Research Progress on Freestanding Window Materials for Extreme Ultraviolet Lithography

ZHAO Guanzhong; ZHAO Chong; LIU Kehai; LIU Chang; LIU Kaihui

doi:10.16490/j.cnki.issn.1001-3660.2025.23.002

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Surface Technology ›› 2025, Vol. 54 ›› Issue (23) : 34-46. DOI: 10.16490/j.cnki.issn.1001-3660.2025.23.002

Special Topic—Atomic-level manufacturing

Research Progress on Freestanding Window Materials for Extreme Ultraviolet Lithography

ZHAO Guanzhong^1,2, ZHAO Chong², LIU Kehai^2,3, LIU Chang^1,*, LIU Kaihui^1,2,*

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Abstract

With the semiconductor industry rapidly advancing toward sub-5-nanometer technology nodes, extreme ultraviolet lithography (EUVL), operating at a wavelength of 13.5 nm, has become a cornerstone technology to sustain Moore's Law and address the increasingly stringent demands for resolution and integration. However, the unique high-energy nature of EUV (92 eV) leads to strong interactions with matters, posing significant challenges for the design and durability of key optical components. Window materials made of freestanding thin films play a critical role. These films include pellicles for mask protection and filters for spectral purity. They must combine four key qualities. First, they need high EUV transmittance. Second, they must span large areas without sagging or tearing. Third, they must resist pressure differences and mechanical stress. Fourth, they must remain stable under high heat loads and in hydrogen plasma.
This review provides a summary of the current research and technological development in freestanding EUV window materials. First, it outlines the fundamental role of such materials in EUVL systems, emphasizing their dual functionality: contamination control and filtering. It elaborates on the specific demands placed on freestanding EUV window materials, including high EUV transmittance, atomic-scale thickness, large-area freestanding structures, and mechanical robustness under pressure differentials. Furthermore, it highlights the need for high thermal conductivity and emissivity to manage heat loads in vacuum environments, as well as chemical resistance against hydrogen plasma used in EUV light sources.
Conventional window materials, such as silicon nitride and metal-based thin films, are adopted in EUVL systems due to their process maturity. However, these materials increasingly fall short in terms of EUV transmittance, heat tolerance, and mechanical strength at ultrathin dimensions. To overcome these limitations, emerging low-dimensional materials, including graphene and carbon nanotubes (CNTs), garner considerable attention. Their intrinsic properties, including atomic-scale thickness, high strength and excellent thermal conductivity, render them promising candidates for next-generation EUV window materials.
The review further explores hybrid architectures combining low-dimensional and traditional materials to synergize their advantages. Graphene pellicles with EUV transmittance exceeding 90% and CNT membranes with superior mechanical strength and thermal emissivity are demonstrated. Composite designs, such as ZrSi₂/SiN_x or TiN-coated graphene, exhibit improved resistance to hydrogen plasma while maintaining optical performance. Nevertheless, challenges remain in terms of large-area uniformity, defect control, and long-term environmental stability.
In addition to surveying fabrication techniques, it discusses characterization methods for evaluating optical, mechanical, thermal, and chemical properties under EUV exposure conditions. It also summarizes industry standards and performance metrics necessary for commercial adoption.
Looking forward, the development of scalable, high-throughput processes for low-dimensional and composite EUV window materials will be essential to meet the demands of future high-power lithographic tools. The development roadmap for EUV window materials emphasizes three phases: refinement of silicon-based films via advanced deposition techniques; optimization of metal-silicide composites for high-power applications; and integration of low-dimensional composites to achieve atomic-scale manufacturing.
By integrating optical, mechanical, thermal, and chemical perspectives, this review delivers a comprehensive guide. It offers a clear path for designing and deploying next-generation freestanding thin films. These window materials will propel future extreme ultraviolet lithography and sustain semiconductor scaling for generations to come.

Key words

extreme ultraviolet lithography / freestanding thin films / window materials / low-dimensional materials / atomic-scale manufacturing

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ZHAO Guanzhong, ZHAO Chong, LIU Kehai, LIU Chang, LIU Kaihui. Research Progress on Freestanding Window Materials for Extreme Ultraviolet Lithography[J]. Surface Technology. 2025, 54(23): 34-46 https://doi.org/10.16490/j.cnki.issn.1001-3660.2025.23.002

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Funding

Guangdong Major Project of Basic and Applied Basic Research (2021B0301030002); National Natural Science Foundation of China (52025023, 12427806, 12304204); National Key R&D Program of China (2022YFA1403500, 2022YFA1403504)