1. A computer implemented method of autogenerating a phase shifted mask pattern from existing circuit design data by a computer comprising the steps of: inputting the existing circuit design data to the computer together with predefined input parameters; scanning by the computer the existing circuit design data to locate small features of the circuit design; defining by the computer areas that need phase assignment; making a first pass phase assignment by the computer for a critical feature and defining by the computer runs of interrelated critical features; propagating by the computer phase assignments through the runs; designing by the computer trim features for the phase shift mask pattern; and generating the phase shift mask pattern based on the designed trim features.