THE ELECTROSTATIC MOVING OBJECTIVE LENS AND OPTIMIZED DEFLECTION SYSTEMS FOR MICROCOLUMNS

The designs of the deflector and the final focusing lens in microcolum ns are constrained by the extremely small physical size, and by the lo w energy of the beam. Because the overall column size and working dist ance are much smaller, the diameter of the deflection field that can b e used without excessive increase in the probe diameter is also smalle r. 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 land ing 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 t he focusing lens. The curvature of field and third order astigmatism m ust also be dynamically corrected by additional fields, and these can be provided outside the lens. The in-lens deflecting fields can be obt ained 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 magnitu des of the deflecting fields can be optimized to maximize the deflecti on field size both with and without vertical landing. In the cases tha t have been analyzed theoretically, the addition of in-lens deflectors offers increased field size in comparison with optimized prelens doub le deflectors. The importance of increasing the held size will determi ne the degree of complexity of the configuration that must be used. (C ) 1996 American Vacuum Society.