SOFT BREAKDOWN OF ULTRA-THIN GATE OXIDE LAYERS

The dielectric breakdown of ultra-thin 3 nm and 4 nm SiO2 layers used as a gate dielectric in poly-Si gate capacitors is investigated. The u ltra-thin gate oxide reliability was determined using tunnel current i njection stressing measurements. A soft breakdown mechanism is demonst rated for these ultra-thin gate oxide layers. The soft breakdown pheno menon corresponds with an anomalous increase of the stress induced lea kage current and the occurrence of fluctuations in the current. The so ft breakdown phenomenon is explained by the decrease of the applied po wer during the stressing for thinner oxides so that thermal effects ar e avoided during the breakdown of the ultra-thin oxide capacitor. It i s proposed that multiple tunnelling via generated electron traps in th e ultra-thin gate oxide layer is the physical mechanism of the electro n transport after soft breakdown. The statistical distributions of the charge to dielectric breakdown and to soft breakdown for a constant c urrent stress of the ultra-thin oxides are compared. It is shown that for accurate ultra-thin gate oxide reliability measurements it is nece ssary to take the soft breakdown phenomenon into account.