High resolution analyzes of resistance behavior in eWLB metal contacts

The embedded Wafer Level Ball Grid Array (eWLB) technology is a new packaging solution that allows a minimum package size for any number of interconnects at a given pitch and the possibility of further system integration in x-, y- and z-direction. The eWLB is a fan-out wafer level package solution realized by a thin film redistribution layer above the semiconductor chip and using standard thin-film processes. The eWLB technology is driven by smaller form factor and better electrical performance with respect to high frequency applications. For optimum electrical performance, it is important to measure and control the contact resistance at interfaces caused by intermediate layers of the electrical re-routing. This paper presents a case study for high resolution analyzes at the eWLB metallization system Al/TiW/Cu with different intermetallic resistance behavior. First, we investigate different eWLB metallization systems after different process step variations. We observe chemically high resistance intermediate layers using Time-of-Flight Mass Spectrometry (ToF-SIMS). These results show that the intermediate layer consists of carbon, oxygen, fluorine, chlorine and sulfur. Second, we applied Focused Ion Beam (FIB) preparation and Transmission Electron Microscopy (TEM) as well as High Resolution Transmission Electron Microscopy (HRTEM) experiments to investigate the metallization interface Al/TiW/Cu. We resolve a porous 2–10 nm thin layer. We show that control of different humidity concentration out of the mold compound and different pre-clean etch rates at warped wafer parts are crucial for optimum contact resistance. The results demonstrate that high resolution analyzes and combinations of different analytical methods (e.g. HRTEM and ToF-SIMS) are very important for optimum process developments in modern package technology.