(172b) Effects of Varying Surface Film Thickness On Particle Adhesion In Semiconductor Material-Based Systems

Effects of Varying Surface Film Thickness on
Particle Adhesion

in Semiconductor Material-based Systems

Abstract

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).

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 (see Fig. 1).  We show d* is
a simple function of D and a
parameter termed the permissible fractional error, ε.  Taking 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 (AFM).  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.

These
developments were then examined on the industrial scale by conducting Particle
Removal Efficiency (PRE) experiments on six 300-mm diameter SiO₂-on-silicon
wafers, onto which varying thicknesses (5, 10, 15, and 20 nm) of titanium
nitride had been deposited.  The
challenge particles were dry-deposited SiO₂ particles having size distribution
peaks at 55 and 110 nm in diameter.
Wafers were cleaned using a commercially available argon/nitrogen
cryogenic aerosol process.  Results from
the PRE experiments (see Fig. 2) revealed that SiO₂ particle removal
efficiency improves with TiN film thickness, implying TiN shields or screens
the adhesion of the SiO₂ contaminants to the underlying layer of SiO₂.  Additionally, PRE improves with particle
size; PREs for the 110-nm challenge particles were 1.5×?4× times those for the
55-nm challenge particles.  Finally, as
TiN film thickness approached d*, changes
in PRE became more drastic.

Figure
1:  variation in AFM pull-off force with
GaN surface film thickness.

Figure 2:  variation in PRE with TiN surface film
thickness.

References

[1] J. N. Israelachvili. (1992). Intermolecular
and Surface Forces (2^nd Ed.). Boston: Academic Press.

[2] R. P. Jaiswal. (2008). Adhesion between particles and
nano-structured films. (Doctoral dissertation). West Lafayette: Purdue
University Press.