The Effect of Ballistic Electron Transport on Copper-Niobium Thermal Interface Conductance

The thermal conductivities of 1μm copper-niobium multilayer films of different period thicknesses are measured by time–domain thermoreflectance at room temperature. The values for thermal conductivity are then used to calculate the thermal conductance between Cu/Nb interfaces using a series resistors model. Results show that Cu/Nb interface conductance increases with the decrease in period thickness reaching a value as high as 20 GWm⁻²K⁻¹ for a 1×1 Cu/Nb multilayer. At shorter period thicknesses, ballistic electron transport dominates the thermal transport across this interface resulting in high interface conductance. The results are well described by a model that accounts for both ballistic and diffusive transport of electrons. This model assumes that an electron on one side of a metal-metal multilayer may not scatter at the interface but rather move ballistically on to the adjacent material and scatter in the adjacent material.