Surface reaction modelling of thermal atomic layer etching on blanket hafnium oxide and its application on high aspect ratio structures

Thermal atomic layer etching is rapidly becoming an important complementary processing technology in the manufacturing of 5 and 3 nm devices in the semiconductor industry. Critically, architectures such as 3D NAND and 3D DRAM require conformal isotropic etching to remove material such as HfO2 in hard-to-reach locations with aspect ratios that can be greater than 50:1. To achieve repeatable device performance throughout a 3D stack, the removal rate (etch per cycle) of the etched material during an etch process needs to be controlled such that the overall etch amount is the same from top to bottom of the device stack. In this work, the reaction kinetics of reactants and byproducts during a cyclical ligand exchange-based atomic layer etching (ALE) process have been modelled. This ALE process consists of two steps: a fluorination step followed by a fluorine-to-chlorine ligand exchange-based removal step. Modeling was performed for each of those steps separately. Experimental data revealed that the fluorine dosing during the fluorination step was predominantly responsible for controlling the etch rate of the ALE process but had only a minimal impact on the etch profile inside high aspect ratio holes. The ligand exchange dosing, on the other hand, predominantly controlled the etch profile (depth loading) with equal etch rates from top-to-bottom, obtained when the step was operated close to saturation. The model predicts that the chemical reaction rate of dimethylaluminum chloride (DMAC) on a fluorinated surface during the ligand exchange step is 9.1 s−1, about 46 times greater than the reaction rate of hydrogen fluoride (HF) on the hafnium oxide surface during the fluorination step (only 0.2 s−1). Furthermore, modeling results revealed that the sticking coefficient for DMAC on a hafnium fluoride surface far exceeded that of HF on a hafnium oxide surface in the conditions modelled (0.94 s−1 for DMAC vs 0.0058 s−1 for HF). With these modeling results, the different roles fluorination and ligand exchange steps have regarding the control of etch rate per cycle and profile inside high aspect ratio holes can be explained.