Computer simulation of hexapole aberration correctors

Abstract

The use of hexapole electron-optical elements to correct the spherical aberrations of the objective lenses of a low-voltage scanning electron microscope is investigated. Compared with the conventional quadrupole-octupole correctors, hexapole systems are simpler in design, easier to tune, and less sensitive to manufacturing imperfections and power supply instabilities. Two configurations of hexapole correctors, RHRHR and HRRH (where R and H stand for round lens and hexapole component, respectively), are considered. Both configurations considerably suppress the spherical aberration of the electron microscope objective lens but cannot correct chromatic aberrations. The second configuration possesses important advantages over the first one: it is mechanically and electrically simpler and also is easier to tune. In addition, as follows from our investigation, the hexapole electrode voltages in the second configuration are lower, the correction accuracy is higher, and the sensitivity to mechanical defects is lower. However, the chromatic aberration in the second configuration is somewhat larger.