Microstructure and Mechanical Behavior of Tantalum Thin Films

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TECHNICAL: Tantalum is an extremely versatile material for use in micro- or nanofabricated devices, and it is used in a wide variety of applications. Ta thin films can be deposited in two different phases each having very different and very useful properties. The alpha phase is the bcc equilibrium phase found in bulk materials. The beta phase is a tetragonal metastable phase that is normally only found in thin film form. At high temperatures, the beta phase spontaneously transforms to the alpha phase, but reported transformation temperatures cover a large range. Ta is known to incorporate a large amount of oxygen and this has been shown to retard the transformation. PIs have found that films deposited in the alpha phase have a typical grain structure of columnar grains with preferred orientation along the film normal and diameters on the order of the film thickness. Alpha-Ta films formed by the beta to alpha phase transformation, however, have a microstructure which, to PIs' knowledge, has never before been reported for a metallic material. This structure is characterized by continuous orientation changes and a discontinuous grain boundary structure. Understanding this microstructure and its properties, as well as understanding the conditions under which the beta phase may be stabilized, are expected not only to contribute to fundamental knowledge, but to generate new possibilities for micro- and nanofabricated devices. The project will combine experimental work with modeling and simulation both at a high level to understand: (1) the structure and stability of the beta phase, (2) the detailed mechanism of the beta to alpha phase transformation, (3) the microstructure of phase-transformed alpha films, (4) plastic deformation and stress levels, and (5) effects of impurities in thin Ta films. PIs have developed specialized equipment over the years that will allow them to generate films with very good control over composition and microstructure, and to interrogate the different constituents of those films in detail using synchrotron x-rays and many other methods. Dislocation- and Molecular-dynamics simulations and density field theory calculations will be done to understand these behaviors across scales from atoms to films. NON-TECHNICAL: The Baker group has been very active in communicating science and technology to a wide range of audiences, and this effort will be continued and expanded in the project. As in the past, PIs expect several undergraduate researchers to participate in this project and to publish their results. PIs will continue to provide a series of outreach experiences to K-12 students and the general public. PIs will generate an activity on crystal structures for middle and high school students, asking the question, can a crystal change its orientation gradually? Baker group graduate students participate extensively in outreach through advising undergraduates and through K-12 outreach activities. PIs will make a virtual reality 3-D materials microstructure tour at the CAVE and the Cornell Theory Center for use by students and scientists alike. As a group PIs have a particular interest in providing high school teachers with opportunities to participate in research. The PI recently generated a program to bring 92 high school teachers to a major technical meeting in Boston for a wide range of activities. PIs will generate research opportunities for teachers through the present project; teachers will come to Cornell and participate in a well defined portion of the project. The PI also participated in the formation of the Nanoscale Informal Science Education (NISE) project which will provide the graduate and undergraduate students in the group with a wide array of opportunities to expand their outreach experience and expertise.