Charge trapping in thin SiO2 layers. Application to the breakdown of MOS

Positive and negative charging is commonly observed in thin SiO2 layers subjected to high electric fields. Under Fowler-Nordheim electron injection that requires fields exceeding 7 MV/cm, positive charging is formed, whereas negative charging is produced at lower fields. In both cases breakdown is obtained after the injection of a critical amount of charges Q(BD), called ''charge to breakdown''. Positive and negative charging induced breakdown is interpreted on the basis of the polarization/relaxation process developed to interpret breakdown in high voltage capacitors. In agreement with experiments, it is demonstrated that negative charging induced breakdown is due to the detrapping of a critical density of electrons. The consequence is that Q(BD) is roughly constant when the current density J is less than a critical value J(c) and decreases very rapidly when J > J(c). When positive charging is formed, breakdown is attributed to the neutrization of positive charges by the injected electrons. in this case breakdown occurs when the product JxN(c)(+) (where N-c(+) is the density of positive trapped charges) reaches a critical value. The charge to breakdown Q(BD) decreases with J according to an expression of the form Q(BD) alpha exp J(o)/J where J(o) is an appropriate parameter characteristic of the dielectric material.