Monte Carlo simulations of atomic layer deposition on 3D large surface area structures: Required precursor exposure for pillar-versus hole-type structures

Due to its excellent conformality, atomic layer deposition (ALD) has become a key method for coating and functionalizing three dimensional (3D) large surface area structures such as anodized alumina (AAO), silicon pillars, nanowires, and carbon nanotubes. Large surface area substrates often consist of arrays of quasi-one-dimensional holes (into which the precursor gas needs to penetrate, e. g., for AAO), or forests of pillars (where the precursor gas can reach the surface through the empty 3D space surrounding the pillars). Using a full 3D Monte Carlo model, the authors compared deposition onto an infinite array of holes versus an infinite array of pillars. As expected, the authors observed that the required exposure to conformally coat an array of holes is determined by the height to width ratio of the individual holes, and is independent of their spacing in the array. For the pillars, the required exposure increases with decreasing center-to-center distance and converges in the limit to the exposure of an array of holes. Our simulations show that, when targeting a specific surface area enhancement factor in the range 20-100, a well-spaced pillar geometry requires a 2-30 times smaller precursor exposure than a hole geometry and is therefore more ALD friendly. The difference in required exposure is shown to depend on the initial sticking probability and structural dimensions. (C) 2016 American Vacuum Society.