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 chip fabrication process. Process variations are detected by measuring the light intensity generated by a hologram designed to project a null image. To detect alignment errors, the mask for the wire segment hologram is distributed between two mask layers. If the two sets of diffracting structures defined by the masks are transferred to the wafer with perfect registration, the result is an area of light cancellation (null) in the image plane. Increased mask misalignment leads to imperfect wavefront cancellation which is manifested as an increase in light intensity in the null region. In order to characterize misalignment under controlled conditions, the two portions of the holographic test structure were initially recombined into a single structure 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 by atomic force microscopy. Initial results indicate the technique is capable of resolving 0.1 μm mask misalignment for a 1 μm minimum feature process