Computer simulation of electromigration in microelectronics interconnect

Electromigration (EM) is a phenomenon that occurs in metal conductor carrying high density electric current. EM causes voids and hillocks that may lead to open or short circuits in electronic devices. Avoiding these failures therefore is a major challenge in semiconductor device and packaging design and manufacturing, and it will become an even greater challenge for the semiconductor assembly and packaging industry as electronics components and interconnects get smaller and smaller. According to the assembly and packaging section of the International Technology Roadmap for Semiconductor (ITRS) developed in 2007 and 2009 [1] [2], EM was a near term threat for the interconnecting part of semiconductor, devices and packaging methods such as flip chip, and Ball Grid Array (BGA). In the industry, EM-aware designs are mainly based on design rules that are derived from empirical laws which do not help understand complicated EM processes and therefore can’t be used to carry out accurate predictions for EM failures of sophisticated components in varied environmental conditions. In this work, novel numerical modelling methods of EM in micro-electronics devices have been developed and the methods have been used to analyse EM process in a lead free solder thin film, and to optimize the design of electronic components in order to reduce the risk of EM relative failure. EM is an atomic diffusion process that is driven by a high density electric current, but it is strongly affected by temperature and its gradient as well as stress distribution. In order to model EM accurately, the interacting electrical, thermal, and mechanical phenomena must all be solved simultaneously. In this work, a novel multi-physics modelling method has been proposed and developed to include all of the above mentioned physical phenomena using unstructured Finite Volume (FV) and Finite Element (FE) techniques. The methods have been implemented on the multi-physics software package PHYSICA. Comparing with existing methods, this fully coupled solution method is a significant improvement that will facilitate further development of electronics design and optimization tool as well as new research work that helps understand EM phenomenon. The developed models can be used to simulate the whole process of EM, predict voids initiation lifetime of electronics products or test specimens. In today’s electronics manufacturing, lead-free solder alloys are used as interconnect. As in copper or aluminium interconnect EM has become a threat to device reliability as current density increase in solder joints with diminishing sizes. In this work, computer simulation methods have been used to analyse the experimentally observed EM process in a thin film solder. The experiment was designed in such a way that effects of temperature and stress gradients can be avoided. The advantage of this experimental method is that the electric current effect is isolated which makes analysis and model validation easier. In this work, the predicted voids locations are consistent with experimental results. In this work, numerical examples are given to illustrate how interconnect designs can be made more EM failure resistant. The ultimate aim of the research is to understand EM and to develop techniques that predict EM accurately so that EM-aware designs can be made easier.