In addition to the surface reaction kinetics, the ALD process also includes the continuous precursor gas transport in the reactor chamber. To capture the detailed process dynamics, the momen- tum, mass and energy transport of gas species need to be coupled with the surface chemical processes. Since the operating tempera- ture of the reactor chamber is maintained at a constant value, in this study, we assume that the energy variation during the ALD operation is negligible, and then an isothermal condition is employed. According to the geometry of Cambridge NanoTech Savannah S100 system, a two-dimensional geometry is applied in the modeling, where the Navier–Stokes equations for compressible fluid flow are applied to describe the momentum conservation in the ALD reactor [18],