Fresnel edge fringes observed in a lensless point projection field-emission electron microscope operating at 90 eV have been studied and found to be formally equivalent to the fringes observed in transmission electron microscopy (TEM) under weak scattering conditions at the edge of an opaque object. The tip-to-spectrum distance z1 plays the role of the objective lens defocus setting Δf in conventional TEM. The image magnification, effective source size, transverse coherence width, instrumental resolution, and source brightness are all obtained from an analysis of the fringe spacings and intensity. The quantum mechanical upper limit on source brightness, as well as relationships among beam brightness, coherence parameters, and degeneracy, are discussed, and the degeneracy measured from experimental Fresnel fringes.
The brightness of nanometer-sized field-emission-electron sources have been measured experimentally. Ultrasharp tungsten (111) single-crystal tips were fabricated in situ using Ne sputtering and field evaporation, and monitored using field ion microscopy. The average brightness of single-atom-terminated nanotips was found to be 3.3×108 A cm−2 sr−1 at 470 V, or 7.7×1010 A cm−2 sr−1 when extrapolated to 100 kV. These results show an improvement of about two orders of magnitude in source brightness over existing cold field-emission-electron sources, and produce a beam with greater particle flux per unit energy than those obtainable using current synchrotron/wiggler/undulator devices.
The electron optical properties of nanometer sized field-emission cathodes are examined for suitability as electron sources for low-voltage scanning electron microscopy, low-voltage transmission point projection microscopy, and low-voltage transmission and reflection electron holography. First-order electron optical properties, aperture and chromatic aberrations, and source coherence are computed using an all-orders numerical method, and compared with analytically computed properties where possible. The electron optical properties of planar emitters, conventional field-emission tips, and new nanotip structures are compared in the absence of space-charge effects. It is found that the spherical and chromatic aberrations of nanotips are dominated by their base structures and that beams produced by nanotips can be considered as totally coherent.
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