Universal mechanisms of Al metallization ageing in power MOSFET devices

• Evolution of unpassivated Al metallization in an aged MOSFET.
• Fine microstructure characterization using FIB and TEM.
• Grain and defect structure monitored during ageing, along with electrical resistance.
• A universal plastic deformation mechanism of Al metallizations is proposed.

Power MOSFET devices are extensively used in the automotive industry, but their modes of ageing are still poorly understood. Here we focus on the physical degradation mechanisms that occur in the upper Al-based metallization layer (source). This layer undergoes thermo-mechanical structural modifications due to the combination of electrical pulses and differences between the various coefficients of thermal expansion. Using electronic and ionic microscopy, we show that ageing can be divided in 2 phases where dislocation-based plasticity and then grain boundary diffusion become predominant. As a result, grain boundary grooving and surface roughening follows a partial division of the later in disconnected Al grains. Such a degradation of the metallization has been widely observed in various devices. It may lead to the observed augmentation of resistivity and also to the focusing of the various current paths, promoting hot spots and subsequent failure.