Breakdown characteristics of ultra-thin gate oxides caused by plasma charging were studied in this work. It is observed that as oxide thickness is scaled down to 4 nm, some traditional monitor parameters may lose their sensitivity for detecting oxide degradation induced by plasma charging damage, due to insignificant trap generation. Even the gate leakage current, although sensitive for 4 nm oxide, may no longer be sensitive enough for even thinner oxide (e.g., 2.6 nm), due to the existence of large tunneling current. Moreover, several soft-breakdown events were found to occur in ultrathin oxide before the final onset of a catastrophic hard-breakdown. Finally, an equivalent local oxide thickness is calculated using local oxide thinning model to estimate the stepwise increase of gate current after soft-breakdown event.