Comparison of charge injection in Si02 and Si3N4 for capacitive RF MEMS switches

Charge injection behaviours in SiO₂ and Si₃N₄ dielectric layers are systematically studied with a Metal-Insulator-Semiconductor (MIS) structure before and after applying a constant dc bias voltage. We found that both the polarity and magnitude of charge accumulation in Si₃N₄ depend on the biasing direction, while the polarity and magnitude of charge accumulation in SiO₂ seems independent on the biasing direction. Charge injection from the Semiconductor to the Si₃N₄ always dominates over charge injection from the metal electrode to the S13N4. Electrons accumulate in the Si₃N₄ when the metal electrode is positively biased, and holes accumulate in the Si₃N₄ when the metal electrode is negatively biased. The hole accumulation is much bigger than the electron accumulation under the same magnitude of bias voltage. However, independent on dc bias polarity, electrons injection in SiO₂ always dominates the charge accumulation either the metal electrode is positively biased or negatively biased. The charge accumulation under negative bias voltage is only slightly different from that under positive bias voltage with same magnitude. Furthermore, the experiment results also show that charge injection level exponentially increases with the applied voltage both in SiO₂ and Si₃N₄. Taking into account the roles of electrons and holes in the process of charge injection, the observed experiment results can be simply explained by the tunnelling barrier at each contact interface.