Cooling future system-on-chips with diamond inter-tiers

Heat spreading is critical in reducing the overall junction tempera-ture of monolithic system-on-chips (SoCs) and high-heat-flux radio frequency (RF) applications. Bulk diamond has the highest thermal conductivity (TC) in nature, but its TC degrades due to the presence of smaller and highly columnar grains. Diamond thin films can be used as the back-end-of-line (BEOL) dielectrics and thermal vias for effective heat spreading if high-quality isotropic diamond growth processes are developed. In this study, we grow large-grain thin-film diamonds (0.3-25 mm) with isotropic TC (300-1,800 W/m/K). This is achieved by controlling the lateral growth and termi-nating smaller grains at the nucleation stage without physically damaging the substrate. The enhancement of TC is achieved by lowering grain boundary density and graphite at the grains. Thermal modeling indicates that incorporating isotropic thin diamond re-duces the temperature in a realistic flip chip and a monolithic 3D deep neural net accelerator by 20% and 50%, respectively.