Comparative Study of Al Metallization Degradation in Power Diodes Under Passive and Active Thermal Cycling

Degradation of Al metallization on Si-based semiconductor chips under operation is a reliability problem known for many years, but the mechanisms of this phenomenon are not fully understood. To quantify contributions of different possible effects, a passive thermal cycling setup has been developed allowing for accelerated tests by varying the device temperature on a short-time scale without applying electrical power. The setup is also capable of testing devices in different atmospheres. The results obtained by the thermal tests of diode chips are compared to those from power cycled diodes with a focus on the degradation of the top metallization layer. The data on the structural and electrical characterization of the samples show that the passive thermal cycling induces metallization degradation very similar to that found for the power-cycled devices. Thus, it can be concluded that the thermal-induced stresses are the dominating mechanisms for the metallization fatigue and the following failure. The role of oxidation and corrosion effects is also studied in the experiments on passive thermal cycling using different environmental conditions. It is suggested that the formation of self-passivating aluminum oxide under ordinary atmospheric conditions can play a positive role in limiting the structural and electrical degradation of the metallization layers. The obtained results represent a considerable contribution to an understanding of major failure mechanisms related to metallization fatigue and reconstruction.