Temperature dependent fracture toughness of glass frit bonding layers

Glass frit bonding is an important technology for the hermetical encapsulation of microsensors. During the manufacturing process or in application the bonding layer is repeatedly exposed to temperature changes. Therefore a meaningful stability assessment must include temperature dependent fracture toughness parameters. This work shows that the influence of temperature changes on the fracture toughness depends on whether the bonding layer is subjected to externally pure tensile or combined tensile and shear loading. The reasons for this effect are discussed by use of finite element simulations. Two different kinds of cracks are compared with regard to the residual stresses that result from the wafer bonding process. These residual stresses have a high influence on the loading conditions at a crack tip, when the crack is kinking into the glass frit material. However the influence of residual stresses on an interfacial crack that propagates parallel to the plane of the bonding layer is almost negligible.