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New development of scanning-type microscope for two-dimensional hydrogen distribution using electron-stimulated desorption method
Surface Science 433–435 (1999) 244–248 www.elsevier.nl/locate/susc
New development of scanning-type microscope for two-dimensional hydrogen distribution using electron-stimulated desorption method K. Ishikawa, K. Ueda *, M. Yoshimura Toyota Technological Institute, Hisakata 2-chome, Tempaku, Nagoya 468-8511, Japan
Abstract Time-of-flight type electron-stimulated desorption (TOF-ESD) spectroscopy enables us to study hydrogen on solid surfaces with high sensitivity. An analyzer for two-dimensional hydrogen distribution has been developed for TOFESD using a thermal field emission (TFE ) gun for scanning electron microscopy. The lateral resolution of analysis is less than 1 mm at the primary-electron energy of 800 eV. A lower electron energy (around 200 eV ) is preferable for inducing the ion desorption. We have improved the TFE gun working at the low primary-electron energy (above 300 eV ) and show several hydrogen images as results of the improvement. The lattice pattern of a copper mesh is clearly resolved even at the energy of 300 eV. In this paper, we also discuss the topographic effects that appeared in scanning ESD images. Roughness of 1.2 mm height affects the ESD yield of H+ ions mainly due to preferential desorption in the direction of the surface normal. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Electron-stimulated desorption; Hydrogen
1. Introduction Hydrogen on solid surfaces is an issue of great interest in chemistry, physics and device technology. For example, the hydrogen-terminated silicon surface is practically useful for hetero-epitaxy and for keeping the surface oxide-free. However, the detection of hydrogen on solid surfaces is difficult or impossible by conventional analytical methods. Several methods are used to study the interaction of hydrogen with solid surfaces, e.g., highresolution electron energy-loss spectroscopy (HREELS) [1], elastic recoil detection analysis ( ERDA) [2,3], attenuated total reflection Fourier * Corresponding author. Fax: +81-52-809-1853. E-mail address: [email protected] ( K. Ueda)
transform infrared spectroscopy (ATR-FTIR) [4], time-of-flight type electron-stimulated desorption ( TOF-ESD) [5,6 ] and scanning tunneling microscopy (STM ) [7]. We have developed a two-dimensional analyzer to investigate the hydrogen distribution on a solid surface using the TOF-ESD method combined with a pencil-type electron gun originally designed for use in scanning electron microscopy (SEM ) [8,9]. The detection limit of desorbed H+ ions depends on the amount of hydrogen in the probing area. To date, imaging the hydrogen distribution at the surface by the ESD method requires a high detection efficiency because of the small ESD yield from a very small probing area. The TOF-ESD system developed in our laboratory has the advantage of high detection efficiency owing to the large
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 04 5 3 -7
K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
solid angle for detection [8]. In our previous report, the primary-electron energy of 800 eV was used because of practical restrictions on the system, such as beam spot size, beam current, cross-section for ion desorption and so on. In general, the maximum cross-section for ion desorption exists around the primary electron energy of 200 eV. Thus, lower primary electron energies are preferable for ESD. In this paper, we report the results of scanning ESD analysis using an improved thermal field emitter ( TFE) gun working at a lower primary energy than that in the previous report. Next we discuss the topographic effect of scanning ESD measurements on the experimental result obtained by scanning ESD system. Finally, we show results of a hydrogen-terminated silicon surface lithographed by a continuous electron beam as a practical application. 2. Experimental All experiments were carried out in an ultrahigh vacuum ( UHV ) chamber (base pressure of
245
2.0×10−8 Pa) equipped with an off-axis type electron gun for low-energy electron diffraction (LEED), a pencil-type electron gun, a photomultiplier tube for SEM and a TOF-type mass analyzer. The experimental apparatus is shown schematically in Fig. 1. The TOF-type mass analyzer is constructed from a three-grid mesh, microchannel plates (MCPs) with effective diameter of 70 mm and a fluorescent screen. The mass analyzer was 112 mm from the sample and positioned normal to the sample surface. The resolution of the mass spectrum is much better if the positive bias voltage applied to the sample is higher. Typically, 20 V was applied to the sample in this study. The pencil-type electron gun was designed for the SEM equipped to the UHV chamber. The TFE provided a fine-focused electron beam in the lowelectron-energy range (e.g., 300 nm in diameter at 800 eV ). An electron beam with this energy range is easily pulsed by a high-speed pulse generator with duration of 220 ns. We improved the electron optics of the TFE gun to obtain a submicrometer electron beam below 800 eV. A diameter of 500 nm has been achieved. The incident angle of the
Fig. 1. Schematic drawing of the scanning TOF-ESD measurement system for two-dimensional hydrogen analysis.
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K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
electron beam is 45° with respect to the surface normal. The repetition cycle of pulsed electronbeam irradiation is about 400 Hz. Each signal of desorbed ions detected by the MCPs was counted and stored in the memory of a personal computer. Accumulation of the signals for 10 000 pulsed electron beams makes a fine TOF spectrum. The yield of desorbed ions is defined by integration of the peak in the TOF spectrum. In order to obtain the image of hydrogen distribution on solid surfaces, the electron beam is scanned digitally over the sample surface, and the yield of each beam position forms a pixel of the image on a gray scale.
3. Results and discussion Fig. 2 shows scanning ESD images using an improved electron gun. The images in 128×128 pixels were obtained on 600 mesh copper grid using the primary-electron energy of (a) 600 eV and (b) 300 eV. The lattice pattern of the copper mesh is clearly resolved even at the energy of 300 eV. The cross-section for ion desorption depends on the adsorbed state of the desorbing atom and also on the primary-electron energy. Thus, we consider that the distribution of hydrogen in different adsorption states was obtained by comparing scanning ESD images of different primary-electron energy. In order to investigate the topographical effects in scanning ESD images, we carried out a linescan analysis on line and space patterns of silicon oxide. Fig. 3a shows a SEM image of lines with a linewidth of 10 mm at a spacing of 10 mm. The height of the lines is about 1.2 mm. The result of a line-scan analysis across the lines is shown in Fig. 3b. The pulses of electron beam have been irradiated from the left-hand side at an incident angle of 45° as shown in the inset of Fig. 3b. The ESD yield of H+ ions has decreased near the left sidewall of the lines and has a local maximum around the right-hand sidewall. The result shown here clearly indicates topographic effects such as observed in SEM contrast. The surface roughness can obstruct both the detection solid angle of the analyzer and irradiation by the incident electron beam. The ESD ions are desorbed mostly in the
Fig. 2. Scanning ESD images of H+ ions desorbed from a 600 mesh copper grid by irradiation of a pulsed electron beam with the primary-electron energy of (a) 600 eV and (b) 300 eV. The scan area is 90 mm×115 mm. Magnification is 1000×.
direction of the surface normal. Thus, it is considered that the lower ESD yields at the left-hand sidewall arise from obstruction of the detector positioned in the direction of the surface normal. Furthermore, at the sidewall of the lines there exists an electric field due to the positive sample bias that normally accelerates the desorbed ions to the detector. This field makes it more difficult
K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
247
Fig. 4. Scanning ESD image of a lithographed circle pattern on the hydrogen-terminated Si(100) surface. The scan area is 128 mm×182 mm. The primary-electron energy of 800 eV was used in both lithography and analysis. Magnification is 700×.
Fig. 3. (a) SEM image of 10 mm line patterns of silicon oxide at a spacing of 10 mm. (b) Line-scan analysis across the line patterns. The primary-electron energy was 800 eV.
to detect the desorbed ions. No ESD yield was obtained from the right-hand sidewall because of the prevention of irradiation by the beam. The electric field at the right-hand sidewall caused the local maximum in line analysis. There is another explanation for topographic effects in ESD measurements. The intensity of the SEM image (shown in Fig. 3a) has changed similar to the ESD yield. It seems that secondary-electron emission plays an important role in H+ desorption. Finally, we demonstrate direct lithography on hydrogen-terminated Si(100) surfaces. Electronbeam lithography on H/Si(100) is of interest for the fabrication methods of device structure [10,11]. An Si(100) surface was prepared by dipping in buffered HF solution prior to evacuation. In UHV, the sample was heated to outgas and followed by
flashing at 1200°C. Then the sample was exposed to hydrogen at 5000 L near a hot tungsten filament to adsorb atomic hydrogen. According to the results of ERDA [3], hydrogen coverage on the Si(100) surface was considered to be less than two monolayers (ML). A circle was drawn by irradiation of a continuous electron beam on this surface, and observed by subsequent scanning ESD measurement using a pulsed electron beam. The primary-electron energy of lithography and analysis was 800 eV. The circle can be clearly confirmed in the scanning ESD image shown in Fig. 4. It is remarkable that the lithographed character was stable not only for 10 h but even after several scanning ESD measurements in spite of less than 2 ML hydrogen coverage. It is suggested that: (1) the effects of adsorption of residual gas are negligible; (2) a trivial level of damage for adsorbed hydrogen is caused by pulsed electron-beam irradiation; and (3) the hydrogen-terminated silicon surface is stable for a long time.
4. Summary We have developed a scanning ESD microscope working in the low-primary-energy range and
248
K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
investigated topographic effects appearing in the ESD measurements. We have concluded that the topographic effects are mainly due to preferential desorption in the direction of the surface normal. A circle was drawn on a hydrogen-terminated Si(100) surface by a continuous electron beam to remove hydrogen locally, and by the subsequent scanning ESD measurement confirmed a clear lithographed pattern. The lithographed pattern not only is stable for 10 h but even persists after several scanning ESD measurements. In order to carry out a quantitative analysis, further investigations are needed such as the desorption mechanism.
Acknowledgements The authors thank Dr Y. Sakai of Jeol Ltd for his help of the development and improvement of the pencil-type field emission gun. This work was supported in part by a Grant-in-Aid for Scientific Research (#10555010) and on Priority Area A
(#10148227) from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] H. Kobayashi, K. Edamoto, M. Onchi, M. Nishijima, J. Chem. Phys. 78 (1983) 7429. [2] R.J. Cullbertson, L.C. Feldman, P.J. Silverman, J. Vac. Sci. Technol. 20 (1982) 868. [3] K. Oura, J. Yamane, K. Umezawa, M. Naitoh, Phys. Rev. B 41 (1990) 1200. [4] Y.J. Chabal, G.S. Higashi, K. Ragharachari, V.A. Burrows, J. Vac. Sci. Technol. A 7 (1990) 2104. [5] K. Ueda, S. Kodama, A. Takano, Surf. Sci. 283 (1993) 195. [6 ] K. Ueda, Jpn. J. Appl. Phys. 33 (1994) 1524. [7] J.J. Boland, Surf. Sci. 244 (1991) 1. [8] K. Ishikawa, M. Yoshimura, K. Ueda, Rev. Sci. Instrum. 68 (1997) 4103. [9] K. Ueda, K. Ishikawa, M. Yoshimura, Jpn. J. Appl. Phys. 36 (1997) 7696. [10] K. Masu, K. Tsubouchi, J. Vac. Sci. Technol. B 12 (1994) 3270. [11] F.Y.C. Hui, G. Eres, D.C. Joy, Appl. Phys. Lett. 72 (1998) 341.
New development of scanning-type microscope for two-dimensional hydrogen distribution using electron-stimulated desorption method K. Ishikawa, K. Ueda *, M. Yoshimura Toyota Technological Institute, Hisakata 2-chome, Tempaku, Nagoya 468-8511, Japan
Abstract Time-of-flight type electron-stimulated desorption (TOF-ESD) spectroscopy enables us to study hydrogen on solid surfaces with high sensitivity. An analyzer for two-dimensional hydrogen distribution has been developed for TOFESD using a thermal field emission (TFE ) gun for scanning electron microscopy. The lateral resolution of analysis is less than 1 mm at the primary-electron energy of 800 eV. A lower electron energy (around 200 eV ) is preferable for inducing the ion desorption. We have improved the TFE gun working at the low primary-electron energy (above 300 eV ) and show several hydrogen images as results of the improvement. The lattice pattern of a copper mesh is clearly resolved even at the energy of 300 eV. In this paper, we also discuss the topographic effects that appeared in scanning ESD images. Roughness of 1.2 mm height affects the ESD yield of H+ ions mainly due to preferential desorption in the direction of the surface normal. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Electron-stimulated desorption; Hydrogen
1. Introduction Hydrogen on solid surfaces is an issue of great interest in chemistry, physics and device technology. For example, the hydrogen-terminated silicon surface is practically useful for hetero-epitaxy and for keeping the surface oxide-free. However, the detection of hydrogen on solid surfaces is difficult or impossible by conventional analytical methods. Several methods are used to study the interaction of hydrogen with solid surfaces, e.g., highresolution electron energy-loss spectroscopy (HREELS) [1], elastic recoil detection analysis ( ERDA) [2,3], attenuated total reflection Fourier * Corresponding author. Fax: +81-52-809-1853. E-mail address: [email protected] ( K. Ueda)
transform infrared spectroscopy (ATR-FTIR) [4], time-of-flight type electron-stimulated desorption ( TOF-ESD) [5,6 ] and scanning tunneling microscopy (STM ) [7]. We have developed a two-dimensional analyzer to investigate the hydrogen distribution on a solid surface using the TOF-ESD method combined with a pencil-type electron gun originally designed for use in scanning electron microscopy (SEM ) [8,9]. The detection limit of desorbed H+ ions depends on the amount of hydrogen in the probing area. To date, imaging the hydrogen distribution at the surface by the ESD method requires a high detection efficiency because of the small ESD yield from a very small probing area. The TOF-ESD system developed in our laboratory has the advantage of high detection efficiency owing to the large
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 04 5 3 -7
K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
solid angle for detection [8]. In our previous report, the primary-electron energy of 800 eV was used because of practical restrictions on the system, such as beam spot size, beam current, cross-section for ion desorption and so on. In general, the maximum cross-section for ion desorption exists around the primary electron energy of 200 eV. Thus, lower primary electron energies are preferable for ESD. In this paper, we report the results of scanning ESD analysis using an improved thermal field emitter ( TFE) gun working at a lower primary energy than that in the previous report. Next we discuss the topographic effect of scanning ESD measurements on the experimental result obtained by scanning ESD system. Finally, we show results of a hydrogen-terminated silicon surface lithographed by a continuous electron beam as a practical application. 2. Experimental All experiments were carried out in an ultrahigh vacuum ( UHV ) chamber (base pressure of
245
2.0×10−8 Pa) equipped with an off-axis type electron gun for low-energy electron diffraction (LEED), a pencil-type electron gun, a photomultiplier tube for SEM and a TOF-type mass analyzer. The experimental apparatus is shown schematically in Fig. 1. The TOF-type mass analyzer is constructed from a three-grid mesh, microchannel plates (MCPs) with effective diameter of 70 mm and a fluorescent screen. The mass analyzer was 112 mm from the sample and positioned normal to the sample surface. The resolution of the mass spectrum is much better if the positive bias voltage applied to the sample is higher. Typically, 20 V was applied to the sample in this study. The pencil-type electron gun was designed for the SEM equipped to the UHV chamber. The TFE provided a fine-focused electron beam in the lowelectron-energy range (e.g., 300 nm in diameter at 800 eV ). An electron beam with this energy range is easily pulsed by a high-speed pulse generator with duration of 220 ns. We improved the electron optics of the TFE gun to obtain a submicrometer electron beam below 800 eV. A diameter of 500 nm has been achieved. The incident angle of the
Fig. 1. Schematic drawing of the scanning TOF-ESD measurement system for two-dimensional hydrogen analysis.
246
K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
electron beam is 45° with respect to the surface normal. The repetition cycle of pulsed electronbeam irradiation is about 400 Hz. Each signal of desorbed ions detected by the MCPs was counted and stored in the memory of a personal computer. Accumulation of the signals for 10 000 pulsed electron beams makes a fine TOF spectrum. The yield of desorbed ions is defined by integration of the peak in the TOF spectrum. In order to obtain the image of hydrogen distribution on solid surfaces, the electron beam is scanned digitally over the sample surface, and the yield of each beam position forms a pixel of the image on a gray scale.
3. Results and discussion Fig. 2 shows scanning ESD images using an improved electron gun. The images in 128×128 pixels were obtained on 600 mesh copper grid using the primary-electron energy of (a) 600 eV and (b) 300 eV. The lattice pattern of the copper mesh is clearly resolved even at the energy of 300 eV. The cross-section for ion desorption depends on the adsorbed state of the desorbing atom and also on the primary-electron energy. Thus, we consider that the distribution of hydrogen in different adsorption states was obtained by comparing scanning ESD images of different primary-electron energy. In order to investigate the topographical effects in scanning ESD images, we carried out a linescan analysis on line and space patterns of silicon oxide. Fig. 3a shows a SEM image of lines with a linewidth of 10 mm at a spacing of 10 mm. The height of the lines is about 1.2 mm. The result of a line-scan analysis across the lines is shown in Fig. 3b. The pulses of electron beam have been irradiated from the left-hand side at an incident angle of 45° as shown in the inset of Fig. 3b. The ESD yield of H+ ions has decreased near the left sidewall of the lines and has a local maximum around the right-hand sidewall. The result shown here clearly indicates topographic effects such as observed in SEM contrast. The surface roughness can obstruct both the detection solid angle of the analyzer and irradiation by the incident electron beam. The ESD ions are desorbed mostly in the
Fig. 2. Scanning ESD images of H+ ions desorbed from a 600 mesh copper grid by irradiation of a pulsed electron beam with the primary-electron energy of (a) 600 eV and (b) 300 eV. The scan area is 90 mm×115 mm. Magnification is 1000×.
direction of the surface normal. Thus, it is considered that the lower ESD yields at the left-hand sidewall arise from obstruction of the detector positioned in the direction of the surface normal. Furthermore, at the sidewall of the lines there exists an electric field due to the positive sample bias that normally accelerates the desorbed ions to the detector. This field makes it more difficult
K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
247
Fig. 4. Scanning ESD image of a lithographed circle pattern on the hydrogen-terminated Si(100) surface. The scan area is 128 mm×182 mm. The primary-electron energy of 800 eV was used in both lithography and analysis. Magnification is 700×.
Fig. 3. (a) SEM image of 10 mm line patterns of silicon oxide at a spacing of 10 mm. (b) Line-scan analysis across the line patterns. The primary-electron energy was 800 eV.
to detect the desorbed ions. No ESD yield was obtained from the right-hand sidewall because of the prevention of irradiation by the beam. The electric field at the right-hand sidewall caused the local maximum in line analysis. There is another explanation for topographic effects in ESD measurements. The intensity of the SEM image (shown in Fig. 3a) has changed similar to the ESD yield. It seems that secondary-electron emission plays an important role in H+ desorption. Finally, we demonstrate direct lithography on hydrogen-terminated Si(100) surfaces. Electronbeam lithography on H/Si(100) is of interest for the fabrication methods of device structure [10,11]. An Si(100) surface was prepared by dipping in buffered HF solution prior to evacuation. In UHV, the sample was heated to outgas and followed by
flashing at 1200°C. Then the sample was exposed to hydrogen at 5000 L near a hot tungsten filament to adsorb atomic hydrogen. According to the results of ERDA [3], hydrogen coverage on the Si(100) surface was considered to be less than two monolayers (ML). A circle was drawn by irradiation of a continuous electron beam on this surface, and observed by subsequent scanning ESD measurement using a pulsed electron beam. The primary-electron energy of lithography and analysis was 800 eV. The circle can be clearly confirmed in the scanning ESD image shown in Fig. 4. It is remarkable that the lithographed character was stable not only for 10 h but even after several scanning ESD measurements in spite of less than 2 ML hydrogen coverage. It is suggested that: (1) the effects of adsorption of residual gas are negligible; (2) a trivial level of damage for adsorbed hydrogen is caused by pulsed electron-beam irradiation; and (3) the hydrogen-terminated silicon surface is stable for a long time.
4. Summary We have developed a scanning ESD microscope working in the low-primary-energy range and
248
K. Ishikawa et al. / Surface Science 433–435 (1999) 244–248
investigated topographic effects appearing in the ESD measurements. We have concluded that the topographic effects are mainly due to preferential desorption in the direction of the surface normal. A circle was drawn on a hydrogen-terminated Si(100) surface by a continuous electron beam to remove hydrogen locally, and by the subsequent scanning ESD measurement confirmed a clear lithographed pattern. The lithographed pattern not only is stable for 10 h but even persists after several scanning ESD measurements. In order to carry out a quantitative analysis, further investigations are needed such as the desorption mechanism.
Acknowledgements The authors thank Dr Y. Sakai of Jeol Ltd for his help of the development and improvement of the pencil-type field emission gun. This work was supported in part by a Grant-in-Aid for Scientific Research (#10555010) and on Priority Area A
(#10148227) from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] H. Kobayashi, K. Edamoto, M. Onchi, M. Nishijima, J. Chem. Phys. 78 (1983) 7429. [2] R.J. Cullbertson, L.C. Feldman, P.J. Silverman, J. Vac. Sci. Technol. 20 (1982) 868. [3] K. Oura, J. Yamane, K. Umezawa, M. Naitoh, Phys. Rev. B 41 (1990) 1200. [4] Y.J. Chabal, G.S. Higashi, K. Ragharachari, V.A. Burrows, J. Vac. Sci. Technol. A 7 (1990) 2104. [5] K. Ueda, S. Kodama, A. Takano, Surf. Sci. 283 (1993) 195. [6 ] K. Ueda, Jpn. J. Appl. Phys. 33 (1994) 1524. [7] J.J. Boland, Surf. Sci. 244 (1991) 1. [8] K. Ishikawa, M. Yoshimura, K. Ueda, Rev. Sci. Instrum. 68 (1997) 4103. [9] K. Ueda, K. Ishikawa, M. Yoshimura, Jpn. J. Appl. Phys. 36 (1997) 7696. [10] K. Masu, K. Tsubouchi, J. Vac. Sci. Technol. B 12 (1994) 3270. [11] F.Y.C. Hui, G. Eres, D.C. Joy, Appl. Phys. Lett. 72 (1998) 341.