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Growth mode of ultrathin gold films deposited on nickel
Applied Surface Science 199 (2002) 138–146
Growth mode of ultrathin gold films deposited on nickel J. Zemeka,*, P. Jiriceka, A. Jablonskib, B. Lesiakb a
Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 53 Prague 6, Czech Republic b Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland Received 18 February 2002; accepted 29 May 2002
Abstract Gold ultrathin overlayers were evaporated on polycrystalline nickel within 17 deposition steps. Mean thicknesses of the deposit in the range from 0.07 to 3.8 nm were determined by a quartz microbalance. Growth mode and morphology were examined using three different methods. First two methods are based on analysis of X-ray induced photoelectron spectra (XPS), the last one on the elastic peak electron spectroscopy (EPES). The Au 4d XPS lines with extended part of background intensity were analysed using the QUASES-Tougaard software. The XPS ratio method, commonly used for uniform overlayers thickness estimation, was applied to the Au 4f and Ni 3p peak areas. The EPES method is based on measurements and Monte Carlo (MC) calculations of elastic electron backscattering probability in the low kinetic energy range (200–1000 eV), where the method has a high surface sensitivity. The results obtained show that an initial growth mode of the gold overlayer is not uniform and indicate formation of gold 3D islands on nickel. # 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk; 79.20.m; 79.20.Kz; 81.15.Ef; 82.80.Pv Keywords: Monte Carlo calculations; X-ray photoelectron spectroscopy; Layer-by-layer growth; 3D island-like growth; Au–Ni system
1. Introduction An initial growth mode and morphology of ultrathin films is a key problem in physics of thin films as well as in wide range of technological applications, in particular in the microelectronics. The problem attracts vast attention of researchers to both, experimental and theoretical aspects of the thin film growth. The useful experimental methods are the ion scattering spectroscopy [1] and the positron induced Auger electron spectroscopy [2], although they are not commonly used. Due to their top-surface sensitivity the Frank– van der Merwe (FM) growth mode (layer-by-layer) * Corresponding author. Fax: þ42-2-3123184. E-mail address: [email protected] (J. Zemek).
can be easily recognised from the Volmer–Weber (VW) growth mode (3D island-like growth on a substrate). Electron-induced Auger electron spectroscopy and X-ray induced photoelectron spectroscopy (XPS) methods are also used for this purpose though their surface sensitivity may be lower, particularly for high kinetic energy of signal electrons [3]. Recently, there has been an increasing interest in the elastic electron backscattering phenomenon such as the elastic peak electron spectroscopy (EPES) mainly due to its application in experimental determination of inelastic mean free paths (IMFP) [4]. Since the elastic electron backscattered intensity depends on target composition [5], the method may be helpful to reveal the growth mode of a deposit. Its high surface sensitivity can be achieved using low kinetic energy of electrons.
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 5 1 - 7
J. Zemek et al. / Applied Surface Science 199 (2002) 138–146
Moreover, in the EPES, the backscattered electron has to overcome the surface/vacuum interface twice. Hence, a probability to excite surface losses and as a consequence, its surface sensitivity further increases [6]. In the present contribution, we examine the growth mode of gold deposit on nickel with XPS and EPES techniques using three different methods: the Au 4d lineshape analysis based on Tougaard‘s theory and a software package QUASES [7], the photoelectron intensity ratio method applied to the Au 4f and Ni 3p peaks (the ratio method has been frequently used for an uniform overlayer thickness estimation in XPS [8]), and measurements of the elastic backscattering probability of primary beam of electrons impinging on the Au/Ni surface. The probability depends strongly on the electron elastic cross-sections for Ni and Au atoms [5], on the mean thickness of a gold deposit and also on a surface morphology [9]. From the measured elastic backscattering probability and that calculated by the Monte Carlo (MC) method we have attempted to obtain information on the initial growth mode of gold on nickel surface. Since electron and photoelectron diffraction complicate relevant theoretical description, we have selected a fine polycrystalline nickel evaporated on a smooth Si wafer. Additionally, the Au/Ni system was selected due to following reasons: (i) It is expected that the Au/Ni interface will be reasonably sharp since the phase diagram of the bulk AuNi alloy shows a wide miscibility gap at temperatures of <1000 K and mutual solubilities of Au and Ni are very limited at lower temperatures [10]. However, it has been reported in the literature [11] that submonolayer quantities of gold on nickel single crystal tend to form a surface alloy. (ii) There is a controversy concerning the growth mode of the ultrathin gold overlayer on Ni. The FM growth mode [12–15], the Stranski–Krastanov (SK) growth model (Au islands grown on the first Au layer(s)) [15,16] and, finally, the VW growth mode [9,15,17,18] have been reported. (iii) Electron scattering properties of gold and nickel are very different. Particularly, differential crosssections for elastic scattering of both elements
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are complicated functions of the scattering angle [5,9]. Due to both, the experimental geometry used in this work and low kinetic energy of electrons, pronounced changes in elastic peak intensity are expected, even for a very small amount of gold deposited on the nickel surface.
2. Experimental In order to produce a smooth initial surface, the 50 nm thick Ni layer was grown by vacuum deposition at room temperature on a Si(1 1 1) wafer. A 15 mm 15 mm Ni/Si sample was introduced to angle-resolved photoelectron spectrometer ADES 400 (VG Scientific, UK). Prior to a gold deposition, the Ni surface was carefully sputter-cleaned in two steps: (a) by 3000 eVoxygen ion beam and then by (b) 3000 eV Ar ion beam. High purity gold was evaporated on Ni clean surface held at room temperature from an effusion cell in 17 successive evaporation steps. A mean thickness of gold films was measured by a quartz microbalance and cross checked by the Au 4d line shape analysis [9]. Moreover, the final thickness of the gold overlayer was tested ex situ by electron probe microanalyser (JEOL JXA-733) in 10 different points along a line across a sample surface. The resulting thicknesses were evaluated as the thickness averaged over 10 measurements (3:57 0:15 nm). This value agrees well with the final thickness of the gold film measured in situ by the quartz microbalance (3.79 nm). Angular-resolved X-ray induced photoelectron spectrometer applying Al Ka radiation and equipped with a hemispherical electron energy analyser operating in the constant pass energy mode of 100 or 50 eV was used to control the surface cleanness of the Ni and Au/Ni surfaces and to record the Ni 3p, Au 4f and Au 4d spectra. The spectra were recorded for each Au film on Ni substrate at two emission angles of 08 and 608 with respect to the surface normal. Prior the QUASES-Tougaard analysis [7], the Au 4d spectra were corrected for the spectrometer transmission function [19]. The elastic peak intensities were measured under fixed experimental geometry. The normal angle of incidence of electron primary beam at energy of 200 and 1000 eV was used. The electron beam current
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was varied from 1 108 to 1 107 A. The beam was defocused with a spot diameter of 5 mm at the sample surface. The hemispherical electron energy analyser was operated in the constant pass energy mode of 5 eV. The analyser axis was set to 258 with respect to the sample normal. The measured elastic peak intensity was corrected for very low inelastic background signal applying the Shirley’s procedure. The width of the elastic peak was typically below 0.5 eV (FWHM).
Table 1 Kinetic energies of the Au 4d, Au 4f and Ni 3p photoelectrons, inelastic (l), transport (ltr) and total (ltot) electron mean free path ˚ ) values used in present model calculations (A Au 4d KE (eV) 1137 l (in Au, TPP) 14.2 l (in Au, recommended) 15.2 ltr (in Au) 26.8 ltot (in Au, TPP) 9.3 ltot (in Au, recommended) 9.7
Au 4f
Ni 3p
Reference
1403 16.5 17.8 32.6 10.9 11.5
1420 16.7 [3] 18.0 [4] 33.0 [24] 11.1 11.6
3. Results 3.1. The Au 4d spectral line shape analysis by QUASES The surface morphology with nanometre depth resolution was quantified by analysis of the Au 4d peak shape and the shape of extended background behind the peak. The method relies on the fact that the energy distribution of emitted electrons depends strongly on the travelled path lengths and thereby also on the in-depth concentration profile. The theoretical framework is described by Tougaard and co-workers [20–22]. The validity of the technique has been established through systematic experimental investigations and comparison to measurements on the same samples by Rutherford backscattering spectrometry (RBS), ion scattering spectrometry (ISS) and atomic force microscopy (AFM) [23]. The corresponding spectra processing is presently facilitated by the software packed QUASES-Tougaard [7]. Analysis of the Au 4d spectral line shapes recorded after each deposition step of Au on Ni was carried out using the growth modes mentioned above: the FM, the SK, and the VW. As a parameter characterising electron transport in a surface region of the sample, the total electron mean free path defined in the next section was used. Its numerical values are summarised in Table 1 for the Au 4d, Au 4f and Ni 3p photoelectrons. Here, it should be noted that the QUASESTougaard software [7] approximates the real SK and the VW surface morphology (3D islands size distribution) by an uniform overlayer of a height H and a coverage y < 1. As a measure of consistency, we have required close simulated and measured mean thickness values of the gold deposit. No such consistency
and in the most cases also a reasonable agreement between the shape of calculated and measured spectra was obtained for the FM and the SK modes. The VW growth mode characterised by the Au coverage and the mean island height has lead to a good agreement between the measured and calculated spectra and, at the same time, to a good consistency with respect to the mean gold thickness measured in situ by the quartz microbalance method (Table 2). Agreement between the measured and calculated Au 4d spectra by using the FM and the VW growth modes of the gold on Ni is illustrated in Fig. 1. For the VW growth mode, the calculated and measured spectra agree well, while for the FM mode the difference due to unrealistic model is clearly discerned in the Au 4d line itself and in the background as well. Fig. 2 displays results of the VW growth model simulations applied to all steps of the gold deposited on nickel. A relative uncovered area of Ni surface is plotted against the gold mean thickness for 08 and 608 emission angles of Au 4d photoelectrons. Both fractions decrease quickly with the increase in thickness. ˚ for The uncovered area of Ni reaches 33% at 10 A ˚. emission angles of 08 and approaches 0% at 29 A Similarly, for emission angle of 608, the uncovered ˚ and approaches 0% at area of Ni reaches 20% at 10 A ˚ . The difference arises due to neglecting elastic 19 A scattering of photoelectrons in the QUASES calculations. The elastic scattering effects of photoelectrons may become pronounced at large emission angles [25]. The structure of the gold overlayer on nickel resulting from Au 4f spectra processing with QUASES-Tougaard software, expressed as a gold islands coverage
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Table 2 The structure of the gold overlayer resulting from Au 4d spectra processing with the QUASES-Tougaard softwarea Evaporation step no.
Au island coverage (%)
Au island ˚) height (A
Mean thickness ˚) QUASES (A
Mean thickness ˚) QCM (A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
9.0 14.9 30.6 30.4 41.9 47.1 49.7 60.6 67.0 77.6 80.2 83.6 85.4 91.9 99.3 100.0 100.0
15 15 11 11 11 12 13 13 14 14 16 17 18 22 27 33 41
1.3 2.2 3.4 3.3 4.6 5.7 6.5 7.9 9.4 10.9 12.8 14.2 15.4 20.2 26.8 33.0 41.0
0.7 1.4 2.3 3.0 3.7 4.4 5.1 6.7 8.1 9.6 11.2 12.7 14.1 18.7 23.4 28.7 37.9
a
The Au 4d spectra were recorded at the normal emission direction.
and a gold islands height, is summarised in Table 2. The resulting mean thickness values of the gold overlayer are compared with those obtained independently by the quartz microbalance. These data are used as the input data for the ratio method and the elastic electron backscattering probability calculations.
3.2. The photoelectron intensity ratio method Initial growth mode of deposited film on a substrate can also be estimated from the ratio of selected peak areas from an overlayer, Io, and a substrate material, Is. The theoretical framework of the method is described
Fig. 1. Comparison of the measured Au 4d line shape (solid line) with that calculated (open circles) using the FM growth mode model (upper spectrum) and the VW model (lower spectrum).
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where ltr is the transport mean free path taken from Jablonski [24]. We applied Au 4f and Ni 3p peak area ratios. Since the kinetic energy of these photoelectrons is almost the same, Eq. (1) can be simplified into lnðR þ 1Þ ¼
Fig. 2. Relative area of Ni substrate uncovered by gold film as revealed by QUASES-Tougaard line shape analysis of the Au 4d lines versus the mean thickness of gold measured by quartz microbalance method. Squares: emission angle of Au 4d photoelectrons 08. Open circles: emission angle 608 with respect to the surface normal.
by Fadley et al. [26]. Below, we present a brief mathematical description of the intensity ratio based on an uniform overlayer model representing the FM growth mode and for the VW growth mode approximated by an uniform overlayer with a substrate coverage y < 1. Exactly the same approximation of the real structure of the gold overlayer is used in the QUASES-Tougaard software. When neglecting electron elastic scattering processes in a surface region of the sample, the FM model is described by the following equation [26] 1 Io I 1 expðt=lo cos aÞ (1) ¼ o1 expðt=ls cos aÞ Is Is where Io1 and Is1 denote the corresponding peak areas from sufficiently thick (bulk) sample of the overlayer and the substrate, respectively, t the (mean) thickness of the overlayer, a the emission angle measured from the surface normal and l is the IMFP of photoelectrons from an overlayer, lo, or from a substrate, ls, passing through the overlayer. Since elastic scattering of photoelectrons in a surface region cannot be neglected without a substantial systematic error [24], we have included a correction of this effect into Eq. (1) by replacing the IMFPs by the total MFPs, ltot, calculated from the transport approximation [27,28] by equation below: 1 1 1 ¼ þ ltot l ltr
(2)
t ; ltot cos a
R¼
Io Is1 Is Io1
(3)
For the more complicated VW growth mode under the approximation described above, we write H 1 Io ¼ Io y 1 exp ; ðlo cos aÞ H 1 1 (4) Is ¼ Is ð1 yÞ þ Is y exp lo cos a where y is the coverage and H is the height of the overlayer. They are provided by the QUASES-Tougaard analysis and are summarised in Table 2. Similar to Eq. (3) and considering Eq. (2), we have 1 lnðR þ 1Þ ¼ ln 1 y þ y expðH=ðltot cos aÞÞ (5) As expected, for y close to 0, lnðR þ 1Þ approaches 0 and for y approaching 1, Eq. (5) transforms into Eq. (3). In Fig. 3, we compare experimental results with those of the FM and the VW model calculations. Note (i) the measured points deviate from a straight line predicted from the FM model and (ii) shapes of the measured dependence and that calculated within the VW model are similar. However, the experimental and data calculated within the both models deviate noticeably. The reasons will be discussed in detail. 3.3. Elastic electron backscattering Jablonski and Tougaard [29] and Jablonski et al. [30] have developed the MC algorithm which could be effectively used in calculations of the elastic backscattering probability for system with overlayers. The thickness dependence of the elastic peak intensity measured using an analyser with a narrow acceptance angle was very well described by theory [30]. Recently, we studied a possibility of determining the overlayer thickness from variations of the elastic backscattering probability for the Au on Ni [9]. In this contribution, we continued our search to establish an effective
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Fig. 3. Dependence of lnðR þ 1Þ versus mean thickness of a gold layer. Open circles: experimental data. Solid line: model calculations (results of the FM model, Eq. (3)). Filled circles: model calculations (the approximated VW growth mode model, Eq. (5)) R ¼ Io Is1 =Is Io1 , where Io ¼ IAu 4f and Is ¼ INi 3p .
Fig. 4. Relative elastic backscattering intensity ratios from Au/Ni system on the mean overlayer gold thickness at 200 eV primary beam electrons. Triangles: experimental data. Open circles: the MC model calculations (the FM model). Filled circles: the MC model calculations (the approximated VW growth mode model).
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Fig. 5. Relative elastic backscattering intensity ratios from Au/Ni system on the mean overlayer gold thickness at 1000 eV primary beam electrons. Triangles: experimental data. Open circles: the MC model calculations (the FM model). Filled circles: the MC model calculations (the approximated VW growth mode model).
procedure for determining the overlayer structure from the elastic peak intensity. We now used the same experimental and theoretical procedure as in [9]. For the FM and the approximated VW models described above and for H and y values resulting from the QUASES-Tougaard analysis summarised in Table 2, the results of MC calculations are shown in Figs. 4 and 5 in comparison with the measured data for kinetic energy of primary electron beam of 200 and 1000 eV. There is a good qualitative agreement between theory and experiment, only.
4. Discussion Due to a complex problem of electron transport at solid surfaces and hence mathematical difficulties, the QUASES-Tougaard analysis makes it possible to apply rather simple structural models of the overlayer which can deviate substantially from the real structure. Specifically, the QUASES-Tougaard software [7] has approximated the real SK and the VW surface morphology (3D islands size distribution) by an uniform overlayer of a height H and a coverage y < 1.
From the same reason, the elastic scattering of signal electrons has been neglected in the software, too. Its effect has, however, been at least partially corrected in this work by using the total electron mean free path instead of the corresponding IMFP, both summarised in Table 1. Our analysis using the QUASES-Tougaard software relays on expected consistency between mean gold overlayer thickness values in situ measured by a well established quartz microbalance method and those resulting from the QUASES-Tougaard software. As shown in Table 2, the QUASES-Tougaard derived thickness values are slightly but systematically higher than those from the independent method used. The difference may be caused by a simplified model calculation used. The photoelectron intensity ratio method, commonly used in photoelectron spectroscopy for overlayer thickness estimations, reveals important deviations between the experimental data and calculated values within the FM and WV models, as shown in Fig. 3. One would expect that the measured and calculated points should be close mutually for a gold mean ˚ , because (a) the Ni 3p thickness exceeding 25 A
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intensity approaches 0 and (b) the Ni substrate is, in accordance with the QUASES-Tougaard predictions of Fig. 2, in this thickness range fully covered by gold. A slope of the straight line resulting from the FM model is governed by the ltot value (Eq. (3)). We used ˚ , the value calculated for the Au 4f photoelec11.5 A trons from the recommended IMFP [4] and the corresponding transport mean free path [24]. A straight line connecting the outset and the last experimental point ˚ . So, the deviation may in Fig. 3 yields in ltot ¼ 9:2 A be caused at least partially by incomplete correction of elastic electron scattering effects and an uncertainty of the IMFP and the transport mean free path values. This conclusion is consistent with a lower slope of a straight line in Fig. 3 calculated within the FM model when the elastic electron scattering effects are neglected. In this case, ltot ¼ l (IMFP) has reached ˚ . Alternatively, the real surface structure of gold 17.8 A deposit is certainly far from the oversimplified model structure. For example, the 3D gold islands having their height larger than the ltot can also modify the Au 4f/Ni 3p peak area ratio. Results of the MC model calculations of the elastic electron backscattering probability differ quantitatively from the measured data, too. Largest discrepancy, however, is observed in the region of small film thickness at 200 eV, i.e., in the region of the initial growth of gold overlayer. Consistently to the results of the photoelectron intensity ratio method, neither the FM nor the VW growth mode results agree with the experiment quantitatively. As the electron transport in a surface region of the sample was properly accounted for in the MC calculations, the reason for quantitative disagreement we find in the oversimplified VW model where size, height and shape distributions of the gold islands were approximated by an uniform overlayer of a height H and a coverage y < 1. The agreement is much better for 1000 eV kinetic energy case shown in Fig. 5, however, this is due to larger penetration depth of primary electrons and much smaller difference in the differential elastic scattering cross-sections of gold and nickel for the scattering geometry used. As a result, the method has lost its superior surface sensitivity at a high energy region. It should be noted that a possible gold–nickel alloying mentioned above can hardly influence present results because the alloying should be limited to a
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contact monolayer at the interface. From the above three methods used in this work, only the elastic electron backscattering has a potential to reveal the gold–nickel interface alloying, due to its superior surface sensitivity. However it needs to be studied further. Similar stability of the results as for the alloying we expect for a possible influence of a surface roughness of the nickel substrate. The nickel layer was evaporated into a silicon wafer held at room temperature during the deposition. A mean surface roughness of the wafer was below 1 nm. So, the final surface roughness of the nickel surface we estimate in the range 1–10 nm. Such value has little influence on recorded photoelectron spectra and surprisingly also on elastically backscattered electron intensities tested on polished and ground silicon wafers with a mean surface roughness of 5.5 and 760 nm, respectively [31].
5. Summary and conclusions Growth mode and morphology of Au/Ni system were studied by XPS, EPES and MC calculations. Present results show that the gold overlayer on nickel is not uniform in early stages of growth. This observation is consistent with results of other authors [15,17,18]. The QUASES-Tougaard analysis excluded both the FM and SK growth modes of gold deposit and revealed that the VW growth mode is a model consistent with spectral line shapes of the Au 4d lines analysed with extended inelastic background. The surface structure parameters, the surface coverage and the mean height of gold islands, obtained from the QUASES-Tougaard analysis were used as input data into two remaining methods applied in this work. Results of the two methods showed qualitative agreement with the experimental data for the VW model of the growth mode. Differences between the photoelectron intensity ratio method predictions and the measured data are caused to some extend by using incomplete corrections for electron elastic scattering and uncertainties in the inelastic and transport mean free paths values. In the elastic electron backscattering method, where electron transport in a surface region of the sample was properly accounted for, the reason for quantitative disagreement is certainly
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in the oversimplified VW model where size, height and shape distributions of the gold islands were approximated by an uniform overlayer of a height H and a coverage y < 1.
Acknowledgements The authors would like to acknowledge the support of KBN grant 2PO3B 03918 (A.J. and B.L.) and GACR 202/02/0237 (J.Z. and P.J.). All the authors would like to acknowledge the support of the KBNCzech Ministry of Education project 50.
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[8] P.J. Cumpson, Surf. Interf. Anal. 29 (2000) 403. [9] A. Jablonski, J. Zemek, P. Jiricek, Surf. Interf. Anal. 31 (2001) 825. [10] R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser, K.K. Kelley, Selected Values of the Thermodynamic Properties of Binary Alloys, American Society for Metals, Metals Park, OH, 1973, p. 294. [11] F. Besenbacher, Rep. Prog. Phys. 59 (1996) 1737. [12] R.C. Jaclevic, Phys. Rev. 30 (1984) 5494. [13] H.S. Hansen, S. Tougaard, Vacuum 41 (1990) 1710. [14] S. Zafeiratos, S. Kennou, Appl. Surf. Sci. 173 (2001) 69. [15] J.M. Guglielmacci, Appl. Surf. Sci. 17 (1984) 337. [16] H.S. Hansen, C. Jansson, S. Tougaard, J. Vac. Sci. Technol. 10 (1992) 2938. [17] S.-I. Yamamoto, H. Yamada, K. Matsushige, Jpn. J. Appl. Phys. 39 (2000) 3717. [18] A.C. Simonsen, J.P. Pohler, C. Jeynes, S. Tougaard, Surf. Interf. Anal. 27 (1999) 52. [19] P. Jiricek, Czech. J. Phys. 44 (1994) 261. [20] S. Tougaard, J. Vac. Sci. Technol. 216 (1989) 343. [21] M. Schleberger, D. Fujita, C. Scharfschwerdt, S. Tougaard, J. Vac. Sci. Technol. 13 (1995) 949. [22] S. Tougaard, J. Vac. Sci. Technol. 14 (1996) 1415. [23] S. Tougaard, Surf. Interf. Anal. 26 (1998) 249. [24] A. Jablonski, Surf. Sci. 364 (1996) 380. [25] A. Jablonski, C.J. Powell, D.R. Penn, J. Electron Spectrosc. Relat. Phenom. 100 (1999) 137. [26] C.S. Fadley, R.J. Baird, W. Siekhaus, T. Novakov, S.A.L. Bergstrom, J. Electron Spectrosc. Relat. Phenom. 4 (1974) 93. [27] J. Zemek, P. Jiricek, S. Hucek, A. Jablonski, B. Lesiak, Surf. Sci. 473 (2001) 8. [28] A. Jablonski, Phys. Rev. 58 (1998) 16470. [29] A. Jablonski, S. Tougaard, J. Vac. Sci. Technol. 8 (1990) 106. [30] A. Jablonski, H.S. Hansen, C. Jansson, S. Tougaard, Phys. Rev. 45 (1992) 3694. [31] J. Zemek, P. Jiricek, A. Jablonski, B. Lesiak, Surf. Interf. Anal. 34 (2002), in press.
Growth mode of ultrathin gold films deposited on nickel J. Zemeka,*, P. Jiriceka, A. Jablonskib, B. Lesiakb a
Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 53 Prague 6, Czech Republic b Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland Received 18 February 2002; accepted 29 May 2002
Abstract Gold ultrathin overlayers were evaporated on polycrystalline nickel within 17 deposition steps. Mean thicknesses of the deposit in the range from 0.07 to 3.8 nm were determined by a quartz microbalance. Growth mode and morphology were examined using three different methods. First two methods are based on analysis of X-ray induced photoelectron spectra (XPS), the last one on the elastic peak electron spectroscopy (EPES). The Au 4d XPS lines with extended part of background intensity were analysed using the QUASES-Tougaard software. The XPS ratio method, commonly used for uniform overlayers thickness estimation, was applied to the Au 4f and Ni 3p peak areas. The EPES method is based on measurements and Monte Carlo (MC) calculations of elastic electron backscattering probability in the low kinetic energy range (200–1000 eV), where the method has a high surface sensitivity. The results obtained show that an initial growth mode of the gold overlayer is not uniform and indicate formation of gold 3D islands on nickel. # 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk; 79.20.m; 79.20.Kz; 81.15.Ef; 82.80.Pv Keywords: Monte Carlo calculations; X-ray photoelectron spectroscopy; Layer-by-layer growth; 3D island-like growth; Au–Ni system
1. Introduction An initial growth mode and morphology of ultrathin films is a key problem in physics of thin films as well as in wide range of technological applications, in particular in the microelectronics. The problem attracts vast attention of researchers to both, experimental and theoretical aspects of the thin film growth. The useful experimental methods are the ion scattering spectroscopy [1] and the positron induced Auger electron spectroscopy [2], although they are not commonly used. Due to their top-surface sensitivity the Frank– van der Merwe (FM) growth mode (layer-by-layer) * Corresponding author. Fax: þ42-2-3123184. E-mail address: [email protected] (J. Zemek).
can be easily recognised from the Volmer–Weber (VW) growth mode (3D island-like growth on a substrate). Electron-induced Auger electron spectroscopy and X-ray induced photoelectron spectroscopy (XPS) methods are also used for this purpose though their surface sensitivity may be lower, particularly for high kinetic energy of signal electrons [3]. Recently, there has been an increasing interest in the elastic electron backscattering phenomenon such as the elastic peak electron spectroscopy (EPES) mainly due to its application in experimental determination of inelastic mean free paths (IMFP) [4]. Since the elastic electron backscattered intensity depends on target composition [5], the method may be helpful to reveal the growth mode of a deposit. Its high surface sensitivity can be achieved using low kinetic energy of electrons.
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 5 1 - 7
J. Zemek et al. / Applied Surface Science 199 (2002) 138–146
Moreover, in the EPES, the backscattered electron has to overcome the surface/vacuum interface twice. Hence, a probability to excite surface losses and as a consequence, its surface sensitivity further increases [6]. In the present contribution, we examine the growth mode of gold deposit on nickel with XPS and EPES techniques using three different methods: the Au 4d lineshape analysis based on Tougaard‘s theory and a software package QUASES [7], the photoelectron intensity ratio method applied to the Au 4f and Ni 3p peaks (the ratio method has been frequently used for an uniform overlayer thickness estimation in XPS [8]), and measurements of the elastic backscattering probability of primary beam of electrons impinging on the Au/Ni surface. The probability depends strongly on the electron elastic cross-sections for Ni and Au atoms [5], on the mean thickness of a gold deposit and also on a surface morphology [9]. From the measured elastic backscattering probability and that calculated by the Monte Carlo (MC) method we have attempted to obtain information on the initial growth mode of gold on nickel surface. Since electron and photoelectron diffraction complicate relevant theoretical description, we have selected a fine polycrystalline nickel evaporated on a smooth Si wafer. Additionally, the Au/Ni system was selected due to following reasons: (i) It is expected that the Au/Ni interface will be reasonably sharp since the phase diagram of the bulk AuNi alloy shows a wide miscibility gap at temperatures of <1000 K and mutual solubilities of Au and Ni are very limited at lower temperatures [10]. However, it has been reported in the literature [11] that submonolayer quantities of gold on nickel single crystal tend to form a surface alloy. (ii) There is a controversy concerning the growth mode of the ultrathin gold overlayer on Ni. The FM growth mode [12–15], the Stranski–Krastanov (SK) growth model (Au islands grown on the first Au layer(s)) [15,16] and, finally, the VW growth mode [9,15,17,18] have been reported. (iii) Electron scattering properties of gold and nickel are very different. Particularly, differential crosssections for elastic scattering of both elements
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are complicated functions of the scattering angle [5,9]. Due to both, the experimental geometry used in this work and low kinetic energy of electrons, pronounced changes in elastic peak intensity are expected, even for a very small amount of gold deposited on the nickel surface.
2. Experimental In order to produce a smooth initial surface, the 50 nm thick Ni layer was grown by vacuum deposition at room temperature on a Si(1 1 1) wafer. A 15 mm 15 mm Ni/Si sample was introduced to angle-resolved photoelectron spectrometer ADES 400 (VG Scientific, UK). Prior to a gold deposition, the Ni surface was carefully sputter-cleaned in two steps: (a) by 3000 eVoxygen ion beam and then by (b) 3000 eV Ar ion beam. High purity gold was evaporated on Ni clean surface held at room temperature from an effusion cell in 17 successive evaporation steps. A mean thickness of gold films was measured by a quartz microbalance and cross checked by the Au 4d line shape analysis [9]. Moreover, the final thickness of the gold overlayer was tested ex situ by electron probe microanalyser (JEOL JXA-733) in 10 different points along a line across a sample surface. The resulting thicknesses were evaluated as the thickness averaged over 10 measurements (3:57 0:15 nm). This value agrees well with the final thickness of the gold film measured in situ by the quartz microbalance (3.79 nm). Angular-resolved X-ray induced photoelectron spectrometer applying Al Ka radiation and equipped with a hemispherical electron energy analyser operating in the constant pass energy mode of 100 or 50 eV was used to control the surface cleanness of the Ni and Au/Ni surfaces and to record the Ni 3p, Au 4f and Au 4d spectra. The spectra were recorded for each Au film on Ni substrate at two emission angles of 08 and 608 with respect to the surface normal. Prior the QUASES-Tougaard analysis [7], the Au 4d spectra were corrected for the spectrometer transmission function [19]. The elastic peak intensities were measured under fixed experimental geometry. The normal angle of incidence of electron primary beam at energy of 200 and 1000 eV was used. The electron beam current
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was varied from 1 108 to 1 107 A. The beam was defocused with a spot diameter of 5 mm at the sample surface. The hemispherical electron energy analyser was operated in the constant pass energy mode of 5 eV. The analyser axis was set to 258 with respect to the sample normal. The measured elastic peak intensity was corrected for very low inelastic background signal applying the Shirley’s procedure. The width of the elastic peak was typically below 0.5 eV (FWHM).
Table 1 Kinetic energies of the Au 4d, Au 4f and Ni 3p photoelectrons, inelastic (l), transport (ltr) and total (ltot) electron mean free path ˚ ) values used in present model calculations (A Au 4d KE (eV) 1137 l (in Au, TPP) 14.2 l (in Au, recommended) 15.2 ltr (in Au) 26.8 ltot (in Au, TPP) 9.3 ltot (in Au, recommended) 9.7
Au 4f
Ni 3p
Reference
1403 16.5 17.8 32.6 10.9 11.5
1420 16.7 [3] 18.0 [4] 33.0 [24] 11.1 11.6
3. Results 3.1. The Au 4d spectral line shape analysis by QUASES The surface morphology with nanometre depth resolution was quantified by analysis of the Au 4d peak shape and the shape of extended background behind the peak. The method relies on the fact that the energy distribution of emitted electrons depends strongly on the travelled path lengths and thereby also on the in-depth concentration profile. The theoretical framework is described by Tougaard and co-workers [20–22]. The validity of the technique has been established through systematic experimental investigations and comparison to measurements on the same samples by Rutherford backscattering spectrometry (RBS), ion scattering spectrometry (ISS) and atomic force microscopy (AFM) [23]. The corresponding spectra processing is presently facilitated by the software packed QUASES-Tougaard [7]. Analysis of the Au 4d spectral line shapes recorded after each deposition step of Au on Ni was carried out using the growth modes mentioned above: the FM, the SK, and the VW. As a parameter characterising electron transport in a surface region of the sample, the total electron mean free path defined in the next section was used. Its numerical values are summarised in Table 1 for the Au 4d, Au 4f and Ni 3p photoelectrons. Here, it should be noted that the QUASESTougaard software [7] approximates the real SK and the VW surface morphology (3D islands size distribution) by an uniform overlayer of a height H and a coverage y < 1. As a measure of consistency, we have required close simulated and measured mean thickness values of the gold deposit. No such consistency
and in the most cases also a reasonable agreement between the shape of calculated and measured spectra was obtained for the FM and the SK modes. The VW growth mode characterised by the Au coverage and the mean island height has lead to a good agreement between the measured and calculated spectra and, at the same time, to a good consistency with respect to the mean gold thickness measured in situ by the quartz microbalance method (Table 2). Agreement between the measured and calculated Au 4d spectra by using the FM and the VW growth modes of the gold on Ni is illustrated in Fig. 1. For the VW growth mode, the calculated and measured spectra agree well, while for the FM mode the difference due to unrealistic model is clearly discerned in the Au 4d line itself and in the background as well. Fig. 2 displays results of the VW growth model simulations applied to all steps of the gold deposited on nickel. A relative uncovered area of Ni surface is plotted against the gold mean thickness for 08 and 608 emission angles of Au 4d photoelectrons. Both fractions decrease quickly with the increase in thickness. ˚ for The uncovered area of Ni reaches 33% at 10 A ˚. emission angles of 08 and approaches 0% at 29 A Similarly, for emission angle of 608, the uncovered ˚ and approaches 0% at area of Ni reaches 20% at 10 A ˚ . The difference arises due to neglecting elastic 19 A scattering of photoelectrons in the QUASES calculations. The elastic scattering effects of photoelectrons may become pronounced at large emission angles [25]. The structure of the gold overlayer on nickel resulting from Au 4f spectra processing with QUASES-Tougaard software, expressed as a gold islands coverage
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Table 2 The structure of the gold overlayer resulting from Au 4d spectra processing with the QUASES-Tougaard softwarea Evaporation step no.
Au island coverage (%)
Au island ˚) height (A
Mean thickness ˚) QUASES (A
Mean thickness ˚) QCM (A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
9.0 14.9 30.6 30.4 41.9 47.1 49.7 60.6 67.0 77.6 80.2 83.6 85.4 91.9 99.3 100.0 100.0
15 15 11 11 11 12 13 13 14 14 16 17 18 22 27 33 41
1.3 2.2 3.4 3.3 4.6 5.7 6.5 7.9 9.4 10.9 12.8 14.2 15.4 20.2 26.8 33.0 41.0
0.7 1.4 2.3 3.0 3.7 4.4 5.1 6.7 8.1 9.6 11.2 12.7 14.1 18.7 23.4 28.7 37.9
a
The Au 4d spectra were recorded at the normal emission direction.
and a gold islands height, is summarised in Table 2. The resulting mean thickness values of the gold overlayer are compared with those obtained independently by the quartz microbalance. These data are used as the input data for the ratio method and the elastic electron backscattering probability calculations.
3.2. The photoelectron intensity ratio method Initial growth mode of deposited film on a substrate can also be estimated from the ratio of selected peak areas from an overlayer, Io, and a substrate material, Is. The theoretical framework of the method is described
Fig. 1. Comparison of the measured Au 4d line shape (solid line) with that calculated (open circles) using the FM growth mode model (upper spectrum) and the VW model (lower spectrum).
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where ltr is the transport mean free path taken from Jablonski [24]. We applied Au 4f and Ni 3p peak area ratios. Since the kinetic energy of these photoelectrons is almost the same, Eq. (1) can be simplified into lnðR þ 1Þ ¼
Fig. 2. Relative area of Ni substrate uncovered by gold film as revealed by QUASES-Tougaard line shape analysis of the Au 4d lines versus the mean thickness of gold measured by quartz microbalance method. Squares: emission angle of Au 4d photoelectrons 08. Open circles: emission angle 608 with respect to the surface normal.
by Fadley et al. [26]. Below, we present a brief mathematical description of the intensity ratio based on an uniform overlayer model representing the FM growth mode and for the VW growth mode approximated by an uniform overlayer with a substrate coverage y < 1. Exactly the same approximation of the real structure of the gold overlayer is used in the QUASES-Tougaard software. When neglecting electron elastic scattering processes in a surface region of the sample, the FM model is described by the following equation [26] 1 Io I 1 expðt=lo cos aÞ (1) ¼ o1 expðt=ls cos aÞ Is Is where Io1 and Is1 denote the corresponding peak areas from sufficiently thick (bulk) sample of the overlayer and the substrate, respectively, t the (mean) thickness of the overlayer, a the emission angle measured from the surface normal and l is the IMFP of photoelectrons from an overlayer, lo, or from a substrate, ls, passing through the overlayer. Since elastic scattering of photoelectrons in a surface region cannot be neglected without a substantial systematic error [24], we have included a correction of this effect into Eq. (1) by replacing the IMFPs by the total MFPs, ltot, calculated from the transport approximation [27,28] by equation below: 1 1 1 ¼ þ ltot l ltr
(2)
t ; ltot cos a
R¼
Io Is1 Is Io1
(3)
For the more complicated VW growth mode under the approximation described above, we write H 1 Io ¼ Io y 1 exp ; ðlo cos aÞ H 1 1 (4) Is ¼ Is ð1 yÞ þ Is y exp lo cos a where y is the coverage and H is the height of the overlayer. They are provided by the QUASES-Tougaard analysis and are summarised in Table 2. Similar to Eq. (3) and considering Eq. (2), we have 1 lnðR þ 1Þ ¼ ln 1 y þ y expðH=ðltot cos aÞÞ (5) As expected, for y close to 0, lnðR þ 1Þ approaches 0 and for y approaching 1, Eq. (5) transforms into Eq. (3). In Fig. 3, we compare experimental results with those of the FM and the VW model calculations. Note (i) the measured points deviate from a straight line predicted from the FM model and (ii) shapes of the measured dependence and that calculated within the VW model are similar. However, the experimental and data calculated within the both models deviate noticeably. The reasons will be discussed in detail. 3.3. Elastic electron backscattering Jablonski and Tougaard [29] and Jablonski et al. [30] have developed the MC algorithm which could be effectively used in calculations of the elastic backscattering probability for system with overlayers. The thickness dependence of the elastic peak intensity measured using an analyser with a narrow acceptance angle was very well described by theory [30]. Recently, we studied a possibility of determining the overlayer thickness from variations of the elastic backscattering probability for the Au on Ni [9]. In this contribution, we continued our search to establish an effective
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Fig. 3. Dependence of lnðR þ 1Þ versus mean thickness of a gold layer. Open circles: experimental data. Solid line: model calculations (results of the FM model, Eq. (3)). Filled circles: model calculations (the approximated VW growth mode model, Eq. (5)) R ¼ Io Is1 =Is Io1 , where Io ¼ IAu 4f and Is ¼ INi 3p .
Fig. 4. Relative elastic backscattering intensity ratios from Au/Ni system on the mean overlayer gold thickness at 200 eV primary beam electrons. Triangles: experimental data. Open circles: the MC model calculations (the FM model). Filled circles: the MC model calculations (the approximated VW growth mode model).
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Fig. 5. Relative elastic backscattering intensity ratios from Au/Ni system on the mean overlayer gold thickness at 1000 eV primary beam electrons. Triangles: experimental data. Open circles: the MC model calculations (the FM model). Filled circles: the MC model calculations (the approximated VW growth mode model).
procedure for determining the overlayer structure from the elastic peak intensity. We now used the same experimental and theoretical procedure as in [9]. For the FM and the approximated VW models described above and for H and y values resulting from the QUASES-Tougaard analysis summarised in Table 2, the results of MC calculations are shown in Figs. 4 and 5 in comparison with the measured data for kinetic energy of primary electron beam of 200 and 1000 eV. There is a good qualitative agreement between theory and experiment, only.
4. Discussion Due to a complex problem of electron transport at solid surfaces and hence mathematical difficulties, the QUASES-Tougaard analysis makes it possible to apply rather simple structural models of the overlayer which can deviate substantially from the real structure. Specifically, the QUASES-Tougaard software [7] has approximated the real SK and the VW surface morphology (3D islands size distribution) by an uniform overlayer of a height H and a coverage y < 1.
From the same reason, the elastic scattering of signal electrons has been neglected in the software, too. Its effect has, however, been at least partially corrected in this work by using the total electron mean free path instead of the corresponding IMFP, both summarised in Table 1. Our analysis using the QUASES-Tougaard software relays on expected consistency between mean gold overlayer thickness values in situ measured by a well established quartz microbalance method and those resulting from the QUASES-Tougaard software. As shown in Table 2, the QUASES-Tougaard derived thickness values are slightly but systematically higher than those from the independent method used. The difference may be caused by a simplified model calculation used. The photoelectron intensity ratio method, commonly used in photoelectron spectroscopy for overlayer thickness estimations, reveals important deviations between the experimental data and calculated values within the FM and WV models, as shown in Fig. 3. One would expect that the measured and calculated points should be close mutually for a gold mean ˚ , because (a) the Ni 3p thickness exceeding 25 A
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intensity approaches 0 and (b) the Ni substrate is, in accordance with the QUASES-Tougaard predictions of Fig. 2, in this thickness range fully covered by gold. A slope of the straight line resulting from the FM model is governed by the ltot value (Eq. (3)). We used ˚ , the value calculated for the Au 4f photoelec11.5 A trons from the recommended IMFP [4] and the corresponding transport mean free path [24]. A straight line connecting the outset and the last experimental point ˚ . So, the deviation may in Fig. 3 yields in ltot ¼ 9:2 A be caused at least partially by incomplete correction of elastic electron scattering effects and an uncertainty of the IMFP and the transport mean free path values. This conclusion is consistent with a lower slope of a straight line in Fig. 3 calculated within the FM model when the elastic electron scattering effects are neglected. In this case, ltot ¼ l (IMFP) has reached ˚ . Alternatively, the real surface structure of gold 17.8 A deposit is certainly far from the oversimplified model structure. For example, the 3D gold islands having their height larger than the ltot can also modify the Au 4f/Ni 3p peak area ratio. Results of the MC model calculations of the elastic electron backscattering probability differ quantitatively from the measured data, too. Largest discrepancy, however, is observed in the region of small film thickness at 200 eV, i.e., in the region of the initial growth of gold overlayer. Consistently to the results of the photoelectron intensity ratio method, neither the FM nor the VW growth mode results agree with the experiment quantitatively. As the electron transport in a surface region of the sample was properly accounted for in the MC calculations, the reason for quantitative disagreement we find in the oversimplified VW model where size, height and shape distributions of the gold islands were approximated by an uniform overlayer of a height H and a coverage y < 1. The agreement is much better for 1000 eV kinetic energy case shown in Fig. 5, however, this is due to larger penetration depth of primary electrons and much smaller difference in the differential elastic scattering cross-sections of gold and nickel for the scattering geometry used. As a result, the method has lost its superior surface sensitivity at a high energy region. It should be noted that a possible gold–nickel alloying mentioned above can hardly influence present results because the alloying should be limited to a
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contact monolayer at the interface. From the above three methods used in this work, only the elastic electron backscattering has a potential to reveal the gold–nickel interface alloying, due to its superior surface sensitivity. However it needs to be studied further. Similar stability of the results as for the alloying we expect for a possible influence of a surface roughness of the nickel substrate. The nickel layer was evaporated into a silicon wafer held at room temperature during the deposition. A mean surface roughness of the wafer was below 1 nm. So, the final surface roughness of the nickel surface we estimate in the range 1–10 nm. Such value has little influence on recorded photoelectron spectra and surprisingly also on elastically backscattered electron intensities tested on polished and ground silicon wafers with a mean surface roughness of 5.5 and 760 nm, respectively [31].
5. Summary and conclusions Growth mode and morphology of Au/Ni system were studied by XPS, EPES and MC calculations. Present results show that the gold overlayer on nickel is not uniform in early stages of growth. This observation is consistent with results of other authors [15,17,18]. The QUASES-Tougaard analysis excluded both the FM and SK growth modes of gold deposit and revealed that the VW growth mode is a model consistent with spectral line shapes of the Au 4d lines analysed with extended inelastic background. The surface structure parameters, the surface coverage and the mean height of gold islands, obtained from the QUASES-Tougaard analysis were used as input data into two remaining methods applied in this work. Results of the two methods showed qualitative agreement with the experimental data for the VW model of the growth mode. Differences between the photoelectron intensity ratio method predictions and the measured data are caused to some extend by using incomplete corrections for electron elastic scattering and uncertainties in the inelastic and transport mean free paths values. In the elastic electron backscattering method, where electron transport in a surface region of the sample was properly accounted for, the reason for quantitative disagreement is certainly
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in the oversimplified VW model where size, height and shape distributions of the gold islands were approximated by an uniform overlayer of a height H and a coverage y < 1.
Acknowledgements The authors would like to acknowledge the support of KBN grant 2PO3B 03918 (A.J. and B.L.) and GACR 202/02/0237 (J.Z. and P.J.). All the authors would like to acknowledge the support of the KBNCzech Ministry of Education project 50.
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