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Title:The atom field ion microscope: Fundamental principles of operation
Author(s):Chambers, Robert Sloane
Doctoral Committee Chair(s):Ehrlich, Gert
Department / Program:Physics
Subject(s):atom probe field ion microscope
chemical composition of a surface
quantitative surface studies
probe-hole field emission
Abstract:The ability of the Atom Probe to characterize the chemical composition of a surface with atomic resolution has been investigated. The experimental techniques necessary for quantitative surface studies will be presented. Introduced to Atom Probe technology are the following capabilities! probe-hole field emission measurements of the work function allowing an independent means for chemical characterization of the surface, automatic temperature measurement and control of the sample down to ~ 200 K, and optical alignment of the sample with the mass spectrometer, eliminating all aiming difficulties in time-of-flight measurements. Then, the physical principles essential to the operation of the Atom Probe, field ionization and time-offlight mass spectrometry, are discussed in detail. First, the kinetics of field ionization are analyzed in three-dimensions, using the transfer Hamiltonian formalism. Quantitative estimates are made for the field ionization rates of atomic and molecular hydrogen and for those imaging gases commonly used in the field ion microscope, helium and neon. In addition, estimates are presented for the post-ionization probability of a molybdenum ion which has been field evaporated from the surface. The conclusion is drawn that post-ionization is not responsible for the occurrence of multiple charge states in the mass spectra of the Atom Probe. Rather, these charge states are the result of the field evaporation process itself" Next, the physical parameters determining the fundamental limits to the Atom Probe's mass and spatial resolution are investigated~ theoretically and experimentally. A universal calibration procedure for relating the measured flight times to actual mass-to-charge ratios is established. This simple procedure, independent of the Atom Probe's particular operating parameters, only requires measurement of the applied static voltage in addition to the time-of-flight measurements. A detailed analysis of pulsed field evaporation indicates that the optimum mass resolution of the time-offlight Atom Probe can be improved to resolve adjacent mass-to-charge ratios. A few experiments still remain, however, to establish quantitative values of the experimental mass resolution. Measurements of the field evaporation rates of tungsten in pulsed fields indicate that the physical mechanism determining these rates is considerably different from the mechanism in static field evaporation: the evaporation rate in pulsed fields (in the nanosecond range) is determined by a surface response to a dynamically increasing electric field instead of the magnitude of the field alone. Measurements of the Atom Probe aiming error for rhodium adatoms on a tungsten (110) substrate indicate that the Probe's spatial resolution can be optimized to be as good as, or better than, the resolution of the field ion microscope. These measurements also reveal that spatial resolution is determined less by the material being identified or the substrate to which it is bound than by the operating conditions of the Atom Probe. Although a few crucial experiments must be completed to quantitatively determine spatial resolution of the lighter adsorbates, the Atom Probe's ability to identify the chemical nature of a specified surface atom is clearly established.
Issue Date:1976
Genre:Dissertation / Thesis
Rights Information:1976 Robert Sloane Chambers
Date Available in IDEALS:2011-07-05
Identifier in Online Catalog:2935747

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