Electrical stresses on ultra-thin gate oxide SOI MOSFETs after irradiation

We present the first experimental data about the wear-out of a 0.1 μm partially depleted SOI CMOS technology after heavy ion irradiation. We show that accelerated life tests based on high electric fields yield significant differences between irradiated and nonirradiated devices, even though no or only minor changes are visible in the characteristics of the devices after irradiation. First, the time to breakdown of the front gate oxide is significantly lower in the irradiated samples. The fresh devices experienced breakdown after 5-8·10²² electrons/cm² were injected across the oxide during high field electrical stress. In the irradiated oxide, the breakdown was reached at much lower injected charge values, between 0.5·10²² electrons/cm² and 1.5-1·10²² electrons/cm² depending on the ion fluences. This translates into a lifetime reduction of 10%-20% of its initial value. Furthermore, the degradation kinetics of the threshold voltage and transconductance peak are strongly affected by ion strikes. In particular, a sudden shift of the threshold voltage and an acceleration in the degradation of the transconductance peak were observed upon the application of an electrical stress, which have no correspondence in nonirradiated devices. An acceleration in the degradation of the parasitic back transistor was observed as well. These phenomena were interpreted in terms of latent damage left by the irradiation in the oxides that make up SOI devices.