Mechanisms of Electromigration-induced Intermetallic Dissolution and Metallization Consumption in Solder Interconnects

Electromigration-induced degradation in solder joints can be classified into two types, pancake-type void formation and metallization consumption, which are driven by two different diffusion mechanisms. The void formation is caused by the Sn electromigration and the combined effect of tensile stress and vacancy supersaturation, while the metallization consumption is caused by the electromigration of metallization atoms (such as Cu or Ni) in the solder from cathode to anode. The rapid development of high-density interconnects raises the electron current density through the solder joints up to 104 A/cm2, resulting in more severe conditions for solder joints reliability. The subjects of the dissertation include three parts. The first part of the study investigates the temperature effect on the electromigration-induced degradation mechanism. Both kinetic analysis and experimental observations demonstrate that temperature is one of the important factors affecting the electromigration behavior. At high temperature, because the magnitudes of Sn and Cu electromigration fluxes are within the same order of magnitude, both void formation and metallization consumption operate simultaneously. At low temperature, however, the Cu electromigration dominates over Sn, so the metallization layer is excessively consumed before the void starts to form and propagate. It implicates that the accelerated electromigration tests at high temperature may not provide a complete picture of electromigration-induced failure mechanism in solder joints. Finally, the kinetic analysis is extended to other fast diffusers considering both temperature and orientation effects. From the first part of the study, it shows that electromigration-induced metallization dissolution or consumption, instead of void formation, can be the dominant degradation under certain conditions. However, current understanding of the electromigration-induced dissolution is not capable of elucidating the microscopic mechanisms and the origin of serrated cathode consumption observed by several research groups recently. In the second part of the study, a purposely designed experiment using Cu/Sn/Cu line-type solder interconnects is implemented in order to reveal the complete microstructural evolution of the consumption process at the cathode. The result shows that a pronounced and unstable grain boundary grooving between Cu6Sn5 grains occurs before the serrated cathode starts to develop. The morphological of the intermetallic compound changes from a layered structure to an unstable scalloped structure. The grooving is driven by the preferential dissolution at the Cu6Sn5 grain boundaries, a process similar to the formation of scallop-type Cu6Sn5 between molten solder and Cu. Because the diffusion and reaction are more significant at the valley of the grooves, the local consumption rate of Cu metallization increases. This non-uniform consumption of Cu eventually leads to a serrated cathode interface. The result indicates that the development of the serrated cathode interface may be very sensitive to the initial microstructure, such as the position and distribution of the Cu6Sn5 grain boundaries. Despite the successful discovery of the microscopic mechanism of current-induced dissolution, its accompanied phenomenon, such as the backfill of Sn atoms, is not completely understood. The only thing that is ascertained is that it may be driven by the electromigration-induced stress field. The stress field has been known to be intimately related to defect kinetics, such as dislocation climb and the microscopic mechanism of diffusion creep. Therefore, in the third part of the study, a microscopic phase field dislocation climb model is proposed in order to uncover the coupling effects of the defect stress field and vacancy diffusion. The capability of the proposed model is further validated by several examples and demonstrations, including the classical Kirkendall effect and interdiffusion. The most promising feature of the model is that the constraint of zero vacancy chemical potential is released, so it is capable of being applied in the simulations with non-equilibrium vacancy concentration. Fast numerical methods to solve the problems of inhomogeneous elasticity and electric conduction are also proposed and validated in this part of the study. It is anticipated that in the future, the proposed model can be applied to the electromigration in solder interconnects, as long as the vacancy thermodynamic databases of the solder systems are developed.Acknowledgements i 摘要 iii Abstract v Contents viii List of Figures x List of Tables xx Chapter 1 Introduction 1 Chapter 2 Literature Review 5 2-1 Electromigration 5 2-2 Electromigration in Metallic Interconnects 8 2-3 Electromigration in Solder Joints 12 2-4 Phase Field Method 23 Chapter 3 Temperature Effect on the Electromigration Behavior of Solder Joints 28 3-1 Motivation 28 3-2 Kinetic Analysis 29 3-3 Experimental 35 3-4 Results and Discussion 37 3-5 Generalization 42 3-6 Summary 52 Chapter 4 Mechanism of Serrated Cathode Dissolution in Cu/Sn/Cu Interconnects 54 4-1 Motivation 54 4-2 Experimental 56 4-3 Results 59 4-4 Discussion 68 4-5 Uncertainties and Limitations 76 4-6 Summary 77 Chapter 5 Development of a Microscopic Phase Field Model for Electromigration with Non-ideal Defect Sources/Sinks 79 5-1 Motivation 79 5-2 Introduction to Microelasticity 81 5-3 Inhomogeneous Electric Conduction 88 5-4 Phase Field Dislocation Climb Model 93 5-5 Limitations and Numerical Difficulties 117 5-6 Summary 118 Chapter 6 Conclusions 120 6-1 Conclusions 120 6-2 Directions for Future Research 122 References 126 Curriculum Vitae 13