The loading effect hinders the precise profile control
during the
selective etching of SiGe in stacked SiGe/Si layers, thereby hindering
optimal gate-all-around (GAA) transistor performance. In this article,
we present a systematic study on the loading effect in the selective
isotropic etching of SiGe in SiGe/Si stacks by varying the structure
density and process conditions, including chamber pressure and etch
time. We measure the lateral SiGe etching depth at different locations
within the stack pillars and evaluate the local etch uniformity. The
results demonstrate that pressure plays an important role in affecting
the isotropic lateral etching performance. Within the tested 10–40
mTorr range, higher pressures lead to increased etch rates but at
the cost of reduced uniformity. A noteworthy observation is that the
uniformity also decreases as the process time increases. To understand
and quantify the phenomena, we propose a physical etch model based
on top-down Monte Carlo ray tracing and simulate the etch profiles.
We calibrate the model with measured data on less dense pillar arrays
with 100 nm spacing and achieve small prediction error on denser pillars
with a spacing of 50 nm. The good agreement between simulations and
experiments demonstrates that the restriction of particle diffusion
in the narrow gap is the major contributor to the loading effect,
and our model is capable of quantitatively characterizing this phenomenon
by predicting the lateral etching profile. This research provides
valuable insights into the etching effects through experiments and
theoretical studies in order to promote the advanced etching technology
development toward GAA transistor manufacturing.