Secondary electron imaging of monolayer steps on a clean Si(111) surface

Surface Science North-Holland

258 (1991) 147-152

‘surface

science

Secondary electron imaging of monolayer steps on a clean Si( 111) surface Y. Homma,

M. Tomita

and T. Hayashi

NTT Applied Electronics Laboratories, Musashino-shi, Tokyo 180, Japan Received

7 February

1991; accepted

for publication

3 June

1991

Secondary electron images of monolayer steps on a 7 X 7 reconstructed Si(ll1) surface are shown to be ultrahigh-vacuum scanning electron microscopy (UHV-SEMI with grazing incidence of the primary electron beam. primary beam, clear contrast for atomic steps can be obtained at incidence angles I 15 ’ with respect to the surface. for atomic steps can still be detected up to 27 O. The step contrast depends strongly on the orientations of the relative to the primary beam incidence, indicating that the contrast is due to a geometrical effect at step edges.

1. Introduction

Ultrahigh-vacuum scanning electron microscopy (UHV-SEM) has shown its ability to characterize surface structures [1,2]. In particular, it has been demonstrated that SEM has the sensitivity required for detecting monatomically high steps on semiconductor or metal surfaces. Ishikawa et al. reported that they had made steps on the silicon (111) surface visible by oxidation of a clean surface in 10e4 Torr air for several tens of minutes [1,2]. They observed fine steps that were probably monolayer steps. Milne [31 recently reported atomic step imaging using a scanning transmission electron microscope (STEM) equipped with a secondary electron detector: a GaAs(ll0) surface cleaved in air (probably oxidized) was imaged by SEM and reflection electron microscopy (REM). They have also shown atomic step images from an oxidized copper (100) surface [3,4]. Kuroda et al. [5,6] have imaged atomic steps on a presumably clean tungsten tip during in situ flashing using a high-resolution field emission SEM. Except for the last example, the surfaces on which atomic steps have so far been observed by SEM were oxidized to some extent. It is therefore of interest whether or not 0039.6028/91/$03.50

0 1991 - Elsevier

Science

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observable by With a 25 keV Faint contrast step direction

atomic steps on clean surfaces, especially semiconductor surfaces, are visible by SEM. In this paper, we present atomic steps on a clean silicon (111) surface imaged by using SEM with a grazing incident electron beam.

2. Experiment Silicon surfaces were observed by using an ultrahigh-vacuum scanning reflection electron microscope (UHV-SREM) equipped with a field emission gun (a modified Hitachi S-800). The configuration near the sample is shown schematically in fig. 1. A 25 keV electron beam (typically

objective

lens SE-detector

_I/

sample

\

Lab

_I/

ov

side view

RHEED Fig. 1. Schematic

B.V. All rights reserved

\

diagram

of SE imaging

(not scaled),

l-3 nA) was incident on the sample surface at ;I low angle, as in reflection high-cncrgy electron diffraction (RHEED). For scanning rcflcction clcctron microscopy (SREM). the 444 apccula~ spot in a diffraction pattern on ;I fluorcxxnt screen was selected by an optical lens and introduccd into a photomultiplier via an optical fiber. SEM images can bc observed simultaneously in the SREM configuration. A secondary clcctron detector was placed near the axis of sample tilting. and the extraction voltage of secondary elcctrons was 10 kV. The working distance bctwccn the objective lens and the sample surface was 35-40 mm. The spatial resolution of the SEM with the normally incident 25 kcV electron beam was 10-20 nm. The base prcssurc in the ionpumped sample chamber was ( l-2) x IO ” ‘I‘ol-r. Using a liquid nitrogen shroud, the chamber pressure was lowered to 7 x IO- I” Torr. A boron-doped Sic II 1) wafer ( - 5 12 cm) misoriented by less than 0.1 ” was uscti in this study. Specimens 0.4 X 5 X 25 mm’ in size wcrc oxidized with a HISO, : H,O1 (4: I) solution and introduced into the chamber through a load lock. A direct electric current (DC) resistively heated the samples. whose temperatures wcrc monitored

a

by an infrared pyromctci-. Oxide on the sample hurfacch was dcsorbcd by heating to 900 ‘- C’. After oxide removal. fine Sic‘ particles wcrc formed. Most 01’ these wcrc rcmovcd by flash heating to 1250 Y’ several times. which produced ;I clean surface with ;I 7 x 7 qtructurc. The chamber prcs\urc was held below h x IO ” Torr during all heating proccsxes. SEM images were observed after cooling to room tcmpcraturc, ‘l’hc image-shortening effect parallel to the incident clcctron beam caused bq the graLing angle, was partially compensated bq reducing the beam raster width in this direction relative to the pcrpcndicular direction (i.c.. by tilt compensation) [7]. ‘l‘hc vertical scale 01’ thcsc SEM images is thus - I / 10 01‘ the horizontal SGIIC.

3. Results

C’lear images of atomic xteps co~~ld be obscr\;cd by SREM after obtaining 7 x 7 rccon structcd surfaces. Figs. 21 and 2b show the images taken by SREM and SEM, respectively. l’oi atomic steps in almost the same arca. This sam-

b

Y Homma et al. / Secondary electron imaging of monolayer steps on Si(lll)

ple was heated to 1200 “C for 20 s and then cooled, at a rate of N 2” C/s between 850 and 750 o C, to room temperature. The current direction was chosen to create a step-bunched surface; i.e., a surface of periodic atomic step bands with several monolayer steps between them [8]. Monolayer steps and step bands are seen in the SREM image. It has been confirmed by Osakabe et al. [9] and Ichikawa et al. [lo] through the observation of screw dislocations that REM (or SREM) step images of a Si(ll1) surface are formed by (111) monolayer steps; i.e., 0.31 nm spacing composed of Si double layers. The secondary electron (SE) image shows the same surface structure as that of SREM. Monolayer steps therefore can be imaged by SEM. Since the SE contrast of the atomic steps was much weaker than the normal topographic contrast, we took the SE image while extremely enhancing the image contrast. Fig. 3 shows the diffraction pattern corresponding to the surface shown in fig. 2. This pattern indicates that the surface has a 7 x 7 reconstructed structure. Fig. 4 shows the mass spectrum of the residual gas in the sample chamber during SEM observations. The total pressure was in the high 10-‘” Torr regime, and the main

Fig. 3. RHEED pattern indicating the Si(lllj7

149

Fig. 4. Mass spectrum of residual gas in the specimen chamber during SEM observation. The shroud in the chamber was filled with liquid nitrogen.

gas components were H, (2 amu) and CO (28 amu). Since the liquid nitrogen shroud was used, the signal for H,O corresponded to less than 3 x lo- l1 Torr. No contrast due to surface contamination was seen in the SEM or SREM images even one hour after sample heating. These

X 7 structure of the surface shown in fig. 2.

two results indicate that the observed surface was free from contamination. The contrast of the atomic steps depends strongly on the angle between the riser of the atomic step staircase and the incident electron beam. Fig. 5 shows SE images for three azimuthal angles of sample rotation. Contrast change is evident. The image of the atomic steps is bright when the electron beam is directed down the staircase (the step-down direction) and dark for the step-up direction. This contrast change is the same as the topographic contrast for a higher step: more secondary electrons arc emitted from a riser faced in the forward scattering direction. In non-Bragg reflection conditions SE images have an advantage over REM images. Especially, a higher angle of incidence (to the sample surface) can be used for SE imaging. Fig. 6 shows atomic step images for incident angles up to 27 “. The tilt compensation tcchniquc was not used fat these images. Atomic steps are clearly seen up to IS”, and are faint but still discernable even at 27 O. At angles greater than 30 O, SEs from the surface are overwhelmed by those from the bulk.

4. Discussion The origin of atomic step contrast in SE images can bc inferred from fig. 5. Bright images arc produced when the primary electron beam goes down the steps and dark images result from the beam going up the atcps. Milne ha4 also shown the same contrast change for atomic steps on Cu using the SEM mode in STEM [3.4]. ‘l’his result strongly indicates that the atomic step contrast is due to the geometrical shape of step edges as small as 0.31 nm. As shown in fig. 7, the difference between the step-down and step-up casts is the amount of SEs emitted from the step-riser part. In the step-down case, a larger amount of SEs arc cxpectcd to hc produced by forward-scattered primary electrons compared to the step-up case where the SEs arc produced by backscattered ciectrons. As already pointed out by Milne [3]. mcchanisms such as a change in work function due to adsorption at steps or charging at steps can bc excluded because they would product contrast independent of the incident beam direction. An-

TeFig. 5. SE images for different indicated

azimuth

angles of sample rotation,

Te‘ [ii21

incidence

2m

(a). [iOl] (h). and [zl I] Cc). ;\n Identical

\tep I\

hy arrows. The primary electron beam is incident from the bottom, and steps are down from left to right. The St‘ tlctrctclt was located on the left \ide of the SE image\.

Y Homma et al. / Secondav

electron imaging of monolayer steps on Sic1 I I)

2 w-n

c

I

e-

d

151

2 m-n

angles of primary electron incidence, - 2.6” (a), - 8” (b), - 15’ Cc), and - 27’ (d). The Fig. 6. SE images with different elec ,tron beam is incident from the bottom, and steps are down from right to left. The SE detector was located on the left si de of the SE images. Tilt compensation was not used.

-+

primary electron

....+ secondary electron

bright

,..I ~~~~’ step-down dark

...d

...4 q;

7#@> step-up Fig. 7. SE contrast

change

Inelastically scattered electron at atomic

steps.

other possible mechanism would be the emission of high-energy scattered electrons from step edges. However, no images could be observed when the SE-extraction voltage was switched off. So, the high-energy scattered electrons do not contribute to the image formation of atomic steps. Ichinokawa et al. reported that they could not obtain an atomic step image from a Sic11 1) surface before exposing the surface to lo-” Torr air [2]. They used the same type of field emission electron gun as we, but at the low acceleration voltage of 2 kV [ll. Their papers did not report the used angle of incidence of the primary elec-

152

Y. Homna

et al. / Secondary electron imaging of monolayer steps on Si(llll

tron beam, but we infer from the images in their papers that a large tilt angle (low grazing incidence) was not used [1,2]. In our experiments as well, atomic step contrast was hardly observed when the primary beam energy was below 5 keV. The reason for the difficulty in observing atomic steps with lower-energy primary electrons is thought to be degradation of step contrast caused by the increase of the primary beam diameter and the reduction of the total amount of SEs due to the decrease of primary beam current. In contrast to this, in the grazing incidence of higherenergy primary electrons, the SE emission is enhanced by both the angle effect (SE yield is proportional to l/cos 8, where 8 is the angle of incidence with respect to the sample normal) and the higher primary current available. This fact might contribute to detect a small contrast change at atomic steps because of a low statistical fluctuation in the SE intensity. SE imaging of atomic steps has several advantages over RE imaging. First, as mentioned in the previous section, the image distortion due to foreshortening is reduced because higher angle of incidence can be used. Second, a convergent beam can be used for imaging, which makes it easier to obtain higher resolution. And third, it is much easier to produce images of a rough surface. These factors should ensure wider application of SE for step imaging.

5. Conclusions It has been demonstrated that atomic steps on a clean Si surface are observable by SEM with grazing incidence of the primary electron beam. Bright contrast of atomic steps is generated with the primary electron beam directed down the steps, and dark contrast with the beam directed up the steps. These results strongly suggest that

the contrast of atomic steps originates from a geometrical effect at the step edge. Optimum configurations for secondary electron imaging of atomic steps is thought to include higher energy and a low angle of incidence (I 15’ to the surface) for the primary electron beam to enhance the SE emission from the surface with the reduction of SEs from the bulk.

Acknowledgements

The authors would like to thank Dr. Masakazu Ichikawa of the Central Research Laboratory, Hitachi Ltd., for his helpful discussions on the instrumentation. Thanks are also due to Drs. Yoshikazu Ishii, Satoru Kurosawa, Mineharu Suzuki, and Robert J. McClelland for their thoughtful discussions and encouragement of this work.

References

Ill

Y. Ishikawa, N. Ikeda, M. Kenmochi and T. lchinokawa. Surf. Sci. 159 (1985) 256. l21T. Ichinokawa, Y. Ishikawa, M. Kenmochi and N. Ikeda. Surf. Sci. 176 (1986) 397. 27 (1989) 433. l31 R.H. Mime, Ultramicroscopy [41 A.L. Bleloch, A. Howie and R.H. Milne. Ultramicroscopy 31 (1989) 99. [Sl K. Kuroda, S. Hosoki and T. Komoda, J. Electron MIcrosc. 34 (1985) 179. lb1 K. Kuroda, S. Hosoki and T. Komoda, Scanning Microsc. 1 (1987) 911. Jpn. J. Appl. Phyx. 21 [71 M. Ichikawa and K. Hayakawa, (1982) 145. and S.I. [81 A.V. Latyshev, A.L. Aseev, A.B. Krasilnikov Stenin, Surf. Sci. 213 (1989) 157. [91 N. Osakabe. Y. Tanishiro, K. Yagi and G. Honjo, Surf. Sci. 102 (19811 424. [lOI M. Ichikawa, T. Doi, M. Ichihashi and K. Hayakawa, Jpn. J. Appl. Phys. 23 (1984) 913.