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High-index progress sprouts at SPIE

Posted on: 05-Mar-2007

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Don’t count out the high-index immersion lithography solutions. That was “Litho Guru” Chris Mack’s conclusion after the SPIE Advanced Lithography conference last week (Feb. 25-March 2nd.) 
Late last year at IEDM, ASML chief scientist Bill Arnold said that finding a high-index bottom lens element would be difficult. And timing is a problem: getting a high-index liquid to replace water, a high-index resist, and particularly the high-index lens material, would be difficult to implement in time for the 32-nm and 22-nm ITRS nodes (45-nm and 32-nm half pitches), Canon researchers said at SPIE.
Indeed, high-index immersion is a complex materials issue. The viscosity of the fluids is much higher than water, making it difficult to achieve the 500- mm/second scan speed needed to achieve high throughputs, IMEC researchers said. Unlike water, the high-index fluids cannot be discarded;  they must be cleaned and recycled within the scanner.  Researchers from JSR reported at SPIE on two recycling methods they have tried out. 
Also, because the optimally steep contact angle of the high-index fluids is difficult to maintain, it is possible that the entire wafer and stage would need to be submerged in order to maintain the proper meniscus.
Nevertheless, the high-index movement is regaining favor, partly because of better-than-expected progress with the bottom lens element. In a Thursday (March 1) session on high-index issues, researchers from Schott Lithotec revealed considerable progress with a high-index material, Lutetium Aluminum Garnet (LuAg), saying that they had improved the transmission of 193-nm-wavelength light by more than one order of magnitude “within only a few months.”
On the fluid side, DuPont researchers disclosed progress with high-index fluids with refractive indices of more than 1.7. “With good progress on the lens element, high index immersion with numerical apertures of 1.55 or above now seems possible,” the DuPont researchers concluded.
Can a high-index system be ready for evaluation in two years? Schott believes it can resolve LuAg crystal growth issues and be ready with prototype bottom lens elements by 2009.  DuPont and JSR scientists also appear confident.
What may still be needed is for one of the Big Three scanner companies to step up and commit to high-index scanners. Nikon said that kind of decision will come this year. ASML is studying the approach, but is heavily committed to EUV.
If a high-index form of immersion can extend 193-nm optical lithography, semiconductor lithography history says that most companies will choose that solution as long as possible.
The race between EUV and high-index immersion is back on, with billions at stake.

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