Propagation (advancement) of a fine slit along a planar grain boundary in an electric field E0, applied parallel to the slit, is investigated by considering electromigration along both the grain boundary and the slit surface. Steady-state solutions of the transport equations are classified according to the value of a parameter β, which, under reasonable assumptions, depends on material constants only: for 5π/4 ≥ β > β2, unique steady-state solutions exist, for β2 > β > β1, multiple steady-state solutions exist, and for β1 ≥ β ≥ π/4, no steady-state solutions exist. For all steadystate solutions, slit width and tip velocity v scale as and , respectively. Generally, v can approach 1 nm/s (3.6 μm/h), thereby representing a likely failure mechanism in fine-line (near bamboo structure) interconnects.

In order to simulate void motion and shape change of electromigration voids a numerical model was developed in which electromigration-driven diffusion on the void surfaces is assumed to act as the primary transport mechanism. The simulation describes the motion and shape evolution of a "two-dimensional void" having a simple initial shape in an isotropic medium. The current density distribution in the vicinity of a void was calculated by the application of a finite element method. Subsequently, the void shape changes by surface diffusion were examined using a finite difference scheme which includes the influence of gradients in curvature along the void surface. The model has been extended to allow other diffusion pathways, such as grain boundaries. The often observed faceting of voids and the formation of slit-like voids are discussed on the basis of simulations in which anisotropic surface tension and anisotropic surface diffusivity were assumed.

Kinetics of mass transfer in thin metal films during electromigration was analyzed as a function of the generated internal stress. Stress evolution was considered for the case when two different stress relaxation mechanisms can operate simultaneously near the anode end of the strip: thresholdless diffusional creep through a fixed aperture, and diffusional creep with a threshold stress, σt, through the same aperture. Stress distribution and electromigration rate were found as explicit functions of the film length, the aperture size, and the current density. The apparent threshold length was analyzed. It was shown that an apparent threshold product can grow with the current density, in agreement with the experimental results.

We explore the effect of faceting on possible mechanisms for mass transport around electromigration voids in aluminum interconnects. Motivated by linear response estimates which suggest that particle flux would be much higher along steps than across terraces on a clean aluminum surface, we study step nucleation in the presence of a small driving force along a surface. We find that step nucleation, even on a nearly defect-free void surface, would be slow if the step energy is equal to that calculated for a clean aluminum surface. In the presence of a uniform electromigration force, the creation of new steps between existing ones should not occur unless the free energy cost of a step is much less than thermal energies. We conclude that voids cannot move intragranularly at μm/hr rates without help from other factors such as local heating and impurities.

We present results of molecular dynamics (MD) simulation studies of Cu segregation to the Σ11 grain boundary (GB) in Al. The simulations were performed with Embedded Atom Method (EAM) potentials for Al and Al-Cu. The results predict that copper atoms tend to order along either side of the interface even at the pure symmetrical GB, forming alternating chains along the direction. The nucleation of the chains is driven by a change in the local atomic level stress induced by the pre-existing Cu atoms at the GB.

We have used transmission electron microscopy to examine in detail how microstructural changes take place in aluminum/copper lines adjacent to tungsten contacts. A structure that could be powered in a probe station was deposited directly on an electron transparent silicon nitride window to allow direct observation of the structure of the line after different intervals of electromigration testing. Observations made on aluminum-4% copper lines show that significant changes in the grain structure surrounding precipitates occur during testing even under conditions where no normal grain growth is observed in the lines. Elecromigration testing at 260°C resulted in grain coarsening and the formation of new diffusion blocking structures in the lines. By making measurements of the volume of individual precipitates as a function of time, the flux of copper through specific segments of the microstructure of the line could be measured. The results indicate that the presence of a single grain boundary can increase the electromigration flux of copper along the line by an order of magnitude. In line segments where a bamboo grain occurred copper diffusion appears to be controlled by diffusion along the Al/oxide interface at the surface of the lines. It is also shown that, in polycrystalline lines, the rate of copper depletion frorr the cathode is largely determined by the rate of dissolution of the precipitates. In mixed structure lines the migration of copper can become quite complex as changes in diffusion paths can occur as precipitates dissolve or grow.

In this paper the results of our investigations of passivated, narrow lines of both Al-Cu and Al-Si-Cu (0.4 at.% Cu, 1.0 at.% Si) by transmission electron microscopy will be presented. Thin alloy films have been deposited on either oxide or refractory-metal films. During the standard anneal treatment of 0.5 h at 400 °C the atomic mobility is relatively high. At this temperature the binary Al-Cu films are one-phase alloys. Depending on the cooling rate, transformation occurs into a two-phase mixture of (initially) finely distributed, Cu-enriched particles and Al. The precipitates eventually form the equilibrium θ-Al2Cu phase, which tends to form large conglomerates upon further annealing. In contrast, the ternary Al-Si-Cu films are two-phase alloys already at 400 °C: at this temperature Si precipitates are pre:sent in the Al matrix, whereas the onset of Cu precipitation is not far below this temperature.

We have studied the behavior of Al2Cu precipitates during electromigration in submicron runners using in situ plan view TEM. Al (0.5 wt. % Cu) runners of widths between 0.3 and 1.0 μ were stressed at temperatures between 200 and 275° C and a current density of 2×106 A/cm2. A greater number of precipitates nucleate per unit length in narrower runners (0.3 and 0.5 μ wide) than in wider runners (0.8 and 1.0 μ wide), although their average size is smaller in the narrower runners. The distribution of precipitates along the length of the runners is skewed towards the anode ends. Their number decreases and the skewing of their distribution increases with increasing temperature and linewidth. Precipitates are three times more likely to be found “downstream” (with respect to the electron flow) of a polygranular region and “upstream” of a blocking grain than the reverse. After their initial nucleation and growth, precipitates either decline in size or continue to grow during electromigration stressing depending on their position in the line and on the local grain structure. Grain boundaries profoundly influence the speed and direction of growth; however, the direction of growth cannot be predicted based on grain boundary inclinations alone. Precipitates tend to stay in one place and grow (or shrink) rather than migrate, in contrast to the behavior of voids which tend to migrate at higher temperatures. When precipitate migration occurs, it is either in association with void migration or by the replacement of small Al grains. Small precipitates remain compact as voids pass over or under them.

This study investigates the effect of solute additions on electromigration in Al-based thin film binary alloys. The “cross-strip” technique was used to observe solute electromigration and its influence on Al electromigration. The results of electromigration tests on five alloy additions, Ag, Au, Cu, Pd and Ni, are presented. It is concluded that beneficial solutes have two characteristics. First, they have a large, negative effective valence (Z**). Second, they have sufficient solubility in Al at test temperature to provide a reservoir of mobile atoms. Ag and Au are relatively ineffective because of their low effective valence. Pd and Ni appears to be relatively ineffective because of their low solubility at test temperature. Only Cu satisfies both criteria.

Patterned 930 nm Al(1%-Si) interconnects over 147 nm of Cu were electromigration lifetime tested at 1.0–1.5 × 105 A/cm2 at 250 °C. The morphology of the surfaces of the electromigrated stripes with different line widths and times to failure were characterized by atomic force microscopy, and changes in surface roughness were compared. The diffusion of copper into the electromigrated aluminum stripes was determined by depth profiling using Auger electron spectroscopy. In particular, areas where hillocks formed were examined and compared to areas of median roughness.

Electromigration tests have been performed on single-crystal Al lines which were predamaged by rows of submicrometer-depth indentations made using a nanoindentation device. Indentations were placed close to each other so that their plastic deformation zones overlapped. During subsequent electromigration testing at 280°C and I to 2 MA/cm2, no damage was observed in non-indented single-crystal lines, while the indented lines showed electromigrationinduced voids at the cathode-side ends of the indented areas, and hillocks at the anode-side ends. The voids grew and moved away from the indentations towards the cathode. Fhe electromigration damage morphology of the indented lines indicates that the mechanical damage generates a local fast diffusion path in the single-crystal lines, which is believed to be due to dislocation core diffusion. A minimum indentation row length was observed, below which no void formed. This electromigration behavior is found to be phenomenologically similar to that of polygranular clusters in near-bamboo lines.

It is known from literature that the properties of thin films greatly depend on their structuare. Therefore, the microstructural design is attractive for control over the properties of thin metal films used for interconnect metallization.

In this paper we discuss the potentialities of the self-ion assisted deposition technique for control over the grain and grain boundary structures of thin metal films and their properties such as resistivity and immunity to electromigration.

It was found that resistivity of aluminum films deposited at the 6 kV bias was virtually equal to resistivity of bulk aluminum. Films deposited at the less bias or without it had higher resistivities. Abnormal grain growth was found in 6 kV-films. In films prepared without bias normal grain growth proceeds.

The microstructure of 0.8.µm, A1(0.5% Cu) lines has been measured both before and after post-pattern annealing in nitrogen at various times and temperatures. Grain boundary inclinations in single grain segments of lines were determined from Focused Ion Beam (FIB) images of two surfaces of the lines. The statistical distributions of inclination angles before and after annealing reveal the tendency for boundaries to rotate to reduce their area. In these experiments the rotation stagnates with the normals to the grain boundaries having a distribution of inclinations with a FWHM of about 33 degrees around the line direction.

This paper reports the analysis of geometrical and microstructural effects on void formation in interconnects. Ion-beam machining is used to define segments for study at the cathode end of test lines. Scanning electron microscopy is used to observe damage development, focused ion beam microscopy to observe the corresponding grain structure. Finite-element calculations of self-consistent current density and temperature distributions in the conductor are used to predict damage locations both for a continuum material and for simulated grain structures. Cross-section changes in the line give temperature variations leading to divergences in atomic flux. Regions of high flux divergence are favoured for electromigration damage, but the precise sites of damage are determined by the grain structure, as shown both in the experiment and in the modelling.

The development of electromigration-induced voids and hillocks in Al - 4 wt. % Cu interconnects is monitored by scanning electron microscopy during interrupted testing and is correlated directly with the actual grain configuration including precipitates. The short segments under study and their grain structures are defined and observed using focused ion beam microscopy. The Cu content in precipitate grains is swept away before electromigration damage, and at most such grains there is subsequent grain thinning. The observations are compared with the results from a computer simulation based on a finite-element calculation of self-consistent current density and temperature distributions. For the first time the simulation uses the actual grain configuration and incorporates Cu atom migration, and back-fluxes driven by stress and concentration gradients. In the simulation the grain-boundary diffusivity is taken to be independent of boundary misorientation or is varied according to randomly assigned orientations. The comparison of the voiding in these two simulated cases and the observations shows that some grain configurations are very susceptible to electromigration damage whatever the diffusivities. For most configurations, however, the misorientation dependence of grainboundary diffusivity is significant and must be included if simulations are to be realistic.

Thin-film Al-2Cu conducting lines with “quasi-bamboo” microstructures were investigated to understand the microstructural mechanism of electromigration failure. Both conventional test structures and electron-transparent lines fabricated on silicon nitride windows were utilized to identify the “weakest” polygranular segments. Even when the current density was reduced to 0.75 MA/cm2 and the segment length was on the order of a few microns, failure occurs at the upstream termination of the longest polygranular segment in the line, at a time that decreases exponentially with the segment length. There is no apparent “Blech length” in quasi-bamboo Al- Cu lines; the longest segments are the failure sites, and their lifetime decreases with segment length in a regular way, even when the longest segments are only a few microns in length. It follows (and is observed) that the time-to-failure distribution of a group of lines is fixed by the distribution of the longest polygranular segments within them. This distribution can be effectively controlled by post-pattern annealing, which can refine the quasi-bamboo structure so that the longest polygranular segments are short and the distribution of longest polygranular segment lengths is narrow. Consequently, post pattern annealing is a very effective method for improving reliability by increasing the time to first failure.

Material anisotropy implies that many film properties are affected by crystallographic orientation in the growth direction (out-of-plane texture) and / or in the plane of growth (in-plane texture). Physical vapor deposited (PVD) Ti and Al-alloy films deposited on silicon dioxide substrates typically exhibit strong fiber textures in the growth direction with little in-plane-texture observed. The strength of these fiber textures has been found to vary substantially depending on the details of the deposition process(es) and, to a lesser degree, on any post-deposition anneals. In this paper the role of the substrate surface roughness in defining film texture is reported. It was found that the substrate surface roughness determines the overlying film crystallographic orientation for Ti and Ti/AlCu films deposited on various oxides. Furthermore, it was found that the texture of the initial metal “seed” layer defines the texture in subsequently deposited films (texture inheritance). Modifications to the oxide surface which decrease the surface roughness lead to an improved crystallographic texture in Ti, AlCu, Ti/AlCu and Ti/TiN/AlCu films. Film orientation was determined from crystallographic pole figures measured using x-ray diffraction (XRD). The oxide surface roughness was measured using atomic force microscopy (AFM), transmission electron microscopy (TEM) and glancing incidence x-ray reflectivity (GIXR).

The evolution of grain structure of AI-1%Si-0.5%Cu film, deposited by sputtering at 150'C substrate temperature has been evaluated, comparing as deposited film microstructure and grain structure at the end of process, both on large metal areas and on narrow strips. Microstructure has been characterized by means of FIB, TEM and XRD analyses. Grain size evolution resulted to be strongly dependent from the type of substrate. In particular TiN substrate appeared to greatly inhibit AlSiCu grain growth. This behavior was attributed to the formation of Ti9AI23 intermetallic compound during the alloy thermal treatment, that pins grain boundaries thus limiting Al grain boundary mobility.

By using TDEAT and ammonia gas, MOCVD TiN films were grown at 300°C and 30torr.After annealing in a vacuum furnace at 500°C, 550°C, and 600°C, the TiN films were investigated by transmission electron microscopy (TEM). After annealing at 550°C, both hexagonal and cubic AIN phase were found at the interface between Al and TiN films. A Ti rich phase such as Al3Ti was also found at Al side. The failure mechanism during high temperature annealing is discussed.

The major sources of impurities in sputtered Al alloy films for interconnects, prepared by physical vapor deposition include those originating from the target material, residual gases present in the vacuum system, and those introduced through the gas delivery system. In this study, we report the effect of impurities incorporated from residual gases present in vacuum systems on the electromigration performance of 0.6 μm wide Al-l%Cu lines. Controlled leaks of isotope gases H2O18, N215, O218, and C13H4, in 10−6 10−9 Torr range, were introduced into a PVD tool during the sputtering process. Using these isotope gases, the impurities originating from residual gases were distinguished from those originating from other sources of impurities. The sputtering target was found to be the major source of H and O impurities in the film, whereas N atoms are introduced in the film mainly through the gas phase. Furthermore, N atoms in the film were found to affect its electromigration behaviour to a larger extent than O and H.