Interference-governed electromagnetic-thermal coupling and heat transport in multilayer nanofilms under pulsed extreme-ultraviolet irradiation

Mo/Si multilayer mirrors are central to extreme ultraviolet (EUV) lithography, where nanoscale optical interference and heat accumulation together constrain reflectivity and operational stability. Conventional surface-flux-based models become inadequate for EUV multilayers because interference produces a strongly modulated volumetric absorption profile across periods, which cannot be fully represented by an equivalent surface heat source. Here we develop an analytical electromagnetic-thermal coupling model that links the energy deposition obtained from the transfer matrix method to transient heat conduction in EUV multilayers. The model reveals a fundamental trade-off whereby increasing the multilayer period number enhances reflectivity but simultaneously elevates temperature by impeding heat dissipation. Because the absorbed energy is distributed within the multilayer, the ETCM predicts differences in the magnitude, position, and temporal evolution of the heat-flux maximum after the pulse compared with a conventional surface-flux model. Systematic analysis establishes approximate scaling relations connecting interfacial thermal resistance, beam size, incident energy density, and multilayer period number to thermal confinement and temperature rise. By incorporating interfacial compaction kinetics, the model can predict long-term interfacial compaction under specified operating conditions and provide a theoretical assessment of mirror lifetime. This work provides a theoretical tool for electromagnetic-thermal design and reliability analysis of multilayer nanostructures under pulsed irradiation.