Since accurate alignment is essential for projection lithography, an extended dual-grating based alignment scheme is proposed. This method is an extension of the basic dual-grating alignment model, the mechanism of which is explained to make it clear how the extended scheme performs in projection lithography. The framework of the extended alignment scheme for projection lithography is constructed, and the process of key parameter determination is then detailed. In both cases, a tiny shift of the wafer during the alignment process can be resolved by a conspicuous displacement or phase variation of corresponding fringes. Analytical results indicate that alignment is independent of the gap between wafer and mask, disturbance from the fluctuation in illumination can be neglected, and alignment resolution in sub-nanometers can be realized with this scheme.
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