Process stability of SiO₂ atomic layer etching in C₄F₆/Ar chemistry

Great interest is shown toward atomic layer etching (ALE) processes due to the better control of the etching process and higher selectivity that they can offer. In order to obtain these benefits, the ALE steps must be self-limited. In the case of SiO₂ ALE, the passivation step often relies on the deposition of a fluorocarbon film on the surface of SiO₂. This reaction is not self-limited, which can lead to a drift of the amount of material etched per cycle with the increasing number of cycles. The drift of these processes can be detected through thickness measurements, but this is often not available in situ in manufacturing tools. For this reason, this study focuses on finding a way to detect the drift of these processes using optical emission spectroscopy (OES) that is more likely available in situ in manufacturing tools. Results presented in this paper first characterize the drift of quasi-ALE of thermal SiO₂ using spectroscopic ellipsometry and x-ray photoelectron spectroscopy. OES spectra are then studied to identify a marker of the drift of the process in agreement with previous measurements. The drift of the process is found to be dependent on the durations of the deposition and activation steps. The intensity of the line of emission at a wavelength of 251 nm, attributed to CF or CF₂, is found to be a marker of the drift of the process.