Patent ID: 11953450
Assignee: ASML NETHERLANDS B.V.
Field: Measurement (Instruments)
Classification: CPC G | IPC G

Claim 11:
12. A system configured to determine an overlay error between a first grating on a surface of a substrate and a second grating superimposed on the first grating, the second grating having substantially identical pitch as the first grating, and the first and second gratings forming a composite grating, the system comprising:
an illumination source configured to:
form a first illumination beam for illuminating the composite grating on the substrate under a first angle of incidence such that the first illumination beam propagates along a direction that has a component along a first horizontal direction along the surface of the substrate, the substrate being in a substrate position, and
form a second illumination beam for illuminating the composite grating on the substrate under a second angle of incidence such that the second illumination beam propagates along a direction that has a component along a second horizontal direction along the surface of the substrate, wherein the second horizontal direction is opposite to the first horizontal direction, the substrate being in the substrate position;
an image detector configured to measure a first signal associated with an intensity of a first order diffracted beam associated with the first illumination beam and a second signal associated with an intensity of a minus first order diffracted beam associated with the second illumination beam; and
a computer program comprising machine readable instructions to determine the overlay error based on the first and the second signal.