Study of DC and AC Electromigration Behavior in Eutectic Pb-Sn Solder Joints

In this study, direct current (DC) and alternating current (AC) electromigration experiments were carried out using solder joints with a Cu/eutectic Pb-Sn/Cu joint configuration. During stressing using DC and AC, a fixed current density of 10⁴ A/cm² was applied to the joints at 150°C. In the joints stressed by DC, electromigration-induced damage occurred, and the corresponding microstructural changes mainly included valley and hillock formation in the solder region, pronounced phase segregation of Pb-rich and Sn-rich domains, asymmetric growth of Cu-Sn intermetallics at the interfaces, and excessive depletion of Cu at the cathode side. In contrast, no significant electromigration was observed after the AC treatments. This was especially true for the treatment with AC frequencies higher than 1 h⁻¹. The dependence of the damage on AC frequency suggests that electromigration in solder joints can be inhibited to a large extent when a proper reverse current is delivered.     

The Hillock Formation in a Cu/Sn-9Zn/Cu Lamella upon Current Stressing

The electromigration (EM) that occurs in a Cu/Sn-9Zn/Cu lamella was investigated for hillock formation at room temperature with a current density of 10³ A/cm² for up to 230 h. Hillocks and cavities grew in the middle of the bulk solder and at the cathode, respectively. The formation of hillocks was ascribed to a compressive stress resulting from the diffusion of Sn atoms driven by electromigration and Cu-Zn compound formation.     

Electromigration in Al/Si contacts—Induced open-circuit failure

Open-circuit failures caused by electromigration in Al/Si contacts are studied. This failure mode is associated with Al depletion or vacancy condensation over the entire position contact area. The contacts exhibiting this failure are those closest to bonding pads. This location preference is attributed to vacancy supplies associated with the large bonding pad. The current acceleration factor for electromigration open failure is found to be 2.5 ± 0.5 and the activation energy is 0.5 ± 0.1 eV. Our empirical data suggests that, for operating temperatures below about 100°C, open-circuit failure will be dominant over junction leakage failure.     

Nonuniform and Negative Marker Displacements Induced by Current Crowding During Electromigration in Flip-Chip Sn-0.7Cu Solder Joints

A quantitative analysis of the nonuniform distribution of current density and nonuniform rate of electromigration has been carried out by measuring the movement of an array of diffusion markers. Tiny marker arrays were fabricated by focused ion beam on the polished surface of flip-chip solder joints near the anode to measure the electromigration rate. The marker velocity at the current-crowding region was found to be at least five times larger than at locations far from the region. Some of the markers in the low-current-density region possess negative velocities, indicating that backflow occurs during the electromigration. The backflow, in which the atomic flow is against the electron flow, is explained by a constant-volume model as well as the back-stress induced by electromigration.     

In-Situ Observation of Electromigration in Eutectic SnPb Solder Lines: Atomic Migration and Hillock Formation

The real-time microstructural evolution during electromigration of eutectic SnPb solder lines with an edge drift structure was examined using an in-situ scanning electron microscope (SEM) technique. The test temperature and the current density were either 100°C or 50°C and 6 × 10⁴ A/cm² or 8 × 10⁴ A/cm², respectively. In-situ microstructural observation of the depleted phases and quantitative analysis of the number of hillock phases made it clear that the dominant migrating element and dominant hillock phase were Sn and Pb at room temperature, respectively, while both dominant migrating element and dominant hillock phase were Pb at 100°C. Such temperature dependence of the dominant hillock phase in the eutectic SnPb solder can be understood by considering the atomic size factors of the metallic solid solutions. We suggest that at high temperature, it is easier for Pb atoms to be injected into the Pb phase (Pb-phase hillocks); while at low temperature, Pb-phase hillocks were squeezed by Sn, which penetrated the Pb phase.     

Lattice electromigration in narrow Al alloy thin-film conductors at low temperatures

At low temperatures, electromigration in polycrystalline Al thin-film conductors has been considered to occur predominantly along grain boundaries. As conductor widths decrease below the average grain size, however, other transport mechanisms will become important. Here we examine electromigration transport mechanisms in narrow AlSiCu conductors in the temperature range 190 to 290°C using the stripe drift technique. For conductors with widths between 0.9 and 2.75 μm both the absolute values of the drift velocity and the activation energy for drift are consistent with a lattice diffusion mechanism. Over this linewidth range the Al microstructure ranges from near-bamboo to approximately 20 μm long polycrystalline segments. The independence of the drift data from the linewidth shows that steady state transport is controlled by the bamboo regions of the conductors, which results from the slower rate of diffusion of Al through the lattice compared to along grain boundaries.     

Kinetics of hillock growth in Al and Al-alloys

Hillock growth kinetics and size distribution were investigated in Al, Al:Si 1% and Al:Si1%:Cu 0.5% layers. Metallization surface was examined by optical, SEM and TEM microscopy, stylus profiling and an automatic method of hillock recognition from a microscope image. The method allowed for counting hillocks in a desired range of their diameter d. Surface density of hillocks was measured as a function of time of furnace annealing at 400°C and as a function of temperature of RTP annealing. A maximum hillock size was found to increase linearly with metallization layer thickness and with logarithm of annealing time. A total area occupied by hillocks was evaluated. Hillock density decreased versus 1/T with an activation energy of 0.28 eV for Al and 0.31 eV for Al:Si. It was found, that a normalized hillock density N may be expressed by a formula N=N₀ exp(−cd). Values for N₀ and c are given together with a short discussion.     

Electromigration mechanisms in aluminum lines

Special test structures were used for investigating electromigration mechanisms. Large-grained Al lines of different lengths and widths were interconnected by varying TiN auxiliary layers. Test current densities lay between 4 × 10⁵ A/cm² and 6 × 10⁵ A/cm² at 200°C. Considering electromigration threshold, grain boundary electromigration was eliminated and interface electromigration appeared, affecting the conductive Al/TiN interface. Interface electromigration clearly contributes to the mass flow of Al lines, and thus can be detrimental for the reliability of metallization. The interface diffusion activation energy is comparable to the grain boundary activation energy. Contrary to a conductive interface, the technical Al surface does not contribute to mass flow. The elimination of interface effects finally brings out homogeneous bulk electromigration. The drift velocity was directly measured after a stress period of 8300 hours at 200°C. For a current density of 4 × 10⁵ A/cm² bulk drift velocity was 7 × 10^?12 cm/s, while grain boundary electromigration surpassed this value by a factor of 300. Electromigration threshold was ascertained for grain boundary as well as for interface and bulk diffusion.     

Dynamics of electromigration induced void/hillock growth and precipitation/dissolution of addition elements studied by in-situ scanning electron microscopy resistance measurements

In the present paper it has been shown that the in-situ SEM resistance measurement technique is a powerful technique to study the dynamics of void/hillock growth and precipitation/dissolution of addition elements in a metal line submitted to a temperature/current stress. The power of the in-situ SEM resistance measurement technique is shown with the first results on Al1wt.%Si0.5wt.%Cu metal lines. During the electromigration experiment, performed in a SEM equipped with a heating stage, back scattered electron images are taken continuously over the entire length of the metal line monitoring a.o. the growth, shape variation and motion of voids/hillocks. The dissolution and motion of Al₂Cu precipitates in the Al1wt.%Si0.5wt.%Cu metal lines can also be monitored since the precipitates appear in the BSE mode as white objects. By comparing the observed electrical resistance drift results with the corresponding SEM micrographs it can be concluded that the resistance changes in the current stressed metal lines are mainly induced by geometrical changes.     

Study of Cu diffusion in an Al–1 wt.%Si–0.5 wt.%Cu bond pad with an Al–1 wt.%Si bond wire attached using scanning electron microscopy

A study with scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) of the Al–1 wt.%Si–0.5 wt.%Cu bond pad metallisation with Al–1 wt.%Si wires bonded to it revealed Cu diffusion from the bond pad into the Al wire after an annealing process. Investigation at different annealing times showed a Cu depletion zone which indicates that the radius R is consistent with a behaviour. The diffusion velocity, as determined experimentally, is close to the velocity of grain boundary diffusion. This observation has important consequences with respect to electromigration testing of Al(Si)Cu metallisations bonded with Al wires. The electromigration (EM) process can be influenced by a bad placement of the Al bond wire with regard to the test lines. If bonded too close to the test line, Cu diffusion out of the test line is possible during annealing, thereby changing the microstructure locally and thus the EM process.     

Effect of Al trace dimension on electromigration failure time of flip-chip solder joints

The effect of Al-trace dimension on electromigration of flip-chip solder joints was investigated. The Al trace dimension was found to have a significant influence on the electromigration failure time. When joints with Al traces 100 μm wide were stressed by 1.0 A at 100°C, failure times were 35 h, 1,700 h, and >3,000 h for joints with Al traces that were 2,550 μm, 1,700 μm, and 850 μm long, respectively. Solder joints with Al traces 40 μm wide and 2,550 μm long failed instantly at 0.6 A. The Joule heating effect was found to be responsible for the huge difference in failure time.     

The applicability of logarithmic extreme value distributions in electomigration induced failures of Al/Cu thin-film interconnects

In electromigration failure studies it is in general assumed that electromigration induced failures may be adequately modelled by a log normal distribution. Further to this it has been argued that a log normal distribution of failure times is indicative of electromigration mechanisms. We have combined post processing of existing life-data from Al/Cu+TiW bilayer interconnects with our own results from Al/Cu interconnects to show that the Log Extreme Value distribution is an equally good statistical model for electromigration failures, even in cases where grain size exceeds the line width. The significance of such a modelling is particularly apparent in electromigration failure rate prediction.     

Effect of Intermetallic on Electromigration and Atomic Diffusion in Cu/SnAg3.0Cu0.5/Cu Joints: Experimental and First-Principles Study

Electromigration phenomena in a one-dimensional Cu/SnAg_3.0Cu_0.5/Cu joint were investigated with current stressing. The special effect of intermetallic compound (IMC) layers on the formation of serious electromigration damage induced by nonuniform current density distribution was discussed based on experimental results. Meanwhile, hillocks were observed both at the anode and near the cathode of the joint, and they were described as the result of diffusion of atoms and compressive stress released along grain boundaries to the relatively free surface. Moreover, the diffusion behavior of Cu at the cathode was analyzed with the electromigration equation, and the stability of Ag atoms in the solder during electromigration was evaluated with a first-principles method.     

Reliability computer-aided design tool for full-chip electromigration analysis and comparison with different interconnect metallizations

We have developed a set of methodologies for thermal aware circuit-level reliability analysis with either Al or Cu metallization in a circuit layout and implemented it in a public domain reliability CAD tool, SysRel. SysRel utilizes a hierarchical reliability analysis flow, with interconnect trees treated as the fundamental reliability unit, that sufficiently captures the differences in electromigration failure between Al and Cu metallizations. Under similar test conditions, the electromigration reliability of Al and Cu interconnect trees demonstrates significant differences because of the differences in interconnect architectural schemes. Using the best estimates of material parameters and an analytical model, we present a detail comparison of electromigration reliability of a sample test-structure as well as of actual circuit layouts with Al and Cu dual-damascene interconnect systems. We also demonstrate fast thermal-analysis in SysRel for circuit performance driven chip-level reliability assessment.     

Effects of Mg additions on the electromigration behavior of Al thin film conductors

Aluminum thin film conductors containing Mg alloying additions have been tested for electromigration failure by formation of electrically open circuits. The test conditions were either 2 × 10⁶ A/cm² or 4 × 10⁶ A/cm² for the current density, and either 175 or 225°C for the temperature. The median lifetimes were found to increase with increasing Mg concentrations up to the highest concentration tested, about 6%. With polycrystalline films the maximum increase in lifetime resulting from Mg additions corresponds to a factor of about 100, as compared to pure Al films. This is about equal to previously reported results obtained with Cu additions. The increase in lifetime has been shown to result from a decrease in the rate of grain boundary diffusion for the Al atoms. Magnesium atoms diffuse at approximately the same rate as Al atoms. Thus the mechanism of failure formation in Al films containing Mg is thought to be different than in Al-Cu films, where Cu atoms diffuse faster than Al atoms and failure ensues upon local Cu depletion.     

纳米压痕技术控制电迁移现象中铝原子积聚位置研究

在利用电迁移现象制备铝纳米线的过程中,铝纳米线的生长位置取决于铝原子的积聚位置。为实现铝原子积聚位置控制,基于纳米压痕技术改变试样中铝膜的横截面结构,制备了铝膜试样。通过输入并调节直流电大小使铝膜内产生电迁移现象。试验结果表明,纳米压痕技术可有效提高局部区域的电流密度,显著增强铝原子的电迁移强度,并在压痕区域出现铝原子积聚现象。     

Sb掺杂对SnAgCu无铅焊点电迁移可靠性的影响

向Sn3.8Ag0.7Cu无铅焊膏中添加质量分数为1%的Sb金属粉末,研究了其焊点在电流密度为0.34×104A/cm2、环境温度150℃下的电迁移行为。通电245h后,阴极处钎料基体与Cu6Sn5IMC之间出现一条平均宽度为16.9μm的裂纹,阳极界面出现凸起带,钎料基体内部也产生了裂纹。结果表明:1%Sb的添加使焊点形成了SnSb脆性相,在高电流密度和高温环境下产生裂纹,缩短了焊点寿命,降低了电迁移可靠性。     

Circuit reliability simulator for interconnect, via, and contactelectromigration

A model for predicting Al interconnect and intermetallic contact/via electromigration time-to-failure under arbitrary current waveform is incorporated in a circuit electromigration reliability simulator. The simulator can (1) generate layout advisory for width and length of each interconnect, and the number of contacts and vias at each node in a circuit, and (2) estimate the overall circuit electromigration failure rate and/or cumulative percent failure as functions of time, temperature, voltage, frequency, and previous stress (e.g., burn-in)     

Pulsed-current duty cycle dependence of electromigration-inducedstress generation in aluminum conductors

The effect of duty cycle of pulsed dc currents on the critical length-current density product, (jl_c), was measured using the Blech-Kinsbron edge-displacement technique [Thin Solid Films 25, 327 (1975)]. Unencapsulated Al edge-displacement segments mere stressed at various duty cycles and the critical lengths, the so-called “Blech lengths”, were measured. It was found that jl_c increased with decreasing duty cycle. We measured a factor of 2.6 increase in jl_c for the 25% duty cycle as compared to dc. This duty cycle dependence of Blech length implies that electromigration resistance for an integrated circuit would be increased for small duty cycle operation by increasing the fraction of interconnects which are sub-Blech-length and are not susceptible to EM damage     

电迁移引起的倒装芯片互连失效

采用倒装芯片组装菊花链器件研究了高电流密度条件下Al互连的失效问题,分析了不同电迁移条件下,由于金属原子的迁移造成的Al互连微结构的变化。在9.7×105A/cm2电流密度强度条件下,钝化窗口位置的Al原子发生电迁移,在电子风力的作用下,Al原子沿电子流方向扩散进入Al互连层下方的焊料中。同时,随着电流加载时间的延长,化学位梯度和内部应力的作用致使焊料成分向Al互连金属扩散,Al互连金属层形成空洞的同时其成分发生变化。