Modified Decohesion Test (MDT) for Interfacial Fracture Toughness Measurement in Microelectronic/MEMS Applications

Delamination of intrinsically or residually stressed thin films is commonly encountered in microelectronics and MEMS systems. Thin films typically accrue stresses through micro structural variations caused by physical vapor deposition, thermally induced stresses imposed due to thermal mismatch, and/or extrinsically introduced forces. These stresses can reach upwards of 1 GPa and can easily exceed the strength of the metal thin film interface. Knowledge of the interfacial fracture toughness (Γ) is necessary to predict if delamination will occur. However, measuring Γ is a challenge for thin film interfaces. Typical testing methods such as bimaterial cantilever, microscratch, peel, bulge, or edge lift-off are limited to organic films, cause complex stress fields, can only measure a single mode mix, or cannot achieve the large energy release rates typical of metal thin film interfaces. A new approach based on the decohesion test, called the modified decohesion test (MDT), eliminates these shortcomings of current testing methods. In this approach, a highly stressed super layer is used to drive delamination and “tune-in” the mode mix at the crack tip. Since the deformations remain elastic, a mechanics-based solution can be used to correlate test parameters to the energy release rate. Common IC fabrication techniques are used to prepare the sample and execute the test, thereby making the test compatible with current microelectronic or MEMS facilities. Varying the crack surface area rather than the energy in the super layer allows the ability to bound Γ using a single test wafer providing a 90% savings in resources and 95% savings in time. Other modifications allow application of the method to highly chemically reactive metals and decrease the sample preparation time. Design, preparation, and execution of the MDT are presented. Results of finite element models are used to validate the approach. Results are shown for a Ti/Al₂O₃ interface.