Formation and Growth of Intermetallic Compounds in Cu–Au and Au–Al Systems for Copper on Gold Bonding

Copper (Cu) ball on gold (Au) bump (COG) bonding is developed as a practicable approach to solving the challenges posed by Cu wire bonding on large integrated circuit devices. In the COG bonding, there are two different interfaces, Cu-Au and Au-Al couples, which is totally different from either Cu or Au wire bonding. In this paper, the COG bonding was studied on a typical experimental chip of 69-μm bond pad opening and four-layer central via pattern design with current wire bonders. Tests of wire pull, ball shear, and crater were adopted in investigating the bonding strength at the interfaces and the potential damage of the underlying metals of bond pads after the COG bonding. The intermetallic compounds (IMCs) formation at the two interfaces, Cu-Au and Au-Al couples, were studied in detail on the cross-sectioned assembled samples. Two reliability tests, high-temperature baking (HTB) and temperature cycle (TC) were also used to investigate the IMC change and growth. The results showed that qualified bonding strength on all of bond pads existed and no damage such as crack and crater was observed on the underlying metals of the bond pads after removing Al metallization. At the Au-Al interface, a comparable Au-Al IMC was formed on as-bonded samples, and it gradually grew thick after the reliability tests and its corresponding content also changed along with temperature and time. At the Cu-Au interface, there was no observable formation of Cu-Al IMC on as-bonded samples. Undergoing 1008 hours HTB at 175°C, only a very thin Cu-Au IMC appeared at the Cu-Au interface. After all, the Cu-Au IMC appearance hardly affects the IC's mechanical performance.