The main goal of the present research is to determine whether there is a significant difference between the aberrations of magnetic and electrostatic electron lenses when they are used in the same way. Comparison of the lenses was done in an electron-optical bench, set up to resemble a scanning electron microscope (SEM).
Five different lenses, two magnetic and three electrostatic, were compared. Each group included two lenses with different lens-field lengths (pole-face spacing in magnetic lenses and interior length in electrostatic lenses). The lenses were used in turn to focus the electron beam into a demagnified image, or probe. The shadowgraph method was used to determine the paraxial properties and the spherical aberration coefficients Cs' of the probes. Graphs of Cs' versus working distance (WD) were used to compare the spherical aberrations of the probes focused by the different lenses. The main difference was shown to be not the type of lens, but rather differences in the lengths of the lens fields. For the same WD, lenses of either type, with shorter lens fields, had higher values of Cs' than did lenses with longer lens fields. An exception occurs when the focal length fo goes through a minimum, causing C s to be lower for a short-field lens.
Theoretical models with the same physical basis for focusing as the respective lens types, but with analytical solutions for the paraxial image and lens properties, were introduced for comparison with the experimental lenses. Lowering the activation of the models brought the curves of model properties into agreement with the corresponding lens graphs. The reduction in model activation needed for agreement was greater for short-field than for long-field lenses, showing that short-field lenses had weaker paraxial fields and higher spherical aberration coefficients.
The analytical curves of lens properties obtained from the reduced activation models were used to evaluate the chromatic coefficients C C' of the probes. Curves of CC' vs. WD show that CC' is higher for the electrostatic lenses than for the magnetic lenses.
|School:||Portland State University|
|School Location:||United States -- Oregon|
|Source:||DAI-B 70/07, Dissertation Abstracts International|
|Keywords:||Electromagnetic aberrations, Electron microscope, Electron optics, Electrostatic lens, Focal properties, Magnetic lens|
Copyright in each Dissertation and Thesis is retained by the author. All Rights Reserved
The supplemental file or files you are about to download were provided to ProQuest by the author as part of a
dissertation or thesis. The supplemental files are provided "AS IS" without warranty. ProQuest is not responsible for the
content, format or impact on the supplemental file(s) on our system. in some cases, the file type may be unknown or
may be a .exe file. We recommend caution as you open such files.
Copyright of the original materials contained in the supplemental file is retained by the author and your access to the
supplemental files is subject to the ProQuest Terms and Conditions of use.
Depending on the size of the file(s) you are downloading, the system may take some time to download them. Please be