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An ion beam facility for HCI-based analysis and materials research
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 249 (2006) 921–923 www.elsevier.com/locate/nimb
An ion beam facility for HCI-based analysis and materials research Gu¨nter Zschornack a,*, Steffen Landgraf a, Frank Großmann b, Ulrich Kentsch b, Vladimir P. Ovsyannikov b, Mike Schmidt b, Falk Ullmann b a
Technische Universita¨t Dresden, Institut fu¨r Angewandte Physik, Mommsenstr 13, D-01062 Dresden, Germany b Leybold Vacuum Dresden GmbH, Zur Wetterwarte 50, D-01109 Dresden, Germany Available online 11 May 2006
Abstract Highly charged ions (HCIs) are a promising tool for the production of structures at a nanometer length scale as well as for surface analysis. We present a room-temperature EBIT (Electron Beam Ion Trap) that produces ions such as Ar18+, Xe44+ and Ir67+. In order to study the physics of the interaction processes, a new ion beam facility has been designed. The HCIs can be separated according to their mass to charge ratio with acceleration, but also with deceleration providing projectiles with kinetic energies ranging from 10 eV times q to 40 keV times q. A beam spot size of some micrometers can be achieved using suitable apertures. The beam can also be swept over an area of about 1 cm2. 2006 Elsevier B.V. All rights reserved. PACS: 07.77.Ka; 29.25.Ni; 41.75.Ak; 79.20.Rf Keywords: Highly charged ions; Ion source; Ion beam line; Materials analysis; Materials modification; Nanostructures
1. Introduction HCIs have extreme physical properties providing new effects in ion–surface interactions and are thus suited for new analysis techniques. Besides the kinetic energy a large amount of potential energy Ep is stored in the ion as a result of the ionization process. The potential energy of HCIs increases with the degree of ionization. Ar18+ stores a potential energy of Ep = 14 keV, Xe44+ of Ep = 51 keV and Ir67+ of Ep = 156 keV, for instance. If the ions are decelerated the potential energy exceeds the kinetic energy. Fig. 1 gives the ratio of the kinetic energy to the stored potential energy of xenon ions in dependence on their charge state for different applied potentials. As an example, the potential energy of a Xe44+ projectile decelerated to 100 eV times q is more than 10 times higher than its kinetic energy. HCIs are very small-sized projectiles. Hydrogenlike germanium Ge31+ is one order of magnitude smaller *
Corresponding author. Tel.: +49 351 260 2212; fax: +49 351 260 3285. E-mail addresses: [email protected], G.Zschornack@ fz-rossendorf.de (G. Zschornack). 0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.164
than atomic hydrogen. Further on HCIs feature in high static electric fields (1014–1016 V/cm2) caused by their charge. These strong electric fields are the basis for the emerging field of ion–surface interactions. 2. Applications of HCIs If HCIs interact with the solid surface the deposition of potential energy leads to ultrafast intense electronic excitations. These interactions result in features on a nanometer length scale, such as blisters and craters. Interaction times are some fs, interaction areas are some 10 nm2. The power density deposited into the surface can reach up to 1014 W/cm2. While kinetic transfer is localized along a deep collision cascade the potential energy of HCIs is released within the first atomic layers of the surface. Furthermore, the stopping power in solids is very effective, e.g. 100 keV/nm for Au69+. Due to their ionic charge HCIs can be accelerated in a very compact setup, which means that large accelerator structures are not required. The acceleration at linear
922
G. Zschornack et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 921–923 Table 1 A selection of ions extracted from the Dresden EBIT Ion 10+
Ne Si8+ Si14+ P15+ Ar16+ Ar18+ Fe24+ Xe44+ Ce42+
Fig. 1. Ratio of the kinetic energy to the potential energy for Xeq+.
accelerators is proportional with q, at circular accelerators with q2. There are increasingly sophisticated demands for highly sensitive materials characterisation at high spatial resolution especially in semiconductor industry. The unique properties of HCIs mentioned above result in the following crucial advantages that has been shown in earlier investigations: • • • •
large secondary ion yield [1], large secondary electron yield [2], high ionization probability of secondary emission [3], high molecular ion yield [4].
Ions/s
Ions/pulse
I/nA (pulse)
240,000 7,800,000 54,000 60,000 1,800,000 7000 8000 6000 16,000
120,000 156,000 54,000 120,000 1,000,000 7000 16,000 18,000 16,000
96 100 60 144 1280 10 31 63 54
As the ions are produced by successive electron impact a certain time is needed to get a desired charge state. Hence the source is operated in pulse mode. The total ion output depends on the pulse repetition rate that is in the order of 1–100 pulses per second. One pulse can contain 105–108 ions according to the ion charge state. While gases are supplied via a high precision leak valve metals are applied from volatile organo-metallic compounds. The Dresden EBIT is able to produce bare ions up to an atomic number of 28, helium-like ions for medium and neon-like ions for heavy elements. Table 1 gives a selection of ions that have been extracted from the ion trap. The new micro-beam facility will deploy the newest Dresden EBIS model, the advanced EBIS (Dresden EBISA). The ion output of the Dresden EBISA is expected to be about 20 times of that of the Dresden EBIT. 4. Micro-beam facility
In connection with TOF-SIMS [5] excellent possibilities for sub 100 nm metrology arise. This can result in an extremely sensitive surface analysis with a up to 1000 times higher ionization probability of secondary particles. Very slow projectiles are available that do not damage deeper layers of the solid. Three-dimensional analysis is possible in combination with sputter processes. Furthermore, there is a slight dependence on matrix effects and inert projectiles such as Xe44+ are available. 3. Production of HCIs at the Dresden EBIT The ions are produced at a further development of the Dresden EBIT that has been presented elsewhere [6–10]. The Dresden EBIT operates at room temperature and is produced in small series. The ion production is based on electron impact ionization in a high-density electron beam that is compressed in a strong magnetic field. The ions are confined radially by the negative space charge of the electron beam. Axial confinement is achieved by additional electrostatic potentials of some 10 V. As the magnetic field is generated by permanent magnets the ion source features a very compact design. The Dresden EBIT is transportable, simple to operate and initial as well as maintenance costs are low. Besides its excellent long-term stability it is characterized by an excellent beam quality with a beam emittance of some mm mrad.
The micro-beam facility (Fig. 2) features the newest generation of the development of the Dresden EBIT. Besides the increase of the ion output the production of higher ion charge states is expected. The small ion beam emittance of the ion source is the precondition for the formation of a micro-beam of HCIs. A beam spot size of some lm2 is achievable using adequate diaphragms. The supply of ion currents that are high enough to use it for investigations of ion–surface interaction is supported by minimized loss at ion guiding. The ions are extracted from the trap by the application of an extraction potential of some kV which simultaneously accounts for the expansion of the electron beam and its subsequent grounding in the electron collector. The beam line employs several ion optics elements that help to guide the ion beam. The ion charge states are separated according to its charge to mass ratio in a 90 double focusing analysing magnet. The beam line can be put on both a negative potential and a positive potential of up to 20 kV. As the ions are produced at a positive potential relative to the beam line potential the trap can be put on nearly ground potential for the production of slow HCIs. Before the ions hit the target they can be decelerated down to 10 eV times q at a two step deceleration lens system. Therewith HCIs are available whose potential energy exceeds their kinetic
G. Zschornack et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 921–923
923
Fig. 2. Micro-beam facility for the investigation of HCIs – solid surface interactions.
energy. The ions can also start on a higher positive potential of up to 40 kV with the kinetic energy of the ions scaling with the charge state. The equipment of the target chamber allows a wide spectrum of investigations of the interactions of HCIs with solid surfaces. 5. Conclusions A beam line is presented that will supply a micro-beam of HCIs. HCIs promise to be a new access to nanostructuring as well as nanometrology. The ion source, an EBIT device which is commercially available from the Leybold Vacuum Dresden GmbH [11], supplies HCIs with a beam of a very small emittance. Operating at room temperature the ion source features a lot of advantages. The beam line is designed to investigate the interaction of HCIs with solid surfaces at an energy range of some 10 eV times q up to 40 keV times q. Acknowledgements The work is supported by the EFRE fund of the EU and by the Freistaat Sachsen (Projects 8945/1450 and 8947/ 1450).
References [1] T. Schenkel, A.V. Hamza, A.V. Barnes, D.H. Schneider, J.C. Banks, B.L. Doyle, Phys. Rev. Lett. 81 (1998) 2590. [2] H. Kurz, F. Aumayr, HP. Winter, D. Schneider, M.A. Briere, J.W. McDonald, Phys. Rev. A 49 (1994) 4693. [3] M. Sporn, G. Libiseller, T. Neidhart, M. Schmid, F. Aumayr, HP. Winter, P. Varga, M. Grether, N. Stolterfoht, Phys. Rev. Lett. 79 (1997) 945. [4] J.W. McDonald, A.V. Hamza, M.W. Newman, J.P. Holder, D.H. Schneider, T. Schenkel, Ultramicroscopy 101 (2004) 225. [5] T. Schenkel, A.V. Hamza, A.V. Barnes, M.W. Newman, G. Machicoane, T. Niedermayer, M. Hattass, J.W. McDonald, D.H. Schneider, K.J. Wu, R.W. Odom, Phys. Scr. T80 (1999) 73. [6] G. Zschornack, V.P. Ovsyannikov, Rev. Sci. Instr. 70 (1999) 2646. [7] U. Kentsch, S. Landgraf, G. Zschornack, F. Großmann, V.P. Ovsyannikov, F. Ullmann, Rev. Sci. Instr. 73 (2002) 660. [8] U. Kentsch, S. Landgraf, M. Schmidt, G. Zschornack, F. Großmann, V.P. Ovsyannikov, F. Ullmann, Nucl. Instr. and Meth. B 205 (2003) 260. [9] U. Kentsch, S. Landgraf, M. Schmidt, H. Tyrroff, G. Zschornack, F. Großmann, V.P. Ovsyannikov, F. Ullmann, Nucl. Instr. and Meth. B 216 (2004) 196. [10]. [11] .
Nuclear Instruments and Methods in Physics Research B 249 (2006) 921–923 www.elsevier.com/locate/nimb
An ion beam facility for HCI-based analysis and materials research Gu¨nter Zschornack a,*, Steffen Landgraf a, Frank Großmann b, Ulrich Kentsch b, Vladimir P. Ovsyannikov b, Mike Schmidt b, Falk Ullmann b a
Technische Universita¨t Dresden, Institut fu¨r Angewandte Physik, Mommsenstr 13, D-01062 Dresden, Germany b Leybold Vacuum Dresden GmbH, Zur Wetterwarte 50, D-01109 Dresden, Germany Available online 11 May 2006
Abstract Highly charged ions (HCIs) are a promising tool for the production of structures at a nanometer length scale as well as for surface analysis. We present a room-temperature EBIT (Electron Beam Ion Trap) that produces ions such as Ar18+, Xe44+ and Ir67+. In order to study the physics of the interaction processes, a new ion beam facility has been designed. The HCIs can be separated according to their mass to charge ratio with acceleration, but also with deceleration providing projectiles with kinetic energies ranging from 10 eV times q to 40 keV times q. A beam spot size of some micrometers can be achieved using suitable apertures. The beam can also be swept over an area of about 1 cm2. 2006 Elsevier B.V. All rights reserved. PACS: 07.77.Ka; 29.25.Ni; 41.75.Ak; 79.20.Rf Keywords: Highly charged ions; Ion source; Ion beam line; Materials analysis; Materials modification; Nanostructures
1. Introduction HCIs have extreme physical properties providing new effects in ion–surface interactions and are thus suited for new analysis techniques. Besides the kinetic energy a large amount of potential energy Ep is stored in the ion as a result of the ionization process. The potential energy of HCIs increases with the degree of ionization. Ar18+ stores a potential energy of Ep = 14 keV, Xe44+ of Ep = 51 keV and Ir67+ of Ep = 156 keV, for instance. If the ions are decelerated the potential energy exceeds the kinetic energy. Fig. 1 gives the ratio of the kinetic energy to the stored potential energy of xenon ions in dependence on their charge state for different applied potentials. As an example, the potential energy of a Xe44+ projectile decelerated to 100 eV times q is more than 10 times higher than its kinetic energy. HCIs are very small-sized projectiles. Hydrogenlike germanium Ge31+ is one order of magnitude smaller *
Corresponding author. Tel.: +49 351 260 2212; fax: +49 351 260 3285. E-mail addresses: [email protected], G.Zschornack@ fz-rossendorf.de (G. Zschornack). 0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.164
than atomic hydrogen. Further on HCIs feature in high static electric fields (1014–1016 V/cm2) caused by their charge. These strong electric fields are the basis for the emerging field of ion–surface interactions. 2. Applications of HCIs If HCIs interact with the solid surface the deposition of potential energy leads to ultrafast intense electronic excitations. These interactions result in features on a nanometer length scale, such as blisters and craters. Interaction times are some fs, interaction areas are some 10 nm2. The power density deposited into the surface can reach up to 1014 W/cm2. While kinetic transfer is localized along a deep collision cascade the potential energy of HCIs is released within the first atomic layers of the surface. Furthermore, the stopping power in solids is very effective, e.g. 100 keV/nm for Au69+. Due to their ionic charge HCIs can be accelerated in a very compact setup, which means that large accelerator structures are not required. The acceleration at linear
922
G. Zschornack et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 921–923 Table 1 A selection of ions extracted from the Dresden EBIT Ion 10+
Ne Si8+ Si14+ P15+ Ar16+ Ar18+ Fe24+ Xe44+ Ce42+
Fig. 1. Ratio of the kinetic energy to the potential energy for Xeq+.
accelerators is proportional with q, at circular accelerators with q2. There are increasingly sophisticated demands for highly sensitive materials characterisation at high spatial resolution especially in semiconductor industry. The unique properties of HCIs mentioned above result in the following crucial advantages that has been shown in earlier investigations: • • • •
large secondary ion yield [1], large secondary electron yield [2], high ionization probability of secondary emission [3], high molecular ion yield [4].
Ions/s
Ions/pulse
I/nA (pulse)
240,000 7,800,000 54,000 60,000 1,800,000 7000 8000 6000 16,000
120,000 156,000 54,000 120,000 1,000,000 7000 16,000 18,000 16,000
96 100 60 144 1280 10 31 63 54
As the ions are produced by successive electron impact a certain time is needed to get a desired charge state. Hence the source is operated in pulse mode. The total ion output depends on the pulse repetition rate that is in the order of 1–100 pulses per second. One pulse can contain 105–108 ions according to the ion charge state. While gases are supplied via a high precision leak valve metals are applied from volatile organo-metallic compounds. The Dresden EBIT is able to produce bare ions up to an atomic number of 28, helium-like ions for medium and neon-like ions for heavy elements. Table 1 gives a selection of ions that have been extracted from the ion trap. The new micro-beam facility will deploy the newest Dresden EBIS model, the advanced EBIS (Dresden EBISA). The ion output of the Dresden EBISA is expected to be about 20 times of that of the Dresden EBIT. 4. Micro-beam facility
In connection with TOF-SIMS [5] excellent possibilities for sub 100 nm metrology arise. This can result in an extremely sensitive surface analysis with a up to 1000 times higher ionization probability of secondary particles. Very slow projectiles are available that do not damage deeper layers of the solid. Three-dimensional analysis is possible in combination with sputter processes. Furthermore, there is a slight dependence on matrix effects and inert projectiles such as Xe44+ are available. 3. Production of HCIs at the Dresden EBIT The ions are produced at a further development of the Dresden EBIT that has been presented elsewhere [6–10]. The Dresden EBIT operates at room temperature and is produced in small series. The ion production is based on electron impact ionization in a high-density electron beam that is compressed in a strong magnetic field. The ions are confined radially by the negative space charge of the electron beam. Axial confinement is achieved by additional electrostatic potentials of some 10 V. As the magnetic field is generated by permanent magnets the ion source features a very compact design. The Dresden EBIT is transportable, simple to operate and initial as well as maintenance costs are low. Besides its excellent long-term stability it is characterized by an excellent beam quality with a beam emittance of some mm mrad.
The micro-beam facility (Fig. 2) features the newest generation of the development of the Dresden EBIT. Besides the increase of the ion output the production of higher ion charge states is expected. The small ion beam emittance of the ion source is the precondition for the formation of a micro-beam of HCIs. A beam spot size of some lm2 is achievable using adequate diaphragms. The supply of ion currents that are high enough to use it for investigations of ion–surface interaction is supported by minimized loss at ion guiding. The ions are extracted from the trap by the application of an extraction potential of some kV which simultaneously accounts for the expansion of the electron beam and its subsequent grounding in the electron collector. The beam line employs several ion optics elements that help to guide the ion beam. The ion charge states are separated according to its charge to mass ratio in a 90 double focusing analysing magnet. The beam line can be put on both a negative potential and a positive potential of up to 20 kV. As the ions are produced at a positive potential relative to the beam line potential the trap can be put on nearly ground potential for the production of slow HCIs. Before the ions hit the target they can be decelerated down to 10 eV times q at a two step deceleration lens system. Therewith HCIs are available whose potential energy exceeds their kinetic
G. Zschornack et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 921–923
923
Fig. 2. Micro-beam facility for the investigation of HCIs – solid surface interactions.
energy. The ions can also start on a higher positive potential of up to 40 kV with the kinetic energy of the ions scaling with the charge state. The equipment of the target chamber allows a wide spectrum of investigations of the interactions of HCIs with solid surfaces. 5. Conclusions A beam line is presented that will supply a micro-beam of HCIs. HCIs promise to be a new access to nanostructuring as well as nanometrology. The ion source, an EBIT device which is commercially available from the Leybold Vacuum Dresden GmbH [11], supplies HCIs with a beam of a very small emittance. Operating at room temperature the ion source features a lot of advantages. The beam line is designed to investigate the interaction of HCIs with solid surfaces at an energy range of some 10 eV times q up to 40 keV times q. Acknowledgements The work is supported by the EFRE fund of the EU and by the Freistaat Sachsen (Projects 8945/1450 and 8947/ 1450).
References [1] T. Schenkel, A.V. Hamza, A.V. Barnes, D.H. Schneider, J.C. Banks, B.L. Doyle, Phys. Rev. Lett. 81 (1998) 2590. [2] H. Kurz, F. Aumayr, HP. Winter, D. Schneider, M.A. Briere, J.W. McDonald, Phys. Rev. A 49 (1994) 4693. [3] M. Sporn, G. Libiseller, T. Neidhart, M. Schmid, F. Aumayr, HP. Winter, P. Varga, M. Grether, N. Stolterfoht, Phys. Rev. Lett. 79 (1997) 945. [4] J.W. McDonald, A.V. Hamza, M.W. Newman, J.P. Holder, D.H. Schneider, T. Schenkel, Ultramicroscopy 101 (2004) 225. [5] T. Schenkel, A.V. Hamza, A.V. Barnes, M.W. Newman, G. Machicoane, T. Niedermayer, M. Hattass, J.W. McDonald, D.H. Schneider, K.J. Wu, R.W. Odom, Phys. Scr. T80 (1999) 73. [6] G. Zschornack, V.P. Ovsyannikov, Rev. Sci. Instr. 70 (1999) 2646. [7] U. Kentsch, S. Landgraf, G. Zschornack, F. Großmann, V.P. Ovsyannikov, F. Ullmann, Rev. Sci. Instr. 73 (2002) 660. [8] U. Kentsch, S. Landgraf, M. Schmidt, G. Zschornack, F. Großmann, V.P. Ovsyannikov, F. Ullmann, Nucl. Instr. and Meth. B 205 (2003) 260. [9] U. Kentsch, S. Landgraf, M. Schmidt, H. Tyrroff, G. Zschornack, F. Großmann, V.P. Ovsyannikov, F. Ullmann, Nucl. Instr. and Meth. B 216 (2004) 196. [10]