Influence of a lift hole-equipped pin chuck on wafer flatness

A site flatness of less than 32 nm will be required when fabricating next-generation devices. A pin chuck is used to flatten a warped wafer. This chuck has three lift holes that are utilized when loading and unloading wafers. These holes deteriorate local flatness. This paper describes the influence of wafer thickness and the diameters of lift holes and chuck pins on a theoretical local deformation of a wafer above a lift hole. It clarifies that local deformation is not caused by deformation between pins and is instead caused by the pin sinking into the back surface of the wafer and the compressive deformation caused by the pin itself. Reducing the lift hole diameter by half, or doubling the diameter of the pin, decreases the raised amount above the lift hole to about 75% or 40%, respectively. In addition, a solution that can be used to distribute the concentrated load above pins without increasing the pin diameter is shown which involves decreasing the pin pitch. It has been determined that local flatness less than 10 nm can be achieved by using a pin chuck with a pitch of 1 mm.