5.4. Effect of the ALD system pressure
The operating pressure of ALD system is another important fac- tor that plays an important role in determining the reaction rates of various surface reactions [2]. According to the experimental setup, the reactor chamber pressure is actually controlled by the balance between inlet and outlet boundaries. Therefore, to simu- late the system pressure at 0.5 Torr, we assume the steady state pressures at the inlet and outlet of ALD reactor are 0.5 Torr and 0.4 Torr respectively ,while the applied partial pressures of injected TMA and H2O will be set at 0.075 Torr and 0.15Torr respectively according to their relations with the ALD system pressure (15% and 30%). Other operating parameters are kept unchanged. In Fig. 8, the TMA +H2O ALD operation is simulated in the first 3 cycles and the variations on surface species coverage are illus- trated. It can be seen that under the pressure of 0.5 Torr, the surface species have not completely reacted with the related pre- cursor gas after each cycle, while under 1 Torr these have fully reacted during each pulse. This is because the reduced system
pressure has limited the surface reaction rates, and moreover, the fast purging process gives little contact time between precursor and substrate surface. As the ALD treatment cycles moving on, the variations of residual surface species are simulated in Fig. 9, which shows that the residual surface species can reach a steady state after a few cycles of ALD operation. It can be seen that nearly 20% |–OH on the substrate surface are not sufficiently reacted dur- ing TMA pulse, while around 30% generated |–Al(CH3)2 and |– Al(CH3)1 groups do not have enough H2O to take reactions during H2O pulse.Itshould be noted here that we have neglected the reac- tions between surface|–OH groups and the generated|–Al(CH3)2 or |–Al(CH3)1 groups during H2O pulse in the modeling. According to the study of Riikka L. Puurunen [3], they indeed can react with each other to produce Al–O–Al on the substrate surface, but will allow much less Al to be deposited on the substrate surface in the following cycles. Therefore, these observations indicate that suit- ably increasing ALD operation pressure not only can improve the completion of surface reactions in the cycling, but also can improve the production of ALD grown film on the silicon wafer surface.