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Nanostructured carbide surfaces prepared by surfactant sputtering
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1398–1402
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nanostructured carbide surfaces prepared by surfactant sputtering H. Hofsäss *, K. Zhang, H. Zutz II. Physikalisches Institut, Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
a r t i c l e
i n f o
Article history: Available online 29 January 2009 PACS: 81.15.Jj 68.55.a Keywords: Sputtering Ripples Surfactant Thin films Metal carbide
a b s t r a c t Nanostructured surface layers of titanium carbide and tungsten carbide were prepared on tetrahedral amorphous carbon (ta-C) films using the surfactant sputtering technique. Surfactant sputtering is a novel ion beam erosion technique, which utilizes the steady state coverage of a substrate surface with foreign atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These foreign atoms act as surfactants, which strongly modify the substrate sputtering yield on atomic to macroscopic length scales. The novel technique allows smoothing of surfaces, the generation of novel surface patterns and nanostructures, controlled shaping of surfaces on the nanometer scale and the formation of ultra-thin compound surface layers. We have sputter eroded ta-C films using 5 keV Xe ions under continuous deposition of either tungsten or titanium surfactants. This leads to the steady state formation of a WxC or a TiC/a-C nanocomposite surface layer of few nm thickness. Depending on the ion angle of incidence the layer is either smooth or nanostructured with a ripple- or dot-like surface topography. We have analyzed the surface topography, the composition and microstructure of the metal-carbon nanocomposites, and compare coverage dependent sputtering yields with SRIM and TRIDYN simulations. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Metal containing diamond-like carbon (DLC) nanocomposite materials have attracted considerable attention because of their improved mechanical, tribological, electrochemical and optical properties compared to pure DLC coating. Metal containing DLC nanocomposites were proposed as wear resistant low friction coatings [1–5], as material for sensor applications [6,7] and as optically selective absorber in thermal solar collectors [8]. The poor adhesion of pure DLC films to metals and polymers, e.g. used in medical prostheses, can be improved using carbon-metal nanocomposites [9]. Of particular interest for tribological coatings are DLC nanocomposites containing tungsten or titanium as metallic component. These nanocomposites have potential as high strength material and their mechanical properties can be adjusted by varying the ratio of carbide/DLC [10–13]. Nanoscale ripple and dot pattern formation by sputter erosion of surfaces has been observed for a variety of different materials [14,15] and may be used to tailor the surface morphology in a controlled way. Ripple formation on surfaces of graphite and other carbon modifications has also been investigated [16–18].
* Corresponding author. Address: Second Institute of Physics, Georg-August University Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany. Tel.: +49 551 39 7669; fax: +49 551 39 4493. E-mail address: [email protected] (H. Hofsäss). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.053
In this work we investigate the formation and structural properties of thin metal carbide layers formed on the surface of ta-C thin films using the novel surfactant sputtering technique developed by our group [19–21]. Surfactant sputtering utilizes the steady-state coverage of the surface with typically <1016 cm 2 of foreign or self atoms simultaneously during ion beam erosion. In the present study the steady-state coverage with W or Ti atoms is achieved by simultaneous sputter deposition of W or Ti onto the ta-C film at low deposition rate. The situation is best described as ion beam assisted deposition beyond the resputtering limit. The deposited atoms act as surface active agents (surfactants) modifying the sputtering yield of substrate atoms in manifold ways and on length scales from the nanometer range to macroscopic dimensions. Earlier work with similar erosion conditions was devoted to cone formation during sputtering of metal and Si targets [22,23] and sputtering yield amplification effects [24]. The method was recently applied to attenuate the sputter erosion for Fe thin films in order to generate films with a thickness gradient [25] and to modify Si surfaces by sputter erosion, yielding to strong surface smoothing by Au surfactants [21]. New approaches to extend the theoretical description of ripple formation to include surfactant effects were recently developed by Kree et al. [26,27]. In the following we present first results on the microstructure and composition of WxC and TiC/a-C nanocomposite surface layers on tetrahedral amorphous carbon (ta-C) substrates generated by surfactant sputtering of ta-C thin films using W or Ti surfactants.
H. Hofsäss et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1398–1402
2. Experimental Tetrahedral amorphous carbon (ta-C) thin films of about 100 nm thickness, grown on Si (1 0 0) substrates by mass selected ion beam deposition of 100 eV 12C+ ions [28], were sputter eroded using mass selected 5 keV Xe ions under simultaneous deposition of W and Ti surfactant atoms. The ion flux was about 2 lA/cm2 and ion fluences were up to 1.2 1017 cm 2. The ion beam system is described in [29]. Surfactant deposition was accomplished using a geometrical arrangement of ta-C film and sputter target both exposed to the ion beam (Fig. 1) [19]. The source of surfactant atoms is a W or Ti foil as sputter target positioned behind the ta-C film, with regard to the ion beam direction, but inclined to the film, so that a fraction of atoms sputtered off the target are directly deposited onto the taC film. The angle between ion beam direction and surface normal of the sputter target was 30°. The angle between ion beam direction and surface normal of the ta-C film was varied between 30° and 70°. Substrate positions given below refer to the scale shown in Fig. 1. The steady-state coverage of surfactants increases from position 0 mm to position 7 mm. This setup offers the possibility to vary the surfactant coverage across the substrate area and to study erosion effects as a function of surfactant coverage. A disadvantage is the possibility of cross-contamination, which is however not relevant for the present studies. A cross-contamination of the target in particular causes a modified steady-state deposition flux of surfactant atoms onto the substrate. In the present studies we compare measured and calculated steady-state surfactant coverages so that the actual steady-state deposition flux is not of primary relevance. The substrates and the films, prior and after sputter erosion, were analyzed by atomic force microscopy (AFM) and Rutherford backscattering spectroscopy (RBS) using a 900 keV He2+. From RBS we determine the amount of surfactant coverage and the sputtering yields. The sputter depth of some samples was also analyzed using a profilometer. The film structure of the samples was analyzed by transmission electron microscopy (TEM). Since TEM sample preparation using a focused ion beam system requires deposition of a Pt layer onto the sample surface, we deposited a few nm of ta-C subsequent to ion beam erosion,
Fig. 1. Geometrical arrangement of ta-C substrate and target with respect to the ion beam. The W or Ti target is the source of surfactant atoms deposited onto the ta-C film with steady-state coverage increasing from d = 0 mm to d = 7 mm.
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so that the surfactant layer appears as a buried layer in the TEM images. Calculation of sputtering yields were performed with SRIM 2006 [30] (The latest SRIM 2008 version underestimates the sputtering yield for target elements Z < 18) and TRIDYN [31]. In TRIDYN simulations the Xe ions as well as low energy (few eV) surfactant atoms with variable ion-to-surfactant ratio were used as incident particles. If possible, the calculated sputtering yields for pure substrates (in this work ta-C but also Si and various metal substrates) were compared to measured sputtering yields, revealing that SRIM sputtering yields are reasonable accurate within a 15% margin or even less. One should have in mind that surface roughening may modify the sputtering yield which cannot be accounted for by simulations. However, in our case the formation of ripples with few nm amplitude on pure ta-C and Si surfaces did not lead to significant differences in measured sputtering yields compared to calculated sputtering yields. TRIDYN simulations were done after adjustment of the surface binding energies to reproduce the SRIM 2006 sputtering yields. For compounds such as TiC or WC also the sputtering yield of the elemental components had to be reproduced.
3. Results and discussion The influence of W surfactants on the ripple formation during sputter erosion of ta-C films is shown in Fig. 2. The Xe ion beam was incident at 70° with respect to the surface normal. The asdeposited ta-C films are quite smooth with a rms roughness <0.8 nm. Erosion without surfactants leads to the well known surface ripples with wavelength k 40 nm in accordance with previous studies [16–18]. With increasing steady state W coverage the ripple pattern coarsens and eventually converts to a dot like pattern. A similar behavior is observed for Ti surfactants. Erosion at incidence angles smaller than 50° results in unstructured rather smooth surfaces with W or Ti area densities between 1015 and 1016 cm 2. Cross section TEM images of samples eroded with 5 keV Xe at 70° using W and Ti surfactants are shown in Fig. 3. A thin layer consisting of W + C and Ti + C, respectively, is clearly seen as dark region. The upper left image is a cut along the ripple direction. The variation of the W + C layer thickness due to the ripple structure is clearly visible. The W + C layer is about 10 nm thick, in accordance with the range of W atoms recoiled by Xe ions, and does not show any crystalline features. In contrast, the Ti + C layer consist of small nanocrystals with planar spacing of about 0.21 nm, which is in accordance to TiC (2 0 0) planes. The Ti + C layer thickness is also in agreement with the range of Ti atoms recoiled by Xe ions. The measured effective carbon sputtering yield as function of surfactant area density is plotted in Figs. 4 and 5 for W and Ti surfactants, respectively. In the case of W surfactants, the sputtering yield is rather independent of the W coverage varying between 1016 and 3 1016 W/cm2 and about 50% reduced compared to the pure ta-C surface. SRIM simulations assuming a thin W layer on ta-C but also TRIDYN simulations taking into account ballistic mixing of W and C due to collision cascade effects cannot explain the magnitude and coverage dependence of the measured sputtering yield. Even the calculated sputtering yield assuming a WC layer on ta-C is somewhat lower than the experimental values. Thus, the W + C layer visible in TEM can be described as amorphous WxC with x smaller but close to 1. The formation of a nearly stoichiometric WC steady-state surface layer requires ion induced diffusion processes in addition to ballistic mixing. Strong ion-induced diffusion was recently also observed during Si erosion using Au surfactants, leading to a buried AuxSi surfactant layer [21].
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Fig. 2. AFM images of as-deposited ta-C and ta-C films irradiated with 5 1016 cm 2 5 keV Xe ions at incidence angle of 70° and co-deposition of W surfactant atoms. The steady-state area density of W as measured by RBS is indicated. The arrow indicates the projected ion beam direction. Upper right diagram: rms roughness and ripple wavelength as function of steady state W coverage.
Due to the lower sputtering yield for Ti (YSRIM 2.1) compared to W (YSRIM 6) for sputtering with 5 keV Xe at 30°, the Ti deposition flux onto the ta-C substrate is smaller and thus the Ti surfactant area density is smaller and varies between 2 1015 and 1.8 1016 Ti/cm2, as measured by RBS. The Ti coverage dependent effective carbon sputtering yield decreases slowly with increasing Ti coverage. This behavior cannot be explained by a TiC/ta-C bilayer system, but is in good agreement with the TRIDYN results taking ballistic mixing into account. Considering the nanocrystals seen in the TEM mages we conclude, that a TiC/a-C nanocomposite is formed as steady-state surface layer with a composition gradient determined by ballistic mixing.
4. Conclusion We have applied surfactant sputtering to modify the surface region of ta-C thin films using Xe ion beam erosion with W and Ti surfactant atoms. Both species lead to the formation of steadystate surface near carbide layers. Few nm thick nanostructured WxC and TiC/a-C thin films are formed as frozen-in steady state surfactant layer after ion beam erosion is stopped. The surfactants strongly influence the ripple formation, leading to a transition from ripple to dot like patterns with increasing metal coverage. Whereas W surfactants lead to the formation of an amorphous WxC surface layer, Ti surfactants lead to the formation of nanocrystalline TiC/a-
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Fig. 3. TEM images of ta-C films irradiated with 5 1016 cm 2 5 keV Xe ions at incidence angle of 70° and co-deposition of W (left column) and Ti (right column) surfactant atoms. The steady-state area density of W and Ti as measured by RBS is indicated. The upper left image shows a cross section parallel to the ripple orientation. The W + C layer shows no crystalline structure, whereas the Ti + C layer consists of nanocrystals with planar spacing corresponding to TiC (2 0 0) planes.
Fig. 4. Comparison of measured sputtering yield as function of W steady state surfactant area density with SRIM simulations for W/a-C and WC/a-C bi-layer systems and TRIDYN simulations for ballistic mixing. The measured sputtering yield is in accordance with the formation of a WC surface layer.
Fig. 5. Comparison of measured sputtering yield as function of Ti steady state surfactant area density with SRIM simulations for a TiC/a-C bi-layer system and TRIDYN simulations for ballistic mixing. The measured sputtering yield is in accordance with the formation of a TiC/a-C nanocomposite layer with composition determined ballistic mixing.
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C composite thin films with composition determined by ballistic mixing. SRIM and TRIDYN simulations are useful tools to analyze and interpret measured sputtering yields as function of surfactant coverage. In general, surfactant sputtering offers manifold possibilities for surface modification to create novel surface patterns, even smooth surfaces, ultrathin films as frozen-in steady state and thin films with thickness gradients. Acknowledgement This work was financially supported by the German Research Society within the priority research program SFB602. References [1] [2] [3] [4] [5] [6] [7]
R.J. Narayan, D. Scholvin, J. Vac. Sci. Technol. B 23 (2005) 1041. Y. Pauleau, E. Thiery, Surf. Coat. Technol. 180 (2004) 313. J. Patscheider, Mrs Bull. 28 (2003) 180. O. Wilhelmsson, M. Rasander, M. Carlsson, Adv. Funct. Mater. 17 (2007) 1611. A.A. Voevodin, J.S. Zabinski, Thin Solid Films 370 (2000) 223. A. Baranov, S. Fanchenko, L. Calliari, et al., Diam. Relat. Mater. 16 (2007) 1365. T. Takeno, T. Takagi, A. Bozhko, et al., Pricm 5: The Fifth Pacific Rim International Conference on Advanced Materials and Processing, Pts 1-5, 475-479 (2005) 2079. [8] P. Oelhafen, A. Schuler, Solar Energy 79 (2005) 110. [9] R.J. Narayan, Diam. Relat. Mater. 14 (2005) 1319.
[10] D. Caschera, F. Federici, S. Kaciulis, et al., Mater. Sci. Eng. C-Biomim. Supramolec. Syst. 27 (2007) 1328. [11] W.J. Yang, T. Sekino, K.B. Shim, et al., Thin Solid Films 473 (2005) 252. [12] B.K. Tay, Y.H. Cheng, X.Z. Ding, et al., Diam. Relat. Mater. 10 (2001) 1082. [13] W.J. Meng, B.A. Gillispie, J. Appl. Phys. 84 (1998) 4314. [14] W.L. Chan, E. Chason, J. Appl. Phys. 101 (2007) 121301. [15] U. Valbusa, C. Boragno, F. Buatier de Mongeot, Mater. Sci. Eng. C23 (2003) 201. [16] S. Habenicht, W. Bolse, K.P. Lieb, K. Reimann, U. Geyer, Phys. Rev. B 60 (1999) R2200. [17] S. Habenicht, W. Bolse, H. Feldermann, U. Geyer, H. Hofsäss, K.P. Lieb, F. Roccaforte, Europhys. Lett. 50 (2000) 209. [18] K. Takahiro, K. Zhang, F. Rotter, D. Schwen, C. Ronning, H. Hofsäss, J. Krauser, Nucl. Instr. and Meth. B 256 (2007) 378. [19] H. Hofsäss, K. Zhang, Appl. Phys. A: Mater. Sci. Process. 92 (2008) 517. [20] H. Hofsäss, K. Zhang, US patent application 61/017, 319 (2007). [21] H. Hofsäss, K. Zhang, Nucl. Instr. and Meth. B 267 (2009) 1398. [22] G.K. Wehner, D.J. Hajicek, J. Appl. Phys. 42 (1971) 1145. [23] X.M. Meng, N.G. Shang, C.S. Lee, I. Bello, S.T. Lee, Phys. Stat. Sol. A 202 (2005) 2479. [24] S. Berg, I.V. Katardjiev, J. Vac. Sci. Technol. A17 (1999) 1916. [25] K. Zhang, M. Uhrmacher, J. Krauser, H. Hofsäss, J. Appl. Phys. 103 (2008) 083507. [26] A. Hartmann, R. Kree, T. Yasseri, J. Phys. Cond. Mater., submitted for publication. [27] R. Kree, T. Yasseri, A. Hartmann, Nucl. Instr. and Meth. B 267 (2009) 1407. [28] H. Hofsäss, H. Binder, T. Klumpp, E. Recknagel, Diam. Relat. Mater. 3 (1994) 137. [29] S. Habenicht, W. Bolse, K.P. Lieb, Rev. Sci. Instr. 69 (1998) 2120. [30] J. Ziegler, J.P. Biersack, M.D. Ziegler, SRIM – The Stopping and Ranges of Ions in Solids, SRIM Co., Chester, 2008.. [31] W. Möller, W. Eckstein, J.P. Biersack, Comput. Phys. Commun. 51 (1988) 355.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nanostructured carbide surfaces prepared by surfactant sputtering H. Hofsäss *, K. Zhang, H. Zutz II. Physikalisches Institut, Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
a r t i c l e
i n f o
Article history: Available online 29 January 2009 PACS: 81.15.Jj 68.55.a Keywords: Sputtering Ripples Surfactant Thin films Metal carbide
a b s t r a c t Nanostructured surface layers of titanium carbide and tungsten carbide were prepared on tetrahedral amorphous carbon (ta-C) films using the surfactant sputtering technique. Surfactant sputtering is a novel ion beam erosion technique, which utilizes the steady state coverage of a substrate surface with foreign atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These foreign atoms act as surfactants, which strongly modify the substrate sputtering yield on atomic to macroscopic length scales. The novel technique allows smoothing of surfaces, the generation of novel surface patterns and nanostructures, controlled shaping of surfaces on the nanometer scale and the formation of ultra-thin compound surface layers. We have sputter eroded ta-C films using 5 keV Xe ions under continuous deposition of either tungsten or titanium surfactants. This leads to the steady state formation of a WxC or a TiC/a-C nanocomposite surface layer of few nm thickness. Depending on the ion angle of incidence the layer is either smooth or nanostructured with a ripple- or dot-like surface topography. We have analyzed the surface topography, the composition and microstructure of the metal-carbon nanocomposites, and compare coverage dependent sputtering yields with SRIM and TRIDYN simulations. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Metal containing diamond-like carbon (DLC) nanocomposite materials have attracted considerable attention because of their improved mechanical, tribological, electrochemical and optical properties compared to pure DLC coating. Metal containing DLC nanocomposites were proposed as wear resistant low friction coatings [1–5], as material for sensor applications [6,7] and as optically selective absorber in thermal solar collectors [8]. The poor adhesion of pure DLC films to metals and polymers, e.g. used in medical prostheses, can be improved using carbon-metal nanocomposites [9]. Of particular interest for tribological coatings are DLC nanocomposites containing tungsten or titanium as metallic component. These nanocomposites have potential as high strength material and their mechanical properties can be adjusted by varying the ratio of carbide/DLC [10–13]. Nanoscale ripple and dot pattern formation by sputter erosion of surfaces has been observed for a variety of different materials [14,15] and may be used to tailor the surface morphology in a controlled way. Ripple formation on surfaces of graphite and other carbon modifications has also been investigated [16–18].
* Corresponding author. Address: Second Institute of Physics, Georg-August University Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany. Tel.: +49 551 39 7669; fax: +49 551 39 4493. E-mail address: [email protected] (H. Hofsäss). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.053
In this work we investigate the formation and structural properties of thin metal carbide layers formed on the surface of ta-C thin films using the novel surfactant sputtering technique developed by our group [19–21]. Surfactant sputtering utilizes the steady-state coverage of the surface with typically <1016 cm 2 of foreign or self atoms simultaneously during ion beam erosion. In the present study the steady-state coverage with W or Ti atoms is achieved by simultaneous sputter deposition of W or Ti onto the ta-C film at low deposition rate. The situation is best described as ion beam assisted deposition beyond the resputtering limit. The deposited atoms act as surface active agents (surfactants) modifying the sputtering yield of substrate atoms in manifold ways and on length scales from the nanometer range to macroscopic dimensions. Earlier work with similar erosion conditions was devoted to cone formation during sputtering of metal and Si targets [22,23] and sputtering yield amplification effects [24]. The method was recently applied to attenuate the sputter erosion for Fe thin films in order to generate films with a thickness gradient [25] and to modify Si surfaces by sputter erosion, yielding to strong surface smoothing by Au surfactants [21]. New approaches to extend the theoretical description of ripple formation to include surfactant effects were recently developed by Kree et al. [26,27]. In the following we present first results on the microstructure and composition of WxC and TiC/a-C nanocomposite surface layers on tetrahedral amorphous carbon (ta-C) substrates generated by surfactant sputtering of ta-C thin films using W or Ti surfactants.
H. Hofsäss et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1398–1402
2. Experimental Tetrahedral amorphous carbon (ta-C) thin films of about 100 nm thickness, grown on Si (1 0 0) substrates by mass selected ion beam deposition of 100 eV 12C+ ions [28], were sputter eroded using mass selected 5 keV Xe ions under simultaneous deposition of W and Ti surfactant atoms. The ion flux was about 2 lA/cm2 and ion fluences were up to 1.2 1017 cm 2. The ion beam system is described in [29]. Surfactant deposition was accomplished using a geometrical arrangement of ta-C film and sputter target both exposed to the ion beam (Fig. 1) [19]. The source of surfactant atoms is a W or Ti foil as sputter target positioned behind the ta-C film, with regard to the ion beam direction, but inclined to the film, so that a fraction of atoms sputtered off the target are directly deposited onto the taC film. The angle between ion beam direction and surface normal of the sputter target was 30°. The angle between ion beam direction and surface normal of the ta-C film was varied between 30° and 70°. Substrate positions given below refer to the scale shown in Fig. 1. The steady-state coverage of surfactants increases from position 0 mm to position 7 mm. This setup offers the possibility to vary the surfactant coverage across the substrate area and to study erosion effects as a function of surfactant coverage. A disadvantage is the possibility of cross-contamination, which is however not relevant for the present studies. A cross-contamination of the target in particular causes a modified steady-state deposition flux of surfactant atoms onto the substrate. In the present studies we compare measured and calculated steady-state surfactant coverages so that the actual steady-state deposition flux is not of primary relevance. The substrates and the films, prior and after sputter erosion, were analyzed by atomic force microscopy (AFM) and Rutherford backscattering spectroscopy (RBS) using a 900 keV He2+. From RBS we determine the amount of surfactant coverage and the sputtering yields. The sputter depth of some samples was also analyzed using a profilometer. The film structure of the samples was analyzed by transmission electron microscopy (TEM). Since TEM sample preparation using a focused ion beam system requires deposition of a Pt layer onto the sample surface, we deposited a few nm of ta-C subsequent to ion beam erosion,
Fig. 1. Geometrical arrangement of ta-C substrate and target with respect to the ion beam. The W or Ti target is the source of surfactant atoms deposited onto the ta-C film with steady-state coverage increasing from d = 0 mm to d = 7 mm.
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so that the surfactant layer appears as a buried layer in the TEM images. Calculation of sputtering yields were performed with SRIM 2006 [30] (The latest SRIM 2008 version underestimates the sputtering yield for target elements Z < 18) and TRIDYN [31]. In TRIDYN simulations the Xe ions as well as low energy (few eV) surfactant atoms with variable ion-to-surfactant ratio were used as incident particles. If possible, the calculated sputtering yields for pure substrates (in this work ta-C but also Si and various metal substrates) were compared to measured sputtering yields, revealing that SRIM sputtering yields are reasonable accurate within a 15% margin or even less. One should have in mind that surface roughening may modify the sputtering yield which cannot be accounted for by simulations. However, in our case the formation of ripples with few nm amplitude on pure ta-C and Si surfaces did not lead to significant differences in measured sputtering yields compared to calculated sputtering yields. TRIDYN simulations were done after adjustment of the surface binding energies to reproduce the SRIM 2006 sputtering yields. For compounds such as TiC or WC also the sputtering yield of the elemental components had to be reproduced.
3. Results and discussion The influence of W surfactants on the ripple formation during sputter erosion of ta-C films is shown in Fig. 2. The Xe ion beam was incident at 70° with respect to the surface normal. The asdeposited ta-C films are quite smooth with a rms roughness <0.8 nm. Erosion without surfactants leads to the well known surface ripples with wavelength k 40 nm in accordance with previous studies [16–18]. With increasing steady state W coverage the ripple pattern coarsens and eventually converts to a dot like pattern. A similar behavior is observed for Ti surfactants. Erosion at incidence angles smaller than 50° results in unstructured rather smooth surfaces with W or Ti area densities between 1015 and 1016 cm 2. Cross section TEM images of samples eroded with 5 keV Xe at 70° using W and Ti surfactants are shown in Fig. 3. A thin layer consisting of W + C and Ti + C, respectively, is clearly seen as dark region. The upper left image is a cut along the ripple direction. The variation of the W + C layer thickness due to the ripple structure is clearly visible. The W + C layer is about 10 nm thick, in accordance with the range of W atoms recoiled by Xe ions, and does not show any crystalline features. In contrast, the Ti + C layer consist of small nanocrystals with planar spacing of about 0.21 nm, which is in accordance to TiC (2 0 0) planes. The Ti + C layer thickness is also in agreement with the range of Ti atoms recoiled by Xe ions. The measured effective carbon sputtering yield as function of surfactant area density is plotted in Figs. 4 and 5 for W and Ti surfactants, respectively. In the case of W surfactants, the sputtering yield is rather independent of the W coverage varying between 1016 and 3 1016 W/cm2 and about 50% reduced compared to the pure ta-C surface. SRIM simulations assuming a thin W layer on ta-C but also TRIDYN simulations taking into account ballistic mixing of W and C due to collision cascade effects cannot explain the magnitude and coverage dependence of the measured sputtering yield. Even the calculated sputtering yield assuming a WC layer on ta-C is somewhat lower than the experimental values. Thus, the W + C layer visible in TEM can be described as amorphous WxC with x smaller but close to 1. The formation of a nearly stoichiometric WC steady-state surface layer requires ion induced diffusion processes in addition to ballistic mixing. Strong ion-induced diffusion was recently also observed during Si erosion using Au surfactants, leading to a buried AuxSi surfactant layer [21].
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Fig. 2. AFM images of as-deposited ta-C and ta-C films irradiated with 5 1016 cm 2 5 keV Xe ions at incidence angle of 70° and co-deposition of W surfactant atoms. The steady-state area density of W as measured by RBS is indicated. The arrow indicates the projected ion beam direction. Upper right diagram: rms roughness and ripple wavelength as function of steady state W coverage.
Due to the lower sputtering yield for Ti (YSRIM 2.1) compared to W (YSRIM 6) for sputtering with 5 keV Xe at 30°, the Ti deposition flux onto the ta-C substrate is smaller and thus the Ti surfactant area density is smaller and varies between 2 1015 and 1.8 1016 Ti/cm2, as measured by RBS. The Ti coverage dependent effective carbon sputtering yield decreases slowly with increasing Ti coverage. This behavior cannot be explained by a TiC/ta-C bilayer system, but is in good agreement with the TRIDYN results taking ballistic mixing into account. Considering the nanocrystals seen in the TEM mages we conclude, that a TiC/a-C nanocomposite is formed as steady-state surface layer with a composition gradient determined by ballistic mixing.
4. Conclusion We have applied surfactant sputtering to modify the surface region of ta-C thin films using Xe ion beam erosion with W and Ti surfactant atoms. Both species lead to the formation of steadystate surface near carbide layers. Few nm thick nanostructured WxC and TiC/a-C thin films are formed as frozen-in steady state surfactant layer after ion beam erosion is stopped. The surfactants strongly influence the ripple formation, leading to a transition from ripple to dot like patterns with increasing metal coverage. Whereas W surfactants lead to the formation of an amorphous WxC surface layer, Ti surfactants lead to the formation of nanocrystalline TiC/a-
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Fig. 3. TEM images of ta-C films irradiated with 5 1016 cm 2 5 keV Xe ions at incidence angle of 70° and co-deposition of W (left column) and Ti (right column) surfactant atoms. The steady-state area density of W and Ti as measured by RBS is indicated. The upper left image shows a cross section parallel to the ripple orientation. The W + C layer shows no crystalline structure, whereas the Ti + C layer consists of nanocrystals with planar spacing corresponding to TiC (2 0 0) planes.
Fig. 4. Comparison of measured sputtering yield as function of W steady state surfactant area density with SRIM simulations for W/a-C and WC/a-C bi-layer systems and TRIDYN simulations for ballistic mixing. The measured sputtering yield is in accordance with the formation of a WC surface layer.
Fig. 5. Comparison of measured sputtering yield as function of Ti steady state surfactant area density with SRIM simulations for a TiC/a-C bi-layer system and TRIDYN simulations for ballistic mixing. The measured sputtering yield is in accordance with the formation of a TiC/a-C nanocomposite layer with composition determined ballistic mixing.
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C composite thin films with composition determined by ballistic mixing. SRIM and TRIDYN simulations are useful tools to analyze and interpret measured sputtering yields as function of surfactant coverage. In general, surfactant sputtering offers manifold possibilities for surface modification to create novel surface patterns, even smooth surfaces, ultrathin films as frozen-in steady state and thin films with thickness gradients. Acknowledgement This work was financially supported by the German Research Society within the priority research program SFB602. References [1] [2] [3] [4] [5] [6] [7]
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