Method for real-time observation of thermo-mechanical induced fatigue in copper thin films

Abstract

Every power electronics device features several layers in their stack, among them one of the most important being the power metallization. It serves as electrical as well as thermal connection with the outer system, in which the device is operated. In cases of short-circuits or switching of inductive loads, these devices need to sustain several thousands of short but very powerful pulses that induce a high temperature increase in the power metallization. Investigating thermo-mechanical fatigue directly in power electronic devices is challenging due to the power metallization not being directly accessible and thermal instabilities inside the semiconductor. Therefore, a simplified structure called polysilicon heater was introduced, which enables direct investigations of the thermo-mechanical behaviour of the power metallization. It mainly consists of a silicon substrate, a polysilicon layer and a copper layer on top. An electrical setup is used to mimic heat-up and cool-down cycles similar to those of a short circuit. The investigations carried out on the polyheaters are based on the acquisition of infrared (IR) thermography pictures of the metal surface, together with electrical measurements, during cycling. All fatigue features visible on the images of a time series need to be quantified, which was achieved by image processing. Developing and programming this image processing routine was the scope of this thesis and it was implemented with the free software "ImageJ".