Conformality of TMA/H2O and TMA/O3 processes evaluated using lateral high aspect ratio structures

Atomic layer deposition (ALD) is a fast-growing technique in manufacturing modern electronics due to its ability to produce uniform and conformal thin films with sub-nanometer precision even within high-aspect-ratio cavities. However, reaction kinetics and deposition parameters set the limits how deep the film can be grown conformally within a high-aspect-ratio structure. In this work, we use lateral high-aspect-ratio structure for comparison of thickness profiles of two aluminum oxide Al2O3 deposition processes: trimethylaluminum Al(CH3)3 (TMA) with either water H2O (denoted as TMA/H2O) or ozone O3 (denoted as TMA/O3) as co-reactants. The processes are performed with Veeco-CNT Savannah S200 ALD reactor on PillarHall® LHAR3 test structures in otherwise the same conditions except for the co-reactant pulse. This structure has a lateral trench with gap height of 500 nm and depth of 1 mm, resulting in aspect ratio of 2000.
The thickness profiles obtained with spectroscopic reflectometry show four main differences between the processes. The initial plateau at low depths describes the growth per cycle (GPC). It is 30% higher for TMA/H2O than for TMA/ O3. Since the TMA dose is the same in both processes, we conclude that O3 generates less hydroxyl groups than H2O, which affects GPC [1]. The plateau is followed by a steep slope. The slope at half-thickness value is related to the sticking coefficient of limiting reactant [2]. The slope in this case corresponds to the sticking coefficient of TMA in both processes, indicating that TMA is the limiting reactant in these deposition conditions. The depth of half-thickness value describes the diffusion length of the reactants under the deposition conditions. This value is slightly higher for TMA/O3 even though the limiting TMA dose is the same in both processes. TMA/O3 diffuses therefore slightly longer into trenches than TMA/H2O. However, the total volume of deposited film remains smaller for TMA/O3 due to smaller GPC. The total area beneath the thickness profile curve, which equals to cross-sectional area of the film, for TMA/O3 is 20% smaller than for TMA/H2O.
Lastly, TMA/H2O is compared to another TMA/H2O process deposited in Picosun R-150 ALD reactor on a similar PillarHall® structure [3]. The processes are otherwise nearly identical except for half-thickness depth. This is due to larger dose used in the Picosun reactor increasing the diffusion length of precursors.