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Storing Matter: A new quantitative and sensitive analytical technique
Applied Surface Science 255 (2008) 1498–1500
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Storing Matter: A new quantitative and sensitive analytical technique T. Wirtz *, 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
A B S T R A C T
Article history:
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 well-known 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, . . .). In order to allow the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM. This paper will provide an introduction to the Storing Matter technique and a description of the prototype. ß 2008 Elsevier B.V. All rights reserved.
Available online 13 May 2008 Keywords: Storing Matter technique Prototype instrument SIMS Quantification
1. Introduction Secondary Ion Mass Spectrometry suffers from one major drawback that gives rise to very significant problems with quantification; the ionisation 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. This phenomenon, which is known as the matrix effect, prevents SIMS from becoming an easy tool for quantitative analysis. The Storing Matter Project aims at developing a new analytical technique, achieving quantification while maintaining high analysis sensitivities, for both organic and inorganic materials together with a dedicated prototype instrument. This paper will provide an introduction to the Storing Matter technique and a description of the prototype instrument. 2. Storing Matter technique 2.1. Principles The Storing Matter technique is based on an idea first proposed by G. Slodzian [1]. It consists in decoupling the sputtering of the
* Corresponding author. E-mail address: [email protected] (T. Wirtz). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.109
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, ions) emitted under the impact of these primary ions are deposited at a sub-monolayer level on a dedicated collector under UHV conditions (Fig. 2). 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 optimisation (deposition of metal films by evaporation, oxidation, . . .) 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 submonolayer level) corresponds in fact to the creation of a new welldefined matrix which is chosen with respect to the subsequent analysis parameters (elements to be analysed, analysis mode, . . .). On the one hand, 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. As a matter of fact, the subsequent analyses are performed in a same and well-defined matrix instead of different matrixes of changing and unknown composition. 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, . . .). The main
T. Wirtz, H.-N. Migeon / Applied Surface Science 255 (2008) 1498–1500
1499
UYStoMat depends on both steps of the Storing Matter technique (sputter-deposition process and analysis) and can be written as follows: UYStoMat ¼ g UY
Fig. 1. The Storing Matter technique consists of two different processes: a deposition process by sputtering the sample and the analysis of the collector.
(1)
g is a factor depending on the sputter-deposition process (ratio of sputtered particles passing through the aperture in front of the collector with respect to the total number of sputtered particles, sticking efficiency on the collector). UY is the useful yield of the analysis of the collector. The factor g is maximised by well positioning the collector and its aperture with respect to the angular distribution of the sputtered particles [2], which depends on the ion bombardment conditions (impact energy and angle of incidence) and on the sample. High values of UY are achieved, as already discussed, by optimising the surface of the collector and thus increasing the ionisation efficiency during the SIMS analysis of the collector. As a result, high values of UYStoMat can be obtained [3]. 3. Prototype instrument
Fig. 2. Schematic drawing of the sputtering-deposition process used in the Storing Matter technique. The rotating collector allows to obtain very diluted deposits (submonolayer regime) and to deposit the information coming from different layers at different locations (in-depth information translated into lateral information).
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. Typical examples for such an optimisation of the collector surface are:
In order to allow the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM (Figs. 3 and 4). Three main sections of the instrument can be distinguished: the preparation of the collectors (ion beam etching, metal deposition by means of thermal evaporation), the sputterdeposition 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 suitcase). 3.1. Preparation of the collectors
The main advantages resulting from this new technique are therefore quantification and improved sensitivities of the analyses. Furthermore, as the initial sputtering of the sample is decoupled from the analysis itself, the primary ion bombardment conditions (impact energy, incidence angle) can be freely chosen and can thus be optimised for instance for optimum depth resolution. In this context, the Storing Matter prototype has been equipped with a specially designed FLIG (Floating Low-Energy Ion Gun) allowing impact energies of the sputtering ion beam down to 200 eV.
In order to assure the accuracy and the reproducibility of the analyses, it is important to avoid pollutants or impurities that could give mass interferences with the deposit. A perfect cleanliness of the collector is thus required. The Storing Matter collectors consist in 1 in. Silicon or Germanium wafers of high purity. The first cleaning and preparation of the collectors occurs in a clean room. In this way pollution, contaminants, dust, etc. are minimised. A cleaning protocol, close to those used in microelectronics, has been defined. The collectors are introduced into the Storing Matter instrument via an airlock system or via a UHV suitcase (see Section 3.3). Once inside the instrument, the collectors are cleaned from any contaminations in a dedicated UHV chamber equipped with an Ar+ sputter gun. Unless collector surfaces of Silicon or Germanium are needed, the cleaned collectors are transferred in a next step into an MBE (Molecular Beam Epitaxy) where they are prepared (coating of their surface) under optimised conditions of cleanliness. The MBE chamber is equipped with four effusion cells, an electron beam evaporator, two quartz microbalances, RHEED (Reflection High Energy Electron Diffraction) and an RGA (Residual Gas Analyser). The working pressure in the chamber is 1010 mbar.
2.2. Useful yield
3.2. Sputter-deposition of sample material on the collector
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.
The prepared collectors are transferred under UHV to the sputter-deposition chamber, in which the sputtering of the sample and the deposition of the sputtered particles on the collector surface are performed under optimised conditions of reproduci-
Oxidised surfaces or metallic surfaces having a high work function for the enhancement of positive secondary ion emission. Metallic surfaces having a low work function for the enhancement of negative secondary ion emission. Gold or silver coated collector surfaces to produce a cationisation for organic information.
1500
T. Wirtz, H.-N. Migeon / Applied Surface Science 255 (2008) 1498–1500
Fig. 3. General layout of the Storing Matter instrument.
tron ion source and filtered by a Wien filter. The gun can be operated at energies from 200 eV to 10 keV, with beam currents up to 50 nA and spot diameters of 20 mm. The ion beam can be rasterscanned over areas of up to 1 mm 1 mm on the sample surface. A complete description of the Storing Matter FLIG will be provided in a separate paper. Further core equipments of the sputter-deposition chamber are a secondary electron detector allowing a visualization of the sputtering ion beam and two motorized high precision stages for the sample and the collector, respectively. The collector is rotating continuously during the deposition process in order to obtain the necessary sub-monolayer deposition. 3.3. UHV transfer of the collectors The collectors are transferred between the different chambers composing the Storing Matter instrument via a 5m long UHV transfer tube (Fig. 4). A UHV transfer system based on portable UHV suitcases allows the collectors to be transferred under UHV conditions – and thus without exposing the collectors to air contaminations – between the Storing Matter prototype and the analytical instruments. For this purpose, dedicated docking stations have been designed and installed on the Storing Matter Instrument and on SAM’s various analytical instruments (Cameca SC-Ultra, Cameca IMS 6f, Cameca NanoSIMS, Cation Mass Spectrometer (CMS), TOF SIMS, Auger Electron Spectroscopy (AES) and X-Ray Photoelectron Spectroscopy (XPS). The sample holders of the different instruments have been modified so that the same transferable unit can be slipped inside. 4. Conclusions
Fig. 4. The Storing Matter prototype instrument.
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. An important point in this technique are the cleanliness and optimisation (deposition of metal films by evaporation, oxidation, . . .) of the collector surface. 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 optimised sensitivities of the analyses. In order to allow the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM. References
bility and cleanliness. This chamber is in particular equipped with a novel, dedicated FLIG (Floating Low-Energy Ion Gun) developed by SAM for the Storing Matter prototype. The Storing Matter FLIG operates with Xe+ or Ar+ ion beams produced with a duoplasma-
[1] Enveloppe Soleau No. 13852 deposited by Cameca on 04/05/1998 on behalf of G. Slodzian. [2] C. Verdeil et al. in these proceedings (SIMS XVI reference: 1094). [3] P. Philipp et al. in these proceedings (SIMS XVI reference: 1092).
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Storing Matter: A new quantitative and sensitive analytical technique T. Wirtz *, 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
A B S T R A C T
Article history:
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 well-known 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, . . .). In order to allow the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM. This paper will provide an introduction to the Storing Matter technique and a description of the prototype. ß 2008 Elsevier B.V. All rights reserved.
Available online 13 May 2008 Keywords: Storing Matter technique Prototype instrument SIMS Quantification
1. Introduction Secondary Ion Mass Spectrometry suffers from one major drawback that gives rise to very significant problems with quantification; the ionisation 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. This phenomenon, which is known as the matrix effect, prevents SIMS from becoming an easy tool for quantitative analysis. The Storing Matter Project aims at developing a new analytical technique, achieving quantification while maintaining high analysis sensitivities, for both organic and inorganic materials together with a dedicated prototype instrument. This paper will provide an introduction to the Storing Matter technique and a description of the prototype instrument. 2. Storing Matter technique 2.1. Principles The Storing Matter technique is based on an idea first proposed by G. Slodzian [1]. It consists in decoupling the sputtering of the
* Corresponding author. E-mail address: [email protected] (T. Wirtz). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.109
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, ions) emitted under the impact of these primary ions are deposited at a sub-monolayer level on a dedicated collector under UHV conditions (Fig. 2). 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 optimisation (deposition of metal films by evaporation, oxidation, . . .) 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 submonolayer level) corresponds in fact to the creation of a new welldefined matrix which is chosen with respect to the subsequent analysis parameters (elements to be analysed, analysis mode, . . .). On the one hand, 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. As a matter of fact, the subsequent analyses are performed in a same and well-defined matrix instead of different matrixes of changing and unknown composition. 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, . . .). The main
T. Wirtz, H.-N. Migeon / Applied Surface Science 255 (2008) 1498–1500
1499
UYStoMat depends on both steps of the Storing Matter technique (sputter-deposition process and analysis) and can be written as follows: UYStoMat ¼ g UY
Fig. 1. The Storing Matter technique consists of two different processes: a deposition process by sputtering the sample and the analysis of the collector.
(1)
g is a factor depending on the sputter-deposition process (ratio of sputtered particles passing through the aperture in front of the collector with respect to the total number of sputtered particles, sticking efficiency on the collector). UY is the useful yield of the analysis of the collector. The factor g is maximised by well positioning the collector and its aperture with respect to the angular distribution of the sputtered particles [2], which depends on the ion bombardment conditions (impact energy and angle of incidence) and on the sample. High values of UY are achieved, as already discussed, by optimising the surface of the collector and thus increasing the ionisation efficiency during the SIMS analysis of the collector. As a result, high values of UYStoMat can be obtained [3]. 3. Prototype instrument
Fig. 2. Schematic drawing of the sputtering-deposition process used in the Storing Matter technique. The rotating collector allows to obtain very diluted deposits (submonolayer regime) and to deposit the information coming from different layers at different locations (in-depth information translated into lateral information).
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. Typical examples for such an optimisation of the collector surface are:
In order to allow the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM (Figs. 3 and 4). Three main sections of the instrument can be distinguished: the preparation of the collectors (ion beam etching, metal deposition by means of thermal evaporation), the sputterdeposition 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 suitcase). 3.1. Preparation of the collectors
The main advantages resulting from this new technique are therefore quantification and improved sensitivities of the analyses. Furthermore, as the initial sputtering of the sample is decoupled from the analysis itself, the primary ion bombardment conditions (impact energy, incidence angle) can be freely chosen and can thus be optimised for instance for optimum depth resolution. In this context, the Storing Matter prototype has been equipped with a specially designed FLIG (Floating Low-Energy Ion Gun) allowing impact energies of the sputtering ion beam down to 200 eV.
In order to assure the accuracy and the reproducibility of the analyses, it is important to avoid pollutants or impurities that could give mass interferences with the deposit. A perfect cleanliness of the collector is thus required. The Storing Matter collectors consist in 1 in. Silicon or Germanium wafers of high purity. The first cleaning and preparation of the collectors occurs in a clean room. In this way pollution, contaminants, dust, etc. are minimised. A cleaning protocol, close to those used in microelectronics, has been defined. The collectors are introduced into the Storing Matter instrument via an airlock system or via a UHV suitcase (see Section 3.3). Once inside the instrument, the collectors are cleaned from any contaminations in a dedicated UHV chamber equipped with an Ar+ sputter gun. Unless collector surfaces of Silicon or Germanium are needed, the cleaned collectors are transferred in a next step into an MBE (Molecular Beam Epitaxy) where they are prepared (coating of their surface) under optimised conditions of cleanliness. The MBE chamber is equipped with four effusion cells, an electron beam evaporator, two quartz microbalances, RHEED (Reflection High Energy Electron Diffraction) and an RGA (Residual Gas Analyser). The working pressure in the chamber is 1010 mbar.
2.2. Useful yield
3.2. Sputter-deposition of sample material on the collector
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.
The prepared collectors are transferred under UHV to the sputter-deposition chamber, in which the sputtering of the sample and the deposition of the sputtered particles on the collector surface are performed under optimised conditions of reproduci-
Oxidised surfaces or metallic surfaces having a high work function for the enhancement of positive secondary ion emission. Metallic surfaces having a low work function for the enhancement of negative secondary ion emission. Gold or silver coated collector surfaces to produce a cationisation for organic information.
1500
T. Wirtz, H.-N. Migeon / Applied Surface Science 255 (2008) 1498–1500
Fig. 3. General layout of the Storing Matter instrument.
tron ion source and filtered by a Wien filter. The gun can be operated at energies from 200 eV to 10 keV, with beam currents up to 50 nA and spot diameters of 20 mm. The ion beam can be rasterscanned over areas of up to 1 mm 1 mm on the sample surface. A complete description of the Storing Matter FLIG will be provided in a separate paper. Further core equipments of the sputter-deposition chamber are a secondary electron detector allowing a visualization of the sputtering ion beam and two motorized high precision stages for the sample and the collector, respectively. The collector is rotating continuously during the deposition process in order to obtain the necessary sub-monolayer deposition. 3.3. UHV transfer of the collectors The collectors are transferred between the different chambers composing the Storing Matter instrument via a 5m long UHV transfer tube (Fig. 4). A UHV transfer system based on portable UHV suitcases allows the collectors to be transferred under UHV conditions – and thus without exposing the collectors to air contaminations – between the Storing Matter prototype and the analytical instruments. For this purpose, dedicated docking stations have been designed and installed on the Storing Matter Instrument and on SAM’s various analytical instruments (Cameca SC-Ultra, Cameca IMS 6f, Cameca NanoSIMS, Cation Mass Spectrometer (CMS), TOF SIMS, Auger Electron Spectroscopy (AES) and X-Ray Photoelectron Spectroscopy (XPS). The sample holders of the different instruments have been modified so that the same transferable unit can be slipped inside. 4. Conclusions
Fig. 4. The Storing Matter prototype instrument.
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. An important point in this technique are the cleanliness and optimisation (deposition of metal films by evaporation, oxidation, . . .) of the collector surface. 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 optimised sensitivities of the analyses. In order to allow the different steps of the Storing Matter technique to be performed under optimised conditions, a dedicated prototype instrument has been developed at SAM. References
bility and cleanliness. This chamber is in particular equipped with a novel, dedicated FLIG (Floating Low-Energy Ion Gun) developed by SAM for the Storing Matter prototype. The Storing Matter FLIG operates with Xe+ or Ar+ ion beams produced with a duoplasma-
[1] Enveloppe Soleau No. 13852 deposited by Cameca on 04/05/1998 on behalf of G. Slodzian. [2] C. Verdeil et al. in these proceedings (SIMS XVI reference: 1094). [3] P. Philipp et al. in these proceedings (SIMS XVI reference: 1092).