Role of Hydrogen in Ultrananocrystalline Diamond Deposition From Argon-Rich Microwave Plasmas

A two-step process consisting of an initial nucleation stage with hydrogen followed by a growth stage without hydrogen is used to examine the role of hydrogen in ultrananocrystalline diamond deposition in Ar-rich microwave plasmas. An addition of 5% of H₂ to Ar-rich/CH₄ in the initial stage increases the nucleation density (up to 10⁹ cm^-2) and the mean grain size (around 10 nm), accompanied by some improvement of crystallinity. However, an addition of H₂ above 10% suppresses nucleation by the etching of nucleation sites. The introduction of hydrogen atoms promotes the termination and stabilization of diamond grain surfaces toward sp³ configuration, which are responsible for enhancing nucleation and crystallization. In contrast, Ar promotes renucleation by increasing the production of C₂ radicals in the plasma, which causes the degradation of the stabilization on the growing surfaces.