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.

The directional atomic layer etching (ALE) of GaN and AlGaN has been developed. The GaN ALE process consists of cyclic Cl 2 plasma chemisorption and Ar ion removal. The etch per cycle (EPC) was 0.4 nm within the self-limiting regime, which is 50 to 100 V. The root-mean-square surface roughness R RMS was 0.6 nm, which was improved from an initial roughness of 0.8 nm. For AlGaN ALE, BCl 3 was added to the chlorine step to obtain a smooth surface with R RMS of 0.3 nm and stoichiometry similar to the initial sample. The ultra smooth surface obtained by etching is promising for use in next-generation power devices.

An experimental procedure to investigate the spatiotemporal characteristics of electrons in the vicinity of mean energy in a radiofrequency plasma is proposed by using optical emission and absorption spectroscopy in Ar. The method employs optical kinetics of long-lived metastable atom Ar(1s5) and short-lived excited Ar(2p9). The electron density distribution ne(z,t) is demonstrated in a typical condition in a two-frequency capacitively coupled plasma, excited at 100MHz and biased at 500kHz in pure Ar. The density shows almost time independent characteristics in the bulk plasma and a strong time dependence in the sheath in front of the bias electrode.

The paper describes the first measurements of beta and diamagnetism in beam heated current-less plasmas of Heliotron E. These measurements were performed with a diamagnetic coil (1984–1985). The inferred volume average beta shows good agreement with the kinetic beta determined from the Thomson scattering electron temperature and density and the ion temperature profiles. A volume average beta of ⟨βτ⟩DIA = 0.7–0.9% is obtained with 3.6 MW of neutral beam injection heating at 0.94 T. The observed maximum diamagnetic beta is close to and slightly below the value predicted by theoretical MHD predictions. The correlation between the diamagnetic beta and observed MHD fluctuations is discussed. The paper includes a general technique used to analyse the diamagnetism of the plasma confined in a helical heliotron device.

When a pair of short- and long-lived excited molecules are coupled with an upper radiative state, it will be possible to derive the number density of the long-lived species from the optical emission spectroscopy of upper-level short-lived species by careful selection of the pair. Two-dimensional density distributions of the long-lived metastable state Ar(1s5) and that of the short-lived excited state Ar(2p2) are observed and compared in the time-averaged form, using a computerized tomography technique of the emissions from a pair of optical transitions in a two-frequency capacitively coupled plasma in pure Ar.