ABSTRACTDiamond is the ultimate candidate for heat-spreading applications because of its extreme thermal management properties. The synthesis of sub-micron diamond films is of great interest for SOD (Silicon On Diamond) wafer technology as well as for specific thermal device applications. The challenge here is the necessity to fabricate ultra-thin layers (down to 100 nm) that are continuous and homogeneous. We studied the Bias Enhanced Nucleation (BEN) pretreatment on microwave plasma reactors in order to have very high nucleation densities (> to 10¹¹ cm⁻²) and optimised the following growth step of the process to obtain sub-micron covering diamond films. In this study, we focus on the bias step parameters to increase the nucleation rate and to limit the growth rate during the bias step. Then, we performed a growth step to control the morphology of the films. We obtained diamond films with thickness lower than 80 nm and with a 6 nm Root-Mean-Square roughness.