Influence of wafer chucking on focus margin for resolving fine patterns inoptical lithography

The fabrication of ULSI devices requires a wide focus margin. A newly devel oped vacuum pin chuck has been used to decrease the margin reduction caused by wafer chucking. This paper describes the influences of pin support and back-surface waviness on site flatness during clamping. Pin support reduces the effect of dust, but degrades the site flatness. It is shown that the d eformation between pins with a 2-mm pin-pitch chuck for an 8 " standard waf er is negligibly small, but the back-surface waviness of the etched wafer d egrades the site flatness by 0.05 mu m in a 30-mm-square field. Reducing th e vacuum pressure and polishing for a short time only slightly improves the site flatness, Therefore, it is difficult to use the present type of etche d wafers and it is necessary to reduce wavinesses on both sides of wafers f or next-generation lithography of 0.13 mu m.