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Feasibility of a cylindrical mirror electron analyzer for structural analysis of crystalline materials using weak ion beams
Nuclear Instruments and Methods in Physics Research B 142 (1998) 402±408
Feasibility of a cylindrical mirror electron analyzer for structural analysis of crystalline materials using weak ion beams Hiroshi Kudo
a,*
, Kiyomitsu Takada a, Kazumasa Narumi b, Shunya Yamamoto b, Hiroshi Naramoto b, Seiji Seki c
a
Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan b Japan Atomic Energy Research Institute, Takasaki 370-12, Japan c SSL Tsukuba Ltd., Ibaraki 305, Japan Received 3 February 1998; received in revised form 10 March 1998
Abstract An electron analyzer of the cylindrical mirror type has been developed for material analysis with MeV ions under low beam-dose conditions. The analyzer was successfully applied to measure the high-energy shadowing eect under extremely low beam current (by a factor of 10ÿ3 ±10ÿ2 ), down to the picoampere range, than that required in typical ion backscattering spectroscopy. Low beam current experiments should be useful for characterization of insulators suffering serious ionization-induced damage. Also, the analyzer might be used for measurements with picoampere beams of secondary particles such as positrons generated by accelerators. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Electron analyzer; Ion-induced electron emission; Ion beam analysis
1. Introduction When a solid target is bombarded with fast ions, the ion-induced electron yield observed at a backward angle is mainly due to recoil of target electrons [1]. The production process of the electrons can be treated with the binary encounter model, which accounts for the observation that the continuum energy spectrum is well characterized by the ion velocity and the spectrum yield
* Corresponding author. Fax: +81 298 53 5205; e-mail: [email protected].
for equal-velocity ions is approximately proportional to the square of the atomic number of the ions [2]. The electron emission is reduced under channeling incidence conditions since atoms near the surface de¯ect the ions away from the aligned atoms and, accordingly, the underlying atoms are eectively shadowed. Therefore, the electron yield re¯ects the high-energy shadowing eect which is the initial stage of ion channeling. For example, the shadowing pattern can be measured by recording the electron yield at a ®xed electron energy, typically in a keV energy range, as a function of the tilt angles of the crystal with respect to the
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 6 2 - 6
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
two independent axes perpendicular to the beam direction [3,4]. The electron measurement is applicable not only to metal and semiconductor targets, but also to insulator ®lms unless the ®lm thickness exceeds the mean penetration distance (projected range) of the ions [3,5]. The analysis with the ion-induced electrons is useful for observation of channeling of high-energy ions that cause serious non-Rutherford scattering in ion backscattering spectroscopy, which is of practical importance for detection of misaligned crystal atoms under a small critical angle of channeling [6]. It is important to note that the electron emission resulting from the recoil of target electrons is isotropic, irrespective of the crystallographic direction of the outgoing path [7]. The shadowing eect can therefore be observed with the ion-induced electrons emitted at any backward angle. The ratio of channeling to random (nonchanneling) yield at a ®xed electron energy can be used as a measure of the shadowing eect. So far, the ion-induced electrons have been energy-analyzed mostly using electrostatic analyzers with a solid angle of acceptance less than ~10ÿ2 sr [2±8]. If the emitted electrons are energy-analyzed and detected over a solid angle on the order of 1 sr, the high-energy shadowing eect is expected to be observed with weak ion beams or extremely low beam doses that have been impractical for backward measurements. For example, an ion
403
beam of picoampere currents might be practically used for analysis of thin insulator ®lms that suer serious radiation damage due to electronic excitations in a typical ion backscattering analysis using nanoampere beams [9,10]. Furthermore, it is of technical interest to investigate the possibility of observing shadowing and related phenomena using a beam of secondary particles such as positrons gererated by accelerators [11]. 2. The cylindrical mirror analyzer The design and operation of the wide acceptance-angle analyzer are dierent from those so far used for peak-search studies such as Auger electron spectroscopy, which require high energy resolution [12,13]. In the present case, the relative energy acceptance (resolution) De/e must be variable up to ~30% and the angular acceptance must be on the order of 1 sr to obtain high count rates. Also, the backscattered ions must be stopped before entering the electron multiplier to reduce background signals. Fig. 1 is a schematic diagram illustrating the electron analyzer of the cylindrical mirror type developed and investigated in the present study. The electrically grounded grid of the cylindrical shape (30-mm diameter) ®xed inside the analyzer consists of an array of 0.2-mm-diameter Cu wires stretched
Fig. 1. Schematic diagram of the cylindrical mirror electron analyzer (cross-sectional view), including examples of the calculated electron orbits for the two pairs of the parameters (a, E/eVp ) (24°, 5.0) and (15°, 7.0).
404
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
~4-mm-apart parallel to the cylinder axis. A microchannel plate (MCP) electron multiplier with a 4mm-diameter center hole (HAMAMATSU F2223-21SH) is used for detection of the energyanalyzed electrons. The incident ion beam passes through the 3-mm-diameter aluminum tube (Al thickness of 0.08 mm) along the cylinder axis and incident on the target crystal. This tube and the 7-mm-diameter disk ®xed at the end of the tube (the left end in Fig. 1) geometrically prevent the backscattered ions from entering the MCP. The three parallel grids in front of the MCP are used to set the lowest measurable electron energy. Negative voltage )Vg is applied to the center grid while the others are electrically grounded. When the distance L from the entrance of the analyzer to the target crystal is 30 mm, the ion-induced electrons emitted from the crystal surface in an angular range of 7°±27° with respect to the cylinder axis enter the analyzer. They are then de¯ected towards the cylinder axis by the electrostatic ®eld between the cylindrical grid and the coaxial cylindrical plate to which negative voltage )Vp is applied. The ring-shape electron repeller ()200 V) is used for accurate measurement of the ion beam current. The emission angle a with respect to the cylinder axis and the kinetic energy E of the electrons that are properly de¯ected and come onto the MCP can be numerically calculated by assuming the logarithmic electrostatic ®eld in the analyzer. For a given value of a, the electron orbit in the analyzer depends on the dimensionless parameter E/ eVp , where e is the electronic charge. Examples of the electron orbits are shown in Fig. 1 for the two pairs of the parameters (a, E/eVp ) (24°, 5.0) and (15°, 7.0). Fig. 2 is a calculated diagram for the initial conditions of the electrons, shown as a function of a and E/eVp . The results are shown for the three values of L. In Fig. 2, the electrons for the initial conditions shown inside the area surrounded by the curve come onto the MCP, while for other cases (outside the surrounded area) the electron orbit goes outside the MCP or is interrupted by the 3-mm-diameter aluminum tube. For the setup at L 30 mm, a wide angular range of approximately 8° 6 a 6 25° with respect to the cylinder axis, which corresponds to a solid angle of 0.53 sr, is accepted for 4.5 6 E/eVp 6 6.0. The
Fig. 2. Calculated diagram for the analyzer showing the transmission condition of the electrons as a function of the angle and kinetic energy in terms of E/eVp . For the initial conditions within the area surrounded by the curve, the electrons come onto the MCP and are counted.
dependence of the acceptance angle on the value of L is seen from the calculated curves for L 27 and 33 mm as well as L 30 mm. A slight change of L by 1 mm, for example, caused by the crystal tilt sensitively aects the measured electron yield only at E/eVp 8. 3. Shadowing measurements Operation and performance of the electron analyzer can be demonstrated by the measurements of the shadowing patterns obtained by tilting the target crystal, as mentioned in Section 1. The maximum depth contributing to the shadowing pattern is typically in the range of 10±100 nm for the present experimental conditions [3,4]. Fig. 3 shows the shadowing pattern around Siá1 0 0ñ axis for an angular range of 3° ´ 3° (tilt angles of h and /), measured using the electrons induced by 1.5 MeV He . In this case, we applied Vp 0.16 kV and Vg 0.82 kV, which lead to the measured range of E/eVp from 0.82/0.16 5.1 to 8.3 (the high-energy end of the measurable area shown in Fig. 2), i.e.,
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
405
Fig. 3. Shadowing pattern around Siá1 0 0ñ for an angular range of 3° ´ 3° (h- and /-tilt), measured with 0.82±1.33 keV electrons induced by 1.5 MeV He . The measurement time is ~20 min under the beam current of 0.45±0.50 nA. Note that the yield axis is taken in the downward direction for a better presentation of the shadowing pattern.
E 0.82±1.33 keV. For the beam spot size of ~0.8mm diameter on target, the He beam current was 0.45±0.50 nA which allows a count rate of ~6000 counts/s (including a negligible contribution from background signals [14]) for random incidence conditions. The shadowing pattern consists of the electron counts for 31 ´ 31 pixels of tilt angle, each of which was measured for 1.0±1.3 s corresponding to a ®xed beam dose of 6.2 ´ 1011 He / cm2 (0.5 nC on the 0.8-mm-diam spot size). The low-energy end of the measured energy range is equal to the binary-encounter peak energy EB 0.82 keV which is the maximum energy transferable from the ion to a rest electron (in a head-on collision). The electron yield at energies higher than EB mainly stems from recoiled inner-shell electrons that have high orbital velocities, compared to the outer-shell or valence electrons. Since the inner-shell electrons are more eectively shadowed than the others, the electron yield at energies above EB is more sensitive to the shadowing eect than below EB .
Fig. 4 shows another example of the shadowing pattern around Geá1 1 1ñ axis for an angular range of 2° ´ 2°, measured using the electrons induced by 20 MeV O4 collimated with a 25-lm-diameter aperture. The measured energy range of 4.1 6 E/ eVp 6 8.3, i.e., E 2.0±4.1 keV was chosen by applying Vp 0.49 kV and Vg 2.0 kV. In this case, the electrons at energies below and above EB 2.7 keV are counted. The O4 beam current on target as low as 7 pA provides a sucient count rate of ~2000 counts/s under random incidence conditions. The counting time for each of the 21 ´ 21 pixels of tilt angle to measure the pattern was ~2 s which corresponds to a ®xed beam dose of 4.5 ´ 1012 O4 /cm2 . If the observation angle is 180° with respect to the ion beam, the random electron yield does not depend on the tilt angle of the crystal because of the approximately straight outgoing-paths of the keV electrons [2]. For a wide observation angle like in the present case, care must be taken for the tilt-angle dependence of the random electron
406
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
Fig. 4. Shadowing pattern around Geá1 1 1ñ for an angular range of 2° ´ 2°, measured with 2.0±4.1 keV electrons induced by 20 MeV O4 collimated with a 25-lm-diameter aperture. The measurement time is 15 min under the beam current of ~7 pA. For each of the 21 ´ 21 tilt angles, the beam dose is 4.5 ´ 1012 O4 /cm2 for the counting time of ~2 s.
yield since it should aect the shadowing pattern. In Figs. 3 and 4, such in¯uence is not discernible in the present angular range of the crystal tilt within ~2° from the surface normal. To investigate the quality of the structural information obtained using the present method, the electron yield from a chemically etched Ni(1 1 0) crystal (~1-mm thickness and ~10-mm diameter) was measured with the 20 MeV O4 beam. The observed channeling direction was á1 1 0ñ perpendicular to the crystal surface. The beam current on target was 8±9 pA after collimated with the 25-lm-diameter aperture. The beam spot was scanned over a 1 ´ 1 mm2 surface area with a step of 50 lm by vertical and horizontal shift of the crystal under channeling incidence conditions. Fig. 5 shows the 1 ´ 1 mm2 images of the Niá1 1 0ñ electron yield measured in this manner for three dierent surface positions. In this case, the measured energy range of 4.5 6 E/eVp 6 8.3, i.e., E 2.2±4.1 keV was chosen by applying Vp 0.49 kV and Vg 2.2 kV. In Fig. 5, the ran-
dom yield corresponds to ~6000 counts (the ratio of á1 1 0ñ to random yield is ~60%). According to a previous analysis of a similarly prepared Ni crystal [6], the electron yield in the channeling case is enhanced mainly by the slightly misaligned atoms in the crystal. Furthermore, measurements of the á1 1 0ñ channeling (shadowing) dip have indicated the absence of macroscopic bend of the crystal within each of the 1 ´ 1 mm2 areas. The imperfect crystallinity of the Ni crystal is well visualized in Fig. 5, demonstrating essentially the same quality of information as obtained by ion backscattering analysis. It is notable that the similar 1 ´ 1 mm2 channeling images for Si crystals of extremely higher crystallinity than the Ni crystal showed uniform contrast within ~3% uncertainty of the electron counts. The clear shadowing patterns shown in Figs. 3 and 4 as well as the channeling images in Fig. 5 con®rm the availability of the electron analyzer of the cylindrical mirror type for analysis of crystalline materials under low beam current, i.e.,
Fig. 5. Typical 1 ´ 1 mm2 images of the Niá1 1 0ñ electron yield measured for three dierent surface positions of the Ni crystal, visualizing the imperfect crystallinity of the Ni crystal. The random yield corresponds to ~6000 counts. The 20 MeV O4 beam was collimated with the 25-lm-diameter aperture. The measurement conditions are approximately the same as for the case shown in Fig. 4.
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408 407
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H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
low beam dose conditions. In the present case, the beam dose required for practical measurement of the electron yield in the channeling case, which provides structural information near the surface (up to a depth of 10±100 nm), is as low as ~1011 ions/cm2 . This value is extremely smaller than those at least required in typical ion backscattering spectroscopy (for example, ~1014 He/cm2 for Si target [9,10]) by a factor of ~10ÿ3 .
4. Conclusions Possible application of low beam current experiments includes the analyses of some insulator ®lms for which electronic excitation by the analyzing beam induces serious damage, and accordingly, usual ion backscattering analyses are not readily applicable. The analyzer of the present or re®ned type might be also useful for channeling and related experiments with secondary particles generated by accelerators, if the available beam current is at least in the picoampere range.
Acknowledgements We thank the accelerator sta at UTTAC and TIARA for their assistance in the experiments, and H. Ohshima for the technical support. This work has been supported in part by the Universities±JAERI Joint Research Project.
References [1] D. Hasselkamp, H. Rothard, K.-O. Groeneveld, J. Kemmler, P. Varga, H. Winter, Particle Induced Electron Emission, vol. II, Springer, Berlin, 1992. [2] H. Kudo, K. Shima, K. Masuda, S. Seki, Phys. Rev. B 43 (1991) 12729; H. Kudo, K. Shima, S. Seki, T. Ishihara, Phys. Rev. B 43 (1991) 12736. [3] T. Inoue, H. Kudo, T. Fukusho, T. Ishihara, T. Ohsuna, Jpn. J. Appl. Phys. 33 (1994) L139. [4] T. Fukusho, A. Tanabe, H. Kudo, T. Ishihara, S. Seki, Jpn. J. Appl. Phys. 35 (1996) 247. [5] H. Kudo, K. Shima, T. Ishihara, Phys. Rev. B 47 (1993) 27. [6] H. Kudo, A. Sakamoto, S. Yamamoto, Y. Aoki, H. Naramoto, T. Inoue, M. Satoh, Y. Yamamoto, K. Umezawa, S. Seki, Jpn. J. Appl. Phys. 35 (1996) L1538. [7] H. Kudo, S. Yamamoto, K. Narumi, Y. Aoki, H. Naramoto, Nucl. Instr. and Meth. B 132 (1997) 41. [8] P.F.A. Alkemade, L. Flinn, W.N. Lennard, I.V. Mitchell, Phys. Rev. A 53 (1996) 886. [9] L.C. Feldman, J.W. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling, Chap. 9, Academic Press, New York, 1982. [10] J.R. Bird, J.S. Williams (Eds.), Ion Beams for Materials Analysis, Chaps. 3 and 6, Academic Press, Sydney, 1989. [11] T. Yamazaki, R. Suzuki, T. Ohdaira, T. Mikado, Y. Kobayashi, in: Proceedings of Seventh International Symposium on Advanced Nuclear Energy Research, Takasaki, Japan, 1996. [12] D. Roy, J.D. Carette, Electron spectroscopy for surface analysis, in: Topics in Current Physics 4, Chap. 2, Springer, Berlin, 1977. [13] D.L. Matthews, Methods of Experimental Physics, vol. 17, Chap. 9.4, Academic Press, New York, 1980. [14] The count rate of the background signal from the MCP, which can be measured with no beam, sensitively depends on the preconditioning of the MCP. In the present case, the ratio of background to signal counts was 3±5%.
Feasibility of a cylindrical mirror electron analyzer for structural analysis of crystalline materials using weak ion beams Hiroshi Kudo
a,*
, Kiyomitsu Takada a, Kazumasa Narumi b, Shunya Yamamoto b, Hiroshi Naramoto b, Seiji Seki c
a
Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan b Japan Atomic Energy Research Institute, Takasaki 370-12, Japan c SSL Tsukuba Ltd., Ibaraki 305, Japan Received 3 February 1998; received in revised form 10 March 1998
Abstract An electron analyzer of the cylindrical mirror type has been developed for material analysis with MeV ions under low beam-dose conditions. The analyzer was successfully applied to measure the high-energy shadowing eect under extremely low beam current (by a factor of 10ÿ3 ±10ÿ2 ), down to the picoampere range, than that required in typical ion backscattering spectroscopy. Low beam current experiments should be useful for characterization of insulators suffering serious ionization-induced damage. Also, the analyzer might be used for measurements with picoampere beams of secondary particles such as positrons generated by accelerators. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Electron analyzer; Ion-induced electron emission; Ion beam analysis
1. Introduction When a solid target is bombarded with fast ions, the ion-induced electron yield observed at a backward angle is mainly due to recoil of target electrons [1]. The production process of the electrons can be treated with the binary encounter model, which accounts for the observation that the continuum energy spectrum is well characterized by the ion velocity and the spectrum yield
* Corresponding author. Fax: +81 298 53 5205; e-mail: [email protected].
for equal-velocity ions is approximately proportional to the square of the atomic number of the ions [2]. The electron emission is reduced under channeling incidence conditions since atoms near the surface de¯ect the ions away from the aligned atoms and, accordingly, the underlying atoms are eectively shadowed. Therefore, the electron yield re¯ects the high-energy shadowing eect which is the initial stage of ion channeling. For example, the shadowing pattern can be measured by recording the electron yield at a ®xed electron energy, typically in a keV energy range, as a function of the tilt angles of the crystal with respect to the
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 6 2 - 6
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
two independent axes perpendicular to the beam direction [3,4]. The electron measurement is applicable not only to metal and semiconductor targets, but also to insulator ®lms unless the ®lm thickness exceeds the mean penetration distance (projected range) of the ions [3,5]. The analysis with the ion-induced electrons is useful for observation of channeling of high-energy ions that cause serious non-Rutherford scattering in ion backscattering spectroscopy, which is of practical importance for detection of misaligned crystal atoms under a small critical angle of channeling [6]. It is important to note that the electron emission resulting from the recoil of target electrons is isotropic, irrespective of the crystallographic direction of the outgoing path [7]. The shadowing eect can therefore be observed with the ion-induced electrons emitted at any backward angle. The ratio of channeling to random (nonchanneling) yield at a ®xed electron energy can be used as a measure of the shadowing eect. So far, the ion-induced electrons have been energy-analyzed mostly using electrostatic analyzers with a solid angle of acceptance less than ~10ÿ2 sr [2±8]. If the emitted electrons are energy-analyzed and detected over a solid angle on the order of 1 sr, the high-energy shadowing eect is expected to be observed with weak ion beams or extremely low beam doses that have been impractical for backward measurements. For example, an ion
403
beam of picoampere currents might be practically used for analysis of thin insulator ®lms that suer serious radiation damage due to electronic excitations in a typical ion backscattering analysis using nanoampere beams [9,10]. Furthermore, it is of technical interest to investigate the possibility of observing shadowing and related phenomena using a beam of secondary particles such as positrons gererated by accelerators [11]. 2. The cylindrical mirror analyzer The design and operation of the wide acceptance-angle analyzer are dierent from those so far used for peak-search studies such as Auger electron spectroscopy, which require high energy resolution [12,13]. In the present case, the relative energy acceptance (resolution) De/e must be variable up to ~30% and the angular acceptance must be on the order of 1 sr to obtain high count rates. Also, the backscattered ions must be stopped before entering the electron multiplier to reduce background signals. Fig. 1 is a schematic diagram illustrating the electron analyzer of the cylindrical mirror type developed and investigated in the present study. The electrically grounded grid of the cylindrical shape (30-mm diameter) ®xed inside the analyzer consists of an array of 0.2-mm-diameter Cu wires stretched
Fig. 1. Schematic diagram of the cylindrical mirror electron analyzer (cross-sectional view), including examples of the calculated electron orbits for the two pairs of the parameters (a, E/eVp ) (24°, 5.0) and (15°, 7.0).
404
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
~4-mm-apart parallel to the cylinder axis. A microchannel plate (MCP) electron multiplier with a 4mm-diameter center hole (HAMAMATSU F2223-21SH) is used for detection of the energyanalyzed electrons. The incident ion beam passes through the 3-mm-diameter aluminum tube (Al thickness of 0.08 mm) along the cylinder axis and incident on the target crystal. This tube and the 7-mm-diameter disk ®xed at the end of the tube (the left end in Fig. 1) geometrically prevent the backscattered ions from entering the MCP. The three parallel grids in front of the MCP are used to set the lowest measurable electron energy. Negative voltage )Vg is applied to the center grid while the others are electrically grounded. When the distance L from the entrance of the analyzer to the target crystal is 30 mm, the ion-induced electrons emitted from the crystal surface in an angular range of 7°±27° with respect to the cylinder axis enter the analyzer. They are then de¯ected towards the cylinder axis by the electrostatic ®eld between the cylindrical grid and the coaxial cylindrical plate to which negative voltage )Vp is applied. The ring-shape electron repeller ()200 V) is used for accurate measurement of the ion beam current. The emission angle a with respect to the cylinder axis and the kinetic energy E of the electrons that are properly de¯ected and come onto the MCP can be numerically calculated by assuming the logarithmic electrostatic ®eld in the analyzer. For a given value of a, the electron orbit in the analyzer depends on the dimensionless parameter E/ eVp , where e is the electronic charge. Examples of the electron orbits are shown in Fig. 1 for the two pairs of the parameters (a, E/eVp ) (24°, 5.0) and (15°, 7.0). Fig. 2 is a calculated diagram for the initial conditions of the electrons, shown as a function of a and E/eVp . The results are shown for the three values of L. In Fig. 2, the electrons for the initial conditions shown inside the area surrounded by the curve come onto the MCP, while for other cases (outside the surrounded area) the electron orbit goes outside the MCP or is interrupted by the 3-mm-diameter aluminum tube. For the setup at L 30 mm, a wide angular range of approximately 8° 6 a 6 25° with respect to the cylinder axis, which corresponds to a solid angle of 0.53 sr, is accepted for 4.5 6 E/eVp 6 6.0. The
Fig. 2. Calculated diagram for the analyzer showing the transmission condition of the electrons as a function of the angle and kinetic energy in terms of E/eVp . For the initial conditions within the area surrounded by the curve, the electrons come onto the MCP and are counted.
dependence of the acceptance angle on the value of L is seen from the calculated curves for L 27 and 33 mm as well as L 30 mm. A slight change of L by 1 mm, for example, caused by the crystal tilt sensitively aects the measured electron yield only at E/eVp 8. 3. Shadowing measurements Operation and performance of the electron analyzer can be demonstrated by the measurements of the shadowing patterns obtained by tilting the target crystal, as mentioned in Section 1. The maximum depth contributing to the shadowing pattern is typically in the range of 10±100 nm for the present experimental conditions [3,4]. Fig. 3 shows the shadowing pattern around Siá1 0 0ñ axis for an angular range of 3° ´ 3° (tilt angles of h and /), measured using the electrons induced by 1.5 MeV He . In this case, we applied Vp 0.16 kV and Vg 0.82 kV, which lead to the measured range of E/eVp from 0.82/0.16 5.1 to 8.3 (the high-energy end of the measurable area shown in Fig. 2), i.e.,
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
405
Fig. 3. Shadowing pattern around Siá1 0 0ñ for an angular range of 3° ´ 3° (h- and /-tilt), measured with 0.82±1.33 keV electrons induced by 1.5 MeV He . The measurement time is ~20 min under the beam current of 0.45±0.50 nA. Note that the yield axis is taken in the downward direction for a better presentation of the shadowing pattern.
E 0.82±1.33 keV. For the beam spot size of ~0.8mm diameter on target, the He beam current was 0.45±0.50 nA which allows a count rate of ~6000 counts/s (including a negligible contribution from background signals [14]) for random incidence conditions. The shadowing pattern consists of the electron counts for 31 ´ 31 pixels of tilt angle, each of which was measured for 1.0±1.3 s corresponding to a ®xed beam dose of 6.2 ´ 1011 He / cm2 (0.5 nC on the 0.8-mm-diam spot size). The low-energy end of the measured energy range is equal to the binary-encounter peak energy EB 0.82 keV which is the maximum energy transferable from the ion to a rest electron (in a head-on collision). The electron yield at energies higher than EB mainly stems from recoiled inner-shell electrons that have high orbital velocities, compared to the outer-shell or valence electrons. Since the inner-shell electrons are more eectively shadowed than the others, the electron yield at energies above EB is more sensitive to the shadowing eect than below EB .
Fig. 4 shows another example of the shadowing pattern around Geá1 1 1ñ axis for an angular range of 2° ´ 2°, measured using the electrons induced by 20 MeV O4 collimated with a 25-lm-diameter aperture. The measured energy range of 4.1 6 E/ eVp 6 8.3, i.e., E 2.0±4.1 keV was chosen by applying Vp 0.49 kV and Vg 2.0 kV. In this case, the electrons at energies below and above EB 2.7 keV are counted. The O4 beam current on target as low as 7 pA provides a sucient count rate of ~2000 counts/s under random incidence conditions. The counting time for each of the 21 ´ 21 pixels of tilt angle to measure the pattern was ~2 s which corresponds to a ®xed beam dose of 4.5 ´ 1012 O4 /cm2 . If the observation angle is 180° with respect to the ion beam, the random electron yield does not depend on the tilt angle of the crystal because of the approximately straight outgoing-paths of the keV electrons [2]. For a wide observation angle like in the present case, care must be taken for the tilt-angle dependence of the random electron
406
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408
Fig. 4. Shadowing pattern around Geá1 1 1ñ for an angular range of 2° ´ 2°, measured with 2.0±4.1 keV electrons induced by 20 MeV O4 collimated with a 25-lm-diameter aperture. The measurement time is 15 min under the beam current of ~7 pA. For each of the 21 ´ 21 tilt angles, the beam dose is 4.5 ´ 1012 O4 /cm2 for the counting time of ~2 s.
yield since it should aect the shadowing pattern. In Figs. 3 and 4, such in¯uence is not discernible in the present angular range of the crystal tilt within ~2° from the surface normal. To investigate the quality of the structural information obtained using the present method, the electron yield from a chemically etched Ni(1 1 0) crystal (~1-mm thickness and ~10-mm diameter) was measured with the 20 MeV O4 beam. The observed channeling direction was á1 1 0ñ perpendicular to the crystal surface. The beam current on target was 8±9 pA after collimated with the 25-lm-diameter aperture. The beam spot was scanned over a 1 ´ 1 mm2 surface area with a step of 50 lm by vertical and horizontal shift of the crystal under channeling incidence conditions. Fig. 5 shows the 1 ´ 1 mm2 images of the Niá1 1 0ñ electron yield measured in this manner for three dierent surface positions. In this case, the measured energy range of 4.5 6 E/eVp 6 8.3, i.e., E 2.2±4.1 keV was chosen by applying Vp 0.49 kV and Vg 2.2 kV. In Fig. 5, the ran-
dom yield corresponds to ~6000 counts (the ratio of á1 1 0ñ to random yield is ~60%). According to a previous analysis of a similarly prepared Ni crystal [6], the electron yield in the channeling case is enhanced mainly by the slightly misaligned atoms in the crystal. Furthermore, measurements of the á1 1 0ñ channeling (shadowing) dip have indicated the absence of macroscopic bend of the crystal within each of the 1 ´ 1 mm2 areas. The imperfect crystallinity of the Ni crystal is well visualized in Fig. 5, demonstrating essentially the same quality of information as obtained by ion backscattering analysis. It is notable that the similar 1 ´ 1 mm2 channeling images for Si crystals of extremely higher crystallinity than the Ni crystal showed uniform contrast within ~3% uncertainty of the electron counts. The clear shadowing patterns shown in Figs. 3 and 4 as well as the channeling images in Fig. 5 con®rm the availability of the electron analyzer of the cylindrical mirror type for analysis of crystalline materials under low beam current, i.e.,
Fig. 5. Typical 1 ´ 1 mm2 images of the Niá1 1 0ñ electron yield measured for three dierent surface positions of the Ni crystal, visualizing the imperfect crystallinity of the Ni crystal. The random yield corresponds to ~6000 counts. The 20 MeV O4 beam was collimated with the 25-lm-diameter aperture. The measurement conditions are approximately the same as for the case shown in Fig. 4.
H. Kudo et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 402±408 407
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low beam dose conditions. In the present case, the beam dose required for practical measurement of the electron yield in the channeling case, which provides structural information near the surface (up to a depth of 10±100 nm), is as low as ~1011 ions/cm2 . This value is extremely smaller than those at least required in typical ion backscattering spectroscopy (for example, ~1014 He/cm2 for Si target [9,10]) by a factor of ~10ÿ3 .
4. Conclusions Possible application of low beam current experiments includes the analyses of some insulator ®lms for which electronic excitation by the analyzing beam induces serious damage, and accordingly, usual ion backscattering analyses are not readily applicable. The analyzer of the present or re®ned type might be also useful for channeling and related experiments with secondary particles generated by accelerators, if the available beam current is at least in the picoampere range.
Acknowledgements We thank the accelerator sta at UTTAC and TIARA for their assistance in the experiments, and H. Ohshima for the technical support. This work has been supported in part by the Universities±JAERI Joint Research Project.
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