Phase-shifting effect of EUV masks with various absorber thicknesses has been studied both by simulations and experiments. In EUV lithography, masks with 180 phase shifting absorber work like embedded attenuated phase-shifting masks. At 66nm thickness of TaN/TaON absorber, 180 degree phase shifting can be achieved in theory. Based on the experiments, we observed that the true180 degree phase shifting can be achieved with absorber thickness between 66 and 76 nm. In this paper, phase shifting impact of the various thickness absorbers has been characterized. Imaging performance of masks with 51 nm, 66 nm and 76 nm thick absorber has been experimentally compared. The process window of various thickness absorber masks are rigorously studied.