Improvement of shot uniformity on a wafer by controlling backside transmittance distribution of a photomask

CD (critical dimension) uniformity on a wafer is affected by several factors such as resist coating, bake, development, etch processes, scanner performance, and photomask CD uniformity. Especially, shot uniformity or in-field CD uniformity is strongly dependent on scanner and photomask. CD error of a photomask and imaging error of a scanner lead to nonuniformity of in-field linewidth distribution. In this paper we propose and demonstrate a shot uniformity improvement method. The shot uniformity improvement method described in this paper utilizes the original shot uniformity map and dose latitude to determine the distribution of illumination intensity drop suitable for correcting CD error on the wafer. The distribution of illumination intensity drop is realized by controlling pattern density of contact hole pattern with 180° phase on the backside of the photomask. We applied this technique to several masks and it was found that global CD uniformity could be excellently improved by the method.