Emissivity of Mo₅Si₃C thin films for radiative cooling in EUV pellicles

In this work, we demonstrate the Nowotny phase of the ternary molybdenum-silicon-carbon system Mo 5 Si 3 C as a high-emissivity constituent material for ultrathin films that require passive radiative cooling. Examples of such films include coatings for integrated circuits, electronic and optical applications of ever shrinking dimensions, which readily heat up during operation such as extreme ultraviolet (EUV) pellicles, dynamic gas lock membranes, and other spectral purity filters used in EUV lithography. Formation of the Mo₅Si₃C phase likely leads to high density of states (DoS) near the Fermi level (E F ), which enhances infrared emissivity and radiative cooling for such freestanding membranes. In comparison to binary crystalline MoSi 2 -based materials, carbon incorporation during synthesis forms Mo–Si–C films containing Mo₅Si₃C crystallites in a Si/C-rich matrix, resulting in a lower EUV absorption and higher emissivity (38% rise in emissivity observed in this work), which is beneficial for limiting thermal loads and loss of light. Consequently, the operating temperature of the membranes drops, which is highly beneficial for their operational lifetime and thermal performance. The annealing temperature evolution showed competition between two trends of emissivity that determine the net emissivity of a thin Mo₅Si₃C-based film: enhancement from an increased DoS at E F when crystallization occurs and emissivity reduction from film interface degradation and partial loss of film continuity at high annealing temperatures. These promising results suggest that Mo₅Si₃C-based films are strong candidates for EUV applications that require radiative cooling.