(Invited) The Use of Plasmas for Isotropic Atomic Layer Etching
Authors: Chittock Nicholas John , Kessels Wilhelmus M. M. (Erwin) , Knoops Harm , Mackus Adrie 
Published: February 2024

Atomic layer etching (ALE) is set to be a vital part of the advanced semiconductor manufacturing toolbox, known for its precise control of the film thickness and minimal damage. These benefits are possible due to the sequential self-limiting half-cycles that are employed within an ALE process. Initially, ALE was underestimated due to low etch rates, but it is now experiencing a renaissance due to the requirements imposed by further downscaling.1

The ALE community is mostly divided into two groups: plasma anisotropic and thermal isotropic etching. 2 In this work, the focus is on exploring isotropic plasma ALE, which is a relatively unexplored direction. This approach involves using plasma radicals to modify and/or etch material isotropically, while minimizing the directional ion component of the plasma. Isotropic plasma ALE can offer several benefits over strictly thermal ALE, such as higher etch rates, lower operating temperatures, and the ability to etch more resistant materials.
In this talk we will first address some of the different techniques employed for plasma isotropic ALE in the literature. Subsequently, we will discuss our own work, which has focused on two isotropic plasma ALE chemistries. The first involves alternating between plasma fluorination and ligand-exchange half-cycles, this ALE chemistry has been found to be effective for etching Al2O3, HfO2, AlN, and GaN films.3,4 Surface contaminants and roughness were found to be reduced post ALE, an aspect that is particularly important for AlN and GaN films that are used in quantum, power semiconductor and LED applications. The second approach involves using diketone (e.g. acetylacetone, hexafluoroacetylacetone) doses followed by either H2 or O2 plasma exposures for ALE of Al2O3 and ZnO.5 The diketone in this process acts to etch the surface, however due to competitive adsorption of different diketone orientations an etch inhibition layer forms. The plasma step is then utilized to clean the surface, removing the self-limiting inhibitor layer, enabling etching to continue in the next cycle. This ALE chemistry diverges from the typical modification/removal ALE cycle, where a modified layer if often observed on the material post ALE. In comparison the diketone ALE chemistry does not rely on modification of the surface making it an ideal process for surface cleaning. This approach provides improved selectivity over the plasma fluorination process and also enables smoothing.
Plasma isotropic ALE is a growing part of the ALE community that can help accelerate the adoption of ALE in academia and industry. The potential benefits of ALE are only just beginning to be explored, and more processes are needed to etch the ever-increasing range of materials utilized in IC manufacturing.

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