Brittle-to-ductile transition in ultrathin Ta/Cu film systems
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Eduard Arzt INM Leibniz Institute for New Materials, D-66123 Saarbru¨cken, Germany
Ralph Spolenakb) Laboratory for Nanometallurgy, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland (Received 20 May 2008; accepted 23 March 2009)
Current semiconductor technology demands the use of compliant substrates for flexible integrated circuits. However, the maximum total strain of such devices is often limited by the extensibility of the metallic components. Although cracking in thin films is extensively studied theoretically, little experimental work has been carried out thus far. Here, we present a systematic study of the cracking behavior of 34 to 506 nm thick Cu films on polyimide with 3.5 to 19 nm-thick Ta interlayers. The film systems have been investigated by a synchrotron-based tensile testing technique and in situ tensile tests in a scanning electron microscope. By relating the energy release during cracking obtained from the stress-strain curves to the crack area, the fracture toughness of the Cu films can be obtained. It increases with Cu film thickness and decreases with increasing Ta film thickness. Films thinner than 70 nm exhibit brittle fracture, indicating an increasing inherent brittleness of the Cu films. I. INTRODUCTION
Flexible electronics have gained widespread interest for numerous applications. Among these are flexible displays,1,2 wearable electronics,3,4 electronic skins,5 and flexible microelectrode arrays (MEAs) usable for in vivo biological applications such as retina implants.6,7 The functional parts of these devices comprise stiff materials (e.g., metals, transparent conductors, and amorphous silicon) deposited on compliant substrates (e.g., polyimide or silicone). When stretched or bent during use, the compliant substrate usually deforms elastically. Brittle coatings or conducting wires may crack or delaminate as a consequence. Mechanical failure is one of the most critical issues in the development of flexible and stretchable electronics. On the other hand, the measurement of fracture toughness of thin films is difficult because of small testing volumes and the challenge of handling small samples. In contrast to bulk materials, there is neither a standard procedure nor a commonly accepted methodology to follow. However, increasing efforts have been made, and several different test methods are proposed and used. The methodologies used to measure the fracture toughness of thin films are bending, buckling, scratching, indentation, and a)
Present address: Universita¨t Karlsruhe, Institut fu¨r Zuverla¨ssigkeit von Bauteilen und Systemen, D-76131 Karlsruhe, Germany. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0252 1906
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 6, Jun 2009 Downloaded: 13 Mar 2015
tensile tests.8 To study multiple cracking phenomena of brittle films on compliant substrates, in situ fragmentation tests have been conducted.9–12 This method, in which the progressive development of crack density i
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