Electromigration-induced Failures Research Articles

Twin-Boundary Reduced Surface Diffusion on Electrically Stressed Copper Nanowires

Surface diffusion is intimately correlated with crystal orientation and surface structure. Fast surface diffusion accelerates phase transformation and structural evolution of materials. Here, through in situ transmission electron microscopy observation, we show that a copper nanowire with dense nanoscale coherent twin-boundary (CTB) defects evolves into a zigzag configuration under electric-current driven surface diffusion. The hindrance at the CTB-intercepted concave triple junctions decreases the effective surface diffusivity by almost 1 order of magnitude. The energy barriers for atomic migration at the concave junctions and different faceted surfaces are computed using density functional theory. We proposed that such a stable zigzag surface is shaped not only by the high-diffusivity facets but also by the stalled atomic diffusion at the concave junctions. This finding provides a defect-engineering route to develop robust interconnect materials against electromigration-induced failures for nanoelectronic devices.

Study of Electromigration-Induced Failures on Cu Pillar Bumps Joined to OSP and ENEPIG Substrates

This work studies electromigration (EM)-induced failures on Cu pillar bumps joined to organic solderability preservative (OSP) on Cu substrates (OSP–bumps) and electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) under bump metallurgy (UBM) on Cu substrates (ENEPIG–bumps). Two failure modes (Cu pad consumption and gap formation) were found with OSP–bumps, but only one failure mode (gap formation) was found with ENEPIG–bumps. The main interfacial compound layer was the Cu6Sn5 compound, which suffered significant EM-induced dissolution, eventually resulting in severe Cu pad consumption at the cathode side for OSP–bumps. A (Cu,Ni)6Sn5 layer with strong resistance to EM-induced dissolution exists at the joint interface when a nickel barrier layer is incorporated at the cathode side (Ni or ENEPIG), and these imbalanced atomic fluxes result in the voids and gap formation. OSP–bumps showed better lifetime results than ENEPIG–bumps for several current stressing conditions. The inverse Cu atomic flux (JCu,chem) which diffuses from the Cu pad to cathode side retards the formation of voids. The driving force for JCu,chem comes from the difference in chemical potential between the (Cu,Ni)6Sn5 and Cu6Sn5 phases.

Numerical failure analysis of current-confined-path current perpendicular-to-plane giant magnetoresistance spin-valve read sensors under high current density

Thermomigration (TM)-induced failures occurred in the current-confined-path (CCP) current perpendicular-to-the plane (CPP) giant magnetoresistance spin valve (GMR SV) read sensors with Cu nanopillar metal paths (∼5 nm in diameter) operating at a high current density (J>2×107 A/cm2) have been numerically studied to explore the magnetic and electrical stability. The Cu interdiffusion (migration) from nanopillars into adjacent magnetic layers (e.g., CoFe) due to thermally induced mass transport was found to be the main physical reason for the magnetic failures of CCP-CPP GMR SV read sensors including the change in interlayer coupling and the reduction in exchange bias field as well as MR. Furthermore, it was numerically verified that the TM-induced failures are more dominant than the electromigration-induced failures at the higher current density beyond J=6×107 A/cm2 in the CCP-CPP GMR SV read sensors. However, all the numerical calculation results demonstrated in this study clearly suggest that these undesirable electrical and magnetic failures occurred in the CCP-CPP GMR SV read sensors can be improved by tuning the path density, the purity (electrical resistivity), and the uniformity of Cu nanopillar metal paths.

Electromigration-induced failures at Cu/Sn/Cu flip-chip joint interfaces

Various EM-induced failures were observed at a current-stressed Cu/Sn/Cu flip-chip solder joint. At the cathode interface, EM-induced Cu-pad consumption occurred at the current-entry point (maximum current-density) and voiding occurred at the other joint corner away from the current-entry point (minimum current-density). At the anode interface, EM-enhanced Kirkendall voids coalesced into a gap at the Cu 3Sn/Cu interface near the current-exit corner. We believe that the above various EM-induced failure modes were resulted from different current-stressing densities at the joint interfaces.

Magnetic instability of giant magnetoresistance spin-valves due to electromigration-induced inter-diffusion

Electromigration-induced magnetic failures in NiFe/(Co)/Cu/(Co)/NiFe spin-valve (SV) multilayers have been investigated. Electrically stressed NiFe/Cu/NiFe SV multilayers showed a dramatic reduction of magnetic moment up to 41%, and a shift of interlayer coupling characteristics, while no obvious magnetic degradation was observed in the NiFe/Co/Cu/Co/NiFe SV multilayers. It was experimentally confirmed that the magnetic degradation of the NiFe/Cu/NiFe SV multilayers is primarily due to the formation of Ni–Cu intermixing caused by the electromigration-induced Cu spacer inter-diffusion. Furthermore, it was demonstrated that an ultra thin Co diffusion barrier at the NiFe/Cu interface is promisingly effective to improve the magnetic stability of NiFe/(Co)/Cu/(Co)/NiFe SV multilayers against electromigration.

Electromigration-induced extrusion failures in Cu/low-k interconnects

Electromigration experiments were conducted to investigate the thresholds required for electromigration-induced extrusion failures in Cu/low-k interconnect structures. Extrusions at the anode were observed after long periods of void growth. Characterization of failure sites was carried out using scanning and transmission electron microscopy, which showed that failures occurred through delamination at the interface between the silicon-nitride-based capping layer diffusion barrier and the underlying Cu, Ta liner, and interlevel dielectric (ILD) materials. This interface is subjected to near tensile (mode I) loading with a mode mixity angle between 4° and 7°, estimated using finite-element-method analysis, as electromigration leads to a compressive stress in the underlying Cu. Comparisons of the fracture toughness for interfaces between the capping layer and individual underlayer materials indicate that the extrusion process initially involves plane-strain crack propagation. As Cu continues to extrude, the crack geometry evolves to become elliptical. An analysis of the critical stress required for extrusions based on these observations leads to a value of approximately 710 MPa, which agrees well with the value determined through estimation of the volume of material extruded and the required stress to accomplish this extrusion. The analysis of the critical stress required for extrusion formation also indicates that sparsely packed, intermediate to wide interconnect lines are most susceptible to electromigration-induced extrusion damage, and that extrusion failures are favored by ILDs with low stiffness (low elastic moduli) and thin liners, both of which are needed in future interconnect systems.

Multi-Via Electromigration Test Structures for Identification and Characterization of Different Failure Mechanisms

AbstractExperiments on three-terminal ‘dotted-I’ test structures (copper metal lines with vias at both ends and an additional via at the center) show that the mortality of a single segment not only depends on the values of its current density and length, but also on the stress conditions in the linked segment. The current density in one 25μm long segment was fixed at 0.5MA/cm2, with electron flow toward the central via. In the othersegment, the current magnitude and sign were varied for different test populations, with the current varied from 2.5MA/cm2 to -2.5MA/cm2 with intermediate values including zero. For all cases, some test structures survived for the full 780 hours of testing and some did not. The percent of the lines that failed increased monotonically with an effective jL product defined as the maximum of the sum of the jL products from all paths through the structure. However, some lines with smaller than expected effective jL products failed, and some lines with relatively large effective jL products did not. Simulations of electromigration and electromigration-induced failures for all test conditions have been carried out. We find that test conditions leading to extreme values of the effective jL product probe different failure mechanisms than those associated with intermediate effective jL products. It is also shown that the definition of the effective jL product must be modified to account for zero current (inactive) segments that act as reservoirs or sinks. Multi-via test structures in general, and dotted-I test structures specifically, are shown to be versatile tools for identification and characterization of different failure mechanisms and length effects through the use of different test conditions with a single fixed structure.

Electromigration study in SnAg3.8Cu0.7 solder joints on Ti/Cr-Cu/Cu under-bump metallization

This paper investigates the electromigration-induced failures of SnAg3.8Cu0.7 flip-chip solder joints. An under-bump metallization (UBM) of a Ti/Cr-Cu/Cu trilayer was deposited on the chip side, and a Cu/Ni(P)/Au pad was deposited on the BT board side. Electromigration damages were observed in the bumps under a current density of 2×104 A/cm2 and 1×104 A/cm2 at 100°C and 150°C. The failures were found to be at the cathode/chip side, and the current crowding effect played an important role in the failures. Copper atoms were found to move in the direction of the electron flow to form intermetallic compounds (IMCs) at the interface of solder and pad metallization as a result of current stressing.

Fast temperature cycling and electromigration induced thin film cracking in multilevel interconnection: experiments and modeling

There is an increasing reliability concern of thermal stress-induced and electromigration-induced failures in multilevel interconnections in recent years. This paper reports our investigations of thinfilm cracking of a multilevel interconnect due to fast temperature cycling and electromigration stresses. The fast temperature cycling tests have been performed in three temperature cycle ranges. The failure times aare represented well by a Weibull distribution. The distributions are relatively well behaved with generally similar slope (shape factor). The failure mechanism is well fitted by the Coffin-Manson equation indicating a uniform acceleration. The observation of cracking in the interlayre dielectric due to fast temperature cycling stress from failure analysis agrees well with the failure mechanism modeling and the calculated Coffin-Manson exponent. Electromigration experiments have shown that devices failed due to extrusion-shorts without increasing of resistance of metal line. The failure times are represented better by the Weibull distribution than by the lognormal distribution (normally used for electromigration data). A simulation of stress buil-up in metal line using an electromigration simulator confirmed that the cracking of interlayer dielectric is the weakest spot and most likely to cause electromigration failure.

Electromigration-induced failure of single layered NiFe Permalloy thin films for a giant magnetoresistive read head

Electromigration-induced failures have been investigated in single layered NiFe Permalloy thin films, which are used in giant magnetoresistance (GMR) read heads in high-density magnetic recording disk drives. The time-to-failure and median-time-to-failure (MTTF, t50) of NiFe thin films were found to be strongly dependent on the applied current density, film thickness, and ambient temperature. The activation energy (ΔQ) of NiFe films derived from the “Black equation” was found to be 0.8 eV. The MTTF and standard deviation (σ) of a NiFe thin film electromigration test stripe with 10 nm thickness, 5 μm width, and 20 μm length were 3.74 and 1.74 h, respectively, at 160 °C ambient temperature. Typical electromigration failures such as “voids and cracks” were observed at the cathode or center region of NiFe film test stripes. These results suggest that the electromigration-induced failures are one of the crucial factors in determining the reliability of GMR spin-valve read heads used in high-density magnetic recording disk drives.

Effects of preexisting voids on electromigration failure of flip chip solder bumps

For the first time, the effects of preexisting voids inside the flip chip solder bump and at the interface of bump/underbump metallization (UBM) are considered on electromigration-induced failures to provide a mechanistic understanding on solder bump electrical reliability. Local peaks of current density due to preexisting voids supported by high operating temperature and temperature gradients and gradients of mechanical stress result in inhomogeneous drift of metal atoms, which leads to the formation of hillocks and more microvoids or large voids in the bump or the bump interfaces. The voids decrease the cross-section of the solder contact, which increases local current density and local resistance. This expected positive feedback cycle can eventually lead to the so-called electromigration failure. In the meantime, the increased volume of the extruded hillocks under electromigration can lead to circuit shorts between neighboring bumps, where the distance becomes closer and closer. A stochastic approach is employed to quantify the effects of various preexisting voids. It is found that preexisting voids at the interface of solder bump/UBM is more prone to growth during electromigration and therefore, a much shorter lifetime of the bump. Voids inside the bump can also speed up the failure of the solder bump and their effects depend on their location, size and total volume fraction. The simulation results can be applied for the reliable design of flip chip solder bumps by modifying the design rule with the consideration of the effects of preexisting voids.

Molecular dynamics study of diffusion and atomic configuration in layered structures for Al circuit interconnects

Diffusion at Al(1 1 1)/underlay-metal interfaces, which is a dominant factor of electromigration-induced open-circuit failures in Al interconnects, was investigated by molecular dynamics simulation. The author focused on interfaces between the (1 1 1) plane in Al and the plane of greatest atomic density in five kinds of metals that have high melting temperatures: Ti, Cr, Mo, W, and Ta. The calculated self-diffusion coefficients of Al atoms near the Al/Ti and Al/Cr interfaces were smaller than those near the other three interfaces. This result is consistent with experimental results by other works that showed that Ti and Cr are effective underlay metals for suppressing the diffusion in Al interconnects. This result on the self-diffusion coefficient was linked to the result on the atomic configurations. The configurations of Al atoms at Al/Ti and Al/Cr interfaces were close to that for bulk Al, while the configurations at the other three interfaces were significantly different from that for bulk Al. It was found that diffusion as well as atomic configuration at the Al/underlay-metal interface is determined by the lattice mismatch between the (1 1 1) plane in Al and the plane of greatest atomic density in the underlay metal.

Electric current effects upon the Sn/Cu and Sn/Ni interfacial reactions

Electromigration-induced failures in integrated circuits have been intensively studied recently; however, electromigration effects upon interfacial reactions have not been addressed. These electromigration effects in the Sn/Cu and Sn/Ni systems were investigated in this study by analyzing their reaction couples annealed at 200°C with and without the passage of electric current. The intermetallics formed were e-(Cu3Sn) and η-(Cu6Sn5) phases in the Sn/Cu couples and Ni3Sn4 phase in the Sn/Ni couples. The same intermetallics were formed in the two types of couples with and without the passage of electric current. The thickness of the reaction layers was about the same in the two types of couples of the Sn/Cu system. In the Sn/Ni system, the growth of the intermetallic compound was enhanced when the flow direction of electrons and that of diffusion of Sn were the same. But the effect became inhibiting if the directions of these two were opposite. Theoretical calculation indicated that in the Sn/Ni system, the electromigration effect was significant and was 28% of the chemical potential effect for the Sn element flux when the Ni3Sn4 layer was 10 µm thick. For the Sn and Cu fluxes in the Sn/Cu reaction couples, similar calculations showed that the electromigration effects were only 2 and 4% of the chemical potential effects, respectively. These calculated results were in good agreement with the experimental observations that in the Sn/Cu system the electric current effects were insignificant upon the interfacial reactions.

Analysis of electromigration-induced failures in high-temperature sputtered Al-alloy metallization

Electromigration-induced failures in high-temperature sputtered Al-alloy metallization have been studied in terms of Al-alloy microstructure, Si nodules, and reaction products between the Al alloy and the underlayer material. Highly reliable interconnects can be realized by high-temperature sputtered Al–Si–Cu-alloy metallization utilizing a Ti underlayer. The interfacial reaction between the growing Al–Si–Cu film and the Ti underlayer during high-temperature sputtering produces an Al3Ti-alloy layer having a resistivity of about 40 μΩ cm, and Si in Al alloy is absorbed in the Al3Ti layer without forming Si nodules. As the result, a bilayer structure of Al alloy/Al–Ti–Si alloy is formed. The electromigration resistance of the resultant TiN/Al alloy/Al–Ti–Si/TiN structure is drastically improved. It is speculated from the experimental results that this is because of the high quality of the Al-alloy film microstructure, i.e., an increase in (111) orientation, a decrease in the standard deviation of the grain distribution, and the superior current bypass effect through the Al–Ti–Si alloy and underlying TiN layer. In particular, an electrical current easily flows through the Al–Ti–Si alloy and TiN layer before a void completely crosses the upper Al alloy part of the metal line because the formation of the AlN high-resistance layer is suppressed by laying the Ti film between the Al–Si–Cu film and the underlying TiN film. Accordingly, the local temperature increase in the vicinity of the void is not essential to the subsequent void growth due to the lower resistivity of the Al–Ti–Si-alloy layer, resulting in the low void growth rate.

Comparative Study of Statistical Distributions in Electromigration-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 lognormal 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 linewidth. The significance of such a modelling is particularly apparent in electromigration failure rate prediction.

Analysis of electromigration-induced failures in multilayered interconnects

The authors studied electromigration-induced failures in multilayered interconnects with overlayer TiN in conjunction with the overlayer-TiN/Al heterointerface evaluation. The electrical contact resistance between Al and TiN changes with fabrication processes. The contact between Al and underlayer TiN shows almost the same resistance as Al/Al direct contact. On the other hand, the contact between overlayer TiN and Al shows higher resistance compared with that of Al/Al direct contact; its value varies with the deposition processes. That is, overlayer-TiN/Al structure successively deposited without breaking vacuum shows one order of magnitude higher contact resistance, while overlayer-TiN/Al structure deposited after Al was exposed to air shows two orders of magnitude higher contact resistance compared with Al/Al direct contact. The high contact resistance between overlayer TiN and Al suppresses the current bypass flow even when it is one order of magnitude higher compared with that of Al/Al direct contact. The simple multilayered structure of overlayered TiN/Ti/Al exhibits high reliability for submicrometer-level interconnects. >

Slit morphology of electromigration induced open circuit failures in fine line conductors

Detailed transmission and scanning electron microscopic images of electromigration-induced open circuit failures are presented for fine line aluminum alloy thin film interconnects. A characteristic slit open circuit, similar to stress migration open circuits in narrow interconnects, is shown for various film compositions, processing, and deposition conditions. It is suggested that slit failure morphology is more generally observed for low (≊1) ratios of conductor line width to film grain size. The slit failures observed often occur near copper rich precipitates. The morphology of several slit voids suggests that they are transgranular across the linewidth, consistent with other recent reports of electromigration induced damage in single crystal interconnects.

Electromigration in a single crystalline submicron width aluminum interconnection

Electromigration properties in a single crystalline submicron width aluminum interconnection formed on Si(111) have been examined by resistance change measurements and in situ observations using scanning electron microscopy. It was observed that single crystalline aluminum has an extremely high resistance to electromigration-induced open circuit failures, when compared to polycrystalline copper and aluminum. The mechanism for the high resistance is considered to be a large activation energy, resulting from lattice diffusion. A tendency for void formation to become parallel to the longitudinal direction of the interconnection assisted the life time prolongation.

Electromigration-induced failures in interconnects with bimodal grain size distributions

Interconnects containing bimodal grain size distributions are known to have lower me-dian times to electromigration-induced failure (MTTF). However, the deviation in the time to failure (DTTF) in such lines has not been well characterized. We find that Al-2%Cu-0.3%Cr interconnects with bimodally distributed grain sizes have MTTF’s which are more than an order of magnitude lower than lines with monomodally distributed small grain sizes. However, the DTTF’s for both types of lines are similar, and in fact slightly lower for lines with bimodal structures. An activation energy of 0.85 eV was obtained both for lines with monomodal large grain structures and bimodal grain struc-tures, suggesting that grain boundary diffusion is the controlling mechanism in both cases. A model based simply on microstructural characteristics,e.g. the distribution of the number of grain boundaries, can explain the lower MTTF’s and DTTF’s for lines with bimodal structures. The implications of bimodal grain size distributions on the reliability of large numbers of lines are discussed. Also, a new, convenient graphical tool for illustrating the failure rate of interconnects with lognormally distributed failure times is presented.

Morphology of electromigration-induced damage and failure in Al alloy thin film conductors

Distinct morphologies of electromigration-induced voids and failures are shown for Al, Al-2%Cu, and Al-2%Cu-l% Si narrow (1–6 µm) unpassivated thin film conductors. SEM and TEM images typically show large non-fatal voids and narrow slit-like open circuit failures for all film conditions and accelerated test conditions. Evidence for transgran-ular slit failures is shown for 1.33 µm wide conductors. A simple model for void growth is presented which accounts for the void morphologies seen. The observed morphologies and the results of void growth modelling suggest that slit voids nucleate after other voids and rapidly produce failure. These conclusions are discussed in terms of ‘classical’ models for electromigration failure processes and resistance and noise power monitoring techniques.