Abstract
EUV lithography is advancing towards high productivity and yield, but the thickness of the interface diffusion layer and surface roughness in Mo/Si multilayer mirrors critically impact optical performance, presenting a major obstacle. This work presents a comprehensive study on controlling the ion beam sputtering deposition parameters to optimize the interface quality, roughness, and microstructure of Mo/Si-based multilayers for EUV applications. Mo/Si and B4C/Mo/B4C/Si multilayers were deposited by ion beam sputtering at 60 °C while systematically varying the Mo ion source voltage and current. The Mo/Si interface diffusion layer thickness increased with higher source voltage in the absence of B4C. Detailed structural analysis revealed that the Mo ion source voltage range of 400-600 V gives the best balance for crystallization, and voltages above 900 V cause excessive intermixing. However, the presence of B4C enhanced diffusion control, maintaining the diffusion layer thickness even at elevated source voltages. Results revealed asymmetric interfacial diffusion in the Mo/Si system, with thicker Mo-on-Si interfaces (0.7 nm) compared to Si-on-Mo (0.46 nm). This asymmetry was found to be strongly correlated to the Mo crystallization behaviour shown in X-ray diffraction patterns, in which a formation of Mo < 110> crystallites inhibited a Si interdiffusion. The incorporation of ultrathin (0.3 nm) B4C barrier layers in the B4C/Mo/B4C/Si system significantly improved the interface definition and suppressed the asymmetric diffusion, resulting in Mo-on-Si and Si-on-Mo interfacial diffusion thicknesses of 0.39 nm and 0.43 nm, respectively. These values are among the lowest interface diffusion thicknesses reported for both Mo/Si and B4C/Mo/B4C/Si multilayer systems. Additionally, the Mo/Si and B4C/Mo/B4C/Si multilayers exhibited extremely low surface roughness values of 0.05-0.08 nm. In addition, the enhanced periodic structure and the interface sharpness were confirmed by X-ray reflectivity measurements which revealed distinctive preservation of higher order Bragg peak intensities for the B4C-containing samples. The achievement of extremely low surface roughness, interface diffusion, and well-controlled crystallization through fine-tuning of the ion beam parameters paves the way for the development of high-performance multilayer optics for next-generation EUV lithography applications.
Cu, D.T., Huang, K.C., Huynh, H.T., Chan, C.H., Nguyen, K. and Kuo, C.C. (2026) Results in Surfaces and Interfaces, 23, p. 100755.