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Thermal desorption of oxides on Si(100): a case study for the scanning photoelectron microscope at MAX-LAB
Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 45–52
Thermal desorption of oxides on Si(100): a case study for the scanning photoelectron microscope at MAX-LAB U. Johansson, H. Zhang1, R. Nyholm Department of Synchrotron Radiation Research, Institute of Physics, Lund University, P.O. Box 118, S-221 00 Lund, Sweden Received 18 June 1996; accepted 16 November 1996
Abstract A scanning photoelectron microscope, utilizing a focused beam of monochromatized photons in the energy range from 15 to 150 eV, has been used to study the thermal desorption of oxide layers on Si(100). The instrument can provide highresolution photoelectron spectra from selected parts of the surface as well as images showing the lateral distribution (on a micrometer scale) of elements in different chemical states by monitoring the photoemission intensity of chemically shifted ˚ ) and thick (200–400 A ˚ ) oxides have been studied. The desorption (at 8408C) of the native core levels. Both native (10–15 A oxide proceeds through a phase of irregular (on a micrometer scale) and diminishing areas of dioxide until a clean surface is obtained. For the thick oxide, annealing to 11008C creates circular voids in the oxide layer which grow linearly in diameter with annealing time. The surface in these voids mainly consists of clean silicon but a small amount of remaining SiO 2 is observed. This remaining dioxide most probably consists of small clusters or particles. For both types of oxide, we find, during and after desorption, a surface-shifted component in the Si 2p core level spectra indicating that at least parts of the surface have an ordered structure which most probably is a 2 × 1 reconstruction. q 1997 Elsevier Science B.V. Keywords: Photoelectron microscopy; Spectromicroscopy; Silicon-oxide
1. Introduction A vast number of experimental studies have been carried out in order to achieve a microscopic understanding of the thermally activated desorption of oxide layers on silicon surfaces. Studies have been made on a variety of different surface oxides, both in terms of the oxide thickness, ranging from thin ˚ ) oxide (sub-)monolayer to thick (several hundred A layers, and in terms of different oxidation procedures [1]. By the use of a number of different microscopic and spectroscopic techniques, a common understanding 1 Present address: Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA.
of the general mechanism of the desorption process has emerged; Si reacts with SiO 2 to form the volatile SiO species, thereby removing SiO 2 from laterally constrained microscopic areas of the initially homogeneously oxidized surface [2–9]. However, experiments capable of simultaneously providing information on both the lateral distribution and the chemical state of silicon(sub-)oxides in the surface layer are very scarce; Scanning Auger Microscopy (SAM) has been applied to the desorption of thick oxides [2] and micro-XANES and SAM measurements on the desorption of thin (native) oxide films have been reported [8,9]. In this paper we use the technique of synchrotronradiation-excited scanning photoelectron microscopy
0368-2048/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S0368-2048(97)00004-2
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U. Johansson et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 45–52
Fig. 1. Schematic layout of the scanning photoelectron microscope. G, plane grating (500 lines mm −1 and 1000 lines mm −1); S1, S2, two Kirkpatrick–Baez mounted spherical mirrors; A, monochromator exit aperture; E, ellipsoidal ring-shaped focusing mirror; SA, sample on x–y–z scanning stage; D, hemispherical sector electron energy analyser.
to reveal, on a lateral scale in the micrometer range, the local chemical state of silicon as the desorption of an oxide layer evolves. We use the inherent chemical sensitivity of core level photoelectron spectroscopy in combination with a well focused exciting photon beam to investigate the lateral distribution of silicon in different chemical states on the surface. The technique is applied to both thin, native, as well as thick, ˚ , oxide layers on Si(100) substrates. several hundred A What is new in the present study is that we have obtained spectroscopic information on these surface structures allowing us to address issues such as: Do voids in the oxide layer expose a clean Si surface and/or do other intermediate-oxidation-state silicon species occur on the surface?
2. Experimental The experiments were performed using the scanning photoelectron microscope at beamline 31 at the MAX I storage ring in Lund, Sweden [10,11]. This beamline is comprised of a 75 mm period undulator in combination with a plane grating monochromator and a ring-shaped ellipsoidal focusing mirror. The system, which is shown schematically in Fig. 1, is capable of providing photoelectron spectroscopy data with a lateral resolution in the micrometer range. The combined undulator source and monochromator can be used efficiently in the photon energy range from 15 eV to 150 eV using two exchangeable 500 lines mm −1 and 1000 lines mm −1 gratings. The photon energy resolution is # 0.1 eV below 100 eV, increasing to
about 0.2 eV at 130 eV used in these experiments. The photon flux at the sample is between 10 9 and 10 10 s −1, depending on photon energy. There are two modes of operation of the scanning photoelectron microscope; either a selected spot on the sample surface is positioned into the focused photon beam to obtain a laterally resolved photoelectron spectrum of the surface layer, or the photoelectron analyser is locked to a specific kinetic energy, e.g. corresponding to a chemically shifted core level, and by scanning the sample through the focused photon beam, while monitoring the photoemission signal, the lateral distribution of this specific chemical component is imaged. The ring-shaped ellipsoidal focusing mirror, operating at a grazing angle of incidence of 28, gives a focal spot with a full width half maximum (FWHM) of about 1.5 mm on the sample surface. However, due to imperfections in the mirror shape and surface smoothness, some light is distributed into tails extending to several tens of micrometers around this focal spot (see further details below). An annular aperture (inner diameter 4.8 mm, outer diameter 5.1 mm) immediately in front of the mirror, in combination with a circular aperture about 10 mm down-stream of the mirror, effectively prevent any light not being reflected by the mirror reaching the sample. The sample is, via a bellows, connected to a piezodriven x–y–z scanning stage placed outside the vacuum system. The scanning range perpendicular to the photon beam is 120 × 120 mm 2. A hemispherical sector electron energy analyser, mounted at a 478 angle to the sample surface normal, is used to record laterally resolved photoelectron spectra and also to monitor the photoemission signal when recording images. Typical recording times for the Si 2p core level spectra shown here are about 30 min, with a total instrumental energy resolution of 0.2 eV. Due to the large chemical shifts for the Si 2p core level investigated here, images could be recorded at lower energy resolution, 0.5 eV, without loss of information. A 120 × 120 mm 2 image, using a step size of 2.4 mm, was typically recorded in 6 min. The silicon substrates (14 × 7 mm 2) used for the experiments were cut from (100)-oriented wafers (p-type, B, 0.02–0.03 Q cm).2 The samples were 2
Virginia Semiconductors, Inc.
U. Johansson et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 45–52
heated by direct current flow through the wafers. The temperature was measured with an optical pyrometer which also allowed the uniformity of the temperature over the surface to be observed. Measurements presented here were made on the central part of the sample where the temperature was uniform within about 6 58C. All annealings were made with the sample in the measuring position which ensured that the same 120 × 120 mm 2 surface area could be observed after repeated heat treatments. All measurements were made after cooling the sample down to room temperature (RT). Measurements at annealing temperatures were not possible since the surface of the ellipsoidal focusing mirror had to be protected from heat radiation from the closely placed sample. The base pressure in the measuring chamber was about 1 × 10 −10 Torr. During annealings to 8408C and 11008C, the pressure increased to about 7 × 10 −10 Torr and 3 × 10 −9 Torr, respectively. Two types of oxides were investigated: native ˚ ) and thick (200–400 A ˚ ) oxide oxides (10–15 A layers. The native oxides were simply prepared by rinsing the wafer in ethanol, after which it was heated in situ to 6008C in order to obtain a clean oxide surface. The thick oxides were prepared by wet oxidation in which the surface was exposed to H 2O vapour in the presence of low concentration HCl at a temperature of 9008C. The exposure time needed to obtain a ˚ oxide thickness was extrapolated from the 200–400 A exposure times for various thicker oxide layers for which the thickness could be estimated by visually observing the colour of the oxidized surface [12]. Also for these samples, a heat treatment (6008C) was used to obtain a clean oxide surface after introduction into the measuring chamber.
3. Results and discussion Before entering into a discussion on the results, we would like to make some general comments on the achievable lateral ‘‘resolution’’ in an experiment of this type. What often, and sometimes rather carelessly, is stated is that a spectromicroscope possesses a certain lateral resolution expressed in nano- or micrometers, without addressing further details. This is generally not satisfactory for spectromicroscopes that use a focused photon beam as the source of
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excitation, since all types of focusing optics give rise to a central focal spot surrounded by tails of radiation due to aberrations, figure errors, diffuse scattering and possibly diffraction which may lead to ambiguous or incorrect interpretations of data, if not properly taken into account. When applying spectroscopic techniques to investigate lateral inhomogeneties on surfaces, as in the present case, one may describe the performance of the instrument in several different ways. We would prefer, for reasons that will become clear in later discussions, to describe the performance of the instrument using the terms detectability, lateral resolution and selectivity. By detectability, we mean the ability to obtain spectroscopic information, with a useful energy resolution and signal-to-noise ratio, from a small isolated structure on the surface. By lateral resolution, we mean the ability to resolve closely spaced structures on the surface as separate objects. Finally, we use the term selectivity to describe the ability to record selectively the spectroscopic information from an area of the sample centred in the focal spot of the exciting radiation without having interference from photoemission from surrounding areas. Although these concepts are not completely independent and might seem to be somewhat arbitrarily chosen, they are still useful concepts when interpreting data, whether it concerns images or spectra. It should be realized that they are not only instrument- but also sample-dependent. Thus, quantitative measures have to be established for each different experimental situation. In the present experiments, selectivity is an important issue and a scheme to address this matter, applicable to the type of samples investigated, is discussed below. Turning to the results from the present study and starting with the native oxide, we observe an Si 2p ˚) core level spectrum characteristic of a thin (10–15 A oxide layer. In agreement with a number of previous investigations (see, e.g., Ref. [13]) we find, apart from the Si 2p signal from the substrate and from SiO 2, 2p components from Si in intermediate oxidation states. With our probe, we find no signs of inhomogeneities in this oxide layer prior to heat treatments. After an annealing to 8408C for about 20 min, we observe that parts of the oxide has desorbed. An image after a total annealing time of 80 minutes is shown in Fig. 2 where the intensity of the Si 2p signal from SiO 2 was
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Fig. 2. Result from a partly desorbed native oxide layer on Si(100) after 80 min of annealing to 8408C. The image was recorded by monitoring the Si 2p core level photoemission signal (hn = 128 eV) from SiO 2, i.e. bright areas have a larger content of SiO 2 than the darker areas. Image size is 18 × 18 mm 2 with a pixel size of 0.4 × 0.4 mm 2. The Si 2p spectra, showing different amounts of dioxide, are recorded from different areas on the surface, as indicated in the image. The absolute kinetic energy scale is not calibrated.
monitored. The image shows an irregular pattern of varying SiO 2 photoemission intensity. Fig. 2 also shows Si 2p spectra from three different areas, as indicated in the image. Upon repeated annealings, the desorption of the oxide proceeds through a phase of irregular but diminishing areas of dioxide, leading to situations where only small patches of dioxide remain. Finally, a clean surface is obtained. For the thick oxide, we find, prior to high temperature anneals, a homogeneous oxide surface. The Si 2p spectra only show one component from the chemically shifted Si 2p component of SiO 2. After repeated high temperature annealings to 11008C, small circular voids in the oxide layer appear.3 These voids grow in diameter with annealing time and finally coalesce into larger areas as seen from the series of images shown in Fig. 3 recorded from SiO 2. Tuning the electron analyser to the clean Si2p peak results in images with a reversed contrast. Fig. 4 shows Si 2p spectra from areas corresponding to the cross-marks in Fig. 3d, i.e. from the centre of a void, from the edge of a void, and from the dioxide layer outside the void. From the images and spectra in Figs 2–4, the native oxide and the thick oxide, at first sight, seem to have a different behaviour upon heat treatment. The first 3
Upon closer inspection, one sees that the circular areas are slightly distorted towards a quadratic shape. This has previously been reported [2] to follow the crystal symmetry of the (100) surface.
˚ thick oxide on Si(100) after Fig. 3. Images obtained from a 300 A repeated annealings to 11008C. The images were recorded by monitoring the Si 2p core level photoemission signal (hn = 130 eV) from SiO 2, i.e. bright areas have a larger content of SiO 2 than the darker areas. Image size is 120 × 120 mm 2 with a pixel size of 2.4 × 2.4 mm 2 and the acquisition time for each image was about 6 min. The dark circular areas are voids in the oxide layer exposing clean Si. The sequence of images a–h shows the same area of the surface after repeated annealings with total annealing times of: a) 18 min, b) 24 min, c) 30 min, d) 36 min, e) 42 min, f) 48 min, g) 54 min, h) 60 min.
U. Johansson et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 45–52
Fig. 4. Si 2p core level spectra (hn = 130 eV) from the three different positions marked by the cross marks in Fig. 3(d). The spectra correspond to; 1) in the centre of the void (30 mm diameter), 2) at the edge of the void, 3) about 30 mm outside the edge. The absolute kinetic energy scale is not calibrated.
observation is the irregular pattern (on a micrometer scale) in which the native oxide desorbs, which is in contrast to the regular circular voids that open up in the thick oxide. The second observation is the appearance of Si in intermediate oxidation states in the native oxide samples, while the thick oxide samples only show the presence of clean Si and SiO 2. For the native oxide, we have used an annealing temperature of 8408C in order to obtain a practical desorption rate; e.g. at 7608C, the desorption rate is extremely low and annealing the sample to 11008C completely removes the oxide in a few minutes.
Fig. 5. Spectrum 1 from Fig. 2 with the 2p 1/2 spin orbit components subtracted. A polynomial background fitted to the spectrum in the energy ranges 20.0–21.5 eV and 29.5–30.0 eV is also shown. The spectrum clearly shows the existence of intermediate oxidation states (see text) and a surface-shifted component at the high kinetic energy side of the main 2p 3/2 peak (inset).
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From a surface like that shown in Fig. 2, where parts of the oxide layer have been desorbed at 8408C, spectra from different areas show differences in the relative intensity between the peaks corresponding to the substrate Si and to the SiO 2, as would be expected. However, there are not two distinct types of areas that would correspond to clean Si and the original native oxide, respectively. Instead we find, when probing areas with remaining oxide, that the Si/SiO 2 intensity ratio varies over the surface. This could indicate that either the oxide thickness varies (which was not the case prior to the heat treatment) or that irregularly spaced voids (smaller than our lateral resolution) have opened up in the oxide layer. From our measurements, we cannot draw any firm conclusion about this. However, there is an interesting observation in the Si 2p core level spectra recorded from all parts of the surface, including areas showing a high intensity from dioxide. The Si 2p component from the substrate Si shows a small extra structure on the high kinetic energy side of the main peak. This is clearly seen in Fig. 5, where spectrum 1 from Fig. 2 is shown with the 2p 1/2 components removed. For a clean Si(100)2 × 1 surface, this structure has been identified as a surface-shifted component related to the dimer atoms [14]. If we make the same interpretation of our Si 2p spectra, this would indicate that there are areas on the surface exposing clean Si in a 2 × 1 reconstruction. In scanning tunneling microscopy measurements, both 2 × 1 and other reconstructions have been observed after desorption of oxide films under similar conditions [7,15]. We are not able to conclude whether these clean Si areas are present in the dioxide-covered areas as voids smaller than our lateral resolution, or if they are exclusively related to the areas with no presence of dioxide (see Fig. 2 and the later discussion on the lateral selectivity of our probe). The Si 2p core level spectral structures from Si in intermediate oxidation states have previously been interpreted to emanate from interface atoms having different coordination to oxygen and to other Si neighbours (see, for example, Ref. [13]). In spectra from different partly oxide-covered areas, we see only weak contributions from these intermediate-oxidation-state peaks. This is exemplified in Fig. 5 where the Si 1+ and Si 2+ oxidation states are weak but clearly observable at 0.9 eV and 1.75 eV [13] lower kinetic energy
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Fig. 6. Spectrum 2 from Fig. 4 recorded from the edge of a void with the 2p 1/2 spin orbit components subtracted. A polynomial background fitted to the spectrum in the energy ranges 19.5– 20.0 eV and 31.5–32.5 eV is also shown. The spectrum shows no evidence of intermediate oxidation states (see text) but the presence of a surface-shifted component at the high kinetic energy side of the main 2p 3/2 peak (inset).
than the main peak. In a previous investigation using the microXANES technique [8,9], the evolution of the intermediate oxidation state peaks was followed as a function of annealing temperature4 and one conclusion was that the Si 2+ state was present until all SiO 2 had desorbed. Our data do not contradict these findings, but in our photoemission data the intermediate oxidation state peaks have an intensity too low to be measured quantitatively with a reliable accuracy. Thus, from the present data we cannot draw any firm conclusions on whether the heat treatment causes a change in the interface structure which would be revealed as a change in relative intensities between the intermediate-oxidation-state peaks. We can conclude that in the desorption process areas of clean (probably 2 × 1 reconstructed) Si appears. Turning now to the thick oxide samples, we first conclude that, prior to the high temperature anneals, the Si 2p spectra (recorded at hn = 130 eV) show only a single component corresponding to SiO 2. In images from various parts of the surface, we find no change in intensity of this oxide signal which means that (within our lateral resolution and detectability limit) the oxide layer is initially free from voids or cracks exposing the substrate. 4 An annealing procedure resulting in a temperature gradient along the surface was used and the temperature-dependent results were obtained by investigating different areas on the sample.
The growth of circular voids in thick oxide layers as a result of high temperature anneals, as seen in Fig. 3, has been observed and extensively studied in the past [2–4]. The mechanism behind the desorption is well understood; once a (micro-)void in the oxide has opened up (probably at a defect or an impurity [2– 4]), the void growth proceeds by elemental Si diffusing to the void edge and reacting with SiO 2 to form volatile SiO. For the oxide layers investigated here, a total annealing time of 18 minutes at 11008C is needed to create voids of 10 mm diameter (Fig. 3a). These voids appear at the same time and they all have very similar diameters. Further heat treatments cause these voids to grow in diameter, but almost no additional voids appear. The growth in diameter is linear with time and has a rate of 0.90 6 0.05 mm min −1 at an annealing temperature of 11008C, up to about 40 mm diameter when the voids start to coalesce. These observations are in agreement with previous reports [3,4]. From Fig. 4 and more clearly from Fig. 6, which shows spectrum 2 from Fig. 4 where the 2p 1/2 component has been removed, we see that the Si 2p spectra consist of only two components: one from the clean substrate and one from Si in SiO 2,5 their relative intensities depending on from which part of the surface the data are collected. There are no signs of any intermediate oxidation state silicon at any place on the surface after cooling the sample to RT. In particular, there is no trace of SiO, which should give rise to a peak at about 0.9 eV lower kinetic energy than the bulk peak, at the edge of a void where the Si + SiO 2 → 2 SiO reaction takes place (see Fig. 6). Thus, from a spectroscopic point of view the surface seems to consists of two distinct regions: areas covered with a thick layer of SiO 2 and areas exposing a clean Si surface. Concerning the clean areas, there are two interesting observations to be made. First, the Si 2p spectrum does show a weak shoulder on the high kinetic energy side of the main peak, see Fig. 6, which is very similar to the result from the native oxide and, therefore, make us believe that at least parts of the surface have regained an ordered Si(100)2 × 1 structure. 5 The binding energy shift between the bulk Si component and the SiO 2 component is much larger in these spectra (about 5 eV) than for the native oxide (about 4 eV) and for surfaces oxidized under vacuum (3.9 eV [13]). This is most probably due to charging effects in the thick oxide layer. We have not seen any signs of this charging affecting our images.
U. Johansson et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 45–52
Fig. 7. The full curve shows the measured intensity distribution (hn = 130 eV) in the focal plane of the ellipsoidal mirror in terms of integrated intensity within a circular area centred on the peak intensity as a function of the diameter of this circle. The symbols show the intensity ratio I(Si)/(I(Si) + 0.55 × I(SiO 2)) measured from photoelectron spectra recorded from the centre of a void as its diameter increases upon repeated annealings (see text for further details).
The observation of areas with an ordered structure is in agreement with the results in Ref. [2] where a 2 × 1 LEED pattern (although with high background intensity) was observed from a surface with a partly ˚ ) dioxide. The second observadesorbed thick (500 A tion is that we always find a small signal from SiO 2 remaining, irrespective of which part of the surface is probed, voids or open, seemingly clean, areas. The question is whether this is due to remaining SiO 2 on the surface or if our probe excites photoelectrons from surrounding areas with remaining SiO 2, i.e. a question of selectivity as defined above. In order to address this question of selectivity in a more quantitative way, we have performed some test measurements to probe the cross-sectional intensity profile of our exciting photon beam. As mentioned earlier, our photon beam has a small central focal spot (1.5 mm FWHM) surrounded by tails of intensity, mainly due to surface roughness and figure errors of the mirror, which may cause photoemission from a limited area outside this central spot. In order to get a quantitative measure of the lateral photon intensity distribution in the focal plane of the ellipsoidal mirror, we replaced the sample with a pin-hole6 and monitored the transmitted photon flux with a Schottky barrier photodiode7 as the pin-hole was scanned in 6 7
Melles Griot, 1.0 +0.5 −0 mm pin-hole. Hamamatsu G1127-02.
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the focal plane. The result of these measurements expressed as integrated intensity within a circular area, centred around the focal spot, is shown in Fig. 7 (full line). We see, for example, that 90% of the total intensity is contained within a diameter of 25 mm. Also shown in Fig. 7 (symbols) is the relative intensity of clean Si compared to SiO 2 expressed as I(Si)/(I(Si) + 0.55 × I(SiO 2)),8 determined from photoemission spectra recorded from the centre of a void as its diameter increases upon repeated annealings. If the observed SiO 2 intensity was only due to a limited selectivity of our probe, i.e. if the SiO 2 intensity would exclusively emanate from the dioxide layer surrounding the void which in itself would expose clean Si, one would expect the I(Si)/(I(Si) + 0.55 × I(SiO 2)) ratio to be equal to the integrated photon intensity as a function of diameter. This is clearly not the case as seen from Fig. 7 and the only reasonable explanation is that an additional SiO 2 photoemission intensity comes from the interior of the void in the oxide layer. In addition to these results we recorded Si 2p spectra from surfaces in the very late stages of oxide desorption, where only small patches of dioxide were observed on the surface. Also in spectra recorded from positions far away from these patches ( . 60 mm), a weak SiO 2 signal was observed. Thus, the conclusion must be that there is some remaining SiO 2 in the seemingly clean areas. However, we find no lateral contrast from this SiO 2 in our images. By comparing the relative intensities of the two Si 2p core level peaks from Si and SiO 2 and using the known electron mean free paths in Si and SiO 2 [13], one can estimate that the measured SiO 2 signal would correspond to an ˚ if there was a un-physical dioxide thickness of 0.3 A homogeneous layer of dioxide remaining on the surface. Also, previous cross-sectional TEM micrographs [2] show that the Si surface inside a void lies below the Si/SiO 2 interface plane, which would be difficult to explain if there was a remaining homogeneous SiO 2 layer in the void. Thus we suggest that the remaining SiO 2 consists of small clusters or particles with dimensions smaller than our probe can detect individually. 8
The factor 0.55 is a correction (taken from Ref. [13]) for the difference in photoemission intensity between clean Si and SiO 2, including the volume density of Si, the photoionization crosssection and the mean free path of the photoelectrons excited with 130 eV photon energy.
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4. Summary
Acknowledgements
In summary, we have been able to follow the desorption of both native and thick oxide layers using laterally resolved photoelectron spectroscopy. Images of surfaces with partly desorbed oxide show a very irregular pattern of remaining dioxide for the native oxide, while the thick oxide desorbs in regular patterns of circular voids. This apparent difference in the desorption process is most probably due to the limited lateral resolution and selectivity of our probe, prohibiting us from observing finer details in the case of the native oxide. It is known (for thicker oxides) that the density of voids increases with decreasing oxide thickness [3]. Thus, we believe that for the native oxide, with a thickness of only ˚ , a large number of voids have already 10–15 A grown and coalesced into larger areas at dimensions too small to be observed with our probe. For the thick oxide layers, the Si 2p core level spectra from different parts of the surface (areas with dioxide, voids or void edges) show only the presence of clean Si and/or SiO 2. In particular there are no traces of SiO left on the surface after annealing and cooling to RT. An other interesting observation is that there are traces of SiO 2 left in the voids created in the thick oxide. From the low intensity of this SiO 2 signal and the lack of contrast in the images, we conclude that this remaining SiO 2 consists of small sub-micron clusters or particles. For both types of oxide, we find during and after desorption a surface-shifted component in the Si 2p core level spectra, indicating that at least parts of the surface have an ordered structure which most probably is a 2 × 1 reconstruction.
We are indebted to Ivan Maximov at the Nanometer Laboratory, Lund University, for assisting us with complementary SEM measurements. This work was supported by the Swedish Research Council for Engineering Sciences.
References [1] For a recent review, see: T. Engel, Surf. Sci. Reports 18(4) (1993). [2] R. Tromp, G.W. Rubloff, P. Balk, F.K. LeGoues, E.J. van Loenen, Phys. Rev. Lett. 55 (1985) 2332. [3] M. Liehr, J.E. Lewis, G.W. Rubloff, J. Vac. Sci. Technol., A5 (1987) 1559. [4] G.W. Rubloff, J. Vac. Sci. Technol. A8 (1990) 1857. [5] Y.-K. Sun, D.J. Bonser, T. Engel, J. Vac. Sci. Technol. A10 (1992) 2314. [6] K.E. Johnson, T. Engel, Phys. Rev. Lett. 69 (1992) 339. [7] K.E. Johnson, P.K. Wu, M. Sander, T. Engel, Surf. Sci. 290 (1993) 213. [8] G.R. Harp, Z.L. Han, B.P. Tonner, Phys. Scr., T31 (1990) 23. [9] G.R. Harp, Z.L. Han, B.P. Tonner, J. Vac. Sci. Technol., A8 (1990) 2566. [10] R. Nyholm, M. Eriksson, K. Hansen, O.-P. Sairanen, S. Werin, A. Flodstro¨m, C. To¨rnevik, T. Meinander, M. Sarakontu, Rev. Sci. Instrum. 60 (1989) 2168. [11] U. Johansson, R. Nyholm, C. To¨rnevik, A. Flodstro¨m, Rev. Sci. Instrum. 66 (1995) 1398. [12] See, for example, S.M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, 1981, p. 393. [13] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff, Phys. Rev. B 38 (1988) 6084. [14] E. Landemark, C.J. Karlsson, Y.-C. Chao, R.I.G. Uhrberg, Phys. Rev. Lett. 69 (1992) 1588. [15] S.M. Gray, M.K.-J. Johansson, L.S.O. Johansson, J. Vac. Sci. Technol. B. 14 (1996) 1043.
Thermal desorption of oxides on Si(100): a case study for the scanning photoelectron microscope at MAX-LAB U. Johansson, H. Zhang1, R. Nyholm Department of Synchrotron Radiation Research, Institute of Physics, Lund University, P.O. Box 118, S-221 00 Lund, Sweden Received 18 June 1996; accepted 16 November 1996
Abstract A scanning photoelectron microscope, utilizing a focused beam of monochromatized photons in the energy range from 15 to 150 eV, has been used to study the thermal desorption of oxide layers on Si(100). The instrument can provide highresolution photoelectron spectra from selected parts of the surface as well as images showing the lateral distribution (on a micrometer scale) of elements in different chemical states by monitoring the photoemission intensity of chemically shifted ˚ ) and thick (200–400 A ˚ ) oxides have been studied. The desorption (at 8408C) of the native core levels. Both native (10–15 A oxide proceeds through a phase of irregular (on a micrometer scale) and diminishing areas of dioxide until a clean surface is obtained. For the thick oxide, annealing to 11008C creates circular voids in the oxide layer which grow linearly in diameter with annealing time. The surface in these voids mainly consists of clean silicon but a small amount of remaining SiO 2 is observed. This remaining dioxide most probably consists of small clusters or particles. For both types of oxide, we find, during and after desorption, a surface-shifted component in the Si 2p core level spectra indicating that at least parts of the surface have an ordered structure which most probably is a 2 × 1 reconstruction. q 1997 Elsevier Science B.V. Keywords: Photoelectron microscopy; Spectromicroscopy; Silicon-oxide
1. Introduction A vast number of experimental studies have been carried out in order to achieve a microscopic understanding of the thermally activated desorption of oxide layers on silicon surfaces. Studies have been made on a variety of different surface oxides, both in terms of the oxide thickness, ranging from thin ˚ ) oxide (sub-)monolayer to thick (several hundred A layers, and in terms of different oxidation procedures [1]. By the use of a number of different microscopic and spectroscopic techniques, a common understanding 1 Present address: Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA.
of the general mechanism of the desorption process has emerged; Si reacts with SiO 2 to form the volatile SiO species, thereby removing SiO 2 from laterally constrained microscopic areas of the initially homogeneously oxidized surface [2–9]. However, experiments capable of simultaneously providing information on both the lateral distribution and the chemical state of silicon(sub-)oxides in the surface layer are very scarce; Scanning Auger Microscopy (SAM) has been applied to the desorption of thick oxides [2] and micro-XANES and SAM measurements on the desorption of thin (native) oxide films have been reported [8,9]. In this paper we use the technique of synchrotronradiation-excited scanning photoelectron microscopy
0368-2048/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S0368-2048(97)00004-2
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Fig. 1. Schematic layout of the scanning photoelectron microscope. G, plane grating (500 lines mm −1 and 1000 lines mm −1); S1, S2, two Kirkpatrick–Baez mounted spherical mirrors; A, monochromator exit aperture; E, ellipsoidal ring-shaped focusing mirror; SA, sample on x–y–z scanning stage; D, hemispherical sector electron energy analyser.
to reveal, on a lateral scale in the micrometer range, the local chemical state of silicon as the desorption of an oxide layer evolves. We use the inherent chemical sensitivity of core level photoelectron spectroscopy in combination with a well focused exciting photon beam to investigate the lateral distribution of silicon in different chemical states on the surface. The technique is applied to both thin, native, as well as thick, ˚ , oxide layers on Si(100) substrates. several hundred A What is new in the present study is that we have obtained spectroscopic information on these surface structures allowing us to address issues such as: Do voids in the oxide layer expose a clean Si surface and/or do other intermediate-oxidation-state silicon species occur on the surface?
2. Experimental The experiments were performed using the scanning photoelectron microscope at beamline 31 at the MAX I storage ring in Lund, Sweden [10,11]. This beamline is comprised of a 75 mm period undulator in combination with a plane grating monochromator and a ring-shaped ellipsoidal focusing mirror. The system, which is shown schematically in Fig. 1, is capable of providing photoelectron spectroscopy data with a lateral resolution in the micrometer range. The combined undulator source and monochromator can be used efficiently in the photon energy range from 15 eV to 150 eV using two exchangeable 500 lines mm −1 and 1000 lines mm −1 gratings. The photon energy resolution is # 0.1 eV below 100 eV, increasing to
about 0.2 eV at 130 eV used in these experiments. The photon flux at the sample is between 10 9 and 10 10 s −1, depending on photon energy. There are two modes of operation of the scanning photoelectron microscope; either a selected spot on the sample surface is positioned into the focused photon beam to obtain a laterally resolved photoelectron spectrum of the surface layer, or the photoelectron analyser is locked to a specific kinetic energy, e.g. corresponding to a chemically shifted core level, and by scanning the sample through the focused photon beam, while monitoring the photoemission signal, the lateral distribution of this specific chemical component is imaged. The ring-shaped ellipsoidal focusing mirror, operating at a grazing angle of incidence of 28, gives a focal spot with a full width half maximum (FWHM) of about 1.5 mm on the sample surface. However, due to imperfections in the mirror shape and surface smoothness, some light is distributed into tails extending to several tens of micrometers around this focal spot (see further details below). An annular aperture (inner diameter 4.8 mm, outer diameter 5.1 mm) immediately in front of the mirror, in combination with a circular aperture about 10 mm down-stream of the mirror, effectively prevent any light not being reflected by the mirror reaching the sample. The sample is, via a bellows, connected to a piezodriven x–y–z scanning stage placed outside the vacuum system. The scanning range perpendicular to the photon beam is 120 × 120 mm 2. A hemispherical sector electron energy analyser, mounted at a 478 angle to the sample surface normal, is used to record laterally resolved photoelectron spectra and also to monitor the photoemission signal when recording images. Typical recording times for the Si 2p core level spectra shown here are about 30 min, with a total instrumental energy resolution of 0.2 eV. Due to the large chemical shifts for the Si 2p core level investigated here, images could be recorded at lower energy resolution, 0.5 eV, without loss of information. A 120 × 120 mm 2 image, using a step size of 2.4 mm, was typically recorded in 6 min. The silicon substrates (14 × 7 mm 2) used for the experiments were cut from (100)-oriented wafers (p-type, B, 0.02–0.03 Q cm).2 The samples were 2
Virginia Semiconductors, Inc.
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heated by direct current flow through the wafers. The temperature was measured with an optical pyrometer which also allowed the uniformity of the temperature over the surface to be observed. Measurements presented here were made on the central part of the sample where the temperature was uniform within about 6 58C. All annealings were made with the sample in the measuring position which ensured that the same 120 × 120 mm 2 surface area could be observed after repeated heat treatments. All measurements were made after cooling the sample down to room temperature (RT). Measurements at annealing temperatures were not possible since the surface of the ellipsoidal focusing mirror had to be protected from heat radiation from the closely placed sample. The base pressure in the measuring chamber was about 1 × 10 −10 Torr. During annealings to 8408C and 11008C, the pressure increased to about 7 × 10 −10 Torr and 3 × 10 −9 Torr, respectively. Two types of oxides were investigated: native ˚ ) and thick (200–400 A ˚ ) oxide oxides (10–15 A layers. The native oxides were simply prepared by rinsing the wafer in ethanol, after which it was heated in situ to 6008C in order to obtain a clean oxide surface. The thick oxides were prepared by wet oxidation in which the surface was exposed to H 2O vapour in the presence of low concentration HCl at a temperature of 9008C. The exposure time needed to obtain a ˚ oxide thickness was extrapolated from the 200–400 A exposure times for various thicker oxide layers for which the thickness could be estimated by visually observing the colour of the oxidized surface [12]. Also for these samples, a heat treatment (6008C) was used to obtain a clean oxide surface after introduction into the measuring chamber.
3. Results and discussion Before entering into a discussion on the results, we would like to make some general comments on the achievable lateral ‘‘resolution’’ in an experiment of this type. What often, and sometimes rather carelessly, is stated is that a spectromicroscope possesses a certain lateral resolution expressed in nano- or micrometers, without addressing further details. This is generally not satisfactory for spectromicroscopes that use a focused photon beam as the source of
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excitation, since all types of focusing optics give rise to a central focal spot surrounded by tails of radiation due to aberrations, figure errors, diffuse scattering and possibly diffraction which may lead to ambiguous or incorrect interpretations of data, if not properly taken into account. When applying spectroscopic techniques to investigate lateral inhomogeneties on surfaces, as in the present case, one may describe the performance of the instrument in several different ways. We would prefer, for reasons that will become clear in later discussions, to describe the performance of the instrument using the terms detectability, lateral resolution and selectivity. By detectability, we mean the ability to obtain spectroscopic information, with a useful energy resolution and signal-to-noise ratio, from a small isolated structure on the surface. By lateral resolution, we mean the ability to resolve closely spaced structures on the surface as separate objects. Finally, we use the term selectivity to describe the ability to record selectively the spectroscopic information from an area of the sample centred in the focal spot of the exciting radiation without having interference from photoemission from surrounding areas. Although these concepts are not completely independent and might seem to be somewhat arbitrarily chosen, they are still useful concepts when interpreting data, whether it concerns images or spectra. It should be realized that they are not only instrument- but also sample-dependent. Thus, quantitative measures have to be established for each different experimental situation. In the present experiments, selectivity is an important issue and a scheme to address this matter, applicable to the type of samples investigated, is discussed below. Turning to the results from the present study and starting with the native oxide, we observe an Si 2p ˚) core level spectrum characteristic of a thin (10–15 A oxide layer. In agreement with a number of previous investigations (see, e.g., Ref. [13]) we find, apart from the Si 2p signal from the substrate and from SiO 2, 2p components from Si in intermediate oxidation states. With our probe, we find no signs of inhomogeneities in this oxide layer prior to heat treatments. After an annealing to 8408C for about 20 min, we observe that parts of the oxide has desorbed. An image after a total annealing time of 80 minutes is shown in Fig. 2 where the intensity of the Si 2p signal from SiO 2 was
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Fig. 2. Result from a partly desorbed native oxide layer on Si(100) after 80 min of annealing to 8408C. The image was recorded by monitoring the Si 2p core level photoemission signal (hn = 128 eV) from SiO 2, i.e. bright areas have a larger content of SiO 2 than the darker areas. Image size is 18 × 18 mm 2 with a pixel size of 0.4 × 0.4 mm 2. The Si 2p spectra, showing different amounts of dioxide, are recorded from different areas on the surface, as indicated in the image. The absolute kinetic energy scale is not calibrated.
monitored. The image shows an irregular pattern of varying SiO 2 photoemission intensity. Fig. 2 also shows Si 2p spectra from three different areas, as indicated in the image. Upon repeated annealings, the desorption of the oxide proceeds through a phase of irregular but diminishing areas of dioxide, leading to situations where only small patches of dioxide remain. Finally, a clean surface is obtained. For the thick oxide, we find, prior to high temperature anneals, a homogeneous oxide surface. The Si 2p spectra only show one component from the chemically shifted Si 2p component of SiO 2. After repeated high temperature annealings to 11008C, small circular voids in the oxide layer appear.3 These voids grow in diameter with annealing time and finally coalesce into larger areas as seen from the series of images shown in Fig. 3 recorded from SiO 2. Tuning the electron analyser to the clean Si2p peak results in images with a reversed contrast. Fig. 4 shows Si 2p spectra from areas corresponding to the cross-marks in Fig. 3d, i.e. from the centre of a void, from the edge of a void, and from the dioxide layer outside the void. From the images and spectra in Figs 2–4, the native oxide and the thick oxide, at first sight, seem to have a different behaviour upon heat treatment. The first 3
Upon closer inspection, one sees that the circular areas are slightly distorted towards a quadratic shape. This has previously been reported [2] to follow the crystal symmetry of the (100) surface.
˚ thick oxide on Si(100) after Fig. 3. Images obtained from a 300 A repeated annealings to 11008C. The images were recorded by monitoring the Si 2p core level photoemission signal (hn = 130 eV) from SiO 2, i.e. bright areas have a larger content of SiO 2 than the darker areas. Image size is 120 × 120 mm 2 with a pixel size of 2.4 × 2.4 mm 2 and the acquisition time for each image was about 6 min. The dark circular areas are voids in the oxide layer exposing clean Si. The sequence of images a–h shows the same area of the surface after repeated annealings with total annealing times of: a) 18 min, b) 24 min, c) 30 min, d) 36 min, e) 42 min, f) 48 min, g) 54 min, h) 60 min.
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Fig. 4. Si 2p core level spectra (hn = 130 eV) from the three different positions marked by the cross marks in Fig. 3(d). The spectra correspond to; 1) in the centre of the void (30 mm diameter), 2) at the edge of the void, 3) about 30 mm outside the edge. The absolute kinetic energy scale is not calibrated.
observation is the irregular pattern (on a micrometer scale) in which the native oxide desorbs, which is in contrast to the regular circular voids that open up in the thick oxide. The second observation is the appearance of Si in intermediate oxidation states in the native oxide samples, while the thick oxide samples only show the presence of clean Si and SiO 2. For the native oxide, we have used an annealing temperature of 8408C in order to obtain a practical desorption rate; e.g. at 7608C, the desorption rate is extremely low and annealing the sample to 11008C completely removes the oxide in a few minutes.
Fig. 5. Spectrum 1 from Fig. 2 with the 2p 1/2 spin orbit components subtracted. A polynomial background fitted to the spectrum in the energy ranges 20.0–21.5 eV and 29.5–30.0 eV is also shown. The spectrum clearly shows the existence of intermediate oxidation states (see text) and a surface-shifted component at the high kinetic energy side of the main 2p 3/2 peak (inset).
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From a surface like that shown in Fig. 2, where parts of the oxide layer have been desorbed at 8408C, spectra from different areas show differences in the relative intensity between the peaks corresponding to the substrate Si and to the SiO 2, as would be expected. However, there are not two distinct types of areas that would correspond to clean Si and the original native oxide, respectively. Instead we find, when probing areas with remaining oxide, that the Si/SiO 2 intensity ratio varies over the surface. This could indicate that either the oxide thickness varies (which was not the case prior to the heat treatment) or that irregularly spaced voids (smaller than our lateral resolution) have opened up in the oxide layer. From our measurements, we cannot draw any firm conclusion about this. However, there is an interesting observation in the Si 2p core level spectra recorded from all parts of the surface, including areas showing a high intensity from dioxide. The Si 2p component from the substrate Si shows a small extra structure on the high kinetic energy side of the main peak. This is clearly seen in Fig. 5, where spectrum 1 from Fig. 2 is shown with the 2p 1/2 components removed. For a clean Si(100)2 × 1 surface, this structure has been identified as a surface-shifted component related to the dimer atoms [14]. If we make the same interpretation of our Si 2p spectra, this would indicate that there are areas on the surface exposing clean Si in a 2 × 1 reconstruction. In scanning tunneling microscopy measurements, both 2 × 1 and other reconstructions have been observed after desorption of oxide films under similar conditions [7,15]. We are not able to conclude whether these clean Si areas are present in the dioxide-covered areas as voids smaller than our lateral resolution, or if they are exclusively related to the areas with no presence of dioxide (see Fig. 2 and the later discussion on the lateral selectivity of our probe). The Si 2p core level spectral structures from Si in intermediate oxidation states have previously been interpreted to emanate from interface atoms having different coordination to oxygen and to other Si neighbours (see, for example, Ref. [13]). In spectra from different partly oxide-covered areas, we see only weak contributions from these intermediate-oxidation-state peaks. This is exemplified in Fig. 5 where the Si 1+ and Si 2+ oxidation states are weak but clearly observable at 0.9 eV and 1.75 eV [13] lower kinetic energy
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Fig. 6. Spectrum 2 from Fig. 4 recorded from the edge of a void with the 2p 1/2 spin orbit components subtracted. A polynomial background fitted to the spectrum in the energy ranges 19.5– 20.0 eV and 31.5–32.5 eV is also shown. The spectrum shows no evidence of intermediate oxidation states (see text) but the presence of a surface-shifted component at the high kinetic energy side of the main 2p 3/2 peak (inset).
than the main peak. In a previous investigation using the microXANES technique [8,9], the evolution of the intermediate oxidation state peaks was followed as a function of annealing temperature4 and one conclusion was that the Si 2+ state was present until all SiO 2 had desorbed. Our data do not contradict these findings, but in our photoemission data the intermediate oxidation state peaks have an intensity too low to be measured quantitatively with a reliable accuracy. Thus, from the present data we cannot draw any firm conclusions on whether the heat treatment causes a change in the interface structure which would be revealed as a change in relative intensities between the intermediate-oxidation-state peaks. We can conclude that in the desorption process areas of clean (probably 2 × 1 reconstructed) Si appears. Turning now to the thick oxide samples, we first conclude that, prior to the high temperature anneals, the Si 2p spectra (recorded at hn = 130 eV) show only a single component corresponding to SiO 2. In images from various parts of the surface, we find no change in intensity of this oxide signal which means that (within our lateral resolution and detectability limit) the oxide layer is initially free from voids or cracks exposing the substrate. 4 An annealing procedure resulting in a temperature gradient along the surface was used and the temperature-dependent results were obtained by investigating different areas on the sample.
The growth of circular voids in thick oxide layers as a result of high temperature anneals, as seen in Fig. 3, has been observed and extensively studied in the past [2–4]. The mechanism behind the desorption is well understood; once a (micro-)void in the oxide has opened up (probably at a defect or an impurity [2– 4]), the void growth proceeds by elemental Si diffusing to the void edge and reacting with SiO 2 to form volatile SiO. For the oxide layers investigated here, a total annealing time of 18 minutes at 11008C is needed to create voids of 10 mm diameter (Fig. 3a). These voids appear at the same time and they all have very similar diameters. Further heat treatments cause these voids to grow in diameter, but almost no additional voids appear. The growth in diameter is linear with time and has a rate of 0.90 6 0.05 mm min −1 at an annealing temperature of 11008C, up to about 40 mm diameter when the voids start to coalesce. These observations are in agreement with previous reports [3,4]. From Fig. 4 and more clearly from Fig. 6, which shows spectrum 2 from Fig. 4 where the 2p 1/2 component has been removed, we see that the Si 2p spectra consist of only two components: one from the clean substrate and one from Si in SiO 2,5 their relative intensities depending on from which part of the surface the data are collected. There are no signs of any intermediate oxidation state silicon at any place on the surface after cooling the sample to RT. In particular, there is no trace of SiO, which should give rise to a peak at about 0.9 eV lower kinetic energy than the bulk peak, at the edge of a void where the Si + SiO 2 → 2 SiO reaction takes place (see Fig. 6). Thus, from a spectroscopic point of view the surface seems to consists of two distinct regions: areas covered with a thick layer of SiO 2 and areas exposing a clean Si surface. Concerning the clean areas, there are two interesting observations to be made. First, the Si 2p spectrum does show a weak shoulder on the high kinetic energy side of the main peak, see Fig. 6, which is very similar to the result from the native oxide and, therefore, make us believe that at least parts of the surface have regained an ordered Si(100)2 × 1 structure. 5 The binding energy shift between the bulk Si component and the SiO 2 component is much larger in these spectra (about 5 eV) than for the native oxide (about 4 eV) and for surfaces oxidized under vacuum (3.9 eV [13]). This is most probably due to charging effects in the thick oxide layer. We have not seen any signs of this charging affecting our images.
U. Johansson et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 45–52
Fig. 7. The full curve shows the measured intensity distribution (hn = 130 eV) in the focal plane of the ellipsoidal mirror in terms of integrated intensity within a circular area centred on the peak intensity as a function of the diameter of this circle. The symbols show the intensity ratio I(Si)/(I(Si) + 0.55 × I(SiO 2)) measured from photoelectron spectra recorded from the centre of a void as its diameter increases upon repeated annealings (see text for further details).
The observation of areas with an ordered structure is in agreement with the results in Ref. [2] where a 2 × 1 LEED pattern (although with high background intensity) was observed from a surface with a partly ˚ ) dioxide. The second observadesorbed thick (500 A tion is that we always find a small signal from SiO 2 remaining, irrespective of which part of the surface is probed, voids or open, seemingly clean, areas. The question is whether this is due to remaining SiO 2 on the surface or if our probe excites photoelectrons from surrounding areas with remaining SiO 2, i.e. a question of selectivity as defined above. In order to address this question of selectivity in a more quantitative way, we have performed some test measurements to probe the cross-sectional intensity profile of our exciting photon beam. As mentioned earlier, our photon beam has a small central focal spot (1.5 mm FWHM) surrounded by tails of intensity, mainly due to surface roughness and figure errors of the mirror, which may cause photoemission from a limited area outside this central spot. In order to get a quantitative measure of the lateral photon intensity distribution in the focal plane of the ellipsoidal mirror, we replaced the sample with a pin-hole6 and monitored the transmitted photon flux with a Schottky barrier photodiode7 as the pin-hole was scanned in 6 7
Melles Griot, 1.0 +0.5 −0 mm pin-hole. Hamamatsu G1127-02.
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the focal plane. The result of these measurements expressed as integrated intensity within a circular area, centred around the focal spot, is shown in Fig. 7 (full line). We see, for example, that 90% of the total intensity is contained within a diameter of 25 mm. Also shown in Fig. 7 (symbols) is the relative intensity of clean Si compared to SiO 2 expressed as I(Si)/(I(Si) + 0.55 × I(SiO 2)),8 determined from photoemission spectra recorded from the centre of a void as its diameter increases upon repeated annealings. If the observed SiO 2 intensity was only due to a limited selectivity of our probe, i.e. if the SiO 2 intensity would exclusively emanate from the dioxide layer surrounding the void which in itself would expose clean Si, one would expect the I(Si)/(I(Si) + 0.55 × I(SiO 2)) ratio to be equal to the integrated photon intensity as a function of diameter. This is clearly not the case as seen from Fig. 7 and the only reasonable explanation is that an additional SiO 2 photoemission intensity comes from the interior of the void in the oxide layer. In addition to these results we recorded Si 2p spectra from surfaces in the very late stages of oxide desorption, where only small patches of dioxide were observed on the surface. Also in spectra recorded from positions far away from these patches ( . 60 mm), a weak SiO 2 signal was observed. Thus, the conclusion must be that there is some remaining SiO 2 in the seemingly clean areas. However, we find no lateral contrast from this SiO 2 in our images. By comparing the relative intensities of the two Si 2p core level peaks from Si and SiO 2 and using the known electron mean free paths in Si and SiO 2 [13], one can estimate that the measured SiO 2 signal would correspond to an ˚ if there was a un-physical dioxide thickness of 0.3 A homogeneous layer of dioxide remaining on the surface. Also, previous cross-sectional TEM micrographs [2] show that the Si surface inside a void lies below the Si/SiO 2 interface plane, which would be difficult to explain if there was a remaining homogeneous SiO 2 layer in the void. Thus we suggest that the remaining SiO 2 consists of small clusters or particles with dimensions smaller than our probe can detect individually. 8
The factor 0.55 is a correction (taken from Ref. [13]) for the difference in photoemission intensity between clean Si and SiO 2, including the volume density of Si, the photoionization crosssection and the mean free path of the photoelectrons excited with 130 eV photon energy.
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4. Summary
Acknowledgements
In summary, we have been able to follow the desorption of both native and thick oxide layers using laterally resolved photoelectron spectroscopy. Images of surfaces with partly desorbed oxide show a very irregular pattern of remaining dioxide for the native oxide, while the thick oxide desorbs in regular patterns of circular voids. This apparent difference in the desorption process is most probably due to the limited lateral resolution and selectivity of our probe, prohibiting us from observing finer details in the case of the native oxide. It is known (for thicker oxides) that the density of voids increases with decreasing oxide thickness [3]. Thus, we believe that for the native oxide, with a thickness of only ˚ , a large number of voids have already 10–15 A grown and coalesced into larger areas at dimensions too small to be observed with our probe. For the thick oxide layers, the Si 2p core level spectra from different parts of the surface (areas with dioxide, voids or void edges) show only the presence of clean Si and/or SiO 2. In particular there are no traces of SiO left on the surface after annealing and cooling to RT. An other interesting observation is that there are traces of SiO 2 left in the voids created in the thick oxide. From the low intensity of this SiO 2 signal and the lack of contrast in the images, we conclude that this remaining SiO 2 consists of small sub-micron clusters or particles. For both types of oxide, we find during and after desorption a surface-shifted component in the Si 2p core level spectra, indicating that at least parts of the surface have an ordered structure which most probably is a 2 × 1 reconstruction.
We are indebted to Ivan Maximov at the Nanometer Laboratory, Lund University, for assisting us with complementary SEM measurements. This work was supported by the Swedish Research Council for Engineering Sciences.
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