Method of focus measurement

The present invention provides a method of focus measurement, wherein a specially designed mask pattern is used to measure the best focus of the exposure process, which is a binary mask layout of bar-like pattern built with BIB patterns including both inner and outside rectangles. When the focused position of the exposure process is different, the center of gravity of both the inner and outside rectangles transferred to the photoresist layer will have different deviations. By plotting this deviation amount against the focus position, an approximated second order equation can be obtained. After differentiating and letting it be 0, the best focus can be obtained.