Patent ID: 8208140

Claim:
A method for aligning a substrate or reticle, comprising: receiving a raw alignment signal produced by illumination of an alignment mark on the substrate or reticle, wherein a first portion of the alignment mark has a first periodicity and a second portion of the alignment mark has a second periodicity that is different from the first periodicity; performing a transformation of the raw alignment signal based at least in part on a Hilbert transform to form a first signal having a first amplitude and a first phase, and a second signal having a second amplitude and a second phase, wherein the first signal and the second signal are associated with the first periodicity and the second periodicity respectively; selecting candidate alignment positions based on local minima in a difference between zero crossings of the first phase of the first signal and zero crossings of the second phase of the second signal; determining an alignment position of the substrate or reticle based on the selected candidate alignment position having a highest weighted combination of the first amplitude and the second amplitude; and aligning the substrate or reticle based on the determined alignment position.