Impact of metal adhesion layer diffusion on thermal interface conductance

Systematic measurements on the impact of interdiffusion between a metal overlayer and adhesion layer on the thermal interface conductance ($G$) at the metal bilayer-dielectric interface are reported. Composition depth profiles quantify the interdiffusion of a Au-Cu bilayer as a function of Cu adhesion layer thickness (0--10 nm), annealing time, and annealing temperature. Optical pump/probe measurements of $G$ quantify the effect of Au-Cu interdiffusion on thermal transport across the (Au-Cu)-$\mathrm{A}{\mathrm{l}}₂{\mathrm{O}}₃$ interface. The enhancement of $G$ between Au and $\mathrm{A}{\mathrm{l}}₂{\mathrm{O}}₃$ through the addition of a Cu adhesion layer decreases as Au-Cu interdiffusion occurs. For example, annealing a 49-nm Au film with a 4.7-nm Cu adhesion layer on $\mathrm{A}{\mathrm{l}}₂{\mathrm{O}}₃$ at 520 K for 30 min, results in a $57\ifmmode\pm\else\textpm\fi{}15%$ drop in $G$. An analytical model of the composition profile is derived with inputs of annealing time, temperature dependent permeabilities of the Au-Cu interface to each species, and the initial thicknesses of the Au and Cu layers. Integrating this model with a diffuse mismatch model defines a methodology for the prediction of $G$ that accounts for interdiffusion in metal bilayers on dielectric substrates, and can be used to evaluate the degradation of $G$ over a device's lifetime.