Charge trapping in thin SiO2 layers: application to the breakdown of MOS

Positive and negative charging is commonly observed in thin SiO ₂ 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 neutralization of positive charges by the injected electrons. In this case breakdown occurs when the product J×N_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 _BDαexpJ₀/J where J₀ is an appropriate parameter characteristic of the dielectric material