Influence of LDD Spacers and H$^+$ Transport on the Total-Ionizing-Dose Response of 65-nm MOSFETs Irradiated to Ultrahigh Doses

The degradation induced by ultrahigh total ionizing dose in 65-nm MOS transistors is strongly gate-length dependent. The current drive decreases during irradiation, and the threshold voltage often shifts significantly during irradiation and/or high-temperature annealing, depending on transistor polarity, applied field, and irradiation/annealing temperature. Ionization in the spacer oxide and overlying silicon nitride layers above the lightly doped drain extensions leads to charge buildup as well as the ionization and/or release of hydrogen. Charge trapped in the spacer oxide or at its interface modifies the parasitic series resistance, reducing the drive current. The released hydrogen transports as H$^{+}$ with an activation energy of ~0.92 eV. If the direction of the electric field is suitable, the H$^{+}$ can reach the gate oxide interface and depassivate Si-H bonds, leading to threshold voltage shifts. Newly created interface traps are most prominent near the source or drain. The resulting transistor responses and defect-energy distributions often vary strongly in space and energy as a result, as demonstrated through current-voltage, charge-pumping, and low-frequency noise measurements.