Controlling Heterogeneous Stress Relaxation in Tin Films: Whiskers, Grain Boundary Sliding, and Beyond

控制锡膜中的非均匀应力松弛：晶须、晶界滑动等

• 批准号: 1610420
• 负责人: Marisol Koslowski
• 金额: $ 48.76万
• 依托单位: Purdue University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-15 至 2020-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1610420&HistoricalAwards=false
• 关键词: Controlling; Heterogeneous; Stress; Relaxation; Tin

Non-Technical AbstractIn microelectronics some of the materials are inherently unstable because they are being used at temperatures close to their melting temperatures. One such material is tin in solder joints: Tin atoms are able to move around (diffuse) relatively quickly at room temperature in response to changes in their environment. Of particular concern is the formation of long Tin whiskers in response to stresses normally occuring in microelectronics. Long Tin filaments can grow spontaneously from surfaces of thin Sn films and can reach lengths of several millimeters. Such whiskers can bridge adjacent contacts and cause short circuits leading to electronic system failures. The question is how to stop them from forming. A new strategy is needed to better understand and specifically to mitigate failure due to whisker growth in Sn films. In particular, forming whiskers is only one of the ways in which the atoms in thin Sn films can respond to stresses. This new strategy needs to take into account the contributions of other processes to relaxing stresses in thin films and to learn how to manipulate them to keep whiskers from forming. The goal of this project is to develop models, numerical simulations and critical experiments at the microscopic scale to quantify the different contributions of these processes in these films. The proposed computational effort is a step forward in reducing the reliance on experimentation to develop new materials or to improve reliability in existing ones. Achieving this goal for materials design requires the development of new predictive simulation tools and training the next generation work force on the use of these advanced tools. The data and simulation tools developed in this project will be broadly available through the NSF supported nanoHUB.org with open access to the materials community including researchers in industry and academy and educators. Demonstration tools for education will be deployed in nanoHUB.org and integrated in the Engineering curriculum at Purdue and will be accessible for universities and industry everywhere. The work proposed provides an excellent opportunity to train graduate students and undergraduates in the integration of materials science and engineering experimental and computational techniques, in developing cross-disciplinary approaches, and in working as members of a multi-disciplinary international research team. Techical AbstractHeterogeneous microstructure-induced stresses that drive stress relaxation are linked to a wide range of failure mechanisms in thin metal films. Whisker and hillock formation are known responses of thin metal films to residual stresses but others include yielding, diffusional and dislocation-mediated creep, grain boundary sliding, cracking, delamination, surface roughening, extrusion-intrusion formation, recrystallization and grain growth. The relative contributions of these multiple operations to stress relaxation frequently switch as stress distributions and microstructures evolve in a dynamic and complex process. A strategy to better understand their changing contributions and specifically to mitigate failure due to whisker growth in Sn films needs to take into account the contributions of these mechanisms to identify: i) the local conditions under which surface grains form whiskers and influence their rate of growth, and ii) what other mechanisms compete with or accelerate whisker formation and growth. The goal of this project is to develop models, numerical simulations and experiments at the microscopic scale to study deformation-microstructure relationships to relax residual stresses in thin Sn films during cyclic bending and thermal cycling, two configurations where multiple processes operate. We propose to develop a framework with simulations of these simultaneous processes with experiments designed to explore the different contributions of these mechanisms. While the proposed framework could be applied to a variety of thin film systems, Sn films not only display a wide range of phenomena that will demonstrate its capabilities, but will also provide the opportunity to test its usefulness in developing mitigation strategies to inhibit tin whisker formation. While the understanding of local stress relaxation processes in thin films and small-scale structures has grown significantly over the past decade, a strategy to examine multiple simultaneous processes, such as dislocation generation, recrystallization, creep, and whisker formation, is still developing. The recent observation that, in Sn films, whiskers nucleate and grow along some grain boundaries during thermal cycling and cyclic bending with other dislocation and diffusion processes being evident offers the opportunity to explore this stress-microstructure-deformation space. The numerical simulations and experiments proposed in this effort are a step forward in answering questions, such as where and how do grains nucleate to form whiskers and hillocks, what local conditions affect their rates of growth and how does whisker growth competes or collaborates with other stress relaxation mechanisms.

非技术摘要在微电子学中，有些材料本质上是不稳定的，因为它们在接近熔化温度的温度下使用。焊点中的锡就是这样一种材料：锡原子能够在室温下相对快速地移动(扩散)，以响应环境的变化。特别值得关注的是，在微电子中通常出现的应力会形成长长的锡晶须。锡丝可以从锡薄膜表面自发生长，长度可达几毫米。这种晶须可以桥接相邻的触点，并导致短路，导致电子系统故障。问题是如何阻止它们的形成。需要一种新的策略来更好地理解并特别是减轻由于锡膜中晶须生长而导致的失效。特别是，形成晶须只是锡薄膜中的原子对应力做出反应的方式之一。这一新的策略需要考虑到其他工艺对缓和薄膜应力的贡献，并学习如何操纵它们以防止晶须的形成。这个项目的目标是在微观尺度上开发模型、数值模拟和关键实验，以量化这些过程在这些薄膜中的不同贡献。拟议的计算工作在减少对实验的依赖以开发新材料或提高现有材料的可靠性方面向前迈进了一步。要实现材料设计的这一目标，需要开发新的预测模拟工具，并培训下一代劳动力使用这些先进工具。在这个项目中开发的数据和模拟工具将通过NSF支持的NanHUB.org广泛提供，并开放访问材料社区，包括工业界和学术界的研究人员和教育工作者。教育演示工具将部署在NanHUB.org中，并整合到普渡大学的工程学课程中，世界各地的大学和行业都可以使用。拟议的工作提供了一个极好的机会，培训研究生和本科生将材料科学与工程实验和计算技术相结合，开发跨学科方法，并作为多学科国际研究小组的成员工作。技术摘要非均质微结构引起的应力驱动应力松弛与金属薄膜的多种失效机制有关。晶须和小丘的形成是已知的金属薄膜对残余应力的响应，但其他包括屈服、扩散和位错介导的蠕变、晶界滑动、裂纹、分层、表面粗化、挤压-侵入形成、再结晶和晶粒长大。这些多重操作对应力松弛的相对贡献经常随着应力分布和微观结构的动态和复杂过程的演变而变化。为了更好地了解它们的变化贡献，特别是减轻锡薄膜中晶须生长造成的失效，需要考虑这些机制的贡献，以确定：i)表面颗粒形成晶须并影响其生长速度的局部条件，以及ii)哪些其他机制与晶须形成和生长竞争或加速晶须的形成和生长。本项目的目标是在微观尺度上开发模型、数值模拟和实验，以研究变形-微结构关系，以缓解循环弯曲和热循环过程中锡薄膜中的残余应力，这两种配置中有多种工艺操作。我们建议开发一个框架，通过实验模拟这些同时进行的过程，以探索这些机制的不同贡献。虽然建议的框架可以应用于各种薄膜系统，但锡膜不仅显示出一系列将展示其能力的现象，而且还将提供机会来测试其在制定抑制锡须形成的缓解策略方面的有效性。尽管对薄膜和小尺度结构中局部应力松弛过程的了解在过去十年中有了很大的发展，但一种研究多个同时过程的策略仍在发展中，例如位错产生、再结晶、蠕变和晶须形成。最近观察到，在锡薄膜中，晶须在热循环和循环弯曲过程中沿某些晶界形核和生长，并有明显的其他位错和扩散过程，这为探索这种应力-组织-变形空间提供了机会。这项工作中提出的数值模拟和实验在回答以下问题方面向前迈进了一步：颗粒在哪里以及如何成核形成晶须和山丘，当地条件如何影响它们的生长速度，晶须生长如何与其他应力松弛机制竞争或协作。