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A new spatial filtering technique is proposed for improving image contrast and depth of focus in projection photolithography. The technique is based on fractional Fourier domain filtering. Unlike the current pupil filtering method, the fractional filter can be placed at any location along the projection optical path other than the pupil plane. The theory of partial coherent diffraction combined with fractional Fourier domain filtering is presented. Phase filters for contact hole and line-space patterns have been designed. Computer simulation of complete imaging process including fractional Fourier domain filtering have been carried out. The simulation has demonstrated that the new filtering technique can significantly improve image fidelity, reduce the optical proximity effect and increase the depth of focus. Because of the flexibility in filter location, it is predicted to be easier to implement in a practical optical lithography system. © 2003 Elsevier Science B.V. All rights reserved.

Original publication




Conference paper

Publication Date





31 - 38