Swiss study shows photons aren’t the problem for hyper-NA EUV

Researchers at the Paul Scherrer Institute (PSI) have created 5 nm half-pitch line/space patterns with 13.5 nm EUV light. For comparison, the theoretical maximum resolution of ASML’s high-NA EUV tool is 8 nm half-pitch. The Swiss approach isn’t interesting for high-volume manufacturing, because it’s slow and can produce only simple and periodic structures rather than a complex chip design. It’s very useful, however, for early development purposes ahead of a new generation of EUV tools, which in this case would be the hyper-NA system ASML has hinted at.

The technique employed by PSI is called EUV interference lithography (EUV-IL), in which two EUV beams are made to interfere with one another to create periodic images. Previously, the Swiss researchers used diffraction gratings to generate patterns down to 6 nm half-pitch. This EUV-IL sub-variety has been extensively used in the development and evaluation of photoresist materials for EUV lithography.

Because of their low diffraction efficiency, EUV-IL gratings require relatively high exposure times, even when using a very bright synchrotron light source. This encourages thermal and mechanical drift, resulting in lower imaging quality.

The boffins at PSI set out to circumvent this issue with an alternative EUV-IL design, in which the interference pattern is created using two mirrors. Although the concept had been previously explored, no one has managed to go below 10 nm half-pitch so far. In Nanoscale, the researchers report 5 nm half-pitch patterns using 13.5 nm EUV light and 4 nm HP using 10.8 nm light. This proves that, even at this scale, the photons are “not the bottleneck for resolution,” the article concludes. “Instead, our work shifts the focus toward developing new types of photoresist materials and optimizing the existing platforms.”