Microstructure and Mechanical Behavior of Tantalum Thin Films

技术：钽是一种用途极其广泛的材料，可用于微米或纳米级器件，用途广泛。Ta薄膜可以在两个不同的相中沉积，每个相都具有非常不同的和非常有用的特性。α相是体相材料中的体心立方平衡相。β相是一种四方亚稳相，通常只以薄膜的形式存在。在高温下，β相自发地转变为阿尔法相，但报道的转变温度覆盖了很大的范围。众所周知，TA含有大量的氧气，这已被证明会延缓转变。PIS研究发现，在α相中沉积的薄膜具有典型的柱状晶结构，沿薄膜法线方向择优取向，直径沿薄膜厚度量级择优取向。然而，通过β到α相转变形成的α-Ta薄膜具有微结构，据PI所知，这种微结构在金属材料中从未被报道过。这种组织具有连续取向变化和不连续的晶界结构的特点。了解这种微观结构及其性质，以及了解β相稳定的条件，不仅有助于基础知识，还将为微米和纳米级器件创造新的可能性。该项目将在高水平上将实验工作与建模和模拟相结合，以了解：(1)β相的结构和稳定性，(2)β相向α相转变的详细机制，(3)相变α膜的微观结构，(4)塑性变形和应力水平，以及(5)Ta薄膜中杂质的影响。多年来，PIS开发了专门的设备，使他们能够生成成分和微观结构控制非常好的薄膜，并使用同步加速器X射线和许多其他方法详细询问这些薄膜的不同成分。将进行位错和分子动力学模拟以及密度场理论计算，以了解从原子到薄膜的这些行为。非技术性：贝克小组一直非常积极地向广泛的受众传播科学技术，这一努力将在项目中继续和扩大。像过去一样，PIS希望几个本科生研究人员参与这个项目并公布他们的结果。PIS将继续为K-12学生和公众提供一系列的外展体验。PIS将为初中生和高中生制作一个关于晶体结构的活动，提出一个问题，晶体可以逐渐改变它的取向吗？贝克集团的研究生通过为本科生提供建议和通过K-12外展活动广泛参与外展。PIS将在洞穴和康奈尔理论中心进行虚拟现实3D材料微结构之旅，供学生和科学家使用。作为一个群体，私人投资机构对为高中教师提供参与研究的机会特别感兴趣。PI最近制定了一项计划，邀请92名高中教师参加在波士顿举行的一次重要的技术会议，开展广泛的活动。PIS将通过目前的项目为教师创造研究机会；教师将来到康奈尔大学，参加该项目明确界定的部分。该小组还参与了纳米尺度非正式科学教育项目的形成，该项目将为该小组的研究生和本科生提供广泛的机会，以扩大他们的外展经验和专门知识。

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.