Solid-State Dewetting of Metallic Thin Films

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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.