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Understanding the Evolution of Residual Stress in Polycrystalline Thin Films
Abstract: Thin films go through a range of stress states during deposition, often changing from compressive to tensile and back again. The final stress state can vary from highly tensile to highly compressive depending on the processing conditions (temperature, growth rate, etc.). The resulting residual stress and stress gradients can severely limit performance (due to cracking, delamination and other failure mechanisms) which provides significant motivation for understanding and controlling stress evolution. We have developed an analytical model that describes residual stress in terms of a kinetic competition between different generation and relaxation mechanisms. The balance between them shifts as the microstructure evolves from isolated islands, through coalescence and finally into a steady state uniform film. For both high mobility and low mobility films the steady state stress scales with the dimensionless parameter D/LR where D is the diffusivity, R is the growth rate and L is the grain size. The model also explains the transition between tensile and compressive stress with thickness of the film. Real-time measurements of stress using wafer curvature are compared with the model predictions to show the validity of this approach.
Biography: Eric Chason is a professor in the School of Engineering at Brown University. He received his Ph.D. degree in physics in 1985 from Harvard University where he worked on the structure and properties of amorphous metals and oxides. After one year of post-doctoral research in Japan, he joined Sandia National Laboratories in 1987 and became a senior member of the technical staff. He became a member of the Brown faculty in 1998.
Chason’s research focuses on the evolution of surfaces and thin films during materials processing. This work has lead to the development of several in situ thin film diagnostics that enable the monitoring of thin film stress, surface morphology, microstructure and interfacial reactions during film growth and ion bombardment. This includes the development of a multi-beam optical technique for monitoring stress evolution in situ during processing. In recent years, his research projects have included residual stress in polycrystalline films, whisker formation in Sn films, ion-induced surface nano-patterning and growth of large-area single crystal substrates by electrodeposition.
Faculty Host: Karl Ludwig
Student Host: Gozde Erdem