Corning Incorporated
Systems for and methods of measuring photomask flatness with reduced gravity-induced error

Abstract:

The methods disclosed herein include recording at near-vertical first and second measurement positions respective first and second interferograms of the photomask surface and defining a difference map as the difference between the first and second interferograms. Respective first and second normal forces on the photomask are also measured at the first and second measurement positions. The change in the normal force is used define a scaling factor, which is applied to the difference map to define a scaled difference map. A compensated flatness measurement with a reduced shape contribution due to gravity is obtained by subtracting the scaled difference map from the first interferogram. An interferometer-based flatness measurement system is also disclosed.