A unique alignment algorithm-hi-cell algorithm is presented. With the alignment scan curve generated from the dark field alignment system, a bi-cell detector implemented in software is used for calculating the misalignment. In the bi-cell detector algorithm the symmetry point over the alignment scan curve is calculated as the ideal alignment point instead of the peak value point or other characteristic value point. Results show that the alignment accuracy better than 0.11 μm (3σ) can be obtained. The princi