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An alternating phase shift mask (alt. PSM) must be fabricated in such a way that imbalances in optical intensities are minimized. The mask structure must be optimized to obtain a balanced distribution of optical intensities and this means that the shifter thickness/quartz depth that corresponds to a phase angle of 180 degrees and the correct amount of undercutting should be estimated. There are two key points in the optimization of an alt. PSM. One is to find the optimum structure in terms of reducing the amount of undercutting. Narrower chrome (Cr) line widths are required for ArF laser lithography than for KrF laser lithography, so the undercutting must be restricted to prevent peeling of the Cr patterns, degradation of cleaning durability, and so on. Another key point is to investigate the effect of Cr line widths and pattern pitches on imbalances in the optical intensities. A variety of pattern pitches and Cr line widths are available from actual devices. All patterns, however, have same shifter thickness and amount of undercutting on each mask produced by a given mask fabrication process. It is thus necessary to study the effect on optical intensities of changes in Cr line widths and pattern pitches so that it is possible to optimize mask structures for a variety of patterns. From our simulation and experimental results, we found that an alt. PSM with vertical sidewalls has advantages in terms of reducing the amount of undercutting and is effective in the fabrication of sub 100-nm devices. We also discovered that imbalances in optical intensities vary periodically with Cr line widths. It was found that a structure for an alt. PSM should be optimized for each Cr line widths on these bases.
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