LENS AND DEFLECTOR DESIGN FOR MICROCOLUMNS

The designs of the deflector and the final focusing lens for an electr on beam microcolumn are constrained by the extremely small physical si ze (less than 4 mm overall), and the low energy of the beam (1 keV). E lectrostatic lenses are more suitable than magnetic lenses because the ir field strengths are more practical. For simplicity of construction, the deflector is also electrostatic. It must be designed in concert w ith the focusing lens to achieve the largest possible deflection field at these low beam energies. Strategies have been developed to optimiz e the performance of symmetric and unsymmetric Einzel lenses together with deflection systems. The lens and deflector dimensions are varied subject to constraints which ensure that the final design can be fabri cated, and nonlinear constraints on the voltages and fields ensure tha t operation is practical. The properties of symmetric and unsymmetric Einzel lenses have been evaluated. Immersion lenses (in which the beam energy is higher in the column than at the target plane) have also be en considered, although they may not be practical in a microcolumn. An other key factor for high performance is the alignment tolerances, sin ce fabrication errors may be a more significant fraction of the lens b ore than for conventional lenses. The aberrations produced by misalign ment have been predicted for the different lens types, and designs can be selected which can tolerate electrode misalignments of approximate ly 1 mu m for the probe sizes and working distances of interest. The f ield which can be covered with a prelens double-deflection system is a nalyzed, and strategies for improving the deflection field are discuss ed. (C) 1995 American Vacuum Society.