Evaluation of coma aberration in projection lens by various measurements

In this paper, evaluation of lens coma aberration in projection lens of a stepper by various measurement methods is described. The measurement methods were based on asymmetry of the printed images caused by coma aberration. We used three measurement methods. They are conventionally used method and two new methods. The three methods are: (a) measuring the CD difference between both ends of line-and-space, (b) observing the side lobe patterns using an attenuated phase shifting mask (a-PSM), and (c) measuring the registration error using overlay patterns that have assist patterns. These measurement patterns are printed on Si wafers by a KrF stepper (NA=0.55) with various sigma. The dependence of the feature size and sigma on the influence of coma aberration was easily measured by these methods. As each method has merits and demerits, proper use of each method is necessary. Then the influence of coma aberration was evaluated by method (a). The influence of coma aberration had two components and they were due to primary coma aberration and de-centering coma aberration respectively. We estimated the influence of coma aberration by simulation and got good agreement with the experimental results. The measurement methods we demonstrated here are applicable for lens evaluation of steppers by the users.