Low-oxygen nitride layers produced by UHV ammonia nitridation of silicon

If chemically vapor deposited high permittivity materials such as TiO₂ and Ta₂O₅ are to gain wide acceptance as alternatives to SiO₂ gates in silicon MOSFETs, the interface between the deposited high-k material and the silicon must be abrupt and have a low density of electrically active defects. Unfortunately, the process for depositing these materials often produces an unacceptably thick, low-permittivity amorphous layer at the interface, which reduces the effectiveness of the high-k material and often contains unacceptably large numbers of charge states. One way to prevent this layer from forming is to deliberately introduce a very thin layer of Si₃N₄ to act as a diffusion barrier prior to deposition of the high-k material. Previous work has shown nitrides to have high concentrations of traps and interface states, but these films also had considerable oxygen contamination, particularly at the nitride-silicon interface. In this paper, we show that direct thermal nitridation of the silicon surface in ammonia can provide a low interface state density surface that is also an excellent diffusion barrier. A key feature of this process is the various techniques needed to obtain very low oxygen incorporation in the Si₃N₄. Even at the Si₃N₄-Si interface, the oxygen content was near the detection limits (0.5%) of Auger Electron Spectroscopy (AES). The nitride films were grown in a range of temperatures that resulted in self-limited thicknesses from a few monolayers to a few nanometers. These films were then characterized by Auger, Time-of-Flight SIMS, and in the case of the thicker films, capacitance-voltage techniques on both n- and p-type silicon substrates. The data shows very low levels of oxygen contamination in the nitride films and low interface state densities in capacitors fabricated from this material.