In this study, we evaluate the defects and charges caused by the ultraviolet (UV)/vacuum ultraviolet (VUV) irradiation in the high-k/metal gate stack structure, especially in HfO₂ layer and at Si/HfO₂ interface. First, we measured the photons irradiating to the surface in the neutral beam etching (NBE) system and in the conventional inductively coupled plasma (ICP) system through optical emission spectroscopy (OES), respectively. By using this method, we evaluate the ability of reducing UV/VUV irradiation damage in the NBE system. As a result, photon intensity detected in the ICP system shows larger magnitude as compared to the NBE system, which indicates the UV/VUV irradiation is more severe in the ICP system. Moreover, in order to understand the twisting of electrical characteristics caused by UV/VUV irradiation, we set the prefabricated metal-oxide-semiconductor (MOS) capacitors in both systems to absorb the irradiation of UV/VUV photons respectively. The electrical characteristics of the etched MOS capacitors and its related plasma-induced damage model are discussed. The result of the devices exposed in the ICP system reveals a greater electrical characteristics shift compared to the devices in NBE such as the interface trap density (Dit) in case of NBE is 3.55621×1012 cm-2eV-1 and in case of ICP is higher i.e., 4.19961×1012 cm-2eV-1.