Advanced Lithography 2011 – A Prologue


In the long view, one thing is clear: the remarkable success of optical lithography at propelling Moore’s Law forward has been a long, steady ride. Moore’s Law has been lithography-limited since the early 1970s, so the steady progress in Moore’s Law over the last 40 years mirrors the steady improvement in resolution that optical lithography has been able to deliver in manufacturing.

But while the results of lithography seem to improve at an astonishingly steady pace, the path to get there has been anything but smooth. There are three big trends driving the improvements in resolution: lowering the wavelength of the imaging light, increasing the numerical aperture (NA) of the imaging lens, and being more clever at squeezing every bit of resolution that physics will allow (including the manipulation of the angle, phase and polarization of the light, as well as significant improvements in the performance of the photoresists used). Let’s look at each trend in more detail.

Lowering wavelength is an obvious way to improve resolution, but also a difficult one. A change in wavelength requires a change in the light source, the lens materials, and the photoresist – that is, almost everything. Since the early days of 436-nm light, we have steadily progressed to 365 nm, then 248 nm, and today’s standard 193 nm. Each change was extremely difficult, but ultimately rewarding. Lithography companies were born and were lost in the transitions. But the story is not quite so linear. The industry spent a fair amount of time and money to develop 157-nm lithography, only to abandon the effort as not worthwhile. And before that a major industry (and government) investment in x-ray lithography become a major failure (and a running joke among lithographers – will the new technology be our savior, or the next x-ray?). Today, the focus is on a another disruptive change in wavelength – to the 13.5-nm wavelength of extreme ultraviolet (EUV) lithography (the wavelength formerly known as soft x-ray). The outcome of that effort is yet to be decided.

Numerical apertures have risen from the Perkin-Elmer Micralign’s 0.16 to today’s best 1.35 (NAs great than 1 required the development of immersion lithography, an absolutely amazing technology). But 1.35 appears to be the limit. An effort to develop high refractive index lens and fluid materials was deemed too difficult and was dumped a few years ago. There’s no more room at the top.

Innovations like phase-shifting masks and off-axis illumination have coupled with significant improvements in photoresist performance and manufacturing process control to allow practical resolution to approach the theoretical limits. While I’m sure there are still innovations to be had (not to mention the dozens of interesting ideas that have been left behind), current performance is so close to the best that physics will allow that there is very little room left to squeeze.

Oh, and by the way, this forty years of (sometimes rocky) progress in resolution has come at no extra cost to make the chips (the cost per square centimeter of finished silicon has stayed about constant for 40 years). Improved yields, larger wafers, and much greater lithography tool throughput has meant that today’s US $50 M lithography scanner can still churn out chips that can be profitably sold for on the order of $10.

So what’s in store for the future of lithography? The only current effort that has the potential to keep us on track using the traditional three scaling approaches is EUV lithography. But the challenges for EUV are still immense, and I remain skeptical. An alternate path is the use of double patterning (or quadruple patterning!), but there the higher costs may prove limiting. This technology is now widely used for making Flash memory chips, but the extension to logic chips is hard.

It is an interesting time in the world of lithography technology – and progress in developing the next bit of technology will be anything but predictable. Companies are beginning to place their bets on competing approaches, and the certainty of success is low. And looming in front of us is possibly the ultimate physical limit: line-edge roughness caused by the stochastic nature of light and chemicals near the molecular scale. It’s fun, and frightening.

But there is one thing everyone in the lithography community agrees on: the place to go to follow the latest progress in the field is the SPIE Advanced Lithography Symposium, which starts Monday in San Jose. How far has EUV lithography progressed? How big are the remaining roadblocks? What innovations in double patterning might make this approach more practical? Has anyone made any progress in reducing line-edge roughness? I’m anxious to learn the answers. That’s why I’m here.