The rapid development of nanotechnology, especially in the field of microelectronics, and ever shrinking dimensions of device components set high requirements for the manufacturing of the necessary nanostructures. Many microscopic components, e.g. transistors, are constructed layer-by-layer from thin film. An important tool 21st century technique for the fabrication of such thin films is the atomic layer deposition. Atomic layer deposition, originally developed in Finland, is based on sequential self-limiting gas-pulses, resulting in a uniform, pin-hole free thin film, with thickness control at the atomic level.
Computational modeling is an important part of modern chemistry. Research can be conducted theoretically - without empirical parameters - with the application of quantum mechanics. With quantum mechanical calculations it is possible to model the electronic structure of molecules and to study the bonding and interactions of molecules as well as different molecular mechanisms. In this work, the deposition of aluminium and zinc oxides were studied using computational chemistry. Both oxides have wide range of applications e.g. in transistors and solar cells.
Aluminium oxide is usually deposited using a trimethylaluminium-water-process. The surface chemistry was studied on a realistic hydroxylated surface model and trimethylaluminium was observed to react rapidly with surface hydroxyl groups to produce monomethylaluminium. Monomethylaluminium was estimated to be relatively inert and to convert to aluminium only at high temperatures. Subsequent water pulse mechanisms were also studied at low methyl-coverage. Direct dimethylaluminium--water reactions were accessible at process conditions, but the elimination of monomethylaluminium by water requires a complex cooperative mechanism.
Zinc oxide is usually deposited using a diethylzinc-water-process. Diethylzinc was found to convert rapidly into monoethylzinc but the elimination of monoethylzinc was found to be a slow process. Based on the calculations, two ethyl-saturated surface structures were constructed, corresponding to low and high temperature estimations. These saturated surfaces were used in a subsequent study on the water pulse reactions, resulting in a reaction network for a complete ALD cycle.
The growth of the zinc oxide thin film was then modeled in macroscopic scale using a kinetic Monte Carlo model. The kinetic modelling enables a direct comparison with experimental measurements. The kinetic model, built upon the theoretical calculations, accurately predicted the temperature-dependency of the film growth. Also, the predicted growth per cycle is in good agreement with experimental data.
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- atomic layer deposition, density functional theory, kinetic Monte Carlo