Stitching enablement for anamorphic imaging: a ~1μm exclusion band and its implications

Anamorphic imaging enables NA=0.55 in future EUV systems. At unchanged reticle size, the maximum on-wafer image size is reduced from the today’s full-field to a half-field of 26mm by16.5mm. Though most of the applications use a chip smaller than a half field, some of them still need a larger chip. To realize an on-wafer full-field with an NA=0.55 EUV system, two half-field images need to be stitched: abutting two images from a single reticle or from two different reticles, depending on the application. Using the ASML NA=0.33 NXE system at imec, “at-resolution stitching” on wafer is used to explore experimentally how CD and pattern placement are affected by abutting images of critical patterns located at the reticle edge. Using various test masks, a pattern placement error is measured within a 10μm range (1x) from the Black Border (BB) edge. Ideally it will be avoided by an adequate mask manufacturing process. We also measure a crosstalk between the two abutting images, that is attributed to a flare crosstalk, impacting the CD of critical patterns. Dummy tiles and a flare OPC need to compensate for this effect similarly to the correction inside the image. Finally, at short range, aerial images of the critical patterns at the very edge of abutting images can crosstalk. To avoid a complex OPC and tight specifications on the BB edge, an exclusion band is recommended to keep those aerial images from interacting. With the adequate placement solution at mask BB edge and with a flare compensation solution implemented, an exclusion band of about 1μm at wafer level is sufficient to support a robust stitching scenario for anamorphic High NA imaging. Its impact on various types of applications is discussed.