Atomic Layer Deposition Conformality and Process Optimization: Transitioning from 2-Dimensional Planar Systems to 3-Dimensional Nanostructures
Robertson Cleveland, Erin Darcy
Rubloff, Gary W
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Conformal coatings are becoming increasingly important as technology heads towards the nanoscale. The exceptional thickness control (atomic scale) and conformality (uniformity over nanoscale 3D features) of atomic layer deposition (ALD) has made it the process of choice for numerous applications found in microelectronics and nanotechnology with a wide variety of ALD processes and resulting materials. While its benefits derive from self-limited saturating surface reactions of alternating gas precursors, process optimization for ALD conformality is often difficult as process parameters, such as dosage, purge, temperature and pressure are often interdependent with one another, especially within the confines of an ultra-high aspect ratio nanopore. Therefore, processes must be optimized to achieve self-limiting saturated surfaces and avoid parasitic CVD-like reactions in order to maintain thickness control and achieve uniformity and conformality at the atomic level while preserving the desired materials' properties (electrical, optical, compositional, etc.). This work investigates novel approaches to optimize ALD conformality when transitioning from a 2D planar system to a 3D ultra-high aspect ratio nanopore in the context of a cross-flow wafer-scale reactor used to highlight deviations from ideal ALD behavior. Porous anodic alumina (PAA) is used as a versatile platform to analyze TiO<sub>2</sub> ALD profiles via ex-situ SEM, EDS and TEM. Results of TiO<sub>2</sub> ALD illustrate enhanced growth rates that can occur when the precursors titanium tetraisopropoxide and ozone were used at minimal saturation doses for ALD and for considerably higher doses. The results also demonstrate that ALD process recipes that achieve excellent across-wafer uniformity across full 100 mm wafers do not produce conformal films in ultra-high aspect ratio nanopores. The results further demonstrate that conformality is determined by precursor dose, surface residence time, and purge time, creating large depletion gradients down the length of the nanopore. Also, deposition of ALD films over sharp surface features are very uniform, and verified by profile evolution modeling. This behavior, in contrast to that in high aspect ratio structures, suggests strongly that detailed dynamics, local flow conditions (e.g. viscous vs molecular), surface residence time, and ALD surface reaction kinetics play a complex role in determining ALD profiles for high aspect ratio features.