Reduction of mask error effect utilizing pupil filter in alternative phase shift lithography

A novel method to reduce a mask error effect in alternative phase shift lithography is proposed, which is derived from the analysis of the pupil plane images generated by masks with and without error. The analysis shows that zeroth order diffraction light, which usually shows zero intensity for precisely formed mask with the same areas for phased and non-phased openings, is generated by mask error both in phase and transmission. So, applying a pupil filter that cuts off rays passing through the central part of pupil is considered to be effective for the reduction of mask error effect. Impacts of the pupil filtering on imaging were evaluated by the numerical calculations of aerial images for various masks and results revealed that by applying a circular pupil filter with same radius as that of non-diffracted illumination image, tolerance in phase and transmission error for US patterns enlarged up to 20 degree and 50%, respectively.