Early Electromigration Failures Research Articles

Vertical interconnects of microbumps in 3D integration

Abstract

Mode II electromigration failure mechanism in Sn-based Pb-free solder joints with Ni under-bump metallization

Early electromigration (EM) failure mechanism, also known as Mode II EM failure in Sn-based Pb-free solder joints with Ni under-bump metallization (UBM) is studied. A numerical model was built to incorporate interdiffusion and EM, and to investigate the effect of Sn crystal orientation. Results show that when the c axis of Sn has a close alignment to the current direction, intermetallic compound (IMC) growth is enhanced by both EM and extremely fast interstitial diffusion of Ni in the solder. The combined process results in IMC dissolution from the cathode side of the solder joint and causes EM failure by UBM depletion. Theoretical predictions are consistent with experimental observations.

Modeling Electromigration Lifetimes of Copper Interconnects

A model for early failure due to electromigration in copper dual-damascene interconnects is proposed. The model is based on analytical expressions obtained from solutions of electromigration stress build-up assuming slit void growth under the interconnect vias. It is demonstrated that the model satisfactorily describes the complex physics of void nucleation and growth of the electromigration damage. Furthermore, it is shown that the simulation results provide reasonable estimates for early electromigration failures.

A compact model for early electromigration failures of copper dual-damascene interconnects

A compact model for early electromigration failures in copper dual-damascene interconnects is proposed. The model is based on the combination of a complete void nucleation model together with a simple mechanism of slit void growth under the via. It is demonstrated that the early electromigration lifetime is well described by a simple analytical expression, from where a statistical distribution can be conveniently obtained. Furthermore, it is shown that the simulation results provide a reasonable estimation for the lifetimes.

Effects of metal stacks and patterned metal profiles on the electromigration characteristics in super-thin AlCu interconnects for sub-0.13 μm technology

The electromigration (EM) characteristics of super-thin aluminum-copper alloy (AlCu) interconnects for sub-0.13μm complementary metal-oxide-semiconductor logic technology were investigated by varying the AlCu underlayers and etched sidewall profile of AlCu wire. Super-thin AlCu wire with a Ti/TiN underlayer and a smooth etched sidewall profile was confirmed to have the best EM resistance in terms of the mean-time-to-failure (MTTF) and the failure distribution. The Ti/TiN underlayer is believed to lead to a longer MTTF by dramatically reducing the effective current density at the super-thin AlCu film in the entire of EM test line with a smaller formation of TiAl3 at the TiN/AlCu interface. The smooth etched sidewall profile is considered to induce a steeper EM failure distribution by removing the early EM failure at the rough sidewall of aluminum. In terms of the location of EM-induced voids, the super-thin AlCu film inhibits the formation of EM-induced voids directly under the tungsten via by insignificant current crowding at the via–metal line corner.

Addressing the challenges in solder resistance measurement for electromigration test

In the continuous drive for smaller chips with more functionality, I/O counts and power requirements increase. This leads to a growing concern on the electromigration (EM) reliability of solder joints in the high-density flip-chip package. For solder EM tests, it is a great challenge to detect early EM failure since the 10% change in resistance is very small due to the small initial resistance of the solder. Furthermore, the small change in resistance can often be masked by the parasitic resistance in the interconnect connecting the solder daisy chain type structures commonly employed in EM tests. The Wheatstone bridge method has been reported to address the inaccuracy associated with the use of Four-probe measurement method in solder EM tests successfully. In this work, we describe the use of Kelvin double bridge configuration that can further increase the accuracy of the bump resistance measured.

Effect of Zn doping on SnAg solder microstructure and electromigration stability

A comprehensive study on the effect of Zn-doped SnAg solders on microstructure and electromigration (EM) is reported. Minor Zn doping, about 0.6 wt %, in a SnAg solder alloy is found to be effective in stabilizing solder microstructure and improving EM reliability. Early EM failure modes in Zn-doped solders are significantly suppressed, resulting in a longer EM lifetime with tight distributions. Analyses using optical microscopy, scanning electron microcopy, transmission electron microscopy, electron microprobe analysis, and electron backscattering diffraction were conducted. Zn addition in SnAg leads to significant changes in solder microstructure, grain structure, and thermal and EM stabilities. The strong reaction of Zn with Cu atoms effectively slowed down the Cu diffusion in β-Sn grains, thus improved the EM stability of Sn-rich solder joints.

Copper alloy seed integration for reliability improvement

Besides cost reduction and yield optimisation, advanced interconnect development is nowadays driven by continuous resistance and capacitance improvement in order to reduce RC-delay with shrinking line dimensions. In order to attain these aggressive goals atomic layer deposition (ALD) has successfully been employed to improve line resistance by depositing atomically thin TaN barriers thereby increasing the copper cross section of the line. However, the reliability is still of concern due to a reduced adhesion of the copper seed to the ALD TaN liner. In this paper a PVD copper alloyed seed and a PVD Ta-flash (∼5 nm) were used to improve the ALD TaN/Cu interface strength and remedy early electromigration failure. The Cu-seed was alloyed with 1 at.% aluminium and integrated into state-of-the-art 90 nm and 65 nm interconnect structures in order to evaluate the extendibility at narrow line widths. Activation energy and mean time to failure were significantly improved upon replacing the Cu-seed layer with the Cu-alloyed seed. The RC product could be reduced with 30% compared to the PVD base line. Physical analyses like de-wetting tests and time of flight secondary ion spectroscopy (TOF-SIMS) were respectively used to gauge the adhesion improvement, and to measure the diffusion of aluminium in copper. It is shown that the aluminium is staying near the ALD TaN interface after anneal explaining the relative small increase in line resistance and the reduced copper mobility near the interface.

An analysis of the weakest-link model for early electromigration failure

The application of the weakest-link or failure-unit model to electromigration failure is discussed in relation to a simple model that should describe the early failures in fine-line, as-patterned aluminium reasonably well. The earliest failures are expected to be due to the presence of at least one polygranular cluster longer than some critical (Blech) length, while the next earliest are assumed to arise from linked shorter clusters. Using a simple model of the microstructure, the probability density function for the number of each of these types of failure-unit is determined as a function of the linewidth. For this simple model the overall failure distribution may be calculated essentially exactly using the Theory of Runs; consequently we can compare these exact results with some common approximations. It is demonstrated through Jensen's inequality that a common approximate method is in fact a rigorous upper bound for the failure distribution. The model exhibits the expected increase in median-time-to-failure and deviation-in-time-to-failure values with decreasing linewidth when forcing a lognormal distribution. Although this would suggest an eventual decrease in reliability with scale reduction, the exact results show that this is not the case.

Effect of cracks in TiN anti-reflection coating layers on early via electromigration failure

Early electromigration failure has been recognized as a generic phenomenon in W-plug via structures. Previous studies have attributed early failure to the properties of the Al layers in contact with the plug, via delamination or to current density distributions in the structure. In this paper, we investigate early failure phenomena in W-plug vias and report a novel early via failure induced by the presence of a crack crossing the lower metal level TiN anti-reflection coating layer in the vicinity of W-plug. The cracks in the TiN shunting layer may cause large resistance jumps of 250–1000 Ω. Resistance simulation results show that the magnitude of resistance jump increases with the size of the crack. The source of the TiN cracks is discussed.

Bath Additive and Current Density Effects on Copper Electroplating Fill of Cu Damascene Structures

AbstractCopper electroplating has become the leading technology for gap fill of damascene structures on advanced interconnects. A key to developing a robust electroplating process that produces deposits free of voids and seams is understanding the role of the additive components, i.e., levelers, brighteners and wetting agents, and their relative diffusion/adsorption characteristics. Additionally, obtaining insight about the cathodic current/potential relationship is critical for maximizing the effectiveness of the additive components.Our results indicate that bath additive composition and the plating parameters (plating pulse frequency, and current density play critical roles in the outcome of the Cu fill. SEM cross sectional analysis of timed partial electroplating fill studies show two types of fill, 1) conformal and 2) bottom-up. Conformal fill of features smaller than 0.25 μm with an aspect ratio (AR) of 4.0 tends to form seam voids in the center of the structure. These seam voids can lead to early electromigration failures. On the other hand, bottom-up fill leads to a void free Cu deposit within the feature.

Bath Additive and Current Density Effects on Copper Electroplating Fill of Cu Damascene Structures

ABSTRACTCopper electroplating has become the leading technology for gap fill of damascene structures on advanced interconnects. A key to developing a robust electroplating process that produces deposits free of voids and seams is understanding the role of the additive components, i.e., levelers, brighteners and wetting agents, and their relative diffusion/adsorption characteristics. Additionally, obtaining insight about the cathodic current/potential relationship is critical for maximizing the effectiveness of the additive components.Our results indicate that bath additive composition and the plating parameters (plating pulse frequency, and current density) play critical roles in the outcome of the Cu fill. SEM cross sectional analysis of timed partial electroplating fill studies show two types of fill, 1) conformal and 2) bottom-up. Conformal fill of features smaller than 0.25 μm with an aspect ratio (AR) of 4.0 tends to form seam voids in the center of the structure. These seam voids can lead to early electromigration failures. On the other hand, bottom-up fill leads to a void free Cu deposit within the feature.

The influence of TiN ARC thickness on stress-induced void formation in tungsten-plug vias

This paper describes a new failure mechanism in W-plug vias, and the process conditions which enhance it. For submicron technologies, the limiting factor in interconnect reliability performance is increasingly dominated by the electromigration resistance of tungsten-plug vias. We have observed that under certain experimental conditions, early electromigration failures can be induced in via-chain test structures. We have demonstrated that these are caused by stress-induced void formation in the metal line immediately beneath the tungsten plug. This is thought to be due to highly localized film stress around the base of the plug, which can be minimized by increasing the thickness of the TiN anti-reflective coating (ARC). This has the effect of reducing the incidence of early failure by suppressing the stress-induced failures.

Early Electromigration Failure in Submicron width, Multilayer Al Alloy Conductors: Sensitivity to Stripe Length

ABSTRACTThe microstructure of Al alloy integrated circuit conductors strongly affects electromigration failure of the metallization. For submicron linewidths, the microstructure is usually near - bamboo, consisting of alternate polycrystalline and bamboo segments. These polycrystalline segments appear to be much more susceptible to electromigration - induced voiding than the bamboo regions. As a result, microstructural related early failures are a significant concern. In this study we systematically vary line - length to investigate the impact of random microstructural variations on the failure distribution parameters of narrow, multilayer (Ti/TiN/Al/TiN) Al alloy conductors. We show that early failures can occur in 0.6 μm wide stripes. The impact of early void formation on conductor failure, however, is dependent on the resistance failure criterion. By increasing the value, early failures may be eliminated in long line - lengths, suggesting that the resistance failure criterion may provide a means to control the impact of early voiding on the electromigration failure.