MICROSCOPY RESEARCH AND TECHNIQUE 20~406-412 (1992)
Observation of Atomic Steps on Single Crystal Surfaces by a Commercial Scanning Electron Microscope YUJI UCHIDA, GISELA WEINBERG, AND GUNTER LEHMPFUHL Fritz-Huber-Znstitut der Mux-Plunck-Gesellschuft, Furudayweg 4-6,1000 Berlin 33, Germany
KEY WORDS
REM, Contrast mechanism, Imaging technique
ABSTRACT Atomic steps on (111) and (100) crystal surfaces of Pt were observed using a commercial scanning electron microscope (SEMI in secondary electron mode. By comparing the SEM images and those by reflection electron microscopy (REM), the observed contrast was confirmed to be that from atomic steps on crystal surfaces. The contrast mechanism is briefly discussed. One application of this imaging technique is also shown. INTRODUCTION The scanning electron microscope (SEM)is one of the most widely used instruments for observing the morphological structure of different specimens. Recently, SEM has achieved remarkable imaging capability through the technical developments in electron source, lens system, electron detector, and improved electronics. Applications of SEM for in situ investigations in the field of surface science are expected. The authors have tried to image monatomic steps on noble-metal surfaces (Pt and Au) with secondary electron detection in a commercial SEM without any special modifications. Imaging of atomic steps in secondary electron imaging mode has been already reported by Milne (1989). In our work we also compared images of atomic steps obtained by SEM and the reflection electron microscopic (REM) technique, but under special conditions for direction of the incident electron beam. Kuroda et al. (1985) had also shown a fine atomic step structure on the tungsten tip. The contrast mechanism of atomic steps in the SEM images could only be assumed.
of about 5 x Pa under operating condition. Therefore, the problem of contamination on the specimen surface during SEM observation is very serious. In addition to this, the oil-diffusion pump is sucked by the continuously operating rotary pump. The mechanical disturbance (e.g., by this rotary pump) poses large problems, which will be discussed later. The secondary electron images were obtained by normal mode operation with a working distance of about 10 mm and an accelerating voltage of 15-25 kV. The crystal was tilted against the incident electron beam so that the incident beam was estimated to satisfy the diffraction condition for intensity enhancement for the chosen electron energy and crystal surface. The orientation of the crystal was only estimated from the position of (111)facets on the spherical crystal. The REM observation was carried out with the Philips EM400T after the specimens had been observed by SEM. The observed areas on the crystal surface were recognized as dark contaminated patches in the REM images.
EXPERIMENTAL RESULTS Figure l a shows the SEM picture of Pt (111)taken EXPERIMENTAL PROCEDURE with an incident electron energy of 25 keV. Besides Spherical single crystals with small (111)and (100) islands, a very faint structures of steps can be recogfacets were used. The noble-metal spherical single nized. Figure l b shows the REM image of the same crystals, Pt and Au, were made by melting one end of a area taken with the enhanced 666 reflection near the 0.2 mm wire with a small hydrogen-oxygen flame, as LO111 crystal zone axis. Here we can clearly recognize already reported elsewhere (Hsu and Cowley, 1983; the atomic steps of the SEM picture. They correspond Uchida et al., 1984). The spherical single crystal of exactly to each other. The shapes of atomic steps in about 0.3-0.4 mm at the end of the wire was clamped both pictures are, of course, a bit different because of by a non-magnetic Cu-Be bronze screw on to the small their different tilting angles and azimuthal directions. specimen holder of aluminum-alloy. The orientation of Atomic steps could only be seen in a relatively small the single crystal could be recognized by observing the focused area in the SEM picture. Dynamical focusing (111) facets on the surface with a stereo microscope. was not used in our experiment in order to minimize The specimen holder with the spherical single crystal the contamination on the specimen surface. was inserted in the rotating-tilting goniometer. Figures 2a-c show SEM and REM pictures of a Pt A Hitachi S-4000 SEM, equipped with a field emission gun (FEG), was used for our investigations. The chamber for the electron source is evacuated by three ion-sputter pumps and attains a vacuum of about 5 x lop9 Pa. The specimen chamber is evacuated by a con- Received June 26, 1990; accepted in revised form September 30, 1990. ventional oil-diffusion pump with a cold-trap which can Address reprint requests to Yuji Uchida, Fritz-Haber-Institut der Maxbe cooled with liquid nitrogen and can reach a vacuum Planck-Gesellschaft, Faradayweg 4-6, 1000 Berlin 33, Germany. 0 1992 WILEY-LISS, INC.
OBSERVATION OF ATOMIC STEPS ON SINGLE CRYSTAL SURFACES
407
Fig. 1. Monatomic steps (indicated by arrows) on the (111)facet of a Pt single-crystal sphere. a: SEM image. b REM image with diffraction pattern. Direction of the incident electron beam is indicated. Beside the atomic steps small particles can be seen in (a)and (b). In
(b) the characteristic doubling of REM imaging is shown. The dark frame in (b) is the field of contamination produced during SEM imaging. The dark bars are the more heavily contaminated areas created by the scanning procedure.
(100) crystal surface. Figure 2a was taken from an area which is indicated with brackets in Figure 2b. The imaged areas are clearly recognized as contaminated rectangles on the (100)facet of the spherical crystal sur-
face. In the SEM picture of Figure 2a, the contrast of atomic steps is much stronger than that in Figure l a . This SEM image was taken with the stronger contrast amplification, which makes the image more noisy, as
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Fig. 2. Monatomic steps on the (100) facet of a Pt single-crystal sphere. a: SEM image at high magnification. b SEM image of the (100) facet at low magnification showing the contaminated fields of the investigated areas. The field in brackets is the area shown in (a).
The oblique contamination field was produced during investigation of the steps under a different angle of incidence. c: REM image of the steps in (a) with diffraction pattern. The image of the field of contamination is tilted because of different condition for imaging.
OBSERVATION OF ATOMIC STEPS ON SINGLE CRYSTAL SURFACES
409
Fig. 3. SEM images of the (100) facet of the Pt single-crystal sphere, which has been cleaned by Ar-ion bombardment and annealed, at low (a)and high (b)magnifications. a: The plane (100) facet can be seen surrounded by curved steps. Within the small-angle fanshaped area on the right-hand side two sets of straight steps can be seen which continue in the curved steps outside of the fan. The dark
rectangle is the field of contamination produced during SEM imaging. Small dark patches of unknown contamination can be seen. b: The two sets of straight steps from the left-hand side of the (100)facet can be seen. The wide dark and bright lines belong to the curved steps on the crystal sphere. Very faint straight atomic steps can just be seen.
can be seen in irregular horizontal lines on the picture. The atomic steps appear as dark or bright lines depending on whether the steps are “up” or “down.” Figures 3a,b show SEM images of a (100) facet of the Pt specimen which was cleaned in an ultra-high vacuum (UHV) chamber by Ar-ion bombardment and annealed. The facet is surrounded by sequences of steps
forming the curvature of the spherical crystal. The continuous curvature of the steps is interrupted within a small-angle fan-shaped area running along a [ O l l l direction. Within this area two sets of straight steps can be seen. Figure 3b shows an enlarged area of the (100) facet. The bright and dark steps belong to the system of curved steps. This area does not correspond to the con-
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Fig. 4. SEM images of the (111) facet of the Pt single-crystal sphere with contamination fields at different angles of observation. 7.3" (a)and 2.3"(b).
taminated area in Figure 3a. It was taken from the opposite (left) side of the (100) facet. The two sets of parallel straight atomic steps can be seen very clearly on the edge part of the (100) facet. Monatomic steps become just visible in this picture. The same structure was observed with the REM technique and was analyzed (Lehmpfuhl and Uchida, 1990). A short discussion will be given later. The following results are not directly concerned with the observation of atomic steps in this work. However, the effect should be closely related with the contrast
mechanism in the secondary electron SEM image. Figures 4a,b show REM images of a Pt (111)facet which were taken with an electron energy of 15 keV under different angles between the incident electron beam and the crystal surface. These angles for Figures 4a,b were estimated to be about 7.3" and 2.3",respectively. Areas which were contaminated before making these observations are seen as dark parallelepipeds on the specimen surface in Figure 4a. Contrary to this, these areas are observed as bright patches in Figure 4b. Heavier contaminations are, however, observed in Fig-
OBSERVATION OF ATOMIC STEPS ON SINGLE CRYSTAL SURFACES
ure 4b as vertical dark bars a t the left side of each bright rectangle. This contrast inversion of the contaminated area should depend on the thickness of contamination, on electron beam energy, and on the inclination of the electron beam to the crystal surface. A detailed investigation has not been carried out yet. Atomic steps were also observed on Au crystal surfaces in secondary electron images. There is no essential difference between the images of atomic steps on Au and those on Pt; therefore, the SEM pictures of atomic steps on Au crystals are not shown here.
DISCUSSION Atomic steps could only be observed in the SEM secondary electron image when the angle between the incident electron beam and the crystal surface was relative small, e.g., 5-10”. This angle corresponds for 25 keV electrons t o the relatively high order Bragg reflection (e.g., the Bragg angle for 25 keV for 666 reflection is about 5.8”).The diffraction effect of the incident electron beam on the crystal clearly exists, but this effect could not be observed in our experiments, because we had no reflection electron detector. The brightness of the crystal surface in the secondary electron SEM image increased continuously with the tilting angle until the surface became almost parallel to the incident electron beam. The specimens were adjusted in such a way that the diffraction condition for intensity enhancement was thought to be satisfied (Uchida et al., 19841. But this could not be ascertained without observation of diffracted electrons. The observation of atomic steps on a semiconductor surface with secondary electron imaging was already reported by Ichinokawa (1986). But the atomic steps could only be imaged after decoration by oxygen atoms. Our metal specimens were molten and recrystallized in air and observed in conventional vacuum. The crystal surface was surely exposed to different gases before observation. However, in our work, the decoration effect for explaining the atomic step contrast was excluded. The dependency of bright and dark contrast of atomic steps cannot be explained reasonably by the decoration effect. The contrast of atomic steps in our work has been found to obey a definite rule, as schematically shown in Figure 5. Atomic steps are seen in the SEM images as bright or dark lines with the narrowest width of about 5 nm. The width of this line contrast increases with defocusing. But the contrast character of atomic steps does not change with defocusing. Atomic steps were estimated to be visible within the defocusing range of about 3 pm. The diameter of the incident electron beam a t this defocusing position is about 9 nm (the semiangle of the illuminating electron probe is about 1.5 x lop3 rad). From these facts the contrast of atomic steps of secondary electron imaging seems to be explained with the simplest model, the standard explanation for the SEM image contrast. The secondary electron yield for the SEM detector depends on the inclination angle 0 of the crystal surface to the incident electron beam. This means that the surface of the crystal becomes brighter as 0 becomes smaller, as shown schematically in Figure
electron
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411
SEM image low intensity (dark)
high intensity (bright)
Fig. 5 . Schematic explanation for detecting intensity variation of secondary electrons from a spherical specimen with atomic steps.
5. This model is clearly acceptable for large atomic steps with step heights so large that the step surfaces can be seen as narrow facets. The steps with large step height were already observed without decoration (Ichinokawa, 1986). It is very interesting that the character of “dark” or “bright” contrast of atomic steps, which depends on whether the steps are “up” or “down,” coincides with that of large atomic steps. Therefore, the contrast of atomic steps may be also explained with this simple model, but quantitative calculations for the explanation of the atomic step contrast remain to be done. As shown in Figure 3, a characteristic structure of atomic steps on the cleaned Pt(100) surface was observed, This structure can only be understood as the modified surface structure controlled by the reconstruction (Lehmpfuhl and Uchida, 1990). It is well known that the Pt(100) surface after UHV cleaning treatment is reconstructed (Moritz, 1984; van Hove et al., 1981). However, the reconstruction itself is lifted during the transport from the UHV chamber to the SEM apparatus or during the first electron beam irradiation in conventional vacuum. However, during UHV treatment the reconstruction controls development of atomic steps along the unit cell of reconstruction. The directions of these two sets of straight parallel atomic steps deviate by 4.4 O from the exact 10111 direction. These directions coincide with the atomic rows with the highest atomic density of the hex-reconstruction on Pt(100) (Lehmpfuhl and Uchida, 1990). The separation of atomic steps is too small to be better resolved in our SEM images. One of the most interesting points is that atomic steps have become visible for the first time in the secondary electron images which were taken by the SEM installed with a FEG. It is not clear if the FEG is essentially important for imaging the atomic steps by SEM. The coherence of the illuminating electron beam from FEG is much better than that from the conventional electron source. Accordingly, it may be concluded that the coherent illumination of electron beam and a t the same time good resolution are necessary for atomic step imaging.
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The contrast inversion through the inclination change of the incident electron beam is not yet explained. A similar phenomenon was already observed in REM images from contaminated crystal surfaces (unpublished observation). Contaminated areas were observed as bright patches on the clean crystal plane. This phenomenon should be closely connected with the refraction effect from the contaminated layer on the crystal surface for the incident electron beam. At first, the authors considered that the effect might be explained in a similar way as in the theory for anti-reflection coating used for light optical lenses. However, the reflectivity of the incident electrons from the crystal surface seems to be too small to explain this effect, if only elastic scattering is considered (the reflectivity of electrons with 15 keV at an inclination of 2" for Pt is about 5%). As mentioned earlier, the electron microscope which we have used has a UHV chamber only for the electron source but a conventional vacuum chamber for specimens. The high contamination rate on the illuminated area of the specimen caused other difficulties for observation of atomic steps not only in the SEM image but also in the REM image for this investigation. Sometimes, no reasonable REM image could be obtained from the contaminated area of the crystal surface because of heavy contamination. The other technical problem was the already mentioned mechanical vibration caused by the rotary pump. The electron microscope was not equipped with a vacuum reservoir system which would allow the rotary pump to be switched off. In addition to this, the tilting stage, which had been used for our experiments, was not sufficiently stable against the mechanical vi-
bration to obtain higher resolution images. The resolution of images may be limited mainly by a mechanical instability of the specimen stage and not by the size of the electron probe.
CONCLUSIONS Atomic steps on (111)and (100) noble-metal crystal surfaces were observed with secondary electron imaging in a commercial SEM without any modification. Atomic steps were observed in the images which were taken with a small angle between the incident electron beam and the crystal surface. The observed contrast of the atomic steps is not due to a decoration effect. Further experiments with SEM must be done under UHV conditions.
REFERENCES Hsu, T., and Cowley, J.M. (1983) Reflection Electron Microscopy (REM) of fcc Metals. Ultramicroscopy, 11:239-250. Ichinokawa, T. (1986) Analytical Scanning Electron Microscopy for Solid Surface. Proceedings of the XIth International Congress on Electron Microscopy, Kyoto, Japan, Vol. 1,pp. 129-132. Kuroda, K., Hosoki, S., and Komoda, T. (1985) Observation for crystal surface of W field emitter tip by SEM. J. Electron Microsc., 34:179-182. Lehmpfuhl, G., and Uchida, Y. (1990) Observation of surface crystallography by reflection electron microscopy. Surface Sci., 235295306. Milne, R.H. (1989) Surface steps imaging by secondary electrons. U1tramicroscopy, 27~433-438. Moritz, W. (1984) Habilitationsschrift, University of Munich. Uchida, Y., Lehmpfuhl, G., and Jager, J. (1984) Observation of surface treatments on single crystalsby reflection electron microscopy. Ultramicroscopy, 15:119-130. Van Hove, M.A., Koestner, R.J., Stair, P.S., Biberian, J.P., Kesmodel, L.L., BartoS, I., and Somorjai, G.A. (1981) The surface reconstructions of the (100) crystal surfaces of iridium, platinum and gold. Surface Sci., 103:189-217.
Observation of Atomic Steps on Single Crystal Surfaces by a Commercial Scanning Electron Microscope YUJI UCHIDA, GISELA WEINBERG, AND GUNTER LEHMPFUHL Fritz-Huber-Znstitut der Mux-Plunck-Gesellschuft, Furudayweg 4-6,1000 Berlin 33, Germany
KEY WORDS
REM, Contrast mechanism, Imaging technique
ABSTRACT Atomic steps on (111) and (100) crystal surfaces of Pt were observed using a commercial scanning electron microscope (SEMI in secondary electron mode. By comparing the SEM images and those by reflection electron microscopy (REM), the observed contrast was confirmed to be that from atomic steps on crystal surfaces. The contrast mechanism is briefly discussed. One application of this imaging technique is also shown. INTRODUCTION The scanning electron microscope (SEM)is one of the most widely used instruments for observing the morphological structure of different specimens. Recently, SEM has achieved remarkable imaging capability through the technical developments in electron source, lens system, electron detector, and improved electronics. Applications of SEM for in situ investigations in the field of surface science are expected. The authors have tried to image monatomic steps on noble-metal surfaces (Pt and Au) with secondary electron detection in a commercial SEM without any special modifications. Imaging of atomic steps in secondary electron imaging mode has been already reported by Milne (1989). In our work we also compared images of atomic steps obtained by SEM and the reflection electron microscopic (REM) technique, but under special conditions for direction of the incident electron beam. Kuroda et al. (1985) had also shown a fine atomic step structure on the tungsten tip. The contrast mechanism of atomic steps in the SEM images could only be assumed.
of about 5 x Pa under operating condition. Therefore, the problem of contamination on the specimen surface during SEM observation is very serious. In addition to this, the oil-diffusion pump is sucked by the continuously operating rotary pump. The mechanical disturbance (e.g., by this rotary pump) poses large problems, which will be discussed later. The secondary electron images were obtained by normal mode operation with a working distance of about 10 mm and an accelerating voltage of 15-25 kV. The crystal was tilted against the incident electron beam so that the incident beam was estimated to satisfy the diffraction condition for intensity enhancement for the chosen electron energy and crystal surface. The orientation of the crystal was only estimated from the position of (111)facets on the spherical crystal. The REM observation was carried out with the Philips EM400T after the specimens had been observed by SEM. The observed areas on the crystal surface were recognized as dark contaminated patches in the REM images.
EXPERIMENTAL RESULTS Figure l a shows the SEM picture of Pt (111)taken EXPERIMENTAL PROCEDURE with an incident electron energy of 25 keV. Besides Spherical single crystals with small (111)and (100) islands, a very faint structures of steps can be recogfacets were used. The noble-metal spherical single nized. Figure l b shows the REM image of the same crystals, Pt and Au, were made by melting one end of a area taken with the enhanced 666 reflection near the 0.2 mm wire with a small hydrogen-oxygen flame, as LO111 crystal zone axis. Here we can clearly recognize already reported elsewhere (Hsu and Cowley, 1983; the atomic steps of the SEM picture. They correspond Uchida et al., 1984). The spherical single crystal of exactly to each other. The shapes of atomic steps in about 0.3-0.4 mm at the end of the wire was clamped both pictures are, of course, a bit different because of by a non-magnetic Cu-Be bronze screw on to the small their different tilting angles and azimuthal directions. specimen holder of aluminum-alloy. The orientation of Atomic steps could only be seen in a relatively small the single crystal could be recognized by observing the focused area in the SEM picture. Dynamical focusing (111) facets on the surface with a stereo microscope. was not used in our experiment in order to minimize The specimen holder with the spherical single crystal the contamination on the specimen surface. was inserted in the rotating-tilting goniometer. Figures 2a-c show SEM and REM pictures of a Pt A Hitachi S-4000 SEM, equipped with a field emission gun (FEG), was used for our investigations. The chamber for the electron source is evacuated by three ion-sputter pumps and attains a vacuum of about 5 x lop9 Pa. The specimen chamber is evacuated by a con- Received June 26, 1990; accepted in revised form September 30, 1990. ventional oil-diffusion pump with a cold-trap which can Address reprint requests to Yuji Uchida, Fritz-Haber-Institut der Maxbe cooled with liquid nitrogen and can reach a vacuum Planck-Gesellschaft, Faradayweg 4-6, 1000 Berlin 33, Germany. 0 1992 WILEY-LISS, INC.
OBSERVATION OF ATOMIC STEPS ON SINGLE CRYSTAL SURFACES
407
Fig. 1. Monatomic steps (indicated by arrows) on the (111)facet of a Pt single-crystal sphere. a: SEM image. b REM image with diffraction pattern. Direction of the incident electron beam is indicated. Beside the atomic steps small particles can be seen in (a)and (b). In
(b) the characteristic doubling of REM imaging is shown. The dark frame in (b) is the field of contamination produced during SEM imaging. The dark bars are the more heavily contaminated areas created by the scanning procedure.
(100) crystal surface. Figure 2a was taken from an area which is indicated with brackets in Figure 2b. The imaged areas are clearly recognized as contaminated rectangles on the (100)facet of the spherical crystal sur-
face. In the SEM picture of Figure 2a, the contrast of atomic steps is much stronger than that in Figure l a . This SEM image was taken with the stronger contrast amplification, which makes the image more noisy, as
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Fig. 2. Monatomic steps on the (100) facet of a Pt single-crystal sphere. a: SEM image at high magnification. b SEM image of the (100) facet at low magnification showing the contaminated fields of the investigated areas. The field in brackets is the area shown in (a).
The oblique contamination field was produced during investigation of the steps under a different angle of incidence. c: REM image of the steps in (a) with diffraction pattern. The image of the field of contamination is tilted because of different condition for imaging.
OBSERVATION OF ATOMIC STEPS ON SINGLE CRYSTAL SURFACES
409
Fig. 3. SEM images of the (100) facet of the Pt single-crystal sphere, which has been cleaned by Ar-ion bombardment and annealed, at low (a)and high (b)magnifications. a: The plane (100) facet can be seen surrounded by curved steps. Within the small-angle fanshaped area on the right-hand side two sets of straight steps can be seen which continue in the curved steps outside of the fan. The dark
rectangle is the field of contamination produced during SEM imaging. Small dark patches of unknown contamination can be seen. b: The two sets of straight steps from the left-hand side of the (100)facet can be seen. The wide dark and bright lines belong to the curved steps on the crystal sphere. Very faint straight atomic steps can just be seen.
can be seen in irregular horizontal lines on the picture. The atomic steps appear as dark or bright lines depending on whether the steps are “up” or “down.” Figures 3a,b show SEM images of a (100) facet of the Pt specimen which was cleaned in an ultra-high vacuum (UHV) chamber by Ar-ion bombardment and annealed. The facet is surrounded by sequences of steps
forming the curvature of the spherical crystal. The continuous curvature of the steps is interrupted within a small-angle fan-shaped area running along a [ O l l l direction. Within this area two sets of straight steps can be seen. Figure 3b shows an enlarged area of the (100) facet. The bright and dark steps belong to the system of curved steps. This area does not correspond to the con-
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Fig. 4. SEM images of the (111) facet of the Pt single-crystal sphere with contamination fields at different angles of observation. 7.3" (a)and 2.3"(b).
taminated area in Figure 3a. It was taken from the opposite (left) side of the (100) facet. The two sets of parallel straight atomic steps can be seen very clearly on the edge part of the (100) facet. Monatomic steps become just visible in this picture. The same structure was observed with the REM technique and was analyzed (Lehmpfuhl and Uchida, 1990). A short discussion will be given later. The following results are not directly concerned with the observation of atomic steps in this work. However, the effect should be closely related with the contrast
mechanism in the secondary electron SEM image. Figures 4a,b show REM images of a Pt (111)facet which were taken with an electron energy of 15 keV under different angles between the incident electron beam and the crystal surface. These angles for Figures 4a,b were estimated to be about 7.3" and 2.3",respectively. Areas which were contaminated before making these observations are seen as dark parallelepipeds on the specimen surface in Figure 4a. Contrary to this, these areas are observed as bright patches in Figure 4b. Heavier contaminations are, however, observed in Fig-
OBSERVATION OF ATOMIC STEPS ON SINGLE CRYSTAL SURFACES
ure 4b as vertical dark bars a t the left side of each bright rectangle. This contrast inversion of the contaminated area should depend on the thickness of contamination, on electron beam energy, and on the inclination of the electron beam to the crystal surface. A detailed investigation has not been carried out yet. Atomic steps were also observed on Au crystal surfaces in secondary electron images. There is no essential difference between the images of atomic steps on Au and those on Pt; therefore, the SEM pictures of atomic steps on Au crystals are not shown here.
DISCUSSION Atomic steps could only be observed in the SEM secondary electron image when the angle between the incident electron beam and the crystal surface was relative small, e.g., 5-10”. This angle corresponds for 25 keV electrons t o the relatively high order Bragg reflection (e.g., the Bragg angle for 25 keV for 666 reflection is about 5.8”).The diffraction effect of the incident electron beam on the crystal clearly exists, but this effect could not be observed in our experiments, because we had no reflection electron detector. The brightness of the crystal surface in the secondary electron SEM image increased continuously with the tilting angle until the surface became almost parallel to the incident electron beam. The specimens were adjusted in such a way that the diffraction condition for intensity enhancement was thought to be satisfied (Uchida et al., 19841. But this could not be ascertained without observation of diffracted electrons. The observation of atomic steps on a semiconductor surface with secondary electron imaging was already reported by Ichinokawa (1986). But the atomic steps could only be imaged after decoration by oxygen atoms. Our metal specimens were molten and recrystallized in air and observed in conventional vacuum. The crystal surface was surely exposed to different gases before observation. However, in our work, the decoration effect for explaining the atomic step contrast was excluded. The dependency of bright and dark contrast of atomic steps cannot be explained reasonably by the decoration effect. The contrast of atomic steps in our work has been found to obey a definite rule, as schematically shown in Figure 5. Atomic steps are seen in the SEM images as bright or dark lines with the narrowest width of about 5 nm. The width of this line contrast increases with defocusing. But the contrast character of atomic steps does not change with defocusing. Atomic steps were estimated to be visible within the defocusing range of about 3 pm. The diameter of the incident electron beam a t this defocusing position is about 9 nm (the semiangle of the illuminating electron probe is about 1.5 x lop3 rad). From these facts the contrast of atomic steps of secondary electron imaging seems to be explained with the simplest model, the standard explanation for the SEM image contrast. The secondary electron yield for the SEM detector depends on the inclination angle 0 of the crystal surface to the incident electron beam. This means that the surface of the crystal becomes brighter as 0 becomes smaller, as shown schematically in Figure
electron
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411
SEM image low intensity (dark)
high intensity (bright)
Fig. 5 . Schematic explanation for detecting intensity variation of secondary electrons from a spherical specimen with atomic steps.
5. This model is clearly acceptable for large atomic steps with step heights so large that the step surfaces can be seen as narrow facets. The steps with large step height were already observed without decoration (Ichinokawa, 1986). It is very interesting that the character of “dark” or “bright” contrast of atomic steps, which depends on whether the steps are “up” or “down,” coincides with that of large atomic steps. Therefore, the contrast of atomic steps may be also explained with this simple model, but quantitative calculations for the explanation of the atomic step contrast remain to be done. As shown in Figure 3, a characteristic structure of atomic steps on the cleaned Pt(100) surface was observed, This structure can only be understood as the modified surface structure controlled by the reconstruction (Lehmpfuhl and Uchida, 1990). It is well known that the Pt(100) surface after UHV cleaning treatment is reconstructed (Moritz, 1984; van Hove et al., 1981). However, the reconstruction itself is lifted during the transport from the UHV chamber to the SEM apparatus or during the first electron beam irradiation in conventional vacuum. However, during UHV treatment the reconstruction controls development of atomic steps along the unit cell of reconstruction. The directions of these two sets of straight parallel atomic steps deviate by 4.4 O from the exact 10111 direction. These directions coincide with the atomic rows with the highest atomic density of the hex-reconstruction on Pt(100) (Lehmpfuhl and Uchida, 1990). The separation of atomic steps is too small to be better resolved in our SEM images. One of the most interesting points is that atomic steps have become visible for the first time in the secondary electron images which were taken by the SEM installed with a FEG. It is not clear if the FEG is essentially important for imaging the atomic steps by SEM. The coherence of the illuminating electron beam from FEG is much better than that from the conventional electron source. Accordingly, it may be concluded that the coherent illumination of electron beam and a t the same time good resolution are necessary for atomic step imaging.
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The contrast inversion through the inclination change of the incident electron beam is not yet explained. A similar phenomenon was already observed in REM images from contaminated crystal surfaces (unpublished observation). Contaminated areas were observed as bright patches on the clean crystal plane. This phenomenon should be closely connected with the refraction effect from the contaminated layer on the crystal surface for the incident electron beam. At first, the authors considered that the effect might be explained in a similar way as in the theory for anti-reflection coating used for light optical lenses. However, the reflectivity of the incident electrons from the crystal surface seems to be too small to explain this effect, if only elastic scattering is considered (the reflectivity of electrons with 15 keV at an inclination of 2" for Pt is about 5%). As mentioned earlier, the electron microscope which we have used has a UHV chamber only for the electron source but a conventional vacuum chamber for specimens. The high contamination rate on the illuminated area of the specimen caused other difficulties for observation of atomic steps not only in the SEM image but also in the REM image for this investigation. Sometimes, no reasonable REM image could be obtained from the contaminated area of the crystal surface because of heavy contamination. The other technical problem was the already mentioned mechanical vibration caused by the rotary pump. The electron microscope was not equipped with a vacuum reservoir system which would allow the rotary pump to be switched off. In addition to this, the tilting stage, which had been used for our experiments, was not sufficiently stable against the mechanical vi-
bration to obtain higher resolution images. The resolution of images may be limited mainly by a mechanical instability of the specimen stage and not by the size of the electron probe.
CONCLUSIONS Atomic steps on (111)and (100) noble-metal crystal surfaces were observed with secondary electron imaging in a commercial SEM without any modification. Atomic steps were observed in the images which were taken with a small angle between the incident electron beam and the crystal surface. The observed contrast of the atomic steps is not due to a decoration effect. Further experiments with SEM must be done under UHV conditions.
REFERENCES Hsu, T., and Cowley, J.M. (1983) Reflection Electron Microscopy (REM) of fcc Metals. Ultramicroscopy, 11:239-250. Ichinokawa, T. (1986) Analytical Scanning Electron Microscopy for Solid Surface. Proceedings of the XIth International Congress on Electron Microscopy, Kyoto, Japan, Vol. 1,pp. 129-132. Kuroda, K., Hosoki, S., and Komoda, T. (1985) Observation for crystal surface of W field emitter tip by SEM. J. Electron Microsc., 34:179-182. Lehmpfuhl, G., and Uchida, Y. (1990) Observation of surface crystallography by reflection electron microscopy. Surface Sci., 235295306. Milne, R.H. (1989) Surface steps imaging by secondary electrons. U1tramicroscopy, 27~433-438. Moritz, W. (1984) Habilitationsschrift, University of Munich. Uchida, Y., Lehmpfuhl, G., and Jager, J. (1984) Observation of surface treatments on single crystalsby reflection electron microscopy. Ultramicroscopy, 15:119-130. Van Hove, M.A., Koestner, R.J., Stair, P.S., Biberian, J.P., Kesmodel, L.L., BartoS, I., and Somorjai, G.A. (1981) The surface reconstructions of the (100) crystal surfaces of iridium, platinum and gold. Surface Sci., 103:189-217.
