CD versus pitch across the slit for multiple 248-nm step-and-scan exposure tools

Proximity effects in optical lithography are under investigation for quite some time. Most of these studies focus on the understanding of the origin of the CD-through- pitch variations and are performed for a single point in the exposure field. Knowing the optical proximity effect, corrections on the reticle can be made to compensate for it (Optical proximity correction, OPC). However, because of the data complexity, corrections for a certain duty cycle are applied independent of location in the exposure field. In order to make CD biasing on the reticle cost effective, the proximity effect variations across the exposure field need to be small. The cycle of measuring the proximity effects, applying corrections to the reticle layout and measuring again on the wafer is very time-consuming. Therefore there is a big gain if proximity corrections, as determined on one individual system, can be applied to other exposure tools without extensive testing and modification of OPC. This implies that proximity effects need to be constant through the slit of an individual tool and constant from one tool to another. In this paper we will study the across slit of the lens and tool-to-tool variation of the optical proximity effect.