A method is described for designing an alternating phase shifted mask
(altPSM) by optimally selecting the width of phase shapes. The selection
of optimal phase shape widths is achieved by providing a lithography
metric that describes the relationship between phase shape width and the
target image dimension such that the metric, such as process window or
across chip linewidth variation (ACLV), is optimized. In a preferred
embodiment, ACLV is computed by Monte Carlo simulation by providing a set
of error distributions for lithographic parameters such as focus, dose,
lens aberrations, and the like. Alternatively, a lookup table of optimal
phase widths associated with target image dimensions may be provided. The
resulting altPSM is characterized by phase shapes having widths that vary
according to the widths of the target image dimensions.
