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Chapter 7:
EUV Mask and Mask Metrology
For the successful implementation of extreme ultraviolet lithography (EUVL) technology for the future semiconductor fabrication process, EUV mask technology is considered to be one of the most critical issues. Since EUV light is strongly absorbed by most materials, reflective optics is applied to all the optical components, including the mask. As a result, the EUV mask structure, based on the mirror-type mask blank, is radically different from that of conventional optical lithography. All the mask components must be prepared with great care to consider the 13.5-nm reflection, which requires nano-scale thin-film technologies. The International Technology Road Map for Semiconductors (ITRS) forecasted that the target technology node for EUVL implementation would be 32 nm and below; today, mask defects are the leading challenge for EUV masks. The reflection of a 13.5-nm wavelength requires a multilayer (ML) structure consisting of many alternating layers of materials having dissimilar EUV optical constants. The Bragg reflection—constructive interference of the partially reflected beam at several interfaces—is the only principle to have EUV reflection, and its efficiency is determined by the structural factor (e.g., the thickness of each layer and their ratios) as well as optical properties (refractive index and extinction coefficient). To produce optical contrast for imaging, the patterns with high EUV absorbance should be formed on the top of this ML mirror. An oblique incident light angle, which is inevitable during image transfer with an EUVL system and causes a shadowing effect, imposes a unique restriction for the EUV mask structure. The ML and absorber specifications are specified in the standard SEMI P38-1103.
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