The production of integrated circuits relies primarily on lithography. Extreme Ultraviolet (EUV) lithography employing a light source at 13.5 nm is currently the most advanced lithography technology for high-volume mass production, which has led to unprecedented progress in the development of integrated circuits (IC). The constant demand for IC chips with higher computing power has increased with the technological development of artificial intelligence in recent years. This requires further improvement in the lithographic resolution for the manufacture of smaller transistors on chips. Beyond extreme ultraviolet lithography (BEUV) at a wavelength of 6.X nm has become a research hotspot according to the Rayleigh criterion.
Light-source technology is indispensable to EUV lithography. To meet the requirements for mass production, an EUV light source must possess key performance characteristics, such as a stable and high-level output power, high energy conversion efficiency, minimum contamination level, and low maintenance cost. Currently, the predominant methods for the emission of BEUV light at a wavelength of 6.X nm include synchrotron radiation/free electron lasers (FEL), laser-produced plasma (LPP), and laser-induced discharge plasma (LDP). The mainstream approach is LPP technology, which utilizes Tb or Gb targets. Meanwhile, FEL exhibit potential as feasible BEUV light sources owing to their high power and efficiency, especially since the recent development of miniaturized X-ray free electron lasers (XFEL).
In particular, the reflective multilayer mirror is a crucial component of the optical system of EUV/BEUV lithography, which determines the exposure efficiency and imaging quality during lithography. To achieve a high reflectivity at the designated wavelength, the multilayer structure generally comprises alternating nanolayers of two materials with high optical contrast and low extinction coefficient. Notably, for a wavelength of 6.X nm, the period thickness of the multilayers is only approximately 3 nm, and the required number of layers exceeds 500. Precise control of the layer thickness and density is essential for ensuring a continuous and stable high reflectivity of the multilayers at the central wavelength. In addition, surface/interfacial roughness and/or intermixing between layers can lead to a decrease in the reflectivity and a shift in the central wavelength of these multilayers. Therefore, many studies have focused on the interfacial barrier layer for suppressing the intermixing or diffusion and mitigating its negative impact on the reflectivity.
Aging and the performance degradation of multilayers have brought considerable engineering challenges to EUV/BEUV lithography. Even a slight decrease in the reflectivity can result in a significant reduction in the power of the entire optical system. Specifically, the thermal stability of the BEUV multilayer is essential for maintaining a longer lifetime, because BEUV mirrors are typically exposed to higher thermal loads (higher power densities) than those used in EUV optics. Such high exposure loads lead to a severe interface diffusion and a reflectivity reduction. In addition, contamination caused by the BEUV light source, including carbon and/or oxidation contamination and plasma-induced damage, significantly shortens the lifetime of BEUV optics.
First, the methods for generating BEUV light (FEL, LPP, and LDP) are introduced in this review. The characteristics of the different BEUV light sources are summarized in Table 1. The advantages and disadvantages of using magnetron sputtering for the deposition of BEUV multilayers are discussed along with typical characterization methods such as X-ray reflectivity (XRR) and transmission electron microscopy (TEM) and their working principles. This review describes suitable spacer (B and B4C) and absorber materials (La and Mo) by examining the refractive index of each material at a wavelength of 6.X nm (Fig.4). From among them, the theoretical reflectivity of the La/B multilayer is the highest (above 80%); however, its measured reflectivity is only approximately 10%, owing to the low sputtering rate of B and its high interface diffusion. Various methods have been proposed to address these issues. For instance, Chkhalo et al. at the Physics of Microstructures of RAS inserted a carbon layer of 0.25?0.3 nm into the interface of La/B4C multilayer to prevent interfacial diffusion and increase the reflectivity to approximately 58.6%. Another typical method involves the passivation of the lanthanum interface with nitrogen, which improves the interfacial optical contrast and inhibits interface diffusion. For example, Kuznets et al. from the University of Twente fabricated a La/LaN/B multilayer with a reflectivity of up to 64%. The structural parameters and properties of other BEUV multilayers developed over the last decade are summarized in Table 2. Regarding the aging and performance degradation of multilayers, we studied the relevant literature on the thermal stability of BEUV multilayer mirrors in recent years, revealing that nitridation treatment of multilayers results in a better thermal stability. The influence of oxidation and contamination on the lifetime of the multilayers is also briefly introduced.
Today, 13.5 nm EUV lithography technology has matured to the stage of widespread use in mass production. However, next-generation BEUV lithography at 6.X nm for commercial applications requires extensive research and further engineering. Notably, there is a lack of comprehensive studies on the development of BEUV light sources and multilayer mirror technologies, both at home and abroad, particularly on the surface contamination and damage mechanism of multilayer mirrors caused by BEUV light. Therefore, all these critical research topics urgently require a joint effort of industry, academia, and research communities with the common goal of developing a BEUV multilayer mirror with a high reflectivity (at 6.X nm) and an industry-relevant level of stability and lifetime, enabling practical applications in commercial use. Finally, we believe that this review will provide an insightful reference for scholars and engineers engaged in domestic research activities related to advanced lithography, while hopefully promoting more in-depth studies on BEUV multilayers.
