Cleavage cracking across triple grain boundary junctions in freestanding silicon thin films
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This article is focused on a fractography study of cleavage cracking at triple grain boundary junctions in freestanding silicon thin films. At a triple junction, as the crystallographic orientations of the two grains ahead of the crack are different by only a few degrees, the cleavage front advance becomes quite jerky. The crack first enters the grain of smaller boundary toughness and then turns into the other grain from the lateral direction. Consequently, the overall fracture resistance cannot be analyzed in the framework of line-average theory. The nonuniform characteristic of crack behavior can be attributed to the increase in local stress intensity. A few typical crack front advance modes are identified.
I. INTRODUCTION
Understanding fracture behaviors of polycrystalline thin films is of great importance to assuring safe performance of micro/nanoelectromechanical systems, integrated circuits, sensors, etc.1 During film growth, if the substrate temperature Tsub is relatively high, the grains are equiaxed; if Tsub is relatively low, because of the difficulty in surface diffusion, the grain structure becomes columnar.2 The growth rate of a grain is highly dependent on its orientation. For instance, in a lowtemperature chemical vapor deposition process, 〈110〉 is the most prevalent out-of-plane direction for silicon.3,4 As a result, as the film becomes thicker, the growth of nuclei of unfavorable orientations is interrupted, and the microstructure is dominated by the grains that can eventually become large.5 The fracture toughness of such a film is governed by the through-thickness grain boundaries.6 If the film is relatively thin, e.g., when the film thickness is only a few micrometers, the influence of buried grains can be significant, as their sizes are comparable with film thickness. When a cleavage front propagates across these boundaries, it must overcome triple junctions, which can have considerable effects on the size dependence of fracture and failure mechanisms and processes at small length scales7–10 as well as the grain boundary behaviors.11,12 There are a few possible cleavage cracking modes at a triple grain boundary junction. It is sometimes assumed that the crack can simultaneously enter the two grains
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0206 J. Mater. Res., Vol. 23, No. 6, Jun 2008
ahead of the front. The overall resistance offered by the triple junction can then be taken as the average of the fracture resistances of the two boundaries.13,14 Another possible crack advance mode is postponed propagation.15 The crack front would first break through the boundary of lower resistance, and the rest is arrested by the one that is of higher resistance. At the protruding section (the convex part) of the front, the local stress intensity is smaller than that at the concave part (the section that is left behind). The latter would eventually overcome the boundary, and the front tends to be straight again. Under this condition, the overall fracture resist
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