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Characterization of iron oxide layers using Auger electron spectroscopy
Applied Surface Science 253 (2007) 3977–3981 www.elsevier.com/locate/apsusc
Characterization of iron oxide layers using Auger electron spectroscopy Milan Bizjak a,*, Anton Zalar b, Peter Panjan b, Benjamin Zorko b, Borut Pracˇek b a
Faculty of Natural Sciences and Engineering, University of Ljubljana, Slovenia b Jozˇef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia
Received 10 July 2006; received in revised form 24 August 2006; accepted 24 August 2006 Available online 16 October 2006
Abstract Metals can form several kinds of oxides. Iron forms wustite (FeO), magnetite (FeO + Fe2O3 or Fe3O4) and haematite (Fe2O3). Iron oxides, especially magnetite, are used for insulation between the lamellas of an electromotor made of electromagnetic sheet. In this work, iron oxide layers were characterized on industrial samples of electromagnetic sheet by AES depth profile analysis, and iron oxides with known chemical composition were used as reference samples, i.e. a magnetite mineral and a standard haematite reference sample. The magnetite mineral was chosen because it can be found in nature in a very pure form. The selection of reference samples was also verified on samples with an oxide layer of known composition, which were prepared by sputter deposition. The composition of the sputtered oxide layers was analysed by the weight-gain method and Rutherford backscattering without the use of standard reference materials (SRM), and the results were then compared with those obtained by AES depth profile analysis. # 2006 Elsevier B.V. All rights reserved. PACS : 81.65; 8170.Jb Keywords: Oxidation; Iron oxides; Auger electron spectroscopy; Rutherford backscattering spectrometry; Weight-gain technique
1. Introduction In electrical engineering in order to protect and insulate electromagnetic sheet, instead of lacquering, controlled preparation of iron oxide layers is becoming an established practice. The required properties of these layers depend on their thickness, chemical composition or type of iron oxide. The qualitative and quantitative composition of iron oxides can be studied by a number of analytical methods. These can provide data on the chemical composition of the oxide layers, but only some of them enable the chemical composition to be traced within the depth of the layer. For the analysis of oxide layers several methods are available and described in the literature [1–12]. Methods such as Rutherford backscattering (RBS), photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and the X-ray diffraction technique (XRD)
* Corresponding author. Tel.: +386 1 470 4500; fax: +386 1 470 4560. E-mail address: [email protected] (M. Bizjak). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.08.045
provide a large amount of data which can then be used in combination to get a more complete picture. AES depth profile analysis enables the qualitative as well as quantitative analysis of thin films and layers. For quantitative analysis, relative sensitivity factors of elements to the Auger transition are used. For pure elements these are provided by the manufacturer of the analytical equipment. This is one of the most frequently used methods of quantitative analysis, and is relatively accurate for specimens in which the elements have not formed compounds [13]. However, in our study we used a method for the determination of oxide layer composition, which based on the use of reference samples with known composition. The reference samples used were magnetite (Fe3O4) and haematite (Fe2O3). The relative sensitivity factors to O (510 eV) and Fe (703 eV) Auger electrons obtained were then used in quantitative calculation of the composition of the oxide layers deposited by sputtering, and in defining the composition of oxidized surfaces of electromagnetic sheet samples. The results obtained for quantitative AES depth profile analysis of sputtered oxides were then compared with those obtained by the weight-gain method and the RBS method.
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M. Bizjak et al. / Applied Surface Science 253 (2007) 3977–3981
2. Experimental Single- and multilayer samples of iron oxides of different chemical compositions were prepared using a Sputron (Balzers) experimental apparatus based on low-voltage plasma beam sputtering. Oxide layers were deposited on silicon wafers and polished alumina plates. During sputtering the target was subjected to a voltage of 1700–1900 V, while the current to the target ranged from 0.6 to 0.9 A. The target made from pure iron was sputtered in an atmosphere of argon and oxygen. The composition of the deposited iron oxides depended on the partial pressure of oxygen. For each single layer sample or deposition of a certain type of oxide on the multilayer sample, the oxygen partial pressure was changed appropriately. Besides sputtered iron oxide layers, samples were also prepared in laboratory conditions by oxidizing electromagnetic sheet in overheated water vapour with a minimum addition of hydrogen. The samples were oxidized for 45 min in a tube furnace at a temperature of 520 8C at an overpressure of 100 Pa. A thermocouple was attached to the sheet so that the sample temperature could be measured directly. After oxidation the sheet was cooled down continuously at a rate of 5 8C/min in argon or in air. Single- and multilayer samples of iron oxides were analysed by Rutherford backscattering of ions. The RBS spectra were taken with He ions with an energy of 1.45 MeV. The scattered ions were detected using the two-detector technique, the detectors being placed at angles of 1208 and 1508. During the measurements the sample was positioned at an angle of 258 with respect to the axis of the incident beam of helium ions impinging upon the surface. The RBS measurements were compared to a computer simulation, in which the RB code program software /6/ was used. The chemical composition of the sputtered oxide layers was also determined by the weight-gain method (WGM). For weight-gain measurements, oxide layers with unknown composition were deposited onto polished alumina substrates. As-deposited samples were inserted into a tube furnace with an oxygen flow at atmospheric pressure. The coating was completely oxidized at high temperature (700–800 8C) and the weight change due to oxidation was determined by a precise microbalance (Mettler, UM3). The composition of the samples was analysed in a PHI SAM 545A scanning Auger microprobe at a base pressure in the vacuum chamber below 1.3 10 7 Pa. A static primary electron beam of 3 keV, 0.5 mA and a diameter of about
40 mm was used. The samples were sputtered using two symmetrically inclined Perkin-Elmer–PHI Mod. 04–191 ion guns. The ion incident angle was about 478 with respect to the normal to the sample surface. The stationary samples were sputtered with 1 keV Ar+ ion beams, rastered across an area of 5 mm 5 mm. The single layer oxides with a thickness ranging from 290 to 550 nm were sputter deposited on silicon wafers. The multilayer oxides of varying composition were deposited in sequence in one direction at a pressure of 0.2 Pa and different partial pressures of oxygen, the electric power on the target being 1700 V/0.6 A. The sputtering process was carried out in an argon–oxygen mixture, while the partial pressure of oxygen was varied to ensure the desired chemical composition of iron oxide deposited on the substrate. Different oxygen proportions in the atmosphere resulted in different oxidation efficiencies. The pressure of the working gas was 0.2 Pa whereas the partial pressure of oxygen ranged from 0.5 to 1.0 10 2 Pa, depending on the desired composition of the oxide layer. The composition of the layer depends on the ratio between the number of sputtered atoms condensing on the substrate and the number of oxygen atoms incorporated in the layer. Reaction between oxygen and iron atoms can take place on the surface of the target, during the transition of sputtered atoms towards the substrate, or on the surface of the substrate. During the sputter process, the deposition parameters were chosen so that the reaction products were created on the substrate surface. The composition of a layer depends on the number of atoms of the target (iron) and the number of oxygen atoms that impinged on the substrate surface per unit of time. In general, it depends on the partial pressure of oxygen, the sputtering rate of iron, the distance between the iron target and the substrate, the substrate temperature and the energy of the sputtered target atoms. The parameters of the preparation of the oxide layers and their basic characteristics are presented in Table 1. 3. Results and discussion 3.1. RBS analysis of oxide layers The concentrations of oxygen and iron in oxide layers were measured by Rutherford backscattering spectroscopy without standard reference materials (SRM). Reliable information on the oxide layer composition was obtained by using a measuring system with two detectors [14].
Table 1 Sputter deposition parameters of iron oxide layers Sample
Layer
Temperature (8C)
Electric power on target (VA)
Partial pressure of O2 (10 3 Pa)
Deposition time (min)
Deposition rate (nm/min)
Layer thickness (nm)
A B C D E
FeO Fe3O4 Fe3O4 a Fe2O3 Fe2O3 a
140 140 150 150 160
1700 V 0.6 A 1700 V 0.6 A 1700 V 0.6 A 1700 V 0.6 A 1700 V 0.6 A
0.15 0.29 9.6 9.0 9.5
20 21 26 20 23
19.4 20.9 21.0 18.5 12.6
387 440 546 370 290
a
Oxides measured by the RBS method corresponding to magnetite and haematite.
M. Bizjak et al. / Applied Surface Science 253 (2007) 3977–3981
The composition of a single layer and the matrix of the target were determined by normalizing the two RBS spectra with respect to each other when taken for the same sample simultaneously. The atomic concentration of an element at a certain depth depends on the stopping power of the material, the number of projectiles scattered in to a given solid angle of the detector, the number of incident ions, the differential crosssection and the experimental parameters [15]. First, a spectrum obtained by the detector 1 is simulated. In this case, the number of projectiles striking the target (substrate coated with oxide layer) during a measurement is determined. The same value is used to simulated the signal obtained by the detector 2. It is undoubtedly that the parameters, which do not depend on the incident and scattering angle, remain unchanged. If the height of the simulated signal of the second spectrum does not match with the measured spectrum, a procedure is carried on by taking new values of relative concentration of the elements in the layer and dose. The first spectrum is simulated again. The procedure is repeated until both simulated spectra match with the corresponding measured spectra with the desired accuracy. The absolute quantitative composition of the layer was determined with an accuracy better than 1 at.%. Fig. 1 shows the measured and calculated RBS spectra of a sputtered single-layer oxide of wustite. The peak of the iron signal changes and depends on the oxygen content in the oxide layer, because the oxygen content may change the density of the helium ion scattering centres. With the use of suitable software, a computer simulation of the process was made. The results of the simulation are shown by the thin curve in the spectrum in Fig. 1. We can see that the spectra match one another well. In the simulation of the RBS spectrum, the input data, such as the layer composition, its thickness and the density of scattering centres in monolayer samples, were changed. The peak of the signal depends solely on oxide composition, whereas the width of the signal depends on the thickness of the layer.
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Table 2 Comparison of the measured values of O/Fe atomic ratios obtained by RBS, AES and WG methods Sample
A B C D E
Oxide
FeO Fe3O4 Fe3O4 Fe2O3 Fe2O3
O/Fe atomic ratio RBS
AES
WG
0.97 1.36 1.53 1.67 1.52
1.08 1.32 1.35 1.45 1.48
1.24 1.32 1.41 1.50 1.51
oxidation in Fe2O3 in the temperature 760 8C for 1 h in oxygen. X-ray diffraction analysis of the oxide layer confirmed only the presence of Fe2O3. Weight-gain method enables calculation of absolute values of the atomic ratio O/Fe. In an oxide layer after oxidation into the oxide Fe2O3 with a known mass, the mass and number of atoms of iron was first calculated. The difference between the mass of the oxide layer after deposition and the mass of iron yields the mass of oxygen in the layer, while from the mass of oxygen the number of oxygen atoms can be calculated. The oxide layer atomic ratios defined in this way are presented in Table 2. We found the atomic ratio between oxygen and the iron atoms in deposited oxide layers to be very close to the measured values obtained by RBS and AES. 3.3. AES depth profile analysis of oxide layers For quantitative AES depth profile analysis of deposited oxide layers it was first necessary to define the relative
3.2. Composition of oxide layers determined by the weightgain method The composition of sputtered oxide layers was defined by measuring the layer mass after deposition and after complete
Fig. 1. RBS spectra obtained with a 1.45 MeV beam on Fe2O3 single layer sample with a thickness of 290 nm.
Fig. 2. Auger spectra of reference samples of iron and magnetite.
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Fig. 3. The O/Fe concentration ratio for Fe2O3 and Fe3O4 with a known composition and sputter deposited single layer oxides.
elemental sensitivity factors for O (510 eV) and Fe (703 eV) Auger transition of electrons. Reference samples of iron, haematite and magnetite with a known composition were ion etched mildly to remove the upper contamination layer, and then at a depth of about 2 nm below the original surface, spectra were taken (Fig. 2). The sensitivity factors SO = 0.500 and SFe = 0.352 were determined from the peak-to-peak heights in the differentiated reference spectra which were not smoothed. Due to preferential removal of oxygen, which depends also on the incident ion energy, the determined sensitivity factors are valid for 1 keV Ar+ ions. These factors were used for the quantitative calculation of the composition of deposited sample and samples prepared by oxidation of electromagnetic sheet in overheated water vapour. In Fig. 3 we can compare the Fe and O concentration distribution with depth of the deposited oxide layers against haematite and magnetite reference samples. We found that the compositions of samples C and E prepared by sputter deposition correlate well with the reference samples.
Fig. 4. AES depth profile of iron oxides in a multilayered structure.
Fig. 5. AES depth profile of a sample oxidized in overheated water vapour and cooled in argon.
The applicability of AES depth profile analysis was also verified on a multilayer structure composed of iron oxide layers with different composition and on the electromagnetic sheet. The AES depth profile in Fig. 4 shows that the oxide multilayer on the silicon substrate consists of three kinds of oxides having a composition very similar to the stoichiometric composition of haematite (on the surface), magnetite (in the middle) and wustite (close to the Si substrate). Due to the high depth resolution obtained by the AES depth profile analysis, the concentration distribution of both elements is clearly seen inside the layer, as well as at the interfaces between the layers. Fig. 5 shows the AES concentration depth profile of an oxide layer formed on the electromagnetic sheet after thermal treatment in overheated water vapour and cooling in argon. The AES depth profile analysis showed that a magnetite layer is formed on the electromagnetic sheet. The transition layer that separates the magnetite from the electromagnetic sheet is
Fig. 6. AES depth profile of a sample oxidized in overheated water vapour and cooled in air.
M. Bizjak et al. / Applied Surface Science 253 (2007) 3977–3981
enriched in iron atoms. The results are in good agreement with the Bauer–Gloessner theory. At a temperature under 570 8C the only possible reaction is 3Fe + 4H2O = Fe3O4 + 4H2. Fig. 6 shows the concentration depth profile throughout the oxide layer formed on the electromagnetic sheet in the laboratory tube furnace in overheated water vapour and cooling in air at a rate of 5 8C/min. We found a double oxide layer with a thin haematite layer on the top and a magnetite layer below it. During cooling the magnetite oxidized into haematite according to the well-known chemical reaction. 4. Conclusions To determine the relative elemental sensitivity factors for O (510 eV) and Fe (703 eV) Auger electrons, samples of magnetite (Fe3O4) and haematite (Fe2O3) with a known composition were used. The factors SO = 0.500 and SFe = 0.352 obtained were used for quantitative calculation of the composition of sputter deposited single layer and multilayer oxides. The results of AES depth profile analysis of the deposited oxide layers were compared with the results obtained by the weight-gain and the RBS methods. We found quite good correlation between measurement results obtained by all three methods. AES depth profile analysis of the oxide layers formed on electromagnetic sheet using two different technological procedures showed the formation of two different oxide layers: (a) the sample cooled in
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argon had a homogenous oxide layer whose composition throughout the profile was close to the stoichiometric magnetite composition (Fe3O4) and (b) the sample cooled in air had an oxide layer composed of a thin layer in the immediate subsurface which had a composition similar to haematite (Fe2O3) and a thicker layer with a composition close to magnetite (Fe3O4). A relatively wide oxide/Fe interface was found. References [1] H.R. Stock, A. Schulz, M. Kopnarski, T. Gross, Surf. Coat. Technol. 98 (1998) 918. [2] D.G. Watson, Surf. Interf. Anal. 15 (1990) 516. [3] S. Hofmann, J. Steffen, Surf. Interf. Anal. 14 (1989) 59. [4] Y. Ishikawa, T. Yoshimura, J. Vac. Sci. Technol. 13 (1995) 1847. [5] C. Leygraf, G. Hultquist, S. Ekelund, Surf. Sci. 51 (1975) 409. [6] C. Leygraf, G. Hultquist, S. Ekelund, Surf. Sci. 46 (1974) 157. [7] C. Leygraf, G. Hultquist, Surf. Sci. 61 (1976) 69. [8] D.R. Baer, Appl. Surf. Sci. 7 (1981) 69. [9] A.G. Sault, Appl. Surf. Sci. 74 (1994) 249. [10] C.J.P. Jensen, D.F. Mitchell, M.J. Graham, Corros. Sci. 22 (1982) 1125. [11] H.J. Mathieu, D. Landolt, Corros. Sci. 26 (1986) 547. [12] A. Vesel, M. Mozeticˇ, A. Zalar, Appl. Surf. Sci. 200 (2002) 94. [13] E.R. Malinowski, D.S. Howery, Factor Analysis in Chemistry, John Wiley and Sons, New York, 1980. [14] P. Panjan, B. Navinsˇek, B. Zorko, A. Zalar, Thin Solid Films 343 (1999) 265. [15] W.K. Chu, J.W. Mayer, M.A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978.
Characterization of iron oxide layers using Auger electron spectroscopy Milan Bizjak a,*, Anton Zalar b, Peter Panjan b, Benjamin Zorko b, Borut Pracˇek b a
Faculty of Natural Sciences and Engineering, University of Ljubljana, Slovenia b Jozˇef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia
Received 10 July 2006; received in revised form 24 August 2006; accepted 24 August 2006 Available online 16 October 2006
Abstract Metals can form several kinds of oxides. Iron forms wustite (FeO), magnetite (FeO + Fe2O3 or Fe3O4) and haematite (Fe2O3). Iron oxides, especially magnetite, are used for insulation between the lamellas of an electromotor made of electromagnetic sheet. In this work, iron oxide layers were characterized on industrial samples of electromagnetic sheet by AES depth profile analysis, and iron oxides with known chemical composition were used as reference samples, i.e. a magnetite mineral and a standard haematite reference sample. The magnetite mineral was chosen because it can be found in nature in a very pure form. The selection of reference samples was also verified on samples with an oxide layer of known composition, which were prepared by sputter deposition. The composition of the sputtered oxide layers was analysed by the weight-gain method and Rutherford backscattering without the use of standard reference materials (SRM), and the results were then compared with those obtained by AES depth profile analysis. # 2006 Elsevier B.V. All rights reserved. PACS : 81.65; 8170.Jb Keywords: Oxidation; Iron oxides; Auger electron spectroscopy; Rutherford backscattering spectrometry; Weight-gain technique
1. Introduction In electrical engineering in order to protect and insulate electromagnetic sheet, instead of lacquering, controlled preparation of iron oxide layers is becoming an established practice. The required properties of these layers depend on their thickness, chemical composition or type of iron oxide. The qualitative and quantitative composition of iron oxides can be studied by a number of analytical methods. These can provide data on the chemical composition of the oxide layers, but only some of them enable the chemical composition to be traced within the depth of the layer. For the analysis of oxide layers several methods are available and described in the literature [1–12]. Methods such as Rutherford backscattering (RBS), photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and the X-ray diffraction technique (XRD)
* Corresponding author. Tel.: +386 1 470 4500; fax: +386 1 470 4560. E-mail address: [email protected] (M. Bizjak). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.08.045
provide a large amount of data which can then be used in combination to get a more complete picture. AES depth profile analysis enables the qualitative as well as quantitative analysis of thin films and layers. For quantitative analysis, relative sensitivity factors of elements to the Auger transition are used. For pure elements these are provided by the manufacturer of the analytical equipment. This is one of the most frequently used methods of quantitative analysis, and is relatively accurate for specimens in which the elements have not formed compounds [13]. However, in our study we used a method for the determination of oxide layer composition, which based on the use of reference samples with known composition. The reference samples used were magnetite (Fe3O4) and haematite (Fe2O3). The relative sensitivity factors to O (510 eV) and Fe (703 eV) Auger electrons obtained were then used in quantitative calculation of the composition of the oxide layers deposited by sputtering, and in defining the composition of oxidized surfaces of electromagnetic sheet samples. The results obtained for quantitative AES depth profile analysis of sputtered oxides were then compared with those obtained by the weight-gain method and the RBS method.
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M. Bizjak et al. / Applied Surface Science 253 (2007) 3977–3981
2. Experimental Single- and multilayer samples of iron oxides of different chemical compositions were prepared using a Sputron (Balzers) experimental apparatus based on low-voltage plasma beam sputtering. Oxide layers were deposited on silicon wafers and polished alumina plates. During sputtering the target was subjected to a voltage of 1700–1900 V, while the current to the target ranged from 0.6 to 0.9 A. The target made from pure iron was sputtered in an atmosphere of argon and oxygen. The composition of the deposited iron oxides depended on the partial pressure of oxygen. For each single layer sample or deposition of a certain type of oxide on the multilayer sample, the oxygen partial pressure was changed appropriately. Besides sputtered iron oxide layers, samples were also prepared in laboratory conditions by oxidizing electromagnetic sheet in overheated water vapour with a minimum addition of hydrogen. The samples were oxidized for 45 min in a tube furnace at a temperature of 520 8C at an overpressure of 100 Pa. A thermocouple was attached to the sheet so that the sample temperature could be measured directly. After oxidation the sheet was cooled down continuously at a rate of 5 8C/min in argon or in air. Single- and multilayer samples of iron oxides were analysed by Rutherford backscattering of ions. The RBS spectra were taken with He ions with an energy of 1.45 MeV. The scattered ions were detected using the two-detector technique, the detectors being placed at angles of 1208 and 1508. During the measurements the sample was positioned at an angle of 258 with respect to the axis of the incident beam of helium ions impinging upon the surface. The RBS measurements were compared to a computer simulation, in which the RB code program software /6/ was used. The chemical composition of the sputtered oxide layers was also determined by the weight-gain method (WGM). For weight-gain measurements, oxide layers with unknown composition were deposited onto polished alumina substrates. As-deposited samples were inserted into a tube furnace with an oxygen flow at atmospheric pressure. The coating was completely oxidized at high temperature (700–800 8C) and the weight change due to oxidation was determined by a precise microbalance (Mettler, UM3). The composition of the samples was analysed in a PHI SAM 545A scanning Auger microprobe at a base pressure in the vacuum chamber below 1.3 10 7 Pa. A static primary electron beam of 3 keV, 0.5 mA and a diameter of about
40 mm was used. The samples were sputtered using two symmetrically inclined Perkin-Elmer–PHI Mod. 04–191 ion guns. The ion incident angle was about 478 with respect to the normal to the sample surface. The stationary samples were sputtered with 1 keV Ar+ ion beams, rastered across an area of 5 mm 5 mm. The single layer oxides with a thickness ranging from 290 to 550 nm were sputter deposited on silicon wafers. The multilayer oxides of varying composition were deposited in sequence in one direction at a pressure of 0.2 Pa and different partial pressures of oxygen, the electric power on the target being 1700 V/0.6 A. The sputtering process was carried out in an argon–oxygen mixture, while the partial pressure of oxygen was varied to ensure the desired chemical composition of iron oxide deposited on the substrate. Different oxygen proportions in the atmosphere resulted in different oxidation efficiencies. The pressure of the working gas was 0.2 Pa whereas the partial pressure of oxygen ranged from 0.5 to 1.0 10 2 Pa, depending on the desired composition of the oxide layer. The composition of the layer depends on the ratio between the number of sputtered atoms condensing on the substrate and the number of oxygen atoms incorporated in the layer. Reaction between oxygen and iron atoms can take place on the surface of the target, during the transition of sputtered atoms towards the substrate, or on the surface of the substrate. During the sputter process, the deposition parameters were chosen so that the reaction products were created on the substrate surface. The composition of a layer depends on the number of atoms of the target (iron) and the number of oxygen atoms that impinged on the substrate surface per unit of time. In general, it depends on the partial pressure of oxygen, the sputtering rate of iron, the distance between the iron target and the substrate, the substrate temperature and the energy of the sputtered target atoms. The parameters of the preparation of the oxide layers and their basic characteristics are presented in Table 1. 3. Results and discussion 3.1. RBS analysis of oxide layers The concentrations of oxygen and iron in oxide layers were measured by Rutherford backscattering spectroscopy without standard reference materials (SRM). Reliable information on the oxide layer composition was obtained by using a measuring system with two detectors [14].
Table 1 Sputter deposition parameters of iron oxide layers Sample
Layer
Temperature (8C)
Electric power on target (VA)
Partial pressure of O2 (10 3 Pa)
Deposition time (min)
Deposition rate (nm/min)
Layer thickness (nm)
A B C D E
FeO Fe3O4 Fe3O4 a Fe2O3 Fe2O3 a
140 140 150 150 160
1700 V 0.6 A 1700 V 0.6 A 1700 V 0.6 A 1700 V 0.6 A 1700 V 0.6 A
0.15 0.29 9.6 9.0 9.5
20 21 26 20 23
19.4 20.9 21.0 18.5 12.6
387 440 546 370 290
a
Oxides measured by the RBS method corresponding to magnetite and haematite.
M. Bizjak et al. / Applied Surface Science 253 (2007) 3977–3981
The composition of a single layer and the matrix of the target were determined by normalizing the two RBS spectra with respect to each other when taken for the same sample simultaneously. The atomic concentration of an element at a certain depth depends on the stopping power of the material, the number of projectiles scattered in to a given solid angle of the detector, the number of incident ions, the differential crosssection and the experimental parameters [15]. First, a spectrum obtained by the detector 1 is simulated. In this case, the number of projectiles striking the target (substrate coated with oxide layer) during a measurement is determined. The same value is used to simulated the signal obtained by the detector 2. It is undoubtedly that the parameters, which do not depend on the incident and scattering angle, remain unchanged. If the height of the simulated signal of the second spectrum does not match with the measured spectrum, a procedure is carried on by taking new values of relative concentration of the elements in the layer and dose. The first spectrum is simulated again. The procedure is repeated until both simulated spectra match with the corresponding measured spectra with the desired accuracy. The absolute quantitative composition of the layer was determined with an accuracy better than 1 at.%. Fig. 1 shows the measured and calculated RBS spectra of a sputtered single-layer oxide of wustite. The peak of the iron signal changes and depends on the oxygen content in the oxide layer, because the oxygen content may change the density of the helium ion scattering centres. With the use of suitable software, a computer simulation of the process was made. The results of the simulation are shown by the thin curve in the spectrum in Fig. 1. We can see that the spectra match one another well. In the simulation of the RBS spectrum, the input data, such as the layer composition, its thickness and the density of scattering centres in monolayer samples, were changed. The peak of the signal depends solely on oxide composition, whereas the width of the signal depends on the thickness of the layer.
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Table 2 Comparison of the measured values of O/Fe atomic ratios obtained by RBS, AES and WG methods Sample
A B C D E
Oxide
FeO Fe3O4 Fe3O4 Fe2O3 Fe2O3
O/Fe atomic ratio RBS
AES
WG
0.97 1.36 1.53 1.67 1.52
1.08 1.32 1.35 1.45 1.48
1.24 1.32 1.41 1.50 1.51
oxidation in Fe2O3 in the temperature 760 8C for 1 h in oxygen. X-ray diffraction analysis of the oxide layer confirmed only the presence of Fe2O3. Weight-gain method enables calculation of absolute values of the atomic ratio O/Fe. In an oxide layer after oxidation into the oxide Fe2O3 with a known mass, the mass and number of atoms of iron was first calculated. The difference between the mass of the oxide layer after deposition and the mass of iron yields the mass of oxygen in the layer, while from the mass of oxygen the number of oxygen atoms can be calculated. The oxide layer atomic ratios defined in this way are presented in Table 2. We found the atomic ratio between oxygen and the iron atoms in deposited oxide layers to be very close to the measured values obtained by RBS and AES. 3.3. AES depth profile analysis of oxide layers For quantitative AES depth profile analysis of deposited oxide layers it was first necessary to define the relative
3.2. Composition of oxide layers determined by the weightgain method The composition of sputtered oxide layers was defined by measuring the layer mass after deposition and after complete
Fig. 1. RBS spectra obtained with a 1.45 MeV beam on Fe2O3 single layer sample with a thickness of 290 nm.
Fig. 2. Auger spectra of reference samples of iron and magnetite.
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Fig. 3. The O/Fe concentration ratio for Fe2O3 and Fe3O4 with a known composition and sputter deposited single layer oxides.
elemental sensitivity factors for O (510 eV) and Fe (703 eV) Auger transition of electrons. Reference samples of iron, haematite and magnetite with a known composition were ion etched mildly to remove the upper contamination layer, and then at a depth of about 2 nm below the original surface, spectra were taken (Fig. 2). The sensitivity factors SO = 0.500 and SFe = 0.352 were determined from the peak-to-peak heights in the differentiated reference spectra which were not smoothed. Due to preferential removal of oxygen, which depends also on the incident ion energy, the determined sensitivity factors are valid for 1 keV Ar+ ions. These factors were used for the quantitative calculation of the composition of deposited sample and samples prepared by oxidation of electromagnetic sheet in overheated water vapour. In Fig. 3 we can compare the Fe and O concentration distribution with depth of the deposited oxide layers against haematite and magnetite reference samples. We found that the compositions of samples C and E prepared by sputter deposition correlate well with the reference samples.
Fig. 4. AES depth profile of iron oxides in a multilayered structure.
Fig. 5. AES depth profile of a sample oxidized in overheated water vapour and cooled in argon.
The applicability of AES depth profile analysis was also verified on a multilayer structure composed of iron oxide layers with different composition and on the electromagnetic sheet. The AES depth profile in Fig. 4 shows that the oxide multilayer on the silicon substrate consists of three kinds of oxides having a composition very similar to the stoichiometric composition of haematite (on the surface), magnetite (in the middle) and wustite (close to the Si substrate). Due to the high depth resolution obtained by the AES depth profile analysis, the concentration distribution of both elements is clearly seen inside the layer, as well as at the interfaces between the layers. Fig. 5 shows the AES concentration depth profile of an oxide layer formed on the electromagnetic sheet after thermal treatment in overheated water vapour and cooling in argon. The AES depth profile analysis showed that a magnetite layer is formed on the electromagnetic sheet. The transition layer that separates the magnetite from the electromagnetic sheet is
Fig. 6. AES depth profile of a sample oxidized in overheated water vapour and cooled in air.
M. Bizjak et al. / Applied Surface Science 253 (2007) 3977–3981
enriched in iron atoms. The results are in good agreement with the Bauer–Gloessner theory. At a temperature under 570 8C the only possible reaction is 3Fe + 4H2O = Fe3O4 + 4H2. Fig. 6 shows the concentration depth profile throughout the oxide layer formed on the electromagnetic sheet in the laboratory tube furnace in overheated water vapour and cooling in air at a rate of 5 8C/min. We found a double oxide layer with a thin haematite layer on the top and a magnetite layer below it. During cooling the magnetite oxidized into haematite according to the well-known chemical reaction. 4. Conclusions To determine the relative elemental sensitivity factors for O (510 eV) and Fe (703 eV) Auger electrons, samples of magnetite (Fe3O4) and haematite (Fe2O3) with a known composition were used. The factors SO = 0.500 and SFe = 0.352 obtained were used for quantitative calculation of the composition of sputter deposited single layer and multilayer oxides. The results of AES depth profile analysis of the deposited oxide layers were compared with the results obtained by the weight-gain and the RBS methods. We found quite good correlation between measurement results obtained by all three methods. AES depth profile analysis of the oxide layers formed on electromagnetic sheet using two different technological procedures showed the formation of two different oxide layers: (a) the sample cooled in
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argon had a homogenous oxide layer whose composition throughout the profile was close to the stoichiometric magnetite composition (Fe3O4) and (b) the sample cooled in air had an oxide layer composed of a thin layer in the immediate subsurface which had a composition similar to haematite (Fe2O3) and a thicker layer with a composition close to magnetite (Fe3O4). A relatively wide oxide/Fe interface was found. References [1] H.R. Stock, A. Schulz, M. Kopnarski, T. Gross, Surf. Coat. Technol. 98 (1998) 918. [2] D.G. Watson, Surf. Interf. Anal. 15 (1990) 516. [3] S. Hofmann, J. Steffen, Surf. Interf. Anal. 14 (1989) 59. [4] Y. Ishikawa, T. Yoshimura, J. Vac. Sci. Technol. 13 (1995) 1847. [5] C. Leygraf, G. Hultquist, S. Ekelund, Surf. Sci. 51 (1975) 409. [6] C. Leygraf, G. Hultquist, S. Ekelund, Surf. Sci. 46 (1974) 157. [7] C. Leygraf, G. Hultquist, Surf. Sci. 61 (1976) 69. [8] D.R. Baer, Appl. Surf. Sci. 7 (1981) 69. [9] A.G. Sault, Appl. Surf. Sci. 74 (1994) 249. [10] C.J.P. Jensen, D.F. Mitchell, M.J. Graham, Corros. Sci. 22 (1982) 1125. [11] H.J. Mathieu, D. Landolt, Corros. Sci. 26 (1986) 547. [12] A. Vesel, M. Mozeticˇ, A. Zalar, Appl. Surf. Sci. 200 (2002) 94. [13] E.R. Malinowski, D.S. Howery, Factor Analysis in Chemistry, John Wiley and Sons, New York, 1980. [14] P. Panjan, B. Navinsˇek, B. Zorko, A. Zalar, Thin Solid Films 343 (1999) 265. [15] W.K. Chu, J.W. Mayer, M.A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978.