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Ta bilayers
Applied Surface Science 252 (2005) 2056–2062 www.elsevier.com/locate/apsusc
AES depth profiling and interface analysis of C/Ta bilayers A. Zalar a,*, J. Kovacˇ a, B. Pracˇek a, S. Hofmann b, P. Panjan a a
b
Jozˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Max Planck Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany
Received 4 November 2004; received in revised form 24 March 2005; accepted 25 March 2005 Available online 22 April 2005
Abstract To study the AES sputter depth profiling of a layered structure with different layer densities and sputtering yields, a bilayer structure of C-graphite (60 nm)/Ta (50 nm) was sputter deposited onto smooth silicon substrates. The sputtering rates of C and Ta and the depth resolution, Dz, at the C/Ta interfaces were investigated using 1 and 3 keV Ar+ ions, respectively, varying the angle of incidence in the range between 228 and 828. It was found that the sputtering rates of Ta and C as well as their ratio are strongly angle dependent. The sputtering induced surface topography deteriorated the depth resolution and was studied by atomic force microscopy (AFM). The ripple structures formed on the surfaces of carbon layers during sputter depth profiling of stationary samples could be avoided by sample rotation. The measured carbon concentration profile revealed a strong electron incidence angle dependent backscattering effect on the C (272 eV) Auger signal. The measured AES depth profile obtained with 1 keVAr+ ions at an angle of incidence of 498 was compared to the theoretical depth profile calculated by the mixing, roughness, information depth (MRI) model taking into account backscattering effect of primary electrons. The measured AES concentration profile agrees well with the simulated one obtained with the MRI model. # 2005 Elsevier B.V. All rights reserved. Keywords: AES depth profiling; C/Ta bilayer; Interfaces; Depth resolution; Sputtering rate; Backscattering
1. Introduction Recently much attention is paid on the synthesis and characterization of novel materials, which are based on carbon in different chemical states, like graphite, carbide, carbonitride, hydrogenated carbon films and composite materials on the basis of metal/ * Corresponding author. Tel.: +386 1 477 34 02; fax: +386 1 477 34 40. E-mail address: [email protected] (A. Zalar).
polymer systems [1–5]. In practice, measurable shifts in Auger energies and shape changes of the C (KLL) Auger peak corresponding to the changes in chemical environment in different carbon compounds were observed [6,7]. Thin film structures and composite materials are often depth profiled with a surface analysis method in combination with ion sputtering [8–10]. However, due to the low density of carbon that has one of the lowest sputtering yields of all elements, the measured concentration of carbon in surface layers, at interfaces and in composite materials may be
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.165
A. Zalar et al. / Applied Surface Science 252 (2005) 2056–2062
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overestimated due to the backscattering effect and preferential sputtering. In this work, we studied the influence of sputtering parameters, i.e. ion energy and ion incidence angle, on the depth resolution at the C/Ta interfaces and on the sputtering rates of Ta and C in C-graphite/Ta bilayers (in text C/Ta bilayers). Furthermore, the measured AES depth profile of C was compared to the theoretical depth profile calculated by the MRI model [11,12], taking into account atomic mixing, sputterinduced surface roughness and information depth of the Auger electrons, and also the backscattering effect of primary electrons.
2. Experimental
Fig. 1. TEM micrograph of the cross-sectioned C (60 nm)/Ta (50 nm) bilayer structure.
C-graphite/Ta bilayers were sputter deposited onto smooth silicon (1 1 1) substrates in a Balzers Sputron plasma chamber. A plasma beam (typically 40 V/ 40 A) is produced between the hot filament and the auxiliary anode around the target. Targets of Ta (99.99%) and pyrolitic graphite with a diameter of 60 mm are interchangeable in situ. The sputtering voltage was held constant at 1700 V with 0.6 A of target current. The substrates were away from the plasma. Therefore, it was possible to keep the substrate temperature during deposition process below 100 8C. The deposition rates of Ta and C were 10 and 30 nm/min, respectively. The thicknesses of the individual thin films were measured using a quartz crystal microbalance during sputter deposition. After deposition, the thickness of the individual thin films and the total thickness were determined by crosssectional transmission electron microscopy (Fig. 1). The bilayer consisted of amorphous C-graphite and crystalline Ta thin films with a thickness of 60 and 50 nm (5%), respectively. AES analysis revealed that in the uppermost layer carbon is in the graphite phase. The amount of carbon in the Ta layer was less than 10 at.% and showed the typical carbide peak shape [6]. Probably carbon was incorporated during sputter deposition of the highly reactive Ta layer from the residual gas at a base pressure of about 5 10 5 Pa and at a working argon atmosphere of about 0.2 Pa. AES depth profiles, with and without sample rotation were measured by recording the intensities of
C (272 eV), Ta (179 eV), O (510 eV) and Si (92 eV) Auger electron signals using a PHI SAM 545A instrument. The argon pressure in the chamber during depth profiling was about 7.3 10 3 Pa and the base pressure was <2 10 7 Pa. We used a static primary electron beam of 3 keV, 2.6 mA with a diameter deb 40 mm. The angle between the ion beam direction and the direction of the primary electron beam was in all the cases about 768. In the stationary samples, the C/Ta interfaces were first investigated with respect to the ion beam incidence angle and to the ion energy. Depth profiles were obtained by ion sputtering with two symmetrically inclined ion beams [13]. Different ion beam incidence angles were achieved by mounting the samples on metal bases attached to the original sample holder. Ion sputtering was performed with 1 and 3 keV, respectively, at seven incidence angles of 228, 298, 368, 498, 628, 718 and 828 with respect to the normal to the sample surface. At these angles the samples were depth profiled with a rastered ion beam (5 mm 5 mm). The ion current was kept constant during all the measurements, whereas the ion current density was changing with the cosine of the ion beam incidence angle. The ion current density was measured at the incidence angle of 498 and was 0.19 A m 2. The sample holder was equipped with a rotation mechanism, which enables sample rotation (1 rev min 1) during simultaneous AES analysis and ion etching [14,15]. To study the development of surface topography during AES depth profiling and its influence on the
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depth resolution, some depth profiles were obtained by rotational depth profiling of the samples. Topography measurements were performed with a DI-Nanoscope IIIa atomic force microscope (AFM) in contact mode. The images were recorded at a scan rate of 2 Hz with a resolution of 512 points/line on a 1 mm 1 mm area. The roughness amplitude Ra of as-deposited carbon layer and some of selected ion-sputtered samples surfaces were measured.
3. Results and discussion The series of AES depth profiles obtained by ion sputtering of stationary samples at seven different incidence angles between 228 and 828 enabled the measurement of depth resolution at the C/Ta interfaces. Because of space restrictions, in this work only two depth profiles of the sample are shown, which are indispensable for the discussion. Fig. 2a shows the measured AES depth profile of an as-deposited C/Ta bilayer obtained with 1 keV Ar+ ions at an incidence
Fig. 2. AES sputter depth profiles of the C/Ta bilayer structure on a smooth silicon substrate, depth profiled with 1 keV Ar+ ions at an incidence angle of 498; (a) as-measured and (b) quantified by applying the relative elemental sensitivity factors, without any further corrections.
angle of 498 and Fig. 2b shows the depth profile of the same sample with the concentration given in the atomic percentage. The Auger peak-to-peak heights of elements were quantified by applying the relative elemental sensitivity factors [16]: SC = 0.18, STa = 0.115, SO = 0.50 and SSi = 0.35, without any further corrections due to other effects, e.g. backscattering effect and preferential sputtering in the contaminated Ta layer. The increase of the intensity of the measured C (272 eV) Auger signal in Fig. 2a is due to the backscattering effect, which in this case is already recognized at the beginning of the depth profile. In the quantitative calculation (Fig. 2b), because of the pure C layer on the top, information about the backscattering effect is lost. In case of a multicomponent C containing layer, such a simplification leads to a wrong quantitative estimation. From both depth profiles in Fig. 2a and b it is concluded that the C/Ta interface is relative sharp, the C-graphite layer is clean but the Ta layer is slightly contaminated by carbon incorporated during the deposition process. To quantify the depth resolution at the C/Ta interfaces in AES depth profiles obtained with 1 and 3 keV argon ions at different ion incidence angles, it is necessary to know the sputtering rates of C and Ta. The set of AES depth profiles enable us to construct the diagrams in Fig. 3a and b which show the dependence of the sputtering rates on the ion incidence angles for the ions of 1 and 3 keV, respectively. For both ion beam energies the sputtering rate of C is lower than for Ta (except for 1 and 3 keV at 828, Fig. 3a and b). As expected the sputtering rates are higher for both components at higher ion beam energy. The ratio between both sputtering rates, z˙Ta/z˙C, is strongly angle dependent; from the maximum values of about 4.3 and 3.7 at 228 the ratio decreases to the values of about 0.5 and 0.8 at the grazing incidence angle of 828 for 1 and 3 keV argon ions, respectively. Fig. 4 shows the depth resolution as a function of ion incidence angle for 1 and 3 keV Ar+ ion sputtering. The total C/Ta interface width Dz (84–16%) can be described as the sum of the contribution at the carbonrich side of the interface, Dz (84–50%) and that at the Ta-rich side of the interface, Dz (50–16%) [17]. The difference in the contribution to the depth resolution (84–16%) of both components is evident. For both ion beam energies of 1 and 3 keV, the contribution to Dz is (except at 828) lower for the carbon-rich side than for
A. Zalar et al. / Applied Surface Science 252 (2005) 2056–2062
Fig. 3. Sputtering rates of C-graphite, z˙C, and Ta, z˙Ta, and the ratio z˙Ta/z˙C as a function of the ion incidence angle; (a) for 1 keV and (b) for 3 keV Ar+ ions.
the Ta-rich side. We explain this phenomenon by the backscattering effect at the carbon-rich side that causes a steeper slope [18] and by the preferential sputtering of Ta at the C/Ta interfaces, which may enhance the increase of the roughness amplitude at the Ta-rich side. The depth profiling at grazing incidence angle of 828 reduces the preferential sputtering and the crystalline orientation effect in metals and promotes the smoothing effect [19] what after our opinion contributes to the improved depth resolution (84– 16%) observed in Fig. 4. In order to follow the evolution of the surface topography during sputtering, AFM measurements were carried out on the surface of the as-deposited sample (Fig. 5a) and after ion sputtering with 1 keV ion beam at an ion incidence angle of 718, at two different depths of the samples, namely in the middle of the carbon layer (Fig. 5b) and at the C/Ta interface (Fig. 5c). The surface of as-deposited C-graphite layer is extremely smooth with a roughness amplitude Ra = 0.13 nm. Fig. 5b and c show the formation of a ripple structure after ion sputtering up to the middle of
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Fig. 4. Total depth resolution, Dz (84–16%), at the C/Ta interfaces and the contributions at the C-rich side, Dz (84–50%), and at the Tarich side of the interfaces, Dz (50–16%), as a function of the ion incidence angle for (a) 1 keV and (b) 3 keV Ar+ ions.
the carbon layer (Fig. 5b) and at the C/Ta interface (Fig. 5c). It is interesting to note that the roughness amplitudes Ra at both depths of about 30 and 60 nm beneath the surface have similar values of 1.45 and 1.56 nm, respectively. Therefore, we conclude that after development of the ripple structure in the first half of the carbon layer, the roughness amplitude from the middle of the layer to the C/Ta interface increases very slowly with the depth. A similar ripple structure and a slow increase of roughness with a depth was also observed on the samples sputtered with 1 keV ions at 498; however, with a lower roughness amplitude Ra of 0.36 and 0.46 nm at half of the depth of the carbon layer and at the C/Ta interface, respectively. Though there is some scatter, the worse depth resolution for 1 and 3 keV ion sputtering was observed at 718. It is worthy to note that the sputtering yield data for C and Ta calculated first from the experimental results obtained in this work and secondly using SRIM calculation (not shown here) showed the maximum
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Fig. 5. AFM images showing the surface structures of carbon layers: (a) of as-deposited C/Ta bilayer, Ra = 0.13 nm, and (b–d) after sputter depth profiling with 1 keVAr+ ions at an incidence angle of 718 for stationary samples (b) in the middle of the carbon layer, Ra = 1.45 nm, (c) at the C/ Ta interface, Ra = 1.56 nm and (d) the same as (c) but with sample rotation during depth profiling, Ra = 0.94 nm.
difference in the region around 708. This can be the reason for the enhanced sputtering induced roughening, resulting in a worse depth resolution [20]. Our AFM investigation of the surface structure formed during AES depth profiling of the samples confirmed this hypothesis: the value of roughness amplitude of the carbon layer ion sputtered at 718 was more than a factor of 3 larger than that at 498. We relate the formation of the ripple structure in the graphite layer to the differences of sputtering yields at the microplanes of the sample surface present in the case of ion sputtering with two symmetrically inclined ion beams. In support to this statement is the AFM image (Fig. 5d), which shows the surface topography at the C/Ta interface after ion sputtering of the rotated sample with 1 keV ions at 718. This surface has a roughness amplitude of 0.94 nm.
For the C/Ta bilayer with different layer densities, an enhancement of the Auger yield in the C-graphite due to backscattered electrons from the Ta layer was observed. Fig. 2a shows an increase of the C (272 eV) Auger signal when approaching the C/Ta interface. The Auger intensities are expected to be proportional to the backscattering factor, 1 + rB(z), which changes within the layer with the depth (z-axis). To simulate the increase of Auger signal of C (272 eV) we calculated the backscattering factor using expression of Barkshire et al., who described backscattered effect with an analytical function of the layer thickness [21]. This function is supposed to give a better agreement with the data than the exponential function used earlier by Hofmann to explain the effect of backscattering on interface broadening [18]. Comparison of calculated backscattering factor with the measured C (272 eV) Auger signal in Fig. 2a showed a similar depth
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dependence. The calculation after Barkshire et al. expression shows that the backscattering effect in the carbon layer occurs at a certain distance before the C/ Ta interface. This distance is dependent on the energy of the primary electron beam. For a 3 keV electron beam at an incidence angle of 308 it is 80 nm, indicating that the backscattering effect took part already at the beginning of the sputtering process in the thinner C layer with a thickness of 60 nm. Additionally, broadening of the C/Ta interface was simulated by the MRI model for the sample depth profiled with 1 keVAr+ ions at an angle of incidence of 498. The MRI model is a theoretical description of depth resolution function taking into account atomic mixing (w), sputter-induced surface roughness (s) and information depth (l) [11,12]. For the first time we combined the MRI-simulation of interface broadening with simulation of the backscattering effect. Fig. 6 shows the C (272 eV) intensity depth profile as measured by AES sputter depth profiling with 1 keV Ar+ ions at an angle of incidence of 498, the model curve for C distribution taking into account the backscattering effect, and the MRI calculated C distribution. First, the input model curve for the MRI simulation was obtained by modification of initial step-like carbon distribution with backscattering factor, 1 + rB(z), using the method described by Barkshire et al. in Ref. [21]. During the MRI fitting the information depth was kept constant, l = 0.65 nm,
Fig. 6. The measured C (272 eV) concentration depth profile (o), model curve for C distribution modified for the backscattering effect (- - -) and MRI reconstructed C distribution (—); the case for AES depth profiling of stationary C/Ta sample using 3 keV electron beam at an incidence angle of 308 and 1 keV Ar+ ions at an incidence angle of 498. Parameters: w = 2.5 nm, s = 0.5 nm, l = 0.65 nm, Dz = 4.4 nm and 1 + rB = 1.36 2.02.
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whereas roughness parameter (s) and atomic mixing (w) were changed. From the MRI simulation we obtained the roughness amplitude at the C/Ta interface to be 0.5 nm and the atomic mixing zone of 2.5 nm for the incidence angle of 498. The depth resolution of 4.4 nm derived from the MRI simulation agrees with the measured depth resolution in Fig. 4a.
4. Conclusions A comparison of depth profiling results for C/Ta bilayers on a smooth Si substrate using AES depth profiling with 1 and 3 keV Ar+ ion beams at seven different ion incidence angles between 228 and 828 showed that: (1) The sputtering rates of Ta and C and their ratio are strongly angle dependent. For 1 and 3 keVAr+ ion sputtering the ratio z˙Ta/z˙C has been changed from the maximum values of about 4.3 and 3.7 at the incidence angle of 228 to the values of about 0.5 and 0.8 at the grazing incidence angle of 828, respectively. The sputtering rate of carbon at 828 was found to be slightly higher than that of Ta. (2) The total interface width Dz (84–16%) was described as a sum of the contribution at the Crich side of the interface, Dz (84–50%), and that at the Ta-rich side of the interface, Dz (50–16%). We ascribed the lesser contribution of the C-side to the backscattering effect and the greater contribution at the Ta-rich side to the preferential sputtering of Ta at the C/Ta interface (except at the ion incidence angle of 828). (3) AFM measurements revealed that the ripple structure formed in the carbon layers during ion sputtering of the stationary samples could be prevented by sample rotation during depth profiling. (4) Owing to the higher density of the Ta layer beneath the carbon layer a strong backscattering of primary electrons took place, which additionally increases the Auger electron emission of C (272 eV). The effect depends on the energy of the primary electron beam. (5) The theoretical calculation of the carbon concentration profile with the MRI model combined the effects of atomic mixing (M), surface rough-
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ness (R), escape depth of the Auger electrons (I) with the method proposed by Barkshire et al. to calculate the backscattering effect of primary electrons (1 + rB). The value of roughness estimated by the MRI model is in reasonable agreement with the AFM roughness amplitude and the experimental depth resolution agrees with value derived from the MRI simulation.
Acknowledgements ˇ eh and Dr. The authors would like to thank Dr. M. C ˇ M. Skarabot (Jozˇef Stefan Institute) for TEM investigation and for help at the AFM measurements. The work was supported by the Ministry of Higher Education, Science and Technology of Slovenia, Ljubljana (Project P2-82).
References [1] A. Grigonis, V. Sablinskas, M. Silinskas, D. Tribandis, Vacuum 75 (2004) 261. [2] O. Wilhelmsson, J.P. Palmquist, T. Nyberg, U. Jansson, Appl. Phys. Lett. 85 (2004) 1066. [3] I. Afanasyev-Charkin, M. Nastasi, Surf. Coat. Technol. 186 (2004) 108.
[4] G. Carotenuto, B. Martorana, P.B. Perlo, L. Nicolais, J. Mater. Chem. 13 (2003) 2927. [5] P. Panjan, M. Cˇekada, D. Kek Merl, M. Macˇek, B. Navinsˇek, A. Jesih, A. Zalar, M. Jenko, Vacuum 71 (2003) 261. [6] T.W. Haas, J.T. Grant, G.J. Dooley, in: F. Ricca (Ed.), Adsorption-Desorption Phenomena, Academic Press, New York, 1972 , p. 359. [7] D.E. Ramaker in, D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, IM Publications and Surface Spectra Lim, Chichester, 2003 , p. 465. [8] H.H. Hwu, B. Fruhberger, J.G.G. Chen, J. Catal. 221 (2004) 170. [9] C. Li, C.B. Cao, H.S. Zhu, Mater. Lett. 58 (2004) 1903. [10] C.A. Taylor, W.K.S. Chiu, Surf. Coat. Technol. 168 (2003) 1. [11] S. Hofmann, Surf. Interface Anal. 21 (1994) 673. [12] S. Hofmann, Surf. Interface Anal. 27 (1999) 825. [13] A. Zalar, P. Panjan, S. Hofmann, Thin Solid Films 181 (1989) 277. [14] A. Zalar, Thin Solid Films 124 (1985) 223. [15] A. Zalar, B. Pracˇek, P. Panjan, Surf. Interface Anal. 30 (2000) 247. [16] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach, R.E. Weber, Handbook of Auger Electron Spectroscopy, in: Physical Electronics Industries, Eden Prairie, 1976. [17] A. Zalar, S. Hofmann, F. Pimentel, D. Kohl, P. Panjan, Vacuum 46 (1995) 1077. [18] S. Hofmann, Mikrochim. Acta Suppl. 8 (1979) 71. [19] A. Barna, P.B. Barna, A. Zalar, Surf. Interface Anal. 12 (1988) 144. [20] A. Barna, M. Menyhard, G. Zsolt, A. Koos, A. Zalar, P. Panjan, J. Vac. Sci. Technol. A 21 (3) (2003) 553. [21] I.R. Barkshire, M. Prutton, D.K. Skinner, Surf. Interface Anal. 17 (1991) 213.
AES depth profiling and interface analysis of C/Ta bilayers A. Zalar a,*, J. Kovacˇ a, B. Pracˇek a, S. Hofmann b, P. Panjan a a
b
Jozˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Max Planck Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany
Received 4 November 2004; received in revised form 24 March 2005; accepted 25 March 2005 Available online 22 April 2005
Abstract To study the AES sputter depth profiling of a layered structure with different layer densities and sputtering yields, a bilayer structure of C-graphite (60 nm)/Ta (50 nm) was sputter deposited onto smooth silicon substrates. The sputtering rates of C and Ta and the depth resolution, Dz, at the C/Ta interfaces were investigated using 1 and 3 keV Ar+ ions, respectively, varying the angle of incidence in the range between 228 and 828. It was found that the sputtering rates of Ta and C as well as their ratio are strongly angle dependent. The sputtering induced surface topography deteriorated the depth resolution and was studied by atomic force microscopy (AFM). The ripple structures formed on the surfaces of carbon layers during sputter depth profiling of stationary samples could be avoided by sample rotation. The measured carbon concentration profile revealed a strong electron incidence angle dependent backscattering effect on the C (272 eV) Auger signal. The measured AES depth profile obtained with 1 keVAr+ ions at an angle of incidence of 498 was compared to the theoretical depth profile calculated by the mixing, roughness, information depth (MRI) model taking into account backscattering effect of primary electrons. The measured AES concentration profile agrees well with the simulated one obtained with the MRI model. # 2005 Elsevier B.V. All rights reserved. Keywords: AES depth profiling; C/Ta bilayer; Interfaces; Depth resolution; Sputtering rate; Backscattering
1. Introduction Recently much attention is paid on the synthesis and characterization of novel materials, which are based on carbon in different chemical states, like graphite, carbide, carbonitride, hydrogenated carbon films and composite materials on the basis of metal/ * Corresponding author. Tel.: +386 1 477 34 02; fax: +386 1 477 34 40. E-mail address: [email protected] (A. Zalar).
polymer systems [1–5]. In practice, measurable shifts in Auger energies and shape changes of the C (KLL) Auger peak corresponding to the changes in chemical environment in different carbon compounds were observed [6,7]. Thin film structures and composite materials are often depth profiled with a surface analysis method in combination with ion sputtering [8–10]. However, due to the low density of carbon that has one of the lowest sputtering yields of all elements, the measured concentration of carbon in surface layers, at interfaces and in composite materials may be
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.165
A. Zalar et al. / Applied Surface Science 252 (2005) 2056–2062
2057
overestimated due to the backscattering effect and preferential sputtering. In this work, we studied the influence of sputtering parameters, i.e. ion energy and ion incidence angle, on the depth resolution at the C/Ta interfaces and on the sputtering rates of Ta and C in C-graphite/Ta bilayers (in text C/Ta bilayers). Furthermore, the measured AES depth profile of C was compared to the theoretical depth profile calculated by the MRI model [11,12], taking into account atomic mixing, sputterinduced surface roughness and information depth of the Auger electrons, and also the backscattering effect of primary electrons.
2. Experimental
Fig. 1. TEM micrograph of the cross-sectioned C (60 nm)/Ta (50 nm) bilayer structure.
C-graphite/Ta bilayers were sputter deposited onto smooth silicon (1 1 1) substrates in a Balzers Sputron plasma chamber. A plasma beam (typically 40 V/ 40 A) is produced between the hot filament and the auxiliary anode around the target. Targets of Ta (99.99%) and pyrolitic graphite with a diameter of 60 mm are interchangeable in situ. The sputtering voltage was held constant at 1700 V with 0.6 A of target current. The substrates were away from the plasma. Therefore, it was possible to keep the substrate temperature during deposition process below 100 8C. The deposition rates of Ta and C were 10 and 30 nm/min, respectively. The thicknesses of the individual thin films were measured using a quartz crystal microbalance during sputter deposition. After deposition, the thickness of the individual thin films and the total thickness were determined by crosssectional transmission electron microscopy (Fig. 1). The bilayer consisted of amorphous C-graphite and crystalline Ta thin films with a thickness of 60 and 50 nm (5%), respectively. AES analysis revealed that in the uppermost layer carbon is in the graphite phase. The amount of carbon in the Ta layer was less than 10 at.% and showed the typical carbide peak shape [6]. Probably carbon was incorporated during sputter deposition of the highly reactive Ta layer from the residual gas at a base pressure of about 5 10 5 Pa and at a working argon atmosphere of about 0.2 Pa. AES depth profiles, with and without sample rotation were measured by recording the intensities of
C (272 eV), Ta (179 eV), O (510 eV) and Si (92 eV) Auger electron signals using a PHI SAM 545A instrument. The argon pressure in the chamber during depth profiling was about 7.3 10 3 Pa and the base pressure was <2 10 7 Pa. We used a static primary electron beam of 3 keV, 2.6 mA with a diameter deb 40 mm. The angle between the ion beam direction and the direction of the primary electron beam was in all the cases about 768. In the stationary samples, the C/Ta interfaces were first investigated with respect to the ion beam incidence angle and to the ion energy. Depth profiles were obtained by ion sputtering with two symmetrically inclined ion beams [13]. Different ion beam incidence angles were achieved by mounting the samples on metal bases attached to the original sample holder. Ion sputtering was performed with 1 and 3 keV, respectively, at seven incidence angles of 228, 298, 368, 498, 628, 718 and 828 with respect to the normal to the sample surface. At these angles the samples were depth profiled with a rastered ion beam (5 mm 5 mm). The ion current was kept constant during all the measurements, whereas the ion current density was changing with the cosine of the ion beam incidence angle. The ion current density was measured at the incidence angle of 498 and was 0.19 A m 2. The sample holder was equipped with a rotation mechanism, which enables sample rotation (1 rev min 1) during simultaneous AES analysis and ion etching [14,15]. To study the development of surface topography during AES depth profiling and its influence on the
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depth resolution, some depth profiles were obtained by rotational depth profiling of the samples. Topography measurements were performed with a DI-Nanoscope IIIa atomic force microscope (AFM) in contact mode. The images were recorded at a scan rate of 2 Hz with a resolution of 512 points/line on a 1 mm 1 mm area. The roughness amplitude Ra of as-deposited carbon layer and some of selected ion-sputtered samples surfaces were measured.
3. Results and discussion The series of AES depth profiles obtained by ion sputtering of stationary samples at seven different incidence angles between 228 and 828 enabled the measurement of depth resolution at the C/Ta interfaces. Because of space restrictions, in this work only two depth profiles of the sample are shown, which are indispensable for the discussion. Fig. 2a shows the measured AES depth profile of an as-deposited C/Ta bilayer obtained with 1 keV Ar+ ions at an incidence
Fig. 2. AES sputter depth profiles of the C/Ta bilayer structure on a smooth silicon substrate, depth profiled with 1 keV Ar+ ions at an incidence angle of 498; (a) as-measured and (b) quantified by applying the relative elemental sensitivity factors, without any further corrections.
angle of 498 and Fig. 2b shows the depth profile of the same sample with the concentration given in the atomic percentage. The Auger peak-to-peak heights of elements were quantified by applying the relative elemental sensitivity factors [16]: SC = 0.18, STa = 0.115, SO = 0.50 and SSi = 0.35, without any further corrections due to other effects, e.g. backscattering effect and preferential sputtering in the contaminated Ta layer. The increase of the intensity of the measured C (272 eV) Auger signal in Fig. 2a is due to the backscattering effect, which in this case is already recognized at the beginning of the depth profile. In the quantitative calculation (Fig. 2b), because of the pure C layer on the top, information about the backscattering effect is lost. In case of a multicomponent C containing layer, such a simplification leads to a wrong quantitative estimation. From both depth profiles in Fig. 2a and b it is concluded that the C/Ta interface is relative sharp, the C-graphite layer is clean but the Ta layer is slightly contaminated by carbon incorporated during the deposition process. To quantify the depth resolution at the C/Ta interfaces in AES depth profiles obtained with 1 and 3 keV argon ions at different ion incidence angles, it is necessary to know the sputtering rates of C and Ta. The set of AES depth profiles enable us to construct the diagrams in Fig. 3a and b which show the dependence of the sputtering rates on the ion incidence angles for the ions of 1 and 3 keV, respectively. For both ion beam energies the sputtering rate of C is lower than for Ta (except for 1 and 3 keV at 828, Fig. 3a and b). As expected the sputtering rates are higher for both components at higher ion beam energy. The ratio between both sputtering rates, z˙Ta/z˙C, is strongly angle dependent; from the maximum values of about 4.3 and 3.7 at 228 the ratio decreases to the values of about 0.5 and 0.8 at the grazing incidence angle of 828 for 1 and 3 keV argon ions, respectively. Fig. 4 shows the depth resolution as a function of ion incidence angle for 1 and 3 keV Ar+ ion sputtering. The total C/Ta interface width Dz (84–16%) can be described as the sum of the contribution at the carbonrich side of the interface, Dz (84–50%) and that at the Ta-rich side of the interface, Dz (50–16%) [17]. The difference in the contribution to the depth resolution (84–16%) of both components is evident. For both ion beam energies of 1 and 3 keV, the contribution to Dz is (except at 828) lower for the carbon-rich side than for
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Fig. 3. Sputtering rates of C-graphite, z˙C, and Ta, z˙Ta, and the ratio z˙Ta/z˙C as a function of the ion incidence angle; (a) for 1 keV and (b) for 3 keV Ar+ ions.
the Ta-rich side. We explain this phenomenon by the backscattering effect at the carbon-rich side that causes a steeper slope [18] and by the preferential sputtering of Ta at the C/Ta interfaces, which may enhance the increase of the roughness amplitude at the Ta-rich side. The depth profiling at grazing incidence angle of 828 reduces the preferential sputtering and the crystalline orientation effect in metals and promotes the smoothing effect [19] what after our opinion contributes to the improved depth resolution (84– 16%) observed in Fig. 4. In order to follow the evolution of the surface topography during sputtering, AFM measurements were carried out on the surface of the as-deposited sample (Fig. 5a) and after ion sputtering with 1 keV ion beam at an ion incidence angle of 718, at two different depths of the samples, namely in the middle of the carbon layer (Fig. 5b) and at the C/Ta interface (Fig. 5c). The surface of as-deposited C-graphite layer is extremely smooth with a roughness amplitude Ra = 0.13 nm. Fig. 5b and c show the formation of a ripple structure after ion sputtering up to the middle of
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Fig. 4. Total depth resolution, Dz (84–16%), at the C/Ta interfaces and the contributions at the C-rich side, Dz (84–50%), and at the Tarich side of the interfaces, Dz (50–16%), as a function of the ion incidence angle for (a) 1 keV and (b) 3 keV Ar+ ions.
the carbon layer (Fig. 5b) and at the C/Ta interface (Fig. 5c). It is interesting to note that the roughness amplitudes Ra at both depths of about 30 and 60 nm beneath the surface have similar values of 1.45 and 1.56 nm, respectively. Therefore, we conclude that after development of the ripple structure in the first half of the carbon layer, the roughness amplitude from the middle of the layer to the C/Ta interface increases very slowly with the depth. A similar ripple structure and a slow increase of roughness with a depth was also observed on the samples sputtered with 1 keV ions at 498; however, with a lower roughness amplitude Ra of 0.36 and 0.46 nm at half of the depth of the carbon layer and at the C/Ta interface, respectively. Though there is some scatter, the worse depth resolution for 1 and 3 keV ion sputtering was observed at 718. It is worthy to note that the sputtering yield data for C and Ta calculated first from the experimental results obtained in this work and secondly using SRIM calculation (not shown here) showed the maximum
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Fig. 5. AFM images showing the surface structures of carbon layers: (a) of as-deposited C/Ta bilayer, Ra = 0.13 nm, and (b–d) after sputter depth profiling with 1 keVAr+ ions at an incidence angle of 718 for stationary samples (b) in the middle of the carbon layer, Ra = 1.45 nm, (c) at the C/ Ta interface, Ra = 1.56 nm and (d) the same as (c) but with sample rotation during depth profiling, Ra = 0.94 nm.
difference in the region around 708. This can be the reason for the enhanced sputtering induced roughening, resulting in a worse depth resolution [20]. Our AFM investigation of the surface structure formed during AES depth profiling of the samples confirmed this hypothesis: the value of roughness amplitude of the carbon layer ion sputtered at 718 was more than a factor of 3 larger than that at 498. We relate the formation of the ripple structure in the graphite layer to the differences of sputtering yields at the microplanes of the sample surface present in the case of ion sputtering with two symmetrically inclined ion beams. In support to this statement is the AFM image (Fig. 5d), which shows the surface topography at the C/Ta interface after ion sputtering of the rotated sample with 1 keV ions at 718. This surface has a roughness amplitude of 0.94 nm.
For the C/Ta bilayer with different layer densities, an enhancement of the Auger yield in the C-graphite due to backscattered electrons from the Ta layer was observed. Fig. 2a shows an increase of the C (272 eV) Auger signal when approaching the C/Ta interface. The Auger intensities are expected to be proportional to the backscattering factor, 1 + rB(z), which changes within the layer with the depth (z-axis). To simulate the increase of Auger signal of C (272 eV) we calculated the backscattering factor using expression of Barkshire et al., who described backscattered effect with an analytical function of the layer thickness [21]. This function is supposed to give a better agreement with the data than the exponential function used earlier by Hofmann to explain the effect of backscattering on interface broadening [18]. Comparison of calculated backscattering factor with the measured C (272 eV) Auger signal in Fig. 2a showed a similar depth
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dependence. The calculation after Barkshire et al. expression shows that the backscattering effect in the carbon layer occurs at a certain distance before the C/ Ta interface. This distance is dependent on the energy of the primary electron beam. For a 3 keV electron beam at an incidence angle of 308 it is 80 nm, indicating that the backscattering effect took part already at the beginning of the sputtering process in the thinner C layer with a thickness of 60 nm. Additionally, broadening of the C/Ta interface was simulated by the MRI model for the sample depth profiled with 1 keVAr+ ions at an angle of incidence of 498. The MRI model is a theoretical description of depth resolution function taking into account atomic mixing (w), sputter-induced surface roughness (s) and information depth (l) [11,12]. For the first time we combined the MRI-simulation of interface broadening with simulation of the backscattering effect. Fig. 6 shows the C (272 eV) intensity depth profile as measured by AES sputter depth profiling with 1 keV Ar+ ions at an angle of incidence of 498, the model curve for C distribution taking into account the backscattering effect, and the MRI calculated C distribution. First, the input model curve for the MRI simulation was obtained by modification of initial step-like carbon distribution with backscattering factor, 1 + rB(z), using the method described by Barkshire et al. in Ref. [21]. During the MRI fitting the information depth was kept constant, l = 0.65 nm,
Fig. 6. The measured C (272 eV) concentration depth profile (o), model curve for C distribution modified for the backscattering effect (- - -) and MRI reconstructed C distribution (—); the case for AES depth profiling of stationary C/Ta sample using 3 keV electron beam at an incidence angle of 308 and 1 keV Ar+ ions at an incidence angle of 498. Parameters: w = 2.5 nm, s = 0.5 nm, l = 0.65 nm, Dz = 4.4 nm and 1 + rB = 1.36 2.02.
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whereas roughness parameter (s) and atomic mixing (w) were changed. From the MRI simulation we obtained the roughness amplitude at the C/Ta interface to be 0.5 nm and the atomic mixing zone of 2.5 nm for the incidence angle of 498. The depth resolution of 4.4 nm derived from the MRI simulation agrees with the measured depth resolution in Fig. 4a.
4. Conclusions A comparison of depth profiling results for C/Ta bilayers on a smooth Si substrate using AES depth profiling with 1 and 3 keV Ar+ ion beams at seven different ion incidence angles between 228 and 828 showed that: (1) The sputtering rates of Ta and C and their ratio are strongly angle dependent. For 1 and 3 keVAr+ ion sputtering the ratio z˙Ta/z˙C has been changed from the maximum values of about 4.3 and 3.7 at the incidence angle of 228 to the values of about 0.5 and 0.8 at the grazing incidence angle of 828, respectively. The sputtering rate of carbon at 828 was found to be slightly higher than that of Ta. (2) The total interface width Dz (84–16%) was described as a sum of the contribution at the Crich side of the interface, Dz (84–50%), and that at the Ta-rich side of the interface, Dz (50–16%). We ascribed the lesser contribution of the C-side to the backscattering effect and the greater contribution at the Ta-rich side to the preferential sputtering of Ta at the C/Ta interface (except at the ion incidence angle of 828). (3) AFM measurements revealed that the ripple structure formed in the carbon layers during ion sputtering of the stationary samples could be prevented by sample rotation during depth profiling. (4) Owing to the higher density of the Ta layer beneath the carbon layer a strong backscattering of primary electrons took place, which additionally increases the Auger electron emission of C (272 eV). The effect depends on the energy of the primary electron beam. (5) The theoretical calculation of the carbon concentration profile with the MRI model combined the effects of atomic mixing (M), surface rough-
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ness (R), escape depth of the Auger electrons (I) with the method proposed by Barkshire et al. to calculate the backscattering effect of primary electrons (1 + rB). The value of roughness estimated by the MRI model is in reasonable agreement with the AFM roughness amplitude and the experimental depth resolution agrees with value derived from the MRI simulation.
Acknowledgements ˇ eh and Dr. The authors would like to thank Dr. M. C ˇ M. Skarabot (Jozˇef Stefan Institute) for TEM investigation and for help at the AFM measurements. The work was supported by the Ministry of Higher Education, Science and Technology of Slovenia, Ljubljana (Project P2-82).
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