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

在微电子学中，有些材料本身就不稳定，因为它们在接近熔化温度的温度下使用。其中一种材料是焊点中的锡：锡原子能够在室温下相对快速地移动（扩散），以响应其环境的变化。特别值得关注的是长锡须的形成 以响应微电子中通常出现的应力。长的锡丝可以从薄锡膜的表面自发生长，并且可以达到几毫米的长度。这样的晶须可以桥接相邻的触点并引起短路，从而导致电子系统故障。问题是如何阻止它们的形成。需要一种新的策略，以更好地理解，特别是减轻失败，由于锡膜晶须生长。 特别是，形成晶须只是薄锡膜中的原子对应力作出反应的方式之一。这种新的策略需要考虑到其他过程对放松薄膜应力的贡献，并学习如何操纵它们以防止晶须形成。该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。拟议的计算工作是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的一个进步。 实现材料设计的这一目标需要开发新的预测模拟工具，并培训下一代工作人员使用这些先进的工具。该项目中开发的数据和模拟工具将通过NSF支持的nanoHUB.org广泛提供，并向材料社区开放访问，包括工业和学术界的研究人员和教育工作者。用于教育的演示工具将部署在nanoHUB.org上，并集成到普渡大学的工程课程中，并将供各地的大学和工业界使用。建议的工作提供了一个很好的机会，培养研究生和本科生在材料科学和工程实验和计算技术的整合，在开发跨学科的方法，并在工作中作为一个多学科的国际研究团队的成员。非均匀微结构引起的应力驱动应力松弛与薄金属膜中广泛的失效机制有关。晶须和小丘形成是薄金属膜对残余应力的已知响应，但其它响应包括屈服、扩散和位错介导的蠕变、晶界滑动、开裂、分层、表面粗糙化、挤压-侵入形成、再结晶和晶粒生长。这些多个操作对应力松弛的相对贡献随着应力分布和微观结构在动态和复杂过程中的演变而频繁切换。为了更好地理解它们的变化贡献，特别是为了减轻Sn膜中晶须生长引起的失效，需要考虑这些机制的贡献，以确定：i）表面晶粒形成晶须并影响其生长速率的局部条件，以及ii）其他机制与晶须形成和生长竞争或加速晶须形成和生长。该项目的目标是在微观尺度上开发模型、数值模拟和实验，以研究变形与微观结构的关系，以在循环弯曲和热循环（两种多个过程运行的配置）期间释放薄锡膜中的残余应力。我们建议开发一个框架，模拟这些同时进行的过程，旨在探索这些机制的不同贡献的实验。虽然所提出的框架可以应用于各种薄膜系统，锡膜不仅显示了广泛的现象，将证明其能力，但也将提供机会，以测试其在开发缓解策略，以抑制锡晶须形成的有用性。虽然在薄膜和小尺度结构的局部应力松弛过程的理解在过去的十年中显着增长，一个策略，检查多个同时进行的过程，如位错生成，再结晶，蠕变和晶须形成，仍在发展。最近的观察，在锡膜，晶须形核和生长沿着一些晶界在热循环和循环弯曲与其他位错和扩散过程是明显的提供了机会，探索这种应力-显微组织-变形空间。在这项工作中提出的数值模拟和实验是向前迈出了一步，在回答问题，如在哪里以及如何做晶粒成核形成晶须和小丘，什么当地条件影响他们的增长速度和晶须生长如何竞争或合作与其他应力松弛机制。

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.