A method includes establishing a simulation process for simulating fabrication of a structure on a wafer. The simulation process includes multiple simulation steps for simulating multiple wafer fabrication steps respectively, and further includes a step of testing the structure that produces a result representing quality of the structure. Each of the simulation steps has a respective adjustable process parameter. The method further includes specifying a respective workable range for each process parameter and running the simulation process in iterations using a wafer process simulator until the result becomes optimal. During the running of the simulation process, every two consecutive iterations either adjust two different process parameters within their workable ranges or adjust a same process parameter at opposite directions within its workable range.