The distribution of radiation-induced charged defects and neutral electron traps in SiO2, and the threshold voltage shift dependence on oxide thickness

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Abstract
As insulated gate field effect transistor dimensions continue to decrease, and fabrication sequences rely increasingly on processes that involve ionizing radiation, it becomes essential to understand the radiation-induced threshold voltage shift (ΔVT) dependence on gate-insulator thickness (tox), since threshold voltage tolerances are required to scale with device dimensions. In the present study, n-channel insulated gate field effect transistor devices were fabricated with gate-insulator thicknesses ranging from 6–50 nm, and were then exposed in an unbiased state to AlKα x-ray radiation to simulate process-induced ionizing radiation exposure. Gate-oxide Coulombic defects and neutral electron traps were measured before and after irradiation using optically assisted electron injection. Following irradiation and injection, the measured voltage shifts indicated that the ‘‘extrinsic’’ defects are localized near, but not at, the Si/SiO2 interface. For oxide thicknesses where the top gate electrode lies in the region above the extrinsic defect volume ΔVT is found to be linear in tox; at thicknesses where the top gate electrode encroaches upon the defect region, ΔVT is found to be quadratic in tox, and for very thin oxides, ΔVT is observed to approach zero. A defect distribution model that applies to process-induced radiation exposure is formulated to explain this behavior. This model also provides a unique and simple method for determining the defect centroid in gate insulators that have been exposed under such irradiation conditions.