Study of quadrupole-octopole C c corrector for the large gap HREM

Abstract

Summary We have considered several aspects of a quadruplet of quadrupole-octopoles aimed at the Cc-correction of a high resolution TEM with large-gap pole piece and low beam energy. The strategy of adding appropriate numbers of octopoles to balance the third order aperture aberrations and correct coma is discussed in detail. We also numerically studied the behavior of several resolution-limiting factors as functions of the focal length and chromatic aberration of the objective lens. For the purpose of comparison, the calculation used on two models: the Gaussian thin lens model and rectangular thick lens model, as extreme cases of the real quadrupole-octopole field distribution. We conclude that for a moderate electrode potential of the electrostatic quadrupole, its power stability poses the the most severe limit on the achievable resolution. It is expected that as long as the power instability of its electrostatic quadrupoles can be maintained below 0.1 ppm, a quadrupole-octopole quadruplet Cc corrector featuring thick field distribution can provide a correction of Cc up to 6.5 mm without inducing any other aberration effects.