Investigation of the Heterogeneous Thermal Conductivity in Bulk CVD Diamond for Use in Electronics Thermal Management

From a materials perspective, diamond exhibits properties that are extremely well suited for use in the thermal management of high power and high heat flux electronic devices. While bulk diamond grown via chemical vapor deposition (CVD) has been demonstrated since the 1980s, and people have measured thermal conductivities ranging from 500–2000 W/m-K, these measurements have typically taken place over a large domain that encompasses numerous diamond grains. However, many of these techniques do not reveal the heterogenous nature of the diamond thermal conductivity which arises due to the local grain structure and orientation. The diamond sample investigated in this study contained a high level of boron doping on the order of 1021cm−3, giving rise to a reduced thermal conductivity measured as 714 W/m-K with a laser flash method. Similar bulk CVD diamond samples that are undoped show thermal conductivity values of greater than 1500 W/m-K with the same measurement technique. Through the use of time-domain thermoreflectance (TDTR) we are able to measure the thermal conductivity of bulk CVD diamond at a spatial resolution smaller than the size of the columnar grains. This allows us to examine significant changes in thermal conductivity as a function of spatial location, which is of great significance when the thermal source from electronics is on the size scale of this variation. Using TDTR, we present an approach involving a variation in the laser spot size using multiple focusing objectives to yield the heterogeneous thermal conductivity in bulk CVD diamond. The data show variations in thermal conductivity near 40% over a diameter of 40 μm. Scanning Electron Microscopy (SEM) and electron backscatter diffraction (EBSD) data are presented which also show variation in microstructure over this length scale giving rise to the heterogeneity.