Size effects on deformation mechanism of nanopillars by FIB-CVD using double-cantilever testing
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Hiro Tanaka Department of Mechanical Engineering, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
Yasuo Kogo Department of Material Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan (Received 29 June 2011; accepted 19 September 2011)
Nanopillars and nanocoils fabricated by chemical vapor deposition using a focused ion beam were used to estimate bending and torsional rigidities under infinitesimal deformation and to investigate nonlinear large deformation behaviors. For the pillars, we performed bending tests using a unique double-cantilever specimen, which was made by joining two pillars together using focused electron beam deposition in a scanning electron microscope. The reproducible load–deflection curves, which were not severely disturbed by the ambiguous chuck condition of the specimens, indicated that the pillar deformation resistance decreased after the linear response (called softening), and it was dependent on the pillar diameter and the ratio of diameter to length. However, all pillars became extremely hardened at large deformation. At diameters of less than 300 nm, and at diameter/length ratios of over 102, this nanopillar size effect (characterized as softening) was consistently observed.
I. INTRODUCTION
Direct deposition using a focused ion beam (FIB) is a prospective method for nanoscopic fabrication.1–3 In particular, FIB-induced chemical vapor deposition (FIB-CVD) has distinct advantages in creating complicated structures.4 Such microstructures are composed of deposited beam members made from amorphous carbon (a-C). Therefore, the overall structural characteristics are, in principle, determined by the mechanical behavior of the member itself. Recently, it was found that nanopillars (hereafter referred to as pillars) made by the above-mentioned technique have a large rubber-like nonlinear deformability before fracture.5 Their curious mechanical and structural properties promote a coherent understanding of a-C pillars as deformable bodies with size-dependent properties, unlike bulky a-Cs [such as diamond-like carbon (DLC)] which are typically brittle materials that fracture with little elongation. We recently developed a unique materials testing procedure, capable of measuring both infinitesimal deformation and large deflection of pillars with high accuracy. To avoid unfavorable nanoscale effects at the loading point due to sticking forces such as Coulomb interactions and meniscus forces, we proposed an original double-cantilever test based on the rigid connection of two pillars using
a focused electron beam (EB) deposition technique in a scanning electron microscope (SEM).6 This original approach enables highly accurate measurement of large deflections by controlling the applied displacement with a commercial cantilever normally used in an atomic force microscope (AFM). In the present report, the effect of size on the large nonlinear deformability of pillars is discussed on the basis of several samples with different diameters and diameter-to-length ratios. In addit
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