Solid-State Dewetting of Metallic Thin Films

摘要：金属薄膜广泛应用于各种设备和系统中，影响着我们的日常生活。它们在用于计算和通信的集成电路、用于传感和生物医学分析的微机电系统（MEMS）、用于信息存储的微磁器件以及用于信息处理、通信和传感的微光子器件和系统中发挥着关键作用。随着这些技术的进步，需要越来越小的金属部件。然而，人们发现，当材料被制造得非常小时，它们的形状往往会随着时间的推移而演变，因为它们试图采用球形，就像水滴一样。这限制了新技术的发展，特别是那些涉及金属部件的技术。在这个项目中，精确控制非常小的金属结构，以研究它们随时间的演变。这些实验研究与解释这种演变所需的理论模型的发展相结合。该项目的目标是开发制造稳定的纳米级金属结构和控制形状演变的新技术，以制造具有新功能的复杂形状的结构。这个项目涉及两个研究小组的学生，一个专注于实验，一个专注于建模。这些学生参加两个小组的会议，并将与德累斯顿工业大学、米兰大学和新南威尔士大学的合作小组的高级和初级成员进行广泛的互动。这个项目的成果被包括在麻省理工学院的课程、工业短期课程以及大规模开放的在线课程中。技术摘要：为了了解晶体各向异性对毛细管驱动的形态演化的影响，对固态脱湿进行了实验和理论研究。单晶薄膜在加热前被平版印刷成图案，以引起形态的演变。发现这种演变强烈地受到图案化特征（如薄膜边缘）的结晶取向的影响。边缘被发现以方向依赖的速率收缩，要么被夹断，留下与收缩边缘平行排列的一系列韧带，要么形成指指不稳定，导致平行韧带沿着收缩方向排列。韧带受到瑞利式不稳定性的影响，导致断裂成颗粒。这种行为是非常可重复的，并导致不同的中间结构，这取决于形状和方向的预图案结构。在所有情况下，晶体各向异性强烈影响观察到的现象。在这个项目中，系统的研究正在进行中，不同材料、厚度和晶体结构的薄膜在平面内晶体取向的范围内具有边缘和其他特征。直边收缩、边缘掐断和指法的动力学研究正在进行中。利用不同的退火环境来了解表面结构和表面能各向异性对脱湿各向异性的影响。所研究的系统的形态演化是由毛细管驱动的表面扩散发生的。在各向同性表面能情况下的二维演化模型已经得到了很好的发展，基于相填充方法的三维模型也正在出现。然而，在脱湿研究中观察到的强各向异性行为表明，需要考虑表面能和扩散各向异性的3D模型。作为该项目的一部分，研究人员正在开发各向异性3D相场模型，该模型将通过与广泛的实验进行比较来进行测试。预计这些基础研究将导致对薄膜和微/纳米结构的毛细管驱动演化的更好理解。这将使材料和系统的设计具有更好的稳定性，并允许使用模板固态脱湿作为生成具有亚光刻特征尺寸的复杂结构的工具。

NON-TECHNICAL ABSTRACT:Metallic thin films are used in a wide range of devices and systems that have an impact on our everyday lives. They play critical roles in the integrated circuits used for computation and communication, microelectromechanical systems (MEMS) used for sensing and biomedical analyses, micromagnetic devices used for information storage, and microphotonic devices and systems used for information processing, communications and sensing. As these technologies advance, smaller and smaller metallic components are required. However, it has been found that when materials are made very small, their shape tends to evolve over time as they try to adopt spherical shapes, like droplets. This is limiting the development of new technologies, especially those involving metallic components. In this project, precisely controlled very small metallic structures are being made to study their evolution over time. These experimental studies are coupled with development of the theoretical models that are needed to explain this evolution. The goals of this project are to develop new techniques for making stable nano-scale metallic structures and for controlling shape evolution to make structures with complex shapes for new functions. This project involves students from two research groups, one focused on experiments and one focused on modeling. These students participate in meetings of both groups and will also extensively interact with senior and junior members of collaborating groups in the Technical University of Dresden, the University of Milano, and the University of New South Wales. Results from this project are included in courses at MIT and short courses for industry, as well as in massively open online courses. TECHNICAL ABSTRACT:Experimental and theoretical studies of solid state dewetting are being carried out to understand the effects of crystalline anisotropy on capillary-driven morphological evolution. Single crystal films have been lithographically patterned before heating to cause morphological evolution. It is found that this evolution is strongly affected by the crystallographic orientation of patterned features such as film edges. Edges were found to retract at orientation-dependent rates and either undergo pinch-off to leave behind sets of ligaments aligned in parallel with the retracting edge, or develop a fingering instability that leads to parallel ligaments aligned along the retraction direction. Ligaments are subject to a Rayleigh-like instability that leads to break-up into particles. This behavior is very reproducible, and leads to different intermediate structures that depend on the shape and orientation of the pre-patterned structures. In all cases, crystalline anisotropy strongly affects the observed phenomenology. In this project, systematic studies are underway in which films of different materials, thickness, and crystallographic texture are patterned with edges and other features within a range of in-plane crystallographic orientations. Kinetic studies of retraction of straight edges, rim pinch-off and fingering are underway. Different annealing ambients are being used to understand the role of surface structure and surface energy anisotropy on dewetting anisotropy. Morphological evolution in the systems under study occurs by capillarity-driven surface diffusion. 2D models for evolution in the case of isotropic surface energies are well developed, and 3D models based on phase filled approaches are emerging. However, the strongly anisotropic behavior that is observed in dewetting studies shows the need for 3D models that account for surface energy and diffusion anisotropy. As part of this project, the investigators are developing anisotropic 3D phase field models that will be tested by comparison with a wide range of experiments. It is anticipated that these basic studies will lead to an improved understanding of capillary-driven evolution of thin films and micro-/nano-structures. This will allow design of materials and systems with improved stability and enable the use of templated solid-state dewetting as a tool for generating complex structures with sub-lithographic feature sizes.