For a lithography process, process windows are conventionally determined based on the amount of CD variation in a focus-exposure matrix (FEM). In a low-k1 region, however, a real process window can be smaller than is determined by the CD variation of FEM, due to a large mask error enhancement factor (MEEF). And the real process window cannot be determined by simply narrowing the process window obtained from a FEM, since MEEF itself is not a constant but a function of various process parameters. All the parameters which can affect MEEF should be considered carefully both in evaluation and in optimization of a real process window. Aerial-image base simulation was avoided in calculation of a process window because aerial-image based simulation cannot properly predict a process window even for simple 1-dimensional line-and-space patterns without introducing a fictitious variable like iso-focal bias, which cannot be extended to general 2-dimensional cases. In this study, a modified procedure for evaluation of process windows of critical layers has been proposed, and the process window was compared to the process window obtained by a conventional procedure. The proposed procedure has been implemented in our in-house lithography simulator to automatically process the evaluation of real process windows. Since the proposed procedure heavily relies on the accuracy of the lithography simulator, consideration of mask corner rounding effect and careful tuning of the physical properties of photoresists among others have also been included to guarantee the overall simulation accuracy.