High-Cycle Fatigue Testing of thin Metal Films on MEMS Cantilever

Aluminium is still one of the most important contact metallisation for power electronic chips like MOSFETs or IGBTs. With a large difference in thermal expansion coefficients (CTEs) between aluminium and silicon or silicon carbide, and the temperatures generated in hot-spots during high power transients, these layers are prone to failure due to thermo-mechanical fatigue. Usually lifetime assessment is done by subjecting dedicated test specimens to standardised stress tests as e.g. thermal cycling.This paper builds on previous work about a method for accelerated stress testing and lifetime modelling of thin aluminium films in the high-cycle fatigue regime by isothermal mechanical loads using Si MEMS cantilevers as sample carriers. By using a new design including two AlN piezos, it is now possible to realize a closed loop control without the need for an external shaker and an optical setup to measure the cantilever amplitude during the fatiguing. It is shown, that a chooseable amplitude could be kept stable for days, allowing more than 10
⁸
cycles. The test method could complement or replace expensive and lengthy thermal cycling and allows in-situ monitoring of failure.