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Design for immersibility

The digs for the annual SPIE Microlithography homecoming may have grown, but the new San      Jose location still barely contained the enormity of the challenges that the industry faces in the litho arena. Although there was the usual range of technical topics, two buzz themes emerged at this year;s event: design for manufacturability (usually referenced by its initials, DFM) and 193-nm immersion lithography (still referred to as "immersion" more than IML). These themes seemed to dominate discussion inside the conference rooms, poster receptions, and exhibit halls, as well as among the attendees milling about in the common areas.

Toward the end of the week, I interviewed an early champion of immersion and a true litho guru, TSMC's Burn Lin. I have seen Burn in action at other conferences and symposia many times over the years, and his technical acumen, outspokenness, and wit have always provided a welcome antidote to what were often dour and dry proceedings. When the industry faced daunting red-brick-wall showstoppers in its hell-bent pursuit of 157 nm as the bridge technology to the next generation, Burn was a voice in the wilderness, crying out for sanity and a thorough examination of the 193-nm wet approach. His wake-up-call presentation in mid-2002 (ironically at a 157-nm workshop) was a critical turning point in what has become the industry's full-court press to get immersion into production in the 2006–2008 time frame.

Burn described the sometimes-arduous task of building a coalition of the willing, both among the suppliers and the chipmakers. "One key point is that we had to convince the equipment suppliers and the materials suppliers to change their roadmaps. Of course it was not easy, but fortunately, TSMC is a big customer of ASML. . . .We had a lot of discussions between ASML and us about going into immersion. The argument I heard most often, almost from all suppliers, is that they have spent so much money [on 157 nm], hundreds of millions of dollars, so I asked them, 'are you still spending it?' They said, 'sure,' so I said, 'the sooner you stop, the sooner you stop wasting your money.' That was my counterargument.

"In order to convince the suppliers, actually I had to line the users up. It was no good if TSMC was the only company that said they needed that. All the users and all the infrastructure would need to be there. So I did have to spend a lot of time talking to other semiconductor companies to convince them that [immersion's] the way to go. Of course, it was not an easy task. But then we got positive feedback, won one company over, which helped me to win over another one, and also it helped to convince the supplier." 

Although he believes that immersion systems will eventually reach acceptable productivity levels, Burn told me that he questioned the wet tools' big price tags. "We are seeing the potential of the immersion scanner approaching the throughput of the dry scanner, even though now it's not the case. But we're not seeing any particular problem that [would prevent it from] approaching that of the dry scanner.

"But the tool itself is a little bit more expensive. It's more expensive than we had calculated. If you think about it, there's no expensive extra part in the scanner. The lens has the same elements and they cut the last element by half and fill it with water, so big deal. There's the water head and so forth, a few mechanical parts, no tremendous high precision required in those. We don't see a reason for extra cost. . . .But it is more expensive, I think probably because of trying to recover the development costs of immersion and maybe also trying to recover the development costs of 157," he added, laughing.

Burn had a lot more to say about the state of immersion during our chat, commenting on many of the technical challenges as well as updating TSMC's own IML roadmap. Look for more of our discussion in an upcoming issue.

Tom Cheyney

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