US Patent 11359916 Darkfield imaging of grating target structures for overlay measurement

A metrology system may include an illumination sub-system to illuminate a metrology target on a sample with illumination having a symmetric off-axis illumination profile, where the symmetric off-axis illumination profile is symmetric along one or more measurement directions, and where the illumination sub-system provides illumination from opposing angles in the symmetric off-axis illumination profile at least one of simultaneously or sequentially. The metrology target may include a first periodic structure on a first layer of the sample and a second periodic structure on a second layer of the sample. The metrology system may further include an imaging sub-system to generate images of the metrology target formed using two non-zero diffraction orders from each point of the symmetric off-axis illumination profile. The metrology subsystem may further determine an overlay error indicative of alignment between the first layer and the second layer based on the one or more images.