Spectromicroscopy of interfaces with synchrotron radiation: multichannel data acquisition

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1 Nuclear Instruments and Methods in Physics Research A (2001) Spectromicroscopy of interfaces with synchrotron radiation: multichannel data acquisition L. Gregoratti a, *, M. Marsi a, G. Cautero a, M. Kiskinova a, G.R. Morrison b, A.W. Potts b a Sincrotrone Trieste, Societa Consoirtile p. Azioni, S.S. 14km in Area Science Park, I Trieste-Basovizza, Italy b Department of Physics, King s College London, Strand, WC2R 2LS London, UK Abstract Probed length scales of sub-micrometer dimensions have been achieved in photoemission spectroscopy owing to the high flux and brightness of the soft X-rays provided by the third generation synchrotron sources and the progress in microfabrication of focusing elements for soft X-rays. The use of multichannel detectors in the recently constructed scanning photoelectron microscopes adds speed and flexibility in data acquisition. Here we present some results obtained with the scanning photoemission microscope at ELETTRA illustrating the importance of the multichannel data acquisition for the interpretation of the data. # 2001 Elsevier Science B.V. All rights reserved. PACS: 79.60; 29.40; J Keywords: Photoemission; Spectromicroscopy; Multichannel detection 1. Introduction *Corresponding author. Tel ; fax: address: gregoratti@elettra.trieste.it (L. Gregoratti). The scanning photoemission microscope (SPEM) is installed at the ESCA microscopy branch beamline. A tunable coherent radiation with maximum intensity in the energy range ev is provided by an undulator. The requirements of the X-ray focusing optics, to preserve the central brightness and cut the incoherent part of the photon flux, are met by a modified version of a spherical grating monochromator with fixed entrance and exit slits. The photon beam is demagnified by means of a Fresnel zone optic to sub-micrometer dimensions. The SPEM is designed to perform X-ray photoelectron spectroscopy (XPS) of microscopic areas of specimen surfaces. In SPEM the photon beam is focused to a spot of diameter mm by means of Fresnel zone plate (ZP) optic, which is in fact a circular grating consisting of alternating transparent and opaque rings whose spacing diminishes with distance from the center. An order-sorting aperture (OSA) is placed between the ZP and the sample to block the light from the zero and higher diffraction orders [1,2]. Recent experiments performed with SPEM at ELETTRA using a zone plate fabricated by CXRO-LNBL (USA) have /01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

2 L. Gregoratti et al. / Nuclear Instruments and Methods in Physics Research A (2001) shown a spatial resolution of 90 nm. The scientific and technological impact of synchrotron radiation spectromicroscopy at sub-micrometer length scales is growing with the continuously shrinking dimensions of many fabricated electronic and magnetic devices and exponentially increasing interest in studies of the relation between the surface morphology and the properties of the materials. The experiments with SPEM consist of image acquisition by collecting photoelectrons with a selected kinetic energy with an hemispherical electron analyser (HA) while scanning the sample. Once the elemental map of the surface has been acquired, photoelectron spectroscopy can be performed from selected spots in order to obtain detailed quantitative information about the lateral variations in the local composition and electronic properties of the object under investigation. The present electron detector of the SPEM mounted after the exit slit of the HA is a 16- channel detector. The flexibility offered by such type of detector combined with electronics and software developed in-house is the main scope of the present report. It will be illustrated using some data obtained with morphologically complex interfaces which show lateral variations in the composition at a length scale accessible by SPEM. 2. Data acquisition system The computer control of all instrumentation incorporated in SPEM is performed using a software bus designed and developed under the Unix operating system by the EECO Beamline Control Group of ELETTRA [2]. It is a flexible and portable software which can synchronize not only the instruments controlling but also integrates the exchange of information between the software packages using different programming languages. Stepper and piezoelectric motors, multimeters, electron and photon detectors are then controlled by a unique graphical user interface which through very simple actions gives a complete control of the experiment set up. Using a 16- channel electron detector in SPEM has reduced substantially the measurement time necessary for the acquisition and the analysis of the data and has provided high efficiency, allowing with a single scan to map different chemical states and/or Fig. 1. Schematic view of the multichannel data acquisition; the 16-channel images can provide both selected energy maps and 16- point spectra from selected areas.

3 886 L. Gregoratti et al. / Nuclear Instruments and Methods in Physics Research A (2001) different elements including the background signal as well. Fig. 1 shows schematically the operation of the multichannel detector. The counts collected by each channel correspond to a specific kinetic energy defined by the kinetic and pass energies at which the analyzer is set; the energy range covered by all the channels is the so-called energy window. Thus one 16-channel image contains 16 energy-separated images. In this way problems of misalignment, drifts of the motors, optics, short lifetime of the probed sample, etc., can be avoided. For example, even a small misalignment between two separately acquired images can produce unreal contrast levels in the processing of the images, which involves subtractions and divisions of the two pictures. The 16 signal values saved for each pixel of an image can also be used to plot a photoemission spectrum of 16 points, as illustrated in Fig. 1; each point corresponds to a specific kinetic energy determined by the selected energy window. 3. Examples of application Here we illustrate the advantage of the 16- channel imaging in the investigation of systems with strong topographic artifacts and systems where several chemically different phases are coexisting. The first example is an investigation of adsorption of sub-monolayer coverages of Au on Ni polycrystalline samples where the strong topographical contrast obscured the contrast related to the non-uniform distribution of Au. The Au Ni system is important in electronics because both metals are commonly used as electrical contacts. Au and Ni are immiscible in the bulk phase but recent STM work [3 7] has demonstrated that submonolayer Au coverages on Ni (1 1 0)- and (1 1 1)- oriented well-ordered surfaces involve atomic exchange processes between the two species. It is found that Au atoms can be incorporated in the surface Ni layers forming a surface alloy. Fig. 2 shows an image obtained collecting the Ni 3p photoelectrons from an atomically clean Ni surface. This image is obtained by summing the signals of all the 16 channels of the detector and should not contain any information about the chemical environment of the nickel which comes from a single chemical state of Ni, because it covers the whole range of Ni 3p photoelectron emission. Thus, the contrast variations in the image are a result of the topography of the surface indicating that the preparation of the samples has led to rich structural morphology. It should be noted that in our microscope the incident focused beam is normal to the sample surface and the analyzer is positioned at a grazing take-off angle of 708 on the right-hand side of the sample. In this construction one gains surface sensitivity but also enhances the artifacts caused by more pronounced surface topographic features. On this surface we deposited 0.4 ML of Au and annealed the interface for a total of 90 min at 620 K in order to study the structural effects on the Au distribution and interaction with Ni. The strong topography-related features dominated the contrast of the Au 4f and Ni 3p images, so that a correction procedure to remove the topographic contribution and leave only the elemental concentration contrast was necessary. The applied processing procedure illustrated in Fig. 2 used the relationship I ¼ I peak I background : I background As peak and background images we selected a few of the 16 image channels which covered the energy window indicated in Fig. 2. The selected channels showed the signal corresponding to the Au 4f 7/2, Ni 3p 3/2 and the corresponding background signal on the high kinetic energy side of each peak. The images at the bottom of Fig. 2 are obtained by applying this correction procedure and can be considered as proportional to the real concentration maps of each element for same area that is shown to be dominated by the surface topography in the top image. We should remark that we did not apply any filtering to the background image which presents a lower signal/noise ratio with respect to the peak one since the channels out of the peak have less statistics for the same counting time. Even with this bare processing the Au 4f

4 L. Gregoratti et al. / Nuclear Instruments and Methods in Physics Research A (2001) Fig. 2. The top image shows the accentuated morphology of a Ni polycrystalline sample. To get the pure elemental maps from these pictures we used the image processing reported in the center selecting as peak and background images corresponding to different channels from the energy window selected and evidenced in the Ni 3p and Au 4f spectra. The two images in the lower part show the result of this processing. concentration map clearly reveals regions with variable contrast that we attributed to a different distribution and adsorption state of Au on the differently oriented grains. The corrected Ni 3p maps appear featureless, which is consistent with the much lower cross section of the Ni 3p photoelectrons and the substantial contribution of the Ni atoms from the bulk to the Ni 3p signal. The second example concerns a system where several phases with different chemical compositions coexist. Using the SPEM we investigated the lateral distribution of the phases formed when 1 ML of Ni dissolved in Si segregates onto the surface. We identified two kinds of 3D islands with different shapes and chemical compositions together with two 2D Ni reconstructions one of them being identified as 1 1 -RC [8 10]. Fig. 3 shows three Si 2p photoemission spectra identifying the NiSi 2, NiSi and (1 1) -RC. The energy window used for the 16-channel imaging is shown as well. The images corresponding to a sum of all the channels, and to the third, seventh and ninth channels are also shown in the figure. The sum is featureless because, as can be judged from the Si 2p spectra, the lateral changes in the total Si 2p signal are negligibly small for the selected energy window. The channel 3 picture shows bright features at the borders of the scanned area. These are the NiSi islands, which, as evidenced by the Si 2p spectra, show higher emission at the energy of the third channel. For kinetic energy corresponding to channel 7 all three spectra have the same intensities and the image is almost uniform, as in the summed image. For the energy corresponding to channel 9 the photoemission from the NiSi 2 phase is the highest, which assigns the bright central feature to a disilicide island while the border NiSi islands appear darker in this case. 4. Conclusions Using two selected cases we demonstrate the importance of multichannel detection and development of correcting procedures for increasing the efficiency of data acquisition and precision of the measurements with scanning photoemission microscopes constructed recently at the third generation synchrotron facilities. This approach also helps to reduce the ambiguity in interpreting the data, especially in the cases when the artifacts are very strong. A new detection system developed at ELET- TRA using 48 channels is to be implemented soon in the SPEM. It will provide the possibility to use in many cases only imaging for quantitative analysis, because for each pixel we will be able to reconstruct with sufficiently good resolution the corresponding photoemission spectrum.

5 888 L. Gregoratti et al. / Nuclear Instruments and Methods in Physics Research A (2001) Fig. 3. The image of the sum of all the channels at the top of the figure reveals the presence of several islands. Analyzing the single channel images it turns out that the islands correspond to different silicide phases. The spectra are three typical spectra for the NiSi 2, NiSi and (1 1) -RC silicide phases. References [1] M. Kiskinova, M. Marsi, E. Di Fabrizio, M. Gentili, Surf. Rev. Lett. 6 (2) (1999) 265. [2] M. Marsi, L. Casalis, L. Gregoratti, S. Gunther, A. Kolmakov, J. Kovac, D. Lonza, M. Kiskinova, J. Electron Spectros. Relat. Phenom. 84 (1997) 73. [3] R. Pugliese, L. Gregoratti, R. Krempska, F. Bille, J. Krempasky, M. Marsi, A. Abrami, J. Synchr. Rad. 5 (1998) 587. [4] J. Jacobsen, L. Pleth Nielsen, F. Basenbacher, I. Stensgaard, E. Lægsgaard, T. Rasmussen, K.W. Jacobsen, J.K. Nrskov, Phys. Rev. Lett. 75 (3) (1995) 489. [5] L.P. Nielsen, I. Steensgaard, F. Besenbacher, E. Lægsgaard, Surf. Rev. Lett. 3 (5/6) (1996) [6] L.P. Nielsen, F. Besenbachar, I. Steensgaard, E. Lægsgaard, C. Engdahl, P. Stoltze, K.W. Jacobsen, J.K. Nrskov, Phys. Rev. Lett. 71 (5) (1993) 754. [7] L.P. Nielsen, I. Steensgaard, E. Lægsgaard, F. Besenbacher, Surf. Sci (1994) 544. [8] L. Gregoratti, S. Gunther, J. Kovac, M. Marsi, R.J. Phaneuf, M. Kiskinova, Phys. Rev. B 59 (3) (1999) [9] L. Gregoratti, S. Gunther, J. Kovac, M. Marsi, M. Kiskinova, Appl. Surf. Sci (1 4) (1999) 255. [10] K. Umezawa, S. Nakanishi, W.M. Gibson, Surf. Sci. 426 (1999) 225.