  Atomic layer deposition allows for precise control over film thickness and
conformality. It is a critical enabler of high aspect ratio structures, such as
3D NAND memory, since its self-limiting behavior enables higher conformality
than conventional processes. However, as the aspect ratio increases, deviations
from complete conformality frequently occur, requiring comprehensive modeling
to aid the development of novel technologies. To that end, we present a model
for surface coverage during atomic layer deposition where incomplete
conformality is present. This model combines existing approaches based on
Knudsen diffusion and Langmuir kinetics. Our model expands the state-of-the art
by (i) incorporating gas-phase diffusivity through the Bosanquet formula as
well as reaction reversibility in the modeling framework first proposed by
Yanguas-Gil and Elam, and (ii) being efficiently integrated within level-set
topography simulators. The model is manually calibrated to published results of
the prototypical atomic layer deposition of Al$_2$O$_3$ from TMA and H$_2$O in
lateral high aspect ratio structures. We investigate the temperature dependence
of the H$_2$O step, thus extracting an activation energy of
$0.178\,\mathrm{eV}$ which is consistent with recent experiments. In the TMA
step, we observe increased accuracy from the Bosanquet formula and we reproduce
multiple independent experiments with the same parameter set, highlighting that
the model parameters effectively capture the reactor conditions.
