Silicon-Framed Tensile Specimens: Techniques and Results

The silicon-framed tensile specimen design has been used in tensile tests of thin films of a variety of metals and epitaxial silicon. A piezo-actuated microtensile test device holds the specimen while the silicon frame is cut just before testing, imposes the tensile displacement, and provides voltages proportional to force and displacement. This technique is appropriate for films that are hundreds of micrometers long, tens to a few hundred micrometers wide, and from 0.3 to 15 micrometers thick. The specimen film must be amenable to lithographic patterning, must adhere well to the silicon substrate, and must resist a silicon etchant. The specimen fabrication is a bulk-micromachining process, because the silicon substrate is etched through underneath the specimen film. Uniaxial yield strength, ultimate tensile strength, and elongation to maximum load can be measured using the microtensile tester. The addition of laser illumination and digital photography allows implementation of electronic speckle pattern interferometry, for accurate measurement of local displacement. This addition allows evaluation of the tensile Young's modulus. Compared to bulk material, thin films of copper and aluminum have lower apparent Young's moduli, higher yield and ultimate tensile strengths because of their fine grain size, and lower elongation to failure. Correlation between properties measured by indentation and by tensile testing needs further study.