| dc.description.abstract | Silicides play a significant role in modern device technology. The operation of electronic devices heavily relies on the specific properties of metal-semiconductor interfaces. Although semiconductor technology has proven very successful in utilizing the properties of materials the knowledge on formation, structure and electrical behaviour of interfaces is still far from complete. In this study an investigation into the Ni-Si binary system was made. Several techniques namely Rutherford Backscattering Spectroscopy, Transmission Electron Microscopy, Scanning Electron Microscopy, Auger Emission Spectroscopy and X-ray Diffraction were employed in the characterization of nickel thin films on silicon and the respective silicides which were formed. Special attention was given to the phase transition from NiSi to NiSi2. First phase formation, namely Ni2Si, was investigated at a vacuum furnace temperature of 290°C. This phase was found to be polycrystalline and grew in layers of uniform thickness with sharp Si-silicide and Ni-silicide interfaces. Growth continued until all the Ni-metal was consumed. Second phase formation (NiSi) was observed at 330°C only after the Ni2Si has grown to its full thickness. This polycrystalline phase also grows in layers. These layers however, are not of uniform thickness, the interfaces between the silicide and silicon substrate therefore being less regular. It was found that NiSi grains could assume one of two crystal structures, orthorhombic or a FeSi cubic structure. Generally it seems as if NiSi initially crystallizes into an orthorhombic crystal structure, before undergoing an allotropic transformation to the FeSi cubic structure. Micro-diffraction was used to characterize individual grains.
Final phase formation (NiSi2) was mainly examined at 750°C. A scanning electron microscopy investigation showed that after 5 minutes of annealing islands of NiSi2 was observed in a NiSi matrix. With longer annealing times these islands grew laterally and eventually joined up with others. Cross-sectional transmission electron microscopy very firmly confirms the presence of NiSi2 surrounded by NiSi. Rutherford backscattering, X-Ray diffraction and Auger electron spectroscopy complement these results. Scanning electron microscopy shows that after the coalescence of individual NiSi2 islands, holes appear on the grain boundaries. These holes probably result from an accumulation of vacancies on the grain boundaries during NiSi2 formation which occurs via Ni diffusion in NiSi into the underlying silicon. As the NiSi2 phase continues to grow these holes increase in size and later take on the same crystal structure as the surrounding cubic NiSi2 grains. Although the reaction: NiSi + Si => NiSi2 is thermodynamically favourable to occur at 750°C, it was found that even after 15 minutes of annealing at 750°C, some grains were still NiSi while many others had switched to NiSi2. Identification was once again done by micro diffraction. This means that there is more than just the thermodynamic aspect involved in deciding when NiSi should transform to NiSi2. A model has therefore been proposed in which the major factors in determining the time lapse for transformation to take place are presented. This model generally presents an atomistic approach which centres around the degree of Ni diffusion across the grain boundary of two individual grains. Observations also suggests that NiSi2 results from NiSi by a diffusion process although nucleation can take place at random. This model must not be seen as contradictory to models proposed in the literature which only allow for non-uniform growth at the Si-NiSi, interface, but must rather be seen as complementing it by allowing for diffusion processes as well. | en_US |
