Why anamorphic lenses are so important to EUV and Quentin Tarantino.
What do Quentin Tarantino and ASML have in common? Anamorphic lenses.
The optical image created by an anamorphic lens is oval, rather than round, with different magnifications along the horizontal and vertical axes. Tarantino used 65mm anamorphic lenses to film The Hateful Eight, and some theaters are also using them to screen the movie. It’s the first fiction feature to use this format since 1966.
The history of anamorphic lenses in semiconductor manufacturing is much less distinguished, but Jan van Schoot and coworkers at ASML and Carl Zeiss believe that there are compelling arguments in their favor. To print sub-8 nm half-pitch features, they said, the industry will need EUV lenses with a larger numerical aperture than the current 0.33 NA standard. However, increasing the numerical aperture will increase the angle of incident light. Further, as EUV uses reflective optics, the outbound and inbound light paths at an element can cross. These two factors contribute to shadowing, reducing the image contrast. More source power may be needed to achieve adequate exposure, a serious concern since EUV throughput is already power-constrained.
Increasing the image magnification from 4x to 8x helps, by reducing the incident light angle. Assuming mask dimensions stay the same, however, increasing the magnification to 8x cuts the image field size to only 0.25x. With 4x as many exposure fields to cover the same wafer area, throughput decreases dramatically. The researchers estimated that the system overhead time — the time needed to simply move the mask and wafer stages — would increase by 16x. Worse, the throughput of such a system would be dominated by overhead time; increasing source power would offer little throughput benefit.
Which is where anamorphic lenses come in. The increased magnification is only needed in the horizontal (scanning) direction. So, the ASML/Zeiss group proposed an 8x/4x anamorphic lens system, giving the anti-shadowing benefits of increased magnification while reducing field size to 0.5x, rather than a quarter-field. Overhead time would still increase, but only by a factor of four, and would be offset by the advantages of improved image contrast.
The Hateful Eight is now in theaters, allowing audiences to evaluate the anamorphic lenses for themselves, but no such EUV lenses have been built. In simulations, though, 8x/4x anamorphic lenses achieved optical performance comparable to that of standard 8x high-NA optics.