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Journal of Electron Microscopy Advance Access originally published online on September 30, 2005
Journal of Electron Microscopy 2005 54(5):413-427; doi:10.1093/jmicro/dfi063
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© The Author 2005. Published by Oxford University Press on behalf of Japanese Society of Microscopy. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

A new evaluation method of electron optical performance of high beam current probe forming systems

Shin Fujita1,2,* and Hiroshi Shimoyama2

1 Production/Design Technology Center, Shimadzu Corporation, 1, Nishinokyo-Kuwabaracho, Nakagyo-ku, Kyoto 604-8511, Japan and 2 Department of Electrical and Electronic Engineering, Faculty of Science and Technology, Meijo University, 1-501, Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan

* To whom correspondence should be addressed. E-mail: fujita{at}shimadzu.co.jp

A new numerical simulation method is presented for the electron optical property analysis of probe forming systems with point cathode guns such as cold field emitters and the Schottky emitters. It has long been recognized that the gun aberrations are important parameters to be considered since the intrinsically high brightness of the point cathode gun is reduced due to its spherical aberration. The simulation method can evaluate the ‘threshold beam current Ith’ above which the apparent brightness starts to decrease from the intrinsic value. It is found that the threshold depends on the ‘electron gun focal length’ as well as on the spherical aberration of the gun. Formulas are presented to estimate the brightness reduction as a function of the beam current. The gun brightness reduction must be included when the probe property (the relation between the beam current lb and the probe size on the sample, d) of the entire electron optical column is evaluated. Formulas that explicitly consider the gun aberrations into account are presented. It is shown that the probe property curve consists of three segments in the order of increasing beam current: (i) the constant probe size region, (ii) the brightness limited region where the probe size increases as , and (iii) the angular current intensity limited region in which the beam size increases rapidly as . Some strategies are suggested to increase the threshold beam current and to extend the effective beam current range of the point cathode gun into micro ampere regime.

Keywords     electron gun, brightness, crossover, aberration, probe size, beam current

Received     29 April 2005, accepted 31 August 2005


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