(253a) Effects of Varying Surface Film Thickness on Particle Adhesion

The
effects of surface film (coating) thickness, d, on the adhesion of a
particle to a surface are investigated.  The
model system in this work includes a pre-manufactured silicon nitride probe
(initial radius of curvature ~15 nm) and a silicon substrate onto which gallium
nitride films have been deposited via atomic layer deposition.  The gallium nitride films have varying
thicknesses (0, 5, 11, 12, 16, and 80 nm).  This system is well approximated by
the idealized sphere-flat plate geometry.

We
develop theory supporting the hypothesis that the coating material will appear
transparent to the particle if d is small in comparison to the
particle-substrate separation distance, D.  However, as d increases a
transition in the particle-substrate interaction will take place.  This implies the existence of a critical coating thickness,
d*, beyond which the adhesion behavior of the coating material will be
equivalent to bulk.  We show
that d* is a simple function of D and a parameter termed the permissible fractional error,
ε.  Assuming D = 0.4 nm at contact, for ε = 0.05
(i.e. 5% permissible fractional error), d* ≈ 28 nm.

The adhesion between the model
system particle and substrate is measured using contact mode atomic force
microscopy.  Experimental results
confirm that for small (i.e. 5, 11, and 12 nm) d, the interaction between
the silicon nitride probe and underlying silicon is relatively unaffected by
the presence of the gallium nitride film; for large (i.e. 80 nm) d, the
interaction parallels that between silicon nitride and bulk (≥ 5 µm)
gallium nitride.  The
d* for this model system is approximately 30 nm, which is in good
agreement with our theoretical predictions.