Modelling the chemical mechanism of the thermal atomic layer etch of aluminium oxide: a density functional theory study of reactions during HF exposure

Abstract

Thermal atomic layer etch, the reverse of atomic layer deposition, uses a cyclic sequence of plasma-free and solvent-free gas-surface reactions to remove ultra thin layers of material with a high degree of control. A theoretical investigation of the hydrogen fluoride pulse in the thermal atomic layer etch of monoclinic alumina has been performed using density functional theory calculations. From experiments, it has been speculated that the HF pulse forms a stable and non-volatile layer of AlF3 on alumina surface. Consistent with this, the desorption of an AlF3 molecule from an HF saturated surface was computed to be energetically unfavourable. HF molecules adsorbed on the alumina surface by forming hydrogen bonds, and either remained intact or dissociated to form Al-F and O-H species. At higher coverages, a mixture of molecularly and dissociatively adsorbed HF molecules in a hydrogen-bonded network was observed. Binding energies converged as the coverage of dissociated F became saturated, consistent with a self-limiting reaction. The formation of H2O molecules in the HF pulse was found to be endothermic with an energy barrier of at least +0.9 eV, but their subsequent desorption was computed to cost as little as +0.2 eV. Based on a model of the saturated Al-F surface, the theoretical maximum of the etch rate was estimated to be -0.57±0.02~Å/cycle (-20.0±0.8 ng/(cm2 cycle)), which matches the range of maximum experimental values. The actual etch rate will, however, be dependent on the specific reagent used in the subsequent step of the atomic layer etch cycle.