טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBarkai Gabbay Odelia
SubjectDynamic Crack Instabilities in Silicon Crystal under
Combined Tensile and Shear Stresses
DepartmentDepartment of Materials Science and Engineering
Supervisor Dr. Dov Sherman
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Two opposing mechanisms dictate crack propagation in brittle crystals : the lower energy of the preferred cleavage plane and the requirement to reduce the shear stresses during propagation. Crack propagation in brittle crystals has been investigated for decades, but the effect of the crack velocity and the stress field at the crack tip on the path selected by the crack and the generated surface instabilities are poorly understood. We investigated crack propagation in brittle crystals in which we intentionally subjected the crack to both tensile and shear stresses in controllable manner. Here we show the dominant role played by the shear stress and the velocity on crack path selection and surface instabilities and the importance of the atomistic arrangement; the two low energy cleavage planes of silicon, {110} and {111}, break fundamentally different, as if they belong to two different materials. The findings necessitate a new theoretical attitude and challenging atomistic simulations on one hand and may assist in better design and failure prevention of devices and chips in the new emerging technologies on the other.

The experimental method consisted of assembling together a thin rectangular and precracked brittle specimen with aluminum plate that serve as the loading device. The specimen was glued to the aluminum frame with epoxy resin, and then heated on electrical heating plate. The thermal expansion coefficients mismatch served as the driving force for   crack propagation.

In order to examine the path selected by a crack, surface perturbations, the effect of velocity and the role played by the atomistic arrangement in brittle single crystals, the two low energy cleavage systems in silicon were investigated as a model material:{110}plane in the <110> direction, and {111}plane in the <112> direction.


It was shown that slow cracks are macroscopically stable while fast cracks are unstable. Cracks propagating along the {110}<110> system show nearly isotropic behavior. On the other hand, a crack along the {111}<112> system is globally stable and only at a certain critical combination of velocity and shear stresses does it deflect abruptly. Local instabilities were observed in the latter system when the crack is sufficiently slow.


The dependence of crack path and surface instabilities on the atomistic arrangement and crack velocity was demonstrated in this investigation. It was shown that in addition to the shear stresses, the atomistic arrangement and crack velocity are the sources of complex and diverse phenomena.