A system, method and recording medium are provided for generating patterns of a paired set of a block mask and a phase shift mask from a data set defining a circuit layout to be provided on a substrate. A circuit layout is inputted and critical segments of the circuit layout are identified. Then, based on the identified critical segments, block mask patterns are generated and legalized for inclusion in a block mask. Thereafter, based on the identified critical segments and the block mask patterns, phase mask patterns are generated, legalized and colored to define a phase shift mask for use in a dual exposure method with the block mask for patterning the identified critical segments of the circuit layout.