Synergistic Interaction between Substrate and Seed Particles in Ultrathin Ultrananocrystalline Diamond Film Nucleation on SiO2 with Controlled Surface Termination

Abstract

We have investigated the effect of surface modification on the dispersion of nanodiamond seeds on the SiO2-coated Si substrate, to enable the ultrathin ultrananocrystalline diamond (UNCD) coating on the substrate, by direct current plasma-assisted chemical vapor deposition (DC-PACVD) using hydrogen-rich chemistry. The ultrasonically dispersed seed density on the SiO2-coated Si wafer was so much lower than that on the pristine Si wafer that the void-free ultrathin UNCD coating was impossible. For surface modification, we have exposed the substrate to (1) the hydrogen/hydrocarbon plasma in the DC-PACVD chamber or (2) the hydrocarbon atmosphere in the hot filament chemical vapor deposition (HF-CVD) chamber, prior to the ultrasonic seeding. The exposure to hydrocarbon or to its plasma greatly reduced the seed density, while exposure to hydrogen plasma drastically enhanced it by a factor of 6, which enabled a void-free ultrathin UNCD coating as thin as 30 nm. The UNCD film and the substrate surface before and after the surface treatment were characterized by XPS, NEXAFS, FTIR, Raman spectroscopy, HR-SEM, HR-TEM, EDX, and the zeta potential analyzer. The effects of the pretreatments on the seed density were explained by Si-OH or Si-CH3 termination and by the consequent change on (1) the zeta potentials of the substrate and (2) that of the nanodiamond particles in the seeding suspension.