Lens and deflector design for microcolumns

The designs of the deflector and the final focusing lens for an electron beam microcolumn are constrained by the extremely small physical size (less than 4 mm overall), and the low energy of the beam (1 keV). Electrostatic lenses are more suitable than magnetic lenses because their field strengths are more practical. For simplicity of construction, the deflector is also electrostatic. It must be designed in concert with the focusing lens to achieve the largest possible deflection field at these low beam energies. Strategies have been developed to optimize 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 fabricated, and nonlinear constraints on the voltages and fields ensure that 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 been considered, although they may not be practical in a microcolumn. Another key factor for high performance is the alignment tolerances, since fabrication errors may be a more significant fraction of the lens bore than for conventional lenses. The aberrations produced by misalignment have been predicted for the different lens types, and designs can be selected which can tolerate electrode misalignments of approximately 1 μm for the probe sizes and working distances of interest. The field which can be covered with a prelens double‐deflection system is analyzed, and strategies for improving the deflection field are discussed.