Papers

AVS1997 Session AS-MoM: Advances in Surface Microscopy

Monday, October 20, 1997 8:20 AM in Room J2

Monday Morning

Start	Invited?	Item
8:20 AM		AS-MoM-1 Variable Temperature AFM on Electrically Insulating Surfaces M. Gutt (University of Bochum, Germany); A. Feltz, M. Sander, T. Berghaus (OMICRON Vakuumphysik GmbH, Germany) AFM at variable temperatures allows the investigation of temperature dependent processes on electrically insulating samples. A novel AFM detection technique, based on the needle-sensor, which originated from Carl Zeiss Jena, Germany, was used for the first time to investigate the nucleation and growth of silver and silicon on silicon dioxide films and glass substrates at variable temperature. The needle-sensor is a pure electrical sensor overcoming all needs for mechanical or optical adjustments in-situ or prior to use. The quartz needle-sensor is operated at its longitudinal resonance. Compensation of temperature dependent changes of the quartz resonator allows to use it in an SPM over a wide temperature range (25 K - 900 K). The deposition of silver and silicon on glass and SiO2 was studied between 50 K and 800 K. Ultra-thin silicon dioxide films, which were derived from a clean silicon surface, exhibited a regular stepped surface. The growth phenomena are discussed with respect to sample temperature and coverage. From RT to 700 K the formation of Ag clusters was observed. No adsorption of Ag was found above 800 K. The cluster density was only weakly temperature dependent. The round, droplet-like shape of the clusters indicates that Ag is not wetting SiO2. This first temperature dependent AFM investigation forms the starting point for further experiments on the technological important system silicon/glass.
8:40 AM		AS-MoM-2 Si-Based Heterolayer and Device Structures Observed by Cross-Sectional STM M. Tao, Z.X. Dai, M.C. Hersam, J.W. Lyding (University of Illinois, Urbana) The rapid advance in Si processing technology has pushed the CMOS device dimension to 0.25 µm, and a feature size well below 0.1 µm is on the horizon. Si-based heterolayers, which have dimensions typically on the order of 10 Å, also provide promising opportunities in RTDs, HBTs, and optoelectronics. Conventional structural and electrical instruments will encounter certain difficulties in characterizing these nanoscale structures. In this paper, we report our development of a new technique, which combine cross-sectional STM and Si V-groove etching, to characterize Si-based heterolayer and device structures. One of the main obstacles in cross-sectional STM of Si-based heterolayer and device structures is the preparation of an atomically flat cleaved Si surface¹. A process has been developed to obtain atomically flat cleaved Si(111) surfaces. It utilizes IC fabrication technology and anisotropic etch to make (111)-faceted V-grooves on Si(100) samples. The V-groove promotes cleave along a single (111) plane, and the cross sections of heterolayer and device structures are exposed on clean surfaces. A home-written edge-locating software can easily find the structures of interest. This technique was first applied to cross-sectional STM of Si. Various step structures and domain boundaries were imaged on Si(111) 2x1, and terraces as large as 1000x1000 Å² were achieved. Several heterolayer and device structures have since been successfully characterized in this ongoing research. SiGe heterolayers are among the first. The interface between Si substrate and SiGe layer is observed. The strain in the heterolayer causes a layered morphology on the cleaved (111) surface. Si/ZnS/Si heterolayers and p-n junctions are also examined. A large density of gap states is observed in the ZnS layer, which is an indicator of its crystal quality. The depletion region is clearly visible in the p-n junction samples and its width is consistent with theory. ¹M.A. Lutz, R.M. Feenstra, and J.O. Chu, Surf. Sci. 328, 215 (1995).
9:00 AM		AS-MoM-3 Effects of Crosslinking on the Spreading of 150 nm Polymer Particles D.D. Woodland, W.N. Unertl (University of Maine) Wetting and spreading processes control the formation and stability of polymer thin films in many industrial applications including magnetic recording media, microelectronics, and microelectromechanics. We describe the effect of crosslinking on the spreading properties of small spherical styrene-butadiene particles. The individual styrene butadiene particles were fabricated to be as nearly identical as possible except for the degree of crosslinking. They had nominal diameters of 150 nm, crosslinking levels of 30% and 91%, and nearly identical glass transition temperatures (312 K and 323 K, respectively). The particles were deposited onto cleaned glass slides from water suspension, dried in air, and stored in vacuum. The as deposited particles were characterized with a scanned probe microscope (SPM). Particle diameters and relative moduli were measured. Spreading was induced by heating to 373 K for 30 minutes in air and the shapes and moduli of the spread particles were determined. The 30% crosslinked particles spread into unusual ring like structures with a small symmetric pit in the center. 91% crosslinked particles spread out to a higher degree laterally than the 30% by collapsing and forming a large crater in the center of the particle. The average height of the two different particle types after spreading is the same within statistical uncertainty. Dialyzation had a minor effect on the particle spreading behavior. The surprisingly complex spreading behavior can effect the mechanical properties and uniformity of thin polymer films due to non-uniform structural formation.
9:20 AM		AS-MoM-4 High Pressure Adsorbate Structures Studied by Scanning Tunneling Microscopy: CO on the Pt(111) Surface J.A. Jensen (Lawrence Berkeley National Laboratory); K.B. Rider (University of California, Berkeley); M. Salmeron, G.A. Somorjai (Lawrence Berkeley National Laboratory) The Pt(111) surface was examined under high pressures of CO with scanning tunneling microscopy (STM). Images obtained in CO pressures of 100 Torr and above indicate the formation of an incommensurate hexagonal close-packed CO layer with a CO spacing of 3.7 Å. At 500 Torr CO, a severe roughening of the Pt(111) surface has been observed. Correlations are made between this work and high pressure CO vibrational spectroscopy measurements of the CO/Pt(111) system.¹ ¹X. Su, P.S. Cremer, Y.R. Shen, and G.A. Somorjai, Phys. Rev. Lett. Vol.77 (1996) 3858
9:40 AM		AS-MoM-5 Evaluation of Phase Separation in PVC/PMMA Blends using Photoelectron and Ion Imaging B.J. Tielsch (Kratos Analytical, United Kingdom); D.J. Surman (Kratos Analytical, Inc.); S.E. Asher (National Renewable Energy Laboratory); E.A. Thomas, J.E. Fulghum (Kent State University) Polymer blends are frequently prepared to obtain material properties not available from the individual constituents. There are a number of possibilities for the morphology of the surface of a binary polymer mixture. The surface composition of the blend may be representative of the bulk composition, of different composition than the bulk but with no apparent phase separation, or show phase separation with surface enrichment of one component. Blends of poly(vinyl chloride)/poly(methyl methacrylate) (PVC/PMMA) have been previously studied using TOF-SIMS imaging and angle-resolved XPS. The XPS work showed a solvent-dependent surface enrichment of PMMA, with no concentration gradient within the XPS sampling depth. The TOF-SIMS images have shown the formation of PVC islands, but some questions remain about the distribution of PMMA in and around the PVC islands. In this work we will compare XPS images with TOF-SIMS images acquired from the same samples. Changes in PVC/PMMA mixing and island formation as a function of composition, solvent and preparation method will be discussed, with an emphasis on the combination of the two techniques to learn about polymer chemistry. This work has been supported by NSF CHE-9631702, NSF DMR89-20147, and 3M.
10:00 AM		AS-MoM-6 Laboratory Size Micro-XPS System with a Laser-Plasma X-ray Source H. Kondo (Nikon Corp., Japan); T. Tomie, H. Shimizu (Electrotechnical Laboratory, Japan) With a progress of semiconductor devices and new material technologies, demand for micro analysis of spatial resolution better than about 1µm is becoming stronger. Although many studies are performed at synchrotron facilities over the world, a compact system is required to be developed, because in-situ observation of devices and materials is crucially important. We have been developing a laboratory size XPS system, in which a laser-produced plasma is employed as an x-ray source, x-ray optics produce an x-ray micro-beam, and photoelectron energy spectrum is obtained by the time-of-flight (TOF) method. Our system is not only compact but also has potential advantages over previous methods. In TOF, all spectral components are recorded in one shot. Because of this nature of TOF, the obtained spectral profiles are highly reliable and very faint changes caused by defects or impurities are easily detectable. Moreover, because the detection solid angle can be made large in TOF, the detection efficiency of photoelectrons can be very high, which relaxes radiation damage of samples which becomes serious in high spatial resolution µ-XPS. High detection efficiency of photoelectrons also leads to fast spectrum acquisition speed. Furthermore, pulsive nature of the x-ray source enables us to observe intermittent and transient phenomena. So far, we observed chemical shifts of Si 2p spectrum in SiO₂ and Si₃N₄ by using monochromatic x rays (λ=4.86 nm, 255 eV) from a boron nitride plasma produced with a Q-switched YAG laser. The spin-orbit splitting of Si 2p electrons was also observed, and the spectral resolution of our XPS system was estimated to be 0.5 eV. We also tried to observe distribution of Al and Si using an x-ray micro-beam generated with an ellipsoidal multilayer mirror. The number of detected photoelectrons in experiments suggested us that, after further development, our XPS system could compete in spectrum acquisition speed even with a system using an undulator source.
10:20 AM		AS-MoM-7 The Application of a Spherical Mirror Analyser to High Resolution Imaging XPS S.C. Page (Kratos Analytical Ltd, United Kingdom) A new imaging X-ray Photoelectron Spectrometer has been developed that combines high energy resolution spectroscopy with fast high spatial resolution imaging XPS. This instrument is based on a novel combination of a conventional hemisperical analyser for high sensitivity spectroscopy with a concentric spherical mirror analyser for fast parallel XPS image acquisition.This is the first use of the spherical mirror analyser in an XPS spectrometer. It has zero spherical aberration and when coupled with the low aberrations of a magnetic immersion objective lens is near ideal as an imaging device. In this spectrometer it is operated at fixed pass energy ensuring an energy resolution independent of kinetic energy.The highly integrated electron optical design allows elemental and chemical state images to be acquired in typically a few seconds with a spatial resolution of a few microns. High sensitivity spectra can then be immediately taken from any number of features observed. The capabilities of this new instrument will be illustrated with examples that highlight its advantages for analysis of a wide variety of specimens.
10:40 AM		AS-MoM-8 Understanding Bonding in Glass Fiber Materials Through a Comparison of Large and Small Area X-ray Photoelectron Images and Spectra J.E. Fulghum (Kent State University); B.J. Tielsch (Kratos Analytical, United Kingdom); D.J. Surman (Kratos Analytical, Inc.) The development of high spatial resolution photoelectron spectrometers makes it possible to obtain a variety of information from heterogeneous samples. By combining large and small area spectral analyses with images, a more complete picture of the sample chemistry can be developed. This approach can also demonstrate that incorrect conclusions may arise if only large or small area analyses are utilized. In this presentation, we will discuss results from two commercially available glass mats, which were coated using different processes. The first sample has a relatively uniform, thick coating of binder on the glass fibers. The second mat has a coating whose apparent uniformity and distribution depend upon the size of the analytical area which is considered. We will compare spectra from large areas (750x350 microns) to images and spectra from fiber bundles (~60 microns) and single fibers (<15 microns). Through this comparison of "macro" and "micro" analyses, it is possible to not only evaluate the distribution of the binder, but to demonstrate phase separation of its components. It develops that analysis of single fibers is required in order to understand the large area analyses and behavior of the final glass material. The information from the fiber bundles is misleading when considered on its own. This work has been supported by NSF CHE-9631702 and NSF CHE-9613880.
11:00 AM		AS-MoM-9 X-ray Spectro-microscopy; A New Technique for Materials Science H.A. Padmore, T. Warwick (Lawrence Berkeley National Lab & Univ. of California, Berkeley) Development of soft x-ray microscopes for spatially resolved spectroscopic surface analysis at the Advanced Light Source will be described in detail. These new instruments make use of the high brightness photon beams from the Advanced Light Source, a third generation electron storage ring designed for the production of soft x-rays. Scanning zone plate microscopes which use Fresnel lenses to produce a sub-micron x-ray spot are operational for high resolution X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectro-microscopy. Large area samples can be navigated and studied by sputter profile XPS techniques in a system which uses a Kirkpatrick-Baez pair of elliptically bent mirrors to produce an x-ray spot of order one micron. A Photo-Electron Emission Microscope (PEEM) is under construction for imaging surfaces at higher spatial resolution by focussing emitted secondary electrons. This system will be used for NEXAFS and Magnetic Circular Dichroism (MCD) studies of thin films and surfaces. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S.Department of Energy, under Contracts No. DE-AC03-76SF00098 and DE-FG02-92ER45468.