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Increasingly, a finer finish polish is being sought in a variety of technologically advanced glass polishing applications. From the production of photomask blanks to the manufacture of precision lenses and prisms, improvements in surface planarity and the reduction of incidents of defectivity are critical to achieving higher levels of device performance. Nanophase Technologies Corporation (NTC) uses a patented plasma arc synthesis technique to produce nanosized cerium dioxide (ceria) particles. The small particle size, narrow particle size distribution, and unique crystal morphology of this ceria allow new performance benchmarks to be realized in the areas of decreased surface roughness (increased planarity), and reduced residual defects and subsurface damage. However, the success or failure of a nanocrystalline material in a particular application depends on the ability to form and maintain a stable dispersion with controlled rheology. The preparation of stable dispersions of nanocrystalline cerium dioxide particles is discussed with the following performance figures of merit - particle size stability, dispersion settling stability, and the ability to manipulate the isoelectric point and the zeta potential of the particles. Aspects of the dispersion specifically engineered for glass polishing, including pH and rheology characteristics, are presented. To illustrate the application of the nanocrystalline ceria dispersion, a specific example of an integrated program to reduce defects in the manufacture of quartz glass substrates and EUV (Extreme Ultraviolet) mask blanks at Schott Lithotech AG in Meiningen is presented. Substrates made of Low Thermal Expansion Materials (LTEM) require modified process formulations to achieve the flatness, roughness and defectivity goals of EUVL (Extreme Ultraviolet Lithography) mask blanks. Results of these processes for several LTEM options establish direction for material improvements, define key polishing process parameters, and guide the identification of important opportunities for future process adaptations. Results of the low defect polishing process for quartz glass substrates will be reported, along with activities directed toward adapting this process for LTEM substrates.
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