Size Effects Determined from Tensile Tests of Perforated MEMS Scale Specimens
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Size Effects Determined From Tensile Tests of Perforated MEMS Scale Specimens Ioannis Chasiotis and Wolfgang G. Knauss1 Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904, U.S.A. 1 Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125, U.S.A.
ABSTRACT A systematic study of small-scale size effects has been conducted on elliptically perforated specimens with minimum radius of curvature of 1 micron. This study aimed at assessing the dependence of failure stress at the tip of a notch on varying: (a) stress concentration for constant radius of curvature, (b) radius of curvature of micro-notches relative to the material grain size and constant stress concentration. The experiments demonstrate a strong influence of notch radius on the failure strength of MEMS scale specimens, while the effect of the stress concentration factor is of rather secondary importance. The local failure strength at the tip of a notch increases when the radius of curvature becomes smaller, which is in accordance with the probabilistic nature of failure. When the notch radius becomes as small as 1 micron (only three times larger than the grain size) then a strong size effect is observed. This effect becomes moderate for larger radii of curvature, up to 8 microns (25 times the grain size), when the failure stress at the notch tip almost reaches the tensile strength recorded for 50 micron wide samples.
INTRODUCTION Mechanical properties measurements of thin films in MicroElectroMechanical Systems (MEMS) are subject to many parameters, such as microfabrication pre- and post-processes, ambient conditions in testing, measurement technique and specimen geometry and size. The latter has been only recently addressed through bending or tension tests [1-3]. Out-of-plane testing inherently incorporates a strong size effect in the measurement of strength due to the transverse stress gradient. In tensile testing, however, the influence of scale is rather buried in the large data scatter and it becomes apparent only when specimens that measure in millimeters are employed [3]. This clear manifestation of size effects in bending also provides the explanation for the puzzling results obtained in the past in round-robin tests [4]. In an effort to demonstrate the existence of such effects in sub-micron stressed material domains and understand their geometric and boundary properties and effect on the mechanical response, we examine the influence of stress raisers, in the form of internal micro-notches, on material failure at the submicron scale. In this systematic approach, polysilicon specimens with different radii of curvature and constant stress concentration (and vice versa) were subjected to in-plane tensile testing. The employment of smooth and regular notches generated elevated, but continuous, stresses, which effectively tested the material at a small size scale. The finite stress concentrations result in bound stresses that mandate the use classical notch stress analyses, rather B2.4.
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