Electromigration Phenomena Research Articles

Monolayer Control of Discotic Liquid Crystal by Electromigration of Dewetted Layers in Thin Film Devices

We show that ultrathin films of a semiconductive discotic liquid crystal, viz. phthalocyanines, can be organized to form a conductive channel tens of microns long between Au electrodes with thickness control over a single monolayer. Our approach exploits the electromigration of the isotropic phase formed starting from the pretransitional region of the columnar-isotropic phase transition. Dewetted isotropic material accumulates to the negative electrode by applying a longitudinal electric field of about 1 V/microm. Dewetting and electromigration expose an ultrathin film, a few monolayers thick, exhibiting columnar liquid crystal order. The layers of this ultrathin film melt progressively above T(C) and can be individually exfoliated by electromigration, starting from the ninth down to the first monolayer. The analysis of the current flowing through the junction as a function of the temperature, together with the comparative imaging of the evolution of morphology, yields a detailed picture of the changes in the dimensionality of the conductive phthalocyanine film and allows us to extract the behavior of the order parameter. The phenomenon of electromigration opens interesting questions on the technological control of individual monolayers on device patterns.

A bimodal three-parameter lognormal mixture distribution for electromigration failure analysis

The electromigration (EM) phenomenon often exhibits two failure mechanisms. The early failures are due to voids developed in the via or at the via-line interface while the wear-out failures are predominantly attributed to voids developed within the interconnect line itself. Presence of these two failure mechanisms necessitates the use of a bimodal mixture distribution for analyzing EM data. Also, due to the presence of incubation time in EM, the three-parameter lognormal distribution should be applied to account for the initial failure-free duration. An efficient global optimization technique called “simulated annealing” is introduced in this work to globally optimize the log-likelihood function of the mixture distribution and determine the values of the lognormal parameters that accurately fit the experimental EM failure data.

Noise and reliability in simulated thin metal films

Many macroscopic aspects of electromigration damage in thin metal films have been investigated by means of Monte Carlo simulations based on simplified physical model. The employed model, can be described as a middle-scale model, in which the physical system is modeled with a high level of abstraction, without a detailed atomic physical model of the system. Among the many effects of the electromigration phenomenon, the simulator has been used to investigate several statistical properties of electromigration failure and the noise behaviour. Notwithstanding this simplicity, it is able to generate results in good agreement with many experimental observations: the lognormal distribution of failures, dependence of the mean time to failure from stress current and film geometry, Black exponent, noise statistics. Furthermore, this simulations confirmed a significant correlation between electromigration noise in the initial phase of stress and time to failure which has been suggested by a few experimentalists. This correlation can be usefully exploited as an early indication of the onset of electromigration damage on a per-sample basis.

Looking at the nanomechanics of electronic devices under the scanning electronic microscope

Modern industry routinely uses sophisticated microscopic methods for quality control and for predicting the performance of manufactured products. These processes typically exploit optical emission and scattering phenomena—such as electron backscattered diffraction and energy-dispersive x-ray spectroscopy—using equipment attached to conventional scanning electron microscopy (SEM). In silicon-based metal oxide semiconductor (MOS) devices, residual stresses may remain from the manufacturing process owing to a mismatch in thermal expansion coefficients between constituent materials. Thermal fatigue and electromigration (displacement of atoms induced by electric current) phenomena can also affect the devices’ micromechanical state. Up to now, SEM has not been applied to these problems for two reasons. First, the effects of strain on electronic transitions arising from band-gap or optically active point defects, as well as their interaction with the electron probe, were not fully understood. Second, no comprehensive algorithm could be found that took into account the superposition of several chemical, physical, and mechanical effects (i.e., temperature gradients, self-absorption, chemical gradients) to extract the stress information from the luminescence spectrum. Yet, the spatial resolution of the SEM would be highly advantageous in residual stress assessment. We recently developed a spectroscopic method for analyzing materials and devices in the SEM that works by visualizing surface and subsurface stress fields. Further, we have used the technique to solve a number of important industrial problems, for example, the stress stored in interconnect structures (like inFigure 1. An SEM image (a) and a CL/PS stress image (b) of the immediate neighborhood of the crack tip in a gallium nitride crystal. The stress map represents the spatial variation of the trace of the stress tensor acquired with a spatial resolution of 30nm.

Encapsulation of Inorganic Pigment and its Electromigration Phenomenon in Dielectric Medium

Encapsulation of inorganic pigments can be useful to protect the pigments from the environment or to prevent particular clustering and aggregation, thereby extending the lifetime of the electrophoretic display when the pigments are used as electrophoretic particles. In this study, inorganic pigments were encapsulated by two step polymerization of methyl methacrylate, ethylene glycol dimethylate, and methacrylic acid. Chemical structure, the morphology and the thermal stability of the encapsulated pigments were investigated by FT-IR, optical and electron microscopies and TGA, respectively. Dispersion characteristics and the electrophoretic response of the encapsulated pigments in dielectric medium were investigated. In comparison with the neat pigments, the modified pigments showed much lower densities and their surface was totally changed.

Studies on solder bump electromigration in Cu/Sn–3Ag–0.5Cu/Cu system

This paper aims to understand the solder bump electromigration phenomenon in the Cu/Sn–3Ag–0.5Cu/Cu system. A temperature of 453 K with a current density of 10 kA/cm 2 was applied. A void nucleated at the highest current density point at the cathode. As the void grew along the cathode side, a solder depletion occurred on the opposite side of the electron entry point, resulting in an open failure. A unique purposely-designed 3D model simulation methodology provides a good understanding of the void nucleation and growth behavior. The temperature of the solder joint during the electromigration test was measured successfully by the resistance change in the junction line between the two joints.

In-situ electromigration study on Sn−Ag−Cu solder joint by digital image speckle analysis

The phenomenon of electromigration in Pb-free Sn-Ag-Cu solder joint specimen subject to high current density was characterized. Electromigration measurements in the direction of the electron flux, via Cu metal lines, ENIG metallization, and Sn-Ag-Cu solder joint is documented. The consequence of this migration phenomenon causes void formation and the significant accumulation of voiding is a serious reliability concern. Digital image speckle analysis (DISA) was used to measure the in-situ micro-deformation of a cross sectioned solder joints, which is subject to electromigration with a current density of 5*10/sup 3/A/cm/sup 2/ under a constant temperature of 150/spl deg/C. The deformation and strain field of the solder joint were analyzed by a digital image correlation software. After 120hrs thermal-current test, higher strain near large voids was detected. The IMC growth behavior with/without current are compared. It was noted that IMC layer growth on anode interface is faster than cathode interface, and both are faster than isothermal aging.

Reservoir effect and the role of low current density regions on electromigration lifetimes in copper interconnects

Electromigration (EM) in copper dual-damascene interconnects with extensions(also described as overhang regions or reservoirs) in the upper metal (M2) were investigated. It was found that as the extension length increases from 0 to 60 nm, the median-time-to-failure increased from 50 to 140 h, representing a ∼200% improvement in lifetimes. However, further increment of the extension length from 60 to 120 nm did not result in any significant improvement in EM lifetimes. Based on calculations of current densities in the reservoir regions and recently reported nucleation, void movement, and agglomeration-based EM phenomena, it is proposed that there is a critical extension length beyond which increasing extension lengths will not lead to longer EM lifetimes.

Effect of Test Conditions on Electromigration Reliability of Sn-Ag-Cu Flip-Chip Solder Interconnects

The high-temperature operation life test (HTOL) was conducted in this paper to study electromigration phenomena of solder interconnects in a flip-chip package assembly. We examined the fatigue reliability and morphological patterns of three solder compositions: Sn-4Ag-0.5Cu, Sn-3.5Ag-1Cu, and Sn-3Ag-1.5Cu, subjected to two test conditions consisting of different average current densities and ambient temperatures (5kA∕cm2 at 150°C and 20kA∕cm2 at 30°C). It is interesting to realize that as the Cu weight content of the solder composition increases, the fatigue life increases under 5kA∕cm2 at 150°C but decreases under 20kA∕cm2 at 30°C. Observed electromigration morphologies along with computed current density and temperature distributions on solder interconnects from the electrothermal coupling analysis were examined, correlated, and discussed.

Electromigration in Al interconnects and the challenges in ultra-deep submicron technology

Electromigration (EM) problem in Al interconnects is one of the most persistent and important challenges in the microelectronic industry. From 1990s, with the scaling down and new processes introduced, ultra-deep submicron (feature size≤0.18 μm) Al interconnects involve more complicated EM issues. In this paper, we summarized the basic formulae of the EM phenomena and got the important flux divergence equations. Based on the equations, the research methods for EM problems were reviewed. The main issues and solutions during the half century of investigation for Al EM challenges were reviewed as well. Finally, the essentials and challenges of current ultra-deep submicron Al EM reliability are analyzed, and some prospective solutions were also proposed.

A study in flip-chip UBM/bump reliability with effects of SnPb solder composition

This paper aims to investigate the electromigration phenomenon of under-bump-metallization (UBM) and solder bumps of a flip-chip package under high temperature operation life test (HTOL). UBM is a thin film Al/Ni(V)/Cu metal stack of 1.5 μm; while bump material consists of Sn/37Pb, Sn/90Pb, and Sn/95Pb solder. Current densities of 2500 and 5000 A/cm 2 and ambient temperatures of 150–160 °C are applied to study their impact on electromigration. It is observed that bump temperature has more significant influence than current density does to bump failures. Owing to its higher melting point characteristics and less content of Sn phase, Sn/95Pb solder bumps are observed to have 13-fold improvement in Mean-Time-To-Failure (MTTF) than that of eutectic Sn/37Pb. Individual bump resistance history is calculated to evaluate UBM/bump degradation. The measured resistance increase is from bumps with electrical current flowing upward into UBM/bump interface (cathode), while bumps having opposite current polarity cause only minor resistance change. The identified failure sites and modes from aforementioned high resistance bumps reveal structural damages at the region of UBM and UBM/bump interface in forms of solder cracking or delamination. Effects of current polarity and crowding are key factors to observed electromigration behavior of flip-chip packages.

Investigations on electromigration phenomena for protein crystallization using crystal growth cells with multiple electrodes: effect of the potential control

Investigations on electromigration phenomena for protein crystallization in gels and solution using a crystal growth cell with multiple electrodes at constant current (2 μA per circuit) are discussed. In this work, the advantages and disadvantages of different electrode dispositions number, and electrode connections inside the cell are shown. A pH change effect was discarded as the main factor affecting the selective growth of the crystals at the cathode and the fact is explained in terms of electromigration phenomena. Additionally, controlled potential experiments were carried out in order to avoid parasitic reactions observed at constant current, resulting in an increase of reproducibility and better control of crystal growth.

Biased resistor network model for electromigration failure and related phenomena in metallic lines

Electromigration phenomena in metallic lines are studied by using a biased resistor network model. The void formation induced by the electron wind is simulated by a stochastic process of resistor breaking, while the growth of mechanical stress inside the line is described by an antagonist process of recovery of the broken resistors. The model accounts for the existence of temperature gradients due to current crowding and Joule heating. Alloying effects are also accounted for. Monte Carlo simulations allow the study within a unified theoretical framework of a variety of relevant features related to the electromigration. The predictions of the model are in excellent agreement with the experiments and in particular with the degradation towards electrical breakdown of stressed Al-Cu thin metallic lines. Detailed investigations refer to the damage pattern, the distribution of the times to failure (TTFs), the generalized Black's law, the time evolution of the resistance, including the early-stage change due to alloying effects and the electromigration saturation appearing at low current densities or for short line lengths. The dependence of the TTFs on the length and width of the metallic line is also well reproduced. Finally, the model successfully describes the resistance noise properties under steady state conditions.

Manipulation and conductivity measurements of gold nanowires

A low drift nanomanipulation device operating in a scanning electron microscope was constructed. The device allowed to manipulate and to characterize the electrical properties of flexible and highly conductive gold nanowires with diameters down to 15nm. Individual nanowires could be electrically contacted either to substrate electrodes or to movable tip electrodes of the nanomanipulation device. Measuring the total resistance of a single nanowire at different lengths gave the intrinsic conductivity (σ ≈ 2.6 × 107S/m) close to that of the bulk material (σ = 4.3 × 107S/m). At high current densities the phenomenon of electromigration could be observed. The problem of spontaneous deposition of insulating layers on the gold objects from contaminations in the SEM chamber is addressed.

Two-dimensional model for soil electrokinetic remediation of heavy metals: Application to a copper spiked kaolin

A two-dimensional numerical model has been developed to simulate the electrokinetic remediation of soils contaminated with heavy metals and has been validated using laboratory experiments performed with a copper spiked kaolin. The model divides the soil into compartments in a Cartesian grid and a non-conductivity barrier encloses the considered area. Basically, it consists in two main parts clearly distinguishable. The first part describes the electromigration phenomenon in the soil, which is represented by a set of electric resistors, following the Cartesian grid and using Kirchoff’s laws of electricity to calculate the voltage drop distribution in the considered area. The second part describes the chemical equilibrium process between the heavy metal and the soil, assuming local equilibrium conditions within the compartments. A good agreement was obtained between the lab scale experimental assays and the model predictions. The model has also been used to examine the effect of the electrolyte neutralization within the scope of the acid-enhanced electrokinetic method. These simulations have foreseen problems related with the system evolution, which would not arise under one-dimensional geometries and are due to the changes of the potential distribution in the two-dimensional arrangement where some kind of short circuit arises, ultimately leading to a decrease of the system efficiency.

Dependence of copper interconnect electromigration phenomenon on barrier metal materials

Dependence of copper interconnect electromigration phenomenon on barrier metal materials

Electromigration Phenomenon of Copper Interconnects on Semiconductor Device

Electromigration Phenomenon of Copper Interconnects on Semiconductor Device

Monte Carlo simulation of electromigration phenomena in metallic lines

The electromigration (EM) of metallic lines is studied in terms of competition between biased processes taking place in a random resistor network. The effects associated with the transport of mass and with the consequent growth of the internal stress are accounted for by stochastic generation and recovery of voids, driven by the external current. Monte Carlo simulations enable us to successfully investigate within a unified theoretical framework a variety of relevant aspects of EM degradation. Here, we present an implementation of the model to rectangular networks. In this manner it is possible to study geometrical effects. An excellent agreement is found between the simulation results and the experimental findings concerning the dependence of the failure process upon the length of the metallic line.

In situ SEM observation of electromigration phenomena in fully embedded copper interconnect structures

An experimental set-up is presented, that allows in situ scanning electron microscope (SEM) investigations of the progress of electromigration damage in fully embedded copper interconnect structures. A LEO Gemini 1550 SEM has been equipped with a heating stage and electrical connections for the experiment. The studied interconnect structures are usually used for reliability testing in electromigration ovens. These test structures are located within the scribelines of wafers. Therefore, they allow the characterization of the electromigration behaviour of products on the wafer. To enable the SEM observation, focused ion beam (FIB) was used to prepare cross-sections of the samples maintaining their electrical functionality. Thereby, a thin layer of passivation was left over in front of the interconnects to keep them fully embedded. The SEM images which were taken at an angle of 60° allow the observation of both the entire via/contact and the connecting lines. Multiple images were recorded during the degradation experiments. The resulting video sequences provide a good visualization of the formation, growth and motion of voids at the stressed interconnects. The dominant diffusion path has been identified.

Characteristic time determination for transport phenomena during the electrokinetic treatment of a porous medium

Electroremediation is a soil decontamination technique. The electric field applied to a porous medium induces the migration of ionic species in solution. The different phenomena appearing in soil are presented. A new methodological approach is carried out in order to determine if preponderant and/or limiting phenomena can be considered. The theoretical study, using dimensional analysis, showed that transport of ionic species in solution could be described only by electro-osmosis and electromigration phenomena. But, other phenomena, such as heterogeneous reactions, can restrict the migration of ionic species which is only possible for chemical species in solution. Then, an experimental study was carried out with a tracer, in order to verify the theoretical characteristic times for transport phenomena, obtained by dimensional analysis and calculated in a synthetic medium. Rigorous experimental procedures were followed to realise well-controlled electroremediation experiments. The experimental results obtained have allowed the measurement of characteristic times of electrokinetic transport and the theoretical times have been validated. In addition, the application of a simple mass balance equation to the soil and an electrolytic compartment has allowed the verification of the migration behaviour of the mass flux as a front advance.