Fracture Strength of Polysilicon thin Films at Stress Concentrations
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Fracture strength of polysilicon thin films at stress concentrations
J. Bagdahn and W. N. Sharpe, Jr. Department of Mechanical Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore MD 21218-2681, U.S.A.
ABSTRACT The fracture strengths of 3.5 microns thick and 20 or 50 microns wide polysilicon specimens were measured. One set of specimens was straight in the gage section, another set had a central hole 5.0 microns in diameter, and the third set had symmetric semicircular notches 2.5 microns in radius on each side. The local maximum fracture stresses of the nonuniform specimens, as calculated from the remote fracture stress with a stress concentration factor, were higher than measured in the straight specimens. This indicates a size effect, which is presumably due to the smaller highly stressed volume or area in the non-uniform specimens. INTRODUCTION Fracture of brittle materials is caused by the unstable propagation of cracks that start at surface or volume defects. If the defects have a random distribution of size, orientation and location, the fracture strength will increase as the size of the affected surface, or volume is reduced. The dependence of the strength on the size is named ‘size effect’. The presence of a size effect in polysilicon have been reported by Tsuchiya et al. [1], Ding et al. [2] and Sharpe et al. /3/ using tensile specimens and in addition Ding et al. [4] showed a size effect during bending tests. Tensile test results from Tsuchiya et al. [2] showed that the tensile strength depends upon the sample length but does not depend on the sample width. Ding et al. [2] and Sharpe et al. [3] found in tensile tests a dependence of the strength on the sample length as well as on the sample width. Shorter samples have a higher strength than longer ones and narrow ones samples have a higher strength than wider ones. The results from bend test showed in addition a dependence of the fracture strength on the length of the cantilever and the sample width [4]. However, microelectromechanical systems (MEMS) have stress concentrations (holes, notches etc.) and these will naturally affect the strength of the microdevice. A study of the effects of stress concentrations on the strength of components is therefore of practical interest. Especially for design considerations, it is of particular interest to predict the strength of a component from strength results from standard test samples like tensile specimens. Experimental results of notched polysilicon samples by Muhlstein et al. [5] and Kahn et al. [6] showed that the local fracture strength at the stress concentrations is higher than the fracture stress derived in tensile tests by other researchers. In this paper we present a comparison of the strength behavior of straight tensile specimens and tensile specimens with center holes and edge notches. Results of the analysis of the stress field invoking finite element analysis and fractographic investigations will be presented.
B9.7.1
EXPERIMENTAL Polysilicon tensile specimens that were fabri
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