Overlay target and measurement method using reference and sub-grids

A metrology tool system comprising a plurality of metrology tools, each metrology tool comprising a stage for securing a lithographically produced semiconductor substrate, an energy source, a lens for directing energy from the energy source onto a surface of the substrate, a lens for capturing an image of a structure lithographically produced on the substrate, and an image processor for measuring distance between points on the image of the lithographically-produced structure on the substrate, each metrology tool stage including an alignment target mounted thereon comprising a first contrasting set of elements forming a first grid pattern having a distance between elements;

• the first grid pattern having a plurality of grid segments in at least one of the x and y directions, and at least one second contrasting set of elements forming a second grid pattern having a distance between elements, the second grid pattern having a plurality of grid segments in the x and y directions, the distance between the second set of elements being less than the distance between the first set of elements, the second grid pattern being disposed within one of the first grid pattern segments, each metrology tool being adapted to be calibrated by measuring a period of the grid segments in the grid patterns on the alignment target permanently mounted to the tool stage.