TECHNICAL FIELD

The present disclosure relates generally to overlay metrology and, more particularly, to darkfield imaging of periodic target structures for overlay metrology.

BACKGROUND

Image-based overlay metrology systems typically generate overlay measurements based on a bright-field image of a metrology target. However, bright-field images are typically sensitive to aberrations in an imaging system. As a result, overlay measurements are also sensitive to the aberrations. Therefore, it would be desirable to provide a system and method for providing overlay metrology that is robust to optical aberrations.

SUMMARY

A metrology system is disclosed in accordance with one or more illustrative embodiments of the present disclosure. In one illustrative embodiment, the system includes an illumination sub-system to illuminate a metrology target 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. In another illustrative embodiment, the metrology target includes a first periodic structure on a first layer of the sample and a second periodic structure on a second layer of the sample. In another illustrative embodiment, the system includes an imaging sub-system to generate one or more images of the metrology target formed using two non-zero diffraction orders from each point of the symmetric off-axis illumination profile. In another illustrative embodiment, the system includes a controller to receive the one or more images of the metrology target from the imaging sub-system associated with the illumination of the metrology target with the symmetric off-axis illumination profile and determine an overlay error indicative of alignment between the first layer and the second layer based on the one or more images.

A metrology system is disclosed in accordance with one or more illustrative embodiments of the present disclosure. In one illustrative embodiment, the system includes a multi-channel illumination source to generate two or more illumination beams, wherein the two or more illumination beams are arranged to illuminate a metrology target on a sample in a symmetric off-axis illumination profile, where the two or more illumination beams are oriented to be symmetric along one or more measurement directions, and where the illumination sub-system provides illumination from opposing illumination beams forming the symmetric off-axis illumination profile at least one of simultaneously or sequentially. In another illustrative embodiment, the metrology target includes a first periodic structure on a first layer of the sample and a second periodic structure on a second layer of the sample. In another illustrative embodiment, the system includes an objective lens configured to collect light from the metrology target in response to the illumination beams. In another illustrative embodiment, the system includes an imaging sub-system to generate one or more images of the metrology target based on two non-zero diffraction orders from each of the two or more illumination beams. In another illustrative embodiment, the system includes a controller to receive the one or more images of the metrology target from the imaging sub-system and determine an overlay error indicative of alignment between the first layer and the second layer based on the one or more images.

A metrology method is disclosed in accordance with one or more illustrative embodiments of the present disclosure. In one illustrative embodiment, the method includes illuminating 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. In another illustrative embodiment, the metrology target includes a first periodic structure on a first layer of the sample and a second periodic structure on a second layer of the sample. In another illustrative embodiment, the method includes generating one or more images of the metrology target based on two non-zero diffraction orders from each point of the symmetric off-axis illumination profile in response to the illumination. In another illustrative embodiment, the method includes determining, with one or more processors, an overlay error indicative of alignment of the first wafer and the second layer based on the one or more images.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention.

