Pattern-integrated interference lithography (PIIL) combines multibeam interference lithography and projection lithography simultaneously to produce two-dimensional (2-D) and three-dimensional (3-D) periodic-lattice-based microstructures in a rapid, single-exposure step. Using a comprehensive PIIL vector model and realistic photolithographic conditions, PIIL exposures for a representative photonic-crystal (PhC) 90 deg bend waveguide are simulated in the volume of the photoresist film. The etched structures in the underlying substrate are estimated as well. Due to the imperfect integration of the photomask within the interference pattern, the interference pattern is locally distorted, thereby impacting the PhC periodic lattice and potentially the device performance. To mitigate these distortions, a photomask optimization method for PIIL is presented in this work. With an improved photomask, pillar-area and pillar-displacement errors in the vicinity of the waveguide are reduced by factors of 3.3 and 2.7, respectively. Furthermore, calculated transmission spectra show that the performance of the PIIL-produced PhC device is as good as that of its idealized equivalent.