Pd-coated Cu Wire Bonding Reliability Requirement for Device Design, Process Optimization and Testing

Abstract

One of the biggest technology drivers in the semiconductor industry today is the fast transition from Au wire bonding to Cu wire bonding. The fast adaptation of Cu and Pd-coated Cu (PCC) 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). Another area that is quite different between Au and Cu is the reliability test requirement. In Au wire bonding, because of the fast intermetallic compound (IMC) growth rate, the 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 3 sets of wire bonding parameters. The bonded samples have IMC coverage between 94% and 97%, well above the industry level of 80%. The reliability (HAST) test passed for all samples at 96 hours. However, there are some failures in the HAST test at 192 hr. There are many factors that can influence reliability outcome including wire bonding and non-wire bonding related factors. The failure analysis identified two potential causes in our case. In one failure case, an abnormally high Chlorine level and void in molding compound were detected next to the failed bond while no Chlorine and void were detected elsewhere. In the 2nd failure case, the bonded ball seems to be off centered and results in poor bonded ball to pad interface. These two factors will be more tightly controlled in future tests to verify the reliability outcome. Intermetallic growth and phase transformation, aluminum oxide, and behavior of palladium in PdCu wire bonds are analyzed using transmission electron microscopy (TEM) of dual beam focused ion beam (FIB) thinned specimens. Results are compared to wire bonding measurement and reliability outcome.