Patent ID: 6064475
Filing Date: 2000-05-16
Classification: G03F,H01L,Y10S

Abstract:
A method of monitoring local focus accuracy, comprising:providing a wafer exposure system having a mask holder, a wafer holder, an objective lens having a principal optical axis, and a controller having data inputs and outputs, wherein said controller positions said wafer holder relative to said objective lens;providing a first number of wafers;selecting a wafer from said first number of wafers wherein said selected wafer has a number of image fields and a layer of resist formed thereon;placing said selected wafer in said wafer holder;providing a mask having an image formed therein;placing said mask in said mask holder;positioning said selected wafer so that, for each of said image fields, said layer of resist is a distance from said objective lens, said layer of resist is rotated a first angle about a first axis perpendicular to said principal optical axis, and said layer of resist is rotated a second angle about a second axis perpendicular to said principal optical axis and perpendicular to said first axis, thereby achieving optimum focus of said mask image on said layer of resist in each of said image fields of said wafer;determining the maximum value of said first angles, the minimum value of said first angles, the mean value of said first angles, the difference between each of said first angles and the mean value of said first angles, the maximum value of said second angles, the minimum value of said second angles, the mean value of said second angles, and the difference between each of said second angles and the mean value of said second angles;selecting another wafer for exposure if said maximum value of said first angles and said maximum value of said second angles are less than a first critical angle, the magnitude of the difference between each of said first angles and said mean value of said first angles are all less than a second critical angle, and the magnitude of the difference between each of said second angles and the mean value of said second angles are all less than said second critical angle, wherein said first critical angle is about 300 micro radians and said second critical angle is about 150 micro radians; andtaking corrective action if said maximum value of said first angles and said maximum value of said second angles are equal to or greater than said first critical angle, the magnitude of the difference between any of said first angles and said mean value of said first angles is equal to or greater than said second critical angle, or the magnitude of the difference between any of said second angles and said mean value of said second angles is equal to or greater than said second critical angle.