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Lithography expert still not sold on EUV

Posted: 19 Dec 2012 ?? ?Print Version ?Bookmark and Share

Keywords:EUV? lithography? semiconductor roadmap?

Jim Blatchford, Texas Instrument's manager of front-end processing, had just finished negotiating for a leading edge 157nm photolithography system, but he was still worried about the unproven technology, so he went to a Society of Photo-Optical Instrumentation Engineers (SPIE) session on 157nm lithography.

When he arrived, however, he was surprised to find the room nearly empty. The rain was pouring down outside making a lonely pitter patter without the usual crowd noises to mask it. Something was wrong at 157nm.

"I remember it so well. We had just finished negotiations on our 157nm tool, but when I walked into a 157nm session at SPIE 2004 the room was almost completely empty. The sound of the rain pouring down on the roof was eerie. I knew right away that something was wrong," said Blatchford.

The mystery was solved when Blatchford found out that the attendees were crowded into the immersion lithography session for 193nm, where presenters were claiming that engineers didn't need to risk going to 157nm. All they had to do was immerse the reduction lens nearest the wafer in water to decrease feature sizes by its index-of-refraction (1.44). And with higher index fluids, it should be possible to extend 193nm lithography even further.

"What was most surprising to me was just how quickly everyone adopted immersion lithography. Once it was established as a feasible technology, everybody got on-board immediately," said Blatchford.

Long immersion history
Immersion lithography was so quickly adopted because it was based on the proven principles of immersion microscopy that stretched back to the 1600s when the English natural philosopher Robert Hooke predicted it. In the 1800s it was demonstrated by Giovanni Battista Amici an Italian astronomer and microscopist. And in the 1900s it was perfected to a science for microscopy.

The principle is that light bends at the interface to a liquid medium!the imitable bend stick in a glass of water!thus magnifying a microscope's image as it comes up through an immersed lens. Likewise, when light passes down through a lithographic reduction lens immersed in a liquid, it shrinks an image by its index-of-refraction. Today we know that combining immersion lithography with multiple-patterning!splitting up a mask into parts that can be exposed in separate steps!the resolution of standard 193nm lithography can be extended to 32nm. And with more sophisticated multiple patterning, and higher index fluids, 193nm lithography could go all the way down.

"Intel has already achieved the 32nm node with triple-patterning, and many engineers are talking about making it all the way to 14nm with multiple patterning," said Brooke. "There are also some other tricks you can play with pitch doubling, making it feasible to achieve 10nm with immersion lithography."

EUV is still being developed, and many semiconductor houses have expressed a willingness to move to EUV when it becomes available, but others are now predicting that immersion lithography, multi-patterning and high-index fluids, will allow semiconductors to make it all the way to the end of the International Technology Roadmap for Semiconductors at 8nm using 193nm lithography.

"It's hard to say whether some revolutionary new architecture will evolve that enables scaling of silicon beyond the end of the current roadmap," said Brooke. "But Intel has publicly said that even if EUV never works, it will be able to make immersion lithography work all the way to the end of the current roadmap."

Nikon, Canon and ASML have all been hard at work trying to make EUV work for almost a decade, since it uses light wavelengths as short as 10nm and thus theoretically could enable sub-five nanometre features only a few molecules wide. However, by then carbon-based electronics may start us down a new roadmap altogether!one that discards lithography's subtractive masks, in favour of additive self-assembly that begin with individual atoms from the bottom-up.

- R Colin Johnson
??EE Times

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