Measurement of lens aberrations by means of image displacements in beam-tilt series

Electron microscope image aberrations are determined by means of the beam tilt/image displacement method with respect to the precision required to obtain a resolution of 1 A. The method simultaneously yields all image aberrations to the fourth order and it is independent of the material used for the procedure. The experimental procedure using amorphous carbon is described and errors in measuring beam-tilt angles, magnifications and image displacements can be kept sufficiently small to achieve the required accuracy. The method is applied to determine aberration constants of a CM300 FEG/UT microscope with correction of the three-fold astigmatism. The coefficient of spherical aberration and the modulus of the three-fold astigmatism were measured to 0.60 (+/- 0.02) mm and 150 (+/- 50) nm, respectively. The beam tilt/image displacement procedure is also computer simulated using an amorphous model structure yielding the same values for the lens aberrations which are used for imaging. However, a coefficient of spherical aberration of 0.67 mm is obtained by applying the focus variation/diffractogram analysis on the same model.