Investigation of new overlay measurement marks for optical lithography

Abstract

New overlay measurement marks that consider the influence of coma aberration have been investigated. The concept behind the marks is the use of patterns that correspond to the actual device pattern rules, because the effect of coma aberration depends on the pattern size and pitch. Registration errors within one layer and overlay errors between two layers have been measured with the marks. The registration measurement results showed that the displacement of the marks with fine line patterns was different from that of the large box pattern. The difference was more than 20 nm. Then the overlay measurement marks were adapted to measure the displacement between a polysilicon gate pattern and an etched oxide layer. The results were compared to the scanning electron microscopy measurement of actual patterns and they showed good agreement. The repeatability of the marks was less than 5 nm and this is sufficient for quarter micron device fabrication. Overlay measurement marks having fine patterns can precisely measure the overlay errors of the actual fine patterns and can improve the overlay accuracy.