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In this paper we describe the off‐line quality control method and its application in optimizing the process for forming contact windows in 3.5‐μm complementary metal‐oxide semiconductor circuits. The off‐line quality control method is a systematic method of optimizing production processes and product designs. It is widely used in Japan to produce high‐quality products at low cost. The key steps of off‐line quality control are: (i) Identify important process factors that can be manipulated and their potential working levels; (ii) perform fractional factorial experiments on the process using orthogonal array designs; (iii) analyze the resulting data to determine the optimum operating levels of the factors (both the process mean and the process variance are considered in this analysis; (iv) conduct an additional experiment to verify that the new factor levels indeed improve the quality control.