Abstract
A photomask technology, Fresnel diffraction mask (FDM) is developed in order to extend the usable depth of focus (DOF) in optical lithography on topographic substrates. By using FDM, a focal plane can be controlled. The Fresnel diffraction image near the photomask is reconstructed on a wafer through a projection lens. The ring-shaped aperture is used as a FDM to form a bright spot. The focal plane of the spot images can be successfully shifted more than 1 μm from the nominal focal plane of the optical system. The FDM produces two light intensity maxima at both sides of the nominal focal plane in the optical axis. By replacing the mask patterns on the highest and/or lowest wafer topography with FDM, the usable DOF on a topographic substrate can be extended because the original mask and FDM have different optimum focal planes.
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