Ion beam sputtering of Mo/Si multilayers with B4C interlayers for EUV optics: Achieving ultrasmooth surfaces and minimal interface diffusion

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 B 4 C/Mo/B 4 C/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 B 4 C. 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 B 4 C 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) B 4 C barrier layers in the B 4 C/Mo/B 4 C/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 B 4 C/Mo/B 4 C/Si multilayer systems. Additionally, the Mo/Si and B 4 C/Mo/B 4 C/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 B 4 C-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. • Systematic optimization of IBS parameters for Mo/Si EUV multilayers. • Ultrathin B 4 C barriers suppress interface diffusion. • Optimal Mo ion energy balances crystallization and interface sharpness. • Achieved very low interface diffusion of 0.39 nm and 0.05 nm roughness.