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Optical Properties of LSI Mask Using Cover Substrate

In order to investigate the optical properties of an LSI mask that uses a cover substrate, we have designed a high-numerical aperture (NA), very low aberration reduction projection lens system. On the basis of this design, it was found that the wavefront aberration of the LSI mask that uses a cover substrate between the mask and projection lens is reduced by optimizing of the mask position and imaging position. A phase-shift mask (PSM) using a cover substrate was formed for the experiments. We measured the intensity profiles of this PSM by Zeiss aerial image measurement system (AIMS). It was found that the intensity profiles of this PSM are better than those of the conventional PSM. We have also found that an LSI mask using a cover substrate of specific thickness is able to achieve better image formation using the projection lens system.