Modeling the impact of incomplete conformality during atomic layer processing

Tobias Reiter, Luiz Felipe Aguinsky, Frâncio Rodrigues, Josef Weinbub, Andreas Hössinger and Lado Filipovic

Atomic layer processing is crucial for the realization of modern microelectronic devices, especially those requiring high aspect ratio (HAR) geometries. In particular, 3D NAND memory stacks depend on the self-limiting nature of atomic layer deposition (ALD) to enhance conformality in ever-increasing HAR trenches and holes. However, as the number of NAND stacks grows and the aspect ratio continues to increase, deviations from full conformality can often occur due to precursor desorption from the surface. We present a model for surface coverage during ALD to include precursor desorption from the surface, which leads to incomplete conformality. The model combines existing Knudsen diffusion and Langmuir kinetics methods and includes the Bosanquet formula for gas-phase diffusivity and reaction reversibility. The parameters for the model are calibrated using published results for the ALD of Al2O3 from trimethylaluminum (TMA) and H2O in HAR geometries. The temperature dependence of the H2O step is likewise analyzed, revealing an activation energy of 0.178 eV, which is consistent with recent experiments. In the TMA step, the Bosanquet formula leads to improved accuracy, and the same parameter set is able to reproduce multiple experiments, demonstrating that the model accurately captures reactor conditions. The model has been incorporated into academic (ViennaPS) and commercial (Silvaco Victory Process) topography simulators, based on the level set approach. The developed model is combined with atomic layer etching (ALE) to simulate the controlled, conformal deposition of HfO2 inside HAR 3D NAND structures.

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