Influence of pin chuck ring seals and polishing steps on wafer flatness

Abstract

A site flatness of less than 32 nm will be required when fabricating next-generation devices. To flatten warped wafers, a pin chuck is used. This chuck has lift holes for loading and unloading wafers and a ring seal around its periphery. It is known that the lift holes and seal deteriorate local flatness. This paper describes the theoretical local deformation of a wafer due to the ring seal and the steps between the pins and ring seals generated by polishing the pin chuck, and the influences of these deformations on wafer flatness. Reducing the pin pitch from 2 mm to 1 mm or doubling the diameter of the pin from 0.2 mm to 0.4 mm decreases the buckling amount due to the ring seal to approximately 25% or 40%, respectively. Increasing the diameter of the ring seal by only 2 mm decreases the buckling from 36 nm to 27 nm. However, deterioration of the wafer flatness caused by several tens of nanometer steps is very large and a step of 30 nm causes a buckling deformation of approximately 100 nm. However, it was determined that local flatness below 10 nm can be achieved by using a static-pressure seal-type pin chuck with a pitch of 1 mm.