Impact of metal adhesion layer diffusion on thermal interface conductance

Abstract

The first systematic measurements on the impact of interdiffusion between a metal overlayer and adhesion layer on the thermal interface conductance (G) at the metal bilayerdielectric 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)-Al 2 O 3 interface. The enhancement of G between Au and Al 2 O 3 through the addition of a Cu adhesion layer decreases as Au-Cu interdiffusion occurs. For example, annealing a 41 nm Au film with a 4.7 nm Cu adhesion layer on Al 2 O 3 at 520 K for 30 minutes, results in a 52 ±16% 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 new 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.

Thermal conductance across harmonic-matched epitaxial Al-sapphire heterointerfaces

“…Metal growth on nonmetal substrates, usually by sputtering or evaporation, suffers from problems like chemical reaction or interfacial mixing with substrates during deposition processes, the inclusion of an oxide or contaminating layers at the interface, polycrystalline metal films with a mixture of different orientations, or poor adhesion with substrates. 16,25,32,[47][48][49] These result in the growth of metalnonmetal interfaces to deviate from the ideal interface often assumed in theoretical modelling, so direct comparison between theoretical and experimental results limits our ability to draw accurate conclusions. 34,50 Additionally, interfacial thermal transport involves multiple fundamental transport mechanisms, including elastic and inelastic phonon transport across the interface, and electron-phonon coupling in the metal and across the interface.…”

“…This is especially true for metal-non-metal interfaces because of the complicated interfacial structures and multiple carrier transport mechanisms near the interfaces. Metal growth on non-metal substrates, usually by sputtering or evaporation, suffers from problems like chemical reaction or interfacial mixing with substrates during deposition processes, the inclusion of an oxide or contaminating layers at the interface, polycrystalline metal films with a mixture of different orientations, or poor adhesion with substrates 18,24,35,36 . These result in the growth of metal-non-metal interfaces to deviate from the ideal interface often assumed in theoretical modeling, so direct comparison between theoretical and experimental results limits our ability to draw accurate conclusions 37,38 .…”

Thickness-Independent Vibrational Thermal Conductance across Confined Solid-Solution Thin Films

“…Several prior works have demonstrated the wide variability of h K with material and interfacial properties such as mismatch of phonon spectra, − interfacial chemistry and bonding, − crystallographic orientation, ,− interfacial structure, ,,− and impurity regions. ,− In most cases, interfacial “imperfections” have been observed to lead to a decrease in thermal boundary conductance. ,, For example, we have observed this decrease in thermal boundary conductance across interfaces with roughness (i.e., geometric disorder), an amorphous layer (i.e., structural disorder), , and elemental mixing resulting in both compositional and structural disorders . In each of these cases, the specific disorder led to a reduction in thermal boundary conductance.…”

“…16,22,49−51 The results in these aforementioned works demonstrate the unique atomic conditions and structures that can lead to increases in thermal boundary conductance but most rely on conditions of atomic disorder or roughness. Still, the experimental realization of the originally posed computational concept of the so-called "phonon bridge" or the ability to insert a third material in between two materials at an interface to increase h K has been limited to metallic adhesion layers 17,18,26,27,36,52 and across self-assembled monolayers with different lengths of alkane chains (that are either sandwiched between thin metallic layers or between a metal layer and a semiconductor substrate). 53−55 For the latter, it has been shown that the length of the self-assembled monolayers does not dictate heat transfer, rather the bonding at the endgroup drives the increase in conductance across the interfaces.…”

“…The results in these aforementioned works demonstrate the unique atomic conditions and structures that can lead to increases in thermal boundary conductance but most rely on conditions of atomic disorder or roughness. Still, the experimental realization of the originally posed computational concept of the so-called “phonon bridge” or the ability to insert a third material in between two materials at an interface to increase h K has been limited to metallic adhesion layers ,,,,, and across self-assembled monolayers with different lengths of alkane chains (that are either sandwiched between thin metallic layers or between a metal layer and a semiconductor substrate). − For the latter, it has been shown that the length of the self-assembled monolayers does not dictate heat transfer, rather the bonding at the end-group drives the increase in conductance across the interfaces. ,− Yet, heat transfer across confined thin films in between two semiconducting or insulating materials has never been studied experimentally, which is the aim of the current work. Here, we experimentally demonstrate that the thermal conductance across confined solid-solution thin films between parent semiconducting materials does not necessarily lead to a change in the overall thermal conductance between the parent materials, which is counterintuitive to the notion that adding a material usually results in a decrease in thermal conductance.…”