In previous editions of this column (Nov. 2002 and Feb. 2003), we examined the impact of phase errors on the printing of alternating phase-shifting masks (PSMs). While alternating-aperture PSMs are important, the overwhelming majority of phase-shift masks used today are attenuated PSMs (also called embedded phase-shift masks, EPSM). EPSM blanks are composed of two or three layers making up the absorber, such as molybdenum/silicon (MoSi), that are then processed in much the same way as a standard chrome-on-glass mask. With the exception of some R&D applications, almost all EPSM blanks have about a 6% intensity transmittance (and of course a nominal 180° phase shift compared to the quartz substrate). The interference of the light transmitted by the EPSM material and that transmitted by the spaces (the quartz) produces a sharper transition from bright to dark at the edge in the resulting aerial image.
展开▼
