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.
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