A method includes inspecting a mask to locate a defect region for a defect of the mask. A phase distribution of an aerial image of the defect region is acquired. A point spread function of an imaging system is determined. One or more repair regions of the mask are identified based on the phase distribution of the aerial image of the defect region and the point spread function. A repair process is performed to the one or more repair regions of the mask to form one or more repair features.