High numerical aperture EUV exposure systems are coming — as soon as 2025 by some estimates. Though certainly a less profound change than the introduction of extreme ultraviolet lithography, high-NA lithography still brings a new set of challenges for photoresists and related materials.
With a higher numerical aperture, photons strike the wafer at a shallower angle. That requires thinner photoresist layers are needed to avoid shadowing. The upside is that a thinner resist layer reduces the risk of pattern collapse, as the aspect ratio of resist features is smaller. However, it also provides less protection for the wafer.
In addition, long etch processes used to create high-aspect ratio wafer features can erode the resist layer, ultimately degrading the transferred pattern. With less material a thinner resist also captures fewer photons, potentially making roughness and other stochastic effects worse.
Chris Mack, CTO of Fractilia, pointed out that stochastic defects are not an inevitable result of high-NA systems. Often those defects are the consequence of a degree of freedom that high-NA provides to lithographers. If feature size is constant, the wider aperture can increase contrast and reduce defects by delivering more photons to a given region. But if, instead, the wider angle is used to increase resolution, printing features that otherwise wouldn’t be reproducible at all, then stochastic effects will likely become worse.