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Abstract and Title of Jens Elstner's Master's Thesis |
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| Title: The Effects of Spherical Aberration Correction on the Resolving Power of Electrostatic Electron-Lenses Abstract: Electron microscopes play a crucial role in scientific research. The fast growing field of nanotechnology as well as biology and engineering rely on the possibility to make images of ever smaller structures. Nowadays Transmission Electron Microscopes achieve a resolution that is close to the ultimate diffraction limit because of the correction of spherical and chromatic lens aberrations. For Photo Electron Emission Microscopes (PEEM) aberration correction is not currently available but is under development by three groups. These are the SMART spectromicroscopy project at BESSY II in Berlin, PEEM3 at the Advanced Light Source (ALS) of Berkeley Labs and most recently the Electron Optics group at Portland State University. All of these groups employ an electrostatic electron mirror for the correction of spherical and chromatic aberration. The purpose of this work is to develop a model that is able to predict the resolution a spherical aberration corrected microscope can achieve. This is accomplished by first examining the effect of a spherical aberration correction on the electron path through the lens system. I derive the geometrical relationship for the focusing distance of an electron behind an uncorrected as well as a comparable corrected lens. This knowledge is then used to calculate the intensity distribution in various image planes behind the two lenses and the gain in resolution and contrast due to the correction. Three models are developed to calculate the intensity distribution in the image of a point-source, the image of an object with finite diameter and the image of a point-source including diffraction. Only the last model which includes diffraction is able to give a definite answer about resolution and contrast enhancement and the reasons for that are discussed. It is found that the type of spherical aberration correction examined here can improve the resolution as well as the contrast of an imaging system. |
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Ray Diagram Illustrating Spherical AbberationThe rays originating from the point source at z at the left do not converge at z'. |
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Intensity of the image of a point source without correction |
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Intensity of the image of a point source with spherical aberration correction |
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