Patent ID: 8248617

Claim:
A system comprising: an interferometer configured to direct test light to an overlay target and subsequently combine it with reference light to form an interference pattern, the test and reference light being derived from a common source; a multi-element detector positioned to detect the interference pattern; one or more optics to image the overlay target on the multi-element detector; a base arranged to position the overlay target relative to the one or more optics; a translation stage arranged to vary a distance between the overlay target and the one or more optics to vary the relative optical path length between the test and reference light when they form the interference pattern; and an electronic processor in communication with the multi-element detector, wherein the overlay target comprises a first pattern and a second pattern and the electronic processor is configured to acquire interferometric information from the multi-element detector while the translation stage varies the distance and to determine information about the relative alignment between the first and second patterns based on an asymmetry of fringes in the interference pattern in a direction across the multi-element detector indicative of the relative alignment, wherein the asymmetry of fringes is attributable to an asymmetry of phase of the interference pattern.