Mo-based multilayer for beyond EUV lithography

Abstract

Beyond EUV (BEUV) lithography has been developing for further downscaling of semiconductor devices. We propose Mo₂N/B multilayer (ML) for designing high-reflectance, high-durability BEUV mirrors. Background: High-reflectance MLs are key components for photomasks and BEUV optic, and their performance depends on optical constants and interfacial stability. La-based ML has reported a reflectivity of over 60%, but it concerns chemical stability to water vapor in air and BEUV-induced hydrogen plasma in lithography. Therefore, alternative materials are required to improve the durability. Aim: We investigate the reflectivity and the durability of Mo₂N/B ML as an alternative of La-based ML. Approach: We fabricated Mo₂N/B ML by unbalanced magnetron sputtering, and evaluated the BEUV reflectivity and EUV-induced hydrogen plasma durability at NewSUBARU synchrotron radiation facility. The profile of Mo₂N/B MLs was assessed by using XRR, and STEM-EDX. Results: BEUV reflectivity obtained 8.5% by ML of 80 pairs and the XRR remained unchanged after storage in air over 1 month, which confirmed its chemical stability. EUV accelerated lifetime tests observed no blister formation during 8 hours of irradiation. Conclusion: This work demonstrated the potential of durability in Mo₂N/B ML, offering a viable Mo-based design. We observed the interfacial state of the ML considering diffusion, roughness, and crystallinity, suggesting the pathway to improve BEUV reflectivity.