| 摘要 |
A method for determining overlay between a first grating (110) and a second grating (120) on a substrate (100), the second grating (120) on top of the first grating (110), the second grating (120) having substantially identical pitch (P1) as the first grating (110), the second and first gratings forming a composite grating (110,120), the method including providing a first illumination beam (IB) for illuminating the composite grating (110,120) under an angle of incidence (β) along a first horizontal direction (D1) along the surface of the substrate, and measuring a first intensity (i+) of a first order diffracted beam (B+) from the composite grating (110,120); providing a second illumination beam for illuminating the composite grating (11θ!i2O) under the angle of incidence (−β) along a second horizontal direction (D2) along the surface of the substrate, wherein the second horizontal direction (D2) is opposite to the first horizontal direction (DI)1 and measuring a second intensity (i−) of a minus first order diffracted beam (B−) from the composite grating (110,120).
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