Mechanical Behavior of Nano-crystalline Metallic Thin Films and Multilayers Under Microcompression
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INTRODUCTION
THIN films and multilayers with layer thickness at the micron/sub-micron scale have been widely applied in semiconductor,[1] solar cell,[2,3] and micro-electromechanical systems (MEMS).[4] Their mechanical properties are critical to the performance and reliability of functional parts and micro-devices made of such materials.[5–7] Due to the mismatch in mechanical properties, constituent layers are implicitly subjected to constraints exerted by neighboring layers or substrate.[5] Fracture of constituent layers usually leads to the failure of the entire multilayer, which often occurs at its intrinsic length scale (e.g., layer thickness).[8,9] Hence, it is crucial to understand how constituent layers can influence the overall mechanical behavior of multilayers compared to monolithic materials. However, characterizing the mechanical properties for thin films and multilayers is usually more difficult than bulk materials due to their small intrinsic length scales.[10–13] The microcompression test, which is a miniaturized version of a conventional uniaxial compression test, has been developed and widely used to investigate the mechanical behavior of materials at small length scales.[8,9,14–20] It has been used to study many deformation mechanisms in thin films and multilayers, such as the intrinsic and extrinsic size effects,[8,9,18,20] the strengthening of brittle/ductile multilayers,[16,17] and the effects of pre-straining and annealing on the flow stress.[19] JIANGTING WANG, Associate Research Fellow, and PETER D. HODGSON, Director, are with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3217, Australia. Contact e-mail: [email protected] CHUNHUI YANG, Associate Professor, is with the School of Computing, Engineering and Mathematics, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia. Manuscript submitted August 17, 2014. Article published online January 4, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
In this work, Cu/Fe and Fe/Cu multilayers were fabricated to model soft-fcc/hard-bcc metallic multilayers by magnetron sputtering, along with monolithic Cu and Fe thin films. Microcompression tests were performed to determine the mechanical behavior of these thin films and multilayers at the micron/submicron scale. The effect of constituent layers on their mechanical behavior is investigated by comparing the Cu/Fe and Fe/Cu multilayers to monolithic Cu and Fe thin films. The fracture and failure modes of these thin films and multilayers are also analyzed with an emphasis on the interaction between soft-Cu and hard-Fe layers in multilayers.
II.
EXPERIMENTAL DETAILS
Cu/Fe and Fe/Cu multilayers (the first element in the pair is the top layer) and monolithic Cu and Fe thin films were fabricated by direct current (DC) magnetron sputtering using high purity Fe (99.99 pct) and Cu (99.99 pct) targets. The thin films and multilayers were deposited on naturally oxidized Si(100) substrates at room temperature. The total thickness of each thin film and multilayer was
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