Session: 13-13-01: Simulations of Material Modeling and Behavior Analysis for MEMS Applications

Paper Number: 149425

149425 - Simulations of Wrinkle Formation in Flexible Thin-Film Structures 

Micro- and nano-scale structures consisting of thin films attached to an elastically compliant substrate commonly exist in flexible electronic devices and packages. When subject to in-plane compression due to mechanical or thermal loading, buckling instability in the form of surface wrinkles can occur. While wrinkle formation is frequently an undesired feature, it is also increasingly exploited to improve device performance or reliability. For instance, a wrinkled surface may enhance light absorption and thus improve optoelectronic efficiency. Another example is that pre-fabricated wrinkles can accommodate stretching by first flattening its surface, leading to a greater capability to withstand tensile strain and thus delaying crack formation. Predicting the formation and evolution of wrinkles thus becomes essential for the design and analysis of soft devices.

Analytical solutions for wrinkling instability exist, but these are limited to straightforward waveforms under the simplest loading condition and constitutive material behavior. More complex geometric, material, and loading scenarios will have to be dealt with by simulation-based techniques. Challenges exist in conventional computational modeling of wrinkling instabilities. A cumbersome multi-step process is typically employed – a modal analysis is performed first to obtain the buckling mode, and then the small waveform is designed into the finite element model as existing geometric defects for subsequent simulations. The surface pattern thus obtained is basically artificially built into the structure.

We recently developed a modeling approach capable of predicting the initiation and temporal development of surface wrinkles in a seamless manner. It is based on embedded imperfections along the film-substrate interface. One or more regular elements (in a prescribed finite element model) immediately below the interface are assigned the properties of the film material instead of the substrate material. Conceptually this may be perceived as having an imperfect interface which inevitably exists in actual physical devices. The simulation technique has been verified with known analytical solutions and proven to be robust, being able to predict wrinkle evolution from the initial flat state through multiple bifurcations (pattern transformations) in a single simulation run.

In this presentation we include our recent studies using this modeling approach in both 2D and 3D. The wrinkling behavior is illustrated using the model system of a semiconducting polymer film bonded to an elastomeric substrate under uniaxial and bi-axial loading. Simple sinusoidal waveforms and complex surface patterns such as checkerboard, herringbone and labyrinth as well as their transitions can be directly simulated. As the substrate thickness decreases, compressive deformation can result in global buckling of the entire structure, which can occur simultaneously with periodic wrinkling in the thin films. With only elastic deformation involved, the wrinkle configuration is shown to be history dependent (e.g., equi-biaxial loading and sequential loading along the two directions can lead to different surface patterns, even if the overall final equi-biaxial deformation states are identical). If plastic yielding takes place in the thin film, a uniform wrinkle geometry may evolve into more sparsely distributed sharp folds or creases. Implications of these findings for flexible device applications will be discussed in this presentation.

Presenting Author: Yu-Lin Shen University of New Mexico

Presenting Author Biography: Yu-Lin Shen is currently Professor and Chair of the Department of Mechanical Engineering at University of New Mexico (UNM). He received his Ph.D. in engineering from Brown University, and M.S. and B.S. in materials science and engineering from National Tsing Hua University in Taiwan. He was a post-doctoral research associate at Massachusetts Institute of Technology before joining UNM in 1996. Professor Shen has been active in research and teaching in the areas of mechanical behavior of materials and solid mechanics. He is particularly well versed in applying modeling techniques to address micro-mechanical problems related to thin films, microelectronic devices and packages, and composite materials. He has published about 200 research papers, mostly in the form of journal articles. His book titled “Constrained Deformation of Materials” was published by Springer in 2010. Professor Shen is a Fellow of ASME. He has been rated among the World's Top 2% Most Influential Scientists List (Career Impact and Single-year Impact) by Elsevier/Stanford University.

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