A 4-POINT BENDING TECHNIQUE FOR STUDYING SUBCRITICAL CRACK-GROWTH IN THIN-FILMS AND AT INTERFACES

A technique was developed to obtain the subcritical crack growth veloc ity in a 4-point bending sample by analyzing the load-displacement cur ve. This was based on the observation that the compliance of a beam in creases as the crack grows. Beam theory was used to analyze the genera l configuration where two cracks propagated in the opposite directions . A simple equation relating the crack velocity to the load and displa cement was established, taking advantage of the fact that the complian ce was linearly proportional to the crack lengths; thus the absolute c rack length was not important. Two methods of obtaining crack velocity as a function of load were demonstrated. First, by analyzing a load-d isplacement curve, a corresponding velocity curve was obtained. Second , by changing the displacement rate and measuring the corresponding pl ateau load, a velocity value was calculated for each plateau load. Whi le the former was capable of obtaining the dependence of crack velocit y versus load from a single test, the latter was found to be simpler a nd more consistent. Applications were made to a CVD SiO2 system. In bo th cases of crack propagation either inside the SiO2 layer or along it s interface with a TiN layer, the crack growth velocity changed with t he stress intensity at the crack tip exponentially. As a result, a sma ll crack will grow larger under essentially any tensile stresses typic ally existing in devices, provided that chemical agents facilitating s tress corrosion mechanisms are also present.