Dependence of Aberration Coefficients of Triode Field Emission Gun on Electrode Geometries for Accelerating and Decelerating Modes

• Published in: Journal of electron microscopy 1995-10, Vol.44 (5), p.319-325
• Main Authors: Morita, Chiaki, Ohye, Toshimi, Shimoyama, Hiroshi
• Format: Article
• Language: English
• Subjects: aberration coefficient; accelerating lens; decelerating lens; field emission gun; gun geometries
• ISSN: 0022-0744; 1477-9986; 1477-9986
• DOI: 10.1093/oxfordjournals.jmicro.a051183

Numerical analysis of spherical and chromatic aberration coefficients have been conducted for a field emission gun consisting of a field emission tip, a 1st anode and a 2nd anode for both accelerating and decelerating modes. A comparison of the aberration coefficients has been made for different electrode geometries such as electrode thickness and electrode aperture angle under the condition of the infinite magnification and a constant voltage ratio of K ≡ V0/V1 where V1 is the tip-to-1st anode voltage and V0 is the tip-to-2nd anode voltage. A preferable electrode geometry under a constant voltage-ratio condition is somewhat different from that under a constant tip-position condition. Under a constant voltage-ratio condition, a combination of a thicker 1st electrode with a thinner 2nd electrode is preferable for the accelerating mode, while the opposite combination of electrode thicknesses is desirable for the decelerating mode. The electrode thickness and electrode aperture angle have been found to have very little effect on the chromatic aberration coefficient for both accelerating and decelerating cases. For the decelerating mode, a smaller distance between the 1st and 2nd anodes is especially suitable for smaller spherical and chromatic aberration coefficients, since the distance can safely be reduced without any serious problem of electric discharge between electrodes.