Low voltage and temperature effects on SILC in stressed ultrathin oxide films

Abstract

It is generally agreed that stress induced leakage current (SILC), in MOS devices, is due to electrons tunneling through stress-induced neutral traps and that SILC has a steady-state component. However, it was observed that SILC, created by positive or negative Fowler–Nordheim injection in 7 and 5 nm thick oxides, decays slowly but continuously when, after stress, the samples are positively or negatively biased at low voltage. The decay is irreversible as long as the gate voltage is less than 4 V. To explain the SILC decay, it has been proposed that some active traps in the tunneling process might possibly be deactivated. The present communication adds first complementary observations on this subject: it shows that the above phenomenon is observed in 3.8 and 3.5 nm thick oxides after a positive stress; that this phenomenon is stable as long as the temperature remains below the SILC annealing temperature threshold which is equal to 200°C; and that during the SILC decay the interface state density does not diminish and can even increase. This communication shows secondly that after a negative stress two SILC components exist in these thin oxides: one which has all the characteristics of the SILC induced by a positive stress and another which does not exist in thicker oxides, which does not decay when a low voltage is applied after stress and which is not annealed at 300°C.