Coherency dependence of projection printing method using a phase-shifting mask

In a projection lithography system having fly-eye elements, a virtual source is created by an array of approximately mutually incoherent point sources. This paper describes a light-amplitude/phase simulation of the mask-projected image for a point source and discusses the dependence of the amplitude and phase on the point source's location. With the phase-shifting method, the mask images illuminated by point sources near the optical axis are better resolved, while point sources far from the optical axis blur small features. These results confirm that smaller-coherency-factor illumination is more effective for optical lithography using the phase-shifting method.