Due to the cross flow in ALD reactor chamber, the injected pre- cursor gas actually cannot reach all surface Sites on substrate at the same time. Therefore, the effect of this position diversity on the surface species formation is simulated in Fig. 4, where the time- dependent local pressures of injected TMA are illustrated. The operating conditions are set the same as the validated case. It can be seen that the local partial pressure of TMA is not uniformly distributed on the substrate surface. If we use point A and point B to denote two edges of the silicon wafer surface, it can be seen that it takes around 0.2 s for point B to get the first contact with TMA gas species after that of point A. The influence of this sequencing treatment on the surface species formation is simulated in Fig. 5, which shows the coverage variations of surface species in one ALD cycle including |–OH, |–Al(CH3)2 and |–Al(CH3)1. It can be seen that, during the TMA half cycle (from t = 0s tot = 10 s), the surface coverage of |–OH is decreased to nearly zero by the production of |–Al(CH3)2 on the silicon wafer surface. Then, the water vapor pulse is applied at t =10 s, where the |–OH is reproduced on the surface, and |–Al(CH3)1 is generated by the surface reactions between |–Al(CH3)2 and adsorbed H2O. After around 1.5s by surface reac- tions, an equilibrium state (saturation) eventually can be reached. Comparing the variations of surface species at two silicon wafer edges (point A and point B), it is obvious that they have nearly the same coverage when the surface reactions reach the equilib- rium state. This can be understood by that the amount of injected precursor is sufficient in the experiment, and thus the position diversity of the silicon wafer surface sites actually has very small effect on the homogeneous alumina film growth.