Analysis of bump resistance and electrical distribution of ultra-fine-pitch microbumps

In this decade, the Moore's law does not fit the trend of technological improvement any more. In order to keep the increasing rate of device density and I/O numbers, the size of devices shrink rapidly. In this kind of condition, the stack of integrated circuit (IC), which is also called three-dimensional integrated circuits (3-D ICs), is a preferable solution of the next-generation products. There are several techniques to conduct the power and signals between the chips in the stack and the ultra-fine pitch micro-bump is the most potential one. The soldering process used in micro-bump is a low temperature process which does not damage the devices in ICs. However, the line-to-bump structure causes the serious current crowding effect near the entrance point of current flow in bump and also makes the bump resistance much larger than expected. In this study, a Kelvin bump structure was fabricated to precisely measure the "REAL" bump resistance of micro-bump. From the past studies, resistance of a flip-chip solder bump is only several milli-Ohms. Unlike the results from flip-chip solder bumps, the bump resistance obtained in this study is about a hundred milli-Ohms, which is larger than flip-chip solder bump by more than ten times. The unexpected high bump resistance will bring significant RC delay between chips and reduce the performance of electric products. This phenomenon totally disobeys the design purpose of 3-D ICs which is to reduce the high resistance caused by the long distance between chips. As a result, investigating a method to reduce the resistance of micro-bump is an important issue in the application of micro-bump and development of 3-D ICs. Moreover, the bump resistances measured from different angles (0°, 60°, 120°, and 180°) were also obtained by our Kelvin bump structures. A finite elements analysis (FEA) is also applied in this study to observe the current density distribution in detail. The results of FEA fit the measurement ones quite well. The FEA shows that the current density distributes uniformly in solder bumps. However, this does not mean that the current crowding effect does not happen anymore. It still takes place in under-bump metallization (UBM) instead. The crowding ratio, a factor to indicate the level of current crowding, is larger than 30 in UBM. This means that the electromigration (EM) failure may occur in UBM, not in solder, so the EM failure will still a serious reliability issue in micro-bumps.