Enhanced Proton Irradiation Resistance of Mo/Si EUV Multilayers with Ultrathin B4C Barriers

Proton-induced structural degradation seriously threatens the long-term reliability of Mo/Si multilayers for extreme ultraviolet (EUV) astronomical observations, which calls for in-depth mechanistic analysis and anti-irradiation optimization schemes. In this work, 40 keV proton irradiation with fluences up to 5 × 1016 p/cm2 was performed on Mo/Si and Mo/B4C/Si/B4C multilayers to characterize the fluence-dependent structural evolution of Mo/Si multilayers and clarify how the interlayers formed from decomposed B4C enhance proton irradiation tolerance. Based on the experimental characterizations, the Mo/Si multilayer exhibited pronounced self-limiting period compaction and asymmetric interface broadening with increasing proton fluence. In contrast, merely sub-angstrom period expansion occurred in the Mo/B4C/Si/B4C multilayer under the maximum fluence exposure. Combined experimental characterizations and Monte Carlo simulations reveal that Si-dominated atomic interdiffusion drives the formation of dense MoSi2 interlayers and subsequent period compaction. Moreover, Si atoms preferentially diffuse forward to Si-on-Mo interfaces, resulting in asymmetric interface broadening. Meanwhile, the progressively thickened MoSi2 interlayers suppress further Si migration and lead to the self-limiting degradation behavior. For the Mo/B4C/Si/B4C multilayer, the reaction between the ultrathin B4C layers and adjacent Mo and Si layers produces boride- and carbide-containing interlayers. Compared with MoSi2 interlayers, these interlayers suppress proton-induced Si diffusion by a factor of about 3.0, which arises from their higher atomic density and larger screened Coulomb scattering cross section. These findings demonstrate that B4C insertion effectively mitigates proton-induced interdiffusion and structural degradation, offering a reliable route to improve the proton irradiation resistance of Mo/Si EUV multilayer mirrors for the forthcoming Xihe-2 mission.