Tilt-modulated spatial phase imaging method for wafer-mask leveling in proximity lithography.

We demonstrate a tilt-modulated phase imaging method to adjust the gap inconsistency for wafer-mask leveling in proximity lithography. Two gratings with close periods are etched on the mask and used as leveling marks. At the illumination of a monochromatic planar wave, the diffracted image of one grating is projected back onto the other one beside it through reflection at the wafer surface. Any wafer-mask tilts in two orthogonal sections are directly modulated into the phase distribution of the interference field and can be directly remedied according to the frequency and angle deviation of the two sets of fringes. Finally, wafer-mask leveling can be achieved at only one spot with preserved accuracy. Computational and experimental results confirm that tilts at the magnitude of 10(-3) rad can be readily resolved by this method.