Papers

AVS1996 Session AS-WeM: Novel Techniques for Surface and Interface Characterization

Wednesday, October 16, 1996 8:20 AM in Room 105B

Wednesday Morning

Start	Invited?	Item
8:20 AM		AS-WeM-1 Micro-Xanes and Micro-Xps of Polymers H. Ade (North Carolina State University) Undulators at second and third generation synchrotron radiation facilities are sufficiently bright sources of soft x-rays to provide diffraction limited probes 50-200 nm in size with sufficient intensity to perform either Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy [1] or X-ray Photoemission Spectroscopy (XPS) at high spatial resolution. Alternatively, spectral features in either NEXAFS or XPS operating mode can be singled out and a particular chemical state of a selected element in the sample can be mapped with a spatial resolution of up to 50 nm in the photon energy range of 250-800 eV. We will review the status of the technology for instrumentation based on zone plate diffractive optics. While many applications to polymers presently employ the simpler transmission geometry, major efforts at both the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Light Source at Lawrence Berkeley National Laboratory are well under way to provide zone plate based surface microscopy on a routine basis. We will provide examples of applications to polymeric materials in the surface and transmission geometry. References: [1] H. Ade et al., Science 258, 972-975 (1992).
9:00 AM		AS-WeM-3 Development of Scanning Photoelectron Microscope Equipped with Wolter-type Focusing Mirror M. Hasegawa, A. Yoneyama (Hitachi, Ltd., Japan) In the manufacture of electric devices, chemical-state analysis in submicron-sized regions is required because of the continuous reduction in the dimensions of device structures. Scanning photoelectron microscopes using a microbeam formed with x- ray optical devices are promising tools for this purpose. We have developed a scanning photoelectron microscope equipped with a Wolter-type grazing incidence mirror for forming soft x-ray microbeam. A sample stage which consisted of a piezo-driven fine movement x-y stage and a stepping-motor-driven coarse movement x-y stage was installed in order to obtain two-dimensional images with high lateral resolution at any point on a sample. We designed the energy analyzer which had a wide acceptance for photoelectrons referring to the concept of the display-type energy analyzer (ref.1) in order to obtain high detection efficiency of photoelectrons. This microscope was installed in the soft x-ray beamline (BL-8A) at the Photon Factory (KEK-PF) in Tsukuba. We tested the lateral resolution of the microscope in the total-photoyield imaging mode and in the energy-selected photoelectron imaging mode by examining Al-stripe patterns on a silicon dioxide layer. The lateral resolution of our scanning photoelectron microscope has reached the sub-micron region in the total-photoyield imaging mode but has remained at the micron-order in the energy-selected imaging mode. This work has been performed under the approval of the Program Advisory Committee of the Photon Factory (Proposal No. 93-Y003) of the National Laboratory for High Energy Physics.1 H. Daimon, Rev. Sci. Instr., 59 (1988) 545
9:20 AM		AS-WeM-4 Fluorescence Studies of Deeply-Buried Thin Films and Other Low-Dimensional Systems at the Advanced Light Source J. Carlisle, L. Terminello (Lawrence Livermore National Laboratory); T. Callcott (University of Tennessee); D. Ederer, R. Perera (Tulane University); F. Himpsel (University of Wisconsin, Madison) Soft x-ray fluorescence is emerging at the Advanced Light Source (ALS) as a powerful probe of complex material systems. In addition to the intrinsic advantages of SXF (bulk sensitivity, elemental selectivity) for probing electronic structure, the high brightness of the ALS has enabled detailed studies of resonant x-ray scattering processes which occur as the incident photons are tuned through a core-level binding energy. In these resonant SXF experiments, a soft x-ray form of resonant Raman scattering is observed, in which the photon absorption and emission events are coupled, and the core electrons tunnel through core-hole intermediate states to final states that contain an electron-hole pair in delocalized valence states. The regimes above and below threshold are quite different, and both allow the crystal momentum resolved band structure of a material to be probed. In this presentation, I will discuss how these effects manifest themselves in several material systems, focusing on graphite and hexagonal boron nitride. These results illustrate how the scattering differs in localized versus delocalized ones. I will also show results from more complex materials systems, including buried thin films, magnetic multilayers, carbon aerogels, and MPCVD diamond films.
9:40 AM		AS-WeM-5 Determination of Coordination Geometries Utilizing a Novel Differential NEXAFS Approach D. Ramaker, H. Sambe, X. Qian (George Washington University); W. O'Grady (Naval Research Laboratory) A novel differential NEXAFS approach has been developed which enables a determination of small distortions in coordination geometry (distortions in bond lengths and angles) about the absorber. The approach requires taking the difference, \Delta\\mu\, between the NEXAFS of the sample and reference material. First, this approach removes the fast changing background in the NEXAFS region. Second, in instances where only the bond angles change, contributions from the single scattering paths cancel, directly revealing the contributions from the double and triple scattering paths which normally transverse one of the altered angles, thus revealing their change. When one of the bond lengths is changed relative to the reference, single scattering contributions may dominate; but only for the bond length which changed. Fourier transforms of \Delta\\mu\ directly reveal the average of the altered path lengths between the sample and reference. More importantly, the amplitude of \Delta\\mu\ directly reflects the magnitude of the changes in the bond lengths or angles. FEFF6 curved-wave, multiple scattering, cluster calculations verify this approach, and help to interpret the data. Application of this technique has been made to the zincate ion in the Zn electrode, where small distortions from the tetrahedral geometry resulting from ion pair interactions can be interpreted directly. Small changes to the Ni site symmetry in the Ni electrode with charging will also be considered. This technique should be generally applicable to solids, liquids, and interfaces. *Supported by the Office of Naval Research
10:00 AM		AS-WeM-6 A Novel Reflectron Time of Flight Analyzer for Surface Analysis using Secondary Ion Mass Spectroscopy and Mass Spectroscopy of Recoiled Ions V. Smentkowski, A. Krauss, D. Gruen (Argonne National Laboratory); J. Holecek, J. Schultz (Ionwerks) We have modeled, designed, built, and tested a novel reflectron time-of-flight (TOF) analyzer, which is capable of performing surface analysis using both secondary ion mass spectroscopy (SIMS) and mass spectroscopy of recoiled ions (MSRI). All elements can be identified, with isotopic resolution, using MSRI. For ions of a given mass, the higher energy ions penetrate further into the reflectron before being turned around while the lower energy ions do not penetrate as deeply. By properly adjusting both the experimental geometry and the reflectron voltages, all ions of a given mass arrive at the detector simultaneously -- resulting in enhanced resolution vs linear TOF detection. SIM spectra are complicated by molecular fragments in addition to elemental ions. In MSRI, only elemental surface ions are detected. As a result, data analysis in MSRI is trivial. Being able to use a single analyzer to selectively obtain SIMS and MSRI data is unique and provides complimentary surface information. MSRI has a number of unique capabilities for surface studies. In-situ, real-time surface analysis can be performed during film growth at pressures greater than 1 mTorr at the substrate by differentially pumping both the ion source and the reflectron analyzer region. It has been demonstrated that the ratio of the positive-to-negative ion yield is phase specific; for example, one can clearly distinguish the different forms of carbon (diamond vs graphite vs amorphous carbon) during film growth. MSRI analysis of poorly conducting surfaces is possible. Work supported by the U.S. Department of Energy, BES-Materials Sciences, under contract W-31-109-ENG-38 and CRADA C9405001.
10:20 AM		AS-WeM-7 Real Time Reflection Electron Microscopy of In/InP(110) Cluster Dynamics M. Gajdardziska-Josifovska, M. Malay (University of Wisconsin, Milwaukee); D. Smith (Arizona State University) Cluster growth dynamics is an important problem in both fundamental and applied surface science. Further progress in this field depends on development of suitable real-time imaging techniques which, in addition to statistical information, would also detect the size and shape evolution of individual clusters. Recently we showed that reflection electron microscopy (REM) is capable of such real-time imaging. Here, we discuss the salient features of this technique, as applied to In clusters produced by non-congruent evaporation of P from InP(110) surfaces. REM employs high energy electrons reflected from a surface at grazing angle, resulting in severe image foreshortening in one direction. This effect is considered the Achilles' heel of REM. However, foreshortening provides a large field of view in one direction, which we show to be an asset for imaging sparse clusters. In addition, by developing a model for interpretation of REM images from clusters, we show that REM provides direct quantification of the height of a 3D cluster, along with one of its base lengths and the contact angle with the surface. A modified UHV transmission electron microscope was used for recording of REM images of heated InP(110) surfaces at video rates. Indium clusters were detected at 650=B0C. These droplets showed no preference for nucleation at surface steps, and the steps appeared stationary throughout the annealing process. Two distinct types of In cluster growth rates and shape evolutions were detected. Contact angle and volume above the InP(110) surface were calculated from the dynamic data. The change of contact angle with time provides evidence for sub-surface cluster etching.
10:40 AM		AS-WeM-8 Use of an IR Laser for Microscopic Characterization of Surface Wettability R. Worley, L. Tender, G. Lopez (University of New Mexico) An optical microscope equipped with an infrared laser focused through the viewing objective can be used to characterize the wettability of micropatterned surfaces by using the laser to nucleate bubbles on the solid surface placed under water. We have used this technique for the characterization the wettability of micropatterned self-assembled monolayers (e.g., formed by microstamping or on interdigitated microelectrode arrays) formed on thin films of gold. By varying the surface composition of the organic monolayers, we found that there is a correlation between both the power necessary to nucleate a bubble and the power necessary to maintain a bubble of a fixed size and the hydrophobicity of the surface. Electrochemical measurements suggest that the power necessary to form bubbles was not sufficient to disrupt the structure of the organic monolayer. We propose this technique as especially useful for resolving microscopic patterns of self-assembled monolayers on gold surfaces.
11:00 AM		AS-WeM-9 Microcantilevers as Physical, Chemical, and Biological Sensors R. Warmack, T. Thundat, P. Oden, G. Chen, P. Datskos (Oak Ridge National Laboratory) Although the physics of adsorption-induced surface stress has known for almost a century, few attempts have been made to exploit this concept for sensors. We have demonstrated novel micromechanical sensors based on adsorption-induced forces on microcantilevers with ppb-to-ppt chemical sensitivity. In contrast to approaches that are based on resonance frequency variations due to mass adsorption, sensors based on chemical adsorption-induced surface stress can be two orders of magnitude more sensitive. Since the cantilevers are very thin they undergo bending due to chemical or biochemical adsorption-induced forces that can be measured with sub-nanometer resolution. Demonstrated examples include detection of vapors of water, mercury, and volatile organic compounds. Since the thermal mass of the cantilever is extremely small, they can be temperature scanned and cooled in few seconds enabling speciation, reaction-rate control, and regeneration. Physical measurements of temperature, viscosity, pressure, and optical and nuclear radiation are possible. Advantages of these sensitive devices include miniature size, simplicity, low-power consumption, potentially very low cost to manufacture, inherent compatibility with array designs, and the ability to operate in gasses and liquids.