H. Stauch et al., IMPACT OF CHUCK FLATNESS ON WAFER DISTORTION AND STEPPER OVERLAY - COMPARISON OF EXPERIMENTAL AND FEM-RESULTS, Microelectronic engineering, 23(1-4), 1994, pp. 197-201
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.