Time-of-flight secondary ion mass spectrometry with transmission of energetic primary cluster ions through foil targets K. Hirata, Y. Saitoh, A. Chiba, K. Yamada, S. Matoba, and K. Narumi Citation: Review of Scientific Instruments 85, 033107 (2014); doi: 10.1063/1.4869036 View online: http://dx.doi.org/10.1063/1.4869036 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature-programed time-of-flight secondary ion mass spectrometry study of 1-butyl-3-methylimidazolium trifluoromethanesulfonate during glass-liquid transition, crystallization, melting, and solvation J. Chem. Phys. 129, 094707 (2008); 10.1063/1.2965526 Maximum likelihood principal component analysis of time-of-flight secondary ion mass spectrometry spectral images J. Vac. Sci. Technol. A 23, 746 (2005); 10.1116/1.1861935 A temperature-programed time-of-flight secondary ion mass spectroscopy study of intermixing of amorphous ethanol and heavy-water films at 15–200 K J. Chem. Phys. 122, 134711 (2005); 10.1063/1.1869372 Highly sensitive time-of-flight secondary-ion mass spectroscopy for contaminant analysis of semiconductor surface using cluster impact ionization Appl. Phys. Lett. 86, 044105 (2005); 10.1063/1.1852715 Quantification of metal contaminants on GaAs with time-of-flight secondary ion mass spectrometry J. Vac. Sci. Technol. B 16, 1002 (1998); 10.1116/1.590058
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 033107 (2014)
Time-of-flight secondary ion mass spectrometry with transmission of energetic primary cluster ions through foil targets K. Hirata,1,a) Y. Saitoh,2 A. Chiba,2 K. Yamada,2 S. Matoba,2 and K. Narumi2 1 National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan 2 Takasaki Advanced Radiation Research Institute (TARRI), Japan Atomic Energy Agency (JAEA), Takasaki, Gumma 370-1292, Japan
(Received 4 October 2013; accepted 8 March 2014; published online 24 March 2014) We developed time-of-flight (TOF) secondary ion (SI) mass spectrometry that provides informative SI ion mass spectra without needing a sophisticated ion beam pulsing system. In the newly developed spectrometry, energetic large cluster ions with energies of the order of sub MeV or greater are used as primary ions. Because their impacts on the target surface produce high yields of SIs, the resulting SI mass spectra are informative. In addition, the start signals necessary for timing information on primary ion incidence are provided by the detection signals of particles emitted from the rear surface of foil targets upon transmission of the primary ions. This configuration allows us to obtain positive and negative TOF SI mass spectra without pulsing system, which requires precise control of the primary ions to give the spectra with good mass resolution. We also successfully applied the TOF SI mass spectrometry with energetic cluster ion impacts to the chemical structure characterization of organic thin film targets. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869036] I. INTRODUCTION
Time-of-flight (TOF) secondary ion (SI) mass spectrometry, which measures the flight time difference of SIs in a drift tube with certain length, is used to determine the mass-tocharge ratio m/z of SIs produced by primary ion impacts on a target. The spectrometry is a powerful tool for surface characterization in various research fields,1 as it allows sensitive detection of SIs over a wide range of m/z and gives chemical specific information about the surfaces of the targets. SIs are emitted from the target surface as a result of the transfer of energy from the incident ion to the target. Consequently, the SI emission yields strongly depend on various parameters of the target and the incident ions. Cluster ion impacts provide different energy transfer from monoatomic ion impacts because the constituent atoms of a primary cluster ion simultaneously impact a very small area of the target surface. We demonstrated that energetic primary cluster ions with energies of the order of sub MeV or greater have various advantages for highly sensitive analysis of the surface chemical structure of organic targets due to the high yields of SIs characteristic of the target materials.2–4 Thus, high SI yields induced by impacts of the energetic primary cluster ions on targets confirms that SI mass spectra can produce valuable information. The measurements of SI flight times by TOF spectrometers require timing signals for both the primary ion incidence on the target (the start signal) and the SI detection (the stop signal). The stop signal is simply obtained from an SI detector placed at the end of the flight path in a TOF drift tube, whereas the start signal measurements require more sophisti-
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(3)/033107/6/$30.00
cated methods. Although using a signal from a pulse generator to trigger the primary ion beam pulse is the most widely employed method to obtain the start signals, this method requires a well-designed pulsing system and precise control of the primary ion beam path to produce an ion beam with a short pulse width in order to give good m/z resolution. The detection signals of secondary electrons (SEs) emitted from the front surface of the target can be used for the start signals. However, in order to detect both the SEs and positive SIs (P-SIs) for the start and stop signals, respectively, the SEs must be sufficiently separated from the P-SIs, which are emitted from the same side of the target with opposite charge from that of the SEs. Thus, P-SI measurements pose some problems. An energetic cluster primary ion penetrates to a certain depth from the front surface and can even be transmitted to the rear surface of a target, depending on the conditions of the target and primary cluster ion, although the primary cluster immediately decomposes into its constituent particles after striking the target surface. As a result of the transmission, the constituent particles originating from the primary cluster, along with the forward secondary particles (F-SPs), including forward SEs (F-SEs) and SIs (F-SIs), are emitted from the rear surface of the target. This emission from the rear surface of the target can provide timing information for the primary ion incidence. The use of start timing signals given by this method should provide both P- and negative-SI (N-SI) TOF mass spectra without a sophisticated pulsing system because the electric potentials to obtain the start and stop signals are independently applied from different sides of the target. In this paper, we present TOF SI mass spectrometry that uses detection signals of the particles emitted from the rear surface of a foil target upon the transmission of energetic primary cluster ions through the target.
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© 2014 AIP Publishing LLC
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High voltage power supply
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SSD mode. The setup for the pulse mode is the same as that previously described in Refs. 6 and 7. SIs emitted from the front surface of the target are accelerated between the target and a TOF mass spectrometer before hitting a 40-mm-diameter MCP (Hamamatsu F-2225) placed at the end of the flight pass. The MCP’s output signals are fed into a fast CFD via a pre-amplifier and used as the stop signals. The cluster ion impacts on targets are expected to give a relatively high total SI yield per impact because, in addition to the enhanced SI yield per incident atom, the number of incident atoms per impact for a cluster ion is n (n: cluster number) times higher than that for the corresponding monoatomic ion. Therefore, we used an SI counting system that can accurately obtain TOF SI mass spectra even when many SIs are detected for one ion impact.8 All SI mass spectra were obtained without intentional charge compensation. The primary ions were injected into the targets at a rate between several tens and several hundreds ions/s.
Computer 1
III. RESULTS AND DISCUSSION (c) Constant fraction discriminator
Counting system
Constant fraction discriminator
Computer 2
FIG. 1. Schematic illustration of TOF SI mass spectroscopy with transmission of energetic primary cluster ions through foil targets.
II. EXPERIMENTAL
Figure 1 is a schematic diagram of the TOF SI mass spectroscopy that transmits energetic primary cluster ions through foil targets. The primary cluster ions are injected into a thin foil target at an angle of 45o to the target surface, then transmitted through the target. Since a primary ion with a higher incident energy can transmit through a thicker target, energetic incident ions are suitable primary ions for the TOF SI mass spectrometry with transmission. Energetic cluster ions with energies on the order of sub MeV or greater, which are produced by a 3 MV tandem accelerator and a 400 kV ion implanter at the Japan Atomic Energy Agency (JAEA) in Takasaki,5 are used in the spectroscopy. As shown in Fig. 1, three types of start signals are available for the SI flight time measurements: (a) detection signals of transmitted constituent particles originating from the primary cluster from a solid-state detector (SSD) (Ortec BU-014-150-100) placed approximately 2 cm downstream from the target (Forward SSD mode: F-SSD mode), (b) detection signals of FSPs from a microchannel plate (MCP) (Hamamatsu F-4655) placed approximately 3 cm in front of the rear side of the target (Forward MCP mode: F-MCP mode), and (c) signals from a pulse generator for triggering the primary ion beam pulse (pulse mode). For both the F-SSD and F-MCP modes, the timing signals from pre-amplifiers (Ortec 142) are transferred to a fast constant fraction discriminator (CFD) to be used as the start signals. Pulse-height measurements for the incident clusters and their constituents are available under the
A. F-SSD mode
Cluster primary ions injected into a foil target are immediately decomposed into their constituents. Due to scattering of the atomic constituents by target atoms and/or a Coulomb explosion between charged constituents, the average distance between the constituents increases in correspondence with the depth from the target surface. Consequently, the distance is large enough to be detected as an isolated atom at a detector located downstream from the target.9, 10 Here, we discuss the use of detection signals of the transmitted constituents originating from primary clusters for TOF SI mass measurements under the F-SSD mode. As a typical example of the SSD detection signals for a primary cluster ion transmitted through foil targets, Figs. 2(a) and 2(b) show pulse-height spectra for 4.0 MeV C8 + transmitted through self-supporting amorphous carbon foils (Arizona Carbon Foil Co. Inc.) with thicknesses of 5 μg/cm2 and 10 μg/cm2 , respectively.11 The pulse-height spectra provide information about the transmission properties of the primary cluster ions through the targets. To investigate the properties of the primary cluster ions before the transmission, Fig. 2(c) shows the pulse-height spectrum for 4.0 MeV C8 + without a target. The relative intensity for each spectrum is obtained by dividing the count for each channel by the total count. The large peak around channel 890 in the spectrum without a target (Fig. 2(c)) is attributed to the direct detection of single C8 . The two weak peaks observed around channels 340 and 570, corresponding to C3 and C5 detection, respectively, indicate that C8 preferentially decomposes into C3 and C5 . The pulse-height spectra for C8 with the 5 μg/cm2 carbon foil (Fig. 2(a)) and the 10 μg/cm2 carbon foil (Fig. 2(b)) exhibit the respective intense peaks around channel 920 and 870 that correspond to detection of all the constituent atoms of the primary C8 cluster (p = 8: p is the number of detected C atoms). The pulse-height channel number of the peak (p = 8) in Fig. 2(b) is lower than that in Fig. 2(a) because an increasing thickness of foil results in an increasing averaged
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FIG. 3. Ratio of the relative intensities of p = 8, 7, and 6 for 4 MeV C8 + transmitted through carbon foils with thicknesses of (a) 5 μg/cm2 and (b) 10 μg/cm2 under the F-SSD mode. The lines are obtained from the expression, p n-p 2 n Cp γ (1 − γ ) , where n = 8, and γ = 0.99 and 0.98 for 5 μg/cm and 10 μg/cm2 , respectively.
C8
10-2 10-3 10-4 200
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MCA channel number FIG. 2. Pulse-height spectra for 4 MeV C8 + transmitted through carbon foils with thicknesses of (a) 5 μg/cm2 and (b) 10 μg/cm2 under the F-SSD mode. Also included in the figure is (c) the pulse-height spectrum for 4.0 MeV C8 + without a target.
energy loss of constituent atoms in the foil. It should be noted that the pulse-height channel number for peak p = 8 in Fig. 2(a) is obviously higher than that for the direct detection of single C8 in Fig. 2(c), although the C8 clusters in Fig. 2(a) have lost a certain amount of their kinetic energy during transmission through the target, i.e., the pulse height for the detection of n atomic constituents with an average energy of Eatom is higher than that of n clusters with an energy of En,cluster , although nEatom < En,cluster . The pulse height for direct cluster detection is lower than expected due to the pulse-height defect that arises from incomplete charge collection in the SSD detector as a result of higher probability of electron-hole recombination in the densely ionized region produced by cluster ion irradiations.9, 10, 12 As the interaction with target atoms and/or between the constituents makes the velocity direction of each constituent deviate from that of the incident cluster, a fraction of the constituents may miss the detector. As a result of the missing constituents, peaks for p = 7, 6 . . . are observed for the pulseheight spectra (with the foil) in Figs. 2(a) and 2(b) in addition to those for p = 8. The relatively higher peaks for p = 3 and 5 (in Figs. 2(a) and 2(b)) are assumed to be primarily due to C3 and C5 , produced by the decomposition of C8 before injection into the targets as mentioned above. The pulse-heights in Fig. 2(b) are smaller than those with corresponding p in Fig. 2(a) because the averaged energy loss of the constituents increases with the increasing foil thickness. It should be noted that the relative peak intensities for p = 8, 7, and 6 are different between Figs. 2(a) and 2(b); the intensity for p = 8 in Fig. 2(b) is lower, but those for p = 7, 6 are higher than the intensities with corresponding p in Fig. 2(a), respectively. This result can be attributed to a lower detection probability of the atomic constituent in Fig. 2(b) than in Fig. 2(a) due to the
increase of a fraction of the constituents missing the detector according to the foil thickness. For comparison of the detection probability of the atomic constituents in Figs. 2(a) and 2(b), Fig. 3 shows the ratio of the relative intensities for p = 8, 7, and 6. The lines are obtained from the following expression, F(γ ) = n Cp γ p (1 − γ )n-p , where γ is the detection probability of the atomic constituent. The experimental data for the 5 μg/cm2 and 10 μg/cm2 foils are well reproduced by the expression for γ = 0.99 and 0.98, respectively. Although the γ value varies with experimental conditions such as the thickness of the target and the distance between the target and the detector, cluster primary ions are advantageous to provide the start timing signals due to multichances of detection for one impact. The undetected probability, obtained by the expression (1 − γ )n , is almost zero for both cases. Figure 4(a) shows the P-SI TOF mass spectrum of carbon foil (5 μg/cm2 ) for 4.0 MeV C8 + under the F-SSD mode. The timing signals, simultaneously provided with the pulse-height signals by the pre-amplifier, were used as the start signals. The major peaks observed in Fig. 4(a) can be attributed to ions of singly charged atomic and molecular hydrogen [H+ (m/z = 1), H2 + (m/z = 2), and H3 + (m/z = 3)], hydrocarbon [e.g., C2 H3 + (m/z = 27), C2 H5 + (m/z = 29), C3 H5 + (m/z = 41), C3 H7 + (m/z = 43)], Na+ (m/z = 23), and K+ (m/z = 39) originating from the target and its surface contaminants. For comparison, the P-SI TOF mass spectrum of the same foil for 4.0 MeV C8 + under the pulse mode is shown in Fig. 4(b). The spectrum in Fig. 4(a) is essentially the same as that in Fig. 4(b), and thus demonstrates that P-SI TOF mass spectra can be obtained using a simple setup without measuring the signals of the primary ion beam pulses. The F-SSD mode is also available for other energetic larger primary cluster ions, including MeV order C60 primary ions, as shown in Fig. 4(c). We also obtained N-SI mass spectra under the FSSD mode (spectra are not shown). The spectra demonstrate that both P- and N-SI TOF mass spectra with cluster ion impacts can be obtained using a simple setup without measuring the signals of the primary ion beam pulses under the F-SSD mode.
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Intensity
Inte ensity
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(b)
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m/z FIG. 5. P-SI TOF mass spectrum of carbon foil (5 μg/cm2 ) for 2.0 MeV C4 + under the F-MCP mode.
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ntensity In
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m/z FIG. 4. P-SI TOF mass spectra of carbon foil (5 μg/cm2 ) for 4.0 MeV C8 + under (a) F-SSD and (b) pulse modes. Also included in the figure is (c) the P-SI TOF mass spectrum of carbon foil (5 μg/cm2 ) for 5.0 MeV C60 + under the F-SSD mode.
B. F-MCP mode
F-SPs such as F-SEs and F-SIs, emitted from the rear surface of targets, can also provide timing information for the primary ion incidence. In the case of the F-MCP mode, the detection signals of F-SEs are more suitable than those of F-SIs, because the F-SEs reach the detector much faster than the F-SIs by applying the extraction potential in detection of charged F-SPs. Here we discuss the TOF SI mass measurements using F-SEs detection signals as the start signals under the F-MCP mode. The SE emission induced by a swift monoatomic ion impact on a solid target can be explained by a three-step process: production of scattered electrons by the incident ion, transport of the electrons in the target, and transmission of the electrons through the surface barrier.13 Cluster ion impacts have been reported to give different F-SE emission phenomena from those for monoatomic ion impacts: the F-SE yields per incident atom for cluster ion impacts on thin carbon foil targets were reduced in comparison with those for a monoatomic ion with the same element and velocity as the cluster.14 In addition, the detailed cluster number dependence of F-SE energy spectra for 0.5 MeV/atom Cn + (n = 1, 2, 3, 4) impacts on carbon foil targets revealed that the F-SE yields per incident atom at the energy range corresponding to the convoy electrons increase with the cluster number, whereas the yields at other energy regions are reduced.15 These differences in the F-SE emission between cluster and monoatomic ion impacts are attributed to the transport or transmission process.14, 15
The F-SE detection signals are available for use as the start signals of TOF SI measurements, although the detailed mechanism that quantitatively explains the F-SE emission by cluster ion impacts is currently unknown. The F-SE yields per incident atom decrease with increasing cluster number. However, their yields per incident ion for cluster ions are still higher than those for the corresponding monoatomic ion as reported above and indicate that enough F-SEs are produced by the transmission of cluster ions. The energy distribution of F-SEs does not affect the start timing very much because even an electron with an initial kinetic energy of 0 eV reaches a detector placed several cm in front of a foil target within several ns by applying an extraction potential of several 100 V. Figure 5 shows the P-SI TOF mass spectrum of selfsupporting amorphous carbon foil (5 μg/cm2 ) for 2.0 MeV C4 + impacts by detecting F-SEs under the F-MCP mode. The spectrum demonstrates that P-SI TOF mass spectra can be obtained using the F-SE detection signals as the start signals for TOF SI measurements, in contrast to using the SEs emitted from the front surface.16 Use of the detection signals under the F-MCP mode has an advantage over those under the F-SSD mode when the kinetic energy of the constituent originating the primary cluster is low. Under the F-SSD mode, the constituent is required to have enough kinetic energy to produce enough electronhole pairs in the detector after penetrating the dead layer of the detector surface. Therefore, the use of detection signals is limited to constituents having enough kinetic energy to be detected by the SSD detector. This limitation can be extended to using F-SEs detection signals as the start signals under the F-MCP mode. Figure 6 shows P-SI and N-SI TOF mass spectra of the carbon foil (1 μg/cm2 ) for 540 keV C60 2+ under the F-MCP mode. The projected range of monoatomic C with incident energy of 9 keV, corresponding to 540 keV C60 , is approximately 30 nm for a silicon matrix, which is shorter than the dead layer thickness of the SSD (approximately 50 nm). In addition to the shorter projected range, the individual constituent C atom loses its kinetic energy in the target, and so it is difficult to obtain the start signals for 540 keV C60 2+ under the F-SSD mode. In contrast, the F-MCP mode can provide the start signals for F-SPs and is available for a constituent having lower kinetic energy at the rear surface of the target. Although the F-MCP mode must apply an extraction potential to increase the velocity of the F-SPs to the MCP for the start signals in addition to the potential for the stop signals,
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m/z 0 FIG. 6. Mass spectra of (a) P-SI and (b) N-SI under F-MCP mode for 540 keV C60 2+ impacts on carbon foil (1 μg/cm2 ).
both the P-SI and N-SI spectra can be obtained as the potentials are independently applied at different surfaces of the foil target. C. Application to chemical structure characterization of organic thin film targets
The energetic cluster impact ionization TOF SI mass spectrometry with transmission assisted timing signals can also provide SI mass spectra of organic thin film targets. As an example of SI mass spectra of organic thin film targets, Fig. 7 shows P-SI TOF mass spectra of (a) poly(methyl methacrylate) (PMMA) and (b) polystyrene (PS) thin-film targets for 4.0 MeV C8 + under the SSD mode. The PMMA and PS targets were produced by dropping toluene solutions of either PMMA or PS (Sigma-Aldrich Co.) on silicon nitride membranes with a thickness of 20 nm (Silison Ltd.). The thicknesses of the PMMA and PS layers are respectively approximated to be 1.7 and 0.7 μg/cm2 , based on pulse-height spectra obtained by the SSD detector. The peaks of atomic hydrogen (H+ ) and hydrocarbon (Cx Hy + ) observed in Figs. 7(a) and 7(b) are typical for polymers containing hydrogen and carbon. The small peak at m/z = 7 in Fig. 7(b) can be attributed to Li+ , which indicates that a lithium containing initiator was probably used in polymerization.17 In addition, characteristic peaks for each polymer are observed in Fig. 7. The respective peaks at m/z = 59 and 69 in Fig. 7(a) represent C2 H3 O2 + and C4 H5 O+ , which are characteristic for the P-SI mass spectrum of PMMA; C2 H3 O2 + is originated from ester group and C4 H5 O+ is related to the main chain of the polymer structure.18–20 In Fig. 7(b), the peak at m/z = 77 (C6 H5 + ) and those at m/z = 39 (C3 H3 + ), 51 (C4 H3 + ) are respectively originated from the phenyl side group in PS and its fragments. This result is characteristic for the P-SI mass spectrum for PS.21 The intense peak at
0
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m/z FIG. 7. P-SI TOF mass spectra of (a) poly(methyl methacrylate) (PMMA) and (b) polystyrene (PS) thin-film targets for 4.0 MeV C8 + under the SSD mode.
m/z = 91 (C7 H7 + ) is also characteristic for PS.20, 21 The agreement of the characteristic peaks in both spectra with the previous typical SI mass spectra demonstrates that the energetic cluster impact ionization TOF SI mass spectrometry with transmission assisted timing signals can be applied to chemical structure characterization of organic thin film targets. IV. CONCLUSIONS
The energetic cluster impact ionization TOF SI mass spectrometry with transmission assisted timing signals was developed to provide informative TOF SI mass spectra without using signals of the primary ion beam pulses. P- and N-SI TOF mass spectra were successfully obtained by the newly developed TOF SI mass spectroscopy that uses detection signals of the particles emitted from the rear surface of foil targets upon the transmission of energetic primary cluster ions through the targets. Although the thickness of a target must be limited so that the particles can be emitted from the rear surface of the target, this spectrometry can be applied to chemical structure characterization of organic thin film targets. Its potentialities and limitations are related to the important subjects of energetic clusters that include transmission properties of energetic cluster ions through foil targets and emission properties of particles from the rear surface of foil targets upon the transmission, which will be studied in the future. The particles emitted from the rear surface contain not only information about timing but also various other information, including the interaction between the incident ion and the target, indicating that the spectrometry can be used in basic and applied studies of energetic cluster ion impacts of target materials by coupling detection and analysis of both the particles and the SIs.
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ACKNOWLEDGMENTS
This work was partially supported by the Interorganization Atomic Energy Research Program. 1 For example, Ion and Neutral Spectroscopy, Practical Surface Analysis Vol.
2, edited by D. Briggs and M. P. Seah (John Wiley and Sons, Chichester, 1992). 2 K. Hirata, Y. Saitoh, K. Narumi, and Y. Kobayashi, Appl. Phys. Lett. 81, 3669 (2002). 3 K. Hirata, Y. Saitoh, A. Chiba, K. Yamada, Y. Takahashi, and K. Narumi, Nucl. Instrum. Methods Phys. Res. B 268, 2930 (2010). 4 K. Hirata, Y. Saitoh, A. Chiba, K. Yamada, and K. Narumi, Appl. Phys. Express 4, 116202 (2011). 5 Y. Saitoh, S. Tajima, I. Takada, K. Mizuhashi, S. Uno, K. Ohkoshi, Y. Ishii, T. Kamiya, K. Yotsumoto, R. Tanaka, and E. Iwamoto, Nucl. Instrum. Methods Phys. Res. B 89, 23 (1994). 6 K. Hirata, Y. Saitoh, A. Chiba, K. Narumi, Y. Kobayashi, and Y. Ohara, Appl. Phys. Lett. 86, 044105 (2005). 7 K. Hirata, Y. Saitoh, A. Chiba, M. Adachi, K. Yamada, and K. Narumi, Nucl. Instrum. Methods Phys. Res. B 266, 2450 (2008). 8 K. Hirata, Y. Saitoh, A. Chiba, K. Yamada, Y. Takahashi, and K. Narumi, Rev. Sci. Instrum. 82, 033101 (2011). 9 K. Narumi, K. Nakajima, K. Kimura, M. Mannami, Y. Saitoh, S. Yamamoto, Y. Aoki, and H. Naramoto, Nucl. Instrum. Methods Phys. Res. B 135, 77 (1998).
Rev. Sci. Instrum. 85, 033107 (2014) 10 K. Baudin, A. Brunelle, M. Chabot, S. Della-Negra, J. Depauw, D. Gardks,
P. Håkansson, Y. Le Beyec, A. Billebaud, M. Fallavier, J. Remillieux, J. C. Poizat, J. P. Thomas, Nucl. Instrum. Methods Phys. Res. B 94, 341 (1994). 11 The thickness in μg/cm2 is often used in discussing the transmission of incident ions through foils. Provided that the density of the foils is 2.25 g/cm3 , the thicknesses of the 5 μg/cm2 and 10 μg/cm2 foils in length are estimated to be 22 nm and 44 nm, respectively. 12 M. Seidl, H. Voit, S. Bouneau, A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, and M. Pautrat, Nucl. Instrum. Methods Phys. Res. B 183, 502 (2001). 13 E. J. Sternglass, Phys. Rev. 108, 1 (1957). 14 Y. Takahashi, K. Narumi, A. Chiba, Y. Saitoh, K. Yamada, N. Ishikawa, H. Sugai, and Y. Maeda, EPL 88, 63001 (2009). 15 S. Tomita, S. Yoda, R. Uchiyama, S. Ishii, K. Sasa, T. Kaneko, and H. Kudo, Phys. Rev. A 73, 060901 (2006). 16 K. Hirata, Y. Saitoh, A. Chiba, K. Yamada, and K. Narumi, Nucl. Instrum. Methods Phys. Res. B 314, 39 (2013). 17 O. W. Webster, Science 251, 887 (1991). 18 M. J. Hearn and D. Briggs, Surf. Interface Anal. 11, 198 (1988). 19 A. M. Leeson, M. R. Alexander, R. D. Short, D. Briggs, and M. J. Hearn, Surf. Interface Anal. 25, 261 (1997). 20 J. G. Newman, B. A. Carlson, R. S. Michael, J. F. Moulder, and T. A. Hohlt, in Static SIMS Handbook of Polymer Analysis, edited by T. A. Hohlt (Perkin-Elmer Corp., 1991). 21 A. Delcorte, X. V. Eynde, P. Bertrand, J. C. Vickerman, and B. J. Garrison, J. Phys. Chem. B 104, 2673 (2000).
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 033107 (2014)
Time-of-flight secondary ion mass spectrometry with transmission of energetic primary cluster ions through foil targets K. Hirata,1,a) Y. Saitoh,2 A. Chiba,2 K. Yamada,2 S. Matoba,2 and K. Narumi2 1 National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan 2 Takasaki Advanced Radiation Research Institute (TARRI), Japan Atomic Energy Agency (JAEA), Takasaki, Gumma 370-1292, Japan
(Received 4 October 2013; accepted 8 March 2014; published online 24 March 2014) We developed time-of-flight (TOF) secondary ion (SI) mass spectrometry that provides informative SI ion mass spectra without needing a sophisticated ion beam pulsing system. In the newly developed spectrometry, energetic large cluster ions with energies of the order of sub MeV or greater are used as primary ions. Because their impacts on the target surface produce high yields of SIs, the resulting SI mass spectra are informative. In addition, the start signals necessary for timing information on primary ion incidence are provided by the detection signals of particles emitted from the rear surface of foil targets upon transmission of the primary ions. This configuration allows us to obtain positive and negative TOF SI mass spectra without pulsing system, which requires precise control of the primary ions to give the spectra with good mass resolution. We also successfully applied the TOF SI mass spectrometry with energetic cluster ion impacts to the chemical structure characterization of organic thin film targets. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869036] I. INTRODUCTION
Time-of-flight (TOF) secondary ion (SI) mass spectrometry, which measures the flight time difference of SIs in a drift tube with certain length, is used to determine the mass-tocharge ratio m/z of SIs produced by primary ion impacts on a target. The spectrometry is a powerful tool for surface characterization in various research fields,1 as it allows sensitive detection of SIs over a wide range of m/z and gives chemical specific information about the surfaces of the targets. SIs are emitted from the target surface as a result of the transfer of energy from the incident ion to the target. Consequently, the SI emission yields strongly depend on various parameters of the target and the incident ions. Cluster ion impacts provide different energy transfer from monoatomic ion impacts because the constituent atoms of a primary cluster ion simultaneously impact a very small area of the target surface. We demonstrated that energetic primary cluster ions with energies of the order of sub MeV or greater have various advantages for highly sensitive analysis of the surface chemical structure of organic targets due to the high yields of SIs characteristic of the target materials.2–4 Thus, high SI yields induced by impacts of the energetic primary cluster ions on targets confirms that SI mass spectra can produce valuable information. The measurements of SI flight times by TOF spectrometers require timing signals for both the primary ion incidence on the target (the start signal) and the SI detection (the stop signal). The stop signal is simply obtained from an SI detector placed at the end of the flight path in a TOF drift tube, whereas the start signal measurements require more sophisti-
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(3)/033107/6/$30.00
cated methods. Although using a signal from a pulse generator to trigger the primary ion beam pulse is the most widely employed method to obtain the start signals, this method requires a well-designed pulsing system and precise control of the primary ion beam path to produce an ion beam with a short pulse width in order to give good m/z resolution. The detection signals of secondary electrons (SEs) emitted from the front surface of the target can be used for the start signals. However, in order to detect both the SEs and positive SIs (P-SIs) for the start and stop signals, respectively, the SEs must be sufficiently separated from the P-SIs, which are emitted from the same side of the target with opposite charge from that of the SEs. Thus, P-SI measurements pose some problems. An energetic cluster primary ion penetrates to a certain depth from the front surface and can even be transmitted to the rear surface of a target, depending on the conditions of the target and primary cluster ion, although the primary cluster immediately decomposes into its constituent particles after striking the target surface. As a result of the transmission, the constituent particles originating from the primary cluster, along with the forward secondary particles (F-SPs), including forward SEs (F-SEs) and SIs (F-SIs), are emitted from the rear surface of the target. This emission from the rear surface of the target can provide timing information for the primary ion incidence. The use of start timing signals given by this method should provide both P- and negative-SI (N-SI) TOF mass spectra without a sophisticated pulsing system because the electric potentials to obtain the start and stop signals are independently applied from different sides of the target. In this paper, we present TOF SI mass spectrometry that uses detection signals of the particles emitted from the rear surface of a foil target upon the transmission of energetic primary cluster ions through the target.
85, 033107-1
© 2014 AIP Publishing LLC
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High voltage power supply
Deflection plates
Pulse generator
Collimators
Gate & Delay MCP
Forward Foil secondary target particles
Collimators
Secondary ion
Preamplifier
MCP
TOF drift tube
Transmitted particles SSD Preamplifier
energy
Amplifier
Preamplifier
timing
Fast timing Fast-timing amplifier (b)
(a)
Multichannel analyzer
SSD mode. The setup for the pulse mode is the same as that previously described in Refs. 6 and 7. SIs emitted from the front surface of the target are accelerated between the target and a TOF mass spectrometer before hitting a 40-mm-diameter MCP (Hamamatsu F-2225) placed at the end of the flight pass. The MCP’s output signals are fed into a fast CFD via a pre-amplifier and used as the stop signals. The cluster ion impacts on targets are expected to give a relatively high total SI yield per impact because, in addition to the enhanced SI yield per incident atom, the number of incident atoms per impact for a cluster ion is n (n: cluster number) times higher than that for the corresponding monoatomic ion. Therefore, we used an SI counting system that can accurately obtain TOF SI mass spectra even when many SIs are detected for one ion impact.8 All SI mass spectra were obtained without intentional charge compensation. The primary ions were injected into the targets at a rate between several tens and several hundreds ions/s.
Computer 1
III. RESULTS AND DISCUSSION (c) Constant fraction discriminator
Counting system
Constant fraction discriminator
Computer 2
FIG. 1. Schematic illustration of TOF SI mass spectroscopy with transmission of energetic primary cluster ions through foil targets.
II. EXPERIMENTAL
Figure 1 is a schematic diagram of the TOF SI mass spectroscopy that transmits energetic primary cluster ions through foil targets. The primary cluster ions are injected into a thin foil target at an angle of 45o to the target surface, then transmitted through the target. Since a primary ion with a higher incident energy can transmit through a thicker target, energetic incident ions are suitable primary ions for the TOF SI mass spectrometry with transmission. Energetic cluster ions with energies on the order of sub MeV or greater, which are produced by a 3 MV tandem accelerator and a 400 kV ion implanter at the Japan Atomic Energy Agency (JAEA) in Takasaki,5 are used in the spectroscopy. As shown in Fig. 1, three types of start signals are available for the SI flight time measurements: (a) detection signals of transmitted constituent particles originating from the primary cluster from a solid-state detector (SSD) (Ortec BU-014-150-100) placed approximately 2 cm downstream from the target (Forward SSD mode: F-SSD mode), (b) detection signals of FSPs from a microchannel plate (MCP) (Hamamatsu F-4655) placed approximately 3 cm in front of the rear side of the target (Forward MCP mode: F-MCP mode), and (c) signals from a pulse generator for triggering the primary ion beam pulse (pulse mode). For both the F-SSD and F-MCP modes, the timing signals from pre-amplifiers (Ortec 142) are transferred to a fast constant fraction discriminator (CFD) to be used as the start signals. Pulse-height measurements for the incident clusters and their constituents are available under the
A. F-SSD mode
Cluster primary ions injected into a foil target are immediately decomposed into their constituents. Due to scattering of the atomic constituents by target atoms and/or a Coulomb explosion between charged constituents, the average distance between the constituents increases in correspondence with the depth from the target surface. Consequently, the distance is large enough to be detected as an isolated atom at a detector located downstream from the target.9, 10 Here, we discuss the use of detection signals of the transmitted constituents originating from primary clusters for TOF SI mass measurements under the F-SSD mode. As a typical example of the SSD detection signals for a primary cluster ion transmitted through foil targets, Figs. 2(a) and 2(b) show pulse-height spectra for 4.0 MeV C8 + transmitted through self-supporting amorphous carbon foils (Arizona Carbon Foil Co. Inc.) with thicknesses of 5 μg/cm2 and 10 μg/cm2 , respectively.11 The pulse-height spectra provide information about the transmission properties of the primary cluster ions through the targets. To investigate the properties of the primary cluster ions before the transmission, Fig. 2(c) shows the pulse-height spectrum for 4.0 MeV C8 + without a target. The relative intensity for each spectrum is obtained by dividing the count for each channel by the total count. The large peak around channel 890 in the spectrum without a target (Fig. 2(c)) is attributed to the direct detection of single C8 . The two weak peaks observed around channels 340 and 570, corresponding to C3 and C5 detection, respectively, indicate that C8 preferentially decomposes into C3 and C5 . The pulse-height spectra for C8 with the 5 μg/cm2 carbon foil (Fig. 2(a)) and the 10 μg/cm2 carbon foil (Fig. 2(b)) exhibit the respective intense peaks around channel 920 and 870 that correspond to detection of all the constituent atoms of the primary C8 cluster (p = 8: p is the number of detected C atoms). The pulse-height channel number of the peak (p = 8) in Fig. 2(b) is lower than that in Fig. 2(a) because an increasing thickness of foil results in an increasing averaged
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10-1
Rev. Sci. Instrum. 85, 033107 (2014)
1
(a)
p=8
10-2
p=7 p=6
Relative inttensity
10-33 10-4 ~ ~
p=3
p=5
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p=7
Rellative intens sity
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10-11 10
/cm / 2
2
10-2
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10-3 p = 3 p = 5 10-4 ~ ~
10-3
6
7
8
p number
(c)
FIG. 3. Ratio of the relative intensities of p = 8, 7, and 6 for 4 MeV C8 + transmitted through carbon foils with thicknesses of (a) 5 μg/cm2 and (b) 10 μg/cm2 under the F-SSD mode. The lines are obtained from the expression, p n-p 2 n Cp γ (1 − γ ) , where n = 8, and γ = 0.99 and 0.98 for 5 μg/cm and 10 μg/cm2 , respectively.
C8
10-2 10-3 10-4 200
400
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1000 1200
MCA channel number FIG. 2. Pulse-height spectra for 4 MeV C8 + transmitted through carbon foils with thicknesses of (a) 5 μg/cm2 and (b) 10 μg/cm2 under the F-SSD mode. Also included in the figure is (c) the pulse-height spectrum for 4.0 MeV C8 + without a target.
energy loss of constituent atoms in the foil. It should be noted that the pulse-height channel number for peak p = 8 in Fig. 2(a) is obviously higher than that for the direct detection of single C8 in Fig. 2(c), although the C8 clusters in Fig. 2(a) have lost a certain amount of their kinetic energy during transmission through the target, i.e., the pulse height for the detection of n atomic constituents with an average energy of Eatom is higher than that of n clusters with an energy of En,cluster , although nEatom < En,cluster . The pulse height for direct cluster detection is lower than expected due to the pulse-height defect that arises from incomplete charge collection in the SSD detector as a result of higher probability of electron-hole recombination in the densely ionized region produced by cluster ion irradiations.9, 10, 12 As the interaction with target atoms and/or between the constituents makes the velocity direction of each constituent deviate from that of the incident cluster, a fraction of the constituents may miss the detector. As a result of the missing constituents, peaks for p = 7, 6 . . . are observed for the pulseheight spectra (with the foil) in Figs. 2(a) and 2(b) in addition to those for p = 8. The relatively higher peaks for p = 3 and 5 (in Figs. 2(a) and 2(b)) are assumed to be primarily due to C3 and C5 , produced by the decomposition of C8 before injection into the targets as mentioned above. The pulse-heights in Fig. 2(b) are smaller than those with corresponding p in Fig. 2(a) because the averaged energy loss of the constituents increases with the increasing foil thickness. It should be noted that the relative peak intensities for p = 8, 7, and 6 are different between Figs. 2(a) and 2(b); the intensity for p = 8 in Fig. 2(b) is lower, but those for p = 7, 6 are higher than the intensities with corresponding p in Fig. 2(a), respectively. This result can be attributed to a lower detection probability of the atomic constituent in Fig. 2(b) than in Fig. 2(a) due to the
increase of a fraction of the constituents missing the detector according to the foil thickness. For comparison of the detection probability of the atomic constituents in Figs. 2(a) and 2(b), Fig. 3 shows the ratio of the relative intensities for p = 8, 7, and 6. The lines are obtained from the following expression, F(γ ) = n Cp γ p (1 − γ )n-p , where γ is the detection probability of the atomic constituent. The experimental data for the 5 μg/cm2 and 10 μg/cm2 foils are well reproduced by the expression for γ = 0.99 and 0.98, respectively. Although the γ value varies with experimental conditions such as the thickness of the target and the distance between the target and the detector, cluster primary ions are advantageous to provide the start timing signals due to multichances of detection for one impact. The undetected probability, obtained by the expression (1 − γ )n , is almost zero for both cases. Figure 4(a) shows the P-SI TOF mass spectrum of carbon foil (5 μg/cm2 ) for 4.0 MeV C8 + under the F-SSD mode. The timing signals, simultaneously provided with the pulse-height signals by the pre-amplifier, were used as the start signals. The major peaks observed in Fig. 4(a) can be attributed to ions of singly charged atomic and molecular hydrogen [H+ (m/z = 1), H2 + (m/z = 2), and H3 + (m/z = 3)], hydrocarbon [e.g., C2 H3 + (m/z = 27), C2 H5 + (m/z = 29), C3 H5 + (m/z = 41), C3 H7 + (m/z = 43)], Na+ (m/z = 23), and K+ (m/z = 39) originating from the target and its surface contaminants. For comparison, the P-SI TOF mass spectrum of the same foil for 4.0 MeV C8 + under the pulse mode is shown in Fig. 4(b). The spectrum in Fig. 4(a) is essentially the same as that in Fig. 4(b), and thus demonstrates that P-SI TOF mass spectra can be obtained using a simple setup without measuring the signals of the primary ion beam pulses. The F-SSD mode is also available for other energetic larger primary cluster ions, including MeV order C60 primary ions, as shown in Fig. 4(c). We also obtained N-SI mass spectra under the FSSD mode (spectra are not shown). The spectra demonstrate that both P- and N-SI TOF mass spectra with cluster ion impacts can be obtained using a simple setup without measuring the signals of the primary ion beam pulses under the F-SSD mode.
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Rev. Sci. Instrum. 85, 033107 (2014)
Intensity
Inte ensity
(a)
0~ ~
(b)
0
0
10
20
30
40
50
60
m/z FIG. 5. P-SI TOF mass spectrum of carbon foil (5 μg/cm2 ) for 2.0 MeV C4 + under the F-MCP mode.
0
0
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30
40
50
60
40
50
60
m/z
ntensity In
(c)
0
0
10
20
30
m/z FIG. 4. P-SI TOF mass spectra of carbon foil (5 μg/cm2 ) for 4.0 MeV C8 + under (a) F-SSD and (b) pulse modes. Also included in the figure is (c) the P-SI TOF mass spectrum of carbon foil (5 μg/cm2 ) for 5.0 MeV C60 + under the F-SSD mode.
B. F-MCP mode
F-SPs such as F-SEs and F-SIs, emitted from the rear surface of targets, can also provide timing information for the primary ion incidence. In the case of the F-MCP mode, the detection signals of F-SEs are more suitable than those of F-SIs, because the F-SEs reach the detector much faster than the F-SIs by applying the extraction potential in detection of charged F-SPs. Here we discuss the TOF SI mass measurements using F-SEs detection signals as the start signals under the F-MCP mode. The SE emission induced by a swift monoatomic ion impact on a solid target can be explained by a three-step process: production of scattered electrons by the incident ion, transport of the electrons in the target, and transmission of the electrons through the surface barrier.13 Cluster ion impacts have been reported to give different F-SE emission phenomena from those for monoatomic ion impacts: the F-SE yields per incident atom for cluster ion impacts on thin carbon foil targets were reduced in comparison with those for a monoatomic ion with the same element and velocity as the cluster.14 In addition, the detailed cluster number dependence of F-SE energy spectra for 0.5 MeV/atom Cn + (n = 1, 2, 3, 4) impacts on carbon foil targets revealed that the F-SE yields per incident atom at the energy range corresponding to the convoy electrons increase with the cluster number, whereas the yields at other energy regions are reduced.15 These differences in the F-SE emission between cluster and monoatomic ion impacts are attributed to the transport or transmission process.14, 15
The F-SE detection signals are available for use as the start signals of TOF SI measurements, although the detailed mechanism that quantitatively explains the F-SE emission by cluster ion impacts is currently unknown. The F-SE yields per incident atom decrease with increasing cluster number. However, their yields per incident ion for cluster ions are still higher than those for the corresponding monoatomic ion as reported above and indicate that enough F-SEs are produced by the transmission of cluster ions. The energy distribution of F-SEs does not affect the start timing very much because even an electron with an initial kinetic energy of 0 eV reaches a detector placed several cm in front of a foil target within several ns by applying an extraction potential of several 100 V. Figure 5 shows the P-SI TOF mass spectrum of selfsupporting amorphous carbon foil (5 μg/cm2 ) for 2.0 MeV C4 + impacts by detecting F-SEs under the F-MCP mode. The spectrum demonstrates that P-SI TOF mass spectra can be obtained using the F-SE detection signals as the start signals for TOF SI measurements, in contrast to using the SEs emitted from the front surface.16 Use of the detection signals under the F-MCP mode has an advantage over those under the F-SSD mode when the kinetic energy of the constituent originating the primary cluster is low. Under the F-SSD mode, the constituent is required to have enough kinetic energy to produce enough electronhole pairs in the detector after penetrating the dead layer of the detector surface. Therefore, the use of detection signals is limited to constituents having enough kinetic energy to be detected by the SSD detector. This limitation can be extended to using F-SEs detection signals as the start signals under the F-MCP mode. Figure 6 shows P-SI and N-SI TOF mass spectra of the carbon foil (1 μg/cm2 ) for 540 keV C60 2+ under the F-MCP mode. The projected range of monoatomic C with incident energy of 9 keV, corresponding to 540 keV C60 , is approximately 30 nm for a silicon matrix, which is shorter than the dead layer thickness of the SSD (approximately 50 nm). In addition to the shorter projected range, the individual constituent C atom loses its kinetic energy in the target, and so it is difficult to obtain the start signals for 540 keV C60 2+ under the F-SSD mode. In contrast, the F-MCP mode can provide the start signals for F-SPs and is available for a constituent having lower kinetic energy at the rear surface of the target. Although the F-MCP mode must apply an extraction potential to increase the velocity of the F-SPs to the MCP for the start signals in addition to the potential for the stop signals,
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Rev. Sci. Instrum. 85, 033107 (2014) 1
(a)
3
(a)
0
0
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m/z
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5
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Intensity
ntensity In
100
m/z
(b)
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69
Inte ensity
Intensity y
59
51 77 39
0
10
20
30
40
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60
m/z 0 FIG. 6. Mass spectra of (a) P-SI and (b) N-SI under F-MCP mode for 540 keV C60 2+ impacts on carbon foil (1 μg/cm2 ).
both the P-SI and N-SI spectra can be obtained as the potentials are independently applied at different surfaces of the foil target. C. Application to chemical structure characterization of organic thin film targets
The energetic cluster impact ionization TOF SI mass spectrometry with transmission assisted timing signals can also provide SI mass spectra of organic thin film targets. As an example of SI mass spectra of organic thin film targets, Fig. 7 shows P-SI TOF mass spectra of (a) poly(methyl methacrylate) (PMMA) and (b) polystyrene (PS) thin-film targets for 4.0 MeV C8 + under the SSD mode. The PMMA and PS targets were produced by dropping toluene solutions of either PMMA or PS (Sigma-Aldrich Co.) on silicon nitride membranes with a thickness of 20 nm (Silison Ltd.). The thicknesses of the PMMA and PS layers are respectively approximated to be 1.7 and 0.7 μg/cm2 , based on pulse-height spectra obtained by the SSD detector. The peaks of atomic hydrogen (H+ ) and hydrocarbon (Cx Hy + ) observed in Figs. 7(a) and 7(b) are typical for polymers containing hydrogen and carbon. The small peak at m/z = 7 in Fig. 7(b) can be attributed to Li+ , which indicates that a lithium containing initiator was probably used in polymerization.17 In addition, characteristic peaks for each polymer are observed in Fig. 7. The respective peaks at m/z = 59 and 69 in Fig. 7(a) represent C2 H3 O2 + and C4 H5 O+ , which are characteristic for the P-SI mass spectrum of PMMA; C2 H3 O2 + is originated from ester group and C4 H5 O+ is related to the main chain of the polymer structure.18–20 In Fig. 7(b), the peak at m/z = 77 (C6 H5 + ) and those at m/z = 39 (C3 H3 + ), 51 (C4 H3 + ) are respectively originated from the phenyl side group in PS and its fragments. This result is characteristic for the P-SI mass spectrum for PS.21 The intense peak at
0
20
40
60
80
100
m/z FIG. 7. P-SI TOF mass spectra of (a) poly(methyl methacrylate) (PMMA) and (b) polystyrene (PS) thin-film targets for 4.0 MeV C8 + under the SSD mode.
m/z = 91 (C7 H7 + ) is also characteristic for PS.20, 21 The agreement of the characteristic peaks in both spectra with the previous typical SI mass spectra demonstrates that the energetic cluster impact ionization TOF SI mass spectrometry with transmission assisted timing signals can be applied to chemical structure characterization of organic thin film targets. IV. CONCLUSIONS
The energetic cluster impact ionization TOF SI mass spectrometry with transmission assisted timing signals was developed to provide informative TOF SI mass spectra without using signals of the primary ion beam pulses. P- and N-SI TOF mass spectra were successfully obtained by the newly developed TOF SI mass spectroscopy that uses detection signals of the particles emitted from the rear surface of foil targets upon the transmission of energetic primary cluster ions through the targets. Although the thickness of a target must be limited so that the particles can be emitted from the rear surface of the target, this spectrometry can be applied to chemical structure characterization of organic thin film targets. Its potentialities and limitations are related to the important subjects of energetic clusters that include transmission properties of energetic cluster ions through foil targets and emission properties of particles from the rear surface of foil targets upon the transmission, which will be studied in the future. The particles emitted from the rear surface contain not only information about timing but also various other information, including the interaction between the incident ion and the target, indicating that the spectrometry can be used in basic and applied studies of energetic cluster ion impacts of target materials by coupling detection and analysis of both the particles and the SIs.
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ACKNOWLEDGMENTS
This work was partially supported by the Interorganization Atomic Energy Research Program. 1 For example, Ion and Neutral Spectroscopy, Practical Surface Analysis Vol.
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