关键词: Overlay(OVL), T 分布, 卡方分布

Abstract: In semiconductor manufacture 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]. Gener-ally, most control limit settings are based on engineering judgment with a tolerance of about 1/3 critical di-mension, and no statistical control assurance is implemented. The goal of this study is to find a statistic method for inline OVL chart control limit setting and thereby set up a reasonable control of OVL chart. It is based on a study of the statistic method “t-distribution & chi-squared distribution”. By studying the Inline OVL data and standard deviation data, the data are offside data which does not follow normal distribution. Control charts of these data are not proper to be controlled by n*sigma, using t-distribution and chi-squared distribution to control OVL chart is strongly recommended here. New control limits are calculated using this method and reasonable OCAP ratio is obtained.

Key words: Overlay (OVL), t-distribution, chi-squared distribution