Reflection electron microscope study of the initial stages of oxidation of Si(111)-7 × 7 surfaces

Ultramicroscopy 18 (1985) 453-462 North-Holland, Amsterdam

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REFLECTION ELECTRON M I C R O S C O P E STUDY OF THE INITIAL STAGES OF OXIDATION OF S i ( l l l ) - 7 x 7 S U R F A C E S N. S H I M I Z U , Y. T A N I S H I R O , K. K O B A Y A S H I , K. T A K A Y A N A G I and K. Y A G I Physics Department, Tokyo Institute of Technology, Oh-Okayama, Meguro, Tokyo 152, Japan Received 10 July 1985; presented at SymposiumJanuary 1985

A gas inlet device was constructed for an ultra-high-vacuum electron microscope and initial stages of oxidation of Si(lll)-7 x 7 surfaces at various temperatures were observed in-situ by reflection electron microscopy(REM). Below500°C the REM images did not show appreciable changes after an introduction of molecular oxygen. Above 500°C hollows with unit depth 0.31 nm were formed and grew in the central part of the terraces between the successive atomic surface steps. At the same time the steps moved so as to shrink the terraces. As oxygenpressure increased, oxide covered the surface and the surface structure transformed to the 1 × 1 structure. The oxide sublimed on heating above 750°C. During the oxygenexposure above 750°C rapid motions of steps and large hollow formations were observed throughout the oxygenexposure process. The growth kinetics of the hollows was analyzed. High-resolution REM observations showing the 7 x 7 lattices during oxidation were also carried out.

1. Introduction The oxidation of Si is one of the most important processes in the microelectronic device industry, and a number of experiments have been carried out in order to understand the details of the process, the formation of defects and the interface structure between the oxide and Si crystal, which is important for transport properties. To know the initial stages of the oxidation process ultra-high-vacuum (UHV) experiments on well defined surfaces are necessary, and many reports have been given following an initial work by Schlier and Farnsworth [1]. At room temperature the sticking coefficient of oxygen has been reported to be small, 1 0 - 4 - 1 0 -1 , and it depends on the surface step density [2]. Later an effect of adsorption of residual gases on the sticking coefficient was reported [3]. Stimulated adsorption and oxidation due to the hot filament of an ionization gauge [2] or irradiating electrons [4] have also been reported. The 7 × 7 reconstructed structure of the clean (111) surface transforms to the 1 x I structure during oxygen exposure. The low-energy electron diffraction

(LEED) pattern shows a strong background, which indicates disordered adsorption [1,5]. There are controversial reports concerning the adsorbed species at the initial stage, especially as to whether molecular oxygen is or is not dissociated (see discussions in a paper by Garner et al. [6] and references therein and in ref. [7]). At elevated temperatures, the sticking coefficient was reported to decrease [8], but a temperature-independent behaviour has also been reported [5]. The chemical reactions, ½ 02 (g) ~ O(ad), Si + O(ad) ~ SiO(g), Si + 2 0 --, SiO2, have been suggested [5]. A high density of steps at the interface between the oxide and Si crystal was reported by Hahn and Henzler [9], although in this case a thick oxide layer was formed at high temperature and atmospheric pressure. Various oxygen chemisorption configurations were suggested by S u e t al. [10]. Previous studies were mainly done by LEED, Auger electron spectroscopy, X-ray photoemission

0304-3991/85/$03.30 © Elsevier SCience Publishers B.V. (North-Holland Physics Publishing Division)

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N. Shimizu et al. / R E M study of initial stages of oxidation of Si(111) - 7 × 7

spectroscopy, work function measurement and high-resolution electron energy-loss spectroscopy, but no microtopographical studies have been done except for high-resolution electron microscopy of the interface between the oxide and Si crystal [11]. In the present study a gas inlet device for an UHV electron microscope was constructed and changes of surface structures on Si(lll)-7 × 7 surfaces by oxygen exposure were observed in-situ by reflection electron microscopy (REM). So far, REM, which can characterize the surface microtopography very sensitively, has been used to observe surface dynamic processes such as phase transitions [12], metal adsorption processes on Si(lll)-7 × 7 surfaces [13,14], but not reaction processes of surfaces with reactive gases.

2. Experimental 2.1. Gas inlet device

Fig. 1 shows a horizontal cross-section of our UHV electron microscope column at a specimen level. A Si (111) crystal (1) in a heating holder (A) is surrounded by a liquid-He-cooled cryotip (2) and liquid-N2-cooled shrouds (3) and (4). All of them have holes so that the specimen (1) can be seen from the two evaporation filaments (5) or two stainless steel tubes (8) in an evaporation chamber (B) from which reactive gases can be leaked onto the specimen. The filaments (5) are also used to produce atomic hydrogen when molecular hydrogen is introduced into the evaporation chamber. A quartz oscillator (7) is used to monitor evaporation rates from the filaments. Fig. 2 schematically illustrates the piping of the gas inlet device. A turbo-molecular pump (1) (50

8

/

Fig. 1. A cr0ss-section of a UHV electron microscope column at the specimen position. A REM holder (A) and an evaporator chamber (B).with filaments (5) and gas inlet tubes (8).

N. Shimizu et al. / R E M study of initial stages of oxidation of Si( l l l )- 7 x 7

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3. Results I

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3.1. Temperature dependence of oxidation I I

I 5

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Fig. 2. A piping for gas inlet device.

l / s ) is used to evacuate a volume between a variable leak valve (7) and valves (5) to high purity sources (2) and (3) (4N oxygen and 5N hydrogen). The pipes can be baked so as to reduce the effect of residual gases in the pipes during the oxygen exposure. Several cycles of an evacuation through a valve (6) and gas inlet through the valves (5) are carried out to reduce impurity gases in the pipes, before an actual operation of the gas inlet through the variable leak valve (7) to the specimen. One important limitation of the present system is that the actual pressure of inlet gases around the specimen cannot be measured because of the narrow space. Therefore the exposure is estimated by monitoring a total pressure rise measured by a Bayart-Alpert gauge and a partial pressure rise of oxygen (in the present case) measured by a quadrupole mass spectrometer in the top-entry specimen chamber. Due to a low conductance of the stainless steel tube (8) in fig. 1, the pressure increases slowly after the start of the gas inlet.

2.2. Specimen preparation Specimen Si crystals, 1 mm wide, 7 mm long and 0.4 mm thick cleaned by an ordinary chemical treatment are clamped in a double tilt REM holder and are cleaned by heating up to 1200°C by DC current fed through the crystals [14]. By the present treatment the surfaces of the crystals show reflection electron diffraction (RED) patterns from the 7 X 7 structure of the clean Si(111) surface. Flat regions with a small step density axe selected for the observations.

Fig. 3 shows REM images before and after an 02 exposure at room temperature. The exposure time is about 5 min with total pressure rise of about ( 2 - 1 0 ) x 10 -s Torr. All of the following observations were carried out with a pressure rise in the top entry specimen chamber of around (2-10) x 10 -s Torr. The REM image of (b) shows no appreciable change from (a), details of step shapes being similar. With a similar 02 exposure at elevated temperatures we always observed surface structure changes, which.indicates that the specimen was actually exposed to 02 gases. The RED patterns did not change so much; the 7 x 7 patterns persisted. However, an area illuminated by the electron beam transformed from the 7 x 7 structure to the 1 x 1 structure *. This did not happen without 02 exposure, which means that an electron-beam-assisted reaction takes place. A similar effect of irradiating electrons was also noted by Ibach and Rowe [4], where the energy and density of irradiating electrons were much smaller than those in the present experiment. The 7 x 7 spots in the RED pattern from unilluminated areas did not disappear, even after 20-30 min of 02 exposure. When the substrate crystal was at about 400°C, the 7 x 7 RED pattern weakened to some extent (thermally-assisted reactions take place)and electron beam irradiation enhanced this transformation. When the substrate temperature was above 500°C the illuminating electrons did not affect the surface processes, and similar REM images and RED patterns were observed from illuminated and unilluminated areas. At the initial stage of the oxygen exposure, hollows with unit step depth 0.31 nm were formed in the middle of terraces between the adjacent steps, and they grew. Fig. 4 shows REM images before and during 02 exposure at 650°C. The arrows indicate the hollows, * The I x 1 structure has not been studied. It simply means that the 7 x 7 reflection spots disappoar and the 1 x 1 fundamental reflection spots remain in the diffraction pattern.

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N. Shimizu et al. / REM study of initial stages of oxidation of S i ( l l l ) - 7 x 7

Fig. 3. REM images of Si.(111)-7 × 7 surfaces before and after 02 exposure at room temperature.

concluded from a contrast analysis of them (the image contrast analysis for the determination of the sense of steps was given in refs. [13,15]). The unit depth was concluded from the interaction of the growing hollows with preexisting steps. Due to the severe foreshortening, the shape of the hollows is not clear, but from images of large hollows they are considered to be close to circular or have polygonal shapes. The hollows were not formed around the steps (see figs. 4 and 7). The R E D patterns showed the 7 × 7 structure of the surface at this stage while the hollows were growing. New hollows were nucleated between the growing hollows. As the substrate temperature increases the hollow density decreases and larger hollows are formed. It was also noted that the preexisting steps move so as to shrink the terraces. The motion is hard to detect in cases of low-temperature exposure but is apparent at higher temperatures. These facts suggest the following surface processes: Oxygen gas reacts with Si atoms on the terraces

and the reaction product, probably SiO as suggested by Lander and Morrison [5], sublimes from the surface. Then, vacancies are formed on the terraces. The vacancies migrate over the terraces and coalesce into hollows near the center of the terraces. The temperature dependence of the hollow density is due to that of the surface diffusion of vacancies and to that of the critical vacancy density for the nucleation of the hollows. The vacancies formed around the surface steps probably migrate to the steps and are annihi'lated, which reduces the probability of hollow formation around the steps and also causes the step motions. At later stages of the oxygen exposure at which the oxygen pressure had increased, images of small speckles appeared over the terraces; at the same time hollows stopped growing and the RED showed a transformation from the 7 × 7 to 1 × 1 patterns. Fig. 5 shows an example of a REM image from such surfaces and a corresponding R E D pattern when the substrate temperature was

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N. Shimizu et al. / R E M study of initial stages of oxidation of Si(111) - 7 x 7

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Fig. 4. REM images (a) and (b) before and during 02 exposure at 650°C and RED pattern (c). The preexisting surface atomic steps are seen in (a). Arrows in (b) indicate hollows.

Fig. 5. A REM image and corresponding RED pattern at later stage of oxidation at 675°C.

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N. Shimizu et al. / R E M study of initial stages of oxidation of Si( l 11) - 7 × 7

Fig. 6. Complicated surface step configuration after 02 exposure above 750°C.

at 675°C. The overgrowth should be an oxide film but the present technique cannot tell the composition. The non-uniform image is due either to inhomogeneity in the oxide or the Si-oxide interface or perhaps to a heterogeneous oxidation. When the oxide-covered silicon crystals were heated to 730-750°C, oxide layers disappeared (by sublimation) and hollows reappeared. In some cases, monolayer-high islands were observed on the terraces. In any case the step density increased on the surfaces of the Si crystals when clean and flat surfaces were exposed to oxygen at higher temperature (above 500°C). When the substrate temperature was above 750°C speckles were not f o r m e d and the 7 × 7 R E D pattern persisted throughout the oxygen exposure. In this case large hollows grew rapidly and touched the neighboring steps, which also moved rapidly; very complicated step configurations were formed as shown in fig. 6. This kind of interaction implies that the hollows were of the unit depth. In some cases we observed the formation of a second hollow near the center of a large hollow. It should be noted here that when the surfaces were not exposed to oxygen, steps moved appreciably only above 850-880°C by sublimation [13].

3.2. Growth kinetics of hollows Fig. 7 shows a sequence of R E M images during oxygen exposure at about 650°C. Between (a) and (b) hollows were formed, which grew ((c) and (d)). Between (b) and (c) new hollows indicated by

arrows were formed which also grew in (d). From such a sequence of micrographs we measured the radius (length in the REM images) of each hollow as a function of the exposure q after nucleation. In the present system, however, the oxygen pressure slightly increased after the start of gas inlet, so that time t after nucleation is not a good measure of the exposure. So we used an integral of Pdt (P is the pressure at t) as a measure of q. Fig. 8 shows a preliminary result, as an example, of logarithmic plots of radius versus exposure q for hollows formed when the substrate temperature was at about 650°C. The plots show a linear change and a slight decrease in the slope at later stages. The slope is about 0.4-0.54. Two mechanisms can be considered for the growth of the hollows. One is to assume that oxygen mainly attacks the periphery (steps) of the hollows following nucleation. In this case we would expect d r / d q to be constant and the slope of the logarithmic plots to be 1.0. On the other hand, if we assume that the oxygen attack at the steps around the hollows is not a dominant mechanism for the growth of them, the growth is controlled by the surface diffusion of the vacancies on the terraces. Then, the problem is similar to that met in nucleation and growth of two-dimensional islands in vacuum deposition (see ref. [16] and references therein). For the detailed analysis of the growth kinetics of the hollows, the diffusion equation around the hollows should be solved around them. However, for the first approximation, we can assume that the vacancies formed in a definite zone

N. Shimizu et al. / R E M study of initial stages of oxidation of Si( l 11) - 7 x 7

459

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Fig. 8. Growth of hollows by 02 exposure at 650°C plotted from a sequence of micrographs similar to that in fig. 7.

area of the zone may be determined by an arrangement of neighbouring hollows when new hollows do not nucleate around the hollow under consideration. The result of fig. 8 seems to support the second mechanism. A decrease of the slope at the later stage is also expected in the second mechanism. The present conclusion does not exclude the preferential reaction of oxygen at the steps. It simply says that the second mechanism predominates over the first one as a whole for the growth of hollows.

3.3. Observations of 7 × 7 lattice fringes during the oxidation

Fig. 7. A sequence of REM images which show hollow growth during 02 exposure at 650°C.

around each hollow diffuse to the hollow and contribute to the growth of it (the complete condensation condition in the case of vacuum deposition [16]). In this case the slope should be 0.5 since we can expect that d(rrr2)/dq is constant, equal to formation rate of the vacancies in the zone. The

In section 3.1 it was mentioned that during the initial stages of oxidation above 500°C hollows were formed, while the 7 × 7 R E D pattern persisted. However, it is not certain whether there are patches of the 1 × 1 structure regions on the surface. To determine this REM images including more than two 7 × 7 spots were taken. Fig. 9 shows high-magnification images of S i ( l l l ) surface during oxidation at about 600°C. The images were taken by including two 7 × 7 spots near to the specular reflection in the aperture. Fringes spaced

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N. Shimizu et al. / R E M study of initial stages of oxidation of Si( l 1 I) - 7 x 7

Fig. 9. High-resolution REM images during 0 2 exposure. 2.3 n m spaced lattice fringes of the Si(lll)-7 x 7 structure are seen all over the surface during 0 2 exposure except places indicated by arrowheads in (b).

about 2.3 nm, which correspond to the lattice fringes of the 7 x 7 structure, are shown in (a) and (b). In (a) clark horizontal lines are hollows, and they have grown in (b). Dark lines indicated by arrows in (b) are new hollows formed during the exposure between (a) and (b). Note that the 7 × 7 lattice fringes are seen not only in flat parts of the surfaces, but also in areas close to the hollows. This means that the density of vacancies on the surface in the present experimental condition is not high enough to destroy the 7 × 7 structure. In some areas, indicated by arrowheads in (b), fringe contrast is weak, which may correspond to an initial stage of the oxide formation.

4. Summary and discussion In the present study the initial stage of the oxidation of Si(lll)-7 x 7 surfaces was studied by in-situ REM. At room temperature no definite contrast changes were noticed. The 7 x 7 spots in RED pattern changed during electron irradiation. This fact and the observation that the hollows were not

formed when the room-temperature-exposed surfaces were reexposed at above 550°C indicate that oxygen molecules or atoms are present on the room-temperature-exposed surface and they poison vacancy formation and migration in the reexposure experiments. The oxygen is probably in a disordered state. It would be interesting to know if there are any changes of the 7 × 7 structure at the initial stage of oxidation at room temperature. For this further detailed observations of changes of relative intensity of the 7 × 7 reflections during oxygen exposure are required. At elevated temperature hollows were formed. Growth kinetics were studied to explore the mechanisms. Preliminary results indicate vacancy formation on the terraces and their diffusion to the hollows. This kind of information is hard to obtain from other surface techniques. It was noticed that the hollow density is temperature dependent. Pressure dependence is also expected as in the case of nucleation phenomenon in vapor depositions [16]. From such kinds Of experiments, it may be possible to deduce diffusion distances of vacancies and an activation energy for the diffusion. Oxide is found only below about 730-750°C. Lander and Morrison [5] reported an O2 pressure dependence at the highest temperature of the oxide formation; and 730-750°C of the present experiment corresponds to an 02 pressure of 10-8-10 -7 T o m This seems to suggest that the local pressure of oxygen around the specimen in the present experiment is 10-8-10 -7 Torr level as measured in the top-entry specimen chamber. Lattice fringes of the 7 × 7 structure were taken in the REM mode to explore the local structure changes of the oxygen-exposed surfaces at a resolution close to the fringe spacing and the method was found to be useful for characterizing the surfaces. Details of the image formation of lattice fringes in REM and its applications will be given elsewhere [17].

References [1] R.E. Schlier and H.E. Farnsworth, J. Chem. Phys. 30 (1959) 917.

N. Shimizu et al. / R E M study of initial stages of oxidation of S i(111) - 7 x 7

[2] H. Ibach, K. Horn, R. Dorn and H. Ll~th, Surface Sci. 38 (1973) 435. [3] N. Kaupke and M. Henzler, Surface Sci. 92 (1980) 407. [4] H. Ibach and J.E. Rowe, Phys. Rev. B10 (1974) 710. [5] J.J. Lander and J. Morrison, J. Appl. Phys. 33 (1962) 2089. [6] C.M. Garner, I. Lindan, C.Y. Su, P. Pianetta and W.E. Spicer, Phys. Rev. B15 (1979) 3944. [7] O.L. Kellog, Appl. Surface Sci. 11/12 (1982) 186. [8] G. Rovida, E. Zanazzi and E. Ferroni, Surface Sci. 14 (1969) 93. [9] P.O. Hahn and M. Henzler, J. Appl. Phys. 52 (1981) 4122. [10] C.Y. Su, P.R. Skeath, I. Lindau and W.E. Spicer, J. Vacuum Sci. Tech. 19 (1981) 481. [11] O.L. Krivanek, T.T. Sheng and D.C. Tsui, Appl. Phys. Letters 32 (1978) 437.

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[12] N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 109 (1981) 353. [13] N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 97 (1980) 393. [14] Y. Tanishiro, K. Takayanagi, K. Kobayashi and K. Yagi, Acta Cryst. A37 (1981) C300. [15] N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 102 (1981) 424. [16] J.A. Venables, in: Proc. 9th Intern. Vacuum Congr. and 5th Intern. Conf. on Solid Surfaces, Madrid, 1983, p. 26. [17] Y. Tanishiro, K. Takayanagi and K. Yagi, J. Microscopy, to be published.