New Atomistic Insights on the Chemical Mechanical Polishing of Silica Glass with Ceria Nanoparticles

Reactive molecular dynamics simulations have been used
to simulate
the chemical mechanical polishing (CMP) process of silica glass surfaces
with the ceria (111) and (100) surfaces, which are predominantly found
in ceria nanoparticles. Since it is known that an alteration layer
is formed at the glass surface as a consequence of the chemical interactions
with the slurry solutions used for polishing, we have created several
glass surface models with different degrees of hydroxylation and porosity
for investigating their morphology and chemistry after the interaction
with acidic, neutral, and basic water solutions and the ceria surfaces.
Both the chemical and mechanical effects under different pressure
and temperature conditions have been studied and clarified. According
to the simulation results, we have found that the silica slab with
a higher degree of hydroxylation (thicker alteration layer) is more
reactive, suggesting that proper chemical treatment is fundamental
to augment the polishing efficiency. The reactivity between the silica
and ceria (111) surfaces is higher at neutral pH since more OH groups
present at the two surfaces increased the Si–O–Ce bonds
formed at the interface. Usually, an outermost tetrahedral silicate
unit connected to the rest of the silicate network through a single
bond was removed during the polishing simulations. We observed that
higher pressure and temperature accelerated the removal of more SiO4 units. However, excessively high pressure was found to be
detrimental since the heterogeneous detachment of SiO4 units
led to rougher surfaces and breakage of the Si–O–Si
bond, even in the bulk of the glass. Despite the lower concentration
of Ce ions at the surface resulting in the lower amount of Si–O–Ce
formed, the (100) ceria surface was intrinsically more reactive than
(111). The different atomic-scale mechanisms of silica removal at
the two ceria surfaces were described and discussed.