Detection and Printability of Shifter Defects in Phase-Shifting Masks II. Defocus Characteristics

One drawback of phase-shifting lithography lies in the high printability of shifter defects. A transparent object on masks (shifter defects) produces lead or lag of the optical phase, which causes an unique printability with respect to focus level. We investigated the printability of shifter defects on phase-shifting masks in out-of-focus conditions. Two types of shifter defects related to the phase angle were investigated: (a) a phase error which is a shifter layer having an incorrect thickness and (b) a phase angle defect which is a defect having a phase angle other than 180°. Phase error causes asymmetrical pattern profiles. It was found that the illumination coherence factor (σ) has a strong influence on the allowance of the shifter thickness (phase error). The effect of phase angle defects located in a large clear area and in small features was investigated. The phase angle defects have an optimum focal plane other than the best focal plane. A defect with a low phase shift located in fine patterns has high printability. In order to establish a mask fabrication process for production-worthy phase-shifting masks, further investigation in the detection of low phase angle shifter defects is necessary.