Diffraction based overlay metrology tool and method of diffraction based overlay metrology

A detection 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 detection 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 aperture stop configured to pass plus and minus first order diffracted beams;
• an image detector configured to receive the plus first order diffracted beam or the minus first order diffracted beam;
• and
• a plurality of lenses arranged along an optical path between the substrate position and the image detector.

A lithographic apparatus comprising a detection 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 second and first gratings forming a composite grating, the lithographic apparatus 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 the 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 a substrate position;
• an aperture stop configured to pass plus and minus first order diffracted beams;
• an image detector configured to receive the plus first order diffracted beam or the minus first order diffracted beam from the composite grating;
• and
• a plurality of lenses arranged along an optical path between the substrate position and the image detector.