In this paper, a method for improving the process window is described by simultaneous source mask optimization (SMO). The method optimizes the source and mask of a critical pattern by optimizing the mask in the frequency domain. The minimum image log slope (ILS) is maximized at fragmentation points in the critical pattern while simultaneously maintaining the printing fidelity. The mask optimized in the frequency domain is then converted into a chromeless phase lithography (CPL) mask. The process window with the optimized source and optimized CPL mask doubles the aerial image contrast in comparison to an attenuating PSM with source optimization only. After optimizing the mask and source for a critical pattern, the remaining parts of the full-chip design are optimized with interference mapping. Another technique for optimizing the source for a full chip is presented in which the source is optimized by using the pitch frequency of the design. From the pitch frequency, the source is optimized by solving an integral equation for the first eigenfunction in which the first eigenfunction is calculated from the sum of coherent system (SOCS) representation of the transfer cross coefficient (TCC).