A chevron specimen for the measurement of mixed-mode interface toughness of wafer bonds

Accurate characterization of interfacial adhesion is essential for the development of reliable wafer bonding processes. In most applications in which wafers are bonded, the interface experiences a combination of shear and normal loading (i.e., mixed-mode loading). When characterizing the fracture properties of a bond, it is important to measure the interface toughness under similar mixed-mode conditions. In the current work, a chevron test specimen composed of bonded cantilever layers of dissimilar thicknesses is analyzed, and the dependence of the mode mixity at the interface is determined as function of the layer thickness ratio. This test geometry is well-suited for the measurement of bonds between typical semiconductor substrates that range in thickness from 0.1 to 1 mm. A nominal specimen geometry with a total layer thickness of 0.5-2 mm and in-plane dimensions of 10 × 10 mm is analyzed using a 3-D finite element (FE) model in combination with the virtual crack closure technique. It is demonstrated that the phase angle (i.e. the degree of mode-mixity) at the interface can be varied from 0° to 35° by changing the layer thickness ratio from 1 to 0.1. The FE results have been fitted to an expression that allows the interface toughness to be easily calculated from experimental data.