Intrinsic Birefringence Won't Halt 157 nm Lithography
Aaron Hand, Managing Editor -- Semiconductor International, 9/1/2001
In addition to the ongoing murmur regarding economic woes, SEMICON West this year was abuzz with concern about the recent finding of inherent birefringence in calcium fluoride (CaF2), a discovery that many lithographers feared could prove a showstopper for 157 nm lithography.
CaF2 is used for some of the optics in 193 nm lithography, but will be relied upon much more heavily in 157 nm lithography because it is one of the few optical materials transparent at that wavelength. Lithographers were already familiar with stress-induced birefringence in CaF2, and were prepared to compensate with relatively complex lens designs, noted Eric Johnson, vice president of technology at Nikon Precision Inc. (Belmont, Calif.). But intrinsic birefringence — which may inflict about 6× more birefringence than systems can handle — is very scary, he said. Phillip Ware of Canon USA Inc.'s Semiconductor Equipment Division (Irving, Texas) also expressed concern about the birefringence issue, noting that — after all the work that's gone into it — intrinsic birefringence had the potential to block the 157 nm effort.
Since then, however, several industry players have come together to assess the situation. Collectively at least, they decided that the inherent birefringence that John Burnett — a physicist with the National Institute of Standards and Technology (NIST, Gaithersburg, Md.) — demonstrated in CaF2 at deep ultraviolet (DUV) wavelengths is a sticky problem that is nonetheless surmountable. More than 50 technologists gathered in San Francisco during SEMICON West to discuss the issue. It certainly will make lens designs more complex, but the fact that this was discovered before all those lenses were made is a good thing, and the birefringence is not likely to be the showstopper people first thought it might be.
What intrinsic birefringence means, essentially, is that the CaF2 crystal refracts light differently depending on how the light hitting it is polarized. The implications, then, are that lithography systems would have to more carefully manage the polarization of the light as it enters a lens — not a small task considering the significant number of lens elements in any given optical lithography lens.
One upshot to come out of the meeting in July is that Optical Research Associates (ORA, Pasadena, Calif.) plans to modify its lens design software, CODE V, to compensate for the effects of intrinsic birefringence. Another technique that Burnett and his colleagues, Zachary Levine and Eric Shirley, have proposed to deal with the effect at a given wavelength is to mix CaF2 with barium fluoride (BaF2) and/or strontium fluoride (SrF2) crystals. BaF2 and SrF2 were also found to have intrinsic birefringence, but of opposite sign as the CaF2 birefringence (Table), so combining the crystals could null out the birefringence. The materials all have the same fluorite crystal structure, so could maintain the same cubic symmetry when combined. Crystal combinations such as Ca1-xBaxF2, Ca1-xSrxF2 or Ca1-x-yBaxSryF2 would be modified to deal with particular wavelengths.
| n × 107 (193 nm) | n × 107 (156 nm) | |||
| Material | Measured | Calculated | Measured | Calculated |
| CaF2 | -3.4 ±0.3 | -1.3 | -11.8 ±0.4 | -18 |
| SrF2 | +9.8 | +7.3 | ||
| BaF2 | +19 ±2 | +27 | +34 ±2 | +52 |
| (Source: NIST) | ||||
Although lithographers gearing for next-generation lithography may be breathing a sigh of relief over this latest wrinkle, they continue to be cautious about CaF2 supply in general. There is still some question, after all, about whether the industry will be able to supply the quantity and quality needed for 157 nm lithography.
Nikon, for one, plans to use its own production of CaF2 for some of its optics supply. Its major supply won't come from in-house, but the company will at least develop the intellectual property for CaF2 and optical coatings to help it get the quality it needs, Johnson said.
With its acquisitions of Optovac and more recently Tropel Corp., Corning Inc. (Corning, N.Y.) is putting the capital in place to tackle CaF2 production, according to Robert Sell, manager of marketing and strategy development. With its CaF2 facility in North Brookfield, Mass., and facilities in France, Russia and New York, the company is not slowing down its R&D on CaF2.
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