- Email: [email protected]
Modification of electrode materials for plasma torches
Surface & Coatings Technology 200 (2005) 254 – 257 www.elsevier.com/locate/surfcoat
Modification of electrode materials for plasma torches I.R. Jankova,T, R.N. Szenteb,T, I.D. Goldmana, M.N.P. Carren˜oc, M.A. Vallec, M. Behard, C.A.R. Costae, F. Galembecke, R. Landersf a Instituto de Fı´sica, Universidade de Sa˜o Paulo, Sa˜o Paulo—SP, Brazil Divisa˜o de Mecaˆnica e Eletricidade, Instituto de Pesquisas Tecnolo´gicas do Estado de Sa˜o Paulo-IPT, Sa˜o Paulo—SP, Brazil c Departamento de Sistemas Eletroˆnicos-PSI, Escola Polite´cnica, Universidade de Sa˜o Paulo, Sa˜o Paulo—SP, Brazil d Instituto de Fı´sica, Universidade Federal de Rio Grande do Sul, Porto Alegre—RGS, Brazil e Instituto de Quı´mica, Universidade Estadual de Campinas, Campinas—SP, Brazil f Instituto de Fı´sica Gleb Wataghin, Universidade Estadual de Campinas, Campinas—SP, Brazil
b
Available online 18 March 2005
Abstract As part of the studies of new materials to be used as electrodes for plasma torches, polycrystalline copper thin film substrates, obtained by depositing copper on silicon wafer using the Electron Beam technique, were implanted with low energy (20–50 keV) alkali ions. The samples, before and after implantation process, were analysed in terms of surface composition and work function changes. Although the implantation doses were low (31015 ions/cm2), relatively high concentrations of alkali metals were detected on the surface, which yielded a work function decrease of 3–9% in relation to the copper value. D 2005 Elsevier B.V. All rights reserved. PACS: 73.30.+y; 79.20.Rf; 52.75.Hn Keywords: Work function; Ion implantation; Plasma torches
1. Introduction Plasma generators or plasma torches are essential tool for various industrial processes, such as: the production of new materials, the treatment of hospital and toxic industrial wastes, refining of metals, treatment of contaminated soil, production of silicon for solar cells, and others. One of the remaining problems inhibiting the further industrial application of plasmas are: short electrode lifetimes, unreliable torch performance and lack of flexibility in torch operation, which are all direct or indirect consequences of relatively high erosion rates of electrodes.
T Corresponding authors. Roberto Nunes Szente is to be contacted at r. Acangueruc¸u 124, Sa˜o Paulo—SP, CEP: 05579-020, Brazil. Tel.: +55 11 37674479; fax: +55 11 3767 4010. Ivan R. Jankov is to be contacted at r. Pires da Mota 1132, apto. 13, Sa˜o Paulo—SP, CEP: 01529-000, Brazil. Tel.: +55 11 3271 1582; fax: +55 11 3091 6749. E-mail addresses: [email protected] (I.R. Jankov)8 [email protected] (R.N. Szente). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.015
The erosion of copper electrodes in concentric cylinder geometry with magnetically driven electric arcs, simulating a plasma torch, was studied for variety of gases and gas mixtures [1–4]. The addition of few percents of polyatomic gases (N2, O2, CO, Cl2, CH4, H2S) to inert gases (Ar or He) caused large variations in the arc velocities, arc voltage, and in the arc movement when compared with the experiments using pure inert gases. It was determined that the contamination of the surface, caused by the decomposition of the polyatomic gases by the electric arc and plasma jet, and the consequent attraction of the positive ions towards the cathode, was the cause for the observed changes. The erosion of the certain region of the cathode material is related with the electron emission current density and the period during which the arc stays in that region. In general, the current density depends on the surface temperature, on the electric field above the surface and on the work function of the surface. Higher temperature and electric field, but lower work function, result in higher current densities. A lower current density and/or lower residence time of the arc
I.R. Jankov et al. / Surface & Coatings Technology 200 (2005) 254–257
on one spot of the cathode surface, which also depends on the work function, decrease the erosion of the cathode. Experimental and theoretical method, using ion beam processing as a btoolQ for controlled modification of the surface region of polycrystalline copper in order to investigate the induced changes (particularly of the work function), had been developed by present authors [5,6] and was used for obtaining the results presented in this article. The aim of the work is to understand the main phenomena controlling the emission of electrons from a metallic surface for future use of that material as electrode in plasma torches. However, the results from these studies could be used in other applications, such as in: catalysis, tribology, oxidation, corrosion, and basic metallurgical studies.
2. Methodology Thin film copper substrates were obtained by depositing electrolytic copper on silicon wafer using the Electron Beam technique, with total thickness of approximately 1100–2300 2 (determined using a perfilometer). The thickness of the film was chosen in such a way that the ions would be completely stopped within the copper film, not reaching the silicon wafer. X-ray diffraction analysis of the substrates confirmed the polycrystalline structure of the copper films. In this work, the copper film substrates were implanted with 31015 ions/cm2 of Na, K, Rb, and Cs ions; see more details in Table 1. During the ion implantation process a specially designed and manufactured mechanical mask was positioned above the substrate. The main objective for using a mechanical mask on top of the sample undergoing an ion implantation process is to create, on the sample surface, a determined pattern of implanted and non-implanted regions, which allows the determination of the changes induced by ion implantation without the need for reference sample. The mask was designed to have rectangular-shaped holes (rectangular area of 60110 Am2 with spacing of 30 Am between them) through which the ions were able to pass and thus, the same pattern of implanted regions was created on the sample surface. The mask also allows the samples to be produced in pairs so that one sample can be analysed exclusively in terms of work function changes and the other by other techniques of interest (e.g., surface and bulk Table 1 KPFM, AES, XPS, and SRIM results for Cu implanted with Na+, K+, Rb+, Cs+ Ion Energy Current R p Work function [keV] [nA] [2] decrease [meV] Na+ K+ Rb+ Cs+ a
20 30 30 50
250 150 100 125
ND—not detected.
153 135 89 109
300F200 140F60 200F100 400F200
Alkali ion concentration [%] 0.5 (AES)
2.4 (XPS) 4.4 NDa 5.7
255
composition, etc.); the implanted area for the additional analyses is sufficiently large to be analysed by various techniques, i.e., 8.55.5 mm2. The mechanical mask is made of silicon and silicon oxide; more details of the method for producing the masks can be seen elsewhere [5]. The surfaces before and after implantation were analysed in terms of work function changes (Kelvin Probe Atomic Force Microscopy—KPFM) and surface composition (Auger Electron Spectroscopy—AES and X-ray Photoelectron Spectroscopy—XPS). The implantation process was simulated using Stopping and Range of Ions in Matter (SRIM) programme (version 2003.10) [7] in order to obtain, among other parameters, the ion range distribution, which can be used to estimate the concentration of implanted alkali metals on the surface. The experimental results of the changes in work function value and surface composition, due to the ion implantation process of different alkali ions, as well as theoretical value for the peak of ion range distribution, R p, are presented in the following section. The principal advantages of KPFM over most of the other techniques for work function measurement (such as: scanning tunnelling microscopy, diode method, contact potential difference method, field emission microscopy, photoelectric method, etc.) are: i) high spatial resolution; ii) the convenience in choosing the area of interest; and iii) the choice of the material to be analysed (i.e., it can be conducting or non-conducting) [8,9]. The drawback is that, if the measurements are not performed in UHV conditions, one needs to take into account the possible contamination or changes (due to the oxidation, for example) of the surface.
3. Results For each sample, eight different areas (7070 Am2) were scanned, using the KPFM equipment, with a resolution of 300300 sampling points and the work function changes were determined, by comparing the surface potential of the implanted and non-implanted regions. It was observed that, for all the samples, the implanted (rectangular-shaped) regions were darker than the non-implanted regions, indicating a decrease of the work function caused by the alkali ion implantation. In the KPFM analyses (an illustration of line analyses of topography and surface potential, for the case of Cu substrates implanted with Cs, are shown in Fig. 1), the mean values for the work function decrease within one area were obtained (area mean values). This process was repeated for all the measurements performed on one sample and the final value for one sample (sample mean value) was calculated as the mean value of all the areas; in Table 1, those sample mean values are shown. The negative sign represents a decrease of the work function of the implanted region when compared to the nonimplanted region. The errors associated with the results are the maximum deviation of the area mean values from the sample mean value. During the ion implantation process, the
256
I.R. Jankov et al. / Surface & Coatings Technology 200 (2005) 254–257
Fig. 1. (a) Topography analysis, (b) surface potential analysis. Typical line analyses of KPFM results for the Cu implanted with alkali metal.
sample surface roughness increases, which contributes to the non-uniformity of the surface concentration of implanted ions and therefore, to the increase of the oscillations in surface potential measurements. Another contribution to the errors of the surface potential results shown in Table 1 is the fact that the substrate has a polycrystalline structure, which can yield, for the case of copper, relatively large variations in the surface potential (each crystalline facet has its work function value) on the non-implanted substrate surface alone [10]. In order to avoid some artefacts which occurred during the KPFM measurements, the images in this article (Fig. 1) were treated using SPMLab software (release 4.0); for example, some levelling processes were employed and the measurement scales were altered by cutting off all the features higher than certain value, so that the contrast between implanted and non-implanted regions could be observed easier. Therefore, the line analyses shown in Fig. 1 are only illustrative. Surface compositions of the samples, determined in the AES analyses, were obtained from the peak-to-peak height of the Auger transition lines plotted in the derivative mode (taking into consideration the mean free path of the Auger electrons and detection sensitivity for each element); in the analyses, primary electron beam of 3 keV was used to irradiate samples and surface compositions were determined as a mean value of three analysis points (each analysed area of approximately 0.2 mm2). The AES spectrum analyses of implanted and non-implanted regions showed that the
variation of the surface concentration of contaminants (O, C, N, Cl, S) was within experimental error (i.e., there was no contamination due to either the ion implantation process or the use of the mechanical mask). Since KPFM technique determines only the relative values (i.e., the changes between the implanted and non-implanted regions), the surface composition results in this article (Table 1) were presented as percent (%) of alkali atom (i.e., as if only copper and alkali atoms were present on the sample surface). Rb was not detected on the sample probably due to the screening of its characteristic peak by (much larger) Cu peak. The sample implanted with Na was also analysed with XPS technique (see Table 1); in XPS analysis, approximately 40 mm2 of the sample was irradiated with X-ray of 1486.6 eV (Al source) and the quantity of different elements was obtained from their respective peak areas, taking into consideration the mean free path of characteristic electrons and analyser resolution. In Table 1, the theoretical values, obtained from the SRIM simulations, for the position (i.e., the distance in 2 from the surface) of peak of ion range distribution, R p, are presented for all the systems studied in this work. After analysing the surface of the sample implanted with Cs, the Ar beam (acceleration voltage 1 keV, angle from the surface normal 108, dose 1.81015 ions/cm2) was used to sputter approximately 6 2 of the sample (the amount of removed material was estimated using theoretical sputtering yield value (obtained from SRIM) for this system of
I.R. Jankov et al. / Surface & Coatings Technology 200 (2005) 254–257
approximately 3 atoms/ion), which caused a decrease in Cs concentration of 77%.
4. Discussion In the surface composition analyses, the analysed volume is determined by the mean free path of characteristic electrons, which are, for the case of AES analysis, approximately 15 2, 7 2, 18 2, and 11 2 for Na, K, Rb, and Cs, respectively; for the XPS analysis, the Na was detected in the first 9 2. Comparing the results for the surface composition of sample implanted with Na, obtained by AES and XPS, one can observe that more Na was detected when smaller sample volume was analysed (2.4%, when 9 2 was analysed (with XPS) and 0.5%, when 15 2 was analysed (with AES); see Table 1), indicating that the Na was concentrated on the surface, dropping very fast towards the bulk of the sample. Similar conclusion can be reached for the case of Cs, where removal of approximately 2 monolayers of copper substrate decreased the Cs concentration for 77% (due to the fact that very small amount of removed substrate material decreased considerably the Cs concentration and, prior to the Ar sputtering, the topography of the surface was irregular (which contributes to the uncertainties of AES analyses) due to the implantation process, any detailed depth-profiling was not be feasible). According to the SRIM simulations, the maximum concentration of implanted ions (see R p in Table 1) for the studied experimental systems should not be on the surface as was observed experimentally (i.e., the concentration for the first few layers should be relatively small, increasing towards the bulk of the sample). The difference between the observed and simulation results can be explained by taking into account other processes which occur during the implantation, such as: Gibbsian Adsorption, Radiation Enhanced Diffusion, Preferential Sputtering, etc., especially when the implanted ions can be considered almost immiscible in substrate (such is the case of alkali atoms in Cu). Preliminary results showed that the heating of the Cs sample accelerates this migration process of the implanted ion specie through the vacancies produced during the implantation, which, upon prolonged exposure to the elevated temperatures, depletes the Cs from the sample. Contrary to the case of surface composition analyses, where the obtained alkali element concentration represent the average value for the first few surface layers, the surface potential (KPFM) measurements are sensitive to the condition of only the first two layers of the surface. Since the alkali element surface concentration decreases fast towards the bulk of the samples, the actual alkali concentrations in the first two layers (which influence KPFM measurements) of all the samples could be different than the values detected by AES and XPS, so the direct comparison between the results obtained by KPFM, on one hand, and AES and XPS techniques, on the other hand, is not possible.
257
Comparing the observed decrease of the work function (see Table 1), it seems that the values increase with the increase of the work function values for the pure polycrystalline alkali metals (2.36 eV, 2.29 eV, 2.26 eV, and 1.95 eV, for Na, K, Rb, and Cs, respectively [10]); the fact that Na does not follow this rule might be attributed to the higher implantation current, which can influence the migration process and thus, Na surface concentration.
5. Conclusion In spite of all the uncertainties that the non-uniformity of implanted regions and the surface potential variations on the non-implanted polycrystalline copper substrate due to the differences in work function values of different copper crystalline facets bring to the surface potential measurements, one can clearly observe that the alkali ion implantation causes relatively large work function decrease in comparison to the polycrystalline copper substrate. Since the work function value depends primarily on the state of first two layers, whereas the analysed volume in AES and XPS is larger (determined by characteristic electron mean free path), and the migration effects could influence the surface and near-surface concentration of alkali impurity, no direct correlation of surface composition and work function decrease is possible at this stage.
Acknowledgements The financial support of FAPESP for this project, grant number 99/03779-0, is greatly appreciated. I.R. Jankov, would like also to thank FAPESP for granting him a scholarship, number 00/09681-0.
References [1] R.N. Szente, R.J. Munz, M.G. Drouet, J. Phys. D: Appl. Phys. 25 (1990) 1193. [2] R.N. Szente, R.J. Munz, M.G. Drouet, Plasma Chem. Plasma Process. 12 (6) (1992) 327. [3] R.N. Szente, R.J. Munz, M.G. Drouet, Pure Appl. Chem. 64 (5) (1992) 657. [4] R.N. Szente, R.J. Munz, M.G. Drouet, J. Phys. D: Appl. Phys. 27 (1994) 1443. [5] I.R. Jankov, R.N. Szente, I.D. Goldman, M.N.P. Carren˜o, J.W. Swart, R. Landers, Br. J. Phys. 31 (4) (2001) 552. [6] I.R. Jankov, R.N. Szente, I.D. Goldman, Vacuum 65 (2002) 547. [7] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [8] M. Nonnenmacher, M.P. O’Boyle, H.K. Wickramasinghe, Appl. Phys. Lett. 58 (25) (1991) 2921. [9] Y. Martin, C.C. Williams, H.K. Wickramasinghe, J. Appl. Phys. 61 (10) (1987) 4723. [10] CRC handbook, 78th ed.,1997–1998.
Modification of electrode materials for plasma torches I.R. Jankova,T, R.N. Szenteb,T, I.D. Goldmana, M.N.P. Carren˜oc, M.A. Vallec, M. Behard, C.A.R. Costae, F. Galembecke, R. Landersf a Instituto de Fı´sica, Universidade de Sa˜o Paulo, Sa˜o Paulo—SP, Brazil Divisa˜o de Mecaˆnica e Eletricidade, Instituto de Pesquisas Tecnolo´gicas do Estado de Sa˜o Paulo-IPT, Sa˜o Paulo—SP, Brazil c Departamento de Sistemas Eletroˆnicos-PSI, Escola Polite´cnica, Universidade de Sa˜o Paulo, Sa˜o Paulo—SP, Brazil d Instituto de Fı´sica, Universidade Federal de Rio Grande do Sul, Porto Alegre—RGS, Brazil e Instituto de Quı´mica, Universidade Estadual de Campinas, Campinas—SP, Brazil f Instituto de Fı´sica Gleb Wataghin, Universidade Estadual de Campinas, Campinas—SP, Brazil
b
Available online 18 March 2005
Abstract As part of the studies of new materials to be used as electrodes for plasma torches, polycrystalline copper thin film substrates, obtained by depositing copper on silicon wafer using the Electron Beam technique, were implanted with low energy (20–50 keV) alkali ions. The samples, before and after implantation process, were analysed in terms of surface composition and work function changes. Although the implantation doses were low (31015 ions/cm2), relatively high concentrations of alkali metals were detected on the surface, which yielded a work function decrease of 3–9% in relation to the copper value. D 2005 Elsevier B.V. All rights reserved. PACS: 73.30.+y; 79.20.Rf; 52.75.Hn Keywords: Work function; Ion implantation; Plasma torches
1. Introduction Plasma generators or plasma torches are essential tool for various industrial processes, such as: the production of new materials, the treatment of hospital and toxic industrial wastes, refining of metals, treatment of contaminated soil, production of silicon for solar cells, and others. One of the remaining problems inhibiting the further industrial application of plasmas are: short electrode lifetimes, unreliable torch performance and lack of flexibility in torch operation, which are all direct or indirect consequences of relatively high erosion rates of electrodes.
T Corresponding authors. Roberto Nunes Szente is to be contacted at r. Acangueruc¸u 124, Sa˜o Paulo—SP, CEP: 05579-020, Brazil. Tel.: +55 11 37674479; fax: +55 11 3767 4010. Ivan R. Jankov is to be contacted at r. Pires da Mota 1132, apto. 13, Sa˜o Paulo—SP, CEP: 01529-000, Brazil. Tel.: +55 11 3271 1582; fax: +55 11 3091 6749. E-mail addresses: [email protected] (I.R. Jankov)8 [email protected] (R.N. Szente). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.015
The erosion of copper electrodes in concentric cylinder geometry with magnetically driven electric arcs, simulating a plasma torch, was studied for variety of gases and gas mixtures [1–4]. The addition of few percents of polyatomic gases (N2, O2, CO, Cl2, CH4, H2S) to inert gases (Ar or He) caused large variations in the arc velocities, arc voltage, and in the arc movement when compared with the experiments using pure inert gases. It was determined that the contamination of the surface, caused by the decomposition of the polyatomic gases by the electric arc and plasma jet, and the consequent attraction of the positive ions towards the cathode, was the cause for the observed changes. The erosion of the certain region of the cathode material is related with the electron emission current density and the period during which the arc stays in that region. In general, the current density depends on the surface temperature, on the electric field above the surface and on the work function of the surface. Higher temperature and electric field, but lower work function, result in higher current densities. A lower current density and/or lower residence time of the arc
I.R. Jankov et al. / Surface & Coatings Technology 200 (2005) 254–257
on one spot of the cathode surface, which also depends on the work function, decrease the erosion of the cathode. Experimental and theoretical method, using ion beam processing as a btoolQ for controlled modification of the surface region of polycrystalline copper in order to investigate the induced changes (particularly of the work function), had been developed by present authors [5,6] and was used for obtaining the results presented in this article. The aim of the work is to understand the main phenomena controlling the emission of electrons from a metallic surface for future use of that material as electrode in plasma torches. However, the results from these studies could be used in other applications, such as in: catalysis, tribology, oxidation, corrosion, and basic metallurgical studies.
2. Methodology Thin film copper substrates were obtained by depositing electrolytic copper on silicon wafer using the Electron Beam technique, with total thickness of approximately 1100–2300 2 (determined using a perfilometer). The thickness of the film was chosen in such a way that the ions would be completely stopped within the copper film, not reaching the silicon wafer. X-ray diffraction analysis of the substrates confirmed the polycrystalline structure of the copper films. In this work, the copper film substrates were implanted with 31015 ions/cm2 of Na, K, Rb, and Cs ions; see more details in Table 1. During the ion implantation process a specially designed and manufactured mechanical mask was positioned above the substrate. The main objective for using a mechanical mask on top of the sample undergoing an ion implantation process is to create, on the sample surface, a determined pattern of implanted and non-implanted regions, which allows the determination of the changes induced by ion implantation without the need for reference sample. The mask was designed to have rectangular-shaped holes (rectangular area of 60110 Am2 with spacing of 30 Am between them) through which the ions were able to pass and thus, the same pattern of implanted regions was created on the sample surface. The mask also allows the samples to be produced in pairs so that one sample can be analysed exclusively in terms of work function changes and the other by other techniques of interest (e.g., surface and bulk Table 1 KPFM, AES, XPS, and SRIM results for Cu implanted with Na+, K+, Rb+, Cs+ Ion Energy Current R p Work function [keV] [nA] [2] decrease [meV] Na+ K+ Rb+ Cs+ a
20 30 30 50
250 150 100 125
ND—not detected.
153 135 89 109
300F200 140F60 200F100 400F200
Alkali ion concentration [%] 0.5 (AES)
2.4 (XPS) 4.4 NDa 5.7
255
composition, etc.); the implanted area for the additional analyses is sufficiently large to be analysed by various techniques, i.e., 8.55.5 mm2. The mechanical mask is made of silicon and silicon oxide; more details of the method for producing the masks can be seen elsewhere [5]. The surfaces before and after implantation were analysed in terms of work function changes (Kelvin Probe Atomic Force Microscopy—KPFM) and surface composition (Auger Electron Spectroscopy—AES and X-ray Photoelectron Spectroscopy—XPS). The implantation process was simulated using Stopping and Range of Ions in Matter (SRIM) programme (version 2003.10) [7] in order to obtain, among other parameters, the ion range distribution, which can be used to estimate the concentration of implanted alkali metals on the surface. The experimental results of the changes in work function value and surface composition, due to the ion implantation process of different alkali ions, as well as theoretical value for the peak of ion range distribution, R p, are presented in the following section. The principal advantages of KPFM over most of the other techniques for work function measurement (such as: scanning tunnelling microscopy, diode method, contact potential difference method, field emission microscopy, photoelectric method, etc.) are: i) high spatial resolution; ii) the convenience in choosing the area of interest; and iii) the choice of the material to be analysed (i.e., it can be conducting or non-conducting) [8,9]. The drawback is that, if the measurements are not performed in UHV conditions, one needs to take into account the possible contamination or changes (due to the oxidation, for example) of the surface.
3. Results For each sample, eight different areas (7070 Am2) were scanned, using the KPFM equipment, with a resolution of 300300 sampling points and the work function changes were determined, by comparing the surface potential of the implanted and non-implanted regions. It was observed that, for all the samples, the implanted (rectangular-shaped) regions were darker than the non-implanted regions, indicating a decrease of the work function caused by the alkali ion implantation. In the KPFM analyses (an illustration of line analyses of topography and surface potential, for the case of Cu substrates implanted with Cs, are shown in Fig. 1), the mean values for the work function decrease within one area were obtained (area mean values). This process was repeated for all the measurements performed on one sample and the final value for one sample (sample mean value) was calculated as the mean value of all the areas; in Table 1, those sample mean values are shown. The negative sign represents a decrease of the work function of the implanted region when compared to the nonimplanted region. The errors associated with the results are the maximum deviation of the area mean values from the sample mean value. During the ion implantation process, the
256
I.R. Jankov et al. / Surface & Coatings Technology 200 (2005) 254–257
Fig. 1. (a) Topography analysis, (b) surface potential analysis. Typical line analyses of KPFM results for the Cu implanted with alkali metal.
sample surface roughness increases, which contributes to the non-uniformity of the surface concentration of implanted ions and therefore, to the increase of the oscillations in surface potential measurements. Another contribution to the errors of the surface potential results shown in Table 1 is the fact that the substrate has a polycrystalline structure, which can yield, for the case of copper, relatively large variations in the surface potential (each crystalline facet has its work function value) on the non-implanted substrate surface alone [10]. In order to avoid some artefacts which occurred during the KPFM measurements, the images in this article (Fig. 1) were treated using SPMLab software (release 4.0); for example, some levelling processes were employed and the measurement scales were altered by cutting off all the features higher than certain value, so that the contrast between implanted and non-implanted regions could be observed easier. Therefore, the line analyses shown in Fig. 1 are only illustrative. Surface compositions of the samples, determined in the AES analyses, were obtained from the peak-to-peak height of the Auger transition lines plotted in the derivative mode (taking into consideration the mean free path of the Auger electrons and detection sensitivity for each element); in the analyses, primary electron beam of 3 keV was used to irradiate samples and surface compositions were determined as a mean value of three analysis points (each analysed area of approximately 0.2 mm2). The AES spectrum analyses of implanted and non-implanted regions showed that the
variation of the surface concentration of contaminants (O, C, N, Cl, S) was within experimental error (i.e., there was no contamination due to either the ion implantation process or the use of the mechanical mask). Since KPFM technique determines only the relative values (i.e., the changes between the implanted and non-implanted regions), the surface composition results in this article (Table 1) were presented as percent (%) of alkali atom (i.e., as if only copper and alkali atoms were present on the sample surface). Rb was not detected on the sample probably due to the screening of its characteristic peak by (much larger) Cu peak. The sample implanted with Na was also analysed with XPS technique (see Table 1); in XPS analysis, approximately 40 mm2 of the sample was irradiated with X-ray of 1486.6 eV (Al source) and the quantity of different elements was obtained from their respective peak areas, taking into consideration the mean free path of characteristic electrons and analyser resolution. In Table 1, the theoretical values, obtained from the SRIM simulations, for the position (i.e., the distance in 2 from the surface) of peak of ion range distribution, R p, are presented for all the systems studied in this work. After analysing the surface of the sample implanted with Cs, the Ar beam (acceleration voltage 1 keV, angle from the surface normal 108, dose 1.81015 ions/cm2) was used to sputter approximately 6 2 of the sample (the amount of removed material was estimated using theoretical sputtering yield value (obtained from SRIM) for this system of
I.R. Jankov et al. / Surface & Coatings Technology 200 (2005) 254–257
approximately 3 atoms/ion), which caused a decrease in Cs concentration of 77%.
4. Discussion In the surface composition analyses, the analysed volume is determined by the mean free path of characteristic electrons, which are, for the case of AES analysis, approximately 15 2, 7 2, 18 2, and 11 2 for Na, K, Rb, and Cs, respectively; for the XPS analysis, the Na was detected in the first 9 2. Comparing the results for the surface composition of sample implanted with Na, obtained by AES and XPS, one can observe that more Na was detected when smaller sample volume was analysed (2.4%, when 9 2 was analysed (with XPS) and 0.5%, when 15 2 was analysed (with AES); see Table 1), indicating that the Na was concentrated on the surface, dropping very fast towards the bulk of the sample. Similar conclusion can be reached for the case of Cs, where removal of approximately 2 monolayers of copper substrate decreased the Cs concentration for 77% (due to the fact that very small amount of removed substrate material decreased considerably the Cs concentration and, prior to the Ar sputtering, the topography of the surface was irregular (which contributes to the uncertainties of AES analyses) due to the implantation process, any detailed depth-profiling was not be feasible). According to the SRIM simulations, the maximum concentration of implanted ions (see R p in Table 1) for the studied experimental systems should not be on the surface as was observed experimentally (i.e., the concentration for the first few layers should be relatively small, increasing towards the bulk of the sample). The difference between the observed and simulation results can be explained by taking into account other processes which occur during the implantation, such as: Gibbsian Adsorption, Radiation Enhanced Diffusion, Preferential Sputtering, etc., especially when the implanted ions can be considered almost immiscible in substrate (such is the case of alkali atoms in Cu). Preliminary results showed that the heating of the Cs sample accelerates this migration process of the implanted ion specie through the vacancies produced during the implantation, which, upon prolonged exposure to the elevated temperatures, depletes the Cs from the sample. Contrary to the case of surface composition analyses, where the obtained alkali element concentration represent the average value for the first few surface layers, the surface potential (KPFM) measurements are sensitive to the condition of only the first two layers of the surface. Since the alkali element surface concentration decreases fast towards the bulk of the samples, the actual alkali concentrations in the first two layers (which influence KPFM measurements) of all the samples could be different than the values detected by AES and XPS, so the direct comparison between the results obtained by KPFM, on one hand, and AES and XPS techniques, on the other hand, is not possible.
257
Comparing the observed decrease of the work function (see Table 1), it seems that the values increase with the increase of the work function values for the pure polycrystalline alkali metals (2.36 eV, 2.29 eV, 2.26 eV, and 1.95 eV, for Na, K, Rb, and Cs, respectively [10]); the fact that Na does not follow this rule might be attributed to the higher implantation current, which can influence the migration process and thus, Na surface concentration.
5. Conclusion In spite of all the uncertainties that the non-uniformity of implanted regions and the surface potential variations on the non-implanted polycrystalline copper substrate due to the differences in work function values of different copper crystalline facets bring to the surface potential measurements, one can clearly observe that the alkali ion implantation causes relatively large work function decrease in comparison to the polycrystalline copper substrate. Since the work function value depends primarily on the state of first two layers, whereas the analysed volume in AES and XPS is larger (determined by characteristic electron mean free path), and the migration effects could influence the surface and near-surface concentration of alkali impurity, no direct correlation of surface composition and work function decrease is possible at this stage.
Acknowledgements The financial support of FAPESP for this project, grant number 99/03779-0, is greatly appreciated. I.R. Jankov, would like also to thank FAPESP for granting him a scholarship, number 00/09681-0.
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