Programmed phase defects, at desirably specified sizes and known locations, for EUV multilayer blanks were successfully fabricated by the following newly developed simple technique; depositing Cr film on a 6025 glass substrate or a Si wafer, generating Cr patterns of isolated lines and/or dots by EB lithography, and depositing Mo/Si multilayer of 40-bilayers by ion beam sputtering over the Cr patterns. Thereby, programmed bump defects were created on the multilayer surface over the Cr pattern at the bottom. The programmed defects were observed by TEM and AFM, of which images revealed behavior of the multilayer growth on the Cr patterns. The observed images show that height and full width at half maximum (FWHM) of the bump on the multilayer surface strongly depended on the Cr pattern in height and width, and also incident angle of the sputtered molecular flux to the substrate surface. The multilayer coating at near-normal (vertical) incidence provides a significant amount of smoothing near the Cr patterns. A bump phase defect of 2-nm height and 60-nm FWHM, as the result, was obtained on the multilayer surface using a 5-nm thick Cr pattern, which corresponded to a minimum killer defect for EUV lithography at 45-nm node. The multilayer blanks with the programmed phase defects can be effectively used as a standard for defect inspection tool development and defect printability study.
This paper describes a simple fabrication process of the programmed phase defects on EUV multilayer blanks, evaluation results on the programmed phase defects, and growth behaviors of multilayer on various patterns (seed of the defects).
The exposure contrast (at wavelength contrast) on an extreme ultraviolet (EUV) mask has been evaluated by an experimental reflectivity measurement and an optical simulation. The experimental contrast showed good agreement with the calculated one for an EUV mask blank with TaBN absorber. The exposure contrast could be precisely estimated for an EUV mask using the simulation. Further, this simulation was used to evaluate the impact of absorber materials (TaBN, Cr and CrN) and 50-nm-thick buffer layers (CrX, SiO2, Ru and C) used to achieve thinner absorber stack. A mask composed of the TaBN absorber and the Cr-based buffer layer showed was the thinnest to achieve thinner absorber stack. A mask composed of the TaBN absorber and the Cr-based buffer layer showed was the thinnest to achieve a contrast of 100, at 81.3-nm. The TaBN absorber and the Cr-based buffer layer were found to be more suitable materials for obtaining lower aspect ratio.
6-inch EUV masks consisting of Mo/Si multilayers and patterned CrX buffer and TaBN absorber layers have recently been developed and evaluated. Mo/Si multilayers with a relatively high EUV reflectivity of 66 percent and an excellent uniformity were obtained on the polished ULE substrates by an ion beam sputtering method. The multilayers showed high durability to the acid abased cleaning and baking at 150 degrees C used in the conventional mask-making process. The Cr based film was optimized as a repair buffer to obtain a high reflectivity of 52 percent at 257 nm and low stress within 100 MPa. TaBN absorbers with a low reflectivity were obtained by optimizing the film compositions, which resulted in a high image contrast to the multilayer for DUV inspection. An EUV contrast level of 99 percent was achieved for a thinner, 100-nm thick absorber stack. Using the optimized mask process, EUV mask with patterns of 180-nm width were successfully obtained, without a significant drop in EUV reflectivity.
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