HEAVY ION IRRADIATION OF THIN GATE OXIDES

We have studied the gate leakage current after heavy ion irradiation of MOS capacitors with thin gate oxides. In 3-nm and 4-nm oxides radiation-induced soft breakdown (RSB) occurs even after ion fluences as small as 100 ion hits on the device surface. The RSB conductive paths likely reproduce the ion hit distribution: some of them can drive a substantial fraction of the whole gate leakage current. The bias applied during irradiation enhances the RSB current intensity but no critical field exists to ignite the RSB, which is observed also under flat-band. The irradiated 3-nm oxides show smaller current variations and random telegraph signal (RTS) noise than the 4-nm oxides, owing to the higher current driven in fresh devices by direct tunneling conduction. The RTS noise increases with the radiation dose; it can be described successfully neither by a Levy nor by a Gaussian distribution. In 6.5-nm and 10-mn thick oxides the defect clusters generated by heavy ion irradiation can produce RSB and RILC (radiation induced leakage current), which have not been observed after low LET irradiation or electrical stresses.