Paper NM-MoM4
Quantum Chemical Molecular Dynamics Approach to Chemical Mechanical Polishing Processes of Gallium Nitride by SiO₂ Abrasive Grain

Monday, December 8, 2014, 9:40 am, Room Hau

Gallium nitride (GaN) is a next-generation semiconductor material with a wide band gap and high electron conductivity. Although the atomic-level planar polished surface is essential for practical GaN devices, it is difficult to polish efficiently the GaN substrate because of its high hardness and chemical stability. The chemical mechanical polishing (CMP) is promising for efficient polishing of the GaN substrate. However, the detailed CMP mechanisms are unclear, and then the design of the processes is difficult. In this study, in order to design the efficient and precise GaN CMP processes, we investigate the GaN CMP via our tight-binding quantum chemical molecular dynamics (TB-QCMD) method.

We perform CMP simulations of a GaN(0001) surface by a SiO₂ abrasive grain in aqueous H₂O₂ solution and aqueous NaOH solution to clarify the chemical reactions of each solution. We reveal that OH radicals and OH^- ions are adsorbed on the GaN surface in aqueous H₂O₂ solution and aqueous NaOH solution, respectively. According to the analysis of the atomic bond population between the Ga atoms in the first layer and the N atoms in the second layer, we elucidate that Ga-N bonds of the GaN substrate in aqueous H₂O₂ solution are weaker than those in aqueous NaOH solution. Therefore, we suggest that the OH radicals are effective for GaN CMP. To confirm the effectivity of OH radicals, we add one OH radical into the solution every 4.0 ps until 64.0 ps during polishing simulation under pure water environment. After 8 OH radicals are added, the 8 added OH radicals are adsorbed on the GaN surface. After 10 OH radicals are added, a surface-adsorbed O atom is generated by the chemical reaction between the surface-adsorbed OH species and a OH radical in the solution. At the friction interface between the GaN substrate and the abrasive grain, the surface-adsorbed O atom is mechanically pushed into the GaN substrate by the abrasive grain. This O atom intrusion induces the dissociation of Ga-N bonds of the GaN substrate. The N-N bond in the GaN substrate is generated due to the Ga-N bonds dissociation. After 16 OH radicals are added, the Ga atom in the first layer binds with 3 OH radicals. Subsequently, Ga(OH)₃ is generated and desorbed from the surface. N₂ molecules are also generated and desorbed from the surface due to the dissociation of Ga-N bonds. We suggested that the GaN CMP process efficiently proceeds by the mechanically induced chemical reactions: a surface-adsorbed O atom is generated and pushed into the GaN bulk by the abrasive grain.