It is known that PSM pattern edge (MoSiON/Qz boundary) of EA-PSM mask is the weakest point against Haze occurrence in real mass production. Based on the understanding of these phenomena, we have developed very efficient ways to protect PSM pattern edge from Haze defect formation even after normal SPM cleaning processes. Oxide layer formulated on the PSM pattern (including pattern top and side) is actively trapping chemical ions existing on the surface and inside bulk of mask substrate, preventing their motion or diffusion toward Haze defect creation during laser exposure. As a result, we are able to reduce cleaning frequency of each EA-PSM mask set without Haze issues and thereby dramatically expand their life time in real mass production.

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Haze issues are getting more serious since size of Haze defect printable on the water surface that could matter is decreasing further with reduced pattern size. Many efforts have been made to reduce the contamination level on the photomask surface by applying wet or dry processes. We have successfully reduced surface contamination down to subppb level for organic and inorganic chemicals. No matter how well the mask surface is cleaned, chemical contaminant cannot be perfectly eliminated from the surface. As long as contaminants exist on the surface, they are getting aggregated around certain points with higher energy to create defects on it during laser exposure. Also, the cleaned mask surface could be contaminated again during following processes such as shipping and storage.Here, we propose a new paradigm for Haze retardation where we severely decelerate defect generation and growth rather than eliminate chemical contaminants on the mask surface. We have made mask surface on which chemical contaminants are hardly accumulated to generate Haze defects even during laser exposure. By creating mask surface insensitive to chemical impurity level up to a certain degree, we are able to retard Haze occurrence much better than by reducing surface impurities down to sub-ppb level. This approach has another advantage of allowing a freedom for mask environment during the process of shipping, storage, and exposure.We further investigate how the treated mask surface should have strong resistance against chemical contaminant aggregation towards Haze defect generation around specific points with high energy.

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