Isotropic Atomic Layer Etching in High Aspect Ratio Structures

Abstract

Isotropic Atomic Layer Etching can be used to remove materials such as high-k oxides or certain metals from high aspect ratio (HAR) structures from sidewalls and even non-line-of-site locations where access via reactive ion etching would be challenging or impossible. Those locations could be sites below the shelves in 3D-NAND stacks or around wires in GAA structures.The etch rate per cycle of Thermal Isotropic ALE should in principle be insensitive to aspect ratio and line-of-sight accessibility if both reaction steps are fully saturated. Deviations from the ideal behavior have been reported previously for Al2O3 ALE with alternating HF and DMAC cycles [1]. Without mitigation, this behavior would ultimately lead to performance differences in devices located at the bottom of the structure compared to those at the top.In this work we present a method to counteract the unwanted depth dependence of the etch rate when etching HfO2 with the HF/DMAC ligand exchange ALE process. To mitigate the depth dependence, we explored the use of operating the ALE process at ultra-high pressures up to 5 Torr and over a wide temperature range. We found that under these conditions, the modification step has been boosted such that it takes place near its saturation regime so that top-to-bottom etch rate differences could be reduced. We show that high pressure and temperature mostly target the fluorine profile along a structure and removal step times needed to be allowed sufficient time to clear the entire modified layer. We give an example of such application in a 50:1 aspect ratio 3D NAND test structure.