Novel Concepts for High-Throughput Electron Lithography
Correcting field aberrations and using multibeam comb lens systems are both
promising methods of increasing the throughput of electron lithography systems.
A proposed hexapole planator compensates for field astigmatism and image curvature
without introducing third-order distortion, coma, and off-axial chromatic aberration.
This planator is well suited to improving the performance of static electron projection
lithography systems (ELPS) provided that the energy width of the beam can be kept
sufficiently small. Connecting two planators via a telescopic round lens transfer
doublet results in a double planator that does not introduce any fourth-order
aberrations. Incorporating this corrector into a coma- and distortion-free projector
system (consisting of four round lenses) yields an electron optical anastigmat
that is free of chromatic aberration of magnification and of all geometrical aberrations
up to the fifth order. The attainable field of view of the corrected EPLS is limited
by the fifth-order field aberrations and by the axial and higher-order off-axial
chromatic aberrations. The limiting geometrical aberrations can be eliminated
by optical means, at least in principle, whereas the chromatic aberrations can
be appreciably reduced only by employing support-free masks. Realizing such masks
is of prime importance in developing high-throughput EPLS.
The multibeam variable-axis comb lens is a promising alternative to ELPS. This
electric lens employs a periodic arrangement of many electrodes in a direction
perpendicular to the initial optic axis. By proper dynamic variation of the potentials
applied to the constituent electrodes, the axis and the focusing field are shifted
such that stigmatic imaging is preserved. Owing to the translational symmetry
of the arrangement, the axis can be laterally displaced by an arbitrary amount.
Moreover, the periodic structure allows the formation and simultaneous shift of
many spatially separated lenses. By placing several comb lenses behind each other,
the entire wafer can be illuminated by at least 100 movable beams. Since these
beams are well-separated spatially, large total currents can be employed.
Formation of a quadrupole field by a twin comb electrode.
The equipotentials are computed by means of the surface charge method.
Quadrupole potential in the region between adjacent sheets