Computational Investigation of the Fate of Aluminum Nitride Particles During Chemical Vapor Deposition

Aluminum nitride (AlN) is a wide bandgap semiconductor material of wide interest. Aluminum nitride films are typically grown using metalorganic chemical vapor deposition. In this process, group-III and group-V precursors, namely tri-methyl-aluminum and ammonia, are injected into a reactor. Subsequently, these reactants react both in the gas-phase as well as at the surface to deposit an epitaxial layer of AlN on a hot substrate (wafer). It has been experimentally observed that AlN nanoparticles are formed in the gas-phase during this process. Although these particles are formed in the vicinity of the hot substrate, they tend to stay away from the hot substrate and are observed to deposit on the cold walls of the reactor. This is undesirable since the particles do not contribute to the growth of the AlN film, and end up damaging the walls of the reactor. In this computational study, the trajectories of the AlN particles are simulated with the goal to understand the mechanisms responsible for their motion. A three-dimensional (3D) Lagrangian Brownian dynamics simulator based on the Langevin equation is first developed. It is then coupled with a 3D computational fluid dynamics solver to simulate the background flow, and the chemical reactions responsible for AlN particle formation. The combined model is first validated, and then exercised for the problem at hand. It is found that thermophoretic forces are primarily responsible for driving the particles away from the hot substrate and depositing them on the cold reactor walls.