In-Situ Degradation Monitoring of Sputtered Thin Al Films on Si Cantilevers Inside SEM During Accelerated Stress Testing using Nano-Indenter Actuation and Vibration Loading

Abstract

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 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 i n t he high-cycle fatigue regime by isothermal mechanical loads using Si MEMS cantilevers as sample carriers. The main focus is on In-situ fatiguing performed inside a scanning electron microscope (SEM) using a nanoindenter (NI), to gain further insight into the development of degradation of the thin aluminium over time. Surface roughness of the fatigued films was previously measured via atomic force microscopy (AFM) as a failure parameter. This was motivated within a physics-of-failure based reliability paradigm by comparison with eqv. plastic strain obtained from finite element simulations, which show a relation between surface roughness and the cumulated plastic strain. Additional to AFM-measurements we present a similar approach using SEM images and compare them with finite e lement simulations.