Dr. Liang C. Chao Profile

Dr. Liang C. Chao

at National Taiwan Univ of Science and Technology

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Proceedings Article | 19 January 2005 Paper

Zonal spherical aberration correction utilizing axial electrodes

Liang Chao

Proceedings Volume 5592, (2005) https://doi.org/10.1117/12.570825

KEYWORDS: Electrodes, Monochromatic aberrations, Lenses, Ion beams, Ions, Ray tracing, Plasma, Lens design, Gallium, Geometrical optics

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Spherical aberration is important in focused ion beam applications where large aperture angles are needed to obtain high beam currents because it results in large tails on the current density distribution. Merwe has shown that for coaxial lenses, negative spherical aberration can be found for rays pass through zonal regions. Merwe’s calculation is valid only for periodic or quasi-periodic lenses and requires a constant axial potential distribution. We have calculated zonal focusing properties of lenses with axial electrodes using nine-point finite difference method and direct ray tracing. Our calculation results indicate that an axial electrode protruding partially into the lens can correct the spherical aberration. When a three-element electrostatic lens is operated at deceleration mode, the introduction of an axial electrode creates zonal regions where the spherical aberration is negative. At deceleration mode, the induced surface charges on the axial electrode have an opposite sign relative to the primary beam. This is in agreement with our previous findings on the study of the correction of spherical aberration utilizing space charges. Same phenomenon was found when an axial electrode is used in conjunction with a cathode lens.

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