Properties of iridium-inserted nickel silicides by thermal annealing of the Ni/Ir bilayer on silicon and polysilicon substrates

Abstract

To improve the thermal stability of the conventional nickel monosilicide, 10 nm-Ni/l nm-Ir/p-Si(100) (or poly-crystalline Si) was thermally annealed using rapid thermal annealing for 40 s at 300–1200°C. The annealed bilayer structure developed into Ni(Ir)Si, and the resulting changes in sheet resistance, microstructure, and composition were investigated using a four-point probe, a scanning electron microscope, a field ion beam, an X-ray diffractometer, and an Auger electron spectroscope. The final thickness of Ni(Ir)Si_x formed on single crystal silicon was approximately 40 nm, and it maintained a sheet resistance of below 20 ω/sq. during the silicidation annealing at 1200°C. The silicide formed on polysilicon had a thickness of 55 nm, and its low resistance was maintained up to 850°C. An additional annealing of silicides at 900°C for 30 min. resulted in a drastic increase in sheet resistance. A possible reason for the improved thermal stability of the silicides formed on single crystal silicion substrate is the role of iridium in preventing NiSi₂ transformation. Iridium also improved the thermal stability of the silicides formed on the polysilicon gate, but this enhancement was lessened due to the formation of NiIrSi_x and also as a result of silicon mixing during high temperature diffusion. In conclusion, the proposed iridium-inserted nickel silicides may be superior to the conventional nickel monosilicides due to improved thermal stability.