Nanoindentation of Au and Pt/Cu thin films at elevated temperatures
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Neville R. Moody Sandia National Laboratories, Livermore, California 94550
William W. Gerberich University of Minnesota, Department of Chemical Engineering and Materials Science, Minneapolis, Minnesota 55455 (Received 24 March 2004; accepted 20 May 2004)
This paper describes the nanoindentation technique for measuring sputter-deposited Au and Cu thin films’ mechanical properties at elevated temperatures up to 130 °C. A thin, 5-nm Pt layer was deposited onto the Cu film to prevent its oxidation during testing. Nanoindentation was then used to measure elastic modulus and hardness as a function of temperature. These tests showed that elastic modulus and hardness decreased as the test temperature increased from 20 to 130 °C. Cu films exhibited higher hardness values compared to Au, a finding that is explained by the nanocrystalline structure of the film. Hardness was converted to the yield stress using both the Tabor relationship and the inverse method (based on the Johnson cavity model). The thermal component of the yield-stress dependence followed a second-order polynomial in the temperature range tested for Au and Pt/Cu films. The decrease in yield stress at elevated temperatures accounts for the increased interfacial toughness of Cu thin films.
I. INTRODUCTION
The microelectronic industry has been growing rapidly over the past 10–20 years, as has its reliance on thin-film deposition techniques for components manufacturing. As modern devices generate quite a bit of heat, and peak temperatures can reach over 100 °C, there is a need to provide adequate cooling for a device to stay operable. Obviously these thermal cycles are unavoidable and eventually lead to thermal fatigue damage and device failure. Consequently, the knowledge of elastic and plastic properties of thin films at elevated temperatures is required for proper chip design and reliability assessments. For the proper stress analysis in conjunction with the thermal cycling characterization, it is important to measure the mechanical properties of thin films at elevated temperatures. Nanoindentation is an alternative test method to the freestanding film mechanical testing1 and microbeam cantilever deflection techniques2,3 for measuring mechanical properties of thin films. First applied over 20 years ago in the hard-drive industry, it is now commonly
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0331 2650
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 9, Sep 2004 Downloaded: 14 Mar 2015
used for microelectronic components and thin films. Nanoindentation is similar to conventional hardness tests, but nanoindentation is performed on a much smaller scale, using special equipment. The force required to press a sharp diamond indenter into the tested material is recorded as a function of indentation depth. Both elastic modulus and hardness can be readily extracted from the nanoindentation curve.4–7 For a metal film, the yield stress (ys), is often approximated as 1/3 of the hardness8 measured by
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