Patent US 5,452,090

In a reverse darkfield (RDF) microlithography alignment system a confocal spatial filtering system discriminates against topographical features other than the alignment mark. A CCD detector array provides flexible confocal filtering via a pixel weighting matched to the optical system. The confocal filtering system employs empirical filter optimization accomplished by correlating stored images with resulting overlays. The empirical optimization of the filter reweights the CCD array and correlates back to measured overlay results. This not only enhances the proven process insensitivity of RDF systems, but combines it with the improved resolution and noise rejection of confocal imaging. Alternatively the means for confocal spatial filtering is comprised of a filter matched to the instantaneous image of an alignment target; for example, a double slit filter matched to the image of a single alignment mark edge.