Electromigration Experiment Research Articles

Applications of Ni and Ag metallizations at the solder/Cu interfaces in advanced high-power automobile interconnects: An electromigration study

An advanced Pb-free solder interconnect, Cu line/Cu pillar/Ni/Sn1.8Ag/Ag/Cu lead frame/Sn3Ag0.5Cu/Au/Ni(P)/Cu trace, using the flip-chip quad flat no‑lead (FCQFN) packaging technology was designed for high-power automobile applications. An electromigration study revealing the Ni and Ag metallization effects on the interfacial reaction was investigated. Current stressing was conducted at a 2 A direct current at 160 °C until the defined 200 %-electrical-resistance-rise failure occurrence. Electromigration induced a two-stage electrical resistance increase mode in the advanced automobile test vehicle with a mechanism transition time of 585 h, as evidenced by the in situ resistance monitoring. Phase transformation from the constituent phases into the predominant (Cu,Ni)6Sn5 and Cu3Sn intermetallic compounds (IMCs) at the interfaces was proposed to explain the rapid resistance rise in the early-stage electromigration. Void formation and coalescence due to the Kirkendall effect and volume shrinkage during the phase transformation were proposed to explain the following steady resistance increase after the critical current stressing time. Furthermore, the phase transformation and growth of the IMCs behaved divergently at different interfaces and electron flow directions. The rapid phase transformation occurred in the downstream Sn1.8Ag solder interconnect due to the accelerated Ni metallization dissolution and formed a complete IMC joint. The anisotropic diffusion driven by the competitive or synergetic effect between the electron wind force and chemical potential gradient was proposed to explain the asymmetric microstructures after the electromigration experiment. The application of Ag metallization benefited the flip-chip soldering process, and the layer-type Ag3Sn IMC served as a diffusion barrier against Sn/Cu interdiffusion at the solder/Cu lead frame interface during the solid-state electromigration. In contrast, the Ni metallization was designed as a diffusion barrier to hinder interdiffusion and the undesirable Cu3Sn growth at the solder/Cu interface. The findings presented in this study could benefit the solder interconnect design with an appropriate metallization combination against undesirable electromigration failure.

Effects of temperature and grain size on diffusivity of aluminium: electromigration experiment and molecular dynamic simulation

Understanding the atomic diffusion features in metallic material is significant to explain the diffusion-controlled physical processes. In this paper, using electromigration experiments and molecular dynamic (MD) simulations, we investigate the effects of grain size and temperature on the self-diffusion of polycrystalline aluminium (Al). The mass transport due to electromigration are accelerated by increasing temperature and decreasing grain size. Magnitudes of effective diffusivity (D eff) and grain boundary diffusivity (D GBs) are experimentally determined, in which the D eff changes as a function of grain size and temperature, but D GBs is independent of the grain size, only affected by the temperature. Moreover, MD simulations of atomic diffusion in polycrystalline Al demonstrate those observations from experiments. Based on MD results, the Arrhenius equation of D GBs and empirical formula of the thickness of grain boundaries at various temperatures are obtained. In total, D eff and D GBs obtained in the present study agree with literature results, and a comprehensive result of diffusivities related to the grain size is presented.

Effect of Novel SAC-Bi Solder Joints on Electromigration Reliability for Wafer Level Chip Scale Packages

Abstract The Sn-4Ag-0.5Cu base solder adding 3 wt.% bismuth (SAC-3Bi) solder has better drop and thermal cycling performance than Sn-4Ag-0.5Cu solder due the excellent mechanical properties in previous study. Therefore, SAC-3Bi has applied in solder joints of wafer level chip scale packaging (WLCSP) in recent year. In this study, SAC-3Bi solder was used on WLCSP device to evaluate electromigration reliability. A specially designed test vehicle for SAC-3Bi solder balls with 200μm diameter Ti/Cu/Cu UBM have been investigated experimentally. The electromigration behavior of solder balls was investigated in terms of the electrical resistance change, and the mechanism was also explored by the microstructure evolutions. The electromigration experiment of the SAC-3Bi solder balls was conducted continuously at 1.2 A to 1.6 A under 170 °C to 185 °C condition. The resistance of solder balls was monitored according to Kelvin structure. An EM prediction model of SAC-3Bi solder balls was built based on Black-type electromigration time to failure equation, followed the JEDEC with five test conditions. The activation energy of SAC-3Bi solder is 0.80 eV ± 0.02 eV, coinciding the activation energy of SAC based solder alloy (0.72 eV ~ 0.89 eV). The microstructure of the solder balls was investigated using a scanning electron microscope (SEM) accompanying energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) for the failure behaviors and phase characterization. There were intermetallic compounds of Cu3Sn and Cu6Sn5 at the interface of copper and solder. We observed two failure modes of SAC-3Bi solder balls during current stressing, the fracture occurred at the UBM/solder ball interface and Cu redistribution layer (RDL), respectively. The failure mechanisms have been investigated in detail from microstructure analysis.

A comprehensive study of electromigration in pure Sn: Effects on crystallinity, microstructure, and electrical property

The literature has accumulated plenty of interesting findings of electromigration-induced phenomena in pure Sn. Most of the researches revealed the thermodynamically steady states of materials under electromigration. We presented a comprehensive study of electromigration in pure Sn at 5.5–7.5 × 103 A/cm2 for 5.5 h revealing the effects on crystallinity, microstructure, and electrical property. The present work provided a divergent explanation about the electrical property variation under electromigration by introducing the crystallinity change aspect, as evidenced by the in situ synchrotron X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) investigations. The in situ XRD analysis showed an integrated intensity decline of diffraction peaks (up to a 90% reduction rate) and the buildup of lattice strain (up to 0.68% beyond the yield point) within the pure Sn strip, revealing a crystallinity degradation phenomenon under electromigration. The atomic-scale lattice appearance showed direct evidence of dislocation production under electromigration as a result of the plastic deformation. The introduction of dislocations formed sub-lattices with various crystal orientations that were responsible for the integrated intensity decline. The increase in electrical resistance after the electromigration experiment corresponded to the consequences of the observed lattice disruption and lattice strain accumulation phenomena. The thermal benchmark experiments evidenced the predominant athermal electromigration effect, rather than the thermal one, on the crystallinity and electrical resistance responses to electromigration.

Athermal and thermal coupling electromigration effects on the microstructure and failure mechanism in advanced fine-pitch Cu interconnects under extremely high current density

The fine-pitch and small line width Cu redistribution lines (RDLs) serve as the key factor in achieving high-density advanced fan-out packaging products. However, the dimension scaling of the next generation Cu RDL will cause more severe electromigration damage to the narrow Cu interconnects. The present study reported the microstructure variations and the failure mechanisms involved in an advanced 2μm/2 μm line/spacing Cu RDL interconnect under the electromigration experiment at an extremely high current density, 6 × 106 A/cm2, to investigate the electrical-thermal coupling interactions. The athermal electromigration effect induced rapid mass transport of Cu atoms, giving rise to RDL width reduction (migrated Cu depletion) and width growth (migrated Cu accumulation) phenomena. Meanwhile, the large amount of Joule heat generation induced the high-temperature material degradation of the polyimide (PI) dielectric layer and the delamination across the RDL/dielectric interface. The thermal stress resulting from the coefficient of thermal expansion mismatch further induced RDL displacement within the degraded PI dielectric layer near the current-stressed Cu RDL. The results suggest the predominant thermal electromigration effects on the PI degradation and RDL displacement.

A Comprehensive Study of Electromigration in Pure Sn: Effects on Crystallinity, Microstructure, and Electrical Property

The literature has accumulated plenty of interesting findings of electromigration-induced phenomena in pure Sn. Most of the researches revealed the thermodynamically steady states of materials under electromigration. We presented a comprehensive study of electromigration in pure Sn at 5.5-7.5 × 103 A/cm2 for 5.5 h revealing the effects on crystallinity, microstructure, and electrical property. The present work provided a divergent explanation about the electrical property variation under electromigration by introducing the crystallinity change aspect, as evidenced by the in situ synchrotron X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) investigations. The in situ XRD analysis showed an integrated intensity decline of diffraction peaks (up to a 90% reduction rate) and the buildup of lattice strain (up to 0.68% beyond the yield point) within the Sn matrix, revealing a crystallinity degradation phenomenon under electromigration. The atomic-scale lattice appearance showed direct evidence of dislocation production under electromigration as a result of the plastic deformation. The introduction of dislocations formed sub-lattices with various crystal orientations that were responsible for the integrated intensity decline. The increase in electrical resistance after the electromigration experiment corresponded to the consequences of the observed lattice disruption and lattice strain accumulation phenomena. The thermal benchmark experiments evidenced the predominant athermal electromigration effect, rather than the thermal one, on the crystallinity and electrical resistance responses to electromigration.

The study on reliability of Ni-coated graphene doped SAC305 lead-free composite solders under high current-density stressing

Purpose The purpose of this paper is to investigate the effect of nickel-plated graphene (Ni-GNS) on the microstructure and mechanical properties of 96.5Sn3Ag0.5Cu (SAC305) lead-free solder joints before and after an electro-migration (EM) experiment. Design/methodology/approach In this paper, SAC305 solder alloy doped with 0.1 Wt.% Ni-GNS was prepared via the powder metallurgy method. A U-shaped sample structure was also designed and prepared to conduct an EM experiment. The EM experiment was carried out with a current density of 1.5 × 104 A/cm2. The microstructural and mechanical evolutions of both solder joints under EM stressing were comparatively studied using SEM and nanoindentation. Findings The experimental results showed that for the SAC305 solder, the interfacial intermetallic compounds (IMC) formulated a protrusion with an average height of 0.42 µm at the anode after 360 h of EM stressing; however, despite this, the surface of the composite solder joint was relatively smooth. During the stressing period, the interfacial IMC on the anode side of the plain SAC305 solder showed a continuous increasing trend, while the IMC at the cathode presented a decreasing trend for its thickness as the stressing time increased; after 360 h of stressing, some cracks and voids had formed on the cathode side. For the SAC305/ Ni-GNS composite solder, a continuous increase in the thickness of the interfacial IMC was found on both the anode and cathode side; the growth rate of the interfacial IMC at the anode was higher than that at the cathode. The nanoindentation results showed that the hardness of the SAC305 solder joint presented a gradient distribution after EM stressing, while the hardness data showed a relatively homogeneous distribution in the SAC305/ Ni-GNS solder joint. Originality/value The experimental results showed that the Ni-GNS reinforcement could effectively mitigate the EM behavior in solder joints under high current stressing. Specifically, the Ni particles that plated the graphene sheets can work as a fixing agent to suppress the diffusion and migration of Sn and Cu atoms by forming Sn-Cu-Ni IMC. In addition, the nanoidentation results also indicated that the addition of the Ni-GNS reinforcement was very helpful in maintaining the mechanical stability of the solder joint. These findings have provided a theoretical and experimental basis for the practical application of this novel composite solder with high current densities.

Interface balance laws, phase growth and nucleation conditions for multiphase solids with inhomogeneous surface stress

In this paper, the thermodynamic configurational force associated with a moving interface is used to derive the conditions for phase growth and nucleation in bodies with multiple diffusing species and arbitrary surface stress at the phase interface. First, the mass, momentum and energy balances are derived on the evolving phase interface. The thermodynamic conditions that result from free energy inequality at the interface are derived leading to the analytical form of the configurational force for bodies subject to mechanical loads, heat and multiple diffusing species. The derived second law condition naturally extends the Eshelby energy–momentum tensor to include species diffusion terms. The above second law restriction is then used to derive the condition for the growth of new phases in a body undergoing finite deformation subject to inhomogeneous as well as anisotropic interface stress, and multiple diffusing species. The growth conditions are derived in both current and reference configurations. The statistical temperature-dependent growth velocity is next derived using the Boltzmann distribution. The derived finite deformation form of growth requirement is simplified to obtain the small deformation diffusive void growth condition. Next, a general, finite deformation, arbitrary surface stress form of phase nucleation condition is derived by considering uncertainty in growth of a small nucleus. The probability of nucleation is shown to naturally depend on a theoretical estimate of critical volumetric energy density, which is directly related to the surface stress. The classical nucleation theory is shown to result from a simplified special case of the general criterion. As an application of the developed theory, the classical Blech electromigration experiment is simulated to estimate the critical energy density corresponding to the onset of electromigration voids at Al–TiN interface.

Solute transport solved with the Nernst-Planck equation for concrete pores with ‘free’ water and a double layer

For calculating ion transport in electro-migration experiments in concrete, the pore space is divided between free pore water and a double layer that compensates the surface charge of minerals lining the pore. The concentrations of counter-ions and co-ions in the double layer are averaged in a Donnan layer, which allows to solve the flux equation of the individual ions. A numerical model is developed for transient states, with the harmonic mean of the chemical activity coefficients and the physical parameters to adapt cell sizes in zones with high concentration gradients. Diffusion coefficients of the ions are corrected for temperature and ionic strength with an empirical equation with three parameters optimized on published transport numbers and specific conductances. The current and the Cl– breakthrough is calculated in an electro-migration experiment with a concrete sample. The surface charge is positive in the experiment, enhancing and reducing the diffusivity of anions and cations.

Effect of nanosilica on chloride permeability in cement mortar

An accelerated electro-migration experiment is carried out to examine the chloride penetration of cement mortar admixed with two types of nanosilica and compared with the natural diffusion measurement. The chloride ion concentration in the destination solution is measured by ultra violet–visible spectrophotometer at different time intervals. In the natural condition, the penetration of chloride ions through a 7-mm thick mortar disc takes approximately 24 h, whereas nanosilica incorporated mortar resists the ingress of chloride ions up to around 72 h. Furthermore, in the accelerated condition, the diffusion coefficient is reduced from 6·8 × 10−11 m2/s to 4·7 × 10−11 m2/s with 3% addition of nanosilica (type I) by weight of cement. The microstructure studies through scanning electron microscopy reveal that the nanosilica refines the pore structure of the mortar and leads to a denser microstructure owing to the formation of additional calcium-silicate-hydrate gel. The pozzolanic reactivity is also determined using a thermogravimetric technique wherein nanosilica and silica fume incorporated cement mortar reduces the calcium hydroxide content by 67 and 30%, respectively. These studies demonstrate that nanosilica efficiently reduces the chloride permeability of cement mortar owing to higher pozzolanic activity.

Electromigration of Tritium in V-4Cr-4Ti

In order to determine effective charge Z* of tritium in V-4Cr-4Ti alloy, an electromigration experiment has been carried out at temperatures of 330, 373 and 423 K in an electric field strength of about 5 V/m. Tritium concentration profile in a steady state was measured with an imaging plate (IP) technique. According to a simple electromigration theory, the effective charge Z* of tritium was determined from the tritium profiles. Z* obtained increases from 0.7 to 0.95 with increasing temperature. Comparison with the value of Z* for pure V is made and influences of alloying elements on tritium electromigration are discussed.

Statistical analysis of electromigration lifetimes and void evolution

Electromigration failure statistics and the origin of the log-normal standard deviation for copper interconnects were investigated by analyzing the statistics of electromigration lifetime and void size distributions at various stages during testing. Experiments were performed on 0.18μm wide Cu interconnects with tests terminated after certain amounts of resistance increase, or after a specified test time. The lifetime and void size distributions were found to follow log-normal distribution functions. The sigma values of these distributions decrease with increasing test time. The statistics of resistance-based void size distributions can be simulated by considering geometrical variations of the void shape. In contrast, the characteristics of time-based void size distributions require consideration of kinetic aspects of the electromigration process. The sigma values of lifetime distributions can be adequately simulated by combining the statistics of both types of void size distributions. Thus, a statistical correlation between electromigration lifetimes and void evolution was established. Using simulation to fit the experimental data, the parameters influencing the electromigration lifetime statistics were identified as variations in void sizes, geometrical and experimental factors of the electromigration experiment, and kinetic aspects of the mass transport process, such as differences in interface diffusivity between the lines. The latter is the result of variations in the copper microstructure at the cathode ends of the interconnects.

Crystal plasticity in Cu damascene interconnect lines undergoing electromigration as revealed by synchrotron x-ray microdiffraction

Plastic deformation was observed in damascene Cu interconnect test structures during an in situ electromigration experiment and before the onset of visible microstructural damage (voiding, hillock formation). We show here, using a synchrotron technique of white beam x-ray microdiffraction, that the extent of this electromigration-induced plasticity is dependent on the linewidth. In wide lines, plastic deformation manifests itself as grain bending and the formation of subgrain structures, while only grain rotation is observed in the narrower lines. The deformation geometry leads us to conclude that dislocations introduced by plastic flow lie predominantly in the direction of electron flow and may provide additional easy paths for the transport of point defects. Since these findings occur long before any observable voids or hillocks are formed, they may have direct bearing on the final failure stages of electromigration.

Analysis of electromigration statistics for Cu interconnects

Electromigration failure statistics and the origin of the lognormal standard deviation for copper interconnects were investigated by analyzing the statistics of electromigration lifetimes and void size distributions at various stages during testing. A statistical correlation between electromigration lifetimes and void evolution was established. Using simulation to fit the experimental data, the parameters influencing the electromigration lifetime statistics were identified as variations in void sizes, geometrical and experimental factors of the electromigration experiment, and kinetic aspects of the mass transport process, such as differences in the interface diffusivity between the lines.

Electromigration-Induced Plastic Deformation in Cu Damascene Interconnect Lines as Revealed by Synchrotron X-Ray Microdiffraction

AbstractThe Scanning X-Ray Submicron Diffraction (μ-SXRD) technique using focused synchrotron radiation white beam developed in the Beamline 7.3.3 at the ALS Berkeley Lab has been used to study the microstructural evolution at granular level of Cu polycrystalline lines during electromigration. Plastic deformation was observed in damascene Cu interconnect test structures during this in situ electromigration experiment and before the onset of visible microstructural damage (voiding, hillock formation). We show here that the extent of this electromigration-induced plasticity is dependent on the line width. In wide lines, plastic deformation manifests itself as grain bending and the formation of subgrain structures, while only grain rotation is observed in the narrower lines. The analysis of the Laue reflections allow for the determination of the geometrically necessary dislocation density in individual grains as well as for the misorientation angles between small angle boundaries generated by polygonization. The deformation geometry leads us to conclude that dislocations introduced by plastic flow lie predominantly in the direction of electron flow and may provide additional easy paths for the transport of point defects. Furthermore, we observe that the rotation axis of this plastic deformation coincides with one of the <112> line directions of the known slip systems for FCC crystal, and that it is always very close (within a few degrees) to the direction of the electron flow. This finding suggests a correlation of the proximity of certain <112> line directions to the direction of electron flow with the occurrence of plastic behavior. One important practical implication of this particular finding is that the grain texture of the line might thus play an important role in giving higher resistance towards early plastic response of the Cu line upon the electromigration loading.

From discrete hopping to continuum modeling on vicinal surfaces with applications to Si(001) electromigration

Coarse-grained modeling of dynamics on vicinal surfaces concentrates on the diffusion of adatoms on terraces with boundary conditions at sharp steps, as first studied by Burton, Cabrera, and Frank (BCF). Recent electromigration experiments on vicinal Si surfaces suggest the need for more general boundary conditions in a BCF approach. We study a discrete one-dimensional hopping model that takes into account asymmetry in the hopping rates in the region around a step and the finite probability of incorporation into the solid at the step site. By expanding the continuous concentration field in a Taylor series evaluated at discrete sites near the step, we relate the kinetic coefficients and permeability rate in general sharp step models to the physically suggestive parameters of the hopping models. In particular we find that both the kinetic coefficients and permeability rate can be negative when diffusion is faster near the step than on terraces. These ideas are used to provide an understanding of recent electromigration experiment on Si(001) surfaces where step bunching is induced by an electric field directed at various angles to the steps.

Quantitative characterization of electromigration-induced plastic deformation in Al(0.5wt%Cu) interconnect

Electromigration-induced failure in metal interconnect constitutes a major reliability problem in the semiconductor industry. Recently, experimental techniques capable of probing grain orientation and stress with a spatial resolution compatible with the dimensions of the lines have emerged. White beam X-ray microdiffraction is particularly well suited to the in situ study of electromigration. The technique was used to probe microstructure in interconnects and recently unambiguously unveiled the plastic nature of the deformation induced by mass transport during electromigration in Al(Cu) interconnect lines even before macroscopic damage. The aim of the present research is to understand the complex dislocation structure arising from electromigration-induced plastic deformation by simulating the shape of the reflections and comparing them with the shape observed in the experimental data. We provide a first quantitative analysis of the dislocation structure generated in individual micron-sized Al grains during an in situ electromigration experiment. Custom software allows us to determine the orientation of the predominant dislocation network in each sample subgrain.

Development of x-ray topographic microscopy for the study of electromigration in interconnects

We have performed x-ray topography experiments in a new configuration using a Fresnel zone plate to form a magnified image of the sample intensity distribution on the detector plane. The main difference from the x-ray topography techniques where no optics is used between the sample and the detector, is the absence of free space propagation effects in the image and the possibility to form a magnified image. Other advantages include large working distance around the sample and low background radiation on the detector. As a first application, we recorded x-ray topographs of an Al interconnect structure as a function of time in an in situ electromigration experiment. The contrast in the images is explained in terms of lattice deformations caused by the stress in the metal films. The results show the evolution of stress due to the electromigration process. The technique should make it possible to obtain best possible spatial resolution allowed by the sample in x-ray topography.

Quantitative analysis of dislocation arrangements induced by electromigration in a passivated Al (0.5 wt % Cu) interconnect

Electromigration during accelerated testing can induce plastic deformation in apparently undamaged Al interconnect lines as recently revealed by white beam scanning x-ray microdiffraction. In the present article, we provide a first quantitative analysis of the dislocation structure generated in individual micron-sized Al grains during an in situ electromigration experiment. Laue reflections from individual interconnect grains show pronounced streaking during the early stages of electromigration. We demonstrate that the evolution of the dislocation structure during electromigration is highly inhomogeneous and results in the formation of unpaired randomly distributed dislocations as well as geometrically necessary dislocation boundaries. Approximately half of all unpaired dislocations are grouped within the walls. The misorientation created by each boundary and density of unpaired individual dislocations is determined. The origin of the observed plastic deformation is considered in view of the constraints for dislocation arrangements under the applied electric field during electromigration.

Electromigration-induced plastic deformation in passivated metal lines

We have used scanning white beam x-ray microdiffraction to study microstructural evolution during an in situ electromigration experiment on a passivated Al(Cu) test line. The data show plastic deformation and grain rotations occurring under the influence of electromigration, seen as broadening, movement, and splitting of reflections diffracted from individual metal grains. We believe this deformation is due to localized shear stresses that arise due to the inhomogeneous transfer of metal along the line. Deviatoric stress measurements show changes in the components of stress within the line, including relaxation of stress when current is removed.