Influence of interfacial structural disorder and/or chemical interdiffusion on thermal boundary conductance for Ti/Si and Au/Si couples

This work aims at experimentally assessing the effect of structural disorder and/or chemical interdiffusion on thermal boundary conductance (TBC) at metal/dielectric interfaces. To this end, the TBC of Au/Si and Ti/Si interfaces was measured by time domain thermoreflectance in the as-deposited state and after various heat treatments. Transmission electron microscopy (TEM) was used to determine the structure of the interface for each heat treatment condition. The TBC at the Au/Si interface was found to be highest in the as-deposited state and to be somewhat reduced after heat treatments. TEM images showed that the highest TBC was obtained when the interface contained a disordered transition layer with some degree of interdiffusion as present in the as-deposited state. Heat treatments led to the separation of species and to more abrupt interfaces with lower TBC. Whether this change in TBC was linked to disorder or to interdiffusion could not be distinguished. The TBC at the Ti/Si interfaces was observed to decrease with the heat treatment duration. TEM investigations revealed the presence of a thin disordered layer that did not evolve much with heat treatments. The evolution of the TBC was hence rationalized by interdiffusion facilitated by heat treatments that most likely modified the diffusion zone thickness and chemical composition, as well as the disordered layer's chemical composition. These changes result in modified interfacial properties, which ultimately affect the TBC. A quantitative exploration of the TBC dependence on the composition was not possible in this work, due to the extreme thinness of the regions under consideration preventing any reliable STEM-energy dispersive x-ray measurement. Published under license by AIP Publishing.

Heat-transfer at the interface between a metal and a non-metal: influence of ultra-thin interlayers, interface chemistry and atomic order

Maïté Blank

The Thermal Boundary Conductance (TBC) at the interface between metals and dielectrics is of key importance for heat transport in structures heterogeneous on a scale from 100 µm downwards. In this thesis the TBC of a large number of metal/dielectric couples was measured using Time-Domain Thermoreflectance (TDTR). Besides creating a large and coherently assessed number of metal dielectric pairs, a particular focus was laid on the effect of ultra-thin interlayers, as well as on the role of interfacial disorder and intermixing on the TBC. For those instances a framework to rationalize the observed phenomena was developed. The large number of consistently evaluated TBC-values allowed comparing the predictive capacity of the widely used Diffuse Mismatch Model (DMM), with the more recently developed approach that combines the Maximum Transmission Limit (MTL) with an average transmission probability that characterizes phonons transmission across real interfaces. It is found that the MTL based prediction of TBC yields consistently good precision whatever substrate is used, while the prediction quality of the DMM decreases with increasing mismatch between the materials on either side of the interface. This work served as a reference to quantify the TBC changes observed in the rest of this work. The influence of a coupling layer on the TBC of a given metal/dielectric couple was experimentally and theoretically assessed by using i) ultra-thin Cu interlayers (1.5 ¿ 25 nm) at the interface between Au and silicon, sapphire and diamond, as well as between Al and diamond, ii) ultra-thin interlayers of Mo and Ni at the interface between Ag or Au and diamond, and iii) intermixed interfaces in the Au-Si and Ti-Si couples with changes in the atomic order and chemical composition induced by various heat treatments. For case i) the TBC was observed to increase (Au-Cu-Si, Au-Cu-Al2O3, Au-Cu-diamond) or decrease (Al-Cu-diamond) monotonically for interlayer thicknesses up to 10 nm roughly, before reaching a plateau very close to the Cu-dielectric TBC value. The evolution of TBC was attributed to the finite electron-phonon coupling factor of the interlayer material for the Au-Cu-Si, Au-Cu-Al2O3, and Au-Cu-diamond cases. The apparent inconsistency between experimental evidence for the Al-Cu-diamond case and theory was rationalized by a variable coverage of the Cu-diamond interface by diffused aluminium evidenced by local chemical analysis. The importance of the electron-phonon coupling factor was confirmed by the use of Ni and Mo interlayers with Au or Ag on diamond, for which the TBC increased monotonically over a much shorter range of interlayer thicknesses than the one observed for Cu, before reaching a plateau. The results were rationalized both with an analytical model developed earlier and a model based on the deviation between temperature of electrons and phonons, a so-called ¿Two Temperature model¿ (TTM) yielding good agreement. In Au/Si samples, a change of the TBC was observed that can be attributed either to disorder or interdiffusion in the interfacial region. Investigations performed on the Ti-Si interface led to significantly decreasing TBC for increased duration of the heat treatment, apparently uncorrelated with the state of crystallization at the interface, but that may be due to interdiffusion.

EPFL2019

Heat-transfer at the interface between a metal and a non-metal: influence of ultra-thin interlayers, interface chemistry and atomic order

Maïté Blank

EPFL2019