Electromigration analysis of high current carrying adhesive-based copper-to-copper interconnections
Khan, Sadia Arefin
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"More Than Moore's Law" is the driving principle for the electronic packaging industry. This principle focuses on system integration instead of transistor density in order to achieve faster, thinner, and smarter electronic devices at a low cost. A core area of electronics packaging is interconnection technology, which enables ultra-miniaturization and high functional density. Solder bump technology is one of the original, and most common interconnection methods for flip chips. With growing demand for finer pitch and higher number of I/Os, solder bumps have been forced to smaller dimensions and therefore, are subjected to higher current densities. However, the technology is now reaching its fundamental limitations in terms of pitch, processability, and current-handling due to electromigration. Electromigration in solder bumps is one of the major causes of device failures. It is accelerated by many factors, one of which is current crowding. Current crowding is the non-uniform distribution of current at the interface of the solder bump and under-bump metallurgy, resulting in an increase in local current density and temperature. These factors, along with the formation of intermetallic compounds, can lead to voiding and ultimately failure. Electromigration in solder bumps has prevented pitch-scaling below 180-210 microns, producing a shift in the packaging industry to other interconnection approaches, specifically copper pillars with solder. This research aims to explore the electromigration resistance of an adhesive-based copper-to-copper (Cu-Cu) interconnection method without solder, which is thermo-compression bonded at a low temperature of 180C. While solder bumps are more susceptible to electromigration, Cu is capable of handling two orders of magnitude higher current density. This makes it an ideal candidate for next generation flip chip interconnections. Using finite element analysis, the current crowding and joule heating effects were evaluated for a 30 micron diameter Cu-Cu interconnection in comparison with two existing flip chip interconnection techniques, Cu pillar with solder and Pb-free solder. A test vehicle (TV) was fabricated for experimental analysis with 760 bumps arranged in an area-array format with a bump diameter of 30 micron. Thermo-mechanical reliability of the test vehicle was validated under thermal cycling from -55C to 125C. The Cu-Cu interconnections were then subjected to high current and temperature stress from 1E4 to 1E6 amps per square centimeter at a temperature of 130C. The results establish the high thermo-mechanical reliability and high electromigration resistance of the proposed Cu-Cu interconnection technology.