Patent ID: 7388675

Claim:
An interferometer for measuring surfaces of large thin, transparent wafers, comprising: a light source emitting light having a coherence length; a holding mechanism for holding a transparent wafer having a first surface located such that a normal to said first surface of said wafer is substantially horizontal; a diffusely reflecting background screen located inclined at an angle to said normal such that said emitted light is diffusely reflected towards said transparent wafer when positioned in said holding mechanism; and a reference flat surface located essentially parallel to said first surface of said transparent wafer when positioned in said holding mechanism and capable of being separated from a first surface of said transparent wafer when positioned in said holder by a distance less than or comparable to one quarter of said coherence length of said light source such that a first pattern of interference fringes representing a difference in flatness between said first surface and said reference surface is localized in a vicinity of said reference surface.