Horacio Espinosa
Department of Mechanical Engineering, Northwestern University, USA

Mechanics of Freestanding Submicron Thin Films-Size Scale Plasticity and Fracture

Over the past decade, there has been a substantial thrust to reduce the size of electronic and electromechanical systems to the micron and sub-micron scale by fabricating devices out of thin film materials. In these applications, successful device development requires a thorough understanding of material mechanical properties as a function of device characteristic dimension. At this scale, specimen geometry and dimensions are similar in size to the material microstructural features. Consequently, new tests and models capable of accurately capturing this effect are highly needed. In this presentation, a new on-chip membrane deflection experiment specially designed to investigate material elastic behavior (including grain anisotropy and morphological effects), plasticity (including size effects in the submicron regime), and fracture will be discussed. Two examples of research recently conducted at Northwestern University will be presented. The first example examines plasticity size effects in freestanding fcc thin films in the absence of macroscopic strain gradients. Experimental results, including transmission electron microscopy, will be presented to demonstrate that indeed strong plasticity size effects exist and to highlight their possible sources. Current shortcomings of plasticity theories at the submicron scale and its implication in the design of micro/nano devices will be discussed. The second example involves the identification of elasticity, strength and fracture properties of ultra-nano-crystalline diamond, a new material poised to revolutionize the development of novel micro and nano-electro-mechanical systems.