ABSTRACT of the Doctoral Thesis
One of the main design challenges for modern power semiconductor devices is the reduction of the device footprint while maintaining or even increasing the power density. This leads to extreme operational conditions, which have to be withstood by the utilized materials and compounds. The lifetime of a device is not limited by the first occurrence of fatigue damage onset. Instead, a certain region of material failure is necessary to impede the heat flow and lead to a critical temperature increase and potential thermal runaway. Therefore, the whole fatigue life needs to be assessed, ranging from early damage formation to damage onset and spatial damage progression. Advanced methods within the framework of the Finite Element Method are developed to describe potential fatigue damage and interface degradation of the power metallization caused by active power cycling with massive overload pulses.
A continuum damage mechanics based approach is formulated to describe fatigue damage inside the ductile power metallization. The approach utilizes a phenomenological multiaxial fatigue criterion for the description of damage onset and a strain energy density based criterion for the assessment of damage progression. Additionally, the effect of damage on the thermal conductivity is included in the model. The approach is implemented into the Finite Element Method allowing for the simulation of spatial damage evolution with respect to the number of load cycles. Furthermore, the change of the thermal field caused by damage degraded heat conduction is considered. The fatigue damage modeling approach is exemplified on simulations of microcantilever beam experiments under low cycle fatigue conditions.
A cyclic cohesive zone model is formulated for the simulation of cyclic delamination between the power metallization and the silicon chip. The model is based on an exponential-traction separation law formulated for monotonic loading conditions. An energy-based fatigue extension is introduced allowing for the prediction of delamination growth under cyclic loading conditions. The whole model formulation utilizes physically interpretable interface properties which can be directly obtained from experimental results. The model accounts for mixed-mode and variable amplitude loading conditions. The thermal conductance of the interface is coupled with the mechanical damage variable resulting in a deterioration of the thermal flux across the interface. The model is implemented into the Finite Element Method allowing for the simulation of structures under thermo-mechanical loading conditions. The mechanical constitutive response of the model is demonstrated on pure and mixed-mode delamination tests undergoing constant and variable amplitude loading.
The fatigue damage modeling approaches are exemplified on a simple, generic submodel of a DMOSFET device. Various simulations are conducted using the damage modeling approaches either independently or in combination with each other. The obtained damage characteristics, their evolution with load pulses, and their interactions are discussed and compared. The developed approaches predict reasonable results and provide a step forward towards physical lifetime models based on numerical simulation techniques for power semiconductor devices.