Interfacial fracture of micro thin film interconnects under monotonic and cyclic loading
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The goal of this research was to develop new experimental techniques to quantitatively study the interfacial fracture of micro-contact thin film interconnects used in microelectronic applications under monotonic and cyclic loadings. The micro-contact spring is a new technology that is based on physical vapor deposited thin film cantilevers with a purposely-imposed stress gradient through the thickness of the film. These "springs" have the promise of being the solution to address near-term wafer level probing and long-term high-density chip-to-next level microelectronic packaging challenges, as outlined by the International Technology Roadmap for Semiconductors. The success of this technology is, in part, dependent on the ability to understand the failure mechanism under monotonic and cyclic loadings. This research proposes two experimental methods to understand the interfacial fracture under such monotonic and fatigue loading conditions. To understand interfacial fracture under monotonic loading, a fixtureless superlayer-based delamination test has been developed. Using stress-engineered Cr layer and a release layer with varying width, this test can be used to measure interfacial fracture toughness under a wide range of mode mixity. This test uses common IC fabrication techniques and overcomes the shortcomings of available methods. The developed test has been used to measure the interfacial fracture toughness for Ti/Si interface. It was found that for low mode mixity Ti/Si thin film interfaces, the fracture toughness approaches the work of adhesion which is essentially the Ti-Si bond energy for a given bond density. In addition to the monotonic decohesion test, a fixtureless fatigue test is developed to investigate the interfacial crack propagation. Using a ferromagnetic material deposited on the micro-contact spring, this test employs an external magnetic field to be able to drive the interfacial crack. Fatigue crack growth can be monitored by E-beam lithography patterned metal traces that are 10 to 40nm wide and 1 to a few µm in spacing. The crack initiation and propagation can be monitored through electrical resistance measurement. In the conducted experiments, it is seen that the interfacial delamination does not occur under fatigue loading, and that the micro-contact springs are robust against interfacial fracture for probing and packaging applications.