Low-Oxygen Nitride Layers Produced by UHV Ammonia Nitridation of Silicon

Abstract

ABSTRACTIf 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.