Control of etch profiles in high aspect ratio holes via precise reactant dosing in thermal atomic layer etching

Thermal atomic layer etching (ALE) was studied in HfO₂-based 3D NAND test structures with an aspect ratio of more than 50:1. Etching was performed via ligand exchange with dimethyl-aluminum chloride (DMAC) after surfaces had been fluorinated with hydrogen fluoride (HF). In these 3D NAND structures, we found that the horizontal etch rate of HfO₂ as a function of depth (depth loading) depended on the DMAC dosing but was nearly independent of the HF dose. The HF dose and the process pressure were keys to increasing the overall etch amount per cycle. With the highest tested HF dose of 192 Torr s and a total process pressure of 8 Torr, we achieved a uniform etch amount of 0.6 nm per cycle. In addition, we investigated the impact of film quality and film coating conformality in these structures on the depth loading in the succeeding ALE processes. The type of precursor, precursor dosing, deposition rate, and substrate temperature played a fundamental role in controlling the film quality and conformality of the deposited HfO₂ layers inside high aspect ratio holes. Fluorination studies on blanket films revealed that fluorination efficiency is improving for pressures in the Torr range compared to previous milliTorr experiments and that only temperatures above 250 °C increased the fluorine concentration in HfO₂ significantly, whereas fluorine levels were unchanged between 150 and 250 °C.