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Low energy scanning electron microscopy combined with low energy electron diffraction
397
Surface Science 176 (1986) 397-414 North-Holland, Amsterdam
LOW ENERGY SCANNING ELECTRON MICROSCOPY WITH LOW ENERGY ELECTRON DIFFRACTION T. ICHINOKAWA, Y. HOSOKAWA Department
Y. ISHIKAWA,
M. KEMMOCHI,
COMBINED
N. IKEDA,
of Applied Physics, Waseda Vniuersity, S-I-I, Ohkubo, Shinjuku-ku,
Tokyo 160, Japan
and J. KIRSCHNER Institut ftir Grenzfliichenforschung und Vakuumphysik, D-5I70 Jiilich I, Fed. Rep. of Germany Received
7 April 1986; accepted
for publication
Kemforschungsanlage
Jiilich GmbH,
13 May 1986
An ultra-high-vacuum (UHV) scanning-electron microscope (SEM) with a field-emission gun (FEG) is operated in a low energy region from 100 eV to 3 keV. The functions of scanning low energy electron diffraction microscopy and scanning Auger microscopy have been implemented and their performance is demonstrated. Observations in the SEM mode, the low energy electron diffraction (LEED) mode and the Auger mode have been made on the following examples: the step structure of the clean Si(ll1) surface, the structure of islands and their movement on the Au evaporated surface of Si(lll), different orientations of grains on the polycrystalline Si surface and the coexistence of different super-structures on the Au-evaporated surface of Si(ll1).
1. Introduction Surface electron microscopy has been intensively developed by many workers using various types of electron optical systems. Typical techniques for surface imaging which have been reported are as follows: (1) reflection electron microscopy (REM) by the conventional transmission electron microscope (CTEM) [l-4]; (2) scanning electron microscopy (SEM) with various types of electron detector system [5-71; (3) photoelectron emission microscopy (PEEM) by the emission electron microscope (EEM) [g-lo]; (4) low energy electron reflection microscopy (LEERM) by the mirror electron microscope (MEM) [ll-131. To obtain meaningful results the specimen has to be in ultra-high vacuum at a pressure of lo- lo Torr and the specimen chamber has to contain surface cleaning facilities. On the other hand, surface microscopic analysis requires information on the chemical composition, the chemical bonding state (electronic structure) and the crystal structure at the surface. 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
39x
T. Ichmoknwa
er ~11.,’ LESEM
combined with LEED
Techniques (1) and (4) described above are very convenient to connect electron diffraction to electron microscopy, while techniques (2) and (3) connect electron spectroscopy to electron microscopy. Microscopic information on the surface crystal structure, e.g.. superstructure domains, crystal defects, steps and orientation of grains, is very important in addition to information on the local variation of chemical composition and electronic structure. Recently, several results of surface electron microscopy combined with reflection high energy electron diffraction (RHEED) have been reported [l-4]. However, most of these techniques used high energy primary electrons and a rather small glancing of incidence onto the surface. In the present experiment, an UHV-SEM with a FEG is operated in an energy range from 100 eV to 3 keV and the new technique of scanning LEED microscopy is added to other techniques; e.g., scanning Auger microscopy (SAM), secondary electron microscopy, electron energy loss microscopy and the others available to the ordinary SEM. We feel that scanning LEED microscopy is of great advantage to surface micro-structure analysis since the requirements of glancing incidence is removed, and since a good surface sensitivity is achieved through the use of low energy electrons. In the present paper, the design and performance of the scanning LEED microscope are presented. Preliminary applications to Si surfaces are given as demonstrations. 2. Equipment 2.1. Outline of the UHV-SEM
with the FEG
A schematic diagram of the UHV-FEG-SEM combined with other surface analytical equipments is shown in fig. 1. The instrument is composed of two chambers in which the vacuum is better than 5 X 10~“’ Torr. The right hand chamber is equipped with standard (macroscopic) surface analysis tools, while the other chamber is used for surface microanalysis and surface imaging. It is equipped with an UHV-FEG-SEM, a moveable cylindrical mirror analyzer (CMA), a two-dimensional detector of diffracted LEED beams, an ion gun, and a deposition source. Signals of elastically diffracted electrons, Auger electrons, inelastically scattered electrons, secondary electrons and backscattered electrons are available to form characteristic images of solid surfaces. The accelerating voltage of the UHV-SEM can be lowered from 30 keV to 100 eV to obtain high surface sensitivity. The electron optical system, the control unit and display circuits are modified versions of the Hitachi S-800 microscope. The UHV-SEM with the FEG was first developed by Crewe and Wall [14]. The optical column of the present microscope is composed of the usual electrostatic lens of Butler type [15] and two magnetic lenses. The electron
T. Ichinokawa
et al. / LESEM
combined with LEED
399
Spoci men Manlpulato~
Fig. 1. Schematic diagram of the UHV-FEG-SEM.
beam is decelerated by the Butler lens when the accelerating voltage is lower than the extraction voltage. The whole optical system is shielded by p-metal from stray electromagnetic fields. The working distance is variable from 3 mm (high resolution specimen position) to 35 mm (analytical specimen position). The specimen is mounted on a manipulator with X, Y and Z translations, two axial rotations and a slight inclination around the other axis. The sample is heated by direct electric current. The other chamber is used for specimen preparation and macroscopic surface analyses and is equipped with a similar specimen manipulator, a specimen heating stage, cleavage equipment, an ion gun, an evaporation cell, AES-LEED optics with an electron gun and a quadrupole mass spectrometer. Both chambers are bakeable up to 200°C and connected by a gate valve. The sample is put into a pre-vacuum chamber for sample exchange by an air-lock system and transferred to the sample stages of both manipulators by a transfer system with magnetic coupling. 2.2. Detection system for scanning LEED microscopy The concept of the scanning LEED microscope comprises two steps: first the formation of a selected-area LEED pattern, and secondary, the generation of raster images with the information contained in the diffraction pattern.
T Lchinokuwu
400
f
et ul. / LXSEM
comhltwd
w:rth L.EED
I_____ f Sample OScill0SC0pe
Fig. 2. The LEED
detector
assembly with two-dimensional
rotatable
from O” to 120°
read-out.
The detector
assembly I\
around the sample.
At a resolution of several hundred A and beam energies of a few hundred eV. the primary beam current is of the order of 10 “’ to 10 ” A. Under these conditions a LEED pattern cannot be observed on a fluorescent screen. Instead, single electron counting techniques have to be used [16]. In the present experiment the LEED detector assembly shown in fig. 2 has been used, which consists of two hemispherical grids, a two-stage channel-plate amplifier, and a position-sensitive detector (Model 2396. Surface Science Lab., 40 x 40 mm’ in active area). The position-sensitive detector (PSD) translates the X--L’ coordinates of each electron diffraction event into two voltages. When these are given to an X-F storage scope. a bright spot is generated for each event, and a LEED diffraction pattern is seen to accumulate on the scope screen. In this way a selected area LEED pattern may be observed from a region of a few 100 A diameter on the surface, with a probe current of lo- ” A or less. The detector assembly is rotatable from 0” to 120” around the sample with a radius of 60 mm. Thus, the angle of incidence of the primary beam may be chosen between grazing and about 30” with respect to the surface normal. The convergence angle of the incident beam is limited by an objective aperture to 1 mrad or less. The spatial resolution of the positionsensitive detector is nominally 200 lines/40 mm, corresponding to an angular resolution of about 3 mrad. The “transfer width” (or inverse momentum resolution) of the system is thus determined by the detector resolution. It is estimated to be comparable to or larger than the linear dimension of the spot illuminated by the primary beam. Placing the primary beam spot at a different position on the sample the same or a different diffraction pattern may be observed, depending on whether the surface structure is the same or not. In this way. for example, the surface orientation of different grains in a polycrystalline sample may be observed. Likewise, one may select a particular beam out of the diffraction
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combined with LEED
PSD - -
‘Y
Position Computing
Window Comparator
AND Gate
Digital Switch
Oscilloscope
Ele_ctronics Fig. 3. Block diagram
of two-dimensional diffraction
analogue comparators spot on the oscilloscope.
to select a window
around
a
pattern and modulate the brightness of the SEM oscilloscope by the intensity of this particular beam. This mode corresponds to the dark field imaging mode in conventional transmission microscopy. Instead of moving mechanically the objective aperture, like in CTEM, an electronic window is set around a diffraction spot using the two-dimensional analogue comparators shown in fig. 3. In this way a dark-field image of the surface structure is formed. The total count rate for the whole area of the PSD is limited to about lo5 counts/s due to electronic dead times. Therefore the maximum count rate inside a selected window is typically less than lo3 counts/s. The scan time for one frame ranges from 10 min to 1 h, depending on the desired signal-to-noise ratio. The data are stored in a micro-computer and an image is formed by 256 x 256 pixels with 8 bit resolution each. If desired, digital background subtraction, contrast enhancement or smoothing may be carried out.
2.3. Detection system for scanning Auger microscopy A single-pass CMA is used for scanning Auger microscopy and electron energy loss microscopy. It is retractable and adjustable in its position from outside the vacuum. The output signal of the CMA is measured in the N’(E) mode by a lock-in amplifier and in the N(E) mode by a pulse counting system. The data processing for Auger electron spectra and scanning Auger images is performed by the micro-computer.
3. Performance of the UHV-FEG-SEM The spatial resolution of electron microscopes depends on the acceleraung voltage. The reason why the lower the accelerating voltage, the lower the spatial resolution is as follows. With decreasing accelerating voltage V,, the
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chromatic aberration (= C,aAP’/v,), the diffraction aberration ( = 0.61 X/a) and the coefficients of spherical and chromatic aberrations of the objective lens (C, and C,) increase. and the brightness of the electron source decreases (n is the semi-divergent angle of the objective aperture, eAT/ is the energy spread, and h the wavelength of the incident electrons). Among these, the effect of AV/V, is most important in the low energy region. A spread of the spot size due to the chromatic aberration is thus inversely proportional to the accelerating voltage. For instance, a probe diameter of 100 A at L’, = 1 kV becomes 1000 A at V, = 100 V.
Fig. 4. Micrographs taken at: (a) E, = 30 keV (magnetic tape), (b) 0.8 keV (latex particles of 1000 and 2000 A diameter), (c) 0.5 keV (particle diameter 2000 A). (d) 0.25 keV (1 pm diameter).
T. Ichinokawa
et al. / LESEM
S-000
403
combined with LEED
FE-SEM
1600 - 30000 1400 -1200
-
/ /
.Q
/
pQ0
-
,
I
/
_‘2OOOo g
0 5
I-
s G
800-
d m : 600-
z -10000
?
LOO200. I 250
0
h 500
ACCELERATION Fig. 5. Spatial
resolution
and optimum
I 750 VOLTAGE
1000 ( V )
magnification versus accelerating UHV-FEG-SEM.
voltage
for the present
The experimental spatial resolution was measured as function of the accelerating voltage by testing samples of magnetic tape and latex particles evaporated by gold. Test images are shown in fig. 4 and the resolution as a function of the accelerating voltage is shown in fig. 5. The experimental resolution of 600 A has been attained at V, = 250 V and of 20 A at VO= 30 kV with a probe current of 2 X lo-” A. The limitations to high resolution were vibrations of the FE tip and of the sample stage. The fluctuation of the emission current is several percent at a total current of 100 PA at a tip temperature of 20°C. The magnitude of the signals for images or spectra is normalized by current picked up by an aperture placed on the electron optical axis. The present tip has been in operation since more than three years. The extraction voltage was initially 2 kV, which increased to 5 kV at present. The rate of increase was large during the first year and is lower since. 4. Experimental results 4.1. UH V-SEA4 observations on Si(l II) surfaces 4.1.1.
Step structure The step structure
of Si(ll1)
formed
by high temperature
annealing
up to
404
Fig. 6. Step structures on the Si(ltl)
surface anaealed
at 1200°C
in UHV.
images: (a) the clean surface and (h) the surface exposed to air of lo-’
Torr
Secondary
electron
for several tens of
minutes at room temperature.
1250°C was reported previously [I‘?]. A number of step bands parallel to [
Fig. 7. SEM images of an island on the Au-evaporated surface of Si(ll1): (b) at 400°C.
(a) at room temperature,
by reflection electron microscopy [1,7] and by transmission electron microscopy using the replication technique [18]. Further studies of the step structure required the dark field imaging capability of the scanning LEED microscope (see below). 4.1.2. Islands on the Au-evaporated Si(lll) surface An island on the Au-evaporated surface of Si(ll1) is observed in the secondary electron image as shown in fig. 7. The island is heterogeneous at room temperature as reported by Oura and Hanawa [19], but becomes homogeneous at a temperature higher than 370°C. The fine texture within the island changes at each melting-cooling cycle. The island moves on the surface at temperatures higher than 400°C as shown in fig. 8. The speed is 0.5-5 ~m/rnin at 550°C depending on surface contamination and surface defects. A large island absorbs small islands during its course and leaves a well-ordered surface of fi X fi structure on its trace. A motive force of the movement is due to the temperature difference at the surface. The islands move towards a direction from lower to higher temperature. Steps may disappear inside the trace as seen from fig. 8c or may be displaced. 4.2. Scanning LEED microscope applications on Si surfaces 4.2.1. Step structure Steps of one or several monolayers height on the terraces of a stepped Si(ll1) surface have not been observed by the secondary electron image. As a first application of scanning LEED microscopy, it was tested whether the fine steps visible in the surface exposed to air, as shown in fig. 6b, are observable or not. A selected area LEED pattern on the terrace in fig. 9b shows th (7 X 7) structure at E, = 250 eV as shown in fig. 9a. Fig. 9b is a secondary
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Fig. 8. The movement of an island on the Au-evaporated surface of Si(l11). Small islands coalesce with the large island and the trace left behind by the movement of the large island shows the fi X fi structure. The step is displaced inside the trace.
electron image. The dark field image was observed by the specular beam spot. indicated by a square mark in fig. 9a. To enhance the contrast, the background is subtracted by image processing. Fig. 9c shows a dark field image at 250 V after image processing. The steps are observable on the terraces and a crystal defect region due to the inclusion shows the dark contrast around it. The image was taken in 6 min. Each pixel contained between 10 and 40 counts, of which 10 were subtracted as a background. These results show the inherently strong contrast in the dark field image (at least a factor of two), as compared to the secondary electron contrast of a few tenths of a percent. The grainy
T. Ichinokawa et al. / LESEM combined with LEED
407
Fig. 9. Scanning LEED microscopy for the step structure on the Si(ll1) surface: (a) a selected area LEED pattern on the terrace (the 7 X 7 structure), (b) a secondary electron image and (c) a dark field image taken by the specular reflection spot in (a). The fine steps are observable on the terraces and a crystal defect region due to the inclusion shows dark contrast around it.
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with LEED
Fig. 10. Scanning LEED microscopy for grains in the polycrystalline Si surface: (a) secondary electron image, (b) selected area LEED pattern of the grain and (c) dark field image taken by the specular spot in (b).
structure of the image is caused this in section 5.
by the counting
statistics.
We comment
on
The polycrystalline Si consists of a number of small grains of less than several pm in size. By annealing up to 700°C the selected area LEED patterns of the 1 x I structure were observed for each grain of different orientation. Fig. 10a shows a secondary electron image and fig. 1Ob a selected area LEED pattern of one grain. Fig. 10~ shows a dark field image taken by the specular spot in fig. lob. The crystal structure and orientation of a grain can be determined with a spatial resolution of - 500 A.
Fig. 11. Scanning LEED microscopy for islands on the Au-evaporated surface of Si(ll1): (a) secondary electron image, (b) and (c) selected area LEED patterns of the substrate (~6 XV? structure) and the island, respectively, and (d) dark field image by means of the specular spot.
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4.2.3. Island structure on the Au-evaporated surf&e of Si(ll1) After Au-evaporation to a thickness of several tens of monolayers and subsequent heating up to 700°C for several tens of minutes, islands of various size were observed in the secondary image as shown in fig. lla. The substrate shows the fi x 6 structure as shown in fig. 11 b by the selected area LEED pattern, while that of the island is diffuse as shown in fig. llc. Heat treatment above 450°C leads to agglomeration of the Au-Si film, producing of the 6 x fi superstructure at the substrate [19] and islands in polycrystalline state. Fig. lid is a dark field image taken by means of the specular beam indicated in fig. llb. The chemical composition of an island shown in fig. 12 is determined by micro-Auger spectroscopy. The Auger electron spectra measured at three points, a, b and c, are shown in fig. 12. According to the Auger micro-analysis, it is found that the island is a heterogeneous mixture of two phases; one phase
a
b
Fig. 12. Micro-Auger electron spectra at three points. a. b and c, on the secondary The island is a heterogeneous mixture of gold and silicon with a eutectic AU 069Sio ?I’
electron image. composition of
T. Ichinokawa
et al. / LESEM
combined with LEED
411
is nearly pure gold and the other nearly pure silicon. The average composition measured with a large diameter of the primary beam is approximately 2 : 1 for Au : Si in atomic concentration. Since islands melt at 370°C as described above, we can find that islands have a eutectic composition of Au0,69Si0,3, as reported by the phase diagram of the Au-Si binary system [20]. The surface inside the trace behind the island movement (at point b) shows a fi x fi LEED pattern, indicating a coverage of a monolayer Au. The fi x 6 structure transforms into the 1 X 1 structure at 750°C consistent with the transition temperature reported by Le Lay et al. [21]. 4.2.4. Coexistence of different superstructures on a Au-evaporated Si(ll I) surface When the clean Si(111)7 X 7 surface is covered by a few monolayers of Au
Fig. 13. Scanning LEED microscopy for the Au-evaporated surface of Si(ll1): (a) secondary electron image, (b) dark field image (the bright area at the central part corresponds to the 5 x 1 structure while the dark area corresponds to the fi x fi structure), (c) and (d) are selected area LEED patterns of the 5 x 1 and fi X fi structures, respectively.
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comlmed
wth LEED
by evaporation and afterwards heated up to 900°C for several tens of minutes. a coexistence of the 5 x 1 and fi x fi structures was observed. The coverage of Au atoms in the 5 x 1 structure is 0.25 monolayer and that in the fi x fi is one monolayer. After Au atoms have evaporated at a temperature above 900°C the surface becomes clean with the 7 X 7 reconstructed structure. In the dark field image of fig. 13b, the bright area in the central part is the 5 X 1 structure and the dark area is the fi X fi structure as illustrated by the selected area LEED patterns in fig. 13~ and 13d, taken in the bright and dark areas, respectively. The coexistence of similar superstructures on the Au-evaporated Si(ll1) surface has been observed by Telieps and Bauer [ll] and Osakabe et al. [l]. The contrast of the superstructures in fig. 13b is the same as those in their images.
5. Discussions
The combination of high energy SEM with RHEED was performed by Ichikawa et al. [7] in the ultra-high vacuum system. The disadvantage of this technique is similar to that of high energy reflection electron microscopy for small glancing angles of incidence. We have developed a combined technique of low energy scanning electron microscopy with LEED for the observation of surfaces at high glancing angle incidence and with a high surface sensitivity. The choice of the primary electron voltage should be a trade-off between achievement of good LEED patterns and limitation of spatial resolution of the raster image. To obtain good LEED patterns with a glancing angle of incidence higher than 30”, the voltage is preferable less than 300 V. The mean free path of electrons in a solid is minimum in an energy range from 60 to 100 eV, which is the most suitable operation region in LEED. The spatial resolution in this energy range is approximately 1500 A as seen from fig. 5. In the range higher than 3 keV, it encounters difficulties such as a small glancing angle of incidence and a poor angular resolution of the selected area LEED patterns caused by the limitation of the spatial resolution of PSD. The compromise between both requirements renders a choice of 200 to 1 keV as the optimum energy region under the conditions of the present experimental arrangement. Then, the highest spatial Oresolution of the present scanning LEED microscope is approximately 500 A. The contrast of the dark field image depends on the type of diffraction beam used for the raster image. While the dark field images shown in the present paper were taken with the specular beam of the fundamental unit mesh, images taken with fractional order beams showed higher contrast. but substantially lower signal-to-noise ratio. The signal-to-noise ratio in the dark field images is so far not limited by the primary beam current, but by the
T. lchinokawa ef al. / LESEM
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413
count rate capability of the position-sensitive detector. An improvement by one order of magnitude may be possible by using faster electronics. Further improvements may be possible by placing a separated channeltron multiplier near the channel-plate or into a center hole of the PSD and by using its output for imaging. It should be noted, however, that a very large increase of the primary beam current, though perhaps feasible, will not be desirable in applications to sensitive systems where electron stimulated desorption may impose severe limitations. A number of applications of the UHV-SEM and the scanning LEED microscope are demonstrated for Si surfaces. Some examples on the microscopic structures in the surface are interesting and have to be discussed in more detail. T,he examples, however, may necessitate further studies to arrive at conclusions. Therefore, we do not advance further discussions on the applications and will publish them elsewhere.
6. Conclusion The design and performance of the UHV-SEM with the FEG are described and preliminary applications to Si surfaces are demonstrated in the SEM mode, the LEED mode and the Auger mode. It is suggested that the technique of scanning LEED microscopy will be available in the following areas of solid surfaces: domains of reconstructed structures (or superstructures), phase transitions on surfaces, initial stages of epitaxial growth, relation between surface structure and surface diffusion, and interactions between surface crystal defects and adsorbates. In situ observations of those processes with changing surface temperature and coverage of adsorbate are interesting to study.
Acknowledgements The present work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education (Japan). One of the authors (JK) appreciates the support by the programme for foreign research fellowship from Waseda University.
References [I] [2] [3] [4] [5]
N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 97 (1980) 393. T. Hsu, Ultramicroscopy 11 (1983) 167. C.E. Warble, Ultramicroscopy 15 (1984) 301. C.E. Cowley and Z.C. Kang, Ultramicroscopy 11 (1983) 131. J.A. Venables, G.D.T. Spiller, D.J. Fathers, C.J. Harland and M. Hanbucken, croscopy 11 (1983) 149.
Ultrami-
414 [6] [7] [8] 191 (lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
T Ichtnokaw
et al. / LESEM
comhmed wth LEED
T. Ichinokawa and Y. Ishikawa, Ultramicroscopy 15 (1984) 193. M. Ichikawa, T. Doi, M. Ichihashi and K. Hayakawa, Japan. J. Appl. Phys. 23 (1984) 913. H. Bethge, Th. Krajewski and 0. Lichtenberger, Ultramicroscopy 17 (1985) 21. I.R. Plummer, H.O. Porter. P.W. Turner. A.J. Dixon. K. Gehring and M. Keenlyside. Nature 303 (1983) 599. J. Cazaux. Ultramicroscopy 17 (1985) 43. W. Telieps and E. Bauer, Ultramicroscopy 17 (1985) 57. A.A. Delong and V. Kolarik. Ultramicroscopy 17 (1985) 67. C. Guittard, J. Phys. DlO (1977) 2331. A.V. Crewe and J. Wall. Rev. Sci. Instr. 39 (1968) 576. J.W. Butler, in: Proc. 6th Intern. Congr. on Electron Microscopy. Kyoto. 1969. Vol. 1. p. 191. P.C. Stair, Rev. Sci. Instr. 51 (1980) 132. Y. Ishikawa. N. Ikeda, M. Kemmochi and T. Ichinokawa, Surface Sci. 159 (1985) 256. H.C. Abbink. R.M. Broundy and G.P. McCarthy, J. Appl. Phys. 39 (1968) 4673. K. Oura and T. Hanawa. Surface Sci. 82 (1979) 202. Selected Values of the Thermodynamic Properties of Binary Alloys. prepared by Ralph Hultgren et al. (American Society for Metals, Metals Park, OH. 1973) p. 317. G. Le Lay. M. Manneville and R. Kern. Surface Sci. 65 (1977) 261.
Surface Science 176 (1986) 397-414 North-Holland, Amsterdam
LOW ENERGY SCANNING ELECTRON MICROSCOPY WITH LOW ENERGY ELECTRON DIFFRACTION T. ICHINOKAWA, Y. HOSOKAWA Department
Y. ISHIKAWA,
M. KEMMOCHI,
COMBINED
N. IKEDA,
of Applied Physics, Waseda Vniuersity, S-I-I, Ohkubo, Shinjuku-ku,
Tokyo 160, Japan
and J. KIRSCHNER Institut ftir Grenzfliichenforschung und Vakuumphysik, D-5I70 Jiilich I, Fed. Rep. of Germany Received
7 April 1986; accepted
for publication
Kemforschungsanlage
Jiilich GmbH,
13 May 1986
An ultra-high-vacuum (UHV) scanning-electron microscope (SEM) with a field-emission gun (FEG) is operated in a low energy region from 100 eV to 3 keV. The functions of scanning low energy electron diffraction microscopy and scanning Auger microscopy have been implemented and their performance is demonstrated. Observations in the SEM mode, the low energy electron diffraction (LEED) mode and the Auger mode have been made on the following examples: the step structure of the clean Si(ll1) surface, the structure of islands and their movement on the Au evaporated surface of Si(lll), different orientations of grains on the polycrystalline Si surface and the coexistence of different super-structures on the Au-evaporated surface of Si(ll1).
1. Introduction Surface electron microscopy has been intensively developed by many workers using various types of electron optical systems. Typical techniques for surface imaging which have been reported are as follows: (1) reflection electron microscopy (REM) by the conventional transmission electron microscope (CTEM) [l-4]; (2) scanning electron microscopy (SEM) with various types of electron detector system [5-71; (3) photoelectron emission microscopy (PEEM) by the emission electron microscope (EEM) [g-lo]; (4) low energy electron reflection microscopy (LEERM) by the mirror electron microscope (MEM) [ll-131. To obtain meaningful results the specimen has to be in ultra-high vacuum at a pressure of lo- lo Torr and the specimen chamber has to contain surface cleaning facilities. On the other hand, surface microscopic analysis requires information on the chemical composition, the chemical bonding state (electronic structure) and the crystal structure at the surface. 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
39x
T. Ichmoknwa
er ~11.,’ LESEM
combined with LEED
Techniques (1) and (4) described above are very convenient to connect electron diffraction to electron microscopy, while techniques (2) and (3) connect electron spectroscopy to electron microscopy. Microscopic information on the surface crystal structure, e.g.. superstructure domains, crystal defects, steps and orientation of grains, is very important in addition to information on the local variation of chemical composition and electronic structure. Recently, several results of surface electron microscopy combined with reflection high energy electron diffraction (RHEED) have been reported [l-4]. However, most of these techniques used high energy primary electrons and a rather small glancing of incidence onto the surface. In the present experiment, an UHV-SEM with a FEG is operated in an energy range from 100 eV to 3 keV and the new technique of scanning LEED microscopy is added to other techniques; e.g., scanning Auger microscopy (SAM), secondary electron microscopy, electron energy loss microscopy and the others available to the ordinary SEM. We feel that scanning LEED microscopy is of great advantage to surface micro-structure analysis since the requirements of glancing incidence is removed, and since a good surface sensitivity is achieved through the use of low energy electrons. In the present paper, the design and performance of the scanning LEED microscope are presented. Preliminary applications to Si surfaces are given as demonstrations. 2. Equipment 2.1. Outline of the UHV-SEM
with the FEG
A schematic diagram of the UHV-FEG-SEM combined with other surface analytical equipments is shown in fig. 1. The instrument is composed of two chambers in which the vacuum is better than 5 X 10~“’ Torr. The right hand chamber is equipped with standard (macroscopic) surface analysis tools, while the other chamber is used for surface microanalysis and surface imaging. It is equipped with an UHV-FEG-SEM, a moveable cylindrical mirror analyzer (CMA), a two-dimensional detector of diffracted LEED beams, an ion gun, and a deposition source. Signals of elastically diffracted electrons, Auger electrons, inelastically scattered electrons, secondary electrons and backscattered electrons are available to form characteristic images of solid surfaces. The accelerating voltage of the UHV-SEM can be lowered from 30 keV to 100 eV to obtain high surface sensitivity. The electron optical system, the control unit and display circuits are modified versions of the Hitachi S-800 microscope. The UHV-SEM with the FEG was first developed by Crewe and Wall [14]. The optical column of the present microscope is composed of the usual electrostatic lens of Butler type [15] and two magnetic lenses. The electron
T. Ichinokawa
et al. / LESEM
combined with LEED
399
Spoci men Manlpulato~
Fig. 1. Schematic diagram of the UHV-FEG-SEM.
beam is decelerated by the Butler lens when the accelerating voltage is lower than the extraction voltage. The whole optical system is shielded by p-metal from stray electromagnetic fields. The working distance is variable from 3 mm (high resolution specimen position) to 35 mm (analytical specimen position). The specimen is mounted on a manipulator with X, Y and Z translations, two axial rotations and a slight inclination around the other axis. The sample is heated by direct electric current. The other chamber is used for specimen preparation and macroscopic surface analyses and is equipped with a similar specimen manipulator, a specimen heating stage, cleavage equipment, an ion gun, an evaporation cell, AES-LEED optics with an electron gun and a quadrupole mass spectrometer. Both chambers are bakeable up to 200°C and connected by a gate valve. The sample is put into a pre-vacuum chamber for sample exchange by an air-lock system and transferred to the sample stages of both manipulators by a transfer system with magnetic coupling. 2.2. Detection system for scanning LEED microscopy The concept of the scanning LEED microscope comprises two steps: first the formation of a selected-area LEED pattern, and secondary, the generation of raster images with the information contained in the diffraction pattern.
T Lchinokuwu
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comhltwd
w:rth L.EED
I_____ f Sample OScill0SC0pe
Fig. 2. The LEED
detector
assembly with two-dimensional
rotatable
from O” to 120°
read-out.
The detector
assembly I\
around the sample.
At a resolution of several hundred A and beam energies of a few hundred eV. the primary beam current is of the order of 10 “’ to 10 ” A. Under these conditions a LEED pattern cannot be observed on a fluorescent screen. Instead, single electron counting techniques have to be used [16]. In the present experiment the LEED detector assembly shown in fig. 2 has been used, which consists of two hemispherical grids, a two-stage channel-plate amplifier, and a position-sensitive detector (Model 2396. Surface Science Lab., 40 x 40 mm’ in active area). The position-sensitive detector (PSD) translates the X--L’ coordinates of each electron diffraction event into two voltages. When these are given to an X-F storage scope. a bright spot is generated for each event, and a LEED diffraction pattern is seen to accumulate on the scope screen. In this way a selected area LEED pattern may be observed from a region of a few 100 A diameter on the surface, with a probe current of lo- ” A or less. The detector assembly is rotatable from 0” to 120” around the sample with a radius of 60 mm. Thus, the angle of incidence of the primary beam may be chosen between grazing and about 30” with respect to the surface normal. The convergence angle of the incident beam is limited by an objective aperture to 1 mrad or less. The spatial resolution of the positionsensitive detector is nominally 200 lines/40 mm, corresponding to an angular resolution of about 3 mrad. The “transfer width” (or inverse momentum resolution) of the system is thus determined by the detector resolution. It is estimated to be comparable to or larger than the linear dimension of the spot illuminated by the primary beam. Placing the primary beam spot at a different position on the sample the same or a different diffraction pattern may be observed, depending on whether the surface structure is the same or not. In this way. for example, the surface orientation of different grains in a polycrystalline sample may be observed. Likewise, one may select a particular beam out of the diffraction
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combined with LEED
PSD - -
‘Y
Position Computing
Window Comparator
AND Gate
Digital Switch
Oscilloscope
Ele_ctronics Fig. 3. Block diagram
of two-dimensional diffraction
analogue comparators spot on the oscilloscope.
to select a window
around
a
pattern and modulate the brightness of the SEM oscilloscope by the intensity of this particular beam. This mode corresponds to the dark field imaging mode in conventional transmission microscopy. Instead of moving mechanically the objective aperture, like in CTEM, an electronic window is set around a diffraction spot using the two-dimensional analogue comparators shown in fig. 3. In this way a dark-field image of the surface structure is formed. The total count rate for the whole area of the PSD is limited to about lo5 counts/s due to electronic dead times. Therefore the maximum count rate inside a selected window is typically less than lo3 counts/s. The scan time for one frame ranges from 10 min to 1 h, depending on the desired signal-to-noise ratio. The data are stored in a micro-computer and an image is formed by 256 x 256 pixels with 8 bit resolution each. If desired, digital background subtraction, contrast enhancement or smoothing may be carried out.
2.3. Detection system for scanning Auger microscopy A single-pass CMA is used for scanning Auger microscopy and electron energy loss microscopy. It is retractable and adjustable in its position from outside the vacuum. The output signal of the CMA is measured in the N’(E) mode by a lock-in amplifier and in the N(E) mode by a pulse counting system. The data processing for Auger electron spectra and scanning Auger images is performed by the micro-computer.
3. Performance of the UHV-FEG-SEM The spatial resolution of electron microscopes depends on the acceleraung voltage. The reason why the lower the accelerating voltage, the lower the spatial resolution is as follows. With decreasing accelerating voltage V,, the
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chromatic aberration (= C,aAP’/v,), the diffraction aberration ( = 0.61 X/a) and the coefficients of spherical and chromatic aberrations of the objective lens (C, and C,) increase. and the brightness of the electron source decreases (n is the semi-divergent angle of the objective aperture, eAT/ is the energy spread, and h the wavelength of the incident electrons). Among these, the effect of AV/V, is most important in the low energy region. A spread of the spot size due to the chromatic aberration is thus inversely proportional to the accelerating voltage. For instance, a probe diameter of 100 A at L’, = 1 kV becomes 1000 A at V, = 100 V.
Fig. 4. Micrographs taken at: (a) E, = 30 keV (magnetic tape), (b) 0.8 keV (latex particles of 1000 and 2000 A diameter), (c) 0.5 keV (particle diameter 2000 A). (d) 0.25 keV (1 pm diameter).
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FE-SEM
1600 - 30000 1400 -1200
-
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.Q
/
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-
,
I
/
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800-
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z -10000
?
LOO200. I 250
0
h 500
ACCELERATION Fig. 5. Spatial
resolution
and optimum
I 750 VOLTAGE
1000 ( V )
magnification versus accelerating UHV-FEG-SEM.
voltage
for the present
The experimental spatial resolution was measured as function of the accelerating voltage by testing samples of magnetic tape and latex particles evaporated by gold. Test images are shown in fig. 4 and the resolution as a function of the accelerating voltage is shown in fig. 5. The experimental resolution of 600 A has been attained at V, = 250 V and of 20 A at VO= 30 kV with a probe current of 2 X lo-” A. The limitations to high resolution were vibrations of the FE tip and of the sample stage. The fluctuation of the emission current is several percent at a total current of 100 PA at a tip temperature of 20°C. The magnitude of the signals for images or spectra is normalized by current picked up by an aperture placed on the electron optical axis. The present tip has been in operation since more than three years. The extraction voltage was initially 2 kV, which increased to 5 kV at present. The rate of increase was large during the first year and is lower since. 4. Experimental results 4.1. UH V-SEA4 observations on Si(l II) surfaces 4.1.1.
Step structure The step structure
of Si(ll1)
formed
by high temperature
annealing
up to
404
Fig. 6. Step structures on the Si(ltl)
surface anaealed
at 1200°C
in UHV.
images: (a) the clean surface and (h) the surface exposed to air of lo-’
Torr
Secondary
electron
for several tens of
minutes at room temperature.
1250°C was reported previously [I‘?]. A number of step bands parallel to [
Fig. 7. SEM images of an island on the Au-evaporated surface of Si(ll1): (b) at 400°C.
(a) at room temperature,
by reflection electron microscopy [1,7] and by transmission electron microscopy using the replication technique [18]. Further studies of the step structure required the dark field imaging capability of the scanning LEED microscope (see below). 4.1.2. Islands on the Au-evaporated Si(lll) surface An island on the Au-evaporated surface of Si(ll1) is observed in the secondary electron image as shown in fig. 7. The island is heterogeneous at room temperature as reported by Oura and Hanawa [19], but becomes homogeneous at a temperature higher than 370°C. The fine texture within the island changes at each melting-cooling cycle. The island moves on the surface at temperatures higher than 400°C as shown in fig. 8. The speed is 0.5-5 ~m/rnin at 550°C depending on surface contamination and surface defects. A large island absorbs small islands during its course and leaves a well-ordered surface of fi X fi structure on its trace. A motive force of the movement is due to the temperature difference at the surface. The islands move towards a direction from lower to higher temperature. Steps may disappear inside the trace as seen from fig. 8c or may be displaced. 4.2. Scanning LEED microscope applications on Si surfaces 4.2.1. Step structure Steps of one or several monolayers height on the terraces of a stepped Si(ll1) surface have not been observed by the secondary electron image. As a first application of scanning LEED microscopy, it was tested whether the fine steps visible in the surface exposed to air, as shown in fig. 6b, are observable or not. A selected area LEED pattern on the terrace in fig. 9b shows th (7 X 7) structure at E, = 250 eV as shown in fig. 9a. Fig. 9b is a secondary
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Fig. 8. The movement of an island on the Au-evaporated surface of Si(l11). Small islands coalesce with the large island and the trace left behind by the movement of the large island shows the fi X fi structure. The step is displaced inside the trace.
electron image. The dark field image was observed by the specular beam spot. indicated by a square mark in fig. 9a. To enhance the contrast, the background is subtracted by image processing. Fig. 9c shows a dark field image at 250 V after image processing. The steps are observable on the terraces and a crystal defect region due to the inclusion shows the dark contrast around it. The image was taken in 6 min. Each pixel contained between 10 and 40 counts, of which 10 were subtracted as a background. These results show the inherently strong contrast in the dark field image (at least a factor of two), as compared to the secondary electron contrast of a few tenths of a percent. The grainy
T. Ichinokawa et al. / LESEM combined with LEED
407
Fig. 9. Scanning LEED microscopy for the step structure on the Si(ll1) surface: (a) a selected area LEED pattern on the terrace (the 7 X 7 structure), (b) a secondary electron image and (c) a dark field image taken by the specular reflection spot in (a). The fine steps are observable on the terraces and a crystal defect region due to the inclusion shows dark contrast around it.
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Fig. 10. Scanning LEED microscopy for grains in the polycrystalline Si surface: (a) secondary electron image, (b) selected area LEED pattern of the grain and (c) dark field image taken by the specular spot in (b).
structure of the image is caused this in section 5.
by the counting
statistics.
We comment
on
The polycrystalline Si consists of a number of small grains of less than several pm in size. By annealing up to 700°C the selected area LEED patterns of the 1 x I structure were observed for each grain of different orientation. Fig. 10a shows a secondary electron image and fig. 1Ob a selected area LEED pattern of one grain. Fig. 10~ shows a dark field image taken by the specular spot in fig. lob. The crystal structure and orientation of a grain can be determined with a spatial resolution of - 500 A.
Fig. 11. Scanning LEED microscopy for islands on the Au-evaporated surface of Si(ll1): (a) secondary electron image, (b) and (c) selected area LEED patterns of the substrate (~6 XV? structure) and the island, respectively, and (d) dark field image by means of the specular spot.
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4.2.3. Island structure on the Au-evaporated surf&e of Si(ll1) After Au-evaporation to a thickness of several tens of monolayers and subsequent heating up to 700°C for several tens of minutes, islands of various size were observed in the secondary image as shown in fig. lla. The substrate shows the fi x 6 structure as shown in fig. 11 b by the selected area LEED pattern, while that of the island is diffuse as shown in fig. llc. Heat treatment above 450°C leads to agglomeration of the Au-Si film, producing of the 6 x fi superstructure at the substrate [19] and islands in polycrystalline state. Fig. lid is a dark field image taken by means of the specular beam indicated in fig. llb. The chemical composition of an island shown in fig. 12 is determined by micro-Auger spectroscopy. The Auger electron spectra measured at three points, a, b and c, are shown in fig. 12. According to the Auger micro-analysis, it is found that the island is a heterogeneous mixture of two phases; one phase
a
b
Fig. 12. Micro-Auger electron spectra at three points. a. b and c, on the secondary The island is a heterogeneous mixture of gold and silicon with a eutectic AU 069Sio ?I’
electron image. composition of
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411
is nearly pure gold and the other nearly pure silicon. The average composition measured with a large diameter of the primary beam is approximately 2 : 1 for Au : Si in atomic concentration. Since islands melt at 370°C as described above, we can find that islands have a eutectic composition of Au0,69Si0,3, as reported by the phase diagram of the Au-Si binary system [20]. The surface inside the trace behind the island movement (at point b) shows a fi x fi LEED pattern, indicating a coverage of a monolayer Au. The fi x 6 structure transforms into the 1 X 1 structure at 750°C consistent with the transition temperature reported by Le Lay et al. [21]. 4.2.4. Coexistence of different superstructures on a Au-evaporated Si(ll I) surface When the clean Si(111)7 X 7 surface is covered by a few monolayers of Au
Fig. 13. Scanning LEED microscopy for the Au-evaporated surface of Si(ll1): (a) secondary electron image, (b) dark field image (the bright area at the central part corresponds to the 5 x 1 structure while the dark area corresponds to the fi x fi structure), (c) and (d) are selected area LEED patterns of the 5 x 1 and fi X fi structures, respectively.
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by evaporation and afterwards heated up to 900°C for several tens of minutes. a coexistence of the 5 x 1 and fi x fi structures was observed. The coverage of Au atoms in the 5 x 1 structure is 0.25 monolayer and that in the fi x fi is one monolayer. After Au atoms have evaporated at a temperature above 900°C the surface becomes clean with the 7 X 7 reconstructed structure. In the dark field image of fig. 13b, the bright area in the central part is the 5 X 1 structure and the dark area is the fi X fi structure as illustrated by the selected area LEED patterns in fig. 13~ and 13d, taken in the bright and dark areas, respectively. The coexistence of similar superstructures on the Au-evaporated Si(ll1) surface has been observed by Telieps and Bauer [ll] and Osakabe et al. [l]. The contrast of the superstructures in fig. 13b is the same as those in their images.
5. Discussions
The combination of high energy SEM with RHEED was performed by Ichikawa et al. [7] in the ultra-high vacuum system. The disadvantage of this technique is similar to that of high energy reflection electron microscopy for small glancing angles of incidence. We have developed a combined technique of low energy scanning electron microscopy with LEED for the observation of surfaces at high glancing angle incidence and with a high surface sensitivity. The choice of the primary electron voltage should be a trade-off between achievement of good LEED patterns and limitation of spatial resolution of the raster image. To obtain good LEED patterns with a glancing angle of incidence higher than 30”, the voltage is preferable less than 300 V. The mean free path of electrons in a solid is minimum in an energy range from 60 to 100 eV, which is the most suitable operation region in LEED. The spatial resolution in this energy range is approximately 1500 A as seen from fig. 5. In the range higher than 3 keV, it encounters difficulties such as a small glancing angle of incidence and a poor angular resolution of the selected area LEED patterns caused by the limitation of the spatial resolution of PSD. The compromise between both requirements renders a choice of 200 to 1 keV as the optimum energy region under the conditions of the present experimental arrangement. Then, the highest spatial Oresolution of the present scanning LEED microscope is approximately 500 A. The contrast of the dark field image depends on the type of diffraction beam used for the raster image. While the dark field images shown in the present paper were taken with the specular beam of the fundamental unit mesh, images taken with fractional order beams showed higher contrast. but substantially lower signal-to-noise ratio. The signal-to-noise ratio in the dark field images is so far not limited by the primary beam current, but by the
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count rate capability of the position-sensitive detector. An improvement by one order of magnitude may be possible by using faster electronics. Further improvements may be possible by placing a separated channeltron multiplier near the channel-plate or into a center hole of the PSD and by using its output for imaging. It should be noted, however, that a very large increase of the primary beam current, though perhaps feasible, will not be desirable in applications to sensitive systems where electron stimulated desorption may impose severe limitations. A number of applications of the UHV-SEM and the scanning LEED microscope are demonstrated for Si surfaces. Some examples on the microscopic structures in the surface are interesting and have to be discussed in more detail. T,he examples, however, may necessitate further studies to arrive at conclusions. Therefore, we do not advance further discussions on the applications and will publish them elsewhere.
6. Conclusion The design and performance of the UHV-SEM with the FEG are described and preliminary applications to Si surfaces are demonstrated in the SEM mode, the LEED mode and the Auger mode. It is suggested that the technique of scanning LEED microscopy will be available in the following areas of solid surfaces: domains of reconstructed structures (or superstructures), phase transitions on surfaces, initial stages of epitaxial growth, relation between surface structure and surface diffusion, and interactions between surface crystal defects and adsorbates. In situ observations of those processes with changing surface temperature and coverage of adsorbate are interesting to study.
Acknowledgements The present work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education (Japan). One of the authors (JK) appreciates the support by the programme for foreign research fellowship from Waseda University.
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T. Ichinokawa and Y. Ishikawa, Ultramicroscopy 15 (1984) 193. M. Ichikawa, T. Doi, M. Ichihashi and K. Hayakawa, Japan. J. Appl. Phys. 23 (1984) 913. H. Bethge, Th. Krajewski and 0. Lichtenberger, Ultramicroscopy 17 (1985) 21. I.R. Plummer, H.O. Porter. P.W. Turner. A.J. Dixon. K. Gehring and M. Keenlyside. Nature 303 (1983) 599. J. Cazaux. Ultramicroscopy 17 (1985) 43. W. Telieps and E. Bauer, Ultramicroscopy 17 (1985) 57. A.A. Delong and V. Kolarik. Ultramicroscopy 17 (1985) 67. C. Guittard, J. Phys. DlO (1977) 2331. A.V. Crewe and J. Wall. Rev. Sci. Instr. 39 (1968) 576. J.W. Butler, in: Proc. 6th Intern. Congr. on Electron Microscopy. Kyoto. 1969. Vol. 1. p. 191. P.C. Stair, Rev. Sci. Instr. 51 (1980) 132. Y. Ishikawa. N. Ikeda, M. Kemmochi and T. Ichinokawa, Surface Sci. 159 (1985) 256. H.C. Abbink. R.M. Broundy and G.P. McCarthy, J. Appl. Phys. 39 (1968) 4673. K. Oura and T. Hanawa. Surface Sci. 82 (1979) 202. Selected Values of the Thermodynamic Properties of Binary Alloys. prepared by Ralph Hultgren et al. (American Society for Metals, Metals Park, OH. 1973) p. 317. G. Le Lay. M. Manneville and R. Kern. Surface Sci. 65 (1977) 261.