Measurement of Poisson's ratio by means of a direct tension test on micron-sized specimens

A methodology developed for measuring Young's modulus and the full stress-strain curve on micron-sized specimens was extended here to measure Poisson's ratio. A dog-bone type specimen was used within a small loading machine with a maximum load of 5 N. The specimen was fabricated from single crystal silicon (SC-Si) with the specimen gage and loading direction in the (0 01) orientation. A silicon on insulator (SOI) wafer was used with a deep reactive ion etching (DRIE) based process. Geometrical parameters of the initial cross-sectional area of the gage were measured by means of image processing on environmental scanning electron microscope (ESEM) images. The test setup also consists of an optical microscope with a monochromatic camera and a data acquisition system. The strains were obtained through the displacement field which was determined by means of digital image correlation (DIC). A speckle pattern was placed on the specimen gage. SC-Si was chosen to study since it is expected that on both the micro and macro-scales, Young's modulus and Poisson's ratio will have the same value. Hence, the accuracy of the method may be examined. The average value of Young's modulus E = 131.4 +/- 2.1 GPa was obtained with the micro-specimens and is consistent with values determined on the macro-scale (E = 130 GPa). The average value of Poisson's ratio on the micro-scale was found as v = 0.23 +/- 0.03 which is lower than the macro-scale value of v = 0.28. The failure stress was determined to be sigma(f) = 1.46 +/- 0.10 GPa. Results for Young's modulus reflect the reliability of the methodology which is suitable for characterization of a large variety of materials exploited in micro-devices for both sensing and actuation. The reasons for the low values measured for v were investigated through emulations of determining the strains. An improvement in the image acquisition system is suggested. (C) 2011 Elsevier B.V. All rights reserved.