Download Microlithography Chapter 2 E Beam Lithography b

FIGURE 2.5. Electrode structure and relevant dimensions for a) LaB6 gun and b) thermal field
emission gun. The electrodes are circularly symmetric about the optical axis. The Wehnelt and
suppressor are biased negative with respect to the cathode, while the anode and extractor are
positively biased. [From Gesley, [17] 1989]
2.2.3 Electron Lenses
Electrons can be focused either by electrostatic forces or magnetic forces. Although electron lenses in
principle behave the same as optical lenses, there are differences. Except in some special cases,
electron lenses can be made only to converge, not diverge. Also, the quality of electron lenses is not
nearly as good as optical lenses in terms of aberrations. The relatively poor quality of electron lenses
restricts the field size and convergence angle (or numerical aperture) that can be used. The two types
of aberrations critical to EBL are spherical aberrations, where the outer zones of the lens focus more
strongly than the inner zones, and chromatic aberrations, where electrons of slightly different energies
get focused at different image planes. Both types of aberrations can be minimized by reducing the
convergence angle of the system so that electrons are confined to the center of the lenses, at the cost
of greatly reduced beam current.
A magnetic lens is formed from two circularly symmetric iron (or some other high permeability
material) polepieces with a copper winding in-between. Fig. 2.6 shows a cross-section through a
typical magnetic lens, along with some magnetic flux lines.