The long-term reliability of modern power MOSFETs is assessed through accelerated electro-thermal aging tests. Previous studies have shown that the source metallization (top metal and wires) is a failure-prone location of the component. To study how the top Aluminum metallization microstructure ages, we have performed ion and electron microscopy and mapped the grain structure before and after avalanche and short-circuit aging tests. The situation under the bond wires is significantly different as the bonding process induces plastic deformation prior to aging. Ion microscopy seems to show two inverse tendencies: grain growth under the wires and grain refinement elsewhere in the metallization. Transmission electron microscopy shows that the situation is more complex. Rearrangement of the initial defect and grain structure happen below and away from the wire. The most harmful fatigue cracks propagate parallel to the wire/metal bonding interface.

A limiting factor for the long-term reliability of power MOSFET-based devices is the electro-thermal and/or thermo-mechanical aging of the metallic parts. In this paper we assess the bonding wire and source metallization degradation of power devices, designed for applications in the automotive industry. Our approach consists in characterizing the metal microstructure before and after accelerated aging tests, by scanning electron microscopy, ion milling and microscopy, focused ion beam tomography, transmission electron microscopy and grain structure mapping. To focus on the wire-metallization bonding interface, we have set up a dedicated sample preparation that allows us to disclose the metallization under the bonding wires. This critical location is significantly different from the naked metallization, as the bonding process induces plastic deformation prior to aging. The main mechanism behind the device failure is the generation and propagation of fatigue cracks in the aluminum metallization. Away and under the wire bonds, they run perpendicularly from the surface down to the silicon substrate following the grain boundaries, due to an enhanced self-diffusion of aluminum atoms. Moreover, initial imperfections in the wire-metallization bonding (small cavities and aluminum oxide residues) are the starting point for harmful cracks that propagate along the wire-metallization interface and can eventually cause the wire lift-off. These phenomena can explain the local increase in the device resistance occurring at failure.

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The long-term reliability of power devices for applications in the automotive industry is limited by the electrothermal and/or thermo-mechanical aging of the metallic parts. In the present work, we characterize the bonding wire and source metallization degradation of power MOSFETs-based devices under accelerated aging conditions, through electron and ion microscopy. The metal degradation is driven by an enhanced self-diffusion of aluminium (Al) atoms along the grain boundaries and a generalized fatigue crack propagation from the surface down to the silicon (Si) bulk. The metallization under the wire bonds is a critical location because it is initially plastically deformed during the bonding process. In addition, the wire-metal interface presents several imperfections, such as small cavities and Al oxide residues. During the electro-thermal cycles, they could be the starting point for harmful cracks that run along the interface (and eventually cause the wire lift-off or the cracking of the substrate). Whichever the propagation direction, the generation of these cracks locally increases the device resistance and temperature, and accelerates the aging process until failure.Mechanisms of power module source metal degradation during electro-thermal aging.