| 摘要 |
A lithographic process is used to form a plurality of target structures (92, 94) distributed at a plurality of locations across a substrate and having overlaid periodic structures with a number of different overlay bias values distributed across the target structures. At least some of the target structures comprise a number of overlaid periodic structures (e.g., gratings) that is fewer than said number of different overlay bias values. Asymmetry measurements are obtained for the target structures. The detected asymmetries are used to determine parameters of a lithographic process. Overlay model parameters including translation, magnification and rotation, can be calculated while correcting the effect of bottom grating asymmetry, and using a multi-parameter model of overlay error across the substrate. |
