Patent ID: 7551296

Claim:
A method for determining the focal position of at least two edges of structures on a substrate, comprises the steps of: acquiring a plurality of images of the at least one structure with at least one measurement window of a detector during the movement of a measurement objective in the Z-coordinate direction traversing a plane with optimal focus, and determining an intensity profile of the structure for each image: determining contrast values for each acquired image and for each edge of the structure located in the measurement window based on the acquired intensity profiles; and determining the position of the optimal focus in the Z-coordinate direction at which the contrast is at a maximum from the contrast values for each edge, wherein the position of the edge in the image is determined during image acquisition, that at the same time a reference position (X 0 , Y 0 ) in the plane created by the X-coordinate direction and the Y-coordinate direction is determined for each image, and that the edge positions in the image and the reference positions may be added to yield a structure edge position, and wherein a functional relationship between the edge positions of the structure and the Z-positions is determined by a fitted function, and wherein the positions of the several edges are each determined by the point of intersection of the position for the optimal focus and the respective functional relationship of the positions of the various edge positions of the structure.