Molecular Dynamics Study of the Formation of Porous Films by Room-Temperature Physical Vapor Deposition of Silica

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