摘要 |
An inspection system is arranged to measure an overlay error by projecting a plurality of radiation beams, differing in wavelength and/or polarization, onto two targets. A first radiation beam is projected onto a first target and the reflected radiation A<SUB>1+</SUB> is detected. The first target comprises two gratings having a bias +d with respect to each other. The first radiation beam is also projected on to a second target, which comprises two gratings having a bias -d with respect to each other, and the reflected radiation A<SUB>1-</SUB> is detected. A second radiation beam, having a different wavelength and/or polarization from the first radiation beam, is projected onto the first target and reflected radiation A<SUB>2+</SUB> is detected and projected onto the second target and reflected radiation A<SUB>2-</SUB> is detected. Detected radiations A<SUB>1+</SUB>, A<SUB>1-</SUB>, A<SUB>2+</SUB>, and A<SUB>2-</SUB> is used to determine the overlay error.
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