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High Resolution Failure Analysis Comparing Au Wire Thermosonic Bonding and Ultrasonic Bonding using an Al-Coated Au Wire for IC Interconnection
Keywords: Gold Wire Bonding, Failure Analyses, Reliability
Being still the dominating technology for IC interconnections, thermosonic and ultrasonic Au wire bonding technologies have to meet different increasing challenges not only in terms of integration density but also with respect to rapidly growing reliability requirements such as in automotive applications. In this paper, different failure mechanisms and reliability risks for Au wire contacts bonded on Al pads of integrated circuits are investigated using TEM and SEM in combination with Focused Ion Beam (FIB-) cross section sample preparation, x-ray spectroscopy (EDS) and surface analysis using TOF-SIMS. Particular respect is given to analyze root causes of ball lift-offs or reduced pull force values related to the interface reactions between Al bond pad and Au ball in thermosonic bonding. By discussing different case examples it is shown that particularly halide contaminations occurring on either the bond pad surface or in the packaging materials can affect the bonding reliability. Here, a key issue is the sensitivity of the Au4Al intermetallic to chemical reactions finally leading to an oxidation of the compound. In addition, also Al oxide films occurring in the bonding interface have carefully to be considered. In addition to Al bond pad properties, Al oxide films may become of specific significance if current Au wires with a 20 nm-thick Al surface coating are used during ultrasonic wire bonding in automotive applications. It could be shown by TEM that, although the Al film improves the initial contact behaviour, it is typically rapidly removed from the bonded interface during the second stage of bonding. Provided appropriate bonding parameters are used, the Al coating on Au wires did not affect the interface microstructure of the investigated bonded interconnects or form an additional reliability risk.
Robert Klengel, Scientific Staff Member
Fraunhofer Institute for Mechanics of Materials
Halle, Saxony-Anhalt 06120,
Germany


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