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

The fabrication of ULSI devices requires a wide focus margin. A newly developed 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 deformation between pins with a 2-mm pin-pitch chuck for an 8 " standard wafer is negligibly small, but the back-surface waviness of the etched wafer degrades the site flatness by 0.05 mu m in a 30-mm-square field. Reducing the vacuum pressure and polishing for a short time only slightly improves the site flatness, Therefore, it is difficult to use the present type of etched wafers and it is necessary to reduce wavinesses on both sides of wafers for next-generation lithography of 0.13 mu m.