Electromigration Phenomena Research Articles

Investigation of the role of compositional effects on electromigration damage of metallic interconnects

Compositional effects are usually observed in the early stages of electromigration of interconnects made of Al alloys like Al–Cu and Al–Si. These effects are due to a variation of the fraction of the solute species (Cu or Si) dissolved into the Al matrix mainly due to heating effects. Here we investigate the role of compositional effects on electromigration phenomena by using a dynamical percolation model that we have recently developed. Our results well reproduce several phenomenological aspects typical of electromigration experiments. This agreement, together with the flexibility of our approach, suggests wide possibilities of further extending the model to study also the geometrical and structural effects.

Electromigration modeling for integrated circuit interconnect reliability analysis

A simple one-dimensional continuum model for electromigration transport has been fairly successful as a tool for understanding many empirical observations of electromigration phenomena in encapsulated interconnect lines on integrated circuits. In this paper, the development of the one-dimensional continuum electromigration model is reviewed, Comparisons are made with previous models, emphasizing the important similarities and differences,.

A percolative simulation of electromigration phenomena

Resistance degradation of metallic strips due to electromigration is studied within a percolative approach based on a random resistor network. Both defect generation and recovery driven by a current stress are considered. The main features of experiments performed on Al–0.5%Cu lines are well reproduced, thus providing a unified description of degradation processes in terms of physical parameters.

A high-voltage scanning electron microscopy system for in situ electromigration testing

An apparatus has been constructed to conduct electromigration tests on realistic specimens while simultaneously observing them at relatively high magnification. A scanning transmission electron microscope has been converted into a high-voltage scanning electron microscope (HVSEM) with a large specimen chamber. By imaging with high-energy electrons (120 keV) and detecting backscattered electrons, voids in metal lines can be viewed through passivation layers. The HVSEM has a resolution of 50 nm through 1 μm of passivation. We have constructed instrumentation to heat and pass current through interconnect structures, while they are inside the electron microscope. Presently, the specimen temperature can be as high as 350 °C and is maintained constant to within 0.1 °C. The resistances of interconnects are measured with a precision of 0.05% during an experiment. Testing the lines at moderately accelerated conditions requires great stability of the microscope and instrumentation as well as full automation of the data collection. These requirements have been met, and metallization lines can be tested for several weeks with minimal operator intervention. Digital images of an entire 300-μm-long test structure as well as electrical data are stored automatically every few minutes during a test. The hundreds to thousands of pictures are analyzed using digital image processing techniques to extract void positions and sizes as a function of time. We use this system to characterize electromigration failure in advanced interconnect structures and to test existing theories on electromigration phenomenon.

Degradation mechanisms in polysilicon emitter bipolar junction transistors for digital applications

This paper is a thorough overview on polysilicon bipolar junction transistors’ (BJTs) reliability, with focus on transistors for digital applications, where the base–emitter junction switches from forward to reverse bias (low fields) and the base–collector junction is reverse biased at high fields. The effects of base–emitter reverse biasing are generation, charging and discharging of traps in silicon oxide or at the Si–SiO 2 interface near the base–emitter junction; their understanding is essential to model transistor current gain degradation and low frequency noise increase. Failure modes and mechanisms, degradation kinetics, lifetime models and physical phenomena related to device aging will be discussed. The base–emitter junction is also stressed by high currents, which lead, for example, to electromigration phenomena. The base–collector junction degradation is mainly due to high field and impact-ionization effects. Reliability constraints are now an important component of a correct design methodology in deep-sub-micron integrated circuits.

In situ observation of electromigration in micrometre-sized gold stripes by scanning force microscopy

In the present study we evaluate the morphology changes in evaporated gold thin stripes of micrometre sizes under high current densities. Using scanning force microscopy to image the time evolution of the metallic stripe, we obtain clear evidence of electromigration by an in situ observation of mass transport from cathode to anode. This mass transport is correlated with the formation of hillocks and voids, which present a spatial distribution that also indicates the influence of temperature gradients. Thus, we introduce a new technique to characterize, in a quantitative manner, the electromigration phenomena as a function of current density, temperature, etc. at the nanometre scale.

Electromigration Characterization Versus Texture Analysis in Damascene Copper Interconnects

AbstractWe have studied the effect of texture (X-ray diffraction pole figures) and grain morphology (Focus Ion Beam cross-sections) on the electromigration performances of copper damascene interconnects. Three different metallizations have been characterized : Chemical Vapor Deposition copper deposited on TiN (process A) and electroplated copper deposited either on Ta (process B) or TaN (process C). The reliability performance of these interconnects has been evaluated using both Wafer Level Reliability (WLR) and Package Level Reliability (PLR) tests on 4 and 0.6 νm wide lines using single metal level test structures. On the basis of the activation energy values and failure analysis observations, we concluded that interfacial diffusion plays a key role in the electromigration phenomenon for processes B and C whereas grain boundaries seem to be the active diffusion path for process A. The existence of several failure mechanisms during electromigration tests (interfacial or grain boundary diffusions), the impact of the damascene architecture on microstructure (sidewall textures and non columnar grain shapes) and the copper propensity for twinning seem to mask the impact of texture on the electromigration reliability of copper damascene interconnects.

Long term noise measurements and median time to failure test for the characterization of electromigration in metal lines

The high sensitivity, which can be obtained by means of noise measurements, is especially useful for the characterization of the early stages of the electromigration phenomenon. In addition, with the noise being strictly dependent on the sample microstructure, it is obvious to expect that it can be used for monitoring the different stages of the degradation process. In this paper, the results obtained by performing noise measurements during lifetime tests are presented, with the aim of investigating the evolution of the electromigration noise during the entire life of the sample. Conventional Median Time to Failure (MTF) tests have been performed on samples belonging to the same set. One of the aims of this work is to investigate the possibility of establishing a new failure criterion based on noise measurement capable of providing, in a shorter time, the same type of information normally obtained from MTF tests.

Modeling electromigration as a fluid–gas system

In this paper two problems in modeling electromigration are addressed. The first problem is an issue of principle concerning counting the number of variables and the number of equations for formulating a well-posed mathematical problem. The second problem deals with setting up a system of equations which are sufficiently detailed for performing computer simulations of the local dynamics governing electromigration phenomena, and which also keep the CPU consumption within acceptable limits. A model for electromigration, which is based on a fluid–gas picture for the material transport, is proposed for dealing with both questions. The flow of matter is described as a combined flow of lattice sites and a flow of vacancies. The gas component is used to describe the vacancy flux.

Electromigration in Al thin films induced by surface acoustic waves: application to imaging

The propagation of a high amplitude surface acoustic wave in an Al thin film induces a large-scale electromigration phenomenon resulting in a permanent etching of the acoustic field in the film. The etched patterns depend on the time of propagation and on the acoustic characteristics. Preliminary observations of a few grooved structures in Al films have been performed by different techniques. A first explanation of this phenomenon based on dynamical Grinfeld instabilities is proposed. By providing permanent pictures of acoustic fields emitted by transducers, this effect could be used to perform imaging of surface acoustic wave propagation.

A stochastic approach to failure analysis in electromigration phenomena

Resistance degradation of thin film conductors is studied within a stochastic approach based on a random resistor network. Both defect generation and recovery are considered and assumed to depend on the stressing current. The main features of available experiments are well reproduced thus providing a unified interpretation of degradation processes and failure in terms of physical parameters.

An x-ray spectromicroscopic study of electromigration in patterned Al(Cu) lines

We studied the surface properties of patterned Al(Cu) lines related to the electromigration phenomena using photoemission spectromicroscopy techniques. We stressed the lines for electromigration in situ in the ultrahigh vacuum microscope chamber and observed the changes on the line surface. Our results show surface precipitation of Cu beneath the Al2O3 layer on the line surface as well as on side walls. Enrichment of grain boundaries in Cu due to electromigration flux was observed in areas downstream of voids with respect to the electron flow.

Stress Effects on Al and Al(Cu) Thin Film Grain-Boundary Diffusion

AbstractStress effects on grain-boundary diffusion in Al and Al(Cu) thin films are evaluated through atomistic simulations. Specifically, the grain-boundary vacancy formation and migration and interstitial migration energetics are obtained as a function of stress states in thin film. In general, the activation energies vary at a rate of 0.1 eV per 1.0 % strain at the grain boundary investigated, indicating the possible impact on electromigration phenomenon in these films.

A New Approach to Calculate Atomic Flux Divergence by Electromigration

In this paper, a new calculation method of the atomic flux divergence used to predict the formation of electromigration-induced void is proposed by considering two-dimensional distributions of current density and temperature and also simply considering the line structure of not only polycrystalline, but also bamboo line. For the verification, electromigration phenomenon near the corner of an angled polycrystalline line, which results in two-dimensional distributions of current density and temperature, is treated as an example. The usefulness of the proposed method is discussed in the light of the comparison of prediction of void formation near the corner with the experimental result.

Electromigration testing of integrated circuit interconnections

The electromigration phenomenon has been one of the most intriguing physical problems in the semiconductor device reliability. The models to explain the phenomenon are here revised, together with the influence of materials and their microstructure. The various measuring techniques are described, including the design of special test patterns, and statistical data analysis is briefly reviewed.

Effect of Residual Stress on the Stability of Gold Thin Film Surfaces

The temporal evolution of the surface topography and x-ray diffractogram of gold thin films was obtained. We found by STM that the surface roughness decreased exponentially with time. X-ray analysis indicates that the microcrystals are strained i.e., the x-ray peaks shift with elapsed time towards the gold standard reflections. The rearrangement of the gold film is induced by surface strains that appear during film growth rather than by the thermal motion of atoms. A DC current was applied to the films to induce surface changes and to study electromigration phenomena. Results show that current effects accelerate film failure by surface diffusion produced by the growth of microcrystals at the expense of mechanically strained microcrystals.

In-Situ X-ray Photoemission Spectromicroscopy of Electromigration in Patterned Al-Cu Lines With Maximum

AbstractWe report the application of a unique photoemission spectromicroscope (MAXIMUM) to the study of electromigration phenomena in Al-Cu interconnects. MAXIMUM is a scanning type photoemission microscope that uses multilayer-coated optics to focus 130 eV x-rays to a sub-0.1 µm spot. An electron energy analyzer collects photoelectrons in a chosen spectral region of interest to form an image of the sample that is sensitive to chemical states of elements on the sample surface. Al-Cu lines were characterized by spectromicroscopy techniques before and after electromigration stressing in the UHV environment of the microscope chamber. We present spectro-micrographs showing the chemical and structural changes on Al-Cu line surfaces as a result of the electromigration process.

Electrical and thermal simulation of local effects for electromigration

The aim of this work is to investigate the distribution of the local current density and temperature gradients along the test patterns employed for the evaluation of the electromigration phenomena in metal tracks. The impact of the line shape is taken into account. The investigation has been performed by means of a 2D simulator based on a FEM solver.

In Situ REM Observation of Step Dynamics on Silicon Surfaces

Abstract Clear understanding of the structural and morphological transformations on the crystal surfaces can only be extracted from considerations of the dynamical properties of surface evolution. So ultra high vacuum reflection electron microscopy (UHV REM) has been applied to in situ studies of step behaviors on the silicon surfaces during various treatments. Dependence of the surface morphology on the number of parameters is reviewed with new results. Special attention is paid to influence of the electromigration phenomena and strain fields on structural evolution of the surfaces during sublimation, phase transition and epitaxial growth. The stability of the atomic step distributions is discussed in the frame of kinetical morphological transitions on silicon surfaces during DC heating of studied crystal [1]. The step motion during sublimation shows a strong influence of stress fields on the step configurations [2]. The importance of surface stress is obvious because in equilibrium conditions there is a residual stress on the surface due to existence of broken bonds.

Diffusion mechanisms at metallic grain boundaries

As the sizes of electronic devices continue to shrink, understanding key atomic phenomena vital to macroscopic processes become increasingly important. Mass transport along grain boundaries (GBs) is such a key process. We have studied diffusion mechanisms at metallic GBs of Ag and Al with the embedded-atom method, with molecular statics (MS) and molecular dynamics (MD), as well as with massively parallel computers at Oak Ridge National Laboratory and Sandia National Laboratory. Formation and migration energies of interstitials and vacancies at the optimal symmetric tilt GBs of different angles in Ag were first obtained using MS at 0 K. Extensive MD simulations were then carried out for selected Ag GBs using massively parallel computers at different finite temperatures up to the melting point. In this way, we were able to determine the dominant diffusion mechanism within different temperature regimes by comparison of the activation energies from MS results for the identified diffusion processes with the MD simulation results. For the first time, the results of this study on Ag GBs had provided realistic explanation and simulation-based evidence for the discrepancy between the activation energies from the recent low-temperature experiments by Ma and Balluffi and those at high temperatures reported in the literature. Preliminary MD results on Al GB diffusion are in excellent agreement with experiment and on-going work on Al and Al-Cu systems aimed to further understand electromigration phenomena will be briefly discussed.