Novel Concepts for High-Throughput Electron Lithography

H.H. Rose

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

February 6, 2002