GoKawiilASML, Intel, Samsung, and TSMC are teaming up to drive the industry transition to 6×12-inch photomasks (reticles). This shift is paramount for High-NA EUV lithography, as the larger stencil would enable printing large chips in a single pass, instead of having to stitch smaller designs together, as ASML explained in a press release this week.
This kind of collaboration between chipmakers isn't entirely unheard of, but it is rare. But when they face an industry-wide challenge, they set aside their rivalry and join forces to move the industry forward. This happened several times in recent decades, first with the failed transition to 450-mm wafers co-funded by GlobalFoundries, IBM, Intel, Samsung, TSMC, and New York State, then with the EUV transition, which was spearheaded by Intel, TSMC, and Samsung.
Higher resolution comes with a nuance
High-NA EUV lithography is a major step forward from today's Low-NA EUV tools. With a numerical aperture of 0.55, High-NA systems can achieve an 8nm single-exposure resolution, compared with 13nm for 0.33-NA EUV scanners. The higher resolution enables chipmakers to pattern smaller, denser features in a single exposure, replacing complex Low-NA EUV multipatterning schemes with a single High-NA exposure. This can reduce the number of masks and process steps, shorten manufacturing cycle times, and potentially improve pattern fidelity and yields, especially on critical layers of next-generation process technologies.
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However, this improvement comes with a significant tradeoff. Conventional 0.33-NA EUV uses 4X reduction optics in both directions, which enables a 26×33 mm exposure field with standard 6×6-inch photomasks. By contrast, High-NA EUV uses 4X/8X anamorphic optics, so the same mask can only expose a 26×16.5 mm half-field, which is hardly a problem for client-oriented designs that are barely larger than 429 mm². However, large dies that fit within a conventional 26×33 mm EUV field must now be patterned using two High-NA exposures stitched together, or split into a multi-chiplet design.
Stitching is a workable near-term solution that all chipmakers, including Intel, Samsung, and TSMC, use, but it comes with multiple drawbacks. First, it reduces the throughput of ASML's Twinscan EXE:5200B scanner from up to 175 wafers per hour for half-field exposures to around 125 wafers per hour when stitching is used. Secondly, chip designers must account for the stitching boundary, which means additional design rules and reduced floor planning freedom.
Finally, the two exposures must be aligned with extreme precision so that features crossing the boundary connect properly. Even tiny alignment errors can distort lines and vias, or compromise interconnects and thus potentially create defects and lower yields. Such yield loss is very expensive in the context of large CPUs and GPUs produced using Low-NA EUV systems. If yield is lost on more expensive High-NA EUV tools, the costs will be even higher, which greatly lowers the appeal of using these scanners.
New photomasks are needed
A Lego version of an ASML Lithography machine. (Image credit: ASML)
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