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Pd-coated Cu Wire Bonding Reliability Requirement for Device Design, Process Optimization and Testing
Keywords: wire bonding, Cu wire bonding, reliability
One of the biggest technology drivers in semiconductor industry today is the fast transition from Au wire bonding to Cu wire bonding. The fast adaptation of Cu and Pd-coated Cu (PdCu) wire has focused the whole packaging industry to develop understanding, equipment and processes that can produce a more reliable and robust Cu wire bonding technology. Although the fundamentals of wire bonding technology are very similar between Au and Cu wire bonding, there are a lot of new challenges in Cu wire bonding. Compared to Au wire bonding, Cu wire bonding needs different bond quality measures and metrology. Traditional ball diameter, ball height and shear measurements are not adequate to quantify a Cu wire bonding process. Some of the additional bond quality measures include pad material push out (pad splash), Al layer peel off (pad peel) and crack in the barrier and dielectric layer (pad crack). The shear value is greatly influenced by the amount of pad splash, so it is no longer a good bond quality indicator. Instead the Intermetallic coverage (IMC) measurement is a key measurement for bond strength as well as the pull test measurement. The pull test failure mode and pull strength is a better indicator of bond strength because it is less affected by the pad splash amount and it is a direct measure of the adhesion between the pad and ball. It is also a good indicator of pad damage. The wire bonding results for two different pad metallizations are presented and analyzed in this paper. Another area that is quite different between Au and Cu are the reliability test requirements. In Au wire bonding, because of the fast IMC growth rate, HTS test is normally the hardest to pass. Due to the corrosion of Cu wire, the HAST test is the most challenging in Cu wire bonding. Reliability requirements still need more knowledge. In this paper, we conduct reliability tests for devices with different wire bonded conditions such as different IMC percentages, different interface shapes and Al remains. The wire bonding response including as bonded condition and after bake condition will be compared to reliability outcome to derive the wire bonding requirement for the passing reliability test. Intermetallic growth and phase transformation, aluminum oxide, and behavior of palladium in PdCu wire bonds are analyzed using high resolution transmission electron microscopy (HRTEM) of dual beam focused ion beam (FIB) thinned specimens. Results are compared to wire bonding measurement and reliability outcome.
Ivy Qin, Engineering Director
Kulicke and Soffa
Fort Washington, PA

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