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A linearly/circularly polarized heterodyne light beam coming from a heterodyne light source with an electro-optic modulator in turn enters a modified Twyman-Green interferometer to measure the surface plane of a GRIN lens. Two groups of periodic sinusoidal segments recorded by a fast complementary metal-oxide semiconductor camera are modified, and their associated phases are derived with the unique technique. The data are substituted into the special equations derived from the Fresnel equations, and the refractive index can be obtained. When the processes are applied to other pixels, the full-field refractive-index distribution can be obtained similarly. Its validity is demonstrated.

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A robust and wafer-less wavelength selection methodology was proposed. An overlay calculation model considering the asymmetric bottom grating structure showed that additional intensities diffracted from the asymmetric structure caused the overlay error. An asymmetry factor was introduced to describe the intensity ratio of the original overlay mark and a mark with bottom grating only. Based on the simulation results, the optimized wavelength was selected by analyzing the wavelength at which there is the minimum variation of the asymmetry factor. Four cases were tested by the simulation, and the maximum overlay error of the optimized wavelength selected by this method was 0.21 nm.

No abstract