Correction of the distorted electron beam spot deflected by electrostatic crossed deflection fields

The aberration constants of a deflected electron beam passing through electrostatic crossed deflection fields due to four circular electrodes arranged round the optical axis are evaluated. The results show that the aberration of a distorted beam spot deflected by the crossed deflection fields is dominated by the second- and third-order astigmatisms, and aberration constants of their astigmatisms are much greater than those for a sequential deflection system with parallel plates. Various caustic patterns of the corrected deflection beam are graphically displayed by computer calculations.