Study on inline Overlay control method

In this paper, we study on Overlay (OVL) control method for inline OVL measurement. In semi conduct manufacturing industry, Lithography (Litho) OVL is defined as the positional accuracy, a criteria parameter and to detect possible inline nonconformance from recipe and equipment shifts [1]. Generally, most control limit setting are based on the engineering judgment, tolerance about 1/3 critical dimension, and no statistical control assurance. The goal of this work is to find a statistic method for inline OVL chart control limit setting, by setting up a reasonable control of OVL chart, a new control method of OVL chart is defined. It is based on a study of the statistic method “t-distribution & chi-squared distribution”. Through studying on Inline OVL mean data and standard deviation data, we found these data are offside data, and does not follow normal distribution. Control charts of these data don't suit be controlled by n*sigma, we study and promote to use t-distribution and chi-squared distribution to control OVL chart. Then we calculate the new control limit with this method, which can provide reasonable control limit and OCAP ratio, thus control OVL process effectively.