Ballistic Thermal Transport in TiN/TiC Multilayers due to Interfacial Elastic Modulus Enhancement

The scale of microelectronics is gradually reducing towards dimensions comparable with the electron mean free path. Since there is a need to dissipate ever increasing amounts of waste heat in microelectronic applications the increased device and interface density is highly undesirable. Thus, the approach of engineering materials with high densities of interfaces to achieve desirable thermal conductivity solids requires a fundamental understanding. In this work, we report on the thermal conductivity of a series of crystalline multilayers composed of alternating layers of titanium nitride and titanium carbide with varying interface densities.We deposit titanium nitride and titanium carbide superlattice films on MgO(001) by reactive magnetron sputtering in an ultra-high vacuum chamber. The alternating nitride carbide layers are formed using reactive mixtures of Ar/N and Ar/CH₄, respectively. The 1 um thick samples are deposited at 1100 °C with alternating layer thicknesses between 1.5 and 15 nm. We directly measure the thermal conductivity of these films via the time-domain thermoreflectance technique. We find that the thermal conductivity increases with an increasing interface density indicating that the heat carriers are scattering less in the interfaces of the multilayers. This finding contradicts the traditional theory and conventional understanding. The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer. The increase in thermal conductivity could be related to their increase in their elastic modulus which was measured by picosecond acoustic and nanoindentation techniques. The increase in interface density leads to a monotonically increasing elastic moduli imply a high quality of interface formation which usually results in a higher thermal conductivity. Our results demonstrate a path toward engineering thermal conductivity, thus providing a novel approach to dissipate the ever-increasing amounts of waste heat in microelectronic devices and alleviate the concern for the continuation of Moore’s law.