A lithography method for forming a plurality of patterns in a photoresist layer. A phase shift mask including a plurality of transparent main features, a plurality of first phase shift transparent regions, and a plurality of second phase shift transparent regions is provided. Each transparent main feature is surrounded by the first phase shift transparent regions and the second phase shift transparent regions interlaced contiguously along a periphery of the transparent main feature. Each of the first phase shift transparent regions has a phase shift relative to each of the second phase shift transparent regions. An exposure process is performed to irradiate the phase shift mask with light so that the patterns corresponding to the transparent main features are formed in the photoresist layer.