double bond. In either case, the erosion is initiated by a H abstraction reaction creating

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Figure 8. Mechanism of the hydrogen-induced erosion via the formation of hydrocarbon radicals (a), and unsaturated hydrocarbon molecules (b), respectively. The asterisk denotes a radical center.

a radical site, whose existence is a prerequisite for the subsequent erosion. The two reaction pathways are compared in Figure 8. Higher hydrocarbon products, C3 to C5 species, exhibit an erosion maximum around 650 K. The activation barrier of these product channels, as indicated by the erosion temperature, seems to be lower than that observed for the methyl radical. This can be attributed to the hyperconjungative stabilization of radicals, i.e., the stabilization of a radical center by neighboring alkyl groups, thus decreasing the C-C bond dissociation energy.35 Relative to the methyl radical, the (CH3)3C* radical exhibits a hyperconjungative stabilization of ∼50 kJ/mol. The same argument holds for the formation of unsaturated hydrocarbon products: the radical center, which is part of the surface (Figure 8), (Cnetwork)3C*, is stabilized by the carbon network. The erosion signals related to the formation of higher hydrocarbon species, Cg3Hy, exhibit a shoulder around 800 K (Figure 2). The QMS signals, monitored during the thermal decomposition of a duplicate film, reveal the desorption of these species in approximately the same temperature range. The release of hydrocarbon radicals from the carbon network during the thermal decomposition of a-C:H proceeds via a thermally activated C-C bond cleavage,14 and exhibits an activation energy of ∼235 kJ/mol: network-C-CxHy f network-C* + CxHy* (* denotes a radical site). The TPE experiment shown in Figure 2 monitors both the thermal decomposition and the hydrogen-induced erosion of a-C:H. Therefore, the question arises, whether the Cg3Hy related signals around 800 K originate from the thermal decomposition of a-C:H, without the assistance of hydrogen atoms and radical intermediates, or represent a new reaction pathway. The intensities of signals monitored during the thermal decomposition are considerably lower than those observed during the erosion (Figure 2). On the other hand, the constant supply of hydrogen atoms through the gas phase provides a continuous production of precursor groups, which then could be released from the carbon network by a thermally activated C-C bond scission without the assistance of a neighboring radical site. It has to be emphasized that the thermally activated erosion at 800 K via this reaction channel is restricted to the formation of higher hydrocarbon species due to the relatively high stability of these radicals. However, the

Zecho et al. contribution of the higher hydrocarbons to the total erosion yield is only small. The methane erosion signal (C1) monitored during the stationary erosion at 900 K (Figure 4) continuously deceases after starting the hydrogen exposure. This observation cannot be explained through the thermally activated decomposition of a-C:H at 900 K leading to a sp2-dominated carbon network structure, or the diffusion of carbon into the platinum bulk (see Results section).The almost linear decrease of the methane signal in the time-slot between 0 and 500 s thus reflects a hydrogen -induced transition from an initially reactive toward an inert a-C:H film surface. This transition probably is a “graphitization” as the erosion yield of graphite has been reported to be ∼100 times lower than that of a-C:H.13 Initially, after annealing at 900 K and prior to the hydrogen exposure (t ) 0 s), the film exhibits a “graphitic”, sp2-dominated carbon network with a high reactivity toward the hydrogen-induced erosion. The hydrogen exposure at 900 K induces a further “graphitization” of the a-C:H film surface, thereby reducing the reactivity of the film toward erosion. In this context the term “graphitization” stands for the growth of larger structural units of graphite. This graphitization probably proceeds via the thermally activated decomposition of radical sites produced by hydrogen abstractions, thereby facilitating the structural rearrangement of the carbon network. A similar process is observed during the postdeposition hydrogen treatment of amorphous hydrogenated silicon (a-Si:H) films at temperatures between 400 and 600 K, called “chemical annealing”, and used to prepare microcrystalline silicon films.36,37 The almost linear decrease of the C1 signal monitored during hydrogen exposure at 900 K is not observed in the ethene (C2) channel (Figure 4). The hydrogen-induced “graphitization” obviously does not suppress the formation of ethene. This probably reflects the structure of ethene, H2CdCH2, which is a structural unit of a sp2-dominated carbon network. At first glance the idea of hydrogen atoms assisting the “graphitization” of the surface of a-C:H films seems to be strange; typically, it is assumed that hydrogen atoms favor the formation of sp3 hybridized carbon rather than assisting graphitization. For example, during the CVD deposition of diamond, hydrogen atoms are necessary to prevent the deposition of graphite via hydrogenation and preferential erosion of graphitic co-deposits. However, one has to keep in mind that the hydrogenation/dehydrogenation equilibrium is flux dependent due to the limited lifetime of the radical intermediates. Thus, using a sufficiently high flux of hydrogen atoms, a sp3-type carbon surface can be stabilized by hydrogen atoms even at high temperatures. The methane signals (C1) shown in Figure 4 and Figure 6, respectively, exhibit an additional, spike-like feature immediately after starting the hydrogen exposure (t ) 0 s) at temperatures above 600 K. This feature seems to originate from either a nonequilibrium concentration of methane precursors, or a hydrogen-induced modification of the carbon network. The HREELS spectra shown in Figure 7 indicate a decreases of the sp3-CHx concentration on the time scale of the transient signal. However, a spike-like methane signal immediately after starting the hydrogen exposure was observed even if the a-C:H film was heated to 1150 K prior to the erosion experiment,38 whereby decomposing all sp3-CH precursor groups toward graphitic structures. Thus, neither the concentration of sp2-CH groups nor the concentration of -CH3 precursor groups can explain the spike-like methane signal. However, both the as-deposited and the annealed a-C:H film should have “exposed” carbon atoms

H-Induced Chemical Erosion of a-C:H Thin Films at the surface due to the roughness of the films. In this context the term “exposed carbon” stands for -CH3, dCH2, and tCH surface groups. Such groups should exhibit an increased probability to react toward methane. The question remains why the spike-like methane signal becomes more pronounced with increasing erosion temperature (Figure 4). According to our erosion model,22,23 the release of a methyl radical from the carbon network is initiated by the formation of a radical site in the vicinity of a methyl precursor group via a hydrogen abstraction. At temperatures above 500 K, the radical site decomposes via cleavage of the neighboring H3C-C bond leading to the formation of a carbon-carbon double bond, H3C-C-C* f H3C* + >CdC