Intermetallic Compound and Void Kinetics Extraction From Resistance Evolution in Copper Pillars During Electromigration Stress Tests

Abstract

Die-to-wafer interconnections such as copper pillars play a vital role in order to enable 3-D integration. This interconnection type allows increasing the density of interconnects but the occurrence of defects, especially intermetallic compounds (IMC) and Kirkendall voids, may reduce the lifetime at elevated operating conditions. This paper investigates the physical degradation mechanisms in copper pillars and micro-bumps, caused by IMC and void formation during stress tests such as electromigration (EM) and high temperature storage. The resistance evolution of the tested interconnections motivated the derivation of a novel analytical model to separate the resistance increases caused by IMC growth and void formation. This allows the real time monitoring of changes in the kinetics, which gives a better understanding of the underlying physics and of the failure mechanisms. In order to validate the model, an additional test series on copper pillars under EM stress with varying conditions is conducted and the model is applied on the monitored resistance evolution. The different stress conditions allowed the extraction of the IMC formation activation energy, which is compared against parameter extraction using classical mean time to failure analysis as well as material parameters of IMC growth that are already reported in the literature.