Atomic layer etching (ALE), a cyclic process of surface modification and removal of the modified layer, is an emerging damage-less etching technology for semiconductor fabrication with a feature size of less than 10 nm. Among the plasma sources, inductively coupled plasma (ICP) can be a candidate for ALE, but there is a lack of research linking discharge physics to the ALE process. In this study, we comprehensively investigated the discharge physics of ICPs with a radio frequency (RF) bias and Ar/C₄F₆ mixture to be considered for the ALE process. Detailed studies on the discharge physics were conducted in each step of ALE (i.e., modification step, removal step) as well as the whole cycle as follows: (1) In the general ALE cycle, plasma properties dependent on the chamber geometry and the discharge mode of the ICP were analyzed; (2) in the modification step, a plasma instability with molecular gas was observed. The timescale for molecular gas removal was also investigated; (3) in the removal step, changes in plasma characteristics with the RF bias power were studied. Based on measurements of these plasma physical parameters, the discharge condition for ALE was optimized. ALE was performed on various thin films, including a-Si, poly c-Si, SiO₂, and Si₃N₄. For each thin film, thicknesses of 0.5–2.0 nm were etched per cycle, as in quasi-ALE. Finally, ALE was performed on a patterned wafer, and the etch thickness of 0.6 nm per cycle and fine etch profile were obtained.

The microwave cutoff probe (CP) is an accurate diagnostic technique to measure absolute electron density even in processing gas plasmas. Because this technique needs the installation of two probe tips and a probe body in the plasma chamber, it may cause plasma perturbation in semiconductor plasma processing; this may increase the uncertainty of the measured value. In this work, a flat CP, which is embedded in the substrate chuck or chamber wall, is proposed to measure electron density without plasma perturbation and to monitor processing plasma in real-time. We first evaluated the performance of various types of flat CPs, such as the point CP, ring CP, and bar cutoff probe (BCP), through electromagnetic (EM) field simulation. The BCP showed better performance with clearer cut-off signal characteristics and minimization of noise signals compared with the other probe types. Therefore, we focused on the characteristics of the BCP through experiments and/or EM simulations and concluded the followings: (i) the measured electron densities of the BCP agree well with those of the conventional CP; (ii) the BCP measures the plasma density near the plasma-sheath boundary layer, which is very closely adjacent to the chamber wall or wafer; (iii) it was demonstrated for the first time that the plasma density can be measured, even though the processing wafers such as un-doped silicon, P type silicon, amorphous carbon, or amorphous carbon/SiO₂ patterned wafers were placed on the flat CP; and (iv) we performed real-time measurements of the electron density using the BCP covered with the wafers in plasmas with various process gases, such as Ar, NF₃, and O₂. These results indicate that the chuck-embed-type or wall-type flat CP can be used as a real-time electron density measurement (monitoring) tool during industrial plasma processing, such as during etching, deposition, sputtering or implantation, and the chuck-embed-type flat CP can measure the plasma density impinging on the wafer in real-time without stopping the processing.

Geometrically symmetric capacitively coupled oxygen plasmas are studied experimentally by optical emission spectroscopy and probe measurements as well as via numerical simulations using the kinetic Particle-in-Cell/Monte Carlo collision (PIC/MCC) approach. The experiments reveal that at a fixed pressure of 20 mTorr and a driving frequency of 13.56 MHz, the central electron density increases with an increased electrode gap, while the time averaged optical emission of atomic oxygen lines decreases. These results are reproduced and understood by the PIC/MCC simulations performed under identical conditions. The simulations show that the electron density increases due to a mode transition from the Drift-Ambipolar-mode to the α-mode induced by increasing the electrode gap. This mode transition is due to a drastic change of the electronegativity and the mean electron energy, which leads to the observed reduction of the emission intensity of an atomic oxygen line. The observed mode transition is also found to cause a complex non-monotonic dependence of the ${\mathrm{O}}_2^{+}$ ion flux to the electrodes as a function of the electrode gap. These fundamental results are correlated with measurements of the etch rate of amorphous carbon layers at different gap distances.

In the semiconductor industry, fluorocarbon (FC) plasma is widely used in SiO₂ etching, with Ar typically employed in the dilution of the FC plasma due to its cost effectiveness and accessibility. While it has been reported that plasmas with other noble gases, namely Kr and Xe, have distinct physical properties such as electron density and temperature, their implementation into plasma etching has not been sufficiently studied. In this work, we conducted SiO₂ etching with FC plasmas diluted with different noble gases, i.e., FC precursors of C₄F₈ and CH₂F₂ with Ar, Kr, or Xe, under various gas flow rates of each as well as plasma diagnostics for the process interpretation. We show that Ar, Kr, and Xe gas mixtures depend on the FC precursor flow rate and the pattern width in a significantly different manner and we elucidate these findings based on plasma diagnostic results. The results of this work are expected to offer a practical etching database for diverse applications including plasma process engineering and the development of plasma simulation in the semiconductor industry.

In the industrial semiconductor plasma processing, plasma density and its spatial distribution are critical for the understanding of discharge physics and the enhancement of the processing yield via monitoring of the status of plasma processing. Thus, real-time monitoring of plasma density and uniformity in the wafer region during wafer processing have been receiving significant attention. In this study, we developed an electrode-embedded flat-cutoff array sensor for the real-time measurement of electron density uniformity during wafer processing. We measured the spatial distribution of electron density on the wafer surface, compared it with the results measured in the bulk plasma region, and found different plasma distributions between the two where this difference can be attributed to local electron kinetics. We also performed plasma etching of amorphous carbon materials, which showed a clear correlation with the electron density distribution measured by the flat-cutoff array sensor. This indicates that the electron density distribution on the wafer surface is crucial for the process result. Thus, the array sensor developed in this study can be applied not only to measure plasma uniformity but also to predict the wafer process beforehand, making it highly significant for industrial plasma processing methods, such as virtual metrology.