Patent ID: 6338926
Filing Date: 2002-01-15
Classification: G03F

Abstract:
A focus measurement method, comprising:providing a mask having a pattern, the pattern composed of a first rectangle and a second rectangle located inside the first rectangle, the first rectangle and the second rectangle having the same center of gravity, each of the first and second rectangles having four bars, each bar having several columns of different pattern width but the same length, wherein adjoining patterns in the same column are separated by a first distance, and adjoining patterns belonging to different columns are separated by second distance, and wherein, for corresponding bars of first and second rectangles, the widths of the patterns belonging to different columns decrease in a first direction in the first rectangle, the widths of the patterns belonging to different columns increase in the first direction in the second rectangle; performing an over-exposure process with different focus settings in a photoresist layer on a wafer by using the mask; measuring an overlay shift of the center of gravity of the first and second rectangles in the photoresist under different focus settings; and fitting the function of the overlay shift to the focus setting by a second order polynomial equation and getting a fitting curve, then taking the derivative of the fitting curve so as to obtain the best focus, which is located at the point where the derivative is zero.