Driving forces and orientation selection during texture transformations in thin metal films

非技术总结金属薄膜(金属层厚度不到人类头发厚度的十分之一)是计算机芯片、光学系统、催化转换器和许多其他高科技设备的基本元件。这些薄膜是由许多微小的金属晶体组成的，这些晶体被称为“颗粒”。由于薄膜很薄，颗粒倾向于自我定向，从而使其晶体结构中的某些方向与薄膜的平面一致。薄膜的性能，以及包含薄膜的器件的性能和可靠性，都非常敏感地取决于这些取向。这个话题已经研究了很多年，但人们还没有能力预测一部电影或一个设备将如何表现。在一个特别令人烦恼的问题上，用一组晶体取向制作的薄膜有时会随着时间的推移改变成不同的取向，从而戏剧性地改变属性。到目前为止，还不可能预测这种转变将在何时发生。一些人提出，初始颗粒必须有缺陷，即晶体结构中的缺陷。这些缺陷代表了过剩的能量，因此，如果新的无缺陷的颗粒可以取代有缺陷的颗粒，薄膜可以达到更稳定、更低的能量状态。然而，这一论点并不能解释为什么应该形成一个新的取向。为了了解这个问题，康奈尔大学的贝克小组将制作具有广泛缺陷结构的薄膜，并将使用诸如康奈尔高能同步加速器源(CHESS)、新墨西哥州阿尔伯克基桑迪亚国家实验室的离子束实验室等复杂工具来表征这些结构和相关的薄膜行为。他们将生成预测性模型来帮助解释他们的结果。在这个项目中产生的知识将有助于继续使下一代纳米软化设备微型化，并应有助于提高所有包含金属薄膜的设备的性能和可靠性。这个项目将涉及霍顿学院的本科生，这是一家位于纽约州北部的小型非授予博士学位的机构。本科生的参与将提高项目的科学产出和这些学生的教育经验。霍顿的学生将在霍顿接受布兰登·霍夫曼教授的建议，但他们将在夏季与康奈尔大学的贝克小组一起工作。贝克集团的研究生和博士后积极参与地区学校和机构的外联活动。当前项目的一个好处是，颗粒取向分布的图像可以非常引人注目，并且通常可以作为艺术独立存在，成为向非科学人员谈论材料科学的很好的破冰者。TECHNICAL SUMMARY金属薄膜是许多微电子、光学、传感器和催化剂等微/纳米制造技术的关键元素。由于尺寸的限制，这种薄膜经常被发现是织构的；也就是说，组成薄膜的单个金属微晶优先地以平行于薄膜平面的某些晶面取向。薄膜在沉积过程中可能形成一种取向分布，但随着时间的推移会转变为另一种取向分布。由于薄膜的性质强烈地依赖于存在的取向，这种纹理转变极大地改变了薄膜的性质。因此，了解织构和织构变化对于了解包含薄膜的器件的性能和可靠性至关重要。一个被广泛引用的模型将纹理转换归因于界面能和应变能之间的竞争。然而，最近的研究表明，这两种驱动力都不起主导作用。因此，有人认为，缺陷能量的降低，就像在块体再结晶中一样，提供了驱动力。虽然这很可能是真的，但方向选择机制并不清楚。事实上，这个概念表明，某些取向应该具有比其他取向更高的缺陷密度。这种与取向有关的缺陷密度的存在还没有报道。为了理解这一点，贝克小组将研究金属薄膜中的缺陷结构及其在织构形成和织构转变中的作用。他们将使用高通量方法生产薄膜，这种方法允许他们在每次薄膜沉积时调查多个参数。他们将改变沉积参数以产生不同的缺陷密度，与离子束实验室和桑迪亚国家实验室合作使用离子轰击诱导点缺陷，并通过改变层错能来改变平面缺陷密度(层错)。薄膜结构将用X射线衍射法和透射电子显微镜方法进行详细的研究。他们将开发将驱动力和织构转变动力学联系起来的模型，以便更好地预测和控制薄膜织构，从而实现性能。

NON-TECHNICAL SUMMARYThin 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. In a particularly vexing problem, films that are made with one set of crystal orientations sometimes change to a different set of orientations over time, dramatically changing the properties. To date, it is not possible to predict when this transformation will occur. A number of people have proposed that the initial grains must have defects, imperfections in their crystal structures. These defects represent excess energy, so if new defect-free grains can replace the defective grains, the film can achieve a more stable, lower energy state. This argument, however, does not explain why a new orientation should form. To understand this problem, the Baker group at Cornell University will make films with a wide range of defect structures and will characterize those structures and the associated film behaviors using sophisticated tools such as the Cornell High Energy Synchrotron Source (CHESS), the Ion Beam Laboratory at Sandia National Labs in Albuquerque, NM, and others. They will generate predictive 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 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. 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.TECHNICAL SUMMARYMetal thin films are critical elements in many micro- and nano-fabricated technologies including microelectronics, optics, sensors, and catalysts. Due to dimensional constraints, such films are frequently found to be textured; that is, the individual metal crystallites comprising the film are preferentially oriented with certain crystal planes parallel to plane of the film. Films may form with one orientation distribution during deposition, but transform to another over time. Since the properties of the film depend strongly on the orientations present, this texture transformation dramatically changes film properties. Understanding texture and texture transformations is thus critical to understanding the performance and reliability of devices containing thin films. A widely quoted model attributes texture transformation to a competition between interfacial and strain energies. However, recent studies suggest that neither of these driving forces play a dominant role. Thus, it has been suggested that reduction in defect energy, as in bulk recrystallization, provides the driving force. While this might well be true, the orientation selection mechanism is not clear. Indeed, this concept suggests that certain orientations should have intrinsically higher defect densities than others. The existence of such orientation dependent defect densities has not yet been reported. To understand this, the Baker group will study the defect structures in thin metal films and their roles in texture formation and texture transformation. They will produce films using a high-throughput method that allows them to investigate multiple parameters with every film deposition. They will vary deposition parameters to produce different defect densities, induce point defects using ion bombardment in collaboration with the Ion Beam Laboratory and Sandia National Laboratories, and vary planar defect density (stacking faults) by varying stacking fault energy. Film structures will be examined in detail using x-ray diffraction and TEM methods. They will develop models that link driving forces and texture transformation kinetics to allow better prediction and control of thin film texture, and therefore properties.