ASYMMETRIC ELECTROSTATIC LENS FOR FIELD-EMISSION MICROPROBE APPLICATIONS

The properties of a physically asymmetric three-element electrostatic lens have been calculated numerically for voltage-symmetric and voltage-asymmetric cases. The lens has low chromatic aberration, Cc∞/f < 0.5, when operated in the voltage-asymmetric mode with voltage ratios Vfinal/Vinitial as low as 2. Calculations of beam diameter versus current with realistic field electron emission parameters of dI/dΩ=1 mA sr-1 and ΔE=2 eV at 5 kV emitter voltage show that at 30-kV final-beam voltage 250 nA current could be focused into a 0.25-μm spot at 5 cm working distance, with an emitter-lens separation of 1.5 cm. It is shown that while the two-element immersion lens is inappropriate for high-current microprobe applications, the three-element asymmetric lens is well suited for this work.