Copper (Cu) Wire Bonding or Copper Wirebonding

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Copper Wirebonding

Copper Wirebonding refers to the wire bonding process that employs copper wires for interconnection, instead of the gold and aluminum wires traditionally used in semiconductor packaging.

Copper is rapidly gaining a foothold as an interconnection material in semiconductor packaging because of its obvious advantages over gold. These advantages include: 1) cost reduction of up to 90%; 2) superior electrical and thermal conductivity; 3) less intermetallic growths; 4) greater reliability of the bond at elevated temperatures; and 5) higher mechanical stability.

Copper is inherently 3 to 10 times cheaper than gold, so substituting gold wires with copper wires can realize tremendous annual cost savings for a semiconductor packaging company.

Copper wire, with an electrical resistivity of 0.017 micro-ohm-m at room temperature, is more electrically conductive by about 25%-30% than gold, which has a resistivity of 0.022 micro-ohm-m at room temperature. This low electrical resistivity of copper results in better electrical performance. In particular, copper wire is a preferred bonding wire material for high-current or high-power applications, since it can carry more current for a given wire diameter.

Copper also has about 25% higher thermal conductivity than gold (385-401 W m^-1 K^-1 for Cu and 314-318 W m^-1 K^-1 for Au). Thus, copper wires dissipate heat within the package faster and more efficiently than gold wire, minimizing the thermal stress to which they are exposed. Excessive heat on the wires can promote grain growth, which lowers the strength of the wires. The heat-affected zone (HAZ) formed on the wire during free air ball formation also tends to be shorter in copper wires because of their better thermal conductivity. The shorter HAZ in copper wires give them better wire looping capability than gold, an important aspect of die stacking.

Another advantage of copper over gold is its lower tendency to form intermetallic compounds with aluminum. The atoms of the gold wire have a high tendency to interdiffuse with those of the aluminum bond pad and form intermetallic compounds (IMC) with them. The high inter-diffusivity between gold and aluminum can create voids at the bond interfaces. The presence of such voids weaken the bond and can lead to bond lifting as well as other wirebond reliability problems. Aside from void formation, some of the intermetallic compounds formed by Au with Al are brittle and are therefore prone to fail by fatigue or stress cracking in the presence of thermo-mechanical loading.

Given the relatively high resistivities of the Au-Al IMC's, these intermetallics can induce additional heating when current is flowing through the wires. The additional heat tend to accelerate the formation of more intermetallics, leading to a vicious cycle of IMC formation and heat generation.

On the other hand, intermetallic compound formation between the copper wire and the aluminum bond pad occurs at a higher temperature than Au-Al IMC formation. Studies by some experts have likewise shown that Cu-Al IMC growth is also 2.5 times slower than Au-Al IMC growth. Because of copper's lower tendency to form intermetallic compounds than gold, copper bonds are deemed to offer a higher reliability at elevated temperatures.

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