Mass transport during electromigration in aluminum-magnesium thin films

Single-crystal and polycrystalline aluminum thin film conductors, containing up to 6 at.% magnesium, were subjected to electromigration experiments at 175° and 225°C with current densities up to 4 × 10⁶ A/cm² and for times up to 50 000 h. These conductors were subsequently examined in order to obtain quantitative data on mass transport during electromigration.The results indicate that in an alloy containing about 6 at.% magnesium the lattice diffusivity of aluminum is larger by an order of magnitude than the corresponding lattice diffusivity in pure aluminum, whereas the grain boundary diffusivity of aluminum in this alloy is two orders of magnitude smaller than the corresponding value in pure aluminum. Further, the lattice and grain boundary diffusivities of magnesium in this alloy are of the same order of magnitude as the aluminum diffusivities, and appear to be slightly smaller in both cases.     

Electromigration of grain boundaries in aluminum

The electromigration of grain boundaries has been investigated with aluminum wires (99.999% purity, 1 mm in diameter) in the temperature range 340°–540°C under a current stress of 6×10³ A cm^-2. Grain boundary migration in aluminum is found to be reduced or enhanced by the currents stress when the grain boundary migrates in or against the current direction, respectively. The effects of additions of copper, silicon and zirconium to aluminum on the electromigration of the grain boundaries have also been examined. The contribution of electromigration to normal grain boundary migration is found to be increased by the addition of solutes. The results are discussed on the basis of the impurity drag mechanism of grain boundary migration.     

Effect of small amount of rare earth addition on electromigration in eutectic SnBi solder reaction couple

Effect of small amount of rare earth (RE) addition on electromigration behavior of Cu/SnBi/Cu solder reaction couple (SRC) was investigated with current density of 5 × 10³ A/cm² at room temperature and 100 °C, respectively. Results indicate that tiny RE addition to eutectic SnBi solder alloy can make the energy of interfaces and grain boundaries decrease, restrain the movement of dislocations and grain boundary sliding. Therefore, phase segregation and IMC growth will be effectively suppressed which enhances the electromigration resistance.     

Interfacial evolution in Sn–58Bi solder joints during liquid electromigration

For Sn–58Bi low temperature solder alloy, local molten induced from electromigration Joule heating might change the atomic diffusion and interfacial behavior. In this paper, the diffusion behavior and interfacial evolution of Cu/Sn–58Bi/Cu joints were studied under liquid–solid (L–S) electromigration in molten solder and were compared with the interfacial behaviors in solid–solid (S–S) electromigration in solid solder. L–S or S–S electromigration was realized by applying a current density of 1.0?×?10⁴ A/cm² to molten solder at 150 °C or solid solder at 25 °C, respectively. During S–S electromigration, Bi atoms were driven towards anode side under electromigration induced flux and then accumulated to form Bi-rich layer near anode interface with current stressing time increasing. During L–S electromigration, Bi atoms were reversely migrated from anode to cathode to produce Bi segregation at cathode interface, while Cu atoms were rapidly dissolved into molten solder from cathode and migrated to form large amounts of Cu₆Sn₅ rod-like phases near anode interface. The reversal in the direction of Bi atoms may be attributed to the reversal in the direction of electromigration induced flux and correspondingly the change on effective charge number of Bi atoms from negative to positive.     

Effect of current stressing on the reliability of 63Sn37Pb solder joints

The effect of current stressing on the reliability of 63Sn37Pb solder joints with Cu pads was investigated at temperatures of −5 °C and 125 °C up to 600 h. The samples were stressed with 3 A current (6.0 × 10² A/cm² in the solder joint with diameter of 800 μm and 1.7 × 10⁴ A/cm² in the Cu trace with cross section area of 35 × 500 μm). The temperatures of the samples and interfacial reaction within the solder joints were examined. The microstructural change of the solder joints aged at 125 °C without current flow was also evaluated for comparison. It was confirmed that the current flow could cause the temperature of solder joints to rise rapidly and remarkably due to accumulation of massive Joule heat generated by the Cu trace. The solder joints stressed at 125 °C with 3 A current had an extensive growth of Cu₆Sn₅ and Cu₃Sn intermetallic compounds (IMC) at both top and bottom solder-to-pad interfaces. It was a direct result of accelerated aging rather than an electromigration or thermomigration effect in this experiment. The kinetic is believed to be bulk diffusion controlled solid-state reaction, irrespective of the electron flow direction. When stressed at −5 °C with 3 A current, no significant change in microstructure and composition of the solder joints had occurred due to a very low diffusivity of the atoms as most Joule heat was eliminated at low temperature. The IMC evolution of the solder joints aged at 125 °C exhibited a subparabolic growth behavior, which is presumed to be a combined mechanism of grain boundary diffusion and bulk diffusion. This is mainly ascribed to the retardant effect against the diffusion course by the sufficiently thick IMC layer that was initially formed during the reflow soldering.     

Electromigration testing of Ti: W/Al and Ti: W/Al-Cu film conductors

Electromigration-induced failures in metal film interconnections influence the reliability of integrated circuits. For shallow (< 1 μm) junction devices a barrier- metal interconnection system such as Ti: W/Al has been proposed to eliminate contact pitting due to silicon-aluminum reactions. The addition of copper to aluminum films is known to improve the electromigration resistance of aluminum film interconnections. Glass-passivated Ti: W/Al and Ti: W/Al-Cu (1.6 wt.% Cu) film conductors (9 μm wide, 1.14 mm long and 170 nm/800 nm thick) on oxidized silicon substrates were subjected to a current stress of 10⁶ A cm^-2 in the temperature range 150–270°C. Mean-time-to-failure data indicate an improvement of approximately a factor of two in electromigration resistance due to the addition of copper. This improvement is smaller than that reported by others. Life test data are consistent with activation energies of 0.61±0.05 and 0.71±0.03 eV for Ti: W/Al and Ti: W/Al-Cu film conductors respectively. Extrapolated mean times to failure are close to 24 and 100 a for Ti: W/Al and Ti: W/Al-Cu films respectively under a current stress of 5 × 10⁵ A cm^-2 at 55°C ambience. Projected failure rates at these operating conditions increase very rapidly with time and approach values of 9 × 10^-7 and 1 × 10^-11 h^-1 for Ti: W/Al and Ti: W/Al-Cu film conductors respectively at 100 000 h.     

Low temperature interdiffusion in the Au-Pd and Au-Rh thin film couples

Interdiffusion profiles in thin polycrystalline multilayer films of Pd-Au and Ti-Rh-Au at temperatures up to 490°C have been measured by Rutherford backscattering. Room temperature grain boundary diffusion of Au into Rh was observed and analyzed to give D_B = 3.5 × 10^-17 cm² sec^-1. The Whipple analysis is applied to our data for the diffusion of Au in Pd; using the lattice diffusivity of Neukam, an activation energy for grain boundary diffusion of 0.9 eV is found. The diffusion of Pd in Au has also been analyzed using the Whipple model, which gives a grain boundary activation energy of 0.6 eV.     

Lateral self-diffusion and electromigration in thin metal films

Two techiques which have been developed to study lateral self-transport in thin films are described and compared. Both are non-destructive and involve the determination of the spatial distribution of suitable radioactive tracers incorporated within the films. A sample application of the previously described digital tracer scanner has been made to self-diffusion and electromigration in Sn films; it is found that at 213°C Sn migrates toward the anode with an effective valence Z¹ of -46.7 and with a diffusivity D of 2.91 × 10^-10 cm²/sec. In addition, a new high resolution autoradiography method is described; it is based on the detection of Ag X-rays emanating from exposed photographic emulsions when they are viewed in the scanning electron microscope. Application to Au films provides unambiguous evidence for grain boundary electrotransport which is anode directed.     

Low temperature interdiffusion in Cu/Ni thin films

Interdiffusion in Cu/Ni thin films was studied by means of Auger electron spectroscopy in conjunction with Ar⁺ ion sputter profiling. The experimental conditions used aimed at simulating those of typical chip-packaging fabrication processes. The Cu/Ni couple (from 10 μm to 60 nm thick) was produced by sequential vapor deposition on fused-silica substrates at 360, 280 and 25 °C in 10^-6 Torr vacuum. Diffusion anneals were performed between 280 and 405 °C for times up to 20 min. Such conditions define grain boundary diffusion in the regimes of B- and C-type kinetics. The data were analyzed according to the Whipple-Suzuoka model. Some deviations from the assumptions of this model, as occurred in the present study, are discussed but cannot fully account for the typical data scatter. The grain boundary diffusion coefficients were determined (for nickel through copper, Q_b = 33.7 kcal mol^-1 (1.46 eV), D_b⁰ = 4.2 × 10^-2 cm²s^-1; for copper through nickel, Q_b = 30.2 kcal mol^-1 (1.3 eV), D_b⁰ = 7.6 × 10^-5 cm²s^-1) allowing calculation of respective permeation distances.     

Electromigration in cobalt films

Structural changes and resistance increases were investigated on Co films of 1 μm thickness, carrying current densities of about 10⁶ A cm^-2. At 360 °C no resistance increase was found after 1000 h. The surface exhibited faceting on a fine scale. Between 540 °C and 640 °C the resistance was found, after a small decrease, to increase linearly with time and then to increase more slowly until failure. The slope p of the linear part increases with temperature with an activation energy of 1.5 eV. The lifetime is inversely proportional to this slope. The current density dependence is as p≈ jⁿ with n ≈ 2. With a.c. no resistance increase was found. Scanning electron microscope observations showed severe grain boundary grooving. The effect of temperature gradients is consistent with electromigration towards the anode. Transmission electron microscope observations on a well-annealed free-hanging film showed only thermomigration. The Soret coefficient was estimated to be about 0.1 K^-1.The resistance increase is described as being due to grain boundary grooving by electromigration on the surface as well as along grain boundaries. The current density dependence is thought to be governed partly by temperature gradients.     

Thermodynamic and mass transport properties of “LiIn”

The EMF of the vario-stoichiometric phase “LiIn” lies between 497 and 145 mV relative to pure lithium at 415°C over the composition range from 46.7 to 63.3 a/o Li, corresponding to a lithium activity varying from 2.29×10^?4 to 8.67×10^?2 at that temperature. On the Li-deficient side of LiIn, the chemical diffusion coefficient increases rapidly with decreasing lithium concentration, approaching the value of 3.98×10^?5 cm²/sec near the In-rich phase boundary of “LiIn”. In the region of positive deviations from the ideal stoichiometry, the chemical diffusion coefficient remains essentially constant up to a composition about 57.8 a/o Li. At higher lithium concentrations, the chemical diffusion coefficient first decreases and then increases as more lithium is added to the sample, and shows a minimum value of 4.73×10^?7 cm²/sec at 62.2 a/o Li.     

Lattice and grain boundary diffusion of copper in thin aluminum films

The lattice D₁ and grain boundary δD_b diffusivities of Cu in Al thin films at 130–185°C are calculated from measurements employing Auger electron spectroscopy and Ar ion beam etching. The calculated values are D₁ = 0.065 cm² s^-1 × exp(-122 kJ/RT) and δD_b = 4.5 × 10^-9 cm³s^-1 exp(-97.4 kJ/RT). The D₁ value is 3–5 times larger at 130–185°C than that predicted by an extrapolation of radioactive tracer measurements of large grain bulk specimens at 433–652°C. The higher value measured here is attributed to the higher density of subgrain defect structures in the thin film.     

Influence of diffusion-annealing temperature on physical and mechanical properties of Cu-diffused bulk MgB2 superconductor

This study reports not only the effect of Cu diffusion on physical and mechanical properties of bulk MgB₂ superconductors with the aid of Vickers microhardness (H_v) measurements but also the diffusion coefficient and the activation energy of copper (Cu) in the MgB₂ system using the resistivity measurements for the first time. Cu diffusion is examined over the different annealing temperature such as 650, 700, 750, 800 and 850 °C via the successive removal of thin layers and resistivity measurement of the sample. Further, Vickers microhardness, elastic modulus, yield strength, fracture toughness and brittleness index values of the samples studied are evaluated from microhardness measurements. It is found that all the results obtained depend strongly on the diffusion annealing temperature and applied load. The microhardness values increase with ascending the annealing temperature up to 850 °C owing to the increment in the strength of the bonds between grains but decreasing with the enhancement in the applied load due to Indentation Size Effect behaviour of the bulk samples. Moreover, the diffusion coefficient is observed to enhance from 2.84 × 10^?8 to 3.22 × 10^?7 cm² s^?1 with the increase of the diffusion-annealing temperature, confirming that the Cu diffusion is more dominant at higher temperatures compared to lower ones. Besides, temperature dependence of the Cu diffusion coefficient is described by the Arrhenius relation D = 2.66 × 10^?3 exp(?1.09 ± 0.05 eV/k_BT) and the related activation energy of the Cu ions in the MgB₂ system is obtained to be about 1.09 eV. Based on the relatively low value of activation energy, the migration of the Cu ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, resulting in the improvement of the physical and mechanical properties of the bulk MgB₂ samples.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

The temperature dependence of stresses in aluminum films on oxidized silicon substrates

In situ measurements were carried out of stress at the AlSiO₂ interface at various temperatures (25–500 °C) and for various film thicknesses (0.2–1.6 microm). These data are complemented with microstructural studies by transmission electron microscopy.For the aluminum films studied, the intrinsic structural component was very small (less than 2 × 10⁸ dyn cm^?2). On heating, thermal mismatch led to a compressive stress, with dσ/dT ≈ ?2 × 10⁷ dyn cm^?2 °C; these films yielded at 6σ6 ; ? 5 × 10⁸ dyn cm^?2, primarily through diffusion creep and grain growth. On cooling from about 450–500 °C, thermal mismatch led to a tensile stress which was limited mainly by dislocation slip. The final room temperature value after thermal cycling ranged from 0.5 × 10⁹ dyn cm^?2 for a 1.5 microm film to 8 × 10⁹ dyn cm^?2 for a 0.2 microm film; these values are believed to represent the critical stress for the generation of dislocation loops within the grains.The grain size of cold-deposited aluminum films ranged from about 0.2 microm for films 0.45 microm thick to about 2 microm for films 1.5 microm thick. Thermal cycling led to an order-of-magnitude increase in the grain size together with the formation of dislocation networks within the grains.     

Grain boundary self-diffusion in evaporated Au films at low temperatures

Direct self-diffusion measurements in vapor-deposited polycrystalline Au films have been made using ¹⁹⁵Au radioactive tracer and an r.f. back-sputtering technique for serial sectioning. A temperature range of 117°–177°C was investigated. It has been demonstrated that self-diffusion in thin Au films at these low temperatures takes place by rapid transport of the tracer atoms along the grain boundaries. The grain boundary self-diffusion parameters are Q_b=1.0±0.1 eV and δD_b⁰ = 9 × 10^?10 cm³/sec, which compare well with those in bulk polycrystalline Au.     

Diffusion in the CuSn binary system: application to Nb3Sn composites

Non-parabolic growth of intermediate phases in CuSn binary diffusion couples has been observed at 220° C. The deviation from parabolic behaviour may be attributed to grain boundary diffusion. Diffusion coefficients for both the∈-(Cu₃Sn) andη-(Cu₆Sn₅) phases are typically of the order of 2×10⁻¹¹ cm² sec⁻¹, and are in general agreement with other published values.     

Reverse polarity effect and cross-solder interaction in Cu/Sn–9Zn/Ni interconnect during liquid–solid electromigration

The diffusion behavior of Zn atoms and Cu–Ni cross-solder interaction in Cu/Sn–9Zn/Ni interconnects during liquid–solid electromigration were investigated under a current density of 5.0 × 10³ A/cm² at 230 °C. Under the combined effect of chemical potential gradient and electron wind, Zn atoms with positive effective charge number would directionally diffuse toward the Cu interface under both flowing directions of electrons. When electrons flowed from Cu substrate to Ni substrate, EM significantly enhanced the diffusion of Cu atoms to the opposite Ni interface, resulting in the formation of interfacial Cu₅Zn₈; while no Ni atoms diffused to the opposite Cu interface. When electrons flowed from Ni substrate to Cu substrate, only a small amount of Cu atoms diffused to the opposite Ni interface, resulting in the formation of a thin interfacial (NiCu)₃(SnZn)₄ (containing 3 wt% Cu); EM significantly accelerated the diffusion of Ni atoms to the Cu interface, resulting in the formation of a large amount of (NiCu)₃(SnZn)₄ at the Cu interface. Even under downwind diffusion, no apparent consumption of Cu substrate was observed due to the formation of a thick and dense Cu₅Zn₈ layer at the Cu interface. It is more damaging with electrons flowing from Ni to Cu than that from Cu to Ni.     

Role of diffusion-annealing temperature on the microstructural and superconducting properties of Cu-doped MgB2 superconductors

This study deals with not only investigate the effect of the copper diffusion on the microstructural and superconducting properties of MgB₂ superconducting samples employing dc resistivity as a function of temperature, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements but also calculate the diffusion coefficient and the activation energy of copper for the first time. Electrical-resistivity measurements indicate that both the room-temperature resistivity value and zero resistivity transition temperatures (T _c ) increase with increasing the diffusion-annealing temperature from 650 to 850?°C. SEM measurements show that not only the surface morphology and grain connectivity improve but also the grain size of the samples increases with the increase in the diffusion-annealing temperature up to 850?°C. As for the XRD results, all the samples contain the MgB₂ phase only and exhibit the polycrystalline superconducting phase with more intensity of diffraction lines, leading to the increasement in the lattice parameter a and c. Additionally, the diffusion coefficient is observed to increase from 6.81?×?10^?8 to 4.69?×?10^?7?cm²?s^?1 as the diffusion-annealing temperature increases, confirming that the Cu diffusion at lower temperatures is much less significant. Temperature dependence of the Cu diffusion coefficient is described with the aid of the Arrhenius relation D?=?3.75?×?10^?3 exp (?1.15?±?0.10?eV/k _B T) and the corresponding activation energy of copper in MgB₂ system is found to be about 1.15?eV. The possible reasons for the observed improvement in microstructural and superconducting properties of the samples due to Cu diffusion are also discussed.