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Depth characterization of nm-layers by low energy ion scattering
Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 578–583 www.elsevier.com/locate/nimb
Depth characterization of nm-layers by low energy ion scattering M. Draxler a,*, S.N. Markin a, R. Beikler b, E. Taglauer b, F. Kastner c, M. Bergsmann c, P. Bauer a a
Institut f€ ur Experimentalphysik, Abteilung Atom- und Oberflaechenphysik, Johannes Kepler Universit€at Linz, Altenbergerstrasse 69, A-4040 Linz, Austria b Max-Planck-Institut f€ur Plasmaphysik, EURATOM Association, D-85748 Garching bei M€unchen, Germany c Hueck Folien GmbH, A-4342 Baumgartenberg, Austria
Abstract Layers with a thickness in the nm range are of growing importance in science and technology. We describe, how the thickness and growth modes of such nm-layers can be characterized by time-of-flight low energy ion scattering (TOFLEIS), when all backscattered projectiles are taken into account. As examples, Cu evaporated on an Al2 O3 and on a PET target with a nominal film thickness of about 1nm are analyzed by means of TOF-LEIS using H and He ions in the keV range as projectiles. 2004 Elsevier B.V. All rights reserved. PACS: 34.50.Bw; 68.55.Jk Keywords: TOF-LEIS; Nanometer layers; Growth modes; Copper; Alumina; PET
1. Introduction Thin films play a dominant role in modern science and technology. They are applicable in very different fields and serve, for example, as coatings, chemical sensors or catalysts. Nowadays, as the miniaturization process of technical devices continues, functional layers with a certain thickness in the nanometer range are essential for many applications. Thin metal films on insulating or
*
Corresponding author. Tel.: +43-732-2468-8520; fax: +43732-2468-8509. E-mail address: [email protected] (M. Draxler).
organic substrates are of particular interest in this field. In this contribution, we focus on the potential of low energy ion scattering (LEIS) on the analysis of nanometer layers, e.g. of ultrathin Cu layers on PET and on Al2 O3 . An overview on the metal/PET adhesion is given, for example in [1]. Ref. [2] presents an XPS study of the, in our case highly relevant, Cu/PET interface. It is stated in these articles that if the chemical bonding is weak (in the case of Cu/PET), the metal is free to diffuse into the polymer layer and form clusters. Another topic of high technological interest is the adsorption of metals on oxides. A summary on this subject is given, for instance, in [3]. An AFM growth study of Cu and Pd on a-Al2 O3 is given in [4]. For the
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.123
M. Draxler et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 578–583
2. Experiment and simulation The experiments were carried out at the experimental setup ACOLISSA, which is described in detail in [14]. In brief, ACOLISSA is a time-offlight low energy ion scattering (TOF-LEIS) setup for primary energies of 500 eV to 10 keV and a backscattering angle of 129. TOF is a well known experimental technique, where the ion beam is chopped into packets and the flight times of the projectiles are recorded. At present, ACOLISSA is
operated at a minimal time resolution of about 20 ns, which yields an energy resolution of about 1% of the primary energy for 1 keV He ions and a Cu target. All charge states can be detected. The separation in time of the backscattered ions and the neutral projectiles is done by applying a postacceleration potential to a drift tube. The resulting spectra are denoted in the following as chargeseparated (CS) spectra. If no separation is made, i.e. no voltage is applied, all charge states have the same flight time, corresponding to their final energy. These spectra are denoted in the following as all-projectiles (AP) spectra. A typical TOF-LEIS-CS spectra for 3 keV He Cu evaporated onto alumina ions incident on 7 A is shown in Fig. 1. The main features of the spectrum are the ion peak in front of the neutral copper peak and a broad spectrum which can be assigned to the alumina substrate. Two different geometries were used for these measurements. On the one hand normal incidence was chosen with a resulting exit angle of 51 with respect to the surface normal (w.r.s.n.). On the other hand an angle of incidence of 76 w.r.s.n. was chosen with a resulting exit angle of 25 w.r.s.n. A small time shift can be clearly observed between the two spectra, corresponding to higher inelastic energy loss on the projectiles way to the sample surface
2500
α=0° α=76°
2000
α
θ
Counts
situation of Cu on Al2 O3 in particular it is stated, that Cu grows rather in 3D islands than layer by layer. LEIS is a standard tool for quantitative surface analysis [5,6]. It is in close analogy to Rutherford backscattering (RBS) [7]. Both techniques make use of ions as projectiles and record energy spectra of projectiles scattered at a certain scattering angle h are evaluated. These energy spectra contain two different types of information: (i) the energy transfer from the projectile to the recoil atom in the close collision yields information on the mass of the scattering partner; (ii) the energy lost by the projectile along its path due to electronic interaction yields information on the depth, in which the scattering took place [8]. The single scattering (SS) model, which assumes that the incoming and outgoing projectile trajectories consist of two straight lines which intersect at the position where the scattering takes place, is used to evaluate the energy spectra. It is most appropriate in the regime of high energies and small depths. Within this concept, the electronic stopping power S ¼ dE=dx [9,10] is used to describe the mean energy loss dE of the projectile per path length dx. The lower the energy, the more important becomes multiple scattering (MS) [11,12]. Due to MS, additional energy is lost, which can be quantified within the nuclear stopping power model. In addition, due to ms also the effective path length is increased and the backscattering event is not well defined anymore [13]. To keep the influence of MS at low energies tolerably low, the target thickness has to be chosen sufficiently small, which may be difficult.
579
Cu
1500
3 keV He+, θ = 129°
1000
500
0 6.0x10
-6
-6
-6
7.0x10 8.0x10 9.0x10 flight time tTOF [s]
-6
1.0x10
-5
Fig. 1. TOF-LEIS-CS spectra of 3 keV He ions incident on 7 A Cu evaporated on alumina. Shown are two different geometries: angle of incidence a ¼ 0 and 76 (with respect to the surface normal). Clearly observable is the time shift between the two spectra which can be assigned to more inelastic energy loss for oblique incidence.
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and a broader neutral copper peak for oblique incidence. In general it is more convenient to evaluate energy spectra rather than TOF spectra. This time-to-energy conversion is done following a standard procedure [15]. When nanometer layers are characterized by TOF-LEIS, quantitative information on the thickness is obtained from the spectrum width with high accuracy only if the SS approximation is applicable, as it is at higher energies, for instance in the MEIS and RBS regime. In order to prove the validity of the SS model, computer simulations have been done, by using the computer code MARLOWE [16], which treats scattering as a series of binary collisions within classical mechanics. The polycrystalline target model was chosen, in which a single crystal is rotated by a random angle after completing the simulation of one trajectory together with the non-local energy loss model. The simulated spectra contain all sorts of scattering processes so that one can evaluate from these simulations under which conditions MS will limit the applicability of the proposed method.
3. Results and discussion In Fig. 2 simulated spectra of 3 keV He ions incident on copper layers of a certain thickness are shown. The most important feature of these spectra is that the spectrum widths correlate well
500 +
θ =129°
1 layer 2 layers 3 layers
400
5 layers 10 layers 15 layers 20 layers
300
5
Counts / inc. 10 particles
3 keV He ,
200
100
0 1500
2000
2500
3000
Energy [eV]
Fig. 2. MARLOWE spectra of 3 keV He ions incident on copper layers of a number of Cu layer thicknesses.
with the corresponding layer thicknesses, similar as in RBS. Knowing the stopping power, the layer thicknesses can in principle be determined from the spectrum widths. The spectra contain also information on the relative importance of SS and MS: The spectrum corresponding to one monolayer of Cu just consists of one well defined peak at the energy corresponding to a binary collision; for thicker Cu layers a background due to MS is observed at high and at low energies. The high energy background is due to specific trajectories that contain at least two rather well defined collisions, while the low energy background is due to multiple collisions by more or less small angles. Qualitatively speaking, for 3 keV He ions and a Cu layer thickness larger than 10 Cu layers, an evaluation of the spectrum width gets impossible, since the SS approximation breaks down. In addition to the 3 keV He spectra, we also did simulations for 1 and 9 keV He projectiles as well. For all three energies, the resulting spectrum widths are evaluated applying standard procedures, the results being shown in Fig. 3. From Fig. 3 it follows that – in accordance with MS theory – for a certain projectile energy only certain layers with a specific thickness can be evaluated within the SS approximation. For a thickness larger than this specific thickness dmax , one has to use lighter projectiles or higher energies. In Fig. 4, results are presented, for 3 keV He Cu deposited by ions backscattered from 7 A evaporation on alumina two different geometries (as depicted in Fig. 1). In Fig. 4(a) the experimental results are shown and in Fig. 4(b) MARLOWE spectra are presented. The main features of the experiment, like the energy shift of the onset and the broadening of the peak for oblique incidence are well reproduced by the simulations. Nevertheless, there are also some differences: (i) the maximum of the copper peak itself is shifted to lower energies in the experimental spectrum and is broader than in the simulation, while the onset of the Cu peak is more or less the same. An intriguing question related to these observations is how appropriate the non-local energy loss model might be that was used in the simulation. To find an answer to this question, further studies are required. (ii) The ‘‘background’’ at about 2 keV is
M. Draxler et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 578–583 100
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α = 0° α = 76°
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+
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mean energy loss per layer [eV/ML]
60
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(a)
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number of analyzed layers 140
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Fig. 3. Mean energy loss per copper layer for He ions and Cu layers as a function of the layer thickness, as evaluated from the MARLOWE spectra (see also Fig. 2). The straight lines represent the electronic energy loss per Cu layer for He ions with primary energies of 1, 3 and 9 keV as calculated from the electronic stopping power used in the simulation in the SS approximation. The dashed lines represent the analogous calculations for the total energy loss per Cu layer (electronic plus nuclear stopping).
2200
+
3 keV He ,
(b)
θ = 129°
80 60 40 20
more pronounced in the experiment than in the simulation, but one has to keep in mind that Cu on alumina grows in a 3D mode, while in the simulation a 2D copper overlayer was used. In Fig. 2, spectra corresponding to a thickness of at least five monolayers exhibit a clearly observable ‘‘surface peak’’, which has been explained [17] by the absence of MS in the outermost atomic layers, where scattering occurs practically exclusively by binary collisions. An important feature of the surface peak (see Fig. 2) is that it requires a sufficiently large layer thickness (10 ML Cu for 3 keV He ions) and the extension of the layer till the outermost atomic layer. To emphasize this latter condition, clean and contaminated thick Cu layers (395 nm Cu evaporated onto a Si single crystal) have been analyzed (see Fig. 5(a)). For the contaminated layer, the Auger electron spectroscopy spectrum showed about 2 ML of hydrocarbons on top of Cu. The amount of CH covering the surface is enough to quench the surface peak completely. This feature is very useful when analyzing thin Cu layers on a PET target: 4 nm (20 ML) of Cu were evaporated onto a PET sample and have been
0 1600
1800
2000
2200
2400
2600
2800
Energy [eV]
Fig. 4. (a) LEIS-AP energy spectra of 3 keV He ions incident Cu evaporated on alumina, corresponding to the TOF on 7 A spectra shown in Fig. 1. (b) MARLOWE spectra corresponding to the experimental spectra shown in (a). Note that in the simulation a lower number of primary particles was used than in the experiment.
analyzed using 3 keV He ions. The resulting LEISAP spectra is shown in Fig. 5(b) as well as a MARLOWE spectrum, which was calculated for a 20 ML thick 2D Cu layer. From the comparison of these spectra it becomes obvious that Cu does not grow in a 2D mode and preferentially diffuses into the bulk. This finding is consistent with the observations of [1,2], where a rather granular morphology for the Cu with extended slabs inside the PET were reported. If the stopping power of the projectiles in PET were known, one could estimate the thickness of the PET surface layer. Note that the present spectrum was obtained using a primary fluence of 3 · 1012 projectiles per cm2 .
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M. Draxler et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 578–583 70
CH covered Cu
60 N(E) [arb. units]
(a)
Clean Cu
50 40 30
AP spectra one may obtain information about the specific growth modes and about the intercalation behavior of nanometer films. Thus, LEIS-AP seems to provide the opportunity to study interface phenomena of nm-layers, with high resolution. With the additional knowledge of accurate stopping powers at low energies a quantitative analysis becomes realistic.
20 10
+
3 keV He ,
0 1000
θ = 129°
Acknowledgements
1500
2000
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70 60 N(E) [arb. units]
(b)
4nm Cu/PET:
This work has been partly supported by the Austrian Science Fund (FWF) under contract number P16173-N08 and P16469-N08. Interesting discussions with Peter Zeppenfeld are gratefully acknowledged.
Experiment Simulation
50
References
40 30 20 10 +
3 keV He , 0 1000
θ = 129°
1500
2000
2500
Energy [eV]
Fig. 5. (a) LEIS-AP energy spectra of 3 keV He ions incident on a clean and a dirty (2 ML hydrocarbons) thick Cu target (395 nm Cu evaporated on a Si wafer). Note that the hydrocarbon overlayer quenches the surface peak completely. (b) LEIS-AP energy spectrum of 3 keV He ions incident on a Cu/ PET target (4 nm of Cu have been deposited onto PET by evaporation). Note that in the simulation only particles with a kinetic energy >1500 eV have been detected.
Thus the measurement can be safely regarded as non-destructive, which is important when analyzing delicate targets as in the present case.
4. Conclusions It has been shown that with the existing setup ACOLISSA nanometer layers can be analyzed non-destructively, due to the low fluence required to measure a spectrum (see above). From LEIS-
[1] J.F. Silvain, J.J. Ehrhardt, Thin Solid films 236 (1993) 230. [2] M. Chtaib, J. Ghijsen, J.J. Pireaux, R. Caudano, R.L. Johnson, E. Orti, J.L. Bredas, Phys. Rev. B 44 (1991) 10815. [3] T.T. Magkoev, G.G. Vladimirov, D. Remar, A.M.C. Moutinho, Solid State Commun. 122 (2002) 341. [4] C.L. Pang, H. Raza, S.A. Haycock, G. Thornton, Surf. Sci. 460 (2000) L510. [5] E. Taglauer, in: J.C. Vickerman (Ed.), Surface Analysis – The Principal Techniques, Wiley–VCH, Weinheim, 1997. [6] P. Bauer, in: H. Bubert, H. Jenett (Eds.), Surface and Thin Film Analysis, Wiley–VCH, Weinheim, 2002, p. 150. [7] L. Palmetshofer, in: H. Bubert, H. Jenett (Eds.), Surface and Thin Film Analysis, Wiley–VCH, Weinheim, 2002, p. 141. [8] J.A. Leavitt, L.C. McIntire, M.R. Weller, in: J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, Pittsburgh, PA, 1995, p. 37. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Range of Ions in Solids, Vol. 1, Pergamon, New York, 1985. [10] E. Rauhala, in: J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, Pittsburgh, PA, 1995, p. 3. [11] P. Sigmund, K.B. Winterbon, Nucl. Instr. and Meth. 119 (1974) 541; P. Sigmund, K.B. Winterbon, Nucl. Instr. and Meth. 125 (1975) 491. [12] G. Amsel, A. LÕHoir, G. Battistig, Nucl. Instr. and Meth. B 201 (2003) 325. [13] Z. Smit, Phys. Rev. A 48 (1993) 2070.
M. Draxler et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 578–583 [14] M. Draxler, S.N. Markin, S.N. Ermolov, K. Schmid, C. Hesch, A. Poschacher, R. Gruber, M. Bergsmann, P. Bauer, Vacuum 73 (1) (2004) 39. [15] T. Buck, G.H. Wheatley, G.L. Miller, D.A.H. Robinson, Y.S. Chen, Nucl. Instr. and Meth. 149 (1978) 591.
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[16] M.T. Robinson, I.M. Torrens, Phys. Rev. B 9 (1974) 5008. [17] M. Draxler, R. Beikler, E. Taglauer, K. Schmid, R. Gruber, S.N. Ermolov, P. Bauer, Phys. Rev. A 68 (2003) 022901.
Depth characterization of nm-layers by low energy ion scattering M. Draxler a,*, S.N. Markin a, R. Beikler b, E. Taglauer b, F. Kastner c, M. Bergsmann c, P. Bauer a a
Institut f€ ur Experimentalphysik, Abteilung Atom- und Oberflaechenphysik, Johannes Kepler Universit€at Linz, Altenbergerstrasse 69, A-4040 Linz, Austria b Max-Planck-Institut f€ur Plasmaphysik, EURATOM Association, D-85748 Garching bei M€unchen, Germany c Hueck Folien GmbH, A-4342 Baumgartenberg, Austria
Abstract Layers with a thickness in the nm range are of growing importance in science and technology. We describe, how the thickness and growth modes of such nm-layers can be characterized by time-of-flight low energy ion scattering (TOFLEIS), when all backscattered projectiles are taken into account. As examples, Cu evaporated on an Al2 O3 and on a PET target with a nominal film thickness of about 1nm are analyzed by means of TOF-LEIS using H and He ions in the keV range as projectiles. 2004 Elsevier B.V. All rights reserved. PACS: 34.50.Bw; 68.55.Jk Keywords: TOF-LEIS; Nanometer layers; Growth modes; Copper; Alumina; PET
1. Introduction Thin films play a dominant role in modern science and technology. They are applicable in very different fields and serve, for example, as coatings, chemical sensors or catalysts. Nowadays, as the miniaturization process of technical devices continues, functional layers with a certain thickness in the nanometer range are essential for many applications. Thin metal films on insulating or
*
Corresponding author. Tel.: +43-732-2468-8520; fax: +43732-2468-8509. E-mail address: [email protected] (M. Draxler).
organic substrates are of particular interest in this field. In this contribution, we focus on the potential of low energy ion scattering (LEIS) on the analysis of nanometer layers, e.g. of ultrathin Cu layers on PET and on Al2 O3 . An overview on the metal/PET adhesion is given, for example in [1]. Ref. [2] presents an XPS study of the, in our case highly relevant, Cu/PET interface. It is stated in these articles that if the chemical bonding is weak (in the case of Cu/PET), the metal is free to diffuse into the polymer layer and form clusters. Another topic of high technological interest is the adsorption of metals on oxides. A summary on this subject is given, for instance, in [3]. An AFM growth study of Cu and Pd on a-Al2 O3 is given in [4]. For the
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.123
M. Draxler et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 578–583
2. Experiment and simulation The experiments were carried out at the experimental setup ACOLISSA, which is described in detail in [14]. In brief, ACOLISSA is a time-offlight low energy ion scattering (TOF-LEIS) setup for primary energies of 500 eV to 10 keV and a backscattering angle of 129. TOF is a well known experimental technique, where the ion beam is chopped into packets and the flight times of the projectiles are recorded. At present, ACOLISSA is
operated at a minimal time resolution of about 20 ns, which yields an energy resolution of about 1% of the primary energy for 1 keV He ions and a Cu target. All charge states can be detected. The separation in time of the backscattered ions and the neutral projectiles is done by applying a postacceleration potential to a drift tube. The resulting spectra are denoted in the following as chargeseparated (CS) spectra. If no separation is made, i.e. no voltage is applied, all charge states have the same flight time, corresponding to their final energy. These spectra are denoted in the following as all-projectiles (AP) spectra. A typical TOF-LEIS-CS spectra for 3 keV He Cu evaporated onto alumina ions incident on 7 A is shown in Fig. 1. The main features of the spectrum are the ion peak in front of the neutral copper peak and a broad spectrum which can be assigned to the alumina substrate. Two different geometries were used for these measurements. On the one hand normal incidence was chosen with a resulting exit angle of 51 with respect to the surface normal (w.r.s.n.). On the other hand an angle of incidence of 76 w.r.s.n. was chosen with a resulting exit angle of 25 w.r.s.n. A small time shift can be clearly observed between the two spectra, corresponding to higher inelastic energy loss on the projectiles way to the sample surface
2500
α=0° α=76°
2000
α
θ
Counts
situation of Cu on Al2 O3 in particular it is stated, that Cu grows rather in 3D islands than layer by layer. LEIS is a standard tool for quantitative surface analysis [5,6]. It is in close analogy to Rutherford backscattering (RBS) [7]. Both techniques make use of ions as projectiles and record energy spectra of projectiles scattered at a certain scattering angle h are evaluated. These energy spectra contain two different types of information: (i) the energy transfer from the projectile to the recoil atom in the close collision yields information on the mass of the scattering partner; (ii) the energy lost by the projectile along its path due to electronic interaction yields information on the depth, in which the scattering took place [8]. The single scattering (SS) model, which assumes that the incoming and outgoing projectile trajectories consist of two straight lines which intersect at the position where the scattering takes place, is used to evaluate the energy spectra. It is most appropriate in the regime of high energies and small depths. Within this concept, the electronic stopping power S ¼ dE=dx [9,10] is used to describe the mean energy loss dE of the projectile per path length dx. The lower the energy, the more important becomes multiple scattering (MS) [11,12]. Due to MS, additional energy is lost, which can be quantified within the nuclear stopping power model. In addition, due to ms also the effective path length is increased and the backscattering event is not well defined anymore [13]. To keep the influence of MS at low energies tolerably low, the target thickness has to be chosen sufficiently small, which may be difficult.
579
Cu
1500
3 keV He+, θ = 129°
1000
500
0 6.0x10
-6
-6
-6
7.0x10 8.0x10 9.0x10 flight time tTOF [s]
-6
1.0x10
-5
Fig. 1. TOF-LEIS-CS spectra of 3 keV He ions incident on 7 A Cu evaporated on alumina. Shown are two different geometries: angle of incidence a ¼ 0 and 76 (with respect to the surface normal). Clearly observable is the time shift between the two spectra which can be assigned to more inelastic energy loss for oblique incidence.
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and a broader neutral copper peak for oblique incidence. In general it is more convenient to evaluate energy spectra rather than TOF spectra. This time-to-energy conversion is done following a standard procedure [15]. When nanometer layers are characterized by TOF-LEIS, quantitative information on the thickness is obtained from the spectrum width with high accuracy only if the SS approximation is applicable, as it is at higher energies, for instance in the MEIS and RBS regime. In order to prove the validity of the SS model, computer simulations have been done, by using the computer code MARLOWE [16], which treats scattering as a series of binary collisions within classical mechanics. The polycrystalline target model was chosen, in which a single crystal is rotated by a random angle after completing the simulation of one trajectory together with the non-local energy loss model. The simulated spectra contain all sorts of scattering processes so that one can evaluate from these simulations under which conditions MS will limit the applicability of the proposed method.
3. Results and discussion In Fig. 2 simulated spectra of 3 keV He ions incident on copper layers of a certain thickness are shown. The most important feature of these spectra is that the spectrum widths correlate well
500 +
θ =129°
1 layer 2 layers 3 layers
400
5 layers 10 layers 15 layers 20 layers
300
5
Counts / inc. 10 particles
3 keV He ,
200
100
0 1500
2000
2500
3000
Energy [eV]
Fig. 2. MARLOWE spectra of 3 keV He ions incident on copper layers of a number of Cu layer thicknesses.
with the corresponding layer thicknesses, similar as in RBS. Knowing the stopping power, the layer thicknesses can in principle be determined from the spectrum widths. The spectra contain also information on the relative importance of SS and MS: The spectrum corresponding to one monolayer of Cu just consists of one well defined peak at the energy corresponding to a binary collision; for thicker Cu layers a background due to MS is observed at high and at low energies. The high energy background is due to specific trajectories that contain at least two rather well defined collisions, while the low energy background is due to multiple collisions by more or less small angles. Qualitatively speaking, for 3 keV He ions and a Cu layer thickness larger than 10 Cu layers, an evaluation of the spectrum width gets impossible, since the SS approximation breaks down. In addition to the 3 keV He spectra, we also did simulations for 1 and 9 keV He projectiles as well. For all three energies, the resulting spectrum widths are evaluated applying standard procedures, the results being shown in Fig. 3. From Fig. 3 it follows that – in accordance with MS theory – for a certain projectile energy only certain layers with a specific thickness can be evaluated within the SS approximation. For a thickness larger than this specific thickness dmax , one has to use lighter projectiles or higher energies. In Fig. 4, results are presented, for 3 keV He Cu deposited by ions backscattered from 7 A evaporation on alumina two different geometries (as depicted in Fig. 1). In Fig. 4(a) the experimental results are shown and in Fig. 4(b) MARLOWE spectra are presented. The main features of the experiment, like the energy shift of the onset and the broadening of the peak for oblique incidence are well reproduced by the simulations. Nevertheless, there are also some differences: (i) the maximum of the copper peak itself is shifted to lower energies in the experimental spectrum and is broader than in the simulation, while the onset of the Cu peak is more or less the same. An intriguing question related to these observations is how appropriate the non-local energy loss model might be that was used in the simulation. To find an answer to this question, further studies are required. (ii) The ‘‘background’’ at about 2 keV is
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α = 0° α = 76°
80
+
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Fig. 3. Mean energy loss per copper layer for He ions and Cu layers as a function of the layer thickness, as evaluated from the MARLOWE spectra (see also Fig. 2). The straight lines represent the electronic energy loss per Cu layer for He ions with primary energies of 1, 3 and 9 keV as calculated from the electronic stopping power used in the simulation in the SS approximation. The dashed lines represent the analogous calculations for the total energy loss per Cu layer (electronic plus nuclear stopping).
2200
+
3 keV He ,
(b)
θ = 129°
80 60 40 20
more pronounced in the experiment than in the simulation, but one has to keep in mind that Cu on alumina grows in a 3D mode, while in the simulation a 2D copper overlayer was used. In Fig. 2, spectra corresponding to a thickness of at least five monolayers exhibit a clearly observable ‘‘surface peak’’, which has been explained [17] by the absence of MS in the outermost atomic layers, where scattering occurs practically exclusively by binary collisions. An important feature of the surface peak (see Fig. 2) is that it requires a sufficiently large layer thickness (10 ML Cu for 3 keV He ions) and the extension of the layer till the outermost atomic layer. To emphasize this latter condition, clean and contaminated thick Cu layers (395 nm Cu evaporated onto a Si single crystal) have been analyzed (see Fig. 5(a)). For the contaminated layer, the Auger electron spectroscopy spectrum showed about 2 ML of hydrocarbons on top of Cu. The amount of CH covering the surface is enough to quench the surface peak completely. This feature is very useful when analyzing thin Cu layers on a PET target: 4 nm (20 ML) of Cu were evaporated onto a PET sample and have been
0 1600
1800
2000
2200
2400
2600
2800
Energy [eV]
Fig. 4. (a) LEIS-AP energy spectra of 3 keV He ions incident Cu evaporated on alumina, corresponding to the TOF on 7 A spectra shown in Fig. 1. (b) MARLOWE spectra corresponding to the experimental spectra shown in (a). Note that in the simulation a lower number of primary particles was used than in the experiment.
analyzed using 3 keV He ions. The resulting LEISAP spectra is shown in Fig. 5(b) as well as a MARLOWE spectrum, which was calculated for a 20 ML thick 2D Cu layer. From the comparison of these spectra it becomes obvious that Cu does not grow in a 2D mode and preferentially diffuses into the bulk. This finding is consistent with the observations of [1,2], where a rather granular morphology for the Cu with extended slabs inside the PET were reported. If the stopping power of the projectiles in PET were known, one could estimate the thickness of the PET surface layer. Note that the present spectrum was obtained using a primary fluence of 3 · 1012 projectiles per cm2 .
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M. Draxler et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 578–583 70
CH covered Cu
60 N(E) [arb. units]
(a)
Clean Cu
50 40 30
AP spectra one may obtain information about the specific growth modes and about the intercalation behavior of nanometer films. Thus, LEIS-AP seems to provide the opportunity to study interface phenomena of nm-layers, with high resolution. With the additional knowledge of accurate stopping powers at low energies a quantitative analysis becomes realistic.
20 10
+
3 keV He ,
0 1000
θ = 129°
Acknowledgements
1500
2000
2500
Energy [eV]
70 60 N(E) [arb. units]
(b)
4nm Cu/PET:
This work has been partly supported by the Austrian Science Fund (FWF) under contract number P16173-N08 and P16469-N08. Interesting discussions with Peter Zeppenfeld are gratefully acknowledged.
Experiment Simulation
50
References
40 30 20 10 +
3 keV He , 0 1000
θ = 129°
1500
2000
2500
Energy [eV]
Fig. 5. (a) LEIS-AP energy spectra of 3 keV He ions incident on a clean and a dirty (2 ML hydrocarbons) thick Cu target (395 nm Cu evaporated on a Si wafer). Note that the hydrocarbon overlayer quenches the surface peak completely. (b) LEIS-AP energy spectrum of 3 keV He ions incident on a Cu/ PET target (4 nm of Cu have been deposited onto PET by evaporation). Note that in the simulation only particles with a kinetic energy >1500 eV have been detected.
Thus the measurement can be safely regarded as non-destructive, which is important when analyzing delicate targets as in the present case.
4. Conclusions It has been shown that with the existing setup ACOLISSA nanometer layers can be analyzed non-destructively, due to the low fluence required to measure a spectrum (see above). From LEIS-
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