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Influence of ion current on the growth of carbon films by ion-beam-assisted deposition
Diamond and Related Materials 8 (1999) 1944–1950 www.elsevier.com/locate/diamond
Influence of ion current on the growth of carbon films by ion-beam-assisted deposition R. Gago*, O. Bo¨hme, J.M. Albella, E. Roma´n Instituto de Ciencia de Materiales de Madrid (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain Received 11 March 1999; accepted 16 June 1999
Abstract Amorphous carbon films have been grown by evaporation of graphite with concurrent assistance of Ar+-ion bombardment. The assisting current was obtained with a 3 cm Kauffman ion gun and was varied between 0 and 15 mA, keeping the evaporation rate and assisting voltage constant. The films have been characterised with Auger electron spectroscopy and Raman spectroscopy. The deposition and post-deposition ion bombardment influence the structure of the films. Bombardment during deposition leads to an increase in the sp3 content up to 40% with an optimum current value around 10 mA. The additional post-deposition bombardment with Ar+ causes damage to the film structure, reducing the sp3 content and increasing the argon content of the film. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon; Growth; Ion-assisted deposition; Spectroscopy
1. Introduction Amorphous carbon films with (a-C:H ) or without (a-C ) hydrogen have been the object of intense research during the last decades because of their unique properties and applications. The properties of this material are controlled by the sp3/sp2 ratio and by the hydrogen content [1]. Normally, amorphous carbon layers present a high sp2 content because this hybridisation is energetically more favourable. The sp3 content gives this material diamond-like (DLC ) properties such as high hardness, infrared (IR) transparency and chemical inertness [2]. However, amorphous films with a high concentration of sp3 bonds present high stress, which limits their adhesion to the substrate [3]. The incorporation of hydrogen into the structure of the films increases the stability by providing stress relief but decreases the mechanical properties of the films, which become more polymeric [4]. The promotion of sp3 bonds in amorphous carbon films can be achieved by momentum transfer to the impinging carbon particles that reach the substrate surface. The change in energy and momentum of the * Corresponding author. Tel.: +34-91-334-9000; fax: +34-91-372-0623. E-mail address: [email protected] (R. Gago)
condensing atoms can be supplied either by direct acceleration or via indirect knock-on bombardment with assisting ions. In both cases, the deposition mechanism is described by a subplantation model [5,6 ]. According to this model, the incident energetic ions that penetrate into subsurface sites provide a quenched-in increase in density which favours the formation of sp3 bonds. Ion-beam-assisted deposition (IBAD) of evaporated or sputtered graphite produces films with a small amount of hydrogen (<5%) as hydrocarbon precursors are not used [7]. The low energy of the evaporated and sputtered carbon atoms, below 0.1 eV and 10 eV, respectively, produces basically sp2-bonded films (~95%) [8]. The addition of the ion-beam assistance provides the momentum transfer necessary to enhance the formation of sp3 bonds by collisions between the assisting ions and the carbon atoms. In this process, the carbon atoms that reach the substrate have a broad distribution of energies, which limits the formation of highly tetrahedral carbon films (ta-C ). In ta-C, the sp3 content can be raised up to 80% when monochromatic beams are employed, although the mechanical stability of the films is very poor (high stress) [9,10]. The knock-on process in IBAD is controlled either with the ion energy, the ion-to-carbon-atom arrival ratio (I/A) or the ion mass. However, the energy spread for the precursor carbon atoms as a result of collisions with
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the incoming ions makes the growth mechanism very complex. For this reason, there are few works dealing with the processes taking part during atom arrival, i.e., atom attachment to the growing surface (either with sp2 or sp3 character) and/or atom re-emission from the surface due to sputtering effects. It has been reported that an increase in the ion flux produces a promotion in the density and sp3/sp2 ratio [11–14]. In attempting to gather these results, it comes out that the ion flux bombarding the growing films has to be set as high as possible in order to improve the properties of the carbon films. The effect of the ion acceleration voltage on the film properties has already been studied in our system [7]. Now, in this work, we complete the study by investigating the influence of the ion-assisting current on the deposition.
2. Experiment The carbon films were grown in a high-vacuum chamber with a base pressure of 2×10−7 mbar on p-type (100)-oriented silicon substrates. The substrates were cleaned successively with trichloroethylene, acetone and ethanol before deposition. The deposition system is equipped with a 5 kW electron gun evaporator and a 3 cm Kauffman assisting ion gun. Prior to deposition, the substrates were sputtered with an Ar+-ion beam of 10 mA and 300 V for 2 min. This procedure eliminates water vapour and contaminants from the substrate surface, thus enhancing the adhesion of the coating to the substrate. The carbon atoms were evaporated from polycrystalline graphite lumps approximately 3 mm in size stored in a 4 cm3 liner. The evaporation rate ˚ s−1 for an electron current of 150 mA obtained was 5 A and an acceleration voltage of 7 keV. The assistance was performed with Ar+ ions accelerated at a voltage of 300 V. The ions impinged the sample with an incident angle of 45° with respect to the substrate normal. The assisting voltage was chosen in the range where the formation of films with a high sp3 content is promoted [7]. The ion current was studied in the range of 0 to 15 mA. Auger electron spectroscopy (AES) was used to obtain the compositional and structural analysis. The measurements were performed in an ultrahigh-vacuum chamber with a base pressure of 5×10−10 mbar. The surface contamination, as estimated by the oxygen signal, was below 5% so the spectra were taken without prior cleaning of the samples. The data acquisition was in the normal mode, using a double-pass cylindrical mirror analyser (CMA). The kinetic energy of the incident electron beam was 3 keV with a current of 75 nA. Raman spectroscopy was performed with a DILOR x–y micro Raman system, using the 514 nm line of an
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argon laser. In this technique, the cross-section for sp2 carbon is about 50 times that of sp3 carbon [15]. This drawback makes difficult to achieve a quantitative analysis, although a good correlation between Raman spectra and film properties has been found [16 ].
3. Results 3.1. Deposition rate The thickness of the deposited films was measured with a Dektak 2020 profiling system. The deposition rate (thickness/deposition time) of the evaporated film without ion assistance (e-C ) was 30±1 nm min−1. The rate for the assisted films fell between 20 and 28 nm min−1 as illustrated in Fig. 1. It is clear from this that the deposition rate decreases when ion assistance is added. In principle, this reduction can be explained by two competitive processes: (1) ion bombardment favours the formation of more energetic bonds (sp3) on the growing films, thus increasing the density of the deposited material; (2) ions accelerated at 300 V produce re-emission of the deposited atoms due to sputtering by the incoming ions. In the last case, the less energetic bonds should be preferentially sputtered, i.e., sp2 bonds. The mechanisms that control the deposition can be answered by comparing the deposition rate with the structural properties of the films, which can be derived from AES and Raman spectroscopy, as discussed below. 3.2. AES The derivative AES spectra for the IBAD films are shown in Fig. 2. The experimental data were smoothed with a five-points fast Fourier transform ( FFT ) filter. Other smoothing techniques, like adjacent averaging,
Fig. 1. Deposition rate for the a-C films grown under different assisting Ar+-ion currents (300 V ).
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Fig. 2. AES spectra for the samples grown under different assisting currents (300 V ).
led to identical results. We have used the Ar LMM line at 215 eV for calibration. In Fig. 2 the elements present in the film can be identified: argon, carbon and oxygen. The composition of the samples was determined using the relative peak-to-peak heights in the Auger spectra and the sensibility factors at a primary energy of 3 keV. The compositions of the IBAD samples studied in this work do not show great differences, with argon and oxygen contents around 3–4%. These values are in agreement with the results from bulk analysis techniques [Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis ( ERDA)] performed in equivalent films [7]. The low percentage of oxygen in the films is related to surface contamination, as will be discussed in Section 3.4. The shape analysis of the C KVV transition was performed with the detailed scan between 200 and 300 eV shown in Fig. 3. We have included the spectra
Fig. 3. AES detailed scan for the same films as in Fig 2. Diamond and graphite are shown for comparison.
of polycrystalline graphite and chemical vapour deposited (CVD) diamond for comparison. The graphite spectra have a main Auger transition at 271 eV due to the p1p fold and a feature around 250 eV assigned to the s1s fold in the heterocyclic ring structures [17]. On the other hand, CVD diamond presents a minimum at 267 eV associated with the s1 s fold and two peaks p p at 247 and 255 eV. The features around 235 and 250 eV found in our films indicate a predominant sp2 content in the samples. In addition, the formation of sp2 bonding is characteristic of the asymmetry in the derivative line [18]. The asymmetry arises from the presence of a shoulder at 278 eV, characteristic of p bonding, and it is attributed to damaged sp2 bonds [19]. On the contrary, diamond has a symmetric line shape as can be seen in Fig. 3. All the films show the shoulder at 278 eV and, hence, have an asymmetry similar to graphite. This corroborates that they are basically sp2-bonded and have a graphite-like structure. The promotion of sp3 bonding with ion assistance can be derived from the position of the main Auger transition. The spectra for the evaporated film without assistance (e-C ) have a transition around 270 eV, indicating an sp2 content close to that of graphite (100%). This transition shifts towards lower energies with the assistance due to the increase of the diamond-like character. Additional information can be obtained from the AES spectra through a quantification of the sp2 content. Quantification of the sp2 content could be directly achieved from a linear extrapolation of the main Auger transition from 267 eV (0%) to 271 eV (100%). However, the small change in the sp3 content should not give an appreciable displacement in the Auger main transition. On the contrary, we have performed a computational calculation based on factor analysis. The procedure consists in fitting the experimental points with a linear combination of the graphite and diamond spectra [19]. For this calculation we have assumed that the Auger sensitivity for both phases is the same. This assumption is reasonable as the intensity of the Auger transition depends in the probability of creating a core hole, which should not depend on the chemical bonding [19]. The values obtained from this calculation are illustrated in Fig. 4. The error bars represent the sum of the fitting and smoothing errors. The evaporated film (e-C ) has an sp2 content of around 90%, in agreement with the literature [2,9,20]. For the assisted films, the sp2 content falls between 60 and 75%. This range is consistent with previous X-ray absorption near edge structure ( XANES) measurements on similar IBAD films deposited at fixed ion-assisting current [7]. AES results indicate a difference between the assisted and non-assisted films. It is clear, from the factor and shape analysis, that ion assistance favours the formation of sp3 bonds up to 40%. However, the values of sp2 content calculated from AES cannot be used to make
R. Gago et al. / Diamond and Related Materials 8 (1999) 1944–1950
Fig. 4. Percentage of sp2 bonds obtained from factor analysis of the Auger spectra. In order to discuss the results, we have plotted a curve according to the Raman results (see text).
an assumption about the behaviour of the film structure with the ion-assisting current. For this reason, the curve plotted in Fig. 4 is only indicative although, as we shall see below, is consistent with Raman results.
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(graphite) at ~1580 cm−1 and D (disordered) at ~1350 cm−1. Although there is not a direct theory that can explain the spectra of amorphous carbon films, there are experimental results that support a good correlation between the Raman peaks and film properties. Particularly, the position of the G peak is proportional to the sp3 content and shifts towards lower frequencies as the sp3 content increases [21]. Also, the increase in the full-width at half-maximum (FWHM ) of this peak indicates a higher diamond-like character of the film [22]. Finally, the relative intensity of the D peak is related to the microcrystalline size of the graphite clusters, where less graphitic amorphous films have a lower I /I value [23]. D G The results of the Raman fitting are illustrated in Fig. 6. The position of the G peak (Fig. 6a) shows a transition towards lower frequencies as we increase the ion current. Even if this shift does not yield quantitative information on the sp3 content, it can be used to determine qualitatively the best films from a diamondlike environment perspective. The behaviour of this parameter indicates that the sp3/sp2 ratio increases with the ion flux until a maximum value, reached around 10 mA. If the ion current is further increased above 10 mA, the sp3 content starts to decrease. The increase in the diamond-likeness with ion assistance is also corroborated by the decrease in the I /I ratio and the D G increase of the FWHM as shown in Figs. 6b and c, respectively. It can be easily noticed that the assistance increases the FWHM, which can be related to the disorder caused by the ion bombardment. Note that the
3.3. Raman spectroscopy Fig. 5 shows the Raman spectra for the set of samples grown with the selected values of the Ar+-ion assisting current. The experimental data have been fitted with two Gaussian distributions associated to the peaks commonly found in amorphous carbon and labelled as G
Fig. 5. Experimental and fitted Raman spectra for the different ionassisting currents.
Fig. 6. Position of the G peak (a), FWHM of the G peak (b) and variation of the relative intensity I /I (c) as obtained from the spectra D G of Fig. 5.
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maximum of disorder is achieved for the same conditions as the maximum of sp3 is reached. 3.4. Post-deposition ion irradiation As we have seen, the compositional analysis of the film surface shows an oxygen content of 3–4%. The bombardment with Ar+ ions prior to measurement is a procedure normally employed to clean the surface. However, this method has been found to produce structural changes in graphite, diamond and amorphous carbon films [24–26 ]. In our case, this procedure gives useful information about the nature of the oxygen content but can not be used as a standard cleaning method. We have bombarded the films with an Ar+-ion beam of 3 keV and 0.07 mA mm−2 in order to clean the sample surface. An Auger spectrum was collected after the cleaning process and compared with that of the as-deposited film. Fig. 7 shows the AES spectra of the sample grown with an assisting current of 7.5 mA, as-deposited and post-deposition bombarded. As we can see, the oxygen signal is not present in the AES spectra after ion bombardment, indicating that the oxygen previously detected is due to surface contamination. Additionally, the comparison between the relative Ar-to-C signal shows that we have incorporated more argon into the structure (up to 6%) with the postdeposition bombardment. As mentioned above, the bombardment process should induce large changes in the structure of the films. The damage to the structure can be derived from the shape analysis of the Auger spectra. The shift of the main Auger transition from 268 to 271 eV, as shown in Fig. 7, indicates that the number of sp2 bonds is
Fig. 7. Auger spectra of the film grown with 7.5 mA of assisting ion current as-deposited and after additional post-deposition ion bombardment. We can appreciate the shift of the Auger transition and the incorporation of more argon into the film.
increased considerably after ion bombardment. Therefore, the structure of the films after ion irradiation turns towards a more graphitic character. This result has been also observed for diamond and a-C:H [25,26 ]. The structural change produced by ion bombardment has also been observed by a bulk technique like Raman spectroscopy. This change can be observed in Fig. 8 for an irradiated film. This result indicates that the changes induced by the ion bombardment are not limited to the surface and affect a thick layer of the film. The shift of the G peak towards 1586 cm−1 and the increase in the relative intensity of the peaks support this increase in the sp2 content of the films, in concordance with AES.
4. Discussion 4.1. Effect of the assisting current The Raman and AES results indicate the formation of a random network consisting of a mixture of sp2 and sp3 bonds. The number of sp3 bonds can be increased with the assisting ion current up to 40%. In addition, the optimum value for promoting sp3 bonds is reached for intermediate currents around 10 mA (see Fig. 6a). The structure of the films, as characterised by Raman and AES, together with the deposition rate measurements (Fig. 1), can give some clues about the mechanism involved in the deposition. For low currents (<3 mA) there is a large reduction in the thickness and, as derived from Raman, the formation of sp3 bonds is small. In this case, the high percentage of carbon atoms with low bonding energies (sp2) is easily sputtered by the incoming ions and the deposition rate suffers a large reduction. As the ion flux is increased the number of knock-on
Fig. 8. Raman spectra for the sample grown with 7.5 mA of ion-beam assisting current before and after post-deposition bombardment. The bombardment induces a graphitisation of the lattice, shifting the G peak towards a higher frequency. The increase in the sp2 content is also followed by an increase in the relative peak intensity.
R. Gago et al. / Diamond and Related Materials 8 (1999) 1944–1950
collisions between the assisting ions and the carbon atoms is increased and, according to the subplantation model, this enhances the formation of sp3 bonds. The enrichment of sp3 bonds reduces the efficiency of sputtering and the deposition rate increases until a maximum is reached. A similar behaviour (increase of the density and sp3/sp2 ratio) as the assisting current is increased has also been observed with other ion-beam-assisted methods [11–13]. In these works it was suggested that the ion current has to be has high as possible to improve the carbon film’s properties. However, for higher ion currents (>10 mA) we have found that the deposition rate and the sp3 content suffer a reduction, as illustrated in Figs 1 and 6, respectively. In this regime the ion-tocarbon ratios should be higher than the ratios normally employed for assistance and, then, the sputtering rate (sputtering yield×ion flux) is high enough to produce a major removal of the deposited atoms. This produces the further decrease in deposition rate observed in Fig. 1. The high ion flux towards the surface should produce a damage analogous to irradiation due to the accumulation of projectiles at the substrate and, hence, reduce the sp3 content. 4.2. Ion irradiation In principle, irradiation with high-energy ions (3 keV ) is supposed to produce a modification of the film structure. Particularly, we have seen that the sp3 content is reduced and the structure turns towards an amorphous graphitic network. This structural modification of the films after post-deposition bombardment is consistent with the deposition mechanism of subplantation, where the carbon atoms must impinge the surface with intermediate energies around 100 eV. In this case, the ion bombardment occurs as a part or during the growth process itself, supplying the energy needed to enhance the formation of sp3 bonds [27]. However, post-deposition bombardment at higher energies leads to a reduction of the sp3 content. This modification of the film structure can be attributed to the increase in local temperature during bombardment and the higher stability of the graphite phase. It is commonly known that amorphous carbon layers have poor thermal stability and suffer graphitisation for annealing temperatures above 200°C [28–30].
5. Conclusions The structure and composition of amorphous carbon films grown by ion-beam-assisted evaporation of graphite have been studied with Raman and Auger spectroscopy. Both techniques show similar trends in the film properties. The carbon films are composed of a mixture of sp2-
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and sp3-hybridised carbon atoms. Ion assistance at low doses (<10 mA) enhances the formation of sp3 bonds with increasing current value. The increase in the diamond-like character is followed by a reduction in the sputtering yield and an increase in the deposition rate. With this method we can obtain films with an sp3 content of up to 40%. For ion currents above 10 mA, sputtering of the deposited film dominates over the deposition and the sp3 content and deposition rate suffer a reduction. Post-deposition ion irradiation causes a change in the structure of the films, reducing the number of sp3 bonds considerably and increasing the argon content. Therefore, the promotion of sp3 bonds is obtained only when ion bombardment occurs during the growth process itself.
References [1] [2] [3] [4] [5]
J. Robertson, Current Opin. Solid State Mater. Sci. 1 (1996) 557. J. Robertson, Adv. Phys. 35 (1986) 317. J.C. Angus, C.C. Hayman, Science 241 (1988) 913. J.C. Angus, F. Jansen, J. Vac. Sci. Technol. A 6 (3) (1988) 1778. Y. Lifshitz, S.R. Kasi, J.W. Rabalais, Phys. Rev. B 41 (15) (1990) 10468. [6 ] J. Robertson, Diamond Relat. Mater. 3 (1994) 361. [7] R. Gago, I. Jimenez, J.M. Albella, A. Climent-Font, L. Terminello, J.C. Banks, B.L. Doyle, Diamond Relat. Mater., submitted for publication. [8] J. Robertson, E.P. O’Reilly, Phys. Rev. B 35 (6) (1987) 2946. [9] M. Weiler, S. Sattel, T. Giessen, K. Jung, H. Ehrhardt, Phys. Rev. B 53 (1996) 1594. [10] J. Robertson, Diamond Relat. Mater. 1 (1992) 397. [11] B. Andre´, F. Rossi, H. Dunlop, Diamond Relat. Mater. 1 (1992) 307. [12] R. Kleber, M. Weiler, A. Kru¨ger, S. Sattel, G. Kunz, K. Jung, H. Ehrhardt, Diamond Relat. Mater. 2 (1993) 246. [13] J. Ullmann, U. Falke, W. Scharff, A. Schro¨er, G.K. Wolf, Thin Solid Films 232 (1993) 154. [14] W. Gissler, P. Hammer, J. Haupt, Diamond Relat. Mater. 3 (1994) 770. [15] P. Bou, J. Vandenbulcke, J. Electrochem. Soc. 138 (10) (1991) 2991. [16 ] M.A. Tamor, W.C. Vassell, J. Appl. Phys. 50 (1994) 3823. [17] K.J. Boyd, D. Marton, S.S. Todorov, A.H. Al-Bayati, J. Kulik, R.A. Zuhr, J.W. Rabalais, J. Vac. Sci. Technol. A 13 (4) (1995) 2110. [18] J.A. Martı´n-Gago, J. Fraxedas, S. Ferrer, F. Comin, Surf. Sci. Lett. 260 (1992) L17. [19] H.J. Steffen, C.D. Roux, D. Marton, J.W. Rabalais, Phys. Rev. B 44 (8) (1991) 3981. [20] E. Grossman, G.D. Lempert, J. Kulik, D. Marton, J.W. Rabalais, Y. Lifshitz, Appl. Phys. Lett. 68 (1996) 1214. [21] S. Prawer, K.W. Nugent, Y. Lifshitz, G.D. Lempert, E. Grossman, J. Kulik, I. Avigal, R. Kalish, Diamond Relat. Mater. 5 (1996) 433. [22] R.O. Dillon, J.A. Woollam, Phys. Rev. B 29 (6) (1984) 3482. [23] H.J. Scheibe, D. Drescher, P. Alers, Fres. J. Anal. Chem. 353 (1995) 695.
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[24] I. Gouzman, R. Brener, C. Cytermann, A. Hoffman, Surf. Interf. Anal. 22 (1994) 524. [25] P. Reinke, G. Franz, P. Oelhafen, Phys. Rev. B 54 (10) (1996) 7067. [26 ] A. Fuchs, J. Scherer, K. Jung, H. Ehrhardt, Thin Solid Films 232 (1993) 51. [27] J. Robertson, Diamond Relat. Mater. 2 (1993) 984.
[28] R. Gago, O. Sa´nchez, A. Climent-Font, J.M. Albella, E. Roma´n, E. Rau¨hala, J. Raisa¨nen, Thin Solid Films 338 (1–2) (1999) 88. [29] D.R. Tallant, J.E. Parmeter, M.P. Siegal, R.L. Simpson, Diamond Relat. Mater. 4 (1995) 191. [30] Z.L. Akkerman, H. Efstathiadis, F.W. Smith, J. Appl. Phys. 80 (5) (1996) 3068.
Influence of ion current on the growth of carbon films by ion-beam-assisted deposition R. Gago*, O. Bo¨hme, J.M. Albella, E. Roma´n Instituto de Ciencia de Materiales de Madrid (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain Received 11 March 1999; accepted 16 June 1999
Abstract Amorphous carbon films have been grown by evaporation of graphite with concurrent assistance of Ar+-ion bombardment. The assisting current was obtained with a 3 cm Kauffman ion gun and was varied between 0 and 15 mA, keeping the evaporation rate and assisting voltage constant. The films have been characterised with Auger electron spectroscopy and Raman spectroscopy. The deposition and post-deposition ion bombardment influence the structure of the films. Bombardment during deposition leads to an increase in the sp3 content up to 40% with an optimum current value around 10 mA. The additional post-deposition bombardment with Ar+ causes damage to the film structure, reducing the sp3 content and increasing the argon content of the film. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon; Growth; Ion-assisted deposition; Spectroscopy
1. Introduction Amorphous carbon films with (a-C:H ) or without (a-C ) hydrogen have been the object of intense research during the last decades because of their unique properties and applications. The properties of this material are controlled by the sp3/sp2 ratio and by the hydrogen content [1]. Normally, amorphous carbon layers present a high sp2 content because this hybridisation is energetically more favourable. The sp3 content gives this material diamond-like (DLC ) properties such as high hardness, infrared (IR) transparency and chemical inertness [2]. However, amorphous films with a high concentration of sp3 bonds present high stress, which limits their adhesion to the substrate [3]. The incorporation of hydrogen into the structure of the films increases the stability by providing stress relief but decreases the mechanical properties of the films, which become more polymeric [4]. The promotion of sp3 bonds in amorphous carbon films can be achieved by momentum transfer to the impinging carbon particles that reach the substrate surface. The change in energy and momentum of the * Corresponding author. Tel.: +34-91-334-9000; fax: +34-91-372-0623. E-mail address: [email protected] (R. Gago)
condensing atoms can be supplied either by direct acceleration or via indirect knock-on bombardment with assisting ions. In both cases, the deposition mechanism is described by a subplantation model [5,6 ]. According to this model, the incident energetic ions that penetrate into subsurface sites provide a quenched-in increase in density which favours the formation of sp3 bonds. Ion-beam-assisted deposition (IBAD) of evaporated or sputtered graphite produces films with a small amount of hydrogen (<5%) as hydrocarbon precursors are not used [7]. The low energy of the evaporated and sputtered carbon atoms, below 0.1 eV and 10 eV, respectively, produces basically sp2-bonded films (~95%) [8]. The addition of the ion-beam assistance provides the momentum transfer necessary to enhance the formation of sp3 bonds by collisions between the assisting ions and the carbon atoms. In this process, the carbon atoms that reach the substrate have a broad distribution of energies, which limits the formation of highly tetrahedral carbon films (ta-C ). In ta-C, the sp3 content can be raised up to 80% when monochromatic beams are employed, although the mechanical stability of the films is very poor (high stress) [9,10]. The knock-on process in IBAD is controlled either with the ion energy, the ion-to-carbon-atom arrival ratio (I/A) or the ion mass. However, the energy spread for the precursor carbon atoms as a result of collisions with
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 1 6 6- 1
R. Gago et al. / Diamond and Related Materials 8 (1999) 1944–1950
the incoming ions makes the growth mechanism very complex. For this reason, there are few works dealing with the processes taking part during atom arrival, i.e., atom attachment to the growing surface (either with sp2 or sp3 character) and/or atom re-emission from the surface due to sputtering effects. It has been reported that an increase in the ion flux produces a promotion in the density and sp3/sp2 ratio [11–14]. In attempting to gather these results, it comes out that the ion flux bombarding the growing films has to be set as high as possible in order to improve the properties of the carbon films. The effect of the ion acceleration voltage on the film properties has already been studied in our system [7]. Now, in this work, we complete the study by investigating the influence of the ion-assisting current on the deposition.
2. Experiment The carbon films were grown in a high-vacuum chamber with a base pressure of 2×10−7 mbar on p-type (100)-oriented silicon substrates. The substrates were cleaned successively with trichloroethylene, acetone and ethanol before deposition. The deposition system is equipped with a 5 kW electron gun evaporator and a 3 cm Kauffman assisting ion gun. Prior to deposition, the substrates were sputtered with an Ar+-ion beam of 10 mA and 300 V for 2 min. This procedure eliminates water vapour and contaminants from the substrate surface, thus enhancing the adhesion of the coating to the substrate. The carbon atoms were evaporated from polycrystalline graphite lumps approximately 3 mm in size stored in a 4 cm3 liner. The evaporation rate ˚ s−1 for an electron current of 150 mA obtained was 5 A and an acceleration voltage of 7 keV. The assistance was performed with Ar+ ions accelerated at a voltage of 300 V. The ions impinged the sample with an incident angle of 45° with respect to the substrate normal. The assisting voltage was chosen in the range where the formation of films with a high sp3 content is promoted [7]. The ion current was studied in the range of 0 to 15 mA. Auger electron spectroscopy (AES) was used to obtain the compositional and structural analysis. The measurements were performed in an ultrahigh-vacuum chamber with a base pressure of 5×10−10 mbar. The surface contamination, as estimated by the oxygen signal, was below 5% so the spectra were taken without prior cleaning of the samples. The data acquisition was in the normal mode, using a double-pass cylindrical mirror analyser (CMA). The kinetic energy of the incident electron beam was 3 keV with a current of 75 nA. Raman spectroscopy was performed with a DILOR x–y micro Raman system, using the 514 nm line of an
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argon laser. In this technique, the cross-section for sp2 carbon is about 50 times that of sp3 carbon [15]. This drawback makes difficult to achieve a quantitative analysis, although a good correlation between Raman spectra and film properties has been found [16 ].
3. Results 3.1. Deposition rate The thickness of the deposited films was measured with a Dektak 2020 profiling system. The deposition rate (thickness/deposition time) of the evaporated film without ion assistance (e-C ) was 30±1 nm min−1. The rate for the assisted films fell between 20 and 28 nm min−1 as illustrated in Fig. 1. It is clear from this that the deposition rate decreases when ion assistance is added. In principle, this reduction can be explained by two competitive processes: (1) ion bombardment favours the formation of more energetic bonds (sp3) on the growing films, thus increasing the density of the deposited material; (2) ions accelerated at 300 V produce re-emission of the deposited atoms due to sputtering by the incoming ions. In the last case, the less energetic bonds should be preferentially sputtered, i.e., sp2 bonds. The mechanisms that control the deposition can be answered by comparing the deposition rate with the structural properties of the films, which can be derived from AES and Raman spectroscopy, as discussed below. 3.2. AES The derivative AES spectra for the IBAD films are shown in Fig. 2. The experimental data were smoothed with a five-points fast Fourier transform ( FFT ) filter. Other smoothing techniques, like adjacent averaging,
Fig. 1. Deposition rate for the a-C films grown under different assisting Ar+-ion currents (300 V ).
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Fig. 2. AES spectra for the samples grown under different assisting currents (300 V ).
led to identical results. We have used the Ar LMM line at 215 eV for calibration. In Fig. 2 the elements present in the film can be identified: argon, carbon and oxygen. The composition of the samples was determined using the relative peak-to-peak heights in the Auger spectra and the sensibility factors at a primary energy of 3 keV. The compositions of the IBAD samples studied in this work do not show great differences, with argon and oxygen contents around 3–4%. These values are in agreement with the results from bulk analysis techniques [Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis ( ERDA)] performed in equivalent films [7]. The low percentage of oxygen in the films is related to surface contamination, as will be discussed in Section 3.4. The shape analysis of the C KVV transition was performed with the detailed scan between 200 and 300 eV shown in Fig. 3. We have included the spectra
Fig. 3. AES detailed scan for the same films as in Fig 2. Diamond and graphite are shown for comparison.
of polycrystalline graphite and chemical vapour deposited (CVD) diamond for comparison. The graphite spectra have a main Auger transition at 271 eV due to the p1p fold and a feature around 250 eV assigned to the s1s fold in the heterocyclic ring structures [17]. On the other hand, CVD diamond presents a minimum at 267 eV associated with the s1 s fold and two peaks p p at 247 and 255 eV. The features around 235 and 250 eV found in our films indicate a predominant sp2 content in the samples. In addition, the formation of sp2 bonding is characteristic of the asymmetry in the derivative line [18]. The asymmetry arises from the presence of a shoulder at 278 eV, characteristic of p bonding, and it is attributed to damaged sp2 bonds [19]. On the contrary, diamond has a symmetric line shape as can be seen in Fig. 3. All the films show the shoulder at 278 eV and, hence, have an asymmetry similar to graphite. This corroborates that they are basically sp2-bonded and have a graphite-like structure. The promotion of sp3 bonding with ion assistance can be derived from the position of the main Auger transition. The spectra for the evaporated film without assistance (e-C ) have a transition around 270 eV, indicating an sp2 content close to that of graphite (100%). This transition shifts towards lower energies with the assistance due to the increase of the diamond-like character. Additional information can be obtained from the AES spectra through a quantification of the sp2 content. Quantification of the sp2 content could be directly achieved from a linear extrapolation of the main Auger transition from 267 eV (0%) to 271 eV (100%). However, the small change in the sp3 content should not give an appreciable displacement in the Auger main transition. On the contrary, we have performed a computational calculation based on factor analysis. The procedure consists in fitting the experimental points with a linear combination of the graphite and diamond spectra [19]. For this calculation we have assumed that the Auger sensitivity for both phases is the same. This assumption is reasonable as the intensity of the Auger transition depends in the probability of creating a core hole, which should not depend on the chemical bonding [19]. The values obtained from this calculation are illustrated in Fig. 4. The error bars represent the sum of the fitting and smoothing errors. The evaporated film (e-C ) has an sp2 content of around 90%, in agreement with the literature [2,9,20]. For the assisted films, the sp2 content falls between 60 and 75%. This range is consistent with previous X-ray absorption near edge structure ( XANES) measurements on similar IBAD films deposited at fixed ion-assisting current [7]. AES results indicate a difference between the assisted and non-assisted films. It is clear, from the factor and shape analysis, that ion assistance favours the formation of sp3 bonds up to 40%. However, the values of sp2 content calculated from AES cannot be used to make
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Fig. 4. Percentage of sp2 bonds obtained from factor analysis of the Auger spectra. In order to discuss the results, we have plotted a curve according to the Raman results (see text).
an assumption about the behaviour of the film structure with the ion-assisting current. For this reason, the curve plotted in Fig. 4 is only indicative although, as we shall see below, is consistent with Raman results.
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(graphite) at ~1580 cm−1 and D (disordered) at ~1350 cm−1. Although there is not a direct theory that can explain the spectra of amorphous carbon films, there are experimental results that support a good correlation between the Raman peaks and film properties. Particularly, the position of the G peak is proportional to the sp3 content and shifts towards lower frequencies as the sp3 content increases [21]. Also, the increase in the full-width at half-maximum (FWHM ) of this peak indicates a higher diamond-like character of the film [22]. Finally, the relative intensity of the D peak is related to the microcrystalline size of the graphite clusters, where less graphitic amorphous films have a lower I /I value [23]. D G The results of the Raman fitting are illustrated in Fig. 6. The position of the G peak (Fig. 6a) shows a transition towards lower frequencies as we increase the ion current. Even if this shift does not yield quantitative information on the sp3 content, it can be used to determine qualitatively the best films from a diamondlike environment perspective. The behaviour of this parameter indicates that the sp3/sp2 ratio increases with the ion flux until a maximum value, reached around 10 mA. If the ion current is further increased above 10 mA, the sp3 content starts to decrease. The increase in the diamond-likeness with ion assistance is also corroborated by the decrease in the I /I ratio and the D G increase of the FWHM as shown in Figs. 6b and c, respectively. It can be easily noticed that the assistance increases the FWHM, which can be related to the disorder caused by the ion bombardment. Note that the
3.3. Raman spectroscopy Fig. 5 shows the Raman spectra for the set of samples grown with the selected values of the Ar+-ion assisting current. The experimental data have been fitted with two Gaussian distributions associated to the peaks commonly found in amorphous carbon and labelled as G
Fig. 5. Experimental and fitted Raman spectra for the different ionassisting currents.
Fig. 6. Position of the G peak (a), FWHM of the G peak (b) and variation of the relative intensity I /I (c) as obtained from the spectra D G of Fig. 5.
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maximum of disorder is achieved for the same conditions as the maximum of sp3 is reached. 3.4. Post-deposition ion irradiation As we have seen, the compositional analysis of the film surface shows an oxygen content of 3–4%. The bombardment with Ar+ ions prior to measurement is a procedure normally employed to clean the surface. However, this method has been found to produce structural changes in graphite, diamond and amorphous carbon films [24–26 ]. In our case, this procedure gives useful information about the nature of the oxygen content but can not be used as a standard cleaning method. We have bombarded the films with an Ar+-ion beam of 3 keV and 0.07 mA mm−2 in order to clean the sample surface. An Auger spectrum was collected after the cleaning process and compared with that of the as-deposited film. Fig. 7 shows the AES spectra of the sample grown with an assisting current of 7.5 mA, as-deposited and post-deposition bombarded. As we can see, the oxygen signal is not present in the AES spectra after ion bombardment, indicating that the oxygen previously detected is due to surface contamination. Additionally, the comparison between the relative Ar-to-C signal shows that we have incorporated more argon into the structure (up to 6%) with the postdeposition bombardment. As mentioned above, the bombardment process should induce large changes in the structure of the films. The damage to the structure can be derived from the shape analysis of the Auger spectra. The shift of the main Auger transition from 268 to 271 eV, as shown in Fig. 7, indicates that the number of sp2 bonds is
Fig. 7. Auger spectra of the film grown with 7.5 mA of assisting ion current as-deposited and after additional post-deposition ion bombardment. We can appreciate the shift of the Auger transition and the incorporation of more argon into the film.
increased considerably after ion bombardment. Therefore, the structure of the films after ion irradiation turns towards a more graphitic character. This result has been also observed for diamond and a-C:H [25,26 ]. The structural change produced by ion bombardment has also been observed by a bulk technique like Raman spectroscopy. This change can be observed in Fig. 8 for an irradiated film. This result indicates that the changes induced by the ion bombardment are not limited to the surface and affect a thick layer of the film. The shift of the G peak towards 1586 cm−1 and the increase in the relative intensity of the peaks support this increase in the sp2 content of the films, in concordance with AES.
4. Discussion 4.1. Effect of the assisting current The Raman and AES results indicate the formation of a random network consisting of a mixture of sp2 and sp3 bonds. The number of sp3 bonds can be increased with the assisting ion current up to 40%. In addition, the optimum value for promoting sp3 bonds is reached for intermediate currents around 10 mA (see Fig. 6a). The structure of the films, as characterised by Raman and AES, together with the deposition rate measurements (Fig. 1), can give some clues about the mechanism involved in the deposition. For low currents (<3 mA) there is a large reduction in the thickness and, as derived from Raman, the formation of sp3 bonds is small. In this case, the high percentage of carbon atoms with low bonding energies (sp2) is easily sputtered by the incoming ions and the deposition rate suffers a large reduction. As the ion flux is increased the number of knock-on
Fig. 8. Raman spectra for the sample grown with 7.5 mA of ion-beam assisting current before and after post-deposition bombardment. The bombardment induces a graphitisation of the lattice, shifting the G peak towards a higher frequency. The increase in the sp2 content is also followed by an increase in the relative peak intensity.
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collisions between the assisting ions and the carbon atoms is increased and, according to the subplantation model, this enhances the formation of sp3 bonds. The enrichment of sp3 bonds reduces the efficiency of sputtering and the deposition rate increases until a maximum is reached. A similar behaviour (increase of the density and sp3/sp2 ratio) as the assisting current is increased has also been observed with other ion-beam-assisted methods [11–13]. In these works it was suggested that the ion current has to be has high as possible to improve the carbon film’s properties. However, for higher ion currents (>10 mA) we have found that the deposition rate and the sp3 content suffer a reduction, as illustrated in Figs 1 and 6, respectively. In this regime the ion-tocarbon ratios should be higher than the ratios normally employed for assistance and, then, the sputtering rate (sputtering yield×ion flux) is high enough to produce a major removal of the deposited atoms. This produces the further decrease in deposition rate observed in Fig. 1. The high ion flux towards the surface should produce a damage analogous to irradiation due to the accumulation of projectiles at the substrate and, hence, reduce the sp3 content. 4.2. Ion irradiation In principle, irradiation with high-energy ions (3 keV ) is supposed to produce a modification of the film structure. Particularly, we have seen that the sp3 content is reduced and the structure turns towards an amorphous graphitic network. This structural modification of the films after post-deposition bombardment is consistent with the deposition mechanism of subplantation, where the carbon atoms must impinge the surface with intermediate energies around 100 eV. In this case, the ion bombardment occurs as a part or during the growth process itself, supplying the energy needed to enhance the formation of sp3 bonds [27]. However, post-deposition bombardment at higher energies leads to a reduction of the sp3 content. This modification of the film structure can be attributed to the increase in local temperature during bombardment and the higher stability of the graphite phase. It is commonly known that amorphous carbon layers have poor thermal stability and suffer graphitisation for annealing temperatures above 200°C [28–30].
5. Conclusions The structure and composition of amorphous carbon films grown by ion-beam-assisted evaporation of graphite have been studied with Raman and Auger spectroscopy. Both techniques show similar trends in the film properties. The carbon films are composed of a mixture of sp2-
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and sp3-hybridised carbon atoms. Ion assistance at low doses (<10 mA) enhances the formation of sp3 bonds with increasing current value. The increase in the diamond-like character is followed by a reduction in the sputtering yield and an increase in the deposition rate. With this method we can obtain films with an sp3 content of up to 40%. For ion currents above 10 mA, sputtering of the deposited film dominates over the deposition and the sp3 content and deposition rate suffer a reduction. Post-deposition ion irradiation causes a change in the structure of the films, reducing the number of sp3 bonds considerably and increasing the argon content. Therefore, the promotion of sp3 bonds is obtained only when ion bombardment occurs during the growth process itself.
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