During the fabrication of MOS-HEMT based on AlGaN/GaN heterostructure, gate patterning is recognized as the most critical step that can lead to electrical degradation of the transistor. In this work, we performed the SiN cap layer plasma etching processes by two fluorine-based plasma processes (SF6/Ar and CHF3/CF4/Ar) with low (≈15 eV) and high (≈260 eV) ion energy. Moreover, we investigate the post-etching treatment using a KOH solution in order to restore the quality of the AlGaN barrier surface after etching. The objective of this article is to evaluate the AlGaN barrier surface damage after the listed plasma etching processes and post-etching strategies by using quasi-in situ Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS), Transmission Electron Microscopy (TEM) and atomic force microscope (AFM). Accordingly, it is found that both high ion energy plasma processes lead to a significant stoichiometric change and modification of the AlGaN barrier layer into a 1.5 nm F-rich AlGaNFx sub-surface reactive layer. The decrease