Conformality of atomic layer deposition analysed via experiments and modelling: case study of zinc oxide for catalytic applications

Abstract

Atomic layer deposition (ALD) has been used in various applications including microelectronics and heterogeneous catalysts. Ideally, ALD enables the growth of homogeneously distributed materials on solid supports including high aspect ratio (HAR) structures. However, to ob-tain conformal ALD coatings on HAR structures, process conditions should be optimized. The goals of this work were (i) to develop and apply a zinc oxide ALD process to prepare diverse copper-zinc oxide on zirconia catalysts for carbon dioxide hydrogenation into methanol and (ii) to investigate the effect of various process parameters on ALD conformality.

Zinc oxide was added on mesoporous zirconia and alumina particles by the reaction of zinc acetylacetonate in a fixed bed ALD reactor. After the reaction, the remaining acac ligands were oxidatively removed in synthetic air at elevated temperatures. The reaction of zinc acetylacetonate on zirconia showed self-terminating behavior with the areal number density of zinc of approximately two atoms per square nanometer. The steric hindrance of bulky acac ligands was likely a saturation-determining factor for zinc obtained by ALD. Meanwhile, an eggshell-type zinc coating was obtained on alumina, and the zinc loading increased when the reactant dose increased. A diffusion–reaction model adapted to spherical supports was used to simulate the effect of reactant exposure on zinc loading. In the simulation, the zinc loading increased with an increase in the reactant exposure. The simulation results fit well with the experimental results. The zinc-after-copper catalyst was superior compared to other copper-after-zinc or copper-only catalysts for carbon dioxide hydrogenation into methanol. The current research showed the importance of tuning of the interaction of zinc and copper for catalytic performance and demonstrated the potential of zinc acetylacetonate as an ALD reactant.

For future ALD conformality studies, a benchmark was proposed using an archetypical trimethylaluminum-water process on lateral HAR microchannels. The effect of process parameters on ALD thickness profiles was investigated using a diffusion–reaction model. For example, penetration depth into microchannels decreased with an increase in the molar mass of ALD reactant and growth per cycle (GPC). The trends of ALD thickness profiles in the free molecular flow regime and transition flow regime were illustrated. This work proposes that the free molecular flow regime and channel filling of less than 5% are the conditions required to obtain fingerprint thickness profile characteristics.