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Storing Matter: A new quantitative and sensitive analytical technique based on sputtering and collection of sample material
Nuclear Instruments and Methods in Physics Research B 267 (2009) 2586–2588
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Storing Matter: A new quantitative and sensitive analytical technique based on sputtering and collection of sample material T. Wirtz *, C. Mansilla, C. Verdeil, H.-N. Migeon Department ‘‘Science and Analysis of Materials” (SAM), Centre de Recherche Public – Gabriel Lippmann, 41 rue du Brill, L-4422 Belvaux, Luxembourg
a r t i c l e
i n f o
Article history: Available online 27 May 2009 PACS: 29.30.h 37.20.+j 65.40.gh 68.49.Sf 79.20.-m 81.70.Jb Keywords: Storing Matter technique Prototype instrument SIMS Quantification
a b s t r a c t The Storing Matter technique, which is a new analytical technique for both organic and inorganic materials, consists in decoupling the sputtering of the specimen from the subsequent analysis step. The surface of the specimen to be analysed is sputtered by means of an ion beam. The particles emitted under the impact of these primary ions are deposited at a sub-monolayer level on a dedicated collector under UHV conditions. It is only in a second step that the deposited matter is analysed in analytical instruments (mainly dynamic and static SIMS). Depositing the matter sputtered from different samples or from different layers of a sample on a same collector makes Storing Matter a powerful tool to circumvent the wellknown matrix effect in SIMS. Moreover, enhanced secondary ion emission can be obtained in the different SIMS analysis modes as the collector surface and thus the matrix is chosen with respect to the elements to be analysed and the analysis mode (M+, M, cationisation for organic information, etc.). In order to allow for the different steps of the Storing Matter technique to be performed under optimized conditions, a dedicated prototype instrument has been developed at SAM. This paper presents the Storing Matter technique and the dedicated prototype instrument with a special focus on the sputter-deposition process used in this new technique. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. The Storing Matter technique
Owing in particular to its excellent sensitivity and its good depth resolution, Secondary Ion Mass Spectrometry (SIMS) constitutes an extremely powerful technique for the analysis of surfaces and thin films [1–3]. Its main fields of application are semi-conductors, metals, glass, organic and composite materials and life sciences. Alongside all its advantages, however, the SIMS technique suffers from one major drawback: the measurements can only be quantified with difficulty. The intensity of the signals measured is generally largely dependent on the sample analysed, given that the ionization yield of a given sputtered element may vary by several orders of magnitude depending on the composition of the matrix in which it is located [4]. This phenomenon is known as the matrix effect. Storing Matter is a new analytical technique we have developed in order to achieve quantification while maintaining high analysis sensitivities, for both organic and inorganic materials. This paper presents the Storing Matter technique and the dedicated prototype instrument with a special focus on the sputterdeposition process used in this new technique.
2.1. Technique
* Corresponding author. E-mail address: [email protected] (T. Wirtz). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.044
The Storing Matter technique consists in decoupling the sputtering of the specimen from the subsequent analysis step (Fig. 1). The surface of the specimen to be analysed is sputtered by means of an ion beam. The particles (atoms, molecules and ions) emitted under the impact of these primary ions are deposited at a submonolayer level on a dedicated collector under UHV conditions. It is only in a second step that the deposited matter is analysed in analytical instruments (mainly dynamic and static SIMS). Two important points in this technique are the cleanliness and the optimization (deposition of metal films by evaporation, oxidation, etc.) of the collector surface. This preparation of the collectors combined to a very diluted deposition of matter (the collector is rotating during the deposition in order to reach a sub-monolayer level) corresponds in fact to the creation of a new well-defined matrix which is chosen with respect to the subsequent analysis parameters (elements to be analysed, analysis mode, etc.). On the one hand, depositing the matter sputtered from different samples or from different layers of a sample on a same (i.e. same matrix) collector makes Storing Matter a powerful tool to circumvent the well-known matrix effect in SIMS: the elements coming from different matrixes are deposited on a same collector. The
T. Wirtz et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2586–2588
2587
The factor c, which is dependent on the sputter-deposition process, is discussed more in detail in Section 3. UY is the useful yield of the analysis of the collector. High values of UY are achieved, as already mentioned, by optimizing the surface of the collector and thus increasing the ionisation efficiency during the SIMS analysis of the collector. Experimental data showing the dependency of UY on the collector surface are presented in [5]. 2.2. Prototype instrument Fig. 1. The Storing Matter technique consists of two different processes: a deposition process by sputtering the sample and the analysis of the collector.
subsequent analyses are now performed in a same and well-defined matrix instead of different matrixes of changing and unknown composition. A typical example of application where the Storing Matter quantification potential is of high interest is the semi-conductor industry. The use of shallow implants (implantation of dopants in the nm range) requires that dopants must be quantitatively analysed not only in the silicon wafer but also in the overlying silicon oxide layer. Here, SIMS faces its major problem: the ionization efficiency of the elements of interest changes by orders of magnitude between the SiO2 and the Si matrixes, thus preventing quantitative measurements of dopants as well as impurities. The Storing Matter technique allows for overcoming this limitation: the semi-conductor sample is sputtered in the depth-profiling mode (see Section 3 and Fig. 2 for details) and the emitted particles are deposited in a monolayer regime on a well-known collector. This collector now constitutes one single well-defined matrix for the subsequent analysis step (instead of two different matrixes, SiO2 and Si, as is the case for traditional SIMS analyses). On the other hand, enhanced secondary ion emission can be obtained in the different SIMS analysis modes as the collector surface and thus the matrix is chosen with respect to the elements to be analysed and the analysis mode (positive secondary ions, negative secondary ions, organic information, etc.). The main concept of the collector treatment is to change the chemical state of the collector surface, which modifies the matrix on which the deposition is done and influences significantly the secondary ion emission. The main advantages resulting from this new technique are therefore quantification and improved sensitivities of the analyses. The Storing Matter useful yield UYStoMat will be defined by the ratio between the total counts of a given element M detected during the analysis of the collector and the number of atoms M initially sputtered from the sample. UYStoMat depends on both steps of the Storing Matter technique (sputter-deposition process and analysis) and can be written as follows:
UY StoMat ¼ c UY:
ð1Þ
A dedicated prototype instrument developed at SAM allows the different steps of the Storing Matter technique to be performed under optimized conditions [6]. Three main sections of the instrument can be distinguished: the preparation of the collectors (cleaning of the collector by ion beam etching, definition of an appropriate matrix by means of metal deposition using thermal evaporation), the sputter-deposition of sample material on the collector (dedicated UHV chamber equipped with a novel floating low-energy ion gun) and the transfer of the collectors in the prototype itself and to the analytical instruments (UHV transfer tube and portable UHV carrier case. 3. The sputter-deposition process The surface of sample to be analysed is sputtered by ion bombardment. For this purpose, the sputter-deposition chamber is equipped with a novel, dedicated FLIG (Floating Low-Energy Ion Gun) [7]. The Storing Matter FLIG operates with Xe+ or Ar+ ion beams produced with a duoplasmatron ion source and filtered by a Wien filter. The gun can be operated at energies from 200 to 10 keV, with beam currents up to 50 nA and spot diameters of 20 lm. The ion beam can be raster-scanned (rastered) over areas of up to 1 mm 1 mm on the sample surface. The positioning and the focussing of the ion beam are performed by using secondary electron imaging. As the initial sputtering of the sample is decoupled from the analysis itself, the primary ion bombardment conditions (impact energy and incidence angle) can be freely chosen and can thus be optimized for instance for optimum depth resolution. The sputtered matter is deposited on the previously prepared and optimized collector, which consists in a 1” wafer. The collector is positioned with a high precision motorised stage (x, y and z, rotation) at a distance of 2 mm in front of the sample surface. A circular aperture mounted 100 lm in front of the collector surface limits the exposed surface of the collector to a diameter of 500 lm. The collector is rotating continuously or sequentially during the deposition process, while the sample and the limiting aperture are kept immobile. On the one hand, this rotation allows for keeping sub-monolayer deposition levels as the flux of sputtered matter
Fig. 2. (a) Schematic drawing of the sputtering-deposition process used in the Storing Matter technique. The rotating collector allows for obtaining very diluted deposits (submonolayer regime) and depositing the information coming from different layers at different locations (in-depth information translated into lateral information). (b) Photo showing the sample holder (1), the collector holder (2) with its limiting aperture (3), the nose of the FLIG (4) and the secondary electron detector (5).
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T. Wirtz et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2586–2588
the sputtered flux coming from the sample. As a matter of fact, studies have shown that the angular distributions of different elements emitted from a same sample are not necessarily identical [9]. A typical example is the angular distributions of In and P emitted under ion bombardment from an InP sample (Fig. 3). The evolution of the ratio shows an enriched In emission in the specular direction (i.e. symmetric emission relative to the normal of the surface) with respect to the incident beam. It has however to be noticed that the non-stoichiometric emission occurs only in a low-emission direction. A stoichiometric collection of matter is thus possible in the Storing Matter technique if the collection aperture is centred on the direction of maximum emission. Finally, variations of the collection efficiency c can be induced by variations of the sticking coefficient of the sputtered elements onto the collector surface. In order to study this aspect, a quantification of the deposited elements on the different substrates will be carried out in a next step by other techniques, e.g. XPS or AES. Fig. 3. Normalised ratio between the indium and the phosphorus signals in function of the emission angle and normalized intensity of the total sputtered flux. The arrow shows the direction of the incident ion beam [4].
constantly deposits on virgin collector areas. On the other hand, the rotation of the collector allows for recording depth profiles of the sample by depositing the matter sputtered from different depths of the sample at different locations and thus by transforming in-depth information into lateral information (Fig. 2). The factor c defined in (1) is dependent on the efficiency of the described sputter-deposition process. c is composed of the ratio of sputtered particles passing through the aperture in front of the collector with respect to the total number of sputtered particles and of the sticking efficiency on the collector. In this respect, a good knowledge of the angular distribution of sputtered particles, which depends on the ion bombardment conditions (impact energy and angle of incidence) and on the sample, is mandatory [8]. The factor c is maximised by well-positioning the collector and its aperture with respect to this angular distribution. In practice, the angular distribution of the sputtered material can be mapped in-situ by scanning a specially developed secondary ion detector based on an electron multiplier detector over the sample surface. Experimental results and simulations show that, depending on the angular distribution of the emitted particles, 10–40% of the sputtered particles enter an aperture of 500 lm in diameter and thus contribute to the deposit. Moreover, special care has to be taken in order to position the aperture in front of the collector on a stoichiometric region of
4. Conclusions The Storing Matter technique consists in decoupling the sputtering of the specimen from the subsequent analysis step. Atoms and molecules are sputtered from the sample to be analysed and are deposited at a sub-monolayer level on a dedicated collector under UHV conditions. Great care has to be taken to optimize the sputter-deposition process with a view to maximum useful yields and quantification (stoichiometry of the deposits). The matter deposited on the collector surface is subsequently analysed, mainly by means of dynamic and static SIMS. The main advantages resulting from this new technique are quantification and optimized sensitivities of the analyses. In order to allow for the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM. References [1] [2] [3] [4] [5] [6]
P.C. Zalm, Vacuum 45 (1994) 753. F.G. Rüdenauer, Anal. Chim. Acta 297 (1994) 197. G. Stingeder, Anal. Chim. Acta 297 (1994) 231. H.A. Storms, K.F. Brown, J.D. Stein, Anal. Chem. 49 (1977) 2023. C. Mansilla, T. Wirtz, C. Verdeil, Nucl. Instr. Meth. B 267 (2009) 2589. T. Wirtz, H.-N. Migeon, Appl. Surf. Sci. (2008) (doi:10.1016/j.apsusc. 2008.05.109). [7] T. Wirtz, C. Mansilla, R. Barrahma, C. Verdeil, Nucl. Instr. Meth. B 267 (2009) 2583. [8] C. Verdeil, T. Wirtz, H. Scherrer, Nucl. Instr. Meth. B 267 (2009) 2745. [9] C. Verdeil et al., Appl. Surf. Sci. (2008) (doi:10.1016/j.apsusc.2008.05.231).
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Storing Matter: A new quantitative and sensitive analytical technique based on sputtering and collection of sample material T. Wirtz *, C. Mansilla, C. Verdeil, H.-N. Migeon Department ‘‘Science and Analysis of Materials” (SAM), Centre de Recherche Public – Gabriel Lippmann, 41 rue du Brill, L-4422 Belvaux, Luxembourg
a r t i c l e
i n f o
Article history: Available online 27 May 2009 PACS: 29.30.h 37.20.+j 65.40.gh 68.49.Sf 79.20.-m 81.70.Jb Keywords: Storing Matter technique Prototype instrument SIMS Quantification
a b s t r a c t The Storing Matter technique, which is a new analytical technique for both organic and inorganic materials, consists in decoupling the sputtering of the specimen from the subsequent analysis step. The surface of the specimen to be analysed is sputtered by means of an ion beam. The particles emitted under the impact of these primary ions are deposited at a sub-monolayer level on a dedicated collector under UHV conditions. It is only in a second step that the deposited matter is analysed in analytical instruments (mainly dynamic and static SIMS). Depositing the matter sputtered from different samples or from different layers of a sample on a same collector makes Storing Matter a powerful tool to circumvent the wellknown matrix effect in SIMS. Moreover, enhanced secondary ion emission can be obtained in the different SIMS analysis modes as the collector surface and thus the matrix is chosen with respect to the elements to be analysed and the analysis mode (M+, M, cationisation for organic information, etc.). In order to allow for the different steps of the Storing Matter technique to be performed under optimized conditions, a dedicated prototype instrument has been developed at SAM. This paper presents the Storing Matter technique and the dedicated prototype instrument with a special focus on the sputter-deposition process used in this new technique. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. The Storing Matter technique
Owing in particular to its excellent sensitivity and its good depth resolution, Secondary Ion Mass Spectrometry (SIMS) constitutes an extremely powerful technique for the analysis of surfaces and thin films [1–3]. Its main fields of application are semi-conductors, metals, glass, organic and composite materials and life sciences. Alongside all its advantages, however, the SIMS technique suffers from one major drawback: the measurements can only be quantified with difficulty. The intensity of the signals measured is generally largely dependent on the sample analysed, given that the ionization yield of a given sputtered element may vary by several orders of magnitude depending on the composition of the matrix in which it is located [4]. This phenomenon is known as the matrix effect. Storing Matter is a new analytical technique we have developed in order to achieve quantification while maintaining high analysis sensitivities, for both organic and inorganic materials. This paper presents the Storing Matter technique and the dedicated prototype instrument with a special focus on the sputterdeposition process used in this new technique.
2.1. Technique
* Corresponding author. E-mail address: [email protected] (T. Wirtz). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.044
The Storing Matter technique consists in decoupling the sputtering of the specimen from the subsequent analysis step (Fig. 1). The surface of the specimen to be analysed is sputtered by means of an ion beam. The particles (atoms, molecules and ions) emitted under the impact of these primary ions are deposited at a submonolayer level on a dedicated collector under UHV conditions. It is only in a second step that the deposited matter is analysed in analytical instruments (mainly dynamic and static SIMS). Two important points in this technique are the cleanliness and the optimization (deposition of metal films by evaporation, oxidation, etc.) of the collector surface. This preparation of the collectors combined to a very diluted deposition of matter (the collector is rotating during the deposition in order to reach a sub-monolayer level) corresponds in fact to the creation of a new well-defined matrix which is chosen with respect to the subsequent analysis parameters (elements to be analysed, analysis mode, etc.). On the one hand, depositing the matter sputtered from different samples or from different layers of a sample on a same (i.e. same matrix) collector makes Storing Matter a powerful tool to circumvent the well-known matrix effect in SIMS: the elements coming from different matrixes are deposited on a same collector. The
T. Wirtz et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2586–2588
2587
The factor c, which is dependent on the sputter-deposition process, is discussed more in detail in Section 3. UY is the useful yield of the analysis of the collector. High values of UY are achieved, as already mentioned, by optimizing the surface of the collector and thus increasing the ionisation efficiency during the SIMS analysis of the collector. Experimental data showing the dependency of UY on the collector surface are presented in [5]. 2.2. Prototype instrument Fig. 1. The Storing Matter technique consists of two different processes: a deposition process by sputtering the sample and the analysis of the collector.
subsequent analyses are now performed in a same and well-defined matrix instead of different matrixes of changing and unknown composition. A typical example of application where the Storing Matter quantification potential is of high interest is the semi-conductor industry. The use of shallow implants (implantation of dopants in the nm range) requires that dopants must be quantitatively analysed not only in the silicon wafer but also in the overlying silicon oxide layer. Here, SIMS faces its major problem: the ionization efficiency of the elements of interest changes by orders of magnitude between the SiO2 and the Si matrixes, thus preventing quantitative measurements of dopants as well as impurities. The Storing Matter technique allows for overcoming this limitation: the semi-conductor sample is sputtered in the depth-profiling mode (see Section 3 and Fig. 2 for details) and the emitted particles are deposited in a monolayer regime on a well-known collector. This collector now constitutes one single well-defined matrix for the subsequent analysis step (instead of two different matrixes, SiO2 and Si, as is the case for traditional SIMS analyses). On the other hand, enhanced secondary ion emission can be obtained in the different SIMS analysis modes as the collector surface and thus the matrix is chosen with respect to the elements to be analysed and the analysis mode (positive secondary ions, negative secondary ions, organic information, etc.). The main concept of the collector treatment is to change the chemical state of the collector surface, which modifies the matrix on which the deposition is done and influences significantly the secondary ion emission. The main advantages resulting from this new technique are therefore quantification and improved sensitivities of the analyses. The Storing Matter useful yield UYStoMat will be defined by the ratio between the total counts of a given element M detected during the analysis of the collector and the number of atoms M initially sputtered from the sample. UYStoMat depends on both steps of the Storing Matter technique (sputter-deposition process and analysis) and can be written as follows:
UY StoMat ¼ c UY:
ð1Þ
A dedicated prototype instrument developed at SAM allows the different steps of the Storing Matter technique to be performed under optimized conditions [6]. Three main sections of the instrument can be distinguished: the preparation of the collectors (cleaning of the collector by ion beam etching, definition of an appropriate matrix by means of metal deposition using thermal evaporation), the sputter-deposition of sample material on the collector (dedicated UHV chamber equipped with a novel floating low-energy ion gun) and the transfer of the collectors in the prototype itself and to the analytical instruments (UHV transfer tube and portable UHV carrier case. 3. The sputter-deposition process The surface of sample to be analysed is sputtered by ion bombardment. For this purpose, the sputter-deposition chamber is equipped with a novel, dedicated FLIG (Floating Low-Energy Ion Gun) [7]. The Storing Matter FLIG operates with Xe+ or Ar+ ion beams produced with a duoplasmatron ion source and filtered by a Wien filter. The gun can be operated at energies from 200 to 10 keV, with beam currents up to 50 nA and spot diameters of 20 lm. The ion beam can be raster-scanned (rastered) over areas of up to 1 mm 1 mm on the sample surface. The positioning and the focussing of the ion beam are performed by using secondary electron imaging. As the initial sputtering of the sample is decoupled from the analysis itself, the primary ion bombardment conditions (impact energy and incidence angle) can be freely chosen and can thus be optimized for instance for optimum depth resolution. The sputtered matter is deposited on the previously prepared and optimized collector, which consists in a 1” wafer. The collector is positioned with a high precision motorised stage (x, y and z, rotation) at a distance of 2 mm in front of the sample surface. A circular aperture mounted 100 lm in front of the collector surface limits the exposed surface of the collector to a diameter of 500 lm. The collector is rotating continuously or sequentially during the deposition process, while the sample and the limiting aperture are kept immobile. On the one hand, this rotation allows for keeping sub-monolayer deposition levels as the flux of sputtered matter
Fig. 2. (a) Schematic drawing of the sputtering-deposition process used in the Storing Matter technique. The rotating collector allows for obtaining very diluted deposits (submonolayer regime) and depositing the information coming from different layers at different locations (in-depth information translated into lateral information). (b) Photo showing the sample holder (1), the collector holder (2) with its limiting aperture (3), the nose of the FLIG (4) and the secondary electron detector (5).
2588
T. Wirtz et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2586–2588
the sputtered flux coming from the sample. As a matter of fact, studies have shown that the angular distributions of different elements emitted from a same sample are not necessarily identical [9]. A typical example is the angular distributions of In and P emitted under ion bombardment from an InP sample (Fig. 3). The evolution of the ratio shows an enriched In emission in the specular direction (i.e. symmetric emission relative to the normal of the surface) with respect to the incident beam. It has however to be noticed that the non-stoichiometric emission occurs only in a low-emission direction. A stoichiometric collection of matter is thus possible in the Storing Matter technique if the collection aperture is centred on the direction of maximum emission. Finally, variations of the collection efficiency c can be induced by variations of the sticking coefficient of the sputtered elements onto the collector surface. In order to study this aspect, a quantification of the deposited elements on the different substrates will be carried out in a next step by other techniques, e.g. XPS or AES. Fig. 3. Normalised ratio between the indium and the phosphorus signals in function of the emission angle and normalized intensity of the total sputtered flux. The arrow shows the direction of the incident ion beam [4].
constantly deposits on virgin collector areas. On the other hand, the rotation of the collector allows for recording depth profiles of the sample by depositing the matter sputtered from different depths of the sample at different locations and thus by transforming in-depth information into lateral information (Fig. 2). The factor c defined in (1) is dependent on the efficiency of the described sputter-deposition process. c is composed of the ratio of sputtered particles passing through the aperture in front of the collector with respect to the total number of sputtered particles and of the sticking efficiency on the collector. In this respect, a good knowledge of the angular distribution of sputtered particles, which depends on the ion bombardment conditions (impact energy and angle of incidence) and on the sample, is mandatory [8]. The factor c is maximised by well-positioning the collector and its aperture with respect to this angular distribution. In practice, the angular distribution of the sputtered material can be mapped in-situ by scanning a specially developed secondary ion detector based on an electron multiplier detector over the sample surface. Experimental results and simulations show that, depending on the angular distribution of the emitted particles, 10–40% of the sputtered particles enter an aperture of 500 lm in diameter and thus contribute to the deposit. Moreover, special care has to be taken in order to position the aperture in front of the collector on a stoichiometric region of
4. Conclusions The Storing Matter technique consists in decoupling the sputtering of the specimen from the subsequent analysis step. Atoms and molecules are sputtered from the sample to be analysed and are deposited at a sub-monolayer level on a dedicated collector under UHV conditions. Great care has to be taken to optimize the sputter-deposition process with a view to maximum useful yields and quantification (stoichiometry of the deposits). The matter deposited on the collector surface is subsequently analysed, mainly by means of dynamic and static SIMS. The main advantages resulting from this new technique are quantification and optimized sensitivities of the analyses. In order to allow for the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM. References [1] [2] [3] [4] [5] [6]
P.C. Zalm, Vacuum 45 (1994) 753. F.G. Rüdenauer, Anal. Chim. Acta 297 (1994) 197. G. Stingeder, Anal. Chim. Acta 297 (1994) 231. H.A. Storms, K.F. Brown, J.D. Stein, Anal. Chem. 49 (1977) 2023. C. Mansilla, T. Wirtz, C. Verdeil, Nucl. Instr. Meth. B 267 (2009) 2589. T. Wirtz, H.-N. Migeon, Appl. Surf. Sci. (2008) (doi:10.1016/j.apsusc. 2008.05.109). [7] T. Wirtz, C. Mansilla, R. Barrahma, C. Verdeil, Nucl. Instr. Meth. B 267 (2009) 2583. [8] C. Verdeil, T. Wirtz, H. Scherrer, Nucl. Instr. Meth. B 267 (2009) 2745. [9] C. Verdeil et al., Appl. Surf. Sci. (2008) (doi:10.1016/j.apsusc.2008.05.231).