Dynamic deformation of a wafer above a lift hole and influence on flatness due to chucking

A site flatness of less than 26nm will be required when fabricating next-generation devices. A pin chuck is used to flatten warped wafers. This chuck has lift holes for loading and unloading wafers and a ring seal around its periphery. The lift holes and seal deteriorate the local flatness. This paper describes the influence of local dynamic deformation on wafer flatness, which is caused by flattening of the warped wafer, and the deterioration in the wafer flatness due to lift holes and due to a polishing step. The repeatability of a stitching method was examined to accurately measure the flatness over a wide region; its value was less than ±3nm. This method was used to measure the local dynamic deformation. The fluctuation in the raised deformation above the lift hole when the wafer was stationary was ±4nm. On the other hand, when the wafer moved it was plus/minus several tens of nanometers and the local dynamic deformation greatly affects the flatness. In addition, it was demonstrated that the polishing step and the deformation difference between the seal and the pin greatly deteriorate the flatness; a 10-mm-diameter ring seal and a 2-mm-pitch pin array cause a wafer deformation of approximately 0.23μm.