- Email: [email protected]
Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility
Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility Toncˇi Tadic´ a, Iva Bogdanovic´ Radovic´ a,⇑, Zdravko Siketic´ a, Donny Domagoj Cosic a, Natko Skukan a, Milko Jakšic´ a, Jiro Matsuo b a b
- Boškovic´ Institute, Bijenicˇka 54, HR-10000 Zagreb, Croatia Laboratory for Ion Beam Interactions, Ruder Quantum Science and Engineering Center, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: MeV SIMS Heavy ion microbeam Molecular imaging Organic samples
a b s t r a c t We describe a new Time-of-flight Secondary Ion Mass Spectrometry (TOF SIMS) setup for MeV SIMS - Boškovic´ application, which is constructed and installed at the heavy ion microbeam facility at the Ruder Institute in Zagreb. The TOF-SIMS setup is developed for high sensitivity molecular imaging using a heavy ion microbeam that focuses ion beams (from C to I) with sub-micron resolution. Dedicated pulse processing electronics for MeV SIMS application have been developed, enabling microbeam-scanning control, incoming ion microbeam pulsing and molecular mapping. The first results showing measured MeV SIMS spectra as well as molecular maps for samples of interest are presented and discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the recent years, molecular imaging by mass spectrometry of secondary molecular ions, resulting from the irradiation of materials with focused MeV heavy ions, is becoming an increasingly important tool for surface characterization of organic materials and biological systems [1–4]. Mass spectrometry using keV ions (SIMS) has been applied for surface characterization and depth profiling of inorganic and organic materials for many years [5,6]. Contrary to keV ions where mostly molecular fragments are detected, MeV ions have the ability to desorb large molecules with 2–3 orders of magnitude higher yield than keV SIMS due to the fact that the main interaction mechanism of MeV ions with a sample surface is through the electronic stopping power. Another advantage is the possibility to use MeV SIMS technique simultaneously with other Ion Beam Analysis (IBA) methods, such as Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) as is already pointed out in Ref. [4]. In the Laboratory for Ion Beam Interactions (LIBI) at the Ruder Boškovic´ Institute in Zagreb, the Heavy Ion Microprobe Facility has been recently upgraded from a triplet to a quintuplet focusing system. Heavy ions with rigidity ME/q2 < 20, obtained from 6 MV EN Tandem Van de Graaff accelerator or 1 MV HVEE Tandetron accelerator, can be focused to a sub-micrometer position
⇑ Corresponding author. Tel.: +385 1 4571 227. E-mail address: [email protected] (I. Bogdanovic´ Radovic´).
resolution, achieving 300 nm resolution for 8 MeV carbon ions in a low current mode [7]. Matching of linear TOF SIMS setup for MeV SIMS with the - Boškovic´ Institute enables studying of microprobe at the Ruder MeV SIMS process using more energetic heavy ions due to the fact that at higher ion energies, higher yields of secondary molecular ions are expected. This, in turn, may provide molecular mapping with a better sensitivity.
2. TOF SIMS setup Linear TOF SIMS setup for MeV SIMS application designed at Kyoto University is mounted on the microbeam chamber at 45° to the incoming heavy ion beam; layout is shown on Fig. 1. The system is operating in vacuum, which is usually in the low 107 mbar region. Secondary molecular ions are extracted from the sample using an acceleration potential difference between the sample surface and a grounded extractor. At the moment, extractor tip is positioned perpendicular to the sample surface at the distance of 10 mm. Linear TOF SIMS setup is attached to an adjustable bellow, enabling fine positioning of the extractor tip to the sample. Insulated sample holder in the chamber enables applying up to ±6 kV to the sample. Flight path for secondary molecular ions consists of acceleration path xa1 = 10 mm between sample and the extractor’s tip, field-free path xf = 437 mm within the extractor (i.e. between the extractor’s tip and MCP’s grid) and acceleration path xa2 = 6.5 mm between MCP’s grid and MCP’s
http://dx.doi.org/10.1016/j.nimb.2014.02.068 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068
2
T. Tadic´ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
System for MeV SIMS has been developed based on a Xilinx Virtex 4 FPGA development board which controls all aspects of the experiment and provides a time resolution of 4.75 ns. Using the Multi-Stop TDC Data Acquisition System, MeV SIMS experiment can be performed in one of the two modes: i. Electron start mode, where secondary electrons, emitted from the sample upon the heavy ion impact, are used as a start signal for time-of-flight measurement and negative secondary molecular ions are detected; and ii. Heavy ion microbeam deflection mode, where a pulse from the fast heavy ion microbeam deflector is used as a start signal for time-of-flight measurement. In most cases, the heavy ion microbeam deflector mode is used for detection of positive secondary molecular ions, with a sample holder charged to +3 kV. 3.1. The fast heavy ion beam deflector
Fig. 1. Layout of the Zagreb microbeam chamber with a mounted TOF SIMS spectrometer.
surface. Typical voltages applied for detection of positive secondary molecular ions on xa1 and xa2 are Va1 = 3 kV and Va2 = 2.2 kV, respectively. Experimentally determined mass resolution from the Leucine mass spectra measured with time resolution of 4.75 ns is shown on Fig. 2. During MeV SIMS experiment, the sample surface is at 45° to the incoming heavy ion microbeam and orthogonal to the axis of the linear TOF SIMS setup. Varying the sample inclination for just ±1° may lead to a large change of secondary molecular ion yield. In order to avoid sample contamination with organic materials, oil-free pumps are installed at the microbeam chamber. Additionally, special care was taken during sample handling and storage to avoid sample contamination with organic materials through the plastic tools and containers. 3. TOF signal processing and heavy ion microbeam deflector A dedicated Field Programmable Gate Array (FPGA) based Multi-Stop Time to Digital Converter (TDC) Data Acquisition
The fast heavy ion beam deflector consists of two pairs of copper plates for vertical and horizontal deflection. Each pair of plates is 120 mm long and 10 mm wide, with 6 mm space between the plates. The highest voltage applied between the plates during our tests was 700 V. Heavy ion microbeam deflector voltage is controlled by dedicated electronics based on MOSFET Push–Pull Switch. Ion pulsing is achieved by passing of the beam over 200 lm 1 mm wide aperture placed in front of the focusing quadrupoles. Duration of the pulse in most cases was set to 2 ns with interval between two pulses of 100 ls. More details on the heavy ion beam deflector can be found in Ref. [8]. 3.2. The Multi-Stop TDC Data Acquisition System The readout chain of the Multi-Stop TDC Data Acquisition System consists of a preamplifier, which is connected either to two Constant Fraction Discriminators (CFDs) (for the electron start mode) or to one CFD (for the heavy-ion deflection start mode) and two fast amplifiers. The fast NIM signals from the CFDs are conditioned and sent into the FPGA board as a digital logic signal (LVCMOS). The FPGA board is installed inside a computer and communicates via a PCI port. Dedicated software was developed enabling communication of the Multi-Stop TDC Data Acquisition System with the heavy ion microbeam deflector. The resulting TDC spectrum obtained from the FPGA is displayed using modified acquisition software SPECTOR [9]. During MeV SIMS experiments key parameters in the control and processing algorithms can be changed by simply setting of the new values in the control software of the acquisition system. Time resolution as low as 4.75 ns was achieved. 4. Testing of TOF SIMS setup for MeV SIMS on organic samples Our MeV SIMS setup was tested on organic samples using 12 MeV Si+5 and 8 MeV O+4 ion beams, focused to a 10 10 lm2 Table 1 Yields for Leucine, Glycine, Dystearoyl Phosphatidylcholine (DSPC) and Polystirene (PS) secondary molecular ions, per incident primary 12 MeV Si+5 ion. Organic compound
Secondary molecular ion mass (Da)
Yield of secondary molecular ions per incident Si+5 ion
Leucine Arginine Glycine DSPC PS
131 175 75 790 104
1.411 0.581 0.118 0.010 0.003
Fig. 2. Experimentally determined mass resolution dM for time resolution dt = 4.75 s.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068
T. Tadic´ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
3
spot. Beam lateral resolution was determined from the 2D maps of Cu X-rays from 125 125 lm2 Cu 200 mesh. Pulsed beam current was typically set to several hundreds of ions per second. It was measured using particle detector placed in the ion beam prior to the MeV SIMS measurement. To accelerate desorbed molecular ions from the target, extraction voltage of +3 kV was applied on the sample holder. Sample surface was tilted at 45° to the incoming pulsed ion microbeam. 4.1. Yield measurement of the secondary molecular ions To measure yields of secondary ions, Si-wafers covered with spin-coated and dried 0.1 mol solutions of Leucine, Glycine, Dystearoyl Phosphatidylcholine (DSPC) and Polystirene (PS) were irradiated by 12 MeV Si+5 ions. TOF MeV SIMS spectra were measured with 19 ns time resolution. Secondary molecular ion yields are given in Table 1. As shown on Fig. 3 at the MeV SIMS spectrum the Leucine molecule monomer at 131 Da, as well as higher order clusters with masses 262, 393, 524, 655 and 786 Da can be clearly seen. Peak intensities of these clusters decrease with increasing mass due to, among the other effects, lower MCP sensitivity for higher molecular masses, as shown by Gilmore and Seah [10]. TOF MeV SIMS mass resolution was tested using 12 MeV Si+5 pulsed ion microbeam and time resolution of 4.75 ns. As can be seen from Fig. 4, Leucine and Leucine’s 13C isotope peaks, with masses 131 and 132 Da, respectively, are well separated.
Fig. 3. MeV SIMS spectrum of spin-coated and dried 0.1 mol solutions of Leucine on Si wafer, measured using 12 MeV Si+5 pulsed ion microbeam. Leucine molecule monomer at 131 Da, as well as higher order clusters can be clearly seen. Time resolution dt = 19 ns.
4.2. Molecular imaging Cu 200 mesh (hole size 97 lm), fastened over dried 0.1 mol solution of Leucine on Si-wafer was used as a sample to test molecular imaging abilities. For the measurement, a 9 MeV O4+ focused ion beam was used. Secondary molecular ions emitted from the organic contamination on the Cu grid (map created from the region in spectra from m/q = 300 to m/q = 500), was used for molecular mapping, as well as secondary molecular ions emitted from the Leucine substrate (map created from the region in spectra from m/q = 73 to m/q = 270). 2D maps (scan size 480 560 lm2) displaying a signal from the Cu mesh and from the Leucine substrate are given in Fig. 5(a) and (b), respectively. Due to fact that angle between the beam and sample is 45°, thickness of the p Cu bars in y direction look thinner for 2/2 then it really is. Position resolution is calculated estimating that organic contamination is equally spread on the Cu mesh surface to be (10 ± 2) microns in x direction and (12 ± 3) microns in y direction. Resolution deterioration can be explained by edge shadowing due to distortion of electric field in presence of vertical boundaries created by the mesh wires, as described by Lee et al. [11].
Fig. 4. Mass resolution measurement using preset time resolution dt = 4.75 s. Peaks for Leucine and Leucine’s 13C isotope at 131 and 132 Da, respectively, are well separated.
Fig. 5. Molecular imaging by MeV SIMS using 12 MeV Si+5 pulsed ion microbeam. Scan size is 480 560 lm2. Sample was Cu mesh with period of 125 125 lm2, fastened over dried 0.1 mol solution of Leucine on Si-wafer: (a) organic contamination on Cu mesh, (b) Leucine substrate.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068
4
T. Tadic´ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
5. Conclusions In the present paper we describe first results obtained using newly developed TOF SIMS setup for secondary ion mass spectrometry using MeV ions – MeV SIMS, installed at the microbeam facility in Zagreb. A dedicated FPGA based Multi-Stop TDC Data Acquisition System for MeV SIMS, which controls all aspects of the experiment and provides a time resolution of 4.75 ns has been developed. Secondary molecular ion yields, measured for several organic samples using 12 MeV Si+5 ions, have shown yield variations from 0.003 up to 1.411 molecular ions per incoming Si+5 ion. Molecular imaging of Leucine covered by 125 125 lm2 Cu mash using focused 9 MeV O4+ beam with lateral resolution of 10 12 lm2 is demonstrated. Acknowledgments This work is supported by the Strategic Japanese-Croatian Cooperative Program on Materials Sciences’’, as a part of Japanese-Croatian Cooperative Research Project ‘‘Enhanced
Molecular Imaging by Focused Swift Heavy Ions’’ and by the European Community project as an Integrating Activity, Support of Public and Industrial Research Using Ion Beam Technology (SPIRIT) under EC contract No. 227012. References [1] Y. Nakata, S. Ninomiya, J. Matsuo, Nucl. Instr. Meth. B 256 (2007) 489–492. [2] H. Yamada, K. Ichiki, Y. Nakata, S. Ninomiya, T. Seki, T. Aoki, J. Matsuo, Nucl. Instr. Meth. B 268 (2010) 1736–1740. [3] M.J. Bailey, B.N. Jones, S. Hinder, J. Watts, S. Bleay, R.P. Webb, Nucl. Instr. Meth. B 268 (2010) 1929–1932. [4] B.N. Jones, V. Palitsin, R. Webb, Nucl. Instr. Meth. B 268 (2010) 1714. [5] S. Hofmann, Prog. Surf. Sci. 36 (1991) 35. [6] A. Benninghoven, Surf. Sci. 299–300 (1994) 246. [7] M. Jakšic´, I. Bogdanovic´ Radovic´, M. Bogovac, V. Desnica, S. Fazinic´, M. Karlušic´, Z. Medunic´, H. Muto, Zˇ. Pastuovic´, Z. Siketic´, N. Skukan, T. Tadic´, Nucl. Instr. Meth. B 260 (2007) 114–118. [8] M. Jakšic´, Z. Medunic´, N. Skukan, Nucl. Instr. Meth. B 201 (2003) 176. [9] M. Bogovac, I. Bogdanovic´, S. Fazinic´, M. Jakšic´, L. Kukec, W. Wilhelm, Nucl. Instr. Meth. B 89 (1994) 219. [10] I.S. Gilmore, M.P. Seah, Int. J. Mass Spectrom. 202 (2000) 217–229. [11] J.L.S. Lee et al., Appl. Surf. Sci. 255 (2008) 1560–1563.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility Toncˇi Tadic´ a, Iva Bogdanovic´ Radovic´ a,⇑, Zdravko Siketic´ a, Donny Domagoj Cosic a, Natko Skukan a, Milko Jakšic´ a, Jiro Matsuo b a b
- Boškovic´ Institute, Bijenicˇka 54, HR-10000 Zagreb, Croatia Laboratory for Ion Beam Interactions, Ruder Quantum Science and Engineering Center, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: MeV SIMS Heavy ion microbeam Molecular imaging Organic samples
a b s t r a c t We describe a new Time-of-flight Secondary Ion Mass Spectrometry (TOF SIMS) setup for MeV SIMS - Boškovic´ application, which is constructed and installed at the heavy ion microbeam facility at the Ruder Institute in Zagreb. The TOF-SIMS setup is developed for high sensitivity molecular imaging using a heavy ion microbeam that focuses ion beams (from C to I) with sub-micron resolution. Dedicated pulse processing electronics for MeV SIMS application have been developed, enabling microbeam-scanning control, incoming ion microbeam pulsing and molecular mapping. The first results showing measured MeV SIMS spectra as well as molecular maps for samples of interest are presented and discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the recent years, molecular imaging by mass spectrometry of secondary molecular ions, resulting from the irradiation of materials with focused MeV heavy ions, is becoming an increasingly important tool for surface characterization of organic materials and biological systems [1–4]. Mass spectrometry using keV ions (SIMS) has been applied for surface characterization and depth profiling of inorganic and organic materials for many years [5,6]. Contrary to keV ions where mostly molecular fragments are detected, MeV ions have the ability to desorb large molecules with 2–3 orders of magnitude higher yield than keV SIMS due to the fact that the main interaction mechanism of MeV ions with a sample surface is through the electronic stopping power. Another advantage is the possibility to use MeV SIMS technique simultaneously with other Ion Beam Analysis (IBA) methods, such as Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) as is already pointed out in Ref. [4]. In the Laboratory for Ion Beam Interactions (LIBI) at the Ruder Boškovic´ Institute in Zagreb, the Heavy Ion Microprobe Facility has been recently upgraded from a triplet to a quintuplet focusing system. Heavy ions with rigidity ME/q2 < 20, obtained from 6 MV EN Tandem Van de Graaff accelerator or 1 MV HVEE Tandetron accelerator, can be focused to a sub-micrometer position
⇑ Corresponding author. Tel.: +385 1 4571 227. E-mail address: [email protected] (I. Bogdanovic´ Radovic´).
resolution, achieving 300 nm resolution for 8 MeV carbon ions in a low current mode [7]. Matching of linear TOF SIMS setup for MeV SIMS with the - Boškovic´ Institute enables studying of microprobe at the Ruder MeV SIMS process using more energetic heavy ions due to the fact that at higher ion energies, higher yields of secondary molecular ions are expected. This, in turn, may provide molecular mapping with a better sensitivity.
2. TOF SIMS setup Linear TOF SIMS setup for MeV SIMS application designed at Kyoto University is mounted on the microbeam chamber at 45° to the incoming heavy ion beam; layout is shown on Fig. 1. The system is operating in vacuum, which is usually in the low 107 mbar region. Secondary molecular ions are extracted from the sample using an acceleration potential difference between the sample surface and a grounded extractor. At the moment, extractor tip is positioned perpendicular to the sample surface at the distance of 10 mm. Linear TOF SIMS setup is attached to an adjustable bellow, enabling fine positioning of the extractor tip to the sample. Insulated sample holder in the chamber enables applying up to ±6 kV to the sample. Flight path for secondary molecular ions consists of acceleration path xa1 = 10 mm between sample and the extractor’s tip, field-free path xf = 437 mm within the extractor (i.e. between the extractor’s tip and MCP’s grid) and acceleration path xa2 = 6.5 mm between MCP’s grid and MCP’s
http://dx.doi.org/10.1016/j.nimb.2014.02.068 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068
2
T. Tadic´ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
System for MeV SIMS has been developed based on a Xilinx Virtex 4 FPGA development board which controls all aspects of the experiment and provides a time resolution of 4.75 ns. Using the Multi-Stop TDC Data Acquisition System, MeV SIMS experiment can be performed in one of the two modes: i. Electron start mode, where secondary electrons, emitted from the sample upon the heavy ion impact, are used as a start signal for time-of-flight measurement and negative secondary molecular ions are detected; and ii. Heavy ion microbeam deflection mode, where a pulse from the fast heavy ion microbeam deflector is used as a start signal for time-of-flight measurement. In most cases, the heavy ion microbeam deflector mode is used for detection of positive secondary molecular ions, with a sample holder charged to +3 kV. 3.1. The fast heavy ion beam deflector
Fig. 1. Layout of the Zagreb microbeam chamber with a mounted TOF SIMS spectrometer.
surface. Typical voltages applied for detection of positive secondary molecular ions on xa1 and xa2 are Va1 = 3 kV and Va2 = 2.2 kV, respectively. Experimentally determined mass resolution from the Leucine mass spectra measured with time resolution of 4.75 ns is shown on Fig. 2. During MeV SIMS experiment, the sample surface is at 45° to the incoming heavy ion microbeam and orthogonal to the axis of the linear TOF SIMS setup. Varying the sample inclination for just ±1° may lead to a large change of secondary molecular ion yield. In order to avoid sample contamination with organic materials, oil-free pumps are installed at the microbeam chamber. Additionally, special care was taken during sample handling and storage to avoid sample contamination with organic materials through the plastic tools and containers. 3. TOF signal processing and heavy ion microbeam deflector A dedicated Field Programmable Gate Array (FPGA) based Multi-Stop Time to Digital Converter (TDC) Data Acquisition
The fast heavy ion beam deflector consists of two pairs of copper plates for vertical and horizontal deflection. Each pair of plates is 120 mm long and 10 mm wide, with 6 mm space between the plates. The highest voltage applied between the plates during our tests was 700 V. Heavy ion microbeam deflector voltage is controlled by dedicated electronics based on MOSFET Push–Pull Switch. Ion pulsing is achieved by passing of the beam over 200 lm 1 mm wide aperture placed in front of the focusing quadrupoles. Duration of the pulse in most cases was set to 2 ns with interval between two pulses of 100 ls. More details on the heavy ion beam deflector can be found in Ref. [8]. 3.2. The Multi-Stop TDC Data Acquisition System The readout chain of the Multi-Stop TDC Data Acquisition System consists of a preamplifier, which is connected either to two Constant Fraction Discriminators (CFDs) (for the electron start mode) or to one CFD (for the heavy-ion deflection start mode) and two fast amplifiers. The fast NIM signals from the CFDs are conditioned and sent into the FPGA board as a digital logic signal (LVCMOS). The FPGA board is installed inside a computer and communicates via a PCI port. Dedicated software was developed enabling communication of the Multi-Stop TDC Data Acquisition System with the heavy ion microbeam deflector. The resulting TDC spectrum obtained from the FPGA is displayed using modified acquisition software SPECTOR [9]. During MeV SIMS experiments key parameters in the control and processing algorithms can be changed by simply setting of the new values in the control software of the acquisition system. Time resolution as low as 4.75 ns was achieved. 4. Testing of TOF SIMS setup for MeV SIMS on organic samples Our MeV SIMS setup was tested on organic samples using 12 MeV Si+5 and 8 MeV O+4 ion beams, focused to a 10 10 lm2 Table 1 Yields for Leucine, Glycine, Dystearoyl Phosphatidylcholine (DSPC) and Polystirene (PS) secondary molecular ions, per incident primary 12 MeV Si+5 ion. Organic compound
Secondary molecular ion mass (Da)
Yield of secondary molecular ions per incident Si+5 ion
Leucine Arginine Glycine DSPC PS
131 175 75 790 104
1.411 0.581 0.118 0.010 0.003
Fig. 2. Experimentally determined mass resolution dM for time resolution dt = 4.75 s.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068
T. Tadic´ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
3
spot. Beam lateral resolution was determined from the 2D maps of Cu X-rays from 125 125 lm2 Cu 200 mesh. Pulsed beam current was typically set to several hundreds of ions per second. It was measured using particle detector placed in the ion beam prior to the MeV SIMS measurement. To accelerate desorbed molecular ions from the target, extraction voltage of +3 kV was applied on the sample holder. Sample surface was tilted at 45° to the incoming pulsed ion microbeam. 4.1. Yield measurement of the secondary molecular ions To measure yields of secondary ions, Si-wafers covered with spin-coated and dried 0.1 mol solutions of Leucine, Glycine, Dystearoyl Phosphatidylcholine (DSPC) and Polystirene (PS) were irradiated by 12 MeV Si+5 ions. TOF MeV SIMS spectra were measured with 19 ns time resolution. Secondary molecular ion yields are given in Table 1. As shown on Fig. 3 at the MeV SIMS spectrum the Leucine molecule monomer at 131 Da, as well as higher order clusters with masses 262, 393, 524, 655 and 786 Da can be clearly seen. Peak intensities of these clusters decrease with increasing mass due to, among the other effects, lower MCP sensitivity for higher molecular masses, as shown by Gilmore and Seah [10]. TOF MeV SIMS mass resolution was tested using 12 MeV Si+5 pulsed ion microbeam and time resolution of 4.75 ns. As can be seen from Fig. 4, Leucine and Leucine’s 13C isotope peaks, with masses 131 and 132 Da, respectively, are well separated.
Fig. 3. MeV SIMS spectrum of spin-coated and dried 0.1 mol solutions of Leucine on Si wafer, measured using 12 MeV Si+5 pulsed ion microbeam. Leucine molecule monomer at 131 Da, as well as higher order clusters can be clearly seen. Time resolution dt = 19 ns.
4.2. Molecular imaging Cu 200 mesh (hole size 97 lm), fastened over dried 0.1 mol solution of Leucine on Si-wafer was used as a sample to test molecular imaging abilities. For the measurement, a 9 MeV O4+ focused ion beam was used. Secondary molecular ions emitted from the organic contamination on the Cu grid (map created from the region in spectra from m/q = 300 to m/q = 500), was used for molecular mapping, as well as secondary molecular ions emitted from the Leucine substrate (map created from the region in spectra from m/q = 73 to m/q = 270). 2D maps (scan size 480 560 lm2) displaying a signal from the Cu mesh and from the Leucine substrate are given in Fig. 5(a) and (b), respectively. Due to fact that angle between the beam and sample is 45°, thickness of the p Cu bars in y direction look thinner for 2/2 then it really is. Position resolution is calculated estimating that organic contamination is equally spread on the Cu mesh surface to be (10 ± 2) microns in x direction and (12 ± 3) microns in y direction. Resolution deterioration can be explained by edge shadowing due to distortion of electric field in presence of vertical boundaries created by the mesh wires, as described by Lee et al. [11].
Fig. 4. Mass resolution measurement using preset time resolution dt = 4.75 s. Peaks for Leucine and Leucine’s 13C isotope at 131 and 132 Da, respectively, are well separated.
Fig. 5. Molecular imaging by MeV SIMS using 12 MeV Si+5 pulsed ion microbeam. Scan size is 480 560 lm2. Sample was Cu mesh with period of 125 125 lm2, fastened over dried 0.1 mol solution of Leucine on Si-wafer: (a) organic contamination on Cu mesh, (b) Leucine substrate.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068
4
T. Tadic´ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
5. Conclusions In the present paper we describe first results obtained using newly developed TOF SIMS setup for secondary ion mass spectrometry using MeV ions – MeV SIMS, installed at the microbeam facility in Zagreb. A dedicated FPGA based Multi-Stop TDC Data Acquisition System for MeV SIMS, which controls all aspects of the experiment and provides a time resolution of 4.75 ns has been developed. Secondary molecular ion yields, measured for several organic samples using 12 MeV Si+5 ions, have shown yield variations from 0.003 up to 1.411 molecular ions per incoming Si+5 ion. Molecular imaging of Leucine covered by 125 125 lm2 Cu mash using focused 9 MeV O4+ beam with lateral resolution of 10 12 lm2 is demonstrated. Acknowledgments This work is supported by the Strategic Japanese-Croatian Cooperative Program on Materials Sciences’’, as a part of Japanese-Croatian Cooperative Research Project ‘‘Enhanced
Molecular Imaging by Focused Swift Heavy Ions’’ and by the European Community project as an Integrating Activity, Support of Public and Industrial Research Using Ion Beam Technology (SPIRIT) under EC contract No. 227012. References [1] Y. Nakata, S. Ninomiya, J. Matsuo, Nucl. Instr. Meth. B 256 (2007) 489–492. [2] H. Yamada, K. Ichiki, Y. Nakata, S. Ninomiya, T. Seki, T. Aoki, J. Matsuo, Nucl. Instr. Meth. B 268 (2010) 1736–1740. [3] M.J. Bailey, B.N. Jones, S. Hinder, J. Watts, S. Bleay, R.P. Webb, Nucl. Instr. Meth. B 268 (2010) 1929–1932. [4] B.N. Jones, V. Palitsin, R. Webb, Nucl. Instr. Meth. B 268 (2010) 1714. [5] S. Hofmann, Prog. Surf. Sci. 36 (1991) 35. [6] A. Benninghoven, Surf. Sci. 299–300 (1994) 246. [7] M. Jakšic´, I. Bogdanovic´ Radovic´, M. Bogovac, V. Desnica, S. Fazinic´, M. Karlušic´, Z. Medunic´, H. Muto, Zˇ. Pastuovic´, Z. Siketic´, N. Skukan, T. Tadic´, Nucl. Instr. Meth. B 260 (2007) 114–118. [8] M. Jakšic´, Z. Medunic´, N. Skukan, Nucl. Instr. Meth. B 201 (2003) 176. [9] M. Bogovac, I. Bogdanovic´, S. Fazinic´, M. Jakšic´, L. Kukec, W. Wilhelm, Nucl. Instr. Meth. B 89 (1994) 219. [10] I.S. Gilmore, M.P. Seah, Int. J. Mass Spectrom. 202 (2000) 217–229. [11] J.L.S. Lee et al., Appl. Surf. Sci. 255 (2008) 1560–1563.
Please cite this article in press as: T. Tadic´ et al., Development of a TOF SIMS setup at the Zagreb heavy ion microbeam facility, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.068