Temperature study of atmospheric-pressure plasma-enhanced spatial ALD of Al2O3 using infrared and optical emission spectroscopy.

Atmospheric-pressure plasma-enhanced spatial atomic layer deposition (PE-s-ALD) is considered a promising technique for high-throughput and low-temperature deposition of ultrathin films for applications where volume and costs are particularly relevant. The number of atmospheric-pressure PE-s-ALD processes developed so far is rather limited, and the fundamental aspects of their growth mechanisms are largely unexplored. This work presents a study of the atmospheric-pressure PE-s-ALD process of Al₂O₃ using trimethylaluminum [TMA, Al(CH₃)₃] and Ar–O₂ plasma within the temperature range of 80–200 °C. Thin-film analysis revealed low impurity contents and a decreasing growth-per-cycle (GPC) with increasing temperature. The underlying chemistry of the process was studied with a combination of gas-phase infrared spectroscopy on the exhaust plasma gas and optical emission spectroscopy (OES) on the plasma zone. Among the chemical species formed in the plasma half-cycle, CO₂, H₂O, CH₄, and CH₂O were identified. The formation of these products confirms that the removal of CH₃ ligands during the plasma half-cycle occurs through two reaction pathways that have a different temperature dependences: (i) combustion reactions initiated by O₂ plasma species and leading to CO₂ and H₂O formation and (ii) thermal ALD-like reactions initiated by the H₂O molecules formed in pathway (i) and resulting in CH₄ production. With increasing temperature, the dehydroxylation of OH groups cause less TMA adsorption which leads to less CO₂ and H₂O from the combustion reactions in the plasma step. At the same time, the higher reactivity of H₂O at higher temperatures initiates more thermal ALD-like reactions, thus producing relatively more CH₄. The CH₄ can also undergo further gas-phase reactions leading to the formation of CH₂O as was theoretically predicted. Another observation is that O₃, which is naturally produced in the atmospheric-pressure O₂ plasma, decomposes at higher temperatures mainly due to an increase of gas-phase collisions. In addition to the new insights into the growth mechanism of atmospheric-pressure PE-s-ALD of Al₂O₃, this work presents a method to study both the surface chemistry during spatial ALD to further extend our fundamental understanding of the method. [ABSTRACT FROM AUTHOR]