Wire bonding is one of the dominant microelectronic connection technologies, which
provides electrical connection between integrated circuits and external leads. A solid free
air ball (FAB), which is fabricated by melting a part of bonding wire in electronic
flame-off (EFO) process, is thermosonically bonded on the aluminum bonding pad at a
high bonding temperature. Because of interdiffusion, intermetallic compounds (IMCs)
form spontaneously at the bonding interface and play a critical role on the ball bonding
strength. In recent years, the utilization of copper bonding wire to replace the traditional
gold bonding wire brings new challenges into this field. The relationship of processing
conditions, microstructure and properties of copper wirebond is therefore investigated in
this thesis work.
First of all, although FABs with same diameter are fabricated using different EFO
settings, same FAB hardness is measured. After thermosonical bonding on the aluminum
pads, different bonding heights indicate these FABs have different resistance to the plastic
deformation. Microstructure investigation demonstrates that different FAB properties are
attributed to the competition between grain size hardening and texture hardening. During
the FAB fabrication, thermal conduction along the bonding wire results in the formation of
heat affected zone (HAZ) connected to the FAB. Different EFO settings are selected in order to obtain straight wire samples with different HAZ length. HAZ boundary is defined
based on the microstructure evolution and hardness variation along the bonding wire. A
novel deflection test was established to accurately measure the HAZ Young’s modulus.
Because of HAZ formation, Young’s modulus drops significantly from 149 GPa in fresh
copper wire to only 110 GPa. Although different HAZ lengths are measured, HAZ
Young’s modulus does not show notable difference.
Then, Cu-Al interfacial IMC corrosion in an accelerated humidity test is investigated.
Although selective IMC corrosion was reported to be the feature of corroded bonding
interface, basic mechanisms behind the selective corrosion are still not fully understood.
After isothermal aging, Al
2Cu, Cu
3Al
2 and Cu
9Al
4 phases are identified at the Cu-Al
bonding interface. Selective IMC corrosion (or selective IMC dealloying) is closely
relating to the chloride. Transmission electron microscopy (TEM) reveals the selective
IMC dealloying process. When Cu
3Al
2 and Cu
9Al
4 are continually attacked by chloride, a
mushroom-shaped corrosion product, with dealloyed copper head and IMC stem, is
undercut from the corroded IMC matrix. The dealloyed copper nano-grain is mounted by
the amorphous Al(OH)
3. A bilayer nanostructure, consisting of Al
2O
3 and aluminum,
forms on the Al
2Cu surface and renders Al
2Cu phase immune to the chloride-induced
corrosion. In addition, IMC stress corrosion and high chloride concentration around
corroded IMC surfaces make the corroded IMC region in a wedge shape.
Finally, an investigation on interfacial IMC corrosion is further conducted for
palladium-coated copper (PCC) wirebond, since nano-scale palladium coating is believed
to improve the corrosion resistant of interfacial IMCs. At the thermally aged interface,
only Cu
9Al
4 and Cu
3Al
2 are confirmed to be Pd-containing IMCs. Inert diffusion behavior
of palladium renders the co-existence of Pd-containing and Pd-absent Cu-Al IMCs, which
are separated by the original ball bonding interface. A Pd-rich Cu-Al IMC (approximately
43 atom % Pd) is found in the annealed Cu-Pd-Al thin film diffusion couple, and
identified as the Cu
3Al
2 solid solubility. Pd-containing Cu-Al IMCs are also attacked by
the chloride in a similar manner of Cu-Al IMC dealloying: a mushroom protuberance,
consisting of Pd-containing copper solid solution as “head” and Pd-containing IMC as
“stem”, is undercut by the attach of chloride in a humid condition. Based on this study, it
is speculated that the palladium coating cannot make the interfacial intermetallics
completely immune to the chloride-induced corrosion in the humidity test.
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