Ronchi lateral shearing interferometry with binary encoded sinusoidal transmittance grating.

The double-grating Ronchi lateral shearing interferometry has attracted much attention as one of the wavefront detection methods of the lithography projection lens. A Ronchi grating is placed in the object plane of the optics under test. Its spatial frequency spectrum determines the interference superposition between the 0th and all odd-order diffraction wavefronts in the detection area. Multi-wave interference means more phase-shifting is required in phase retrieval, which compromises real-time detection accuracy and increases phase-shifting errors. The sinusoidal transmission grating only generates 0th and conjugate 1st diffraction orders. By replacing the object grating, the interference field will be reduced to a superposition of two coherent interference patterns. This simplification enables a significant reduction in the number of phase shifts. In this work, we analyze interference fields resulting from the introduction of sinusoidal gratings based on scalar diffraction theory for low-NA optical systems under test. Considering challenges in fabricating sinusoidal transmission gratings, we introduce the binary encoded methods to achieve closer adherence to target transmittance distributions. The proposed interferometry exhibits superior real-time capability and demonstrates higher resistance to phase-shifting error and environmental noise than the double-grating Ronchi system.