A theoretical analysis is given of ultimate-resolution imaging in optical lithography by off-axis illumination, a common approach to image quality improvement. On this basis, the optimization of light-source aperture configuration is addressed. The types of aperture considered are the circular dipole, the circular qua-drupole, the simple annular aperture, the annular dipole, and the annular quadrupole. They are compared in terms of simultaneous transfer of transverse and longitudinal mask features.
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