EUV mask technology is considered to be one of the most critical issues for the successful implementation of EUVL in the semiconductor fabrication process. Since EUV light is strongly absorbed by most materials, reflective optics is applied to all of the optical components, including the mask structure. As a result, the EUV mask structure, based on the multilayer mirror (MLM), is radically different from that of conventional optical lithography. All of the mask films must be prepared with great care, which requires nanoscale thin-film technologies. The target technology node for EUVL implementation is 7 nm and below; today, mask defects are one of the leading challenges associated with EUV mask fabrication.
A ML structure consisting of many alternating layers of materials showing dissimilar EUV optical constants is essential for reflecting 13.5-nm wavelength light. Bragg reflection—constructive interference of the partially reflected beam at several interfaces—is necessary to maximize EUV reflection at near–normal incidence, and its efficiency is determined by the structure (e.g., 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 are formed on the top of this MLM. However, an oblique incident light angle, which is required for image transfer with a reflective system, causes various mask 3D effects, as will be discussed in detail later in the chapter.
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