Holger Maltor

Use of Nanocrystalline Ceria in EUV Lithography Optics Polishing

EUV lithography promises large gains in resolution as a result of the extremely short wavelength. However, the requirement of aspherical off-axis mirrors dramatically increases the challenge of the optics manufacture relative to refractive designs. For example, because of the short wavelength of only 13.5 nm, a homogenous roughness and RMS values of 2 angstroms and below are necessary for sufficient throughput and high uniformity on these parts, and these specifications can only be achieved obtained through complex polishing processes. Because of these exacting microroughness requirements, fabrication technology is being driven to the exploration of new areas. An example of one of these new technology areas involves the use of nanocrystalline cerium oxide made using a patented plasma arc process that produces particles with very well defined physical properties. Because of the unique manufacturing process, these particles have highly controlled surface chemistry which results in the ability to prepare extremely stable dispersions in water. As such, these dispersion are useful in a variety of polishing processes where a small particle and a tightly controlled particle size distribution are required to access increasingly stringent surface roughness requirements. Carl Zeiss SMT has evaluated a number of cerium oxide slurries manufactured by Nanophase Technologies Corporation for improving polishing processes. The objective was to obtain reproducible low roughness values over a wide range of spatial frequencies. Results show that a significant improvement of the surface roughness was achieved with Nanophase ceria slurry CE-6068 in all spatial frequencies.