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Ion beam sputtering of Ag – Angular and energetic distributions of sputtered and scattered particles
Nuclear Instruments and Methods in Physics Research B 316 (2013) 198–204
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Ion beam sputtering of Ag – Angular and energetic distributions of sputtered and scattered particles René Feder ⇑, Carsten Bundesmann, Horst Neumann, Bernd Rauschenbach Leibniz-Institut für Oberflächenmodifizierung e. V., Permoserstr. 15, D-04318 Leipzig, Germany
a r t i c l e
i n f o
Article history: Received 1 August 2013 Received in revised form 6 September 2013 Available online 10 October 2013 Keywords: Ion beam sputtering ESMS Angular distribution Energetic distribution
a b s t r a c t Ion beam sputter deposition (IBD) provides intrinsic features which influence the properties of the growing film, because ion properties and geometrical process conditions generate different energy and spatial distribution of the sputtered and scattered particles. A vacuum deposition chamber is set up to measure the energy and spatial distribution of secondary particles produced by ion beam sputtering of different target materials under variation of geometrical parameters (incidence angle of primary ions and emission angle of secondary particles) and of primary ion beam parameters (ion species and energies). A representative set of Ag thin films is deposited arranged on a substrate holder equatorial to the Ag target in steps of 10° and characterized concerning their film thickness by profilometry to determine the angular distribution of the sputtered particles. The film thickness distributions show a tilted, cosine-like shape and a shifting of the maximum position depending on the primary particle energy and incidence angle of the primary ions. The energy distributions of sputtered and scattered ions and of sputtered neutrals are measured with an energy-selective mass spectrometer. The average energy of the sputtered ions increases with increasing emission angle and also increases with increasing incidence angle of the primary ions. In contrast, the average energy of the sputtered ions is nearly unaffected by the primary particle energy and particle species. The energy distribution of the scattered Ar ions reveals high energetic maxima which shift with increasing emission angle to higher energies. These maxima are not observed for Xe bombardment. The total energies of sputtered and scattered ions show significant differences between the two bombarding species. The maximum of the energy distribution of sputtered Ag neutrals is used to conclude on the surface binding energy of Ag (2.72 eV). All experimental data are compared with Monte Carlo simulations done with the well-known TRIM.SP. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ion beam sputter deposition (IBD), an important technique for high quality thin film production, is influenced by primary and secondary process parameters. Primarily the energy and mass of the primary particles, the mass of the target atoms and the process geometry lead to different thin film properties [1,2]. These differences are caused by different angular and/or energy distributions of the sputtered and scattered particles. Even IBD is used for decades, the full capabilities of this technique has not been investigated systemically or specifically used yet. Therefore the systematic analysis of the properties of the secondary, film forming particles described in this paper is reasonable for further process adaptation. ⇑ Corresponding author. Address: Leibniz-Institut für Oberflächenmodifizierung e. V., Permoserstr. 15, D-04318 Leipzig, Germany. Tel.: +49 (0) 341 235 4021; fax: +49 (0) 341 235 2313. E-mail address: [email protected] (R. Feder). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.09.007
The present report focuses on the properties of the sputtered and scattered particles for ion beam sputtering a Ag target with Ar and Xe energetic particles. The flux distributions of sputtered Ag particles, the energy distributions of sputtered Ag ions and the energy distributions of scattered Ar and Xe ions is measured under variation of the process geometry (incidence and emission angle) and the primary particle energy (500–1500 eV). These data are compared with simulation results, based on the commonly used Monte Carlo code TRIM.SP [3]. Additionally, the energy distribution of sputtered Ag neutrals is measured to estimate the surface binding energy of the Ag target material. The energy distributions of secondary ions sputtered from different metal targets were studied before by Kosyachkov [4], whereby higher primary energies of 4–6 keV but only one incidence angle were used. Oechsner [5] studied the energy distributions of sputtered neutrals from different polycrystalline metal targets under Ar ion bombardment only for normal incidence angle. Goehlich et al. [6,7] used LiF to study the energy distribution of sputtered Ti, W and Al, reporting of deviations from the linear
R. Feder et al. / Nuclear Instruments and Methods in Physics Research B 316 (2013) 198–204
cascade theory. There also exist theoretical studies predicting the anisotropic energy distribution of sputtered particles for oblique incidence [8]. None of these contributions studied the secondary particle properties under systematic variation of the primary particle energy, mass and incidence angle. Recently, first results for Ar ion bombardment, fixed ion incidence angle and fixed primary ion energy were carried out with the same setup [9].
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All experimental data are compared with simulations which are done using the Monte Carlo simulation code TRIM.SP [3]. The input parameters are taken from Eckstein [11]. The number of simulated primary particles is 108 at each single simulation case. For the energy spectrum of sputtered atoms from a collision cascade it is well established that the Thompson relationship [12] can be used, predicting the energy of a sputtered particle to be proportional to E/(E + U)3, where U is the surface binding energy of the target atoms.
2. Experimental conditions and simulation Fig. 1 shows a schematic geometry sketch of the vacuum deposition chamber. The set up provides the possibility to vary the primary ion incidence angle (a) and the polar emission angle (b). The in house developed broad beam ion source for generating the primary energetic ion beam is of RF type with a three grid multiaperture extraction system and an open diameter of 16 mm [10]. Ar and Xe are used as sputter gas. The mass flow is about 3.5 sccm for Ar and 1.1 sccm for Xe. The RF-power is set to 70 W for generation of both gas discharges. Under this conditions total ion beam currents of about 10 mA are produced. The ion source as well as the target is mounted on rotary tables on identical rotation axes. Additionally, a sample holder can be mounted in the chamber for thin film deposition. A more detailed view on the experimental set up is given elsewhere [9]. Multiple sets of Ag films are deposited by sputtering a Ag target by an Ar respectively a Xe ion beam under variation of the primary ion incidence angle (a = 0°, 30° and 60°) and the primary ion energy (Eion = 0.5 keV, 1.0 keV and 1.5 keV) to determine the particle flux distributions. The substrates (Si with a 50 nm Ti adhesive layer) are placed on the sample holder under polar emission angles between 40° and 90° in steps of 10°. Profilometry is used to measure film thicknesses upon measuring the step height between the film and the substrate after one deposition process set up. For step generation, a part of the substrate is covered during the deposition process. The particle flux can be calculated with help of the film thickness and the sputter time. An energy selective mass spectrometer (ESMS) is used to measure the energy distributions of sputtered and scattered particles. The ESMS operates in a mass range from 1 up to 512 amu and an energy up to 500 eV with a resolution of 1 amu and 0.5 eV, respectively. The spectrometer is mounted on a CF-flange and protrudes 200 mm inside the chamber. Caused by the dimension of the ion source and the ESMS, the minimal detectable emission angle (b) is 60°, 30° or 0° for primary ion incidence angles of 0°, 30° or 60° respectively. The energy distributions of sputtered (Ag) and scattered (Ar, Xe) ions are measured at emission angles in steps of 10° for primary ion energies of 1 keV and 1.5 keV and incidence angles of 0°, 30° and 60° in respect to the target normal.
Fig. 1. Schematic sketch of the ion beam sputter setup.
3. Results and discussion 3.1. Particle flux distribution The collision cascade theory predicts an isotropic distribution of recoil atoms in the target if it is bombarded by ions at normal incidence. In this case a cosine-type angular distribution of sputtered particles is predicted [13]. Additionally, simulations and experimental results indicate an energy dependency of the angular distribution [14–16], because for low primary ion energies the collision cascade is not completely developed. Consequently, the angular distribution of recoil atoms is not isotropic, resulting in a changed angular distribution (heart-, under- or over-cosine types) [17]. In Fig. 2, the experimental and simulated particle flux distributions of sputtered Ag particles are shown under variation of different process parameters. All particle flux distributions show the expected, tilted, cosine-like shape with a maximum depending on the primary particle energy and the incidence angle. Fig. 2(a) demonstrates the Ag particle flux U by sputtering a Ag target with Ar ions for different times tspu (15 min, 30 min, 60 min). There is no difference in the flux for the different sputter times. Fig. 2(b) shows the flux distribution of Ag particles for sputtering with Xe. The maximum of U is about 30% higher than for sputtering with Ar, as expected from the higher total sputter yield and the higher energy transfer due to the mass ratio of primary particles (mion) and target atoms (mtarget) [18,19]. In Fig. 2(c) and (d) the influence of the primary ion incidence angle a on the particle flux distribution for both inert gases is outlined. Similarly to Fig. 2(b), the particle flux is always higher for sputtering with Xe. The particle flux also increases by changing the incidence angle from 0° to 30°, because the total sputter yield increases. For the largest investigated incidence angle (a = 60°) the particle flux is significantly lower. In Fig. 2(e) and (f) the influence of the primary ion energy Eion is shown for an incidence angle of a = 30° for sputtering with Ar and Xe, respectively. The Ag particle flux increases with increasing primary ion energy and is higher for sputtering with Xe ions than for sputtering with Ar ions. The maximum of U shifts to higher angles up to b = 30° for the lowest investigated primary ion energies and decreases with increasing primary ion energy. The particle fluxes calculated from simulation results are in good agreement with the experimental values in most cases. The simulation results are about two times higher for an incidence angle of 60° (c, d). For Eion = 0.5 keV (e, f), the simulated particle flux is much higher than the experimental U for sputtering with Ar ions as well as for sputtering with Xe ions. For Eion = 1.5 keV experimental values are higher than the simulated distributions. The deviations between the simulated and the experimental values, as well as the decreasing particle flux for a = 60°, are not an effect of the applied particle flux at the effective area of the sputter target. The influence of the beam diameter and divergence and the angle between target and beam are taken into account. Other possible origins of the deviations are given by the unknown influence of the target’s surface roughness and the mixing of different incident angles due to the beam divergence. Measurements on
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Fig. 2. Experimental (symbols) and simulated (lines) Ag particle flux distributions under variation of process parameters (time (a), ion species (a, c, e vs. b, d, f), incidence angle (c, d), primary particle energy (e, f)).
a Ge target show a similar behavior. Further measurements are needed to clarify these influences. 3.2. Sputtered Ag ions Fig. 3 demonstrates the energy distributions of Ag ions sputtered from a Ag target with Ar (a) and Xe (b) ions with a primary ion energy of Eion = 1 keV under an incidence angle of a = 30° at selected emission angles b. Due to the mass ratio between primary ion and target atom, the maximum energy of sputtered Ag ions is higher for sputtering with Xe than for sputtering with Ar. The energy of the Ag ions shows the expected Eb behavior according
to Thompson formula for emission angles b 6 70° for both primary ion species. For b = 80° the shape of the curves changes due to the fact that the production of direct recoils is possible. These direct recoiled particles have higher energies than the particles sputtered in a collision cascade and can only occur if a + b > 90° [9,20]. In Fig. 4, the average energy hEi of the sputtered Ag ions is shown as a function of the emission angle for different incidence angles a (0°, 30°, 60°) and primary particle energies Eion (1.0 keV, 1.5 keV). The values are calculated from experimental energy distributions, e.g. in Fig. 3. The average energy hEi of the sputtered Ag ions increases with increasing emission angle b for both primary ion species. Also, hEi increases slightly with increasing the
Fig. 3. Experimental energy distributions of Ag ions sputtered from a Ag target with Ar (a) and Xe (b) ions at selected emission angles b.
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Fig. 4. Average energy of Ag ions sputtered by Ar (a) and Xe (b) ions from a Ag target as a function of the emission angle b, under variation of incidence angle a and primary particle energy Eion. Data are calculated from energy distributions as shown in Fig. 3.
primary particle energy. For a combination of large incidence and large emission angles, e.g. a = 60° and b = 80°, the maximum energy of the sputtered Ag particles exceeds the energy range of the ESMS (500 eV). Therefore, the average energy cannot be calculated for these data points from measured data. In Fig. 5 the average energies hEi of sputtered Ag particles as a function of the emission angle calculated from the simulated energy distributions (not shown here) for sputtering with Ar (a) and Xe (b) for different primary ion energies Eion (0.5 keV, 1.0 keV, 1.5 keV) and incidence angles a (0°, 30°, 60°) are outlined. All curves show an increase with increasing emission angle b up to a maximum between 60° and 80°, followed by a rapid decrease till an emission angle of 90°. The average particle energy increases significantly with increasing incidence angle and slightly with the primary ion energy for both primary ion species. The shape of the curves is similar for sputtering with Ar and Xe, but for Xe the average particle energy is higher. The difference is small for low primary ion energies and small incidence angles, but increases with increasing a, b and Eion. The simulated curves reproduce the shape of the experimental curves well and the absolute values of the average particle energies are comparable. 3.3. Scattered Ar and Xe ions The energy distributions of Ar and Xe ions backscattered from the Ag target for different emission angles (Eion = 1 keV; a = 30°)
are demonstrated in Fig. 6. The maximum energy of a backscattered particle increases with increasing emission angle for both ion species. In the energy distributions of scattered Ar ions additional well defined high energetic maxima occur, which shift with increasing emission angle to higher energies. These maxima originate from a direct scattering process near or at the target surface and can be calculated as described elsewhere [9]. In contrast, there are no high energetic maxima in the energy distributions of scattered Xe ions. This is explained by the condition for detectable directly scattered particles: the mass ratio mtarget/mion must be larger than the sinus of the scattering angle [9]. In the geometry chosen here, the scattering angle is c = 180°ab. Therefore the maximum scattering angle for Xe at a Ag atom is about 55°. Thus, directly scattered Xe ions can only be detected for a = 60° and b = 70° and 80° for the parameters used in this setup. The average energy of the scattered particles could not be calculated from experimental data, because for most of the spectra the maximum energy of the scattered particles exceeds the energy range of the ESMS. In Fig. 7 the average energies hEi of scattered primary particles Ar (a) and Xe (b) reflected from a Ag target are outlined as a function of the emission angle calculated from simulation results for different combinations of Eion and a. For both primary ions and all combinations of Eion and a the average energy of the scattered particles increases with increasing emission angle. hEi also increases with increasing primary ion energy and primary ion
Fig. 5. Average energies of sputtered Ag particles as a function of the emission angle b calculated from simulation results for sputtering with Ar (a) and Xe (b) for different combinations of primary particle energy Eion and incidence angle a.
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Fig. 6. Experimental energy distributions of backscattered primary Ar (a) and Xe (b) ions reflected from a Ag target at selected emission angles.
Fig. 7. Average energies of scattered primary particles Ar (a) and Xe (b) reflected from a Ag target as a function of the emission angle b calculated from simulation results for different combinations of primary particle energy Eion and incidence angle a.
incidence angle for both species. The average energy of scattered Ar particles is significantly higher than for Xe in every case. This could be an effect of high energetic directly scattered particles. There is a change of the shape in one of the curves for Xe (a = 60°; Eion = 1 keV) for b > 65°. For this combination of incidence and emission angle, the condition for direct scattered particles described above is fulfilled. These high energetic particles cause an additional energy input and increase the average particle energy during the growing process. 3.4. Total energies of sputtered and scattered particles The total energy Etot is the integrated energy distribution of the Ag particles sputtered from the Ag target calculated from simulation results, normalized by the number of primary particles. Fig. 8 shows the normalized total energy as a function of the emission angle for sputtering with Ar (a) and Xe (b) under variation of primary ion energy and incidence angle. All curves are of similar shape for both ion species. Like the average energy of the sputtered particles, the normalized total energy increases significantly with increasing incidence angle and slightly with the primary ion energy. Additionally, the normalized total energies are higher for sputtering with Xe than for sputtering with Ar, especially for the highest investigated primary ion energies and high incidence angles. Fig. 9 shows the normalized total energies of the scattered Ar (a) and Xe (b) particles as a function of the emission angle calculated from the simulation results for the same combinations of primary
ion energy and incidence angle as in Fig. 8. The normalized total energy increases with increasing incidence angle and slightly with increasing primary ion energy. For sputtering with Ar, the normalized total energies of the scattered particles are comparable with the normalized total energies of the sputtered Ag particles. For sputtering with Xe, a large influence of the primary ion incidence angle is obvious. For a = 0°, the normalized total energies of the sputtered particles are too small and therefore out of the scale. For a = 30° and the three different primary ion energies the normalized total energies of the scattered Xe ions are about one order of magnitude lower than the normalized total energies of the scattered Ar ion energies, and also more than 10 times lower than the normalized total energies of the sputtered Ag particles. For a = 60°, the curve gets a different shape, because especially for high emission angles additional energy contributions from directly scattered particles occurs. In this case, the energy input from the scattered particles gets higher than that from the sputtered particles. 3.5. Sputtered Ag neutrals Fig. 10 shows an example for the energy distributions of sputtered neutral Ag atoms from a Ag target in relation to the energy distribution of water molecules from the residual gas inside the vacuum chamber. The maximum in the energy distribution of the water molecules is needed for calibration, because the internal ion source of the ESMS operates at a voltage of about 100 V. Therefore, the maximum in the energy distribution of the water molecules at thermal energy occurs at about 100 eV. As known from the Thomp-
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Fig. 8. Total energies of sputtered Ag particles divided by the number of primary particles as a function of the emission angle b calculated from simulation results for different combinations of ion species (Ar (a), Xe (b)), primary particle energy Eion and incidence angle a.
Fig. 9. Total energies of scattered primary particles divided by the number of primary particles as a function of the emission angle b calculated from simulation results for different combinations of ion species (Ar (a), Xe (b)), primary particle energy Eion and incidence angle a.
angles and energies, which do not affect the energetic position of the maximum of the energy distribution the sputtered neutrals. From these measurements the surface binding energy of Ag is calculated to be 2.72 ± 0.30 eV, what is in good agreement to the theoretical values from [21] as well as to the heat of sublimation of Ag, which is often used for simulations.
4. Summary
Fig. 10. Energy distributions of sputtered neutral Ag atoms from the Ag target in relation to the energy distribution of water molecules from the residual gas. Note the different scales for the counts.
son formula [12], the maximum in the energy distribution of the sputtered Ag neutrals is located at the half of the surface binding energy, additionally shifted by the voltage of the internal ionizer. So, the difference in the position of the two maxima is equal to the half of the surface binding energy of the target material. Several measurements are carried out with different primary ion incidence
The described experimental setup represents a proper tool to study the properties of sputtered and scattered particles upon ion beam sputtering. It could be demonstrated exemplarily for sputtering Ag with Ar and Xe ions under systematic variation of the primary and secondary process parameters primary particle energy, incident angle, emission angle and ion species. The particle flux distributions for sputtered Ag particles are in a good agreement with the values calculated from the TRIM.SP simulations. The energy distributions of sputtered Ag particles show a strong dependence on the emission angle. The values of the average energy of the sputtered particles increase with increasing emission angle, increasing primary ion incidence angle and slightly with increasing primary ion energy. The average energies of the sputtered particles calculated from simulation results show the same behavior and are of comparable value. The experimental energy distributions of scattered Ar and Xe ions differ significantly. In the energy distributions of the Ar ions, additional maxima from di-
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rect scattering processes occur. For Xe, these maxima are missing. The average energy of the Ar particles scattered from the Ag target are much higher than those of the Xe particles as a result of the direct scattered particles. Simulations and experimental data feature a considerable difference in the ratio of the normalized total energies of sputtered and scattered particles between sputtering with Ar or Xe, originating in the different energy distributions of backscattered particles. The maximum of the energy distribution of sputtered Ag neutrals is used to calculate its surface binding energy to be (2.72 ± 0.3) eV. In the future, the results presented in this paper will be related to the thin film properties like electrical resistivity and optical properties. Additionally, the behavior of other target materials, such as the semiconductor material Ge, will be investigated in the future in the same way, in order to address systematic similarities or differences with the metal Ag.
Acknowledgements The authors want to thank the D.F.G. for financial support (Project BU2625/1-1), F. Scholze, I. Herold, P. Hertel, M. Müller, R. Woyciechowski and the IOM Workshop for technical support.
References [1] C. Bundesmann, I.-M. Eichentopf, S. Mändl, H. Neumann, Thin Solid Films 516 (2008) 8604. [2] C.A. Davis, Thin Solid Films 226 (1993) 30. [3] J.P. Biersack, W. Eckstein, Appl. Phys. A: Mater. Sci. Process. 34 (1984) 73. [4] A.A. Kosyachkov, Vacuum 42 (1991) 143. [5] H. Oechsner, Phys. Rev. Lett. 24 (1970) 583. [6] A. Goehlich, N. Niemöller, H.F. Döbele, Phys. Rev. B 62 (2000) 9349. [7] A. Goehlich, D. Gillmann, H.F. Döbele, Nucl. Instr. Meth. B 179 (2001) 351. [8] M. Stepanova, S.K. Dew, Nucl. Instr. Meth. B 215 (2004) 357. [9] R. Feder, F. Frost, H. Neumann, C. Bundesmann, B. Rauschenbach, Nucl. Instr. Meth. B. 317 (2013) 136. [10] M. Zeuner, F. Scholze, B. Dathe, H. Neumann, Surf. Coat. Technol. 142–144 (2001) 39. [11] W. Eckstein, IPP Report 9/132, 2002, pp. 193–195. [12] M.W. Thompson, Philos. Mag. 18 (1968) 377. [13] R. Behrisch, Sputtering by Particle Bombardment, vol. 1, Springer, Berlin, 1981. [14] G.K. Wehner, D. Rosenberg, J. Appl. Phys. 31 (1960) 177. [15] Y. Yamamura, Nucl. Instr. Meth. 194 (1982) 515. [16] J. Roth, J. Bohdansky, W. Eckstein, Nucl. Instr. Meth. 218 (1983) 751. [17] H.H. Andersen, B. Stenum, T. Sorensen, H.J. Whitlow, Nucl. Instr. Meth. B 6 (1985) 459. [18] H. Oechsner, Z. Physik. 261 (1973) 37. [19] M. Tartz, T. Heyn, C. Bundesmann, C. Zimmermann, H. Neumann, Eur. Phys. J. D 61 (2011) 587. [20] W. Eckstein, Nucl. Instr. Meth. B 27 (1987) 78. [21] Y. Kudriavtsev, A. Villegas, A. Godines, R. Asomoza, Appl. Surf. Sci. 239 (2005) 273.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Ion beam sputtering of Ag – Angular and energetic distributions of sputtered and scattered particles René Feder ⇑, Carsten Bundesmann, Horst Neumann, Bernd Rauschenbach Leibniz-Institut für Oberflächenmodifizierung e. V., Permoserstr. 15, D-04318 Leipzig, Germany
a r t i c l e
i n f o
Article history: Received 1 August 2013 Received in revised form 6 September 2013 Available online 10 October 2013 Keywords: Ion beam sputtering ESMS Angular distribution Energetic distribution
a b s t r a c t Ion beam sputter deposition (IBD) provides intrinsic features which influence the properties of the growing film, because ion properties and geometrical process conditions generate different energy and spatial distribution of the sputtered and scattered particles. A vacuum deposition chamber is set up to measure the energy and spatial distribution of secondary particles produced by ion beam sputtering of different target materials under variation of geometrical parameters (incidence angle of primary ions and emission angle of secondary particles) and of primary ion beam parameters (ion species and energies). A representative set of Ag thin films is deposited arranged on a substrate holder equatorial to the Ag target in steps of 10° and characterized concerning their film thickness by profilometry to determine the angular distribution of the sputtered particles. The film thickness distributions show a tilted, cosine-like shape and a shifting of the maximum position depending on the primary particle energy and incidence angle of the primary ions. The energy distributions of sputtered and scattered ions and of sputtered neutrals are measured with an energy-selective mass spectrometer. The average energy of the sputtered ions increases with increasing emission angle and also increases with increasing incidence angle of the primary ions. In contrast, the average energy of the sputtered ions is nearly unaffected by the primary particle energy and particle species. The energy distribution of the scattered Ar ions reveals high energetic maxima which shift with increasing emission angle to higher energies. These maxima are not observed for Xe bombardment. The total energies of sputtered and scattered ions show significant differences between the two bombarding species. The maximum of the energy distribution of sputtered Ag neutrals is used to conclude on the surface binding energy of Ag (2.72 eV). All experimental data are compared with Monte Carlo simulations done with the well-known TRIM.SP. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ion beam sputter deposition (IBD), an important technique for high quality thin film production, is influenced by primary and secondary process parameters. Primarily the energy and mass of the primary particles, the mass of the target atoms and the process geometry lead to different thin film properties [1,2]. These differences are caused by different angular and/or energy distributions of the sputtered and scattered particles. Even IBD is used for decades, the full capabilities of this technique has not been investigated systemically or specifically used yet. Therefore the systematic analysis of the properties of the secondary, film forming particles described in this paper is reasonable for further process adaptation. ⇑ Corresponding author. Address: Leibniz-Institut für Oberflächenmodifizierung e. V., Permoserstr. 15, D-04318 Leipzig, Germany. Tel.: +49 (0) 341 235 4021; fax: +49 (0) 341 235 2313. E-mail address: [email protected] (R. Feder). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.09.007
The present report focuses on the properties of the sputtered and scattered particles for ion beam sputtering a Ag target with Ar and Xe energetic particles. The flux distributions of sputtered Ag particles, the energy distributions of sputtered Ag ions and the energy distributions of scattered Ar and Xe ions is measured under variation of the process geometry (incidence and emission angle) and the primary particle energy (500–1500 eV). These data are compared with simulation results, based on the commonly used Monte Carlo code TRIM.SP [3]. Additionally, the energy distribution of sputtered Ag neutrals is measured to estimate the surface binding energy of the Ag target material. The energy distributions of secondary ions sputtered from different metal targets were studied before by Kosyachkov [4], whereby higher primary energies of 4–6 keV but only one incidence angle were used. Oechsner [5] studied the energy distributions of sputtered neutrals from different polycrystalline metal targets under Ar ion bombardment only for normal incidence angle. Goehlich et al. [6,7] used LiF to study the energy distribution of sputtered Ti, W and Al, reporting of deviations from the linear
R. Feder et al. / Nuclear Instruments and Methods in Physics Research B 316 (2013) 198–204
cascade theory. There also exist theoretical studies predicting the anisotropic energy distribution of sputtered particles for oblique incidence [8]. None of these contributions studied the secondary particle properties under systematic variation of the primary particle energy, mass and incidence angle. Recently, first results for Ar ion bombardment, fixed ion incidence angle and fixed primary ion energy were carried out with the same setup [9].
199
All experimental data are compared with simulations which are done using the Monte Carlo simulation code TRIM.SP [3]. The input parameters are taken from Eckstein [11]. The number of simulated primary particles is 108 at each single simulation case. For the energy spectrum of sputtered atoms from a collision cascade it is well established that the Thompson relationship [12] can be used, predicting the energy of a sputtered particle to be proportional to E/(E + U)3, where U is the surface binding energy of the target atoms.
2. Experimental conditions and simulation Fig. 1 shows a schematic geometry sketch of the vacuum deposition chamber. The set up provides the possibility to vary the primary ion incidence angle (a) and the polar emission angle (b). The in house developed broad beam ion source for generating the primary energetic ion beam is of RF type with a three grid multiaperture extraction system and an open diameter of 16 mm [10]. Ar and Xe are used as sputter gas. The mass flow is about 3.5 sccm for Ar and 1.1 sccm for Xe. The RF-power is set to 70 W for generation of both gas discharges. Under this conditions total ion beam currents of about 10 mA are produced. The ion source as well as the target is mounted on rotary tables on identical rotation axes. Additionally, a sample holder can be mounted in the chamber for thin film deposition. A more detailed view on the experimental set up is given elsewhere [9]. Multiple sets of Ag films are deposited by sputtering a Ag target by an Ar respectively a Xe ion beam under variation of the primary ion incidence angle (a = 0°, 30° and 60°) and the primary ion energy (Eion = 0.5 keV, 1.0 keV and 1.5 keV) to determine the particle flux distributions. The substrates (Si with a 50 nm Ti adhesive layer) are placed on the sample holder under polar emission angles between 40° and 90° in steps of 10°. Profilometry is used to measure film thicknesses upon measuring the step height between the film and the substrate after one deposition process set up. For step generation, a part of the substrate is covered during the deposition process. The particle flux can be calculated with help of the film thickness and the sputter time. An energy selective mass spectrometer (ESMS) is used to measure the energy distributions of sputtered and scattered particles. The ESMS operates in a mass range from 1 up to 512 amu and an energy up to 500 eV with a resolution of 1 amu and 0.5 eV, respectively. The spectrometer is mounted on a CF-flange and protrudes 200 mm inside the chamber. Caused by the dimension of the ion source and the ESMS, the minimal detectable emission angle (b) is 60°, 30° or 0° for primary ion incidence angles of 0°, 30° or 60° respectively. The energy distributions of sputtered (Ag) and scattered (Ar, Xe) ions are measured at emission angles in steps of 10° for primary ion energies of 1 keV and 1.5 keV and incidence angles of 0°, 30° and 60° in respect to the target normal.
Fig. 1. Schematic sketch of the ion beam sputter setup.
3. Results and discussion 3.1. Particle flux distribution The collision cascade theory predicts an isotropic distribution of recoil atoms in the target if it is bombarded by ions at normal incidence. In this case a cosine-type angular distribution of sputtered particles is predicted [13]. Additionally, simulations and experimental results indicate an energy dependency of the angular distribution [14–16], because for low primary ion energies the collision cascade is not completely developed. Consequently, the angular distribution of recoil atoms is not isotropic, resulting in a changed angular distribution (heart-, under- or over-cosine types) [17]. In Fig. 2, the experimental and simulated particle flux distributions of sputtered Ag particles are shown under variation of different process parameters. All particle flux distributions show the expected, tilted, cosine-like shape with a maximum depending on the primary particle energy and the incidence angle. Fig. 2(a) demonstrates the Ag particle flux U by sputtering a Ag target with Ar ions for different times tspu (15 min, 30 min, 60 min). There is no difference in the flux for the different sputter times. Fig. 2(b) shows the flux distribution of Ag particles for sputtering with Xe. The maximum of U is about 30% higher than for sputtering with Ar, as expected from the higher total sputter yield and the higher energy transfer due to the mass ratio of primary particles (mion) and target atoms (mtarget) [18,19]. In Fig. 2(c) and (d) the influence of the primary ion incidence angle a on the particle flux distribution for both inert gases is outlined. Similarly to Fig. 2(b), the particle flux is always higher for sputtering with Xe. The particle flux also increases by changing the incidence angle from 0° to 30°, because the total sputter yield increases. For the largest investigated incidence angle (a = 60°) the particle flux is significantly lower. In Fig. 2(e) and (f) the influence of the primary ion energy Eion is shown for an incidence angle of a = 30° for sputtering with Ar and Xe, respectively. The Ag particle flux increases with increasing primary ion energy and is higher for sputtering with Xe ions than for sputtering with Ar ions. The maximum of U shifts to higher angles up to b = 30° for the lowest investigated primary ion energies and decreases with increasing primary ion energy. The particle fluxes calculated from simulation results are in good agreement with the experimental values in most cases. The simulation results are about two times higher for an incidence angle of 60° (c, d). For Eion = 0.5 keV (e, f), the simulated particle flux is much higher than the experimental U for sputtering with Ar ions as well as for sputtering with Xe ions. For Eion = 1.5 keV experimental values are higher than the simulated distributions. The deviations between the simulated and the experimental values, as well as the decreasing particle flux for a = 60°, are not an effect of the applied particle flux at the effective area of the sputter target. The influence of the beam diameter and divergence and the angle between target and beam are taken into account. Other possible origins of the deviations are given by the unknown influence of the target’s surface roughness and the mixing of different incident angles due to the beam divergence. Measurements on
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Fig. 2. Experimental (symbols) and simulated (lines) Ag particle flux distributions under variation of process parameters (time (a), ion species (a, c, e vs. b, d, f), incidence angle (c, d), primary particle energy (e, f)).
a Ge target show a similar behavior. Further measurements are needed to clarify these influences. 3.2. Sputtered Ag ions Fig. 3 demonstrates the energy distributions of Ag ions sputtered from a Ag target with Ar (a) and Xe (b) ions with a primary ion energy of Eion = 1 keV under an incidence angle of a = 30° at selected emission angles b. Due to the mass ratio between primary ion and target atom, the maximum energy of sputtered Ag ions is higher for sputtering with Xe than for sputtering with Ar. The energy of the Ag ions shows the expected Eb behavior according
to Thompson formula for emission angles b 6 70° for both primary ion species. For b = 80° the shape of the curves changes due to the fact that the production of direct recoils is possible. These direct recoiled particles have higher energies than the particles sputtered in a collision cascade and can only occur if a + b > 90° [9,20]. In Fig. 4, the average energy hEi of the sputtered Ag ions is shown as a function of the emission angle for different incidence angles a (0°, 30°, 60°) and primary particle energies Eion (1.0 keV, 1.5 keV). The values are calculated from experimental energy distributions, e.g. in Fig. 3. The average energy hEi of the sputtered Ag ions increases with increasing emission angle b for both primary ion species. Also, hEi increases slightly with increasing the
Fig. 3. Experimental energy distributions of Ag ions sputtered from a Ag target with Ar (a) and Xe (b) ions at selected emission angles b.
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Fig. 4. Average energy of Ag ions sputtered by Ar (a) and Xe (b) ions from a Ag target as a function of the emission angle b, under variation of incidence angle a and primary particle energy Eion. Data are calculated from energy distributions as shown in Fig. 3.
primary particle energy. For a combination of large incidence and large emission angles, e.g. a = 60° and b = 80°, the maximum energy of the sputtered Ag particles exceeds the energy range of the ESMS (500 eV). Therefore, the average energy cannot be calculated for these data points from measured data. In Fig. 5 the average energies hEi of sputtered Ag particles as a function of the emission angle calculated from the simulated energy distributions (not shown here) for sputtering with Ar (a) and Xe (b) for different primary ion energies Eion (0.5 keV, 1.0 keV, 1.5 keV) and incidence angles a (0°, 30°, 60°) are outlined. All curves show an increase with increasing emission angle b up to a maximum between 60° and 80°, followed by a rapid decrease till an emission angle of 90°. The average particle energy increases significantly with increasing incidence angle and slightly with the primary ion energy for both primary ion species. The shape of the curves is similar for sputtering with Ar and Xe, but for Xe the average particle energy is higher. The difference is small for low primary ion energies and small incidence angles, but increases with increasing a, b and Eion. The simulated curves reproduce the shape of the experimental curves well and the absolute values of the average particle energies are comparable. 3.3. Scattered Ar and Xe ions The energy distributions of Ar and Xe ions backscattered from the Ag target for different emission angles (Eion = 1 keV; a = 30°)
are demonstrated in Fig. 6. The maximum energy of a backscattered particle increases with increasing emission angle for both ion species. In the energy distributions of scattered Ar ions additional well defined high energetic maxima occur, which shift with increasing emission angle to higher energies. These maxima originate from a direct scattering process near or at the target surface and can be calculated as described elsewhere [9]. In contrast, there are no high energetic maxima in the energy distributions of scattered Xe ions. This is explained by the condition for detectable directly scattered particles: the mass ratio mtarget/mion must be larger than the sinus of the scattering angle [9]. In the geometry chosen here, the scattering angle is c = 180°ab. Therefore the maximum scattering angle for Xe at a Ag atom is about 55°. Thus, directly scattered Xe ions can only be detected for a = 60° and b = 70° and 80° for the parameters used in this setup. The average energy of the scattered particles could not be calculated from experimental data, because for most of the spectra the maximum energy of the scattered particles exceeds the energy range of the ESMS. In Fig. 7 the average energies hEi of scattered primary particles Ar (a) and Xe (b) reflected from a Ag target are outlined as a function of the emission angle calculated from simulation results for different combinations of Eion and a. For both primary ions and all combinations of Eion and a the average energy of the scattered particles increases with increasing emission angle. hEi also increases with increasing primary ion energy and primary ion
Fig. 5. Average energies of sputtered Ag particles as a function of the emission angle b calculated from simulation results for sputtering with Ar (a) and Xe (b) for different combinations of primary particle energy Eion and incidence angle a.
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Fig. 6. Experimental energy distributions of backscattered primary Ar (a) and Xe (b) ions reflected from a Ag target at selected emission angles.
Fig. 7. Average energies of scattered primary particles Ar (a) and Xe (b) reflected from a Ag target as a function of the emission angle b calculated from simulation results for different combinations of primary particle energy Eion and incidence angle a.
incidence angle for both species. The average energy of scattered Ar particles is significantly higher than for Xe in every case. This could be an effect of high energetic directly scattered particles. There is a change of the shape in one of the curves for Xe (a = 60°; Eion = 1 keV) for b > 65°. For this combination of incidence and emission angle, the condition for direct scattered particles described above is fulfilled. These high energetic particles cause an additional energy input and increase the average particle energy during the growing process. 3.4. Total energies of sputtered and scattered particles The total energy Etot is the integrated energy distribution of the Ag particles sputtered from the Ag target calculated from simulation results, normalized by the number of primary particles. Fig. 8 shows the normalized total energy as a function of the emission angle for sputtering with Ar (a) and Xe (b) under variation of primary ion energy and incidence angle. All curves are of similar shape for both ion species. Like the average energy of the sputtered particles, the normalized total energy increases significantly with increasing incidence angle and slightly with the primary ion energy. Additionally, the normalized total energies are higher for sputtering with Xe than for sputtering with Ar, especially for the highest investigated primary ion energies and high incidence angles. Fig. 9 shows the normalized total energies of the scattered Ar (a) and Xe (b) particles as a function of the emission angle calculated from the simulation results for the same combinations of primary
ion energy and incidence angle as in Fig. 8. The normalized total energy increases with increasing incidence angle and slightly with increasing primary ion energy. For sputtering with Ar, the normalized total energies of the scattered particles are comparable with the normalized total energies of the sputtered Ag particles. For sputtering with Xe, a large influence of the primary ion incidence angle is obvious. For a = 0°, the normalized total energies of the sputtered particles are too small and therefore out of the scale. For a = 30° and the three different primary ion energies the normalized total energies of the scattered Xe ions are about one order of magnitude lower than the normalized total energies of the scattered Ar ion energies, and also more than 10 times lower than the normalized total energies of the sputtered Ag particles. For a = 60°, the curve gets a different shape, because especially for high emission angles additional energy contributions from directly scattered particles occurs. In this case, the energy input from the scattered particles gets higher than that from the sputtered particles. 3.5. Sputtered Ag neutrals Fig. 10 shows an example for the energy distributions of sputtered neutral Ag atoms from a Ag target in relation to the energy distribution of water molecules from the residual gas inside the vacuum chamber. The maximum in the energy distribution of the water molecules is needed for calibration, because the internal ion source of the ESMS operates at a voltage of about 100 V. Therefore, the maximum in the energy distribution of the water molecules at thermal energy occurs at about 100 eV. As known from the Thomp-
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Fig. 8. Total energies of sputtered Ag particles divided by the number of primary particles as a function of the emission angle b calculated from simulation results for different combinations of ion species (Ar (a), Xe (b)), primary particle energy Eion and incidence angle a.
Fig. 9. Total energies of scattered primary particles divided by the number of primary particles as a function of the emission angle b calculated from simulation results for different combinations of ion species (Ar (a), Xe (b)), primary particle energy Eion and incidence angle a.
angles and energies, which do not affect the energetic position of the maximum of the energy distribution the sputtered neutrals. From these measurements the surface binding energy of Ag is calculated to be 2.72 ± 0.30 eV, what is in good agreement to the theoretical values from [21] as well as to the heat of sublimation of Ag, which is often used for simulations.
4. Summary
Fig. 10. Energy distributions of sputtered neutral Ag atoms from the Ag target in relation to the energy distribution of water molecules from the residual gas. Note the different scales for the counts.
son formula [12], the maximum in the energy distribution of the sputtered Ag neutrals is located at the half of the surface binding energy, additionally shifted by the voltage of the internal ionizer. So, the difference in the position of the two maxima is equal to the half of the surface binding energy of the target material. Several measurements are carried out with different primary ion incidence
The described experimental setup represents a proper tool to study the properties of sputtered and scattered particles upon ion beam sputtering. It could be demonstrated exemplarily for sputtering Ag with Ar and Xe ions under systematic variation of the primary and secondary process parameters primary particle energy, incident angle, emission angle and ion species. The particle flux distributions for sputtered Ag particles are in a good agreement with the values calculated from the TRIM.SP simulations. The energy distributions of sputtered Ag particles show a strong dependence on the emission angle. The values of the average energy of the sputtered particles increase with increasing emission angle, increasing primary ion incidence angle and slightly with increasing primary ion energy. The average energies of the sputtered particles calculated from simulation results show the same behavior and are of comparable value. The experimental energy distributions of scattered Ar and Xe ions differ significantly. In the energy distributions of the Ar ions, additional maxima from di-
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rect scattering processes occur. For Xe, these maxima are missing. The average energy of the Ar particles scattered from the Ag target are much higher than those of the Xe particles as a result of the direct scattered particles. Simulations and experimental data feature a considerable difference in the ratio of the normalized total energies of sputtered and scattered particles between sputtering with Ar or Xe, originating in the different energy distributions of backscattered particles. The maximum of the energy distribution of sputtered Ag neutrals is used to calculate its surface binding energy to be (2.72 ± 0.3) eV. In the future, the results presented in this paper will be related to the thin film properties like electrical resistivity and optical properties. Additionally, the behavior of other target materials, such as the semiconductor material Ge, will be investigated in the future in the same way, in order to address systematic similarities or differences with the metal Ag.
Acknowledgements The authors want to thank the D.F.G. for financial support (Project BU2625/1-1), F. Scholze, I. Herold, P. Hertel, M. Müller, R. Woyciechowski and the IOM Workshop for technical support.
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