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TECHNICAL PAPERS

# Fracture Toughness and Subcritical Crack Growth in Polycrystalline Silicon

[+] Author and Article Information
I. Chasiotis

Aerospace Engineering,  University of Illinois at Urbana-Champaign, Urbana, IL 61801chasioti@uiuc.edu

S. W. Cho

Materials Science and Engineering,  University of Virginia, Charlottesville, VA 22904

Aerospace Engineering,  University of Illinois at Urbana-Champaign, Urbana, IL 61801

A crack tip very close to a grain boundary or at a triple junction point would complicate this argument; these precracks were responsible for the upper bound of $KIc,PolySi$ values $(<1.24MPa√m)$ reported here.

J. Appl. Mech 73(5), 714-722 (Dec 10, 2005) (9 pages) doi:10.1115/1.2172268 History: Received March 30, 2005; Revised December 10, 2005

## Abstract

The fracture behavior of polycrystalline silicon in the presence of atomically sharp cracks is important in the determination of the mechanical reliability of microelectromechanical system (MEMS) components. The mode-I critical stress intensity factor and crack tip displacements in the vicinity of atomically sharp edge cracks in polycrystalline silicon MEMS scale specimens were measured via an in situ atomic force microscopy/digital image correlation method. The effective (macroscopic) mode-I critical stress intensity factor for specimens from different fabrication runs was $1.00±0.1MPa√m$, where $0.1MPa√m$ is the standard deviation that was attributed to local cleavage anisotropy and grain boundary effects. The experimental near crack tip displacements were in good agreement with the linearly elastic fracture mechanics solution, which supports $K$ dominance in polysilicon at the scale of a few microns. The mechanical characterization method implemented in this work allowed for direct experimental evidence of incremental (subcritical) crack growth in polycrystalline silicon that occurred with crack increments of $1–2μm$. The variation in experimental effective critical stress intensity factors and the incremental crack growth in brittle polysilicon were attributed to local cleavage anisotropy in individual silicon grains where the crack tip resided and whose fracture characteristics controlled the overall fracture process resulting in different local and macroscopic stress intensity factors.

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## Figures

Figure 1

Preparation and testing of MEMS-scale fracture specimens. (a) Specimen before indentation, (b) indenter tip brought in contact with the specimen substrate near the polysilicon specimen, (c) specimen with edge precrack after indentation, (d) freestanding specimen with edge precrack after substrate removal (release), (e) specimen loaded with a glass grip attached to the specimen paddle, and (f) fracture of specimen.

Figure 2

Indentation created via a Vickers indenter on SiO2 (substrate). The crack propagated (vertically) into the polycrystalline silicon specimen.

Figure 3

(a) Edge precrack in a tension specimen imaged by AFM before and (b) after removal of the sacrificial SiO2 layer. All precracks were clearly visible in 5–10μm AFM images recorded before HF release but were invisible after SiO2 removal.

Figure 4

U-displacement fields in the vicinity of a crack tip for increasing KI,polySi as obtained by AFM/DIC and LEFM, respectively. The crack tip is located at the root of the contours. The left-to-right order of gray levels in the contour plots is the same as in the contour legends.

Figure 5

Subcritical crack growth in a polysilicon specimen. Note the steps at the crack tip in (b) and (c).

Figure 6

AFM images showing subcritical (incremental) crack growth in polysilicon and the corresponding deformation fields. (a) Initial crack tip location at KI,PolySi=0.809MPa√m, (b) crack at KI,PolySi=0.835MPa√m and (c) KI,PolySi=0.896MPa√m. The applied far field stress is normal to the path of crack propagation. After (c) the crack maintained its length until KI,PolySi=1.063MPa√m when it grew catastrophically. The arrows point to the crack tip at each stress intensity level. Because of the fine crack line the AFM images were enhanced for publication purposes via an edge finder filter. The deformation field contour plots span wider fields of view compared to the AFM images. The left-to-right order of gray levels in the contour plots is the same as in the contour legends.

Figure 7

Crack location during subcritical growth as seen in AFM (top image) and SEM (bottom image) micrographs that show the top specimen surface and the crack surface, respectively. The arrows point to the location of the crack at different macroscopic stress intensities according to Fig. 6.

Figure 8

SEM images of the fracture cross section of a polysilicon specimen. The section marked by dashed lines in (a) indicates the segment of incremental crack growth. After crack initiation hackle lines are apparent in (b) and (c), and the surface roughness increased dramatically resulting in crack branching in (d). Each cross section is comprised of a large number of high-resolution SEM images. The specimen cross sections are tilted with respect to the viewing angle. The specimen thickness is 2μm.

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