Atomic layer etching (ALE) is a technique for removing thin layers of material using sequential reaction steps that are self-limiting. ALE has been studied in the laboratory for more than 25 years. Today, it is being driven by the semiconductor industry as an alternative to continuous etching and is viewed as an essential counterpart to atomic layer deposition. As we enter the era of atomic-scale dimensions, there is need to unify the ALE field through increased effectiveness of collaboration between academia and industry, and to help enable the transition from lab to fab. With this in mind, this article provides defining criteria for ALE, along with clarification of some of the terminology and assumptions of this field. To increase understanding of the process, the mechanistic understanding is described for the silicon ALE case study, including the advantages of plasma-assisted processing. A historical overview spanning more than 25 years is provided for silicon, as well as ALE studies on oxides, III–V compounds, and other materials. Together, these processes encompass a variety of implementations, all following the same ALE principles. While the focus is on directional etching, isotropic ALE is also included. As part of this review, the authors also address the role of power pulsing as a predecessor to ALE and examine the outlook of ALE in the manufacturing of advanced semiconductor devices.

Thermal atomic layer etching (ALE) can be performed using sequential reactions based on surface modification followed by volatile release of the modified surface layer. Surface modification can be accomplished using fluorination. Volatile release can then be achieved using precursors that undergo ligand-exchange reactions with the fluorinated surface layer. Metal fluorides can be employed to model the fluorinated surface layer. The ligand-exchange reaction between the precursor and the metal fluoride can lead to spontaneous etching of the metal fluoride. A new reactor with in situ quadrupole mass spectrometry (QMS) was constructed to observe the volatile etch products from the reaction of ligand-exchange precursors with metal fluoride powders. The metal fluoride powders were AlF3, HfF4, GaF3, InF3, and SnF4. The ligand-exchange precursors were Al­(CH3)3, SiCl4, and TiCl4. A variety of studies were conducted including Al­(CH3)3 + AlF3, SiCl4 + HfF4, SiCl4 + InF3, TiCl4 + SnF4, Al­(CH3)3 + GaF3, and SiCl4 + AlF3. The temperature-dependent in situ QMS studies revealed the many possibilities that occur during the ligand-exchange reaction of precursors with metal fluoride powders. Various categories of behavior were observed from these studies: (i) Ligand exchange occurs at low temperature, but metal etch products from the substrate are not observed until high temperature. (ii) Ligand-exchange and metal etch products from the substrate are observed at similar temperatures. (iii) Ligand exchange occurs, but no metal etch products from the substrate are observed up to a limiting temperature. Knowledge of these possibilities for the ligand-exchange reaction between precursors and metal fluoride powders during spontaneous etching helps to further the understanding of thermal ALE.

Atomic layer etching (ALE) is a multistep process used today in manufacturing for removing ultrathin layers of material. In this article, the authors report on ALE of Si, Ge, C, W, GaN, and SiO2 using a directional (anisotropic) plasma-enhanced approach. The authors analyze these systems by defining an “ALE synergy” parameter which quantifies the degree to which a process approaches the ideal ALE regime. This parameter is inspired by the ion-neutral synergy concept introduced in the 1979 paper by Coburn and Winters [J. Appl. Phys. 50, 5 (1979)]. ALE synergy is related to the energetics of underlying surface interactions and is understood in terms of energy criteria for the energy barriers involved in the reactions. Synergistic behavior is observed for all of the systems studied, with each exhibiting behavior unique to the reactant–material combination. By systematically studying atomic layer etching of a group of materials, the authors show that ALE synergy scales with the surface binding energy of the bulk material. This insight explains why some materials are more or less amenable to the directional ALE approach. They conclude that ALE is both simpler to understand than conventional plasma etch processing and is applicable to metals, semiconductors, and dielectrics.

This article reviews the state-of-the art status of thermal atomic layer etching of various materials such as metals, metal oxides, metal nitrides, semiconductors, and their oxides. We outline basic thermodynamic principles and reaction kinetics as they apply to these reactions and draw parallels to thermal etching. Furthermore, a list of all known publications is given organized by the material etched and correlated with the required reactant for each etch process. A model is introduced that describes why in the nonsaturation mode etch anisotropies may occur that can lead to unwanted performance variations in high aspect ratio semiconductor devices due to topological constraints imposed on the delivery of reactants and removal of reactant by-products.

In this article, we have performed detailed investigations of the 193 nm photoresist transformations after exposure to the so-called HBr and Ar plasma cure treatments using various characterization techniques (x-ray photoelectron spectroscopy, Fourier transformed infrared, Raman analyses, and ellipsometry). By using windows with different cutoff wavelengths patched on the photoresist film, the role of the plasma vacuum ultraviolet (VUV) light on the resist modifications is clearly outlined and distinguished from the role of radicals and ions from the plasma. The analyses reveal that both plasma cure treatments induce severe surface and bulk chemical modifications of the resist films. The synergistic effects of low energetic ion bombardment and VUV plasma light lead to surface graphitization or cross-linking (on the order of 10 nm), while the plasma VUV light (110–210 nm) is clearly identified as being responsible for ester and lactone group removal from the resist bulk. As the resist modification depth depends strongly on the wavelength penetration into the material, it is found that HBr plasma cure that emits near 160–170 nm can chemically modify the photoresist through its entire thickness (240 nm), while the impact of Ar plasmas emitting near 100 nm is more limited. In the case of HBr cure treatment, Raman and ellipsometry analyses reveal the formation of sp2 carbon atoms in the resist bulk, certainly thanks to hydrogen diffusion through the resist film assisted by the VUV plasma light.