Low Temperature RF Plasma Annealing Using A NH3-N2 Gas Mixture

Abstract

Techniques such as plasma etching, electron-beam lithography, X-ray lithography, ion etching, sputtering and ion implantation are used to Increase the density of Integration of semiconductor devices. All these techniques introduce undesirable radiation damage into the processed device. Annealing techniques are used to reduce or remove completely the radiation induced defects. The conventional postmetallization low temperature (400–450 °C) annealing using Forming gas (10% H2-90% N2) does not anneal all the charge pentres in many device structures. We have employed a novel low temperature (350°C) rf plasma technique using a NH3-N2 gas mixture to anneal bipolar structures. Vertical pnp transistors made with high energy ion implantation and having poor electrical characteristics have been dramatically improved after 30 min annealing with this new technique. The value of the ideality factor of the base current which was about 1.4 before annealing approached the ideal value of 1.0 after 30 min annealing. Optical emission spectroscopy of the NH3-N2 glow discharge shows the presence of NH radicals, atomic hydrogen and nitrogen ion molecules during the plasma annealing. The atomic hydrogen passivates electrically active defects in the oxide and at the Si/SiO2 interface. A nitridation process occurs at the surface of the top BPSG layer, where a thin silicon nitride film is formed and plays the role of a capping layer which inhibits saturation phenomena. Angle-resolved XPS and ellipsometry have been used to analyze the surface of a silicon wafer exposed to this plasma annealing process. A qualitative model is also proposed to explain the mechanisms involved in this novel NH3-N2 rf plasma annealing process.