Patent ID: 7477396

Claim:
A method for measuring overlay error, comprising: A] placing a first target component on a first layer of a substrate; B] placing a second target component on a second layer of a substrate, with the first target component and the second target component forming a target; C] acquiring an image of the target; D] measuring light intensity along a first scan line of the image; E] calculating a first symmetry value of the first scan line based on the measured light intensity along the first scan line; F] measuring light intensity along a subsequent scan line of the image; G] calculating a subsequent symmetry value of the subsequent scan line of the image based on the measured light intensity along the subsequent scan line; H] repeating steps F and G to obtain a symmetry value for substantially all scan lines of the image; and I] calculating offset between the first and second target components using the minimum or maximum symmetry value from the symmetry values determined in steps D-H.