Stitching insights towards high numerical aperture extreme ultraviolet lithography: an experimental study

Abstract

The anamorphic nature of the projection optics of the high NA EUV scanners results in a “high NA field” with a maximum size of 26 × 16.5 mm2 at the wafer level. Therefore, to create a die larger than the high NA full field, two images are stitched together. The so-called in-die stitching is enabled by a combination of design, process, mask, optical proximity correction (OPC), and scanner solutions. Various imaging effects toward the edge of the black border (BB) need to be considered and are discussed in this paper, supported with experimental stitching data using the ASML NXE:3400 EUV scanner at IMEC. The design enablement, consisting of creating a stitching band free of critical patterns, still needs to take various imaging constraints into account. The process enablement of “at-resolution” stitching is also discussed based on a representative experimental study at NA ¼ 0.33, in preparation for NA ¼ 0.55. For a pitch 28-nm array of vertical lines and spaces, optimum conditions are confirmed experimentally to create a robust stitch utilizing either a Ta-based or a low-n absorber. An example of a stitched and OPC’ed use case is shown. In the case of a low-n mask, the higher reflectivity is experimentally mitigated using sub-resolution gratings. We also quantify the imaging impact of the transition between the absorber and the BB in the stitching region. A better understanding of the key stitching effects discussed in this paper helps with the enablement of high NA EUV imaging solutions.