Tensile strength of zinc oxide films measured by a microbridge method
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D.R. Lu State Key Laboratory of Transducer Technology, Shanghai Institute of Metallurgy, Academy of Sciences, Shanghai, People’s Republic of China (Received 28 May 2003; accepted 16 July 2003)
Double-layered ZnO/silicon nitride microbridges were fabricated for microbridge tests. In a test, a load was applied to the center of the microbridge specimen by using a microwedge tip, where the displacement was recorded as a function of load until the specimen broke. The silicon nitride layer in the structure served to enhance the robustness of the specimen. By fitting the data to a theory, the elastic modulus, residual stress, and tensile strength of the ZnO film were found to be 137 ± 18 GPa, −0.041 ± 0.02 GPa, and 0.412 ± 0.05 GPa, respectively. The analysis required the elastic modulus, internal stress, and tensile strength of the silicon nitride layer. They were measured separately by microbridge tests on single-layered silicon nitride microbridges. The measured tensile strength of the ZnO films represents the maximum tolerable tensile stress that the films can sustain when they are used as the functional component in devices.
I. INTRODUCTION
There is a recent intention to use zinc oxide (ZnO) films to serve as the piezoelectric actuating or sensing elements in microelectromechanical systems, transducers, surface acoustic wave filters, and thin film bulk acoustic wave resonators.1–3 A ZnO film is inherently brittle and has the risk of fracture when it is used in a micro-sized device. Therefore, the knowledge of the fracture properties of thin piezoelectric ceramic films is important when using them in the above-mentioned devices.4 To date, most of the experiments for examining the fracture properties of ceramics are designed for bulk samples,5,6 but investigations on film samples are seldom reported. This is due to the difficulty of extracting the mechanical properties of a thin film from the data of a measurement with the presence of the substrate. Although in principle, measurements can be made directly on a free-standing film sample obtained by removing the substrate material (e.g., substrate-free tensile tests and bulge tests),7,8 nevertheless, the tests are highly tedious because of the fragility of the substrate-free specimens. In this paper, we present the results of the measurements of the elastic modulus, internal stress, and tensile strength of magnetron-sputtered ZnO films by using a a)
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J. Mater. Res., Vol. 18, No. 10, Oct 2003 Downloaded: 16 Mar 2015
microbridge method.9,10 The tensile strength of the ZnO films is most concerned because it represents a limitation of the maximum stress that can be sustained by a film used in a device. In a microbridge test, a sample was made in the form of a double-layered ZnO/silicon nitride microbridge. The silicon nitride layer was introduced to enhance the robustness of the specimen. During the test, a load was applied to the center of the microbridge un
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