Patent ID: 7889323

Claim:
A wafer stage comprising: a wafer chuck adapted to hold a wafer during a fabrication process, the wafer chuck comprising a body, a plurality of chucking protrusions disposed on a central portion of an upper surface of the body and adapted to contact the wafer when the wafer is seated on the wafer chuck, and an outer edge ring comprising concentrically disposed first and second edge rings disposed on the upper surface of the body and adapted to contact an outer edge of the wafer; lift pins adapted to pass through the wafer chuck, move vertically, and support the wafer as the wafer is placed on the wafer chuck during a wafer loading operation before the fabrication process, and as the wafer is removed from the wafer chuck during a wafer unloading operation after the fabrication process; a vacuum unit adapted to apply suction to a first portion of the outer edge of the wafer through a plurality of vacuum holes formed through the first edge ring to seat the wafer on the wafer chuck; an air expulsion unit adapted to expel air towards a second portion of the outer edge of the wafer through a plurality of air expulsion holes formed in the second edge ring to prevent the wafer from sagging during the wafer loading operation; and a controller adapted to control operation of the air expulsion unit such that air is expelled towards the second portion of the outer edge of the wafer only after suction is applied to the first portion of the outer edge of the wafer, and such that suction on the first portion of the outer edge of the wafer and expulsion of air towards the second portion of the outer edge of the wafer are coincidently applied during the wafer loading operation.