Controlling Adhesion Between Stiff Surfaces by Tailoring Interface Geometry

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Stiff and hard materials with flat and clean surfaces can spontaneously adhere when contacted even under minimal pressure. This "direct bonding" process is used to join ultra-flat glass optics and to bond large-diameter semiconductor wafers. It is also essential in emerging technologies based on wafer-scale semiconductor layer transfer processes and micro-transfer printing, including 3D integrated circuits, microelectromechanical systems (MEMS), and flexible hybrid electronics. This project focuses on understanding adhesion of interfaces formed by bonding relatively stiff materials with surfaces containing engineered features that give rise to imperfect adhesion. The hypothesis is that surface patterning can be used to realize controlled and directionally-dependent adhesion between stiff materials. Even though nearly all interfaces have imperfect adhesion, there remains a fundamentally incomplete understanding of the mechanics of imperfect adhesion, both from scientific and engineering points of view. Consequently, at present the design of surfaces with desired adhesion and toughness is mostly trial and error based. This project will advance the science of adhesion and will enhance the fabrication of soft and hard electronics as well as MEMS, impacting national health, prosperity, and welfare and securing national defense. From an education and outreach standpoint, this project will lead to new modules and hands-on demos on imperfect adhesion that will be broadly used to educate at all levels: graduate and undergraduate students, K-12 students, and the public. Through a combination of experiments and rigorous mechanics analyses incorporating traction-separation laws that are physically motivated and experimentally calibrated, research will be conducted to design methodologies to control interfacial toughness. The adhesion of (1) micro- and nano-patterned silicon surfaces with various geometries, and (2) surfaces with engineered waviness will be investigated. Atomic force microscopy-based measurements will be used to characterize the traction-separation adhesion law and determine parameters such as the work of adhesion and interaction length scale. Micro- and nanopatterning will be used to introduce adhesion anisotropy and asymmetry, yielding interfaces that have dramatically different toughness depending on the direction of loading and crack propagation. An accurate characterization of the traction-separation law governing adhesion, which is required to predict toughness, will be obtained from a combination of AFM measurements and experiments on wavy interfaces coupled with detailed analyses. In addition, use simulation tools will be developed and used to incorporate the experimentally-verified adhesion laws in parametric studies to design surface patterns to meet specific objectives. The work will lead to models that allow for the design of interfaces with controlled, anisotropic, and asymmetric toughness. The understanding of imperfect adhesion will enable advances in micro- and nano-patterning to be exploited to realize innovative approaches to control interface toughness through geometry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.