Thin metal films are critical elements in many nano- and micro-fabricated technologies including microelectronics, optics, sensors, and catalysts. Due to dimensional constraints, such films are often found to be textured; that is, the individual metal crystallites comprising the film are preferentially oriented with certain types of crystal planes parallel to the film plane. Since the properties of the film depend strongly on the orientations present, understanding texture is critical to understanding the performance and reliability of devices containing thin films. There is good general agreement on the driving forces leading to texture formation. However existing models predict that only one texture component should occur at equilibrium, while mixed textures are common. To understand this, the kinetics of texture transformations in real films must be understood. In this project, the Baker group will study the kinetics of the transformation from as-deposited to as-annealed texture in thin metal films. Preliminary work suggested that inhomogeneous 3-D stress states that arise in such films may stabilize the mixed texture. To investigate this, the Baker group will produce films with very good control over film structure and chemistry, characterize film structure, including measurements of grain size distributions as a function of orientation, and will determine the volume fractions of the different texture components, and stress states in them, using in-situ synchrotron x-ray diffraction and TEM methods. They will use a high-throughput method that allows them to investigate multiple parameters with every film deposition. Detailed finite element simulations will be conducted to study the stress distributions within individual crystallites, and analytical models will be used to link stress states, kinetic models, and thermodynamic models into a construct that should strongly enhance prediction and control of thin film texture, and therefore properties.
NON-TECHNICAL SUMMARY: Thin metal films---metal layers less than one tenth the thickness of a human hair---are essential elements in computer chips, optical systems, catalytic converters, and many other high-tech devices. These films are made up of many tiny metal crystals, called "grains". Because the films are so thin, grains tend to orient themselves so that certain directions in their crystal structure align with the plane of the film. The properties of thin films, and therefore the performance and reliability of devices containing thin films, depend very sensitively on these orientations. This topic has been studied for many years but people do not yet have the ability to predict how a film or a device will behave. The Baker group at Cornell University has proposed that the way that loads are distributed across the grains can stabilize different combinations of orientations. To study this, they will make films, characterize their structure and behavior using sophisticated tools such as the Cornell High Energy Synchrotron Source (CHESS) and will generate computer models to help interpret their results. The knowledge generated in this project will help make it possible to continue to miniaturize the next generation of nanofabricated devices and should help to improve performance and reliability in all devices that contain thin metal films. This project will involve undergraduates at both Cornell and at Houghton College, a small non-PhD-granting institution in upstate New York. Undergraduate participation will enhance both the scientific output of the project and the educational experience of those students. Undergraduates will participate fully in the research, and will present their work in presentations at professional society meetings and in peer-reviewed papers. Houghton students will be advised at Houghton by Prof. Brandon Hoffman, but will spend summers working with the Baker group at Cornell. Baker group graduate students and post-docs are active in outreach activities to area schools and institutions. A benefit of the current project is that images of grain orientation distributions can be quite striking and can often stand on their own as art, making a nice icebreaker for talking about materials science to non-scientists.
