Development of a New Copper Corrosion Screening Methodology for Application in Microelectronics

Abstract

Due to the constant miniaturization of microelectronics, the aim of the semiconductor industry has been to increase the packaging density of devices. Wire bonds form the primary interconnects between the integrated circuit chip and the metal lead frame in semiconductor packaging. Gold (Au) wire has been used for wire bonding in the electronics industry because of its mechanical and electrical properties, high reliability, and ease of assembly. However, due to the increasingly high cost of Au, alternative wire bonding materials have been considered. Copper (Cu) is gradually replacing Au because it exhibits not only a high resistance to electromigration, but also has excellent electrical conductivity and a low resistivity. The benefit of copper’s conductivity is that it reduces heat generated by joule heating, thus reducing RC delay. However, copper also has its drawbacks; tiny amounts of contaminant can corrode Cu interconnects, leading to the failure of entire microelectronic devices. On-chip corrosion within Cu interconnect microstructures can result in increased defectiveness, which causes serious reliability issues and decreases production yield. Copper is susceptible to corrode in acidic and strong alkaline solutions in the presence of oxygen and other oxidants. Effective corrosion monitoring is critical as an early alert before the onset of corrosion-related failure. In this study, we report a new method for monitoring the corrosion of copper relevant to the microelectronics industry through the combination of electrical resistance measurement changes of Cu wire and microscopic time-lapse imaging. Industry-grade 99.99% Cu bonding wire (diameter = 25μm) was exposed to the harsh, oxidizing conditions of ammonium persulfate, and its change in resistance was monitored. As the corrosion of Cu bonding wire took place, the cross-sectional area of the wire decreased, causing an increase in its resistance (via R= ρ•l/A). During the resistance monitoring, the simultaneous microscope time-lapse imaging enabled Cu corrosion to be observed in real time. Due to the combined quantitative and qualitative nature of this metrology, it provides detail on the corrosion process that might not be obtained by traditional corrosion tests. While ammonium persulfate will almost never be encountered by an IC device during normal use, it provides a rapid screening environment that can quickly distinguish between corrosion inhibition and no protection at all. This metrology was used to test both bare Cu bonding wire and corrosion-inhibitor coated bonding wire, and showed a drastic difference in their corrosion rate and behavior. This work ultimately aids with inhibitor selection and inhibitor coating design that will help ensure long term reliability of electronics used in industrial applications. Figure 1