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Wire Bonding Semiconductor Devices for Metallurgists
Keywords: Intermetallic, Fine Pitch, Wire Bonding
In 2008 more than 10^13 semiconductor interconnects will be formed. Of this quantity, more than 90% will be produced by wire bonding. Wire bonding is a high speed, ultrasonic welding process. In the more common variation, ball bonding, fine diameter (average diameter is now < 25m) gold or copper wire is welded to a thin (1m) Al-1%Si-0.5%Cu bond pad on the semiconductor device. Typical bond pads are now < 75m square and devices like graphic processors may have more than 1000 on a single device. State-of-the-art bonders now operate at rates > 16 wires/second (32 welds + high speed motions between the two welds that define each wire). Placement accuracy must be better than 2.5m. Even though the wires are a small fraction of the diameter of a hair (100m) and the speed is high, the process is a welding process and must be understood from a metallurgical perspective. At the start of the process, formation of a ball on the tip of the wire by a spark from the Electronic Flame Off (EFO) is a melting/casting process. Thermal transfer to solidify the ball is >90% up the wire and the grains are clearly columnar, with a few equiaxed grains at the very bottom tip of the ball (the last region to solidify). The influence of ultrasonics on metals has been shown to significantly change the physical properties, lowering the flow stress and Youngs Modulus, allowing both easy flow of slip planes with massive plastic deformation of the ball and also significant deformation of the underlying bond pad. Gold bonding wire is normally 99.99% (4-9s, 100 ppm total impurities) pure and is produced by refining gold to 5-9s purity, and alloying within the 100ppm allowable impurity range. Absolute control of the alloying is required to allow the dynamics of high-speed loop formation (Z- axis accelerations of >300 gs) without loss of control or damage to the wire. Intermetallic formation in the Au-Al and Cu-Al systems has been widely studied and much work has gone into controlling distribution of phases. The use of copper ball bonding is growing rapidly, especially in the larger wire diameter applications where the replacement of Au by Cu provides a large cost advantage. Copper has some advantages (cost, conductivity, stiffness) but it is also harder mechanically and can cause damage to the underlying Si, is more difficult to bond (the intermetallic has a much lower growth rate) and requires a reducing atmosphere to eliminate oxidation.
Lee Levine, Consultant
Process Solutions Consulting
New Tripoli, PA

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