To maintain throughput, ASML’s engineers had to seriously beef up the stages in high-NA scanners while maintaining stringent positional control.
The optics design of high-NA scanners required a major overhaul to accommodate the changes that accompany a move to a higher numerical aperture. Unmitigated, this would result in an exposure field size that’s four times smaller than what has been standard for years in the industry. The exposure field refers to the wafer area being exposed in a single exposure step. These steps need to be repeated many times to expose the entire wafer.
You can see the problem here: an exposure field that’s four times as small means that four times as many steps need to be completed to fill up a wafer, even if these steps are a little smaller. To not take a hit in throughput, this would require the scanner’s moving parts – ie the wafer and reticle stages – to perform these steps roughly four times as fast. “Impossible! That was never going to happen,” says Frits van der Meulen, function architect for EUV wafer positioning at ASML.
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