Desirable reticle flatness from focus deviation standpoint in optical lithography

We performed precise and systematic approaches to clarify what reticle flatness should be from the standpoint of focal deviation in optical lithography. The impact of reticle warpage on focus deviation was measured by an aerial image sensor to obtain any tiny shift of reticle-induced focus precisely. We clarified the criteria of reticle flatness after chucking. An optimum free-standing shape that would become the desired shape after chucking was obtained by simulation and an analytical approach. The flatness of the chucked reticle was found to be determined by both the free-standing plate shape inside the reticle holder and the shape of the plate facing the holder. Reticle flatness was redefined according to the results. Requirements with respect to the newly defined flatness for each technology node were clarified by focus budget analysis.