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Study of Formation and Change of Intermetallic Compounds in Cu-Au and Au-Al Systems for Copper on Gold Bonding
Keywords: COG (copper bonded ball on gold bump), Intermetallic formation and growth, Cu-Au and Au-Al systems
In microelectronic packaging, thermosonic bonding technology is playing a leading method for making electrical interconnection between chips and lead frames or substrates. Gold (Au) wire is the most predominant material used in thermosonic bonding. With gold price continuously soaring and packaging cost competitiveness, copper (Cu) wire is widely regarded as an alternative interconnection material that serves as a competent successor to gold wire due to many advantages in mechanical/electrical characteristics and cost efficiency. However, its well-known disadvantage, higher hardness offers a main challenge to introduction of copper wire bonding on high-end integrated circuits because it inevitably induces crater, peeling and underlying crack of bond pads. A new wire bonding approach called COG (Copper ball on Gold bump) bonding is developed to solve this challenge along with conventional copper wire bonding as a practicable method. The COG bonding is to bond a gold bump on bond pad first using current wire bonder, and then to introduce conventional copper wire bonding to micro-weld a copper ball onto the gold bump. The prefab gold bump serves as a cushion to absorb majority of mechanical stress from copper wire bonding by its body deformation so that bond pad only receive the minority stress that is no more than its sustaining capability. With respect to the COG bonding, it has two interfaces, Au-Al and Cu-Au systems at the first bonds, which is significant different from conventional wire bonding with either copper or gold wire. So there is a general concern with performance of the initial intermetallic compound of Cu-Au after thermosonic bonding and its growth through encapsulation and reliability tests. The paper aims to study of integrity of initial intermetallic compound of both Cu-Au and Au-Al systems on as-bonded devices with the COG bonding and effect of the Cu-Au and Au-Al IMC growth through reliability tests. Temperature can significantly affect behavior of IMC growth on as-bonded balls. High temperature can promote atomic diffusion at the Au-Al and Cu-Au systems. Refer to JEDEC criteria, High temperature baking and temperature cycles are chosen to investigate the behaviors of the IMC growth for reliability assessment with the COG bonding. OM, SEM and EDS are used to check the behavior of the two interfaces on the sectioned samples. In the study, it is disclosed that the Cu-Au system does not formed an intermetallic product while clear multiple intermetallic products are generated at the Au-Al system after wire bonding process done. However, In the Cu-Au interface, atomic diffusion definitely happens by performing DES test and the Cu-Au system tends to a eutectic type due to no structural changes when the solute atoms substitute the solvent atoms in the crystal. Undergoing the two reliability tests, a very thin Cu-Au IMC is presented finally at the Cu-Au interface while a quick growth of the Au-Al IMC is observed along with some small Kirkendall voids appearing at the Au-Al interface on the sectioned samples. HTB and TC can promote atomic diffusion in the Cu-Au system, but this atomic diffusion is much slower than that of the Al-Au system due to low vacancy-solute binding energy and atomic radii mismatch of Cu-Au. The whole study result reveals an extensive foreground of copper wire bonding on high-end integrated circuit devices by using the COG bonding due to much lower Cu-Au IMC growth and no differentia in Au-Al IMC change from that of conventional Au wire bonding throughout ICs lifetime.
Yingwei Jiang, Section Manager in Global Packaging Engineering
Freescale Semiconductor Inc.
Tianjin 300385,
China


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