Characterizing lens distortion to overlay accuracy by using fine measurement pattern

Previous work has shown that the lens distortion will contribute significantly to overlay error. Since the resolution has gone below 0.18 um with 70 nm overlay tolerance, the lens aberration prays a more important role in the tolerance budget. Originally, the line size of overlay measurement target is about 3 um, which is much larger than the circuit dimension. Because the smaller pattern is more sensitive to lens distortion, an overlay difference may exist between real devices and the measurement target. In this study, we compared the overlay measurement result of fine pattern and traditional box in box target. With strong lens distortion effect, a fine measurement target may result in an overlay value which is close to the image shift of real circuit. Several types of target design were examined using overlay measuring signal, and the COMA effect was detected by the method of line width measurement. Experiment showed that lens aberration might cause over 15 nm overlay displacement. Because the lens distortion is a random error, software should subtract it while analyzing measurement data. Finally, the difficulties in using a fine target were also discussed and possible solutions were derived to increase the measurement accuracy.