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'The End of Optical Lithography' has arrived - now what?

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CPTF 2012 The optical lithography that etches the chips in your digital devices is reaching its limits, but exactly when its oft-touted replacement – extreme ultraviolet lithography, commonly known as EUV – will be ready for prime time remains unclear.

"There are still some technical challenges which, of course, lead to a certain degree of uncertainty as to when, exactly, EUV will become available," said IBM Distinguished Engineer Lars Liebmann, speaking at the Common Platform Technology Forum 2012 on Wednesday in Santa Clara, California.

Liebmann should know. As part of IBM's semiconductor R&D team, he focuses on research into "design technology and co-optimization for sub-resolution patterning of leading-edge technology nodes" – meaning that he's figuring out how to etch chips with smaller and smaller features.

Currently, "leading-edge technology nodes" are etched with the optical immersion-lithography technology known as 193i, which TSMC and IBM started using at 45nm, and Intel began using at 32nm. Unfortunately, as chip process sizes are shrinking to 14 nanometer and beyond, 193i is reaching the end of its usability.

Tellingly, Liebmann's talk was entitled "The End of Optical Lithography", and its core focus was on how the transition would be made from 193i to EUV – and on the challenges of getting from here to there.

EUV is a radically different lithography technology from 193i optical, Liebmann explained. Not only is its wavelength significantly shorter at 13.5-14nm compared with optical's 193nm, but the light is derived not from the argon fluoride (ArF) excimer laser used for 193i but, instead, from a plasma light source.

In addition, the EUV operation happens in an extreme vacuum, and not in ambient atmosphere, and instead of photons being the etching agents, Liebmann said, "you're actually relying on secondary electrons to trigger the reactions."

Liebmann also explained a number of other differences – such as the use of reflective masks and a completely different etching chemistry, but his core message was that the move to EUV is not an evolutionary step as was the move to 193i, but instead a revolutionary change.

"I just mention that," he said, "because early on when some brilliant mind renamed projection x-ray lithography to EUV lithography, people got this impression that, 'Oh, deep EUV? It's pretty much the same thing.' No, it's a fundamentally different approach."

Significant challenges remain to be sorted out, he emphasized. For one, at its current state of development, EUV is currently "at least one, maybe two orders of magnitude too low on the intensity," he said.

Some of EUV's complexities are due to how its light is generated. "Exploding microdroplets of tin in a vacuum with a high-powered laser to make light is a very complicated process," Liebmann said, in quite the understatement.

Part of the problem in EUV's research and development process, he said, is that "until you get sufficient flux out of your lightbulb, it's very difficult to develop the chemistry" of the resists being etched.

Diving deeper into process technology geekery, he explained this problem, saying, "If you improve the sensitivity of the resist to make up for the low source power, now you get into shot noise, and you end up with very rough sidewalls."

What's an EUV boffin to do, eh?

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