Patent ID: 8670118

Claim:
A method for determining an overlay error between a first grating on a surface of a substrate and a second grating superimposed or the first grating, comprising: providing a first illumination beam having a first asymmetrical illumination mode for illuminating a composite grating under a first angle of incidence, the composite grating comprising the first and second superimposed gratings with similar pitch on the surface of the substrate, and the substrate being in a fixed position; measuring, at an image plane, a first intensity of a diffracted beam from the composite grating based on the first illumination beam; providing a second illumination beam having a second asymmetrical illumination mode for illuminating the composite grating under a second angle of incidence, wherein a horizontal directional component of the second illumination beam is opposite to a horizontal directional component of the first illumination beam along the surface of the substrate, the substrate being in the fixed position; and measuring a second intensity of a diffracted beam from the composite grating at the image plane based on the second illumination beam.