The electrostatic moving objective lens and optimized deflection systems for microcolumns

Abstract

The designs of the deflector and the final focusing lens in microcolumns are constrained by the extremely small physical size, and by the low energy of the beam. Because the overall column size and working distance are much smaller, the diameter of the deflection field that can be used without excessive increase in the probe diameter is also smaller. This field size can be improved by incorporating deflectors within the focusing lens. The theory of the "moving objective lens" can be extended to apply to electrostatic lenses and deflectors, and this can lead to greatly increased deflection field sizes when a vertical landing beam is required. The implementation of the moving objective lens requires a pre-deflector, as well as an electrostatic deflecting field proportional to the second derivative of the axial potential inside the focusing lens. The curvature of field and third order astigmatism must also be dynamically corrected by additional fields, and these can be provided outside the lens. The in-lens deflecting fields can be obtained by segmenting the lens electrodes so that they can provide both focusing and deflecting fields, and adding shields to shape the fields correctly. In the absence of dynamic correction, the relative magnitudes of the deflecting fields can be optimized to maximize the deflection field size both with and without vertical landing. In the cases that have been analyzed theoretically, the addition of in-lens deflectors offers increased field size in comparison with optimized prelens double deflectors. The importance of increasing the field size will determine the degree of complexity of the configuration that must be used. © 1996 American Vacuum Society.