A lithography alignment measurement system using sinusoidal phase modulation with a vortex beam

Overlay accuracy is a key performance metric of lithography systems. Achieving high overlay accuracy requires precise alignment between the mask and wafer before exposure. Consequently, accurate position measurement of wafer alignment marks (AMs) is indispensable for lithographic alignment. Among various optical position measurement techniques, sinusoidal phase-modulated interferometry (SPMI) offers high-precision position measurement. However, the SPMI measurement accuracy is affected by the phase α of the interference signal. Based on the existing vortex beam alignment principle, a new SPMI-based acquisition and demodulation strategy is developed for lithographic alignment measurement. By switching the topological charge of the vortex beam, the phase α of the phase-modulated interference signal can be shifted away from low-accuracy regions. Experimental results demonstrate that the proposed method achieves a standard deviation of 0.65 nm for AM displacement measurements over a 500 nm displacement range.