(310f) Molecular Dynamics Simulations of Plasma-Surface Interactions: Nanoscale Feature Etching on a Silicon Substrate

Classical
molecular dynamics (MD) simulations have been employed to model the fabrication
of very small ( < 5 nm ) features in silicon. As scale-down continues in the
semiconductor and other thin-film device industries, fundamental understanding
of etch mechanisms at these smaller scales is becoming increasingly important
for process design and control.  In our MD approach, confined beams of ions and
radicals have been used to approximate a perfect masking layer, using either
cylindrical (hole) or rectangular (trench) geometries with a nominal
confinement size of ~2 nm.  Bombarding species (chosen from typical plasma
processing gases) include Ar⁺ and CF_x⁺ ions,
as well as F and CF_x radicals. The nano-feature evolution is tracked
over several thousand impacts, allowing direct examination of the etch process
as a function of ion/radical fluence. By varying the ratios of the incoming
species, various physical and chemical contributions can be examined and their
corresponding roles in the etch process and sidewall formation can be studied. 
For example, we show that ion bombardment alone is sufficient to create a dense,
mixed damaged/passivation layer along the sidewalls of the feature being etched
that is ~1 nm thick.  However, for some bombarding chemistries, radical
deposition along the feature also plays an important role in determining the
final sidewall composition and subsequent etch characteristics. We elucidate
which mechanisms are most critical under the various conditions studied.  Additional
simulations have been carried out to examine the effects of an explicit masking
layer (i.e. amorphous carbon deposited on top of the silicon) on both the final
geometry of the feature and to overall feature size limitations. For example,
ions impinging on the masking layer's sidewalls can lead to sputtering of the
masking material into the feature, where it mixes into the substrate layer
during subsequent bombardment, changing the effective etch rate.  Further, the
mask can collapse at the very small length scales examined, limiting the transport
of radicals and ions to the substrate material.  Mask erosion under certain
chemistries can lead to loss of critical dimension control or mask impurity
incorporation into the substrate. Future developments to capture more realistic
phenomena (such as surface charging effects, longer-than-MD-timescale diffusion
effects, etc.) are also discussed, including the parallelization of the code to
allow for atomistic simulation of larger scale features.