Molecular Dynamics\nStudy of the Formation of Porous\nFilms by Room-Temperature Physical Vapor Deposition of Silica

Low-temperature physical vapor deposition is studied\nby means of\nmolecular dynamics (MD) simulations using the reactive force field\n(ReaxFF) potential. In contrast with prior MD studies of this process,\nwhich employed nonreactive, rigid ion force fields, our approach allows\nfor accurate modeling of the reactive incorporation of impinging particles,\ngenerated by the vaporization process, into the growing film. The\nuse of the ReaxFF also enables us to properly model the charges of\nimpinging particles and their electrostatic interactions with the\natoms, forming the topmost layers of the growing film. In order to\nbetter evidence the associated effects, we focus on the conditions\nof growth when the impinging particles have moderate kinetic energies\n(below a couple of electron-volts). We thus evidence that under these\nconditions, impacting particles tend to be strongly deflected during\nthe last stages of their approach to the film, and all the more so\nwhen they are slow. As a result, they tend to be captured by denser\nregions, leading to the formation of porous microstructures. This\nphenomenon results from the polarization of Si–O bonds and\nthe tendency of silicon atoms to surround themselves with oxygen atoms,\nleading to an overall polarization of the interface between dense\nmatter and pores.