Abstract
The fabrication of ULSI devices on a 12" silicon wafer requires a site flatness of within 0.1μm. However, the flatness is affected by the vacuum pin chucks used for planarizing wafers in optical lithography or for final polishing. This paper describes the contact between the pin tops and the wafer back-surface during clamping. Complete contact between the back-surface and the pin tops is difficult to achieve with a 775-μm-thick standard 12" wafer, even when the back is the mirror surface. The contact ratio between the wafer and the pin tops was measured. It depends on the wafer thickness. These experimental results closely agreed with theoretical results. The pin deformation that degrades the flatness was also analyzed. If the pressure loaded on all pins was uniform, the pin deformation is only 8nm. However, when the pressure loaded on each pin varied in peak to valley values by a factor of 16, the effect of the pin deformation on the flatness became significant.
