CPC G03F 9/7088 (2013.01) [G03F 9/7076 (2013.01)] | 13 Claims |
1. A method of alignment, the method comprising:
providing an alignment mark formed on or in a planar substrate, the mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell divided into adjacent first and second portions along a direction of periodicity, the first portion having a first effective refractive index and a first length along the direction of periodicity, and the second portion having a second effective refractive index, which is lower over its optical path compared to the first effective refractive index, and a second length along the direction of periodicity;
illuminating the alignment mark with radiation having a predetermined wavelength, wherein the effective refractive indexes and lengths of the portions of the alignment mark are configured to provide: a) an optical path length of the unit cell in the direction of periodicity that essentially equals an integer multiple of the predetermined wavelength, and b) an optical path length of the second portion in the direction of periodicity that is selected from 0.35 to 0.45 of the predetermined wavelength, such that scattering occurs mainly by excitation of a waveguiding mode in the periodic structure;
detecting radiation scattered by the alignment mark arising from the illumination; and
determining a position of the alignment mark using the detected radiation.
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