Ultrananocrystalline diamond (UNCD) is characterized by the dimensions of its ultra-nanoscale grains and is typically synthesized under high nucleation conditions using an argon-rich hydrogen/methane/argon gas mixture. UNCD films consist of crystalline grains of at least 95% sp³-bonded carbon that are 2-10 nm in size with a surface roughness ranging between 20 and 40 nm. UNCD films exhibit most of the outstanding properties of diamond such as an exceptional hardness, high mechanical strength, extremely low friction coefficient, chemical inertness, thermal stability and high electrical conductivity at room temperature when doped with nitrogen. While all these properties are intrinsic of the material, the required roughness scale of the UNCD film needs to be carefully tailored and customized on an application-by-application basis. For instance, a low roughness is key to manufacturing adequate micro-electromechanical systems (MEMS), wear-resistant low friction coatings, X-ray lithography masks and biomedical devices where stringent tolerance and topological precision are of utmost importance.

Here, a new process for controlled smoothening of ultrananocrystalline diamond films using H₂/O₂ plasma is presented. Diamond films were exposed to oxygen, hydrogen and hydrogen/oxygen mixture (19:1) plasmas. Evolution of morphology and thickness were monitored ex-situ using scanning electron microscopy (SEM) while roughness was measured using atomic force microscopy (AFM). Neither pure oxygen nor pure hydrogen plasmas were found suitable for smoothening the films. The H₂/O₂ mixture plasma exposure resulted in a constant roughness decrement of 0.043 nm/min with a slow etching rate (0.07 µm/h) allowing for the control of the final film roughness without sacrificing too much of the material thickness.