A Review of Tension Test Methods for Thin Films
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A Review of Tension Test Methods for Thin Films William N. Sharpe, Jr. Mechanical Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD, 21218 ABSTRACT Test methods for mechanical property measurement of the thin films used in MEMS have been developed and refined over the last decade. This brief review considers only tensile testing since that is the preferred method for measuring Young’s modulus, strength, etc. for macroscale structural materials. There are basically two kinds of tensile specimens – framed specimens whose support strips are cut after mounting and semidetached specimens fastened to the substrate at one end. The loading and force measurement systems are similar and use commercial transducers. Strain is measured either by overall grip displacement or by digital imaging in most cases. Initial works are described and followed by descriptions of some recent applications. The paper concludes with a suggested test method that could be amenable to standardization. INTRODUCTION This paper is to a great extent an update of a presentation at the 2001 MRS Symposium [1]. It is not a comprehensive review; in fact, most of the references are from the past two years. They are chosen to present a ‘state-of-the-art’ view of the topic. Some readers may want a more thorough overview of the various approaches (not limited to tensile testing or to thin films) over the years. This author published such a review in 2001 [2], but it is now somewhat out of date. Hemker and Sharpe [3] have a recent overview that summarizes test methods and discusses results from a materials science viewpoint. Review articles by Haque and Saif [4] and Srikar and Spearing [5] will give the reader an appreciation for the innovation and sophistication of this area of experimental research. The first issues in mechanical testing are how to manufacture a specimen and mount it in a testing machine; these become more challenging as the dimensions get smaller. A clear pattern of approaches has emerged for thin films. The tensile film specimen remains fastened at either one or both ends to the substrate upon which it has been deposited. In the first case, the substrate is fastened to one grip of the test machine and the free end was somehow fastened to the other grip. In the second case, the substrate ends are fastened to the test machine and temporary substrate supports are removed before testing. These will be referred to as ‘semidetached’ and ‘framed’ in this review. The next and equally challenging issue is strain measurement. The first approach was obviously grip displacement measurement. Macroscale tensile testing avoids this because it is easy to apply resistance strain gages or clip gages in most cases – high temperature or other hostile environments being exceptions. There is a need to measure strain directly on the specimen, and interferometry was used early on. However, digital imaging has emerged as the predominant and preferred method; the images can be optical or otherwise such as from an AFM.
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