|M.Sc Student||Strassberg Rotem|
|Subject||Diffusion-Induced Microstructure Stabilization in|
Thin Nanocrystalline Films
|Department||Department of Materials Science and Engineering||Supervisor||Professor Eugen Rabkin|
Mechanical and electrical properties of thin films such as hardness, yield strength, electrical resistance and stability against electromigration strongly depend on their microstructure. Since almost all thin films are used in contact with a substrate and many also have contacts with overlayers, it is important to consider how interactions with other materials affect the film microstructure, particularly grain growth process.
In this research, grain boundary diffusion and its effects on microstructure was investigated. We studied the grain growth in thin nanocrystalline Au films deposited on Sapphire substrate with and without ultrathin Ti underlayer (adhesion promoter). We selected the thin Au films on Ti underlayer for our studies because of the technological relevance of this system, and because the parameters of grain boundary diffusion and oxidation in this system are well-known. The samples were annealed at 200°C for 2 h in ambient air. Surface topography of the Au thin films was analyzed utilizing Atomic Force Microscopy (AFM). The microstructure was determined with the aid of Transmission Electron Microscopy (TEM), and depth composition profile of Ti in the Au thin films was carried out by Time of Flight - Secondary Ion Mass Spectrometry (TOF-SIMS). The Au films with and without Ti underlayer were compared.
The reference thin Au film without Ti underlayer exhibited a significant grain growth during annealing, whereas no changes in microstructure of Au layer were observed in the Ti/Au bilayers. This stabilization of microstructure of Au layer was attributed to the thermal grain boundary grooves on the Au surface that were filled with Ti oxide. The grooves exhibited an elongated morphology characterized by low value of apparent dihedral angle, atypical for thermal grain boundary grooves in pure metals. The grooves of this morphology were very efficient in pinning of grain boundary motion. A quantitative model of grain boundary grooving coupled with simultaneous grain boundary interdiffusion of Ti and Au in the thin bilayer films was developed. The model predicts the formation of long and narrow grooves at the grain boundaries, with very different apparent and true dihedral angles at the groove root. The apparent dihedral angle decreased at longer annealing times while the true dihedral angle remained constant. Finally, we demonstrated that the low value of apparent dihedral angle of the grooves is responsible for suppressing the grain growth in thin nanocrystalline Au films.