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High resolution imaging for charged particles using CR-39 and atomic force microscopy
Nuclear Instruments and Methods in Physics Research A 422 (1999) 751—755
High resolution imaging for charged particles using CR-39 and atomic force microscopy H. Takahashi *, K. Amemiya , Y. Kaizuka , M. Nakazawa , N. Yasuda, M. Yamamoto, T. Sakai, T. Kamiya, S. Okada Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan National Institute of Radiological Sciences, Anagawa-4, Inage-ku, Chiba 263-8555, Japan Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, 370-12 Gunma, Japan
Abstract Recently Atomic Force Microscopy (AFM) has been used for a readout of Solid State Nuclear Track Detectors (SSNTD). In this technique, the SSNTD is etched for very short time and scanned by an AFM to observe track etch pits. Etch pits of several hundred nm in diameter can be observed. We applied this technique to charged particle imaging using CR-39. CR-39 plates were etched for about 5 min in 27% NaOH solution at 70°C. Then very small etch pits were measured. A typical diameter of etch pits was below 1 lm and a typical etch pit depth was about several hundred nm. The obtained image has demonstrated a position resolution of less than 100 nm. Ion microbeam profiles were also measured by the system. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Recently new surface observation techniques have become available such as Scanning Tunnel Microscopy (STM), Atomic Force Microscopy (AFM), and Confocal Scanning Laser Microscopy (CSLM). AFM has been widely used for the observation of insulator because it provides a very precise image of a three dimensional surface structure. Thus, AFM has recently been applied to Solid State Nuclear Track Detectors (SSNTD) by several
* Corresponding author. Tel.: #81-3-3812-2111 ext.7007; fax: #81-3-5802-3341; e-mail: [email protected].
researchers and its excellent characteristics have been reported [1—4]. In this technique, the SSNTD is etched for very short time (normally about several minutes). Then the etched surface of the SSNTD is examined by the AFM. Consequently, very small etch pits of several hundred nm in diameter can be observed. Our group has studied this method for the dosimetry of high flux charged particle beams in the heavy ion therapy [4]. However, the technique can also provide very high resolution imaging of charged particles without a complicated setup. This paper describes characteristics of the imaging technique for charged particles using CR-39 and AFM.
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 0 3 0 - 4
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H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
2. AFM readout method of CR-39 We have tested the AFM readout technique of CR-39 for charged particle imaging. HARZLAS CR-39 plates from FUKUVI company were used in our experiments. CR-39 plates were etched for 5 min in 27% NaOH solution at 70°C. Then the plates were set into an AFM (Digital Instruments, Nanoscope III). The AFM was operated in a tapping mode. Although the initial track etching rate was found to be smaller than that observed after several hours [5], etching was successfully performed and very small track etch pits were observed. A typical etch pit size was below 1 lm depending on the restricted energy loss (REL) of incident charged particles and a typical etch pit depth was about several hundred nm. To examine the performance of observing submicrometer scale
images of charged particles, CR-39 plates were irradiated by several kinds of ions using a Tandetron charged particle accelerator at RCNST, the University of Tokyo. Ions were incident at 7° on the plates due to the setup of the irradiation port. Fig. 1 shows an example of a surface image obtained with 1 MeV carbon ion irradiation where carbon ions were irradiated through a silver mesh mask with a 10 lm grid (3.8 lm thick). To clearly show etch pit shapes, the depth from the surface is given in z-axis. There exist etch pits of several hundred nm in diameter. Fig. 2 is a typical cross section of an etch pit obtained with 1 MeV carbon ion irradiation and Fig. 3 shows an etch pit shape of a 1 MeV proton where the same etching condition as Fig. 2 was applied. Because the initial track etching rate is slower than the track etch rate after several hours, the etch pit shape is often observed as
Fig. 1. An image obtained with 1 MeV carbon irradiation.
H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
an imperfect cone shape. However, Fig. 3 clearly demonstrates the position resolution of several tens of nm. Fig. 4 shows modulation transfer functions (MTF) calculated from point spread functions for two kinds of charged particles (1 MeV protons and
753
1 MeV carbon ions). A spatial frequency of more than 10 000 line pairs/mm can be resolved for 1 MeV proton tracks. Moreover, adjusting etching time to the optimum one, we can obtain the best
Fig. 4. Modulation transfer functions for etch pits irradiated by 1 MeV protons and 1 MeV carbons. Fig. 2. Typical etch pit shape measured with 1 MeV carbon ion irradiation.
Fig. 3. Typical etch pit shape measured with 1 MeV proton irradiation.
Fig. 5. An optical microscope image obtained with 15 MeV nickel ion microbeam irradiation.
VII. APPLICATIONS
754
H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
spatial resolution for a particular particle and energy. Since we have kept the same etching condition to make a systematic comparison among results, the etch pit size can be more smaller than the demonstrated result. Because the AFM has an ability to observe a single atom, the surface ruggedness of CR-39 plates after etching and the etch pit shape determine the ultimate resolution of the system. A typical value of the surface ruggedness is about several nm and, for the previous proton data, the measured surface ruggedness was 1.6 nm rms. Thus, the ultimate resolution of this method is thought to be about the order of several nm. Etch pit shapes are dependent on REL of incident charged particles. Besides the excellent spatial resolution, the use of AFM provides three dimen-
sional information on the etch pit shape which is difficult to be obtained with conventional optical microscopes [2]. This information might be used for the identification or discrimination among various incident particles. This is another advantage over other methods of the CR-39 readout, such as the SEM readout. As an example of high resolution imaging, 15 MeV nickel ion microbeam was irradiated on a CR-39 plate. Fig. 5 shows an image obtained with a conventional optical microscope. Each etch pit corresponds to a single ion. An AFM image is shown in Fig. 6 which was obtained by scanning a part of the irradiated area. Although a gap in the cone shape of each etch pit is observed in Fig. 6, the reason of this effect is under investigation.
Fig. 6. An AFM image obtained with 15 MeV nickel ion microbeam irradiation.
H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
755
3. Conclusion
Acknowledgements
The AFM readout of CR-39 plates has been applied to the high resolution charged particle radiography. The spatial resolution of less than 100 nm has been observed for 5 min etching time in 27% NaOH solution at 70°C. The ultimate resolution of the method is dependent on the surface ruggedness and the REL of incident charged particles. This method is a passive detection method, however, it can scan a large area up to several cm with very high resolution. Applications of this method include microbeam monitoring, microdosimetry in biological samples, etc.
The authors would like to thank Prof. K. Ogura of College of Industrial Technology, Nihon University.
References [1] [2] [3] [4] [5]
G. Binnig et al., Phys. Rev. Lett. 56 (1986) 930. M. Drndic et al., Nucl. Instr. and Meth. B 93 (1994) 52. K. Havancsak et al., Rad. Meas. 28 (1997) 65. M. Yamamoto et al., Rad. Meas. 28 (1997) 227. N. Yasuda et al., Nucl. Instr. and Meth. B 142 (1998) 111.
VII. APPLICATIONS
High resolution imaging for charged particles using CR-39 and atomic force microscopy H. Takahashi *, K. Amemiya , Y. Kaizuka , M. Nakazawa , N. Yasuda, M. Yamamoto, T. Sakai, T. Kamiya, S. Okada Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan National Institute of Radiological Sciences, Anagawa-4, Inage-ku, Chiba 263-8555, Japan Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, 370-12 Gunma, Japan
Abstract Recently Atomic Force Microscopy (AFM) has been used for a readout of Solid State Nuclear Track Detectors (SSNTD). In this technique, the SSNTD is etched for very short time and scanned by an AFM to observe track etch pits. Etch pits of several hundred nm in diameter can be observed. We applied this technique to charged particle imaging using CR-39. CR-39 plates were etched for about 5 min in 27% NaOH solution at 70°C. Then very small etch pits were measured. A typical diameter of etch pits was below 1 lm and a typical etch pit depth was about several hundred nm. The obtained image has demonstrated a position resolution of less than 100 nm. Ion microbeam profiles were also measured by the system. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Recently new surface observation techniques have become available such as Scanning Tunnel Microscopy (STM), Atomic Force Microscopy (AFM), and Confocal Scanning Laser Microscopy (CSLM). AFM has been widely used for the observation of insulator because it provides a very precise image of a three dimensional surface structure. Thus, AFM has recently been applied to Solid State Nuclear Track Detectors (SSNTD) by several
* Corresponding author. Tel.: #81-3-3812-2111 ext.7007; fax: #81-3-5802-3341; e-mail: [email protected].
researchers and its excellent characteristics have been reported [1—4]. In this technique, the SSNTD is etched for very short time (normally about several minutes). Then the etched surface of the SSNTD is examined by the AFM. Consequently, very small etch pits of several hundred nm in diameter can be observed. Our group has studied this method for the dosimetry of high flux charged particle beams in the heavy ion therapy [4]. However, the technique can also provide very high resolution imaging of charged particles without a complicated setup. This paper describes characteristics of the imaging technique for charged particles using CR-39 and AFM.
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 0 3 0 - 4
VII. APPLICATIONS
752
H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
2. AFM readout method of CR-39 We have tested the AFM readout technique of CR-39 for charged particle imaging. HARZLAS CR-39 plates from FUKUVI company were used in our experiments. CR-39 plates were etched for 5 min in 27% NaOH solution at 70°C. Then the plates were set into an AFM (Digital Instruments, Nanoscope III). The AFM was operated in a tapping mode. Although the initial track etching rate was found to be smaller than that observed after several hours [5], etching was successfully performed and very small track etch pits were observed. A typical etch pit size was below 1 lm depending on the restricted energy loss (REL) of incident charged particles and a typical etch pit depth was about several hundred nm. To examine the performance of observing submicrometer scale
images of charged particles, CR-39 plates were irradiated by several kinds of ions using a Tandetron charged particle accelerator at RCNST, the University of Tokyo. Ions were incident at 7° on the plates due to the setup of the irradiation port. Fig. 1 shows an example of a surface image obtained with 1 MeV carbon ion irradiation where carbon ions were irradiated through a silver mesh mask with a 10 lm grid (3.8 lm thick). To clearly show etch pit shapes, the depth from the surface is given in z-axis. There exist etch pits of several hundred nm in diameter. Fig. 2 is a typical cross section of an etch pit obtained with 1 MeV carbon ion irradiation and Fig. 3 shows an etch pit shape of a 1 MeV proton where the same etching condition as Fig. 2 was applied. Because the initial track etching rate is slower than the track etch rate after several hours, the etch pit shape is often observed as
Fig. 1. An image obtained with 1 MeV carbon irradiation.
H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
an imperfect cone shape. However, Fig. 3 clearly demonstrates the position resolution of several tens of nm. Fig. 4 shows modulation transfer functions (MTF) calculated from point spread functions for two kinds of charged particles (1 MeV protons and
753
1 MeV carbon ions). A spatial frequency of more than 10 000 line pairs/mm can be resolved for 1 MeV proton tracks. Moreover, adjusting etching time to the optimum one, we can obtain the best
Fig. 4. Modulation transfer functions for etch pits irradiated by 1 MeV protons and 1 MeV carbons. Fig. 2. Typical etch pit shape measured with 1 MeV carbon ion irradiation.
Fig. 3. Typical etch pit shape measured with 1 MeV proton irradiation.
Fig. 5. An optical microscope image obtained with 15 MeV nickel ion microbeam irradiation.
VII. APPLICATIONS
754
H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
spatial resolution for a particular particle and energy. Since we have kept the same etching condition to make a systematic comparison among results, the etch pit size can be more smaller than the demonstrated result. Because the AFM has an ability to observe a single atom, the surface ruggedness of CR-39 plates after etching and the etch pit shape determine the ultimate resolution of the system. A typical value of the surface ruggedness is about several nm and, for the previous proton data, the measured surface ruggedness was 1.6 nm rms. Thus, the ultimate resolution of this method is thought to be about the order of several nm. Etch pit shapes are dependent on REL of incident charged particles. Besides the excellent spatial resolution, the use of AFM provides three dimen-
sional information on the etch pit shape which is difficult to be obtained with conventional optical microscopes [2]. This information might be used for the identification or discrimination among various incident particles. This is another advantage over other methods of the CR-39 readout, such as the SEM readout. As an example of high resolution imaging, 15 MeV nickel ion microbeam was irradiated on a CR-39 plate. Fig. 5 shows an image obtained with a conventional optical microscope. Each etch pit corresponds to a single ion. An AFM image is shown in Fig. 6 which was obtained by scanning a part of the irradiated area. Although a gap in the cone shape of each etch pit is observed in Fig. 6, the reason of this effect is under investigation.
Fig. 6. An AFM image obtained with 15 MeV nickel ion microbeam irradiation.
H. Takahashi et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 751—755
755
3. Conclusion
Acknowledgements
The AFM readout of CR-39 plates has been applied to the high resolution charged particle radiography. The spatial resolution of less than 100 nm has been observed for 5 min etching time in 27% NaOH solution at 70°C. The ultimate resolution of the method is dependent on the surface ruggedness and the REL of incident charged particles. This method is a passive detection method, however, it can scan a large area up to several cm with very high resolution. Applications of this method include microbeam monitoring, microdosimetry in biological samples, etc.
The authors would like to thank Prof. K. Ogura of College of Industrial Technology, Nihon University.
References [1] [2] [3] [4] [5]
G. Binnig et al., Phys. Rev. Lett. 56 (1986) 930. M. Drndic et al., Nucl. Instr. and Meth. B 93 (1994) 52. K. Havancsak et al., Rad. Meas. 28 (1997) 65. M. Yamamoto et al., Rad. Meas. 28 (1997) 227. N. Yasuda et al., Nucl. Instr. and Meth. B 142 (1998) 111.
VII. APPLICATIONS