Study of Formation and Change 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-scale integrated circuit devices. In the COG bonding, there are two interfaces, Cu-Au and Au-Al couples, which is totally different from either Cu or Au wire bonding. In this paper, the COG bonding is studied on an experimental chip of 69um bond pad opening and 4-layer central via pattern design with current wire bonders.

Tests of wire pull, ball shear and crater are 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 (IMC) formation at the two interfaces, Cu-Au and Au-Al couples, are studied on the cross-sectioned assembled samples. Two kinds of stress tests, High Temperature Baking (HTB) and Temperature Cycle (TC) are used to investigate the IMC change and growth.

The results show that qualified bonding strength on all of bond pads exists and no damage such as crater is observed on the underlying metals of the bond pads after chemical etching. At the Au-Al interface, a comparable Au-Al IMC is formed on as-bonded samples, and it gradually grows thick after the stress tests and its corresponding content also changes along with temperature and time. But in contrast there is no observable formation of Cu-Al IMC at the Cu-Au interface on as-bonded samples. Undergoing 1008 hours HTB at 175oC, only a very thin Cu-Au IMC appears at the Cu-Au interface. After all, the Cu-Au IMC appearance hardly affects the IC’s mechanical performance.