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 ºC, and the reaction cross section σ was determined. The cross section at 300 ºC was fairly large (σ=3x10-19 cm2) and it decreased with decreasing temperature. Below 200 ºC, the cross section also clearly varied with the surface coverage. For example, at 100 ºC, the cross section started at σ=1x10-20 cm2 for a full –CH3coverage and decreased to σ=3x10-21 cm2 for a 60% coverage. This coverage dependence of the reaction kinetics also explains the presence of the persistent –CH3groups 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 –CH3groups were not incorporated into the film as a contaminant species. The absolute –CH3 coverage was measured for ALD between 100 and 450 ºC. 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 ºC, and this number decreased to 0.8 at 100 ºC. These insights into the underlying growth mechanism augment the understanding of Al2O3 ALD and reveal several nuances in this well-studied ALD process.
|Tijdschrift||Journal of Vacuum Science and Technology A: Vacuum, Surfaces, and Films|
|Nummer van het tijdschrift||5|
|Vroegere onlinedatum||21 jul 2017|
|Status||Gepubliceerd - 1 sep 2017|