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

非技术概述薄金属膜（金属层厚度小于人类头发的十分之一）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要素。这些薄膜由许多微小的金属晶体组成，称为“晶粒”。 由于薄膜很薄，晶粒倾向于自我定向，从而使其晶体结构中的某些方向与薄膜平面对齐。薄膜的性质，以及因此包含薄膜的器件的性能和可靠性，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电影或设备的行为。在一个特别令人烦恼的问题中，用一组晶体取向制成的薄膜有时会随着时间的推移而改变为一组不同的取向，从而显着改变性质。到目前为止，还无法预测这种转变何时发生。许多人提出，最初的晶粒必须有缺陷，在其晶体结构中的缺陷。这些缺陷代表多余的能量，所以如果新的无缺陷的晶粒可以取代有缺陷的晶粒，薄膜可以达到更稳定，更低的能量状态。然而，这一论点并没有解释为什么要形成一个新的方向。为了理解这个问题，康奈尔大学的贝克小组将制作具有各种缺陷结构的薄膜，并将使用诸如康奈尔高能同步加速器源（CHESS）、位于新墨西哥州阿尔伯克基的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制造设备的制造成为可能，并应有助于提高所有包含薄金属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，这是一个位于纽约北部的小型非博士授予机构。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，但他们将在康奈尔大学的贝克小组工作一个夏天。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。目前项目的一个好处是，晶粒取向分布的图像可以是相当惊人的，往往可以站在自己的艺术，使一个很好的破冰船谈论材料科学的非科学家。技术摘要金属薄膜是许多微和纳米制造技术，包括微电子，光学，传感器和催化剂的关键要素。由于尺寸限制，经常发现这样的膜是织构化的;也就是说，构成膜的各个金属微晶优先取向为具有平行于膜平面的某些晶面。膜可以在沉积期间形成为具有一种取向分布，但是随着时间的推移转变为另一种取向分布。由于膜的性质强烈地依赖于存在的取向，因此这种纹理变换显著地改变膜的性质。因此，理解纹理和纹理变换对于理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属性纹理转换之间的界面能和应变能的竞争。然而，最近的研究表明，这两种驱动力都没有发挥主导作用。因此，有人建议，减少缺陷能量，在体再结晶，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，这一概念表明，某些取向应该具有比其他取向更高的固有缺陷密度。这种依赖于取向的缺陷密度的存在尚未被报道。为了理解这一点，贝克小组将研究薄金属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，使他们能够在每次薄膜沉积时研究多个参数。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，并通过改变堆垛层错能量来改变平面缺陷密度（堆垛层错）。薄膜结构将使用X射线衍射和TEM方法详细检查。他们将开发连接驱动力和纹理转换动力学的模型，以更好地预测和控制薄膜纹理，从而提高性能。

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.