Measurement of axial geometrical aberrations of the probe-forming lens by means of the shadow image of fine particles

Abstract

A method using a shadow image of fine particles, proposed for the stigmation and the measurement of spherical aberration of the probe-forming lens, was further developed for the accurate measurement of spherical aberration and of non-rotationally symmetrical axial aberrations. From the radii of the rings of infinite magnification in a pair of shadow images, the coefficient of two- or three-fold astigmatism or axial coma can be measured together with the spherical aberration coefficient. A theoretical evaluation and an experiment carried out with a probe-forming lens of 1.5 nm resolution showed that the error of measurement is within several percent for the spherical aberration coefficient and is less than 30 nm for the coefficient of two-fold astigmatism. The three-fold astigmatism remaining after the correction of spherical aberration was also measured and was found to be of considerable importance in limiting the performance of the spherically corrected lens.