Metrology system using optical phase

Abstract

Misalignment error between two periodic structures such as two overlay targets placed side-by-side is measured. The two structures are illuminated by coherent radiation and the positive and negative diffraction beams of the input beam by the two structures are detected to discover the optical phase difference between the positive and negative diffraction beams. The misalignment between the two structures may then be ascertained from the phase difference.

1 . A method for detecting misalignment of two periodic structures placed next to each other so that they are periodic substantially along a first line, comprising: 
 illuminating a portion of each of the two structures using radiation that is substantially coherent, each of said portions having a dimension along the first line larger than the period of the corresponding structure;    detecting diffracted radiation signals from the illuminated portions of the structures to provide at least one output signal; and    determining from the at least one output signal a misalignment between the structures.    
   
   
       2 - 43 . (canceled)