Early Stages Of Electromigration Research Articles

Measuring residual stresses in individual on-chip interconnects using synchrotron nanodiffraction

As the dimensions of interconnects in integrated circuits continue to shrink, an urgent need arises to understand the physical mechanism associated with electromigration. Using x-ray nanodiffraction, we analyzed the stresses in Blech-structured pure Cu lines subjected to different electromigration conditions. The results suggest that the measured residual stresses in the early stages of electromigration are related to relaxation of stresses caused by thermal expansion mismatch, while a developing current-induced stress leads to reductions in the residual stress after longer test times. These findings not only validate the feasibility of measuring stress in copper lines using nanodiffraction but also highlight the need for a further understanding, particularly through in situ electromigration experiments with x-ray nanodiffraction analysis.

Study on 1/<i>f</i> <sup>γ</sup> Noise Characterization of Metallic Interconnection Electromigration

Metallic interconnection electromigration is a common phenomenon in integral circuit, and it influences circuit reliability seriously as rapid miniaturization of circuit size. Resistance is a traditional electromigration characterization parameter, but it requires strict testing condition, and it is not sensitive to vacancies accumulation at early stage of electromigration. Some recent researches show that noise is very sensitive to metallic thin film electromigration, it can be used as an effective inspect method for electromigration. So noise wais used to characterize damage degree of Al thin film electromigration in this paper., Tthe results showeds both noise amplitude and frequency exponent increased during electromigration process, and when void nucleation occuredhappens, frequency exponent increased sharply. noise can also reflect electromigration degree under different environment temperature. Through comparing with resistance, noise can be proved a better characterization parameter for electromigration

Plastic deformation in Al (Cu) interconnects stressed by electromigration and studied by synchrotron polychromatic x-ray microdiffraction

We report here an in-depth synchrotron radiation based white beam x-ray microdiffraction study of plasticity in individual grains of an Al (Cu) interconnect during the early stage of electromigration (EM). The study shows a rearrangement of the geometrically necessary dislocations (GNDs) in bamboo typed grains during that stage. We find that about 90% of the GNDs are oriented so that their line direction is the closest to the current flow direction. In nonbamboo typed grains, the Laue peak positions shift, indicating that the grains rotate. An analysis in terms of force directions has been carried out and is consistent with observed EM induced grain rotation and bending.

In-situ early stage electromigration study in Al line using synchrotron polychromatic X-ray microdiffraction

ABSTRACTElectromigration is a phenomenon that has attracted much attention in the semiconductor industry because of its deleterious effects on electronic devices (such as interconnects) as they become smaller and current density passing through them increases. However, the effect of the electric current on the microstructure of interconnect lines during the very early stage of electromigration is not well documented. In the present report, we used synchrotron radiation based polychromatic X-ray microdiffraction for the in-situ study of the electromigration induced plasticity effects on individual grains of an Al (Cu) interconnect test structure. Dislocation slips which are activated by the electric current stressing are analyzed by the shape change of the diffraction peaks. The study shows polygonization of the grains due to the rearrangement of geometrically necessary dislocations (GND) in the direction of the current. Consequences of these findings are discussed.

Fractal character of noise in electromigration in metel interconnection

Based on fractal theory , through the fractal dimension of electro-diffusion, the dimensions of noise time series and grain boundary are corelated. The change of noise dimension is shown to be useful for studying the damage mechanism and determining the parameters for electromigration. The picture of grain boundary becomes complicated in the early period of electromigration, but after the voids are informed, it becomes more regular. So the fractal dimension of time series increases during the early stage of electromigration and goes down after the voids are informed. The theoretical analysis is confirmed by experiment, the dimension of noise time series may be a new characteristic parameter for metallic interconnection electromigration.

Quantitative analysis of dislocation arrangements induced by electromigration in a passivated Al (0.5 wt % Cu) interconnect

Electromigration during accelerated testing can induce plastic deformation in apparently undamaged Al interconnect lines as recently revealed by white beam scanning x-ray microdiffraction. In the present article, we provide a first quantitative analysis of the dislocation structure generated in individual micron-sized Al grains during an in situ electromigration experiment. Laue reflections from individual interconnect grains show pronounced streaking during the early stages of electromigration. We demonstrate that the evolution of the dislocation structure during electromigration is highly inhomogeneous and results in the formation of unpaired randomly distributed dislocations as well as geometrically necessary dislocation boundaries. Approximately half of all unpaired dislocations are grouped within the walls. The misorientation created by each boundary and density of unpaired individual dislocations is determined. The origin of the observed plastic deformation is considered in view of the constraints for dislocation arrangements under the applied electric field during electromigration.

High resolution X-ray tomography with applications in biology and materials science

With the new tomography setup developed for the x-ray microscope XM-I installed at the Advanced Light Source, tomography of immunolabelled frozen-hydrated cells to detect protein distributions inside of cells was performed. The distribution of the nuclear protein, male specific lethal 1 (MSL-1) in the Drosophila meianogaster cell was studied. Another application field for high resolution tomography which is of fundamental interest in materials science is electromigration in advanced copper interconnects. In this work, quantitative time-resolved x-ray microscopy mass transport studies of the early stages of electromigration in an inlaid Cu line/via structure were performed with 40 nm sptial resolution at 1.8 keV photon energy. Correlation of the real time x-ray microscopy images with post mortem high voltage electron micrographs of the sample shows that the void nucleation occurs at the site of grain boundaries in Cu and that the voids migrate along these grain boundaries during electromigration. To provide 3D information about the exact location (bulk or interface) of void nucleation and migration during an EM experiment, as well as to measure quantitatively the mass transport in the volume, future experiments must be based on time-resolved x-ray tomography.

Early stage electromigration in gold thin films

The early stage of electromigration in thin gold films on polyimide has been investigated at room temperature using the resistometric technique. While the resistance increase is initially linear, a saturation tendency is observed for longer stressing times at all stress current densities. A simple model is described which relates the saturation behavior in the resistance change to the buildup of mechanical stress gradients, which produce a counterflux of metal ions proportional to the stress gradient. The stress gradients arise due to nonuniformity in the grain size in the polycrystalline thin metal films which produces cluster regions of small grain size alternating with regions of large or near-bamboo grain size, which have larger and smaller metal ion diffusivities, respectively. The dependence of the maximum level of the resistance’s change and the rate of resistance increase on the stress current density are experimentally characterized and compared with the model’s predictions with good agreement.

Characterization of the early stages of EM in Al-based metal lines by means of a HRRMT based on an extremely stable ambient temperature

AbstractA high resolution resistometric measurement technique (HRRMT) is used to study electromigration (EM) in different Al-based metal lines. Instead of using a bridge-method, the high resolution is now obtained by performing experiments in a very stable temperature environment. The main advantage of this technique is that resistance variations are measured, due to processes that occur in the tested metal line only. During EM, both resistivity and geometrical variations can occur, both resulting in a resistance change. A new method is proposed in order to separate these two contributions on-line during the whole EM-experiment. It is shown that even at low current stress, geometrical changes are the main cause for the EM-induced resistance increase.

Activation energy in the early stage of electromigration in Al-1% Si/TiN/Ti bamboo lines

The activation energy of electromigration in 'bamboo'-type, passivated Al-1% Si/TiN/Ti lines was determined by means of high resolution resistance measurements at wafer level. Both the very early stages, with a nonlinear behaviour of the resistance, and the subsequent stages, characterized by an approximately constant rate of resistance change, were analysed with an existing model that correlates electromigration and mechanical stress evolution. An activation energy could be extracted only in the phase of linear resistance increase, where a value of 0.95 eV was found. Diffusion at the interface between Al-Si and the barrier metal or the passivation could be responsible for this value of the activation energy.

Electromigration at gold–aluminium interfaces and in thin aluminium tracks

The in situ method to study accelerated ageing of electronic interconnections under current stress has been applied to (a) Au ball bonds on A11% Si metallization, and (b) thin aluminium tracks. For the Au ball bonds, it has been shown that direct current stresses above 108 Am−2 cause electromigration effects at the Au-Al interface, and that the ageing behaviour depends on the direction of the applied direct current stress. It was also shown that the addition of 1 per cent Cu to the metallization strongly retards void formation, and hence open contact failure. For the thin aluminium tracks, it has been shown that the early stage of electromigration, which can be studied accurately with the in situ technique, is closely related to the stress relaxation within the track. It is therefore concluded that the kinetics of electromigration can only be described if the kinetics of stress relaxation are well understood.

Electromigration in Thin Films of Au on GaAs

AbstractEvolution of the fractional change of electrical resistance ΔR/R in thin films of Au on (001) substrates of semi-insulating GaAs has been investigated as a function of time t, temperature T, and current density j. Initially, ΔR/R increases linearly with increasing t for constant T and j, and exponentially with increasing T for constant t and j, characterized by an activation energy of 0.73 eV. An analytical model is developed to evaluate ΔR/R for the early stages of electromigration. This model is based on flux divergence at grain boundary triple junctions resulting from variations of grain boundary inclination and/or diffusivity. Using a Monte Carlo method, conducting lines containing a prescribed number of random triple junctions are simulated, wherein distribution of mass flux divergence determines initial values of ΔR/R. Moreover, by selection of an appropriate failure criterion, the sequence of cumulative failures is characterized by a log-normal-like distribution, which defines mean time to failure and corresponding standard deviation. In general, the model is in good agreement with experimental observations.

Effect of diffusivity variation on capillarity and electromigration-induced surface and grain boundary diffusion

The authors present a detailed analysis of capillarity effect in the presence of surface diffusivity variation and use this as a basis to describe evolution of the surface profiles generated by the electromigration-induced surface and grain boundary (GB) diffusion in a bicrystal film. Thermal GB grooving is asymmetric in the presence of diffusivity variation across the GB, but the extent of asymmetry is rather significant, less than 0.14% from the symmetric case, but up to 60% in diffusivity variation considered. Surface profiles of the electromigration-induced surface and GB diffusion for the samples with and without prior heat treatments have been carefully analysed and compared. The surface morphological changes can be understood in terms of the combined action of the following effects: (i) Dynamic balance between the surface tension and the GB tension incorporation with surface diffusivity variation across the GB generates further disparity in the profile changes. The effect lasts as long as the sample is under power stressing. (ii) The non-vanishing surface flux divergence with the GB acting either as a sink or source of the flux contributes dominantly to the GB disfiguration. (iii) Four additional transient terms originating from the capillarity effect aggravate the surface profile changes in the early stage of electromigration, causing surface undulation and so on, but gradually, the effect is overshadowed. Electromigration damages are mainly dictated by a quantity M which in turn is determined by the interplay between the GB flux and the surface diffusivity variation. With a negative M the profile starts with a sharp cusp which in time turns obtuse, then the minimum moves downstream into the grain. When M is positive, the GB profiles change rapidly with a small increase in M and in power stressing time. The profiles contract and protrude like a mound, and quickly rise above the x axis. Beyond this point, the shape of the surface morphology becomes relatively unchanged. Furthermore, when two or more GB diffusions are activated simultaneously, the superpositional effect of the GB fluxes may lead to trapezoidal growth and/or faceted voids, among other types of profiles. Finally, the problems with respect to further quantification of the quantity M are briefly discussed.

Void Growth in the Early Stages of Electromigration

Recently, a TEM study has shown that a high density(l010 to 1012/cm2) of small(<50A) voids are present in both as-deposited(l), and annealed(2) evaporated gold films. Using the defocus contrast technique, Lloyd and Nakahara(1) were able to make void size measurements which allowed a systematic study of the dependence of void formation on deposition parameters.In order to study the role of pre-existing voids in electromigration(EM) damage, ^lOOOA-thick gold films with initial void densities of lO10and1011/cm2 were obtained by evaporation onto KC1 substrates held at 280°C and 80°C, respectively. This choice of substrate temperatures was based on our previous study (l) of the effect of deposition parameters on void density.All films studied were found to contain many small voids which were revealed with the use of defocus contrast. EM-induced void growth was studied by comparing two films deposited onto a substrate held at 80°C during the same evaporation.