Revisiting the growth mechanism of atomic layer deposition of Al₂O₃: A vibrational sum-frequency generation study

The growth mechanism of the prototypical atomic layer deposition (ALD) process of Al₂O₃ using Al(CH₃)₃ (TMA) and H₂O has been revisited on the basis of insights obtained with the nonlinear optical analysis technique of broadband sum-frequency generation (BB-SFG). With BB-SFG spectroscopy, both the –CH₃ and –OH surface groups ruling the growth of Al₂O₃ by ALD were detected and could be monitored during the ALD process with submonolayer sensitivity. Several remaining questions pertaining to the growth mechanism of Al₂O₃ were addressed. The reaction kinetics of the H₂O half-cycle were studied for ALD between 100 and 300 °C, and the reaction cross section σ was determined. The cross section at 300 °C was fairly large (σ = 3 × 10⁻¹⁹ cm²) and it decreased with decreasing temperature. Below 200 °C, the cross section also clearly varied with the surface coverage. For example, at 100 °C, the cross section started at σ = 1 × 10⁻²⁰ cm² for a full –CH₃ coverage and decreased to σ = 3 × 10⁻²¹ cm² for a 60% coverage. This coverage dependence of the reaction kinetics also explains the presence of the persistent –CH₃ groups at low temperatures which are no longer reactive toward H₂O. By a dedicated study using x-ray photo-emission spectroscopy, it was demonstrated that the persistent –CH₃ groups were not incorporated into the film as a contaminant species. The absolute –CH₃ coverage was measured for ALD between 100 and 450 °C. With this data, steric hindrance was ruled out as the cause of the self-limiting behavior in the TMA half-cycle on basis of the decrease observed in the –CH₃ coverage with temperature. The self-limiting behavior was attributed to the depletion of under coordinated O during the TMA half-cycle. Moreover, the chemisorption of TMA on the -OH surface groups during the TMA half-cycle was investigated. On average, 1.5 –CH₃ ligands remained on the surface per deposited Al atom after the TMA half-cycle at 300 °C, and this number decreased to 0.8 at 100 °C. These insights into the underlying growth mechanism augment the understanding of Al₂O₃ ALD and reveal several nuances in this well-studied ALD process.