Alternating Phase-Shift Mask (Alt-PSM) is one of the key technologies for 0.15 micrometer or below rule device fabrication. But it is not yet widely utilized because of difficulty on phase controllability and defect controllability. Through more than 7 years at our commercial base operation and feedback from customers, we have improved our Alt-PSM both on its performance on wafer resist image and defect minimization. We have focused on the two elements, defects detection and repair, that made it difficult to control defect quantity on Alt-PSMs. In this paper, we describe results of experiment and optimization method that aims to assure zero defect on Alt-PSM for DUV lithography. We prepared evaluation plates. The plates contain series of programmed quartz defect on 0.6 micrometer line & space, each has phase errors of 60, 120 and 180 degrees at KrF wavelength. We used several latest models of inspection tools to evaluate phase shift quartz defect detectability, which are KLA353UV, STARlight, 9MD84SR(i). Micrion8000 was used as the repair tool. MSM-100/AIMS was used to evaluate wafer CD error of defect area before and after repair. As results, we found that inspection by short wavelength, especially by 9MD84SR(i), was effective for detection of phase shift defects, and that if this method were combined with STARlight inspection, detectability of the phase shift defect would be improved. With combination of this inspection method and our FIB repair, which is optimized for premeasured height of each phase shift defect by use of AFM, we would be able to supply zero defect Alt-PSMs for 0.15 micrometer design rule devices.