Patent ID: 8339595
Filing Date: 2012-12-25
Classification: G01N,G03F

Abstract:
1. A method for determining an overlay error between a first pattern on a surface of a substrate and a second pattern superimposed on the first pattern, the first pattern comprising a first grating and the second pattern comprising a second grating on top of the first grating, the second grating having substantially identical pitch as the first grating, the second and first gratings forming a composite grating, the method comprising: providing a first illumination beam for illuminating at least the composite grating under an angle of incidence such that the first illumination beam propagates along a direction that has a component along a first horizontal direction along the surface of the substrate, the substrate being in a fixed position; measuring, at an image plane, a first intensity of a diffracted beam from the composite grating; providing a second illumination beam for illuminating at least the composite grating under the angle of incidence such that the second illumination beam propagates along a direction that has a component along a second horizontal direction along the surface of the substrate, wherein the second horizontal direction is opposite to the first horizontal direction, the substrate being in the fixed position; and measuring a second intensity of a diffracted beam from the composite grating at the image plane.