IMPACT OF CHUCK FLATNESS ON WAFER DISTORTION AND STEPPER OVERLAY - COMPARISON OF EXPERIMENTAL AND FEM-RESULTS

Overlay accuracy is one of the most important lithographic topics for ULSI device production [1]. Significant contributions to the overlay l ike alignment accuracy and mask distortion are well known and subject of continuous investigations [2]. Below an accuracy range of 100 nm ad ditional influences must be taken into account [3,4], which were usual ly neglected for relaxed design rules. One of these influences is dire ctly related to wafer distortions induced by flatness inaccuracies of wafer chucks used for instance in X-ray or optical stepper. This impac t was characterised by investigating the elastic behaviour of 4'' wafe rs, fixed on a wafer chuck. The resulting strains and elongations in t he wafer surface were measured by exposure experiments and compared wi th additional FEM calculations. From these results we estimate that fo r design rules below quarter micron the flatness accuracy between diff erent wafer chucks has to be smaller than 0.25 mu m.