The present disclosure generally relates to semiconductor structures and, more particularly, to mask structures and methods of manufacture. The method includes determining a plane through a frontside surface and a backside surface of a mask, each plane representing a flatness of the frontside surface and the backside surface, respectively; subtracting, using at least one computing device, a difference between the plane of the frontside surface and the plane of the backside surface to find a thickness variation; generating, using the at least one computing device, a fitting to fit the thickness variation; and subtracting, using the at least one computing device, the fitting from the thickness variation to generate a residual structure for collecting a residual flatness measurement.