EUV lithography for the semiconductor industry and beyond

Over the last few decades, remarkable progress has been made in the field of multilayer coatings for the EUV spectral region, mainly due to the demands of EUV lithography for the semiconductor industry. The progress is associated with a deep understanding of the multilayer film growth, application of advanced smoothing technologies for multilayer interfaces, as well as the development of modern deposition and characterization techniques. The projection optics precisely coated by Mo/Si mirrors with 70 % reflectivity is the heart of the performance of industrial EUVL steppers recently developed by ASML for high-volume chip production. While the EUVL at 13.5 nm has just launched into the market, the first high-performance LaN/B multilayer mirrors with reflectivity up to 61 % were developed for the next emerging lithography generation with a reduced wavelength of 6.6 nm. Recent technological progress in EUVL optics and radiation sources triggered the development of compact microscopes in the water window spectral range (2.4 – 4.4 nm). The optimization of the multilayer design and deposition process of Cr/V, Cr/Sc and Cr/C mirrors resulted in reflectivity of 14.5% at 2.42 nm, 27.0 % at 3.16 nm, and 15,6% at a wavelength of 4.42 nm, respectively. These promising and still not limiting results indicate a large potential for future application of multilayer coated optics in various application fields such as microscopy in the water window, plasma diagnostics, spectroscopy, and astrophysics. SUMMARY Induced mainly by the production of more powerful and energy-efficient electronic circuits with the aid of Extreme Ultraviolet Lithography (EUVL), operated at the wavelength of 13.5 nm, optics developments in recent years have pushed the boundaries for reflection coatings at short light wavelengths further away [1]. Since all materials and gases absorb EUV radiation, the reflective optics make use of the constructive interference of the light partially reflected from many interfaces within the coated multilayer stack to achieve high reflectivity of more than 70 %, while a single surface reflects less than 1 % of EUV light near-normal incidence. The high absorption also requires that the EUVL process be carried out in a vacuum environment. Interference coatings made from molybdenum (Mo) and silicon (Si) enable theoretical peak reflectance values of up to 75.4 % at 13.5 nm. A challenge, however, is to achieve such high reflectance values in practice and combine this with high functional stability in order to realize an efficient operation of an EUV wafer stepper system. The projection optics precisely coated by Mo/Si mirrors with 70 % reflectivity is the heart of the performance of EUV wafer steppers [2] recently developed by ASML for high volume chip production with printing feature sizes of 7 nm and beyond [3]. In 2022 ASML hopes to ship a next-generation EUV system with increased numerical apertures, which is targeted for a printing resolution down to 3 nm in 2023 [3]. This paper summarizes the current progress and the present knowledge collected at Fraunhofer IOF in the design, fabrication, and characterization of EUV multilayer coatings with excellent optical performance and functional stability. FIGURE 1: General view (left) and substrate loading (right) of sputtering system NESSY-3 designed for the precision coating of EUV optics with lateral thickness gradient. [4]