Bonding mechanism in ultrasonic gold ball bonds on copper substrate
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I. INTRODUCTION
WIRE bonding is the most utilized technique for making microelectronic interconnects from an integrated circuit (IC) to the substrate. Currently, on an annual basis, more than 4 trillion wire bonds are made.[1] It is the flexibility and cost effectiveness of wire bonding that makes it widely accepted in industry. Among the variations of wire bonding techniques (i.e., ball and wedge bonding), the predominant method used today is thermosonic ball bonding of gold wire onto aluminum metallization. Thermosonic ball bonding utilizes a normal bond force simultaneously with thermal and ultrasonic energy to form the first bond (ball) on the IC, followed by the second bond (crescent) on the substrate. However, despite its wide industry acceptance, there is a lack of a quantitative understanding of the bonding mechanisms.[1] Aluminum is the metallization typically used for thermosonic ball bonding. With the greater demand in microelectronics for ever-increasing clock speeds, methods need to be found to accommodate this trend. Copper possesses much better electrical properties than aluminum, such as lower electrical resistance[2] and the ability to support increased signal speeds. However, there are problems with bonding on copper. Copper forms an oxide layer at the elevated temperatures of thermosonic bonding, which hinders bonding.[3] Although an oxide layer also forms on aluminum, it does not pose a problem for bonding, as it is brittle and easily breaks up and is dispersed.[1] On the other hand, the oxide that forms on copper is soft and, hence, does not fracture, which poses problems for bonding. However, researchers have shown that it is possible to produce reliable bonds on copper using a shielding-gas atmosphere[4] or by bonding at ambient temperatures.[5] In a development study of gold ball bonding on a copper substrate at ambient temperatures,[5] it was found that optimal bonding in terms of bond shape and bond shear force was obtained at I. LUM, Graduate Student, J.P. JUNG, Visiting Professor, and Y. ZHOU, Canada Research Chair in Microjoining, are with the Deparment of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada. Contact e-mail: [email protected] J.P. JUNG’s permanant address is with the Department of Materials Science and Engineering, University of Seoul, Seoul, 130-743, Korea. Manuscript submitted August 9, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
a bonding power, force, and time of 325 mW, 35 gf, and 1000 ms, respectively. At present, a quantitative understanding of the bonding mechanisms is lacking and, specifically, there are no published studies of the bonding mechanisms in gold ball bonding on a copper substrate. Therefore, this study was undertaken to gain a better understanding of the role of process parameters in the ball-bond formation in thermosonic gold ball bonding on a copper substrate at ambient temperatures. The bond footprints left on the substrate were observed with scanning electron microscopy (SEM) for microwelded regions and changes in substr
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