Electromigration Behavior Research Articles

Precisely regulating ion convection to counteract electro-migration for H+–OH- separation with enabling efficient electrochemical water softening

Conventional electrochemical precipitation softening often suffers from the low utilization of OH– for hardness precipitation on the cathode surface in traditional undivided electrolytic cell. In this study, an asymmetric cathode was designed to confine the occurrence of H2 evolution reaction at its backside in an undivided electrolytic cell, where the electrodes were placed aslant at an angle of 5° to facilitate the upward motion of H2 bubbles and the water flowing distribution network was used to inhibit the circumfluence of water. This specific cell configuration successfully decoupled the ion convection and electro-migration behaviors, and regulated the convection behaviors of H+ and OH– to counteract their electro-migration behaviors. Specially, theoretical calculations revealed that the OH– convection rate (Vw,b = 0.033 ∼ 0.043 cm s−1) under the combined effects of H2 gas bubbling and water flowing-through cathode at current densities of 1 ∼ 15 mA cm−2 was larger than its electro-migration rate (Ve = 0.0062 ∼ 0.019 cm s−1), which effectively promoted the transport of OH– away from the cathode backside into the bulk solution opposite to the direction toward the anode side. The H+ electro-migration toward the cathode could be also offset by the convection mediated by water flowing-through anode. Thus, the H+–OH– neutralization reaction was effectively inhibited, favoring the hardness precipitation. Under the optimum condition, the developed cell showed a Ca hardness removal efficiency of 80.8 ± 1.18 % and a current efficiency of up to 73 % at an energy consumption of 2.67 ± 0.13 kWh kg−1 CaCO3, which were superior to most electrolytic systems with or without membranes. Generally, this study provides a precisely engineered electrodes configuration for energy-efficient electrochemical water softening in circulating cooling water system.

Atomic-Scale Investigation of Electromigration Behavior in Cu-to-Cu Joints at High Current Density

Three-dimensional integrated circuit (3D IC) packaging offers significant advantages, such as enhanced computational efficiency through greater integration and shorter interconnection distances. The effective interconnection of layers, particularly through redistribution layer (RDL) technology, is crucial for the successful operation of 3D ICs. While copper remains the preferred material for interconnections, often in conjunction with through-silicon via (TSV) technology for chip-to-chip links, the shrinking dimensions of components present challenges. In particular, the reduction in size may lead to elevated current densities in individual joints, giving rise to issues like electromigration (EM) and potential failure. EM-induced atomic migration may lead to the formation of voids in the interconnection. The accumulation of voids to a critical point may result in an open circuit in the joints, significantly impacting the conductor's reliability. This work delves into the micro-scale aspects of these challenges, complementing previous macroscopic investigations. We aim to elucidate the behavioral mechanisms through atomic-scale observations, employing an in-situ high-resolution transmission electron microscope (in-situ HRTEM) to observe atomic-scale images of EM. The insights gained from this work not only contribute to the academic understanding of industry challenges but hold potential applications in real-world manufacturing processes. Fig. 1. (a) Schematic diagram of TEM sample preparation. (b) Cross-sectional HRTEM image of copper bonding. (c, d) TEM images showing voids expansion at bonding interface. Figure 1

The impact of titanium alloying on altering nanomechanical properties and grain structures of sputter-deposited cobalt for electromigration reliability enhancement

Cobalt (Co) has recently become a highlighted on-chip interconnection material for advanced metallization scheme of microelectronic circuits. Here, electromigration behaviors of sputter-deposited Co and Co(Ti; 0.9 at%) interconnection lines embedded in Si are comparatively studied. These lines were annealed (500 °C/30 min) prior to further accelerated electromigration testing under high current densities in the range of 3 × 107–9 × 108 A cm−2. A collection of the obtained electromigration failure lifetimes, current scaling factors and activation energies confirms a noticeable enhancement in the electromigration reliability of the Co lines by the added Ti. Results by scanning transmission electron microscopy (STEM) and nanoscratch testing reveal that 3–5 nm-sized Ti crystallites are uniformly dispersed in the Co-interconnect matrix, effectively inhibiting Co grain growth, promoting the formation of nanotwinned Co and elevating film’s mechanical properties for ensuring the electromigration reliability. With deteriorated hardness and adhesion energy, reduced planar defects and many big grains all spanning transversely the entire interconnect line, Co is vulnerable to electromigration failures, also giving accelerated thermally-induced interfacial diffusion and chemical reactions with the surrounding Si, as proven by depth-profiling secondary spectrometry and STEM analysis.

Electromigration Effects in Overcurrent PVC-Insulated Copper Wire: Failure and Deformation Impacts

Electromigration is a critical issue in materials science and electrical engineering, significantly impacting the reliability and efficiency of electrical systems. This study investigates the electromigration behavior of PVC-insulated copper wires under various overcurrent conditions, focusing on material degradation and electrical performance. Copper cables, identified as 046620.3 Eterna CU/PVC 1.5 mm2, were subjected to currents ranging from 0 to 110 A. The mean time to failure (MTTF) was calculated using Black’s equation, revealing a sharp decline in MTTF with increasing current density. Surface morphology analysis using SEM showed the formation of voids and hillocks at higher currents, indicating severe electromigration damage. XRF analysis demonstrated significant changes in the elemental composition, particularly a reduction in copper content and an increase in chlorine and other elements, suggesting degradation of the PVC insulation. FTIR spectroscopy revealed substantial chemical changes in the PVC material, especially under extreme overcurrent conditions, highlighting dehydrochlorination and carbonyl group formation. There is a clear relationship between overcurrent conditions and electromigration phenomena, as evidenced by the observed damage to surface morphology, changes in elemental composition, and alterations in the chemical structure of PVC. The mechanisms and causes of electromigration are explained comprehensively in this work, illustrating how increased overcurrent accelerates the electromigration process, leading to the formation of voids and hillocks in the copper conductor. This damage is accompanied by a significant reduction in copper content and an increase in chlorine levels, indicating the degradation of PVC insulation. FTIR spectra further confirmed these findings by showing chemical changes such as dehydrochlorination and carbonyl group formation under high current stress. The MTTF values reflect the severity of these impacts, with samples exposed to higher currents showing drastically reduced lifespans. For instance, samples subjected to 100 A and 110 A currents exhibited MTTF values of 0.2 minutes and 0.004 minutes, respectively.

Effect of ethanolamine as a steel corrosion inhibitor in realkalization of carbonated concrete

ABSTRACT This paper assessed the electromigration behavior of ethanolamine and its efficiency as a corrosion inhibitor of rebar during the realkalization. Cylindrical concrete specimen (diameter 45 mm, height 20 mm) with steel rebar (6 mm in diameter, 35 mm in length) were artificially carbonated and corroded in an accelerated manner with impressed anodic current. The specimens were realkalized in the electrolyte solution containing ethanolamine, while the steel rebars were connected to the cathodic current (200 mA·h, 1 A/m2). After 25 days of monitoring the repair process, experimental data revealed notable changes in electrochemical parameters. The corrosion potential increased from −627 mV to −236 mV, signifying a reduction in corrosion activity. The cathodic exchange current density decreased from 4.9 μA/cm2 to 2.3 μA/cm2, showing a decrease in the corrosion rate. Also, the pH value of the solution dropped to a comparatively lower level, the ethanolamine and potassium ion concentrations decreased from 22 to 30.54 g/L to 3.22 and 10.23 g/L, respectively. Moreover, nitrogen detection on the steel surface and varied levels of ethanolamine concentration at different depths in the concrete were also measured, both indicating a clear migration of inhibitor to the steel surface to form a passive film.

Electromigration behavior of Sn–3.0Ag–0.5Cu/Sn–37Pb hybrid solder joints with various mixed percentages for aerospace electronics

Electromigration behavior of Sn–3.0Ag–0.5Cu/Sn–37Pb hybrid solder joints with various mixed percentages for aerospace electronics

Solder-less Fine Pitch Copper-to-Copper Bonding

The industry has been looking to increase pitch scaling through various methods. One of these methods is an alternative to solid-state solder bonding. However, very few processes can compete with solder bonding’s fast speed, low cost, and flexibility. The approach with the biggest potential for connecting fine pitch interconnects in high-density semiconductor packaging is solder-less copper-to-copper direct bonding. There are many benefits to using copper-to-copper direct bonding, including the ability to achieve ultra-fine-pitch (bump pitch 10 μm), higher reliability (both thermo-mechanically and better electromigration behavior), low resistance, as well as the lack of concern for solder hierarchy for multiple reflow. This copper-to-copper direct bonding technology enables a variety of 2.5D/3D advanced packaging architectures and is also suitable for multi-die stacking. In this study, we report the investigation of a direct copper-to-copper bonding methodology and reliability of the bonded components. Our process differs from common hybrid bonding copper-to-copper techniques, in that it is designed for low volume highly complex products. This technology allows a die-to-die and die-to-wafer process using thermocompression bonding under a deoxidizing vapor flow for direct copper-to-copper bonding. The process does not require high temperature oven annealing and can be done in a normal atmospheric environment. The investigated bonding process is ideal for high power, high thermal, high density 3D stacking and high-speed digital communication applications. We demonstrated the bonding procedure using a test vehicle with 40 μm diameter pillars and 80-μm bump pitch. We investigated the effects of bonding force and evaluated joint reliability. Interconnect/bond strength was determined through destructive shear testing and cross-sectional analysis. A scanning electron microscope was used to inspect and examine the bonded joint quality. Finally, completed assemblies were thermal cycled to evaluate degradation of joint strength under induced stress, and resistance measurements were performed to evaluate the electrical performance of the bonded copper joint.

Electromigration of Power Devices Part 2: Power Device in Applications in Controlled Environment

In the power semiconductor industry, there is a continuous development toward a higher current capability of devices while device’s dimensions shrink. Although higher current is preferred, the reliability risk of the electromigration (EM) remains challenging. In the present study, a special EM test vehicle is used to investigate the pure EM behavior of power device, following our previous work. The selection of the test condition is discussed in detail, which is defined based on the temperature and current profiles of the device in the actual application, focusing on the applications of the power devices in a controlled environment, i.e., data centers or telecommunication servers. The current and temperature profiles of the device are discussed based on the device’s maximum power dissipation and its respective thermal design. The results show that the different combinations of currents and temperatures can lead to different ratios of acceleration factor between the temperature and the current. Consequently, an inappropriate selection of the testing condition can result in the over-domination of temperature over the current, leading to wrong conclusions. The effects of the PCB surface finish (both Ni and Sn plating) as well as different electron flow directions on the EM were studied. It is concluded that the maximum current that can be applied to a power device in the application within a controlled environment is rather limited by the cooling capability of the system than the EM induced device failure.

Improving Electromigration Resistance of Fine‐Pitch Redistributed Layer Using Graphene‐Doped Twinned Copper Composites

The electromigration (EM) behavior of graphene‐doped twinned copper composite (GC–Cu) and nanotwinned copper (NT–Cu) redistributed layers (RDLs) are investigated. Firstly, the GC–Cu has longer lifetimes under various test conditions. For example, the GC–Cu RDL took 291 h to reach a 20% resistance increase, while the NT–Cu RDL took 117 h under the same condition of 120 °C and 106 A cm−2. Secondly, thinner oxidation layer and slighter voiding phenomena are detected on GC–Cu RDLs. When the line width and spacing (L/S) of RDL decreased from 20/20 μm to 10/10 μm, new failure modes, i.e., fracture of oxide layer, separation between the oxide layer and copper matrix are also detected, demonstrating the increasing thermal mismatch. Combined with the finite element analysis (FEA), there is simultaneous current aggregation and stress concentration around the voids, which should further cause the degradation of RDL. Above all, this study gives insights into the material design in the fine‐pitch RDL structure.

Characterization of electromigration-induced short-range stress development in Al(0.25 at. % Cu) conductor line

Scanning x-ray microbeam topography and fluorescence experiments were conducted in situ to study the electromigration behavior of a 0.5 μm thick, 10 μm wide, and 200 μm long Al(0.25 at. % Cu) conductor line with 1.5 μm-thick SiO2 passivation on a single crystal Si substrate. The strain sensitivity of x-ray topography measurement allowed detailed examination of the electromigration-induced stress distribution and evolution in the conductor line in response to the depletion of Cu solute early in the electromigration process. Upon electromigration at 0.4 MA/cm2 and 303 °C, a short-range stress gradient was quickly induced by Al migration in the Cu-depleted cathode region to counteract further Al flow. The stress gradient was fully developed during the 5.3 h incubation time, extending over the critical Blech length of about 66 μm from the cathode end. Plastic deformation then occurred at the downstream end of the Cu-depleted region. The preferential electromigration of Cu did not cause detectable stress change outside the Cu-depleted region, except for the significant stress development from the Al2Cu precipitation at the anode end which appeared to initiate the fracture in the passivation. Preliminary finite difference modeling was undertaken to simulate the experimental observations, from which important parameters dictating electromigration in Al(Cu) line were extracted: an apparent effective valence of −5.6 and −1.9 for Cu and Al in Al(Cu), respectively, and a critical Cu concentration of 0.16 at. % above which Al grain boundary diffusion is effectively blocked.

Study on Phase Electromigration and Segregation Behavior of Cu-Cored Sn-58Bi Solder Interconnects under Electric Current Stressing

Cu-cored solder interconnects have been demonstrated to increase the performance of interconnect structures, while the quantitative understanding of the effect of the Cu-cored structure on microstructure evolution and atomic migration in solder interconnects is still limited. In this work, the effect of the Cu-cored structure on phase migration and segregation behavior of Sn-58Bi solder interconnects under electric current stressing is quantitatively studied using a developed phase field model. Severe phase segregation and redistribution of Bi-rich phase are observed in the Cu-cored Sn-58Bi interconnects due to the more pronounced current crowding effect near the Cu core periphery. The average current density and temperature gradient in Sn-rich phase and Bi-rich phase decrease with an increase in the diameter of the Cu core. The temperature gradient caused by Joule heating is significantly reduced owing to the presence of the Cu core. Embedding of the Cu core in the solder matrix could weaken the directional diffusion flux of Bi atoms, so that the enrichment and segregation of the Bi phase towards the anode side are significantly reduced. Furthermore, the voltage across the solder interconnects is correspondingly changed due to the phase migration and redistribution.

Understanding electromigration failure behaviors of narrow cobalt lines and the mechanism of reliability enhancement for extremely dilute alloying of manganese oxide

Self-forming barriers for copper (Cu) interconnect metallization are well developed. However, the self-forming-barrier process and associated electromigration behaviors of narrow lines of cobalt (Co), a new interconnect wiring material for sub-10-nm semiconductor device nodes, are yet to be explored. In this study, we present an all-wet electroless trench-filling process to fabricate narrow (100 nm) lines of Co and MnO-added Co [Co(MnO)], then thermally annealed to some extent (400 °C/30 min) prior to electromigration testing. Empirical data obtained from accelerated electromigration testing, including failure lifetimes, current-density scaling factors, activation energies and physical failure profiles, consistently show that electromigration reliability of the Co lines is markedly enhanced by 0.1% added MnO. Transmission electron microscopy and focused ion beam imaging, along with dynamic adhesion strength data, reveal that the enhancement is related to annealing-induced segregation and dispersion of MnO particles at the interface with SiO2, ensuring adhesion of Co on SiO2, preventing interfacial interdiffusion and stuffing electromigration pathways. The adhesion strength, microstructures and Joule-heating data of Co and Co(MnO) are provided for the discussion of the differences in their electromigration performance and failure mechanism. Comments on electroless-plated Co as a new interconnect material are also given.

Electromigration-induced microstructure evolution and failure mechanism of sintered nano-Ag joint

Nanosized silver (nano-Ag) has been proposed as a flip-chip interconnection material owing to its low sintering temperature and impressive working performance. With ever-shrinking size of electronic devices, electromigration (EM) becomes an important reliability issue. However, the EM behavior of nano-Ag joints has rarely been investigated before. In this study, a flip-chip structured nano-Ag joint was examined at a current density of 1.0 × 104 A/cm2 at 150 °C and its microstructural change at different experimental times was analyzed. Severe delamination between the Cu substrate and sintered Ag occurred at the cathode, and the extent of delamination increased with increasing EM time. Fractured surfaces revealed the presence of a large number of whiskers on the opposite side of the cathode owing to stress release during the EM process. For comparison, a Sn-3Ag-0.5Cu solder joint was examined under identical experimental conditions. Although failure in both samples originated from the cathode, the failure mechanism was different. The root cause for the difference in the EM phenomenon observed in the two types of materials was determined in this study.

High temperature electromigration behavior of cobalt lines observed by in situ transmission electron microscopy

As industrial practices shift away from Cu as the initial back-end-of-line interconnect material due to size limitations, new candidate metals are being tested and characterized. Electromigration resistance is particularly important in ultra-narrow lines and an in situ study provides unique insight into the formation and progression of electromigration damage. This, in turn, helps to inform device design so that electromigration resistant circuits can be produced efficiently. In this work, the authors demonstrate an in situ transmission electron microscopy technique for electromigration analysis of Cu replacement metals in microelectronic interconnects. Using this method, candidate metal lines can be tested at high current densities, ∼5×106 A/cm2, at controllable temperatures over the range of 300–1000 °C. In this work, cobalt lines are tested in the range of the effective valence inversion temperature. The analysis examines void nucleation, growth, and migration as a function of temperature and line geometry. We find that there is a relative insensitivity of failure time to operating temperature, with samples tested between approximately 600 and 900 °C having roughly equivalent failure times. We ascribe this result to a combination of linewidth effects and a decrease in the magnitude of the effective valence approaching the inversion temperature. Failure mechanism is also not affected by temperature in this range, with the primary determining factor being the linewidth and corresponding availability of grain boundaries for diffusive mass transport. We also observe increased lifetimes of devices with uniform temperatures compared to those in which large thermal gradients exist.

Microstructure and Properties of Electromigration of Sn58Bi/Cu Solder Joints with Different Joule Thermal Properties

Electromigration is one of the most important research issues affecting the reliability of solder joints. Current-induced Joule heating affects the electromigration behavior of solder joints. Solder joints with different cross-sectional areas were designed to obtain different Joule heating properties. The effects of the interfacial intermetallic compound (IMC) and mechanical properties of Sn58Bi/Cu solder joints were studied for different Joule heating properties. The results showed that as the cross-sectional area of the Sn58Bi/Cu solder joints increased, the Joule heating of the joint increased. The anode IMC thickness of the joint thickened and transformed into a planar shape. The Bi migrated to the anode region to form a Bi-rich layer and gradually increased in thickness. The cathode IMC thickness first increased, then decreased, and gradually dissolved. The Sn-rich layer formed near the solder side and gradually increased in thickness, with microcracks occurring when the cross-sectional area of the joint increased to 0.75 mm2. The joint shear fracture path moved from the soldering zone near the cathode IMC layer to the interfacial IMC layer. The fracture mechanism of the joint changed from a mixed brittle/tough fracture, dominated by deconstruction and secondary cracking, to a brittle fracture dominated by deconstruction. The joint shear strength was reduced by 60.9% compared to that in the absence of electromigration.

Interfacial Reaction and Electromigration Failure of Cu Pillar/Ni/Sn-Ag/Cu Microbumps under Bidirectional Current Stressing.

The electromigration behavior of microbumps is inevitably altered under bidirectional currents. Herein, based on a designed test system, the effect of current direction and time proportion of forward current is investigated on Cu Pillar/Ni/Sn-1.8 Ag/Cu microbumps. Under thermo-electric stressing, microbumps are found to be susceptible to complete alloying to Cu6Sn5 and Cu3Sn. As a Ni layer prevents the contact of the Cu pillar with the solder, Sn atoms mainly react with the Cu pad, and the growth of Cu3Sn is concentrated on the Cu pad sides. With direct current densities of 3.5 × 104 A/cm2 at 125 °C, the dissolution of a Ni layer on the cathode leads to a direct contact reaction between the Cu pillar and the solder, and the consumption of the Cu pillar and the Cu pad shows an obvious polarity difference. However, with a bidirectional current, there is a canceling effect of an atomic electromigration flux. With current densities of 2.5 × 104 A/cm2 at 125 °C, as the time proportion of the forward current approaches 50%, a polarity structural evolution will be hard to detect, and the influence of the chemical flux on Cu-Sn compounds will be more obvious. The mechanical properties of Cu/Sn3.0Ag0.5Cu/Cu are analyzed at 125 °C with direct and bidirectional currents of 1.0 × 104 A/cm2. Compared with high-temperature stressing, the coupled direct currents significantly reduced the mechanical strength of the interconnects, and the Cu-Sn compound layers on the cathode became the vulnerable spot. While under bidirectional currents, as the canceling effect of the electromigration flux intensifies, the interconnect shear strength gradually increases, and the fracture location is no longer concentrated on the cathode sides.

Electromigration failure in inkjet-printed Ag conductive lines

Electromigration (EM) is crucial to the reliability of most conductive lines used in electronics. In the present study, the EM characteristics of inkjet-printed Ag conductive lines were analyzed under various EM acceleration conditions to comprehend the EM failure behaviors associated with inkjet-printed Ag lines with nanoparticle inks. The evolution of the porosity level in the microstructure of the inkjet-printed Ag lines during the EM test was investigated to locate the EM failure positions in the line and identify the main driving force for EM mass transport. Two theoretical models (resistometric and Black’s) were employed to analyze the activation energy and expected lifetime of inkjet-printed Ag lines. This study indicates that the EM of Ag cations is directed toward the cathode by the direct force resulting from the electric field–ion interaction, resulting in EM failure near the anode and hillock formation near the cathode of the inkjet-printed Ag lines. The activation energy computed from the theoretical models suggests that the surface diffusion of Ag through the inkjet-printed line plays an important part in the EM failure mechanism. This research was a pioneering attempt to experimentally investigate the EM performance of inkjet-printed Ag lines.

Experimental and Numerical Investigation of Electromigration Behavior of Printed Silver Wire Under High Current Density

Abstract The electromigration (EM) damage is becoming a severe problem in the printed flexible electronics as the printed circuits are fabricated thinner and thinner due to the development of printing technology. In this work, the EM behavior of printed silver wires was investigated by EM experiments and numerical simulations. The EM tests showed that voids are generated in the cathode area and hillocks are formed in the anode area for a wire with a small length. However, with the increase of wire length, hillocks tend to occur on the two sides of the silver wire middle part. The results of numerical simulations based on the atomic flux divergence (AFD) method revealed that the formation of the hillocks on the printed wire is caused by not only the mechanism of electron wind but also the strong temperature gradient along the wire length and width direction. Also, it can be concluded that the temperature gradient induced by Joule heating plays a more important role than electron wind in the atomic migration of the printed silver wire subjected to a high current density. The influence of the printed silver wire size on the EM behavior was also analyzed by numerical simulation, and the results demonstrated that the printed silver wires with a larger length and a smaller width-to-thickness ratio are more likely to develop hillocks on the two sides of silver wire middle part while subjected to a high current density.

Phase field modelling of the electromigration behaviour in sintered silver

Phase field modelling of the electromigration behaviour in sintered silver

Synchrotron X-ray study of electromigration, whisker growth, and residual strain evolution in a Sn Blech structure

Synchrotron X-ray analysis of the Sn electromigration behavior and Sn whisker growth in a Blech structure using nano-X-ray fluorescence microscopy and white beam Laue nanodiffraction was conducted. Sn depletion at the cathode and whisker/extrusion formation at the anode were characterized in-situ, and the results obeyed the electromigration kinetics. This electromigration scenario gradually decayed because of the counterbalance between electron wind force and back stress. White beam Laue nanodiffraction analysis showed that a noticeable compressive deviatoric stress in the direction of electron flow built up at the anode of Sn strips, particularly in the roots of Sn whiskers, confirming that electromigration-induced atomic accumulation occurred downstream in a strip and that Sn whiskering was closely related to internal stress resulting from atomic accumulation in confined segments. Finally, a theoretical model based on fundamental electromigration theory revealed that Sn diffused predominately through lattice and grain boundary paths at Sn homologous temperature of 0.6.