Electromigration Conditions Research Articles

Spontaneous formation of a system of highly ordered germanium nanoislands upon epitaxial deposition on profiled (111) silicon substrates under electromigration conditions

Atomic-force microscopy was used to study the surface topography of SiGe structures grown by epitaxial deposition of Ge on profiled Si(111) substrates under electromigration conditions. Systems of highly ordered germanium nanosized islands with dimensions of 10–20 nm and a density of 6×1010 cm−2 were obtained. It is shown that the geometrical parameters of self-organizing nanoislands can be controlled by a proper choice of the growth and postgrowth annealing conditions for these structures.

Modeling and Simulation for Microelectronic Materials Research

Recent development in modeling and simulation for microelectronic materials research in the areas of Cu alloy interconnect is presented. A model for defect generation and diffusion through a Frenkel pair formation process at grain boundaries under electromigration conditions was developed. The model has consistently predicted the activation energies well for all the interconnect materials, namely Al, Al-Cu, Cu, and Cu alloys. A virtual simulation procedure was developed to screen a beneficial dopant to Cu based on the assumption that grain boundary (GB) diffusion dominantly controls the electromigration performance. The procedure covers dopant segregation to GB, dopant bulk diffusion, dopant and Cu diffusion along GB, and the effect of presence of a dopant on Cu diffusion at GB by dragging and blocking (stuffing) mechanisms. The success of the procedure has convincingly shown that modeling and simulation is the first choice to design advanced materials systems for next generation of interconnect.

Approximate Green's functions in boundary integral analysis

It is well known that employing a Green's function which satisfies the prescribed conditions on a part of the boundary is advantageous for boundary integral calculations. In this paper, it is shown that an approximate Green's function, one in which the known data is nearly reproduced, can also be highly beneficial in implementations of the boundary-element method. This approximate Green's function approach is developed herein for solving the Laplace equation, and applied to the modeling of void dynamics under electromigration conditions in metallic thin-film interconnects used in integrated circuits.

Non-linear analysis of the morphological evolution of void surfaces in metallic thin films under surface electromigration conditions

A theoretical analysis is presented of the non-linear dynamics of void surface morphology in metallic thin films under the action of an electric field that induces surface electromigration. Self-consistent dynamical simulations aided by the conclusions of linear stability theory reveal the important role of the anisotropy of void surface diffusivity, the strength of the applied electric field, and the void size in the dynamics. Special emphasis is placed on void dynamics in cases of low symmetry of surface diffusion anisotropy. Our simulations predict void faceting and wedge-shaped void formation, failure of the metallic films due to propagation of slit-like features from the void surface, as well as propagation of soliton-like features on void surfaces prior to failure. The simulation predictions are in excellent agreement with recent experimental observations.

The Determination of Copper Composition Profiles in Semiconductor Device Aluminum Interconnect Electromigration Test Lines using Electron Probe Microanalysis (EPMA)

Abstract Extensive long range transport of copper has been observed in polycrystalline aluminum-copper (Al-Cu) interconnects subjected to accelerated electromigration testing. Cu depletion is believed to be the rate limiting factor for electromigration damage accumulation. It is not clear if this is also the case for Al-Cu interconnects with bamboo/near-bamboo microstructures. In order to understand this phenomenon, Cu transport under electromigration conditions in bamboo microstructures has to be characterized quantitatively. This paper describes the procedure used to determine Cu distributions on a series of bamboo test structures subjected to accelerated electromigration (EM) testing. Copper composition profiles were determined for these structures through the use of electron probe microanalysis (EPMA) techniques and customized corrections for non-standard interaction volumes. Aluminum (0.5 wt.% Cu) interconnect test line structures were fabricated with four metal layers and surrounding silicon dioxide dielectric. Following EM testing, the samples were deprocessed to expose the test lines at metal layer three, as shown in Figure 1.

Theoretical Analysis of Faceted Void Dynamics in Metallic Thin Films Under Electromigration Conditions

AbstractA theoretical analysis is presented of the electromigration-induced dynamics of transgranular voids in metallic thin films. The analysis is based on self-consistent dynamical simulations of current-driven void surface propagation coupled with the distribution of the electric field in the metallic film. The simulation predictions highlight the rich nonlinear dynamics of current-driven evolution of voids that become faceted due to the strongly anisotropic nature of surface diffusion. The numerical results are analyzed based on approximate analytical solutions to faceted void migration and a linearized theory for the morphological stability of planar void facets.

Relationship Between Structure and Electromigration Characteristics of Pure Aluminum Films

ABSTRACTAluminum films were deposited from a high purity aluminum source by the self-ion assisted technique onto oxidized silicon wafers with TiN sub-layers. The ions were accelerated toward the substrate by potentials of 0, 3 and 6 kV. The films were patterned into strips 670 μm long and 8 μm wide using photo-lithographic procedures and wet etching. Average drift velocities were measured in the films tested under electromigration conditions. Electromigration activation energy was obtained for the films. It was found that electromigration activation energy increased with the acceleration potential. The strength of the (111) fiber texture, however, decreased with the acceleration potential. Therefore, the weaker textures resulted in higher electromigration activation energies. These results can be explained in terms of grain boundary structure, which controls electromigration behavior. By using orientation imaging microscopy to characterize the structures, it was shown that the weaker textured specimens contained a high fraction of low angle and low diffusivity grain boundaries.

Morphological stability of a heterophase interface under electromigration conditions

The evolution of the interface between two mutually insoluble metallic phases, under the influence of a strong electric field is examined. A slight perturbation of the interface away from a plane y=h(x) leads to a component of the electric field along the interface. This creates a diffusion flux of the individual atoms along the interface which, in turn, leads to an increase in the amplitude of the initial perturbation and thus to an interfacial profile instability. The processes is expected to be controlled by interface diffusion in response to three distinct driving forces: the electric field, internal stresses (which arise due to the accumulation or depletion of matter at the interface), and the interfacial curvature. The stress distribution along the interface was found from a self-consistent solution of the elastic problem. For the instability to occur, differences in effective atomic charges, elastic moduli and/or atomic mobilities of the two constituent metals are required. Small sinusoidal corrugations are shown to grow with time for a range of wavelengths. The corrugations can grow monotonically or vary in oscillatory manner, depending on their wavelength.

Analysis of damage formation and propagation in metallic thin films under the action of thermal stresses and electric fields

A quantitative analysis is presented of void formation, morphological stability, and evolution in metallic thin films, which are problems that underlie serious reliability issues in interconnections used in integrated circuits. The analysis is based on the coupling of bulk and interfacial mass transport phenomena with elastic deformations and current stressing. The formation and growth of intergranular voids in bamboo-structure conductor lines due to stresses that develop during processing is investigated. The investigation is aided by self-consistent simulation of bulk and grain-boundary diffusional processes using bicrystal models with elastic grains. A systematic analysis is presented of the morphological evolution of transgranular voids in passivated and unpassivated aluminum lines under current densities that are typical of electromigration testing. The effects of the electric field and surface properties on the morphological stability of voids are examined and morphologies that bifurcate from rounded or wedge-like void shapes are predicted. The theoretical results are discussed in the context of experimental data of void propagation under electromigration conditions.

Dynamics of transgranular voids in metallic thin films under electromigration conditions

A theoretical analysis is presented for the morphological evolution and stability of transgranular wedge-shaped voids in conductor lines with bamboo grain structure under an applied electric field. A linear stability analysis identifies void shapes that become unstable with respect to the growth of perturbations with wavelengths longer than a critical wavelength. The analysis predicts a critical wavelength that is consistent with experimental observations in unpassivated aluminum lines. The predicted changes in shape during the morphological evolution of the void surfaces are in agreement with the experimental observations.

Observations of electromigration induced void nucleation and growth in polycrystalline and near-bamboo passivated Al lines

Electromigration voiding in passivated pure Al lines was observed in situ using high voltage scanning electron microscopsy. Two different types of lines were investigated; one deposited under high purity conditions with a near-bamboo microstructure, and one deposited under conventional conditions with a polycrystalline microstructure with stress voids present. The samples were observed while being tested under accelerated electromigration conditions. Samples were then thinned and analyzed with transmission electron microscopy techniques to investigate the relationship between line microstructure and void nucleation. Electromigration voids and stress voids were seen to nucleate at very specific, unique, microstructural sites that require the intersection of a grain boundary with the line sidewall. Void movement after initiation is also dictated by microstructure, with voids only growing into and causing failure in grains oriented with a {111} plane near perpendicular to the line. The conditions for void nucleation suggest that heterogeneities at the line sidewall are necessary for void nucleation.

Theory and Computer Simulation of Grain-Boundary and Void Dynamics in Polycrystalline Conductors

AbstractMicrostructure evolution in polycrystalline metallic thin films under stress and electric current underlies many reliability issues in microelectronics. This paper presents a quantitative analysis of grain-boundary and void dynamics based on the coupling between bulk and interfacial mass transport phenomena with elastic deformations and current stressing. The formation and growth of intergranular voids in bamboo-structure conductor lines due to stresses that develop during processing is analyzed. The analysis is aided by self-consistent simulation of bulk and grain-boundary diffusional processes using bicrystal models with elastic grains. A systematic analysis is presented of the morphological evolution of transgranular voids in aluminum lines under current densities that are typical of electromigration testing. The effects of the electric field and surface properties on the morphological stability of voids are examined and morphologies that bifurcate from rounded or wedge-like void shapes are predicted. The theoretical results are discussed in the context of experimental data for void propagation under electromigration conditions.

Morphology of Damage in Al Films Tested Under Electromigration Conditions Using the Drift Velocity Method

AbstractPure Al films deposited by the partially ionized beam technique at bias Iavels of 0, 3 and 6 kV onto TiN sublayers were patterned for measurement for direct drift velocities. Then films were annealed in a vacuum 10−4 Pa for 1 hour. Electromigration tests were performed in an air atmosphere at a temperature of 280°C. The current density during tests was 6·105 A/cm2. Examinations in the SEM showed that the morphology of the damage on cathode and anode ends of the Al stripes depends on the bias during deposition. In the case of 0 kV bias, voids nucleate on the back and side edges of the cathode end and propagate toward the anode end. Hillock and whisker formation was observed on the anode end. The height and diameter of the whiskers were about 50 and 1–2 μm, respectively. In the case of 3 and 6 kV bias the drifting edge was smooth. Only hillocks (no whiskers) grew on the anode edge. The morphological differences of these films are discussed on the basis of self-ion bombardment effects.

Stress That Counteracts Electromigration: Threshold Versus Kinetic Approach

AbstractThe paper analyzes electromigration (EM) conditions and material properties that determine the maximum EM induced stress, σa, and stress gradient, ∇σ, which counteract EM flow in interconnects.The first systematic data on the drift velocity vs. stripe length, L, current density, j, and temperature are presented for Al lines. In contrast to the conventional approach to the Blech problem with σa taken to be a material constant (“yield strength”), the observations suggest that σa increases with j. The stress adjustment is shown to result from the imperative coupling of the net flux of material directed to the downwind end of the stripe with the flux of plastic flow (creep) responsible for stress relaxation. The effect of parameters of the constitutive equation assumed to describe the plastic flow kinetics, namely that of strain rate exponent, threshold stress, and creep, effective viscosity, on the stress cya is considered. To account for the creep viscosity, η, obtained unpassivated aluminum stripes from EM experiments, a model for the attachment-controlled Coble creep is suggested.

The Microstructural Nature of Electromigration and Mechanical Stress Voids in Integratedcircuit Interconnect

AbstractIt is well known that stress and electromigration induced voiding is of major concern for integrated circuit interconnect reliability. However, there has been little systematiccharacterization of void morphology and crystallography in ever more technologically important narrow, “near-bamboo” conducting lines. Prior reports indicate thatvoids are typically wedge or slit shaped, with failure often associated with slit voids.Void face habit plane is most often reported to be {111}. Wedge and slit void morphology and crystallography have been studied in comb/serpentine and parallel line array test structures. In virtually all cases, void faces are {111} oriented. In contrast to earlier studies, intragranular wedge stress voids have been observed. All electromigration opens were due to slit voids; these were typically intragranular, in contradiction to current theories of void formation, and likely are mechanical fractures. Under accelerated test conditions, non-grain boundary diffusion paths appear to operate at distances of tens of micrometers. Relative displacement between wedge voids and attached grain boundaries occurs where a wedge face lies on a near common {111} plane for the two grains. It is suggested that slit voids are intragranular under both stress and electromigration conditions and likely associated with local interconnect depassivation. Based solely on appearance and crystallography, no void can uniquely be identified as due to stress alone or electromigration alone.

On void nucleation and growth in metal interconnect lines under electromigration conditions

Electromigration failure in rigidly passivated metal interconnect lines is studied with particular reference to the vacancy supersaturations and hydrostatic stresses that can be developed at blocking grain boundaries under electromigration conditions. It is shown that the high stresses needed for homogeneous void nucleation to occur are probably too high to be developed by electromigration and that failure is more likely to involve the growth of pre-existing voids. We also show that the amount of void growth that can occur at a blocking grain boundary by electromigration of vacancies down the adjoining grain boundaries is small relative to the dimensions of the line unless the adjoining grain boundaries are continuous in a very long section of the line. This suggests that other mechanisms of void growth are responsible for electromigration failure. An analysis of the electromigration of small pre-existing voids shows that above a critical size, large voids migrate faster than smaller ones. This leads to a catastrophic process in which large voids can catch up with and coalesce with smaller ones, growing in size and migrating more rapidly as they do so. We conclude that the migration and coalescence of preexisting voids is a more likely mechanism of electromigration failure.

On void nucleation and growth in metal interconnect lines under electromigration conditions

On void nucleation and growth in metal interconnect lines under electromigration conditions

Atomistic and computer modeling of metallization failure of integrated circuits by electromigration

Nonstationary and stationary concentration profiles of vacancies in a grain boundary are calculated for the case of electromigration and various conditions of vacancy formation and annihilation. If supersaturation of vacancies is assumed for void formation, current densities necessary for voiding have to be larger than a critical value jcrit, which is related to the length l of the grain boundary by the relation ljcrit=const. An estimation of the value of the constant in the last equation is in agreement with experimental values. An approximate equation for the median time to failure (MTF) is derived from the model for grain boundaries conducting the flux of matter parallel or in series. The results explain many features of experimental studies. First values of MTF for a network of grain boundaries obtained by computer simulations are presented as well, and the effect of resistance heating and nonstationarity on the current exponent is discussed in some detail.