An experimental fracture mechanics study of a strong interface: The silicon/glass anodic bond
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The fracture behavior of the silicon/Pyrex glass anodic bond was investigated using the methods of linear elastic bimaterial fracture mechanics. Due to the high bond strength, interfacial cracks were invariably observed to kink away from the interface into the more compliant glass under approximately mode I remote tensile loading. Kink angles measured by profilometry increased from 14 to 28° as bonding temperature increased from 300 to 450 °C. A regime of stable cracking accompanied penetration of cracks into the glass, with maximum load and corresponding fracture toughness measurement occurring at a location significantly removed from the interface. Approximately mode I, near-interface, plane-strain fracture toughness values (KIC) measured by rising load testing of chevron-notched and straight-thru-cracked compact-tension specimens increased from 0.63 to 0.68 MPa-m1/2 and 0.66 to 0.75 MPa-m1/2, respectively, as bonding temperature increased from 300 to 450 °C. In addition, XPS measurements revealed a sodium depletion zone of decreasing size and depletion magnitude with increasing bonding temperature over the same range. The near-interface region of the glass also experiences compressive residual stresses which decrease linearly with distance from the interface according to linear elastic computations. These stresses increase in magnitude with increasing bonding temperature due to enhanced differential thermal contraction upon cooling to room temperature. It is proposed that the trends in toughness and in kink angle with bonding temperature can be at least partially accounted for by variation of crack-tip shielding with compressive residual stress magnitude, the effects of interfacial crack-tip shear stresses induced by the thermal mismatch, and by an increase in Young's modulus of the near-interface glass accompanying sodium depletion. I. INTRODUCTION Anodic bonding has a variety of commercial applications including pressure sensors, photovoltaics, and microelectronic device packaging. This process has been used to bond a wide variety of material combinations, including ceramic/metal, ceramic/semiconductor, ceramic/ceramic, and semiconductor/semiconductor pairs. The process differs from conventional diffusion bonding primarily in that the layers to be bonded are not only heated, but are also subjected to a strong electric field. Consequently, during charging, a strong electrostatic attraction develops between the layers, facilitating good contact, and obviating the need for application of external pressure. The field also permits bonding at significantly lower temperatures so that materials with larger differences in thermal expansion coefficient and lower melting temperature can be successfully bonded. J. Mater. Res., Vol. 10, No. 2, Feb 1995 http://journals.cambridge.org
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The silicon/Pyrex glass anodic bond was the original combination and is the focus of this investigation.1'2 A significant amount of research has been done on the process of anodic bonding, including modeling of the curr
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