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High Resolution Analysis of Resistance Behaviour in eWLB Metal Contacts
Keywords: eWLB, high resolution failure analyses, increased resistance
We present a case study for high resolution analysis of the Al/TiW/Cu metallization system in an embedded Wafer Level Ball Grid Array (eWLB). The embedded Wafer Level Ball Grid Array (eWLB) technology is a new packaging solution that allows increased I/O density and further system integration. The eWLB is a fan-out package solution realized by a thin film redistribution layer above the semiconductor chip using standard thin-film processes. The eWLB technology is driven by cost reduction, smaller form factor and better electrical performance in respect to high frequency applications and the possibility to realize any number of interconnects in a given pitch on the Wafer Level Package. For electrical performance interface control in the electrical rerouting is needed to avoid intermediate layers that might cause increase of the contact resistance. High resolution analyses have to be performed to detect thin resistive layers. This paper presents a case study for high resolution analysis at the eWLB metallization system Al/TiW/Cu with different resistance behavior. In a first step different eWLB metallization systems were investigated after different process step variations. High resistance intermediate layers were observed chemically using Time-of-Flight Mass Spectrometry (ToF-SIMS). These results show that the intermediate layer consists of Carbon, Oxygen, Fluorine and Sulfur. In the second step focused ion beam (FIB) preparation and transmission electron microscopy (TEM) as well as high resolution transmission electron microscopy (HRTEM) investigation are applied to investigate the metallization interface Al/TiW/Cu. We resolve a porous 2-10nm thin layer. The investigations indicate three factors, which are influencing the contact resistance RC of eWLB: • Humidity out of mold compound is out-gasing during sputtering and being split in H2 and O2 by the plasma in the pre-clean chamber; the O2 is re-oxidizing the aluminum pads during the pre-clean • Pre-clean etch rate is reduced at warped parts of the wafer • A third factor observed by ToF-SIMS is distillation of water vapor out of the mold compound, which takes parts of the dielectric with it and contaminates the aluminum pads. Usage of dry eWLB wafers and appropriate warpage control avoid the formation of interface layers.
Sandy Klengel (Bennemann),
Fraunhofer Institute for Mechanics of Materials IWM
Halle, Sachsen-Anhalt 06116,
Germany


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