Compact On-Chip Microtensile Tester With Prehensile Grip Mechanism

Abstract

Accurate measurement of material strength at small scales is of critical importance for the design and manufacture of reliable micro- and nanoscale devices. Many materials of interest, e.g., polycrystalline silicon (polysilicon) and silicon carbide, are brittle, with statistical strength distributions. Therefore, strength quantification requires a large number of tests accurately performed on a practical platform. Here, analysis and testing of a compact on-chip microtensile test system, recently presented in a brief communication [Hazra et al., Journal of Micromechanics & Microengineering, 2009], is expanded significantly. We present new data on strength, alignment precision, temperature distribution, compliance calibration, stiffness ratio, force calibration, and effect of monolayer film coverage. High force (up to 30 mN) is applied to a 70-μm-long freestanding tensile bar (nominally of 2 μm width and 2.25 μm height) by cooling a thermal actuator (TA) that has gripped a crosshead via a prehensile mechanism. According to finite element analysis, specimen heating is small (<; 45°C above ambient). The system features a relatively small area occupied on the chip (500 × 700 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ); excellent alignment resulting in in-plane and out-of-plane stress gradients of 2.2% and 1.5%, respectively, no sensitivity to residual stress or to cross-sectional shape, and high strain resolution (2.3 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> ). A compliance calibration factor, obtained from finite element analysis, is used to convert measured fracture displacement to fracture strain. The grip mechanism must be much stiffer than the tensile bar. A value of 6.1 is determined for this stiffness ratio. We show that the TA acts as a nonlinear spring that can be modeled to determine the applied force. Assuming a value of Young's modulus <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> = 164 GPa, we find a characteristic strength of 2.45 GPa, with a Weibull modulus of 12.04, reflecting a sample size of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> = 34 polysilicon microtensile bars.