Chromatic Aberration Correction of a Low-Voltage SEM using a Wien Filter: Adjusting the Correction Strength

Abstract

The resolution of state-of-the-art low-voltage scanning electron microscopes (LV SEM), which is currently limited by the chromatic and spherical aberrations of the objective lens, can be improved by incorporating an aberration correcting device. At present four different concepts are discussed in literature: Zach and Haider demonstrated that a quadrupole/octupole corrector can correct both chromatic and spherical aberration. Rose proposed a Wien filter for chromatic aberration correction, which has relaxed stability requirements. Recently, we reported a simplified version of this corrector and showed that a spherical aberration corrector can be integrated in a Wien filter. Henstra and co-workers suggested a purely electrostatic corrector that can correct both chromatic and spherical aberration.For all these concepts problems may arise when the lens-to-sample (working) distance for an aligned corrector is to be changed. in general, the corrector settings depend on the ratio Cc/f2, where Cc and f denote the coefficient of the chromatic aberration and the focal length of the objective lens, respectively. When the working distance is changed, this ratio is no longer perfectly matched to the corrector settings. The tedious realignments and readjustments, which then seem necessary, can be avoided by using a doublet objective lens as illustrated schematically in Figure 1.