Total-Ionizing-Dose Effects and Low-Frequency Noise in 90 nm, 3.3 V Input/Output CMOS Devices

This study investigates total-ionizing-dose (TID) effects and low-frequency noise (LFN) in 90 nm, 3.3 V input/output (I/O) n-channel metal oxide semiconductor field effect transistors (MOSFETs) following exposure to 60Co γ irradiation. Devices were tested across a range of geometries to evaluate the evolution of DC current-voltage characteristics, LFN, and random telegraph noise (RTN) as a function of TID. After irradiation, a distinct hump emerges in the subthreshold region of the DC drain current versus gate voltage response, indicating the formation of a secondary conduction mechanism. This leakage is independent of device width, suggesting that the mechanism is unlikely to be caused by defect buildup in the gate oxide, which would typically scale with active gate area. Instead, the results suggest that the leakage originates from donor-like defects located in the transition oxide between the gate and the shallow trench isolation (STI) dielectric, i.e., the STI corner. RTN measurements reveal the activation of an electrically active subset of switching traps at biases corresponding to the onset of the subthreshold hump. The hump is attributed to a broader population of localized post-irradiation defects in STI-corner regions. These distributions include both fixed charge and switching traps. The transition of the LFN spectra from ~1/f in as-processed devices to ~1/f2 after irradiation is consistent with the emergence of a dominant trap concentration at a discrete energy. Technology computer-aided design (TCAD) simulations incorporating localized border traps at the STI corner reinforce this interpretation. In contrast, I/O (3.3 V) p-channel MOSFETs investigated under the same irradiation conditions do not show an anomalous subthreshold hump or RTN.