Interfacial fracture toughness measurement for thin film interfaces

Delamination of intrinsically or residually stressed thin films is commonly encountered in microelectronics and MEMS systems. Knowledge of the interfacial fracture toughness (Γ) is necessary to predict if delamination will occur. A new approach based on the decohesion test, called the modified decohesion test (MDT), eliminates shortcomings of current testing methods. In this approach, a highly stressed superlayer is used to drive delamination and produce any 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. MDT uses the change in crack surface area to vary the available energy per unit area for crack growth and thus to bound the interfacial fracture toughness. Therefore, this technique uses a single sample to measure the interfacial fracture toughness, as opposed to the decohesion test that uses several samples to be able to bound the interfacial fracture toughness. 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 discussed. Finite element model results of MDT sites are used to validate the approach. Preliminary results of the test show that for a Ti/alumina interface, at a mode mixity of −14.5°, the interfacial fracture toughness is greater than 34 J/m2 and for a Ti/Si interface, at a mode mixity of 23°, the interfacial fracture toughness is 8.9 J/m2 ⩽ Γ(23°) ⩽ 9.89 J/m2.