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To: FJB who wrote (24931)	5/31/2000 11:57:00 AM
From: BillyG	Read Replies (1) \| Respond to of 25960

Calcium-Fluoride Crystals Play Key Role in Lifting Chip Performance Virtually every PC-owner is familiar with the problem: Their brand-new computer becomes obsoletes by faster, easier-to-use models before they have even had a chance to make much use of it. This rapid progress in the fabrication of semiconductor circuit components is largely attributable to advances in optical microlithography, the technology employed for creating transistors and memory modules on silicon wafers. Boosting microchips' performance requires optics capable of generating ever finer features. Advances in miniaturization and the resultant steadily rising performance of integrated circuits (IC) are thus closely related to the spatial resolutions of the optical systems employed for their fabrication. As the state-of-the-art in microlithography technology has progressed, light of shorter and shorter wavelengths has been used to "write" features on microchips, which in turn has dictated the need for modifying the optics of the microlithography systems employed to suit those shorter wavelengths. Ultrahomogeneous optical glasses proved suitable for use with the 365nm line, the so-called "i-line," of the light sources that were used in the first half of the 1980s. However, fused silica, which withstands high power densities better and has a higher transmittance at short wavelengths, had to be used for the high-powered 248nm lasers that replaced them. From 193nm to 157nm The latest generation of microlithography systems employs lasers operating at 193nm, a wavelength that allows writing features as narrow as 100nm, i.e. 1/10,000 of a millimeter. This technology is currently being readied for mass-production applications in the microchip industry. Meanwhile, a German initiative supported by the Federal Ministry for Education, Research, and Technology launched a project aimed at implementing 157nm microlithography, which will lower the limit on microchip features to as little as 70nm. According to current project scheduling, the necessary basic research work will have been concluded by mid-2001, followed by process development and systems integration. Fabrication of the first semiconductor circuit components employing this new technology should thus start sometime in late 2003. Employment of 157nm microlithography is intended to close the expected technology gap between conventional fabrication of semiconductor circuit components employing photolithographic methods and the "extreme ultraviolet" microlithography employing wavelengths ranging from 11nm to 13nm. Employment of soft-X-ray sources will allow fabricating microstructures as fine as 30nm or thereabouts, the limit imposed by the grain structure of silicon. Calcium Fluoride vs Quartz However, quartz has a low transmittance at that wavelength. Yet another problem that arises is "compaction," which is a change of transmitted light due to localized density variations that cause spatial variations in refractive index. Therefore, lens elements fabricated from calcium fluoride are already being employed in this application. This problem becomes even worse at shorter wavelengths, which is why crystalline calcium fluoride is the only material suitable for use with lasers operating at 157nm, a wavelength that will allow fabricating the next generation of microchips, which will have features as fine as 70nm. No other material is able to withstand irradiation by short-wavelength UV as well as calcium fluoride. Its superior resistance to chemical attack in comparison to other fluorides is yet another reason for its employment in this particular application. Schott ML GmbH (http://www.schott.de) has experience in growing high-purity crystals. Employment of new types of vacuum furnaces now allows it to fabricate calcium-fluoride blanks in final diameters of up to 350mm and thicknesses in excess of 100mm. Their crystal lattices are uniform and free of any defects or dislocations, in spite of their large dimensions, which is why they are called "single crystals." The results of initial testing indicate that they exhibit stress-induced birefringence of less than 1nm/cm, a new record for optical quality. Demand for calcium fluoride is expected to rise sharply over the next 3 to 4 years. The semiconductor industry estimates that it will need as much as 50 metric tons of high-quality calcium-fluoride crystals annually within 3 to 4 years' time. by Dr Ewald Morsen, Schott ML GmbH; and Manfred Weber, Science Journalist, Germany (May 2000 Issue, Nikkei Electronics Asia) nikkeibp.asiabiztech.com