TEM microstructural analysis of As-Bonded Al–Au wire-bonds Adi Karpel Giyora Gur Ziv Atzmon Wayne D. Kaplan Received: 20 January 2007 / Accepted: 7 February 2007 / Published online: 12 March 2007 Ó Springer Science+Business Media, LLC 2007 Abstract In this study the interface morphology of a model 99.999% (5N) Au wire bonded to Al pads in the as-bonded state was examined by scanning/transmis- sion electron microscopy with energy dispersive spectroscopy. Specimens for transmission electron microscopy were prepared using the lift-out method in a dual-beam focused ion beam system. Analysis of the bond microstructure was conducted as a function of the Al pad content and as a function of the bonding tem- perature. Additions of Si and Cu to the Al pad affect the morphology and the uniformity of the interface. A characteristic-void line is formed between two inter- metallic regions with different morphologies in the as-bonded samples. According to the morphological analysis it was concluded that a liquid phase forms during the bonding stage, and the void-line formed in the intermetallic region is the result of shrinkage upon solidification and not the Kirkendall effect. Introduction Wire-bonding is the one of the key industrial tech- niques to connect semiconductor devices to the mac- roscopic circuit arranged on a circuit board [1]. Wire- bonding provides an electric connection, for example, of an aluminum pad on a silicon die to a lead frame. The most commonly used system is high-purity Au 99.99% (4N) wires bonded to Al pads. A combination of thermal and ultrasonic energies is employed to form a bond between the Au wire and the Al pad, as well as between the wire and the lead frame [1]. Given the nominally low-temperature process (usually in the range of 120–240 °C), it is usually assumed that the bond is formed in the solid state via the formation of intermetallics by solid-state diffusion [1]. During the wire bonding process an interface region that is composed of Al and Au is formed [1–6]. Previous studies have shown that Al–Au intermetal- lics are formed during the bonding stage and during the subsequent packaging processes [7]. The Al–Au intermetallics that were found to form during wire- bonding and packaging processes of Au wire to pure Al pads are the thermodynamically stable Al–Au intermetallics [2, 4, 6]. In order to increase the electromigration resistance of the Al pad, Si and Cu are added [1]. The Si–Cu additions reduce the uni- formity of the intermetallic coverage, and can influ- ence which intermetallics are formed during the bonding process [6–8]. The Al–Au intermetallic region of the bond contains flaws in the form of voids. Previous studies of the morphology of the interface region that is formed at the bonding stage suggested that the flaws result from contaminates and residual native oxide layers that formed prior to bonding. The oxide layers are believed to prevent complete contact between the Au ball bond and the Al pad [2–5]. Other studies have suggested that during the life-time of the device, growth of the A. Karpel  W. D. Kaplan (&) Department of Materials Engineering, Technion—Israel Institute of Technology, Haifa 32000, Israel e-mail: kaplan@tx.technion.ac.il G. Gur  Z. Atzmon Kulicke & Soffa Bonding Tools, Yokneam Elite 20692, Israel 123 J Mater Sci (2007) 42:2334–2346 DOI 10.1007/s10853-007-1592-z