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Ion Projection Lithography is one promising candidate for a next generation IC technology. Within this field of research one of the most critical aspects is the development of open stencil masks. The stress formation during the mask fabrication process affects the critical dimensions (CD) of the structures to be formed. The stress measurement in the mask blanks is performed by the well known bulging method. The accuracy of the membrane bulging method depends on the correct description of the bulged membrane shape. The accuracy of the method can be improved by including the deviations from spherical approximation by correcting the membrane bulging formula. Additionally, the shape ofthe membrane deformation and the improved bulging formula are compared with FE simulations. The commonly used interferometer technique can not be used to determine the absolute zero point of the membrane (e.g. plane membrane). With the diffraction image technique the zero point is determined by the geometry. Additionally, by bulging of the membrane in two directions, the zero point can be deduced from the anti-symmetry ofthe pressure vs. deflection curve.

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