Electrical reliability aspects of HfO_2 high-k gate dielectrics with TaN metal gate electrodes under constant voltage stress

摘要

The dielectric breakdown property of ultrathin 2.5 and 5.0 nm hafnium oxide (HfO_2) gate dielectric layers with metal nitride (TaN) gate electrodes for metal oxide semiconductor (MOS) structure has been investigated. Reliability studies were performed with constant voltage stressing to verify the processing condition effects (film thicknesses and post metal annealing temperatures) on times to breakdown. The leakage current characteristics are improved with post metal annealing temperatures (PMA) for both 2.5 and 5.0 nm HfO_2 physical thicknesses. However, it is more prominent (～2 orders of magnitudes) for 2.5 nm HfO_2 film thickness. The values of oxide-trapped charge density and interface-state density are also improved for 2.5 nm HfO_2 film. The different stages of charge-trapping behaviors, i.e., stress-induced leakage current, soft and hard breakdown mechanisms have been detected. During constant voltage stress of the MOS capacitors, an increase in the time-dependent gate current is observed, followed by the occurrence of several fluctuations. The amplitude of the fluctuations is much larger in the 5.0 nm HfO_2 gate dielectric layer compared to the 2.5 nm HfO_2 layer. After the occurrence of such fluctuations, the current-voltage characteristics exhibited an increased in gate current compared to the fresh (unstressed) devices.