Null holographic test structures for the measurement of overlay and its statistical variation

Results are presented on the use of null wire segment holograms for the in- line assessment of mask alignment errors in the integrated circuit fabricat ion process. Process variations are detected by measuring the light intensi ty generated by a hologram designed to project a null image. To detect alig nment errors, the mask for the wire segment hologram (WSH) is distributed b etween two mask layers. If the two sets of diffracting structures defined b y the masks are transferred to the wafer with perfect registration, the res ult is an area of light cancellation (null) in the image plane. Increased m ask misalignment leads to imperfect wavefront cancellation, which is manife sted as an increase in light intensity in the null region. In order to char acterize misalignment under controlled conditions, the two portions of the holographic test structure were initially recombined into a single structur e but with intentional misalignment between the two portions designed into the mask. The technique was then used to characterize the alignment errors between two separate masks with the actual fabricated offsets measured usin g atomic force microscopy. Initial results indicate the technique is capabl e of resolving 0.1-mu m mask misalignment for a 1-mu m minimum feature proc ess.