Roughness Formation During Plasma Etching of Composite Materials: A Kinetic Monte Carlo Approach

The focus of this article is on the plasma-etching behavior of films of composite materials consisting of two randomly distributed phases with different etch rates. Two etching modes are examined as follows: 1) with anisotropic flux of ions (sputtering) and 2) with isotropic flux of etchants (chemical etching). First, analytical relations are derived to describe etch rate and surface coverage (with difficult to etch material) versus selectivity s (etch rate ratio) and volume fraction p of the easily etched material. Then, the evolution of surface roughness is calculated by means of kinetic Monte Carlo simulation of the process. In both modes, a critical time t
_c
proportional to s is found which defines two regimes of roughness behavior. For t > t
_c
, the rms surface roughness increases during etching as a power law with an exponent similar to that characterizing the etching of one phase homogeneous film. The exponent is 0.5 for the anisotropic-etching case and much lower (~0.17) for chemical-etching due to shadowing phenomena. On the contrary, the presence of two phases is evidenced for t <; t
_c
. At t <; t
_c
and small s, roughness is maximized for p ≈ 0.9. For large s, the maximum becomes sharper and moves to larger p values (~0.99 for s = 100). Furthermore, in chemical etching, at large s, a dynamic transition at t ~ t
_c
from a region where local surface fluctuations are enhanced with etching time (anomalous scaling) to normal behavior is found. Finally, it is shown that an appropriate version of the model captures the basic features of the roughness evolution of a composite film measured experimentally.