A failure criterion for interface notches in silicon/glass anodic bonds

A common approach for joining silicon - structured using anisotropic etching - and glass is anodic bonding. The combination of these two techniques (etching and bonding) lead to problems concerning the reliability of the resulting microelectromechanical systems during mechanical loading caused by mechanical failure starting at sharp interfacial corners conditional to etching technology. Therefore a failure concept is required that can be applied for the design of reliable microsystems. Based on the fracture mechanic concept of critical stress intensity factors it is possible to develop such a criteria for fracture initiation at interface corners of silicon/glass anodic bonds. For examination of this approach we designed and fabricated a series of special test structures -silicon/glass anodic bonds with different notch geometries. These structures are based on typical structures found in microelectromechanical systems. We performed a large number of symmetric and asymmetric four-point flexure tests at different temperatures and loading rates to eliminate the influence of sub critical crack growth. The measured critical failure stresses were used as input data for a finite element model to calculate the singular stress field of the interface notch tip and its corresponding notch stress intensity factors A1C (mode I) and A2C (modeII). Using the results of the asymmetric four-point flexure tests it can be shown that failure is mostly characterised by A1C. Furthermore, using optical and scanning electron microscopy we were able to show, that the crack path is well approx imated by the path of maximum radial stresses calculated using finite element analysis. The so determined A1C values may then be used to characterise the strength of a microsystem during the layout phase; meaning that already in the stage of design reliability of a microsystem can be assured.