The relative change in resistance due to electromigration was studied in thin (0.7 μm) film conductors of AI-0.5% Cu Alloy passivated with a 1 μm thick glass passivation using a sensitive AC bridge technique. In contrast to previous experiments performed on unpassivated structures where a roughly linear resistance increase was observed, a saturation value for the resistance increase was observed which was seen to be a function of temperature and the applied current density. The results were found to be consistent with a Shatzkes and Lloyd electromigration model.

To compare the results obtained by 1/f noise, high-resolution resistance and electromigration lifetime measurements, the effect of annealing at 400 °C in forming gas of Al and AI(Si) thin films has been studied. All samples of a given composition have been fabricated simultaneously. Upon annealing, the 1/f noise measured at room temperature decreases by a factor of two whereas the lifetime increases by a factor 3 to 6. The activation energies derived from the 1/f noise and the high-resolution resistance measurements during and after stressing with a DC current are closely correlated and compare well to values reported for lifetime measurements. Comparing the data of the different experiments suggest that the lifetime experiments predominantly probe the presence of weak spots in the films whereas the noise measurements respond more to local disorder.

The 1/f noise in heat-treated Al and AI-Cu thin films was investigated at a current density of 20 mA/μm2 from approximately RT to 200 C. The results were consistent with the Dutta-Dimon-Hom model, i.e., the noise is the result of a collection of two-state systems. From the temperature dependence of the noise, a modified Dutta-Dimon-Hom model was used to extract the energy of formation, ΔHf, and the energy of migration, ΔHm, of the noise process. The development as well as simulations of this model will be discussed. It will be shown that ΔHm can be determined through investigation of the temperature at which the noise magnitude peaks for many frequencies. It will also be shown that ΔHf can be determined by investigation of the temperature at which the noise magnitude peaks and the temperature at which a, the negative of the logarithmic slope of the power spectral density, passes through 1. For both the Al and Al-Cu films, ΔHm was approximately 0.7 eV. The results also indicated that ΔHf was ≤ 0.1 eV for the Al films, and only slightly higher, i.e., 0.1 - 0.2 eV, for the Al-Cu films. The temperature at which cx passes through I was similar for both the Al and AI-Cu films. These results strongly suggest that the l/f noise is caused by the motion of Al atoms along the grain boundaries.

A computer model is presented and used to simulate the motion of vacancies in a crystalline lattice. The lattice is modeled as a resistive array connected in series/parallel with vacancies represented as elements of high resistance in the array. Time series and spectra are generated by allowing the vacancies to move and these are consistent with actual waveforms produced by thin metal films. Simulation of temperature change yields dependencies similar to those predicted by Dutta and Horn (1979). By varying the density of vacancies the model predicts a variable Hooge parameter that saturates to a constant beyond a critical value.

This paper summarizes the main ingredients of a general theory for a quantitative description of dynamical processes in heterogeneous materials under stresses, thermal cycling, or current. The theory is derived analytically from the atomic scale using the principles of quantum mechanics and statistical mechanics, without any empirical postulates. The equations describe the cross-coupled phenomena of stress-induced diffusion, diffusion-induced stress, electromigration, void growth, dislocation climb, and slip. The laws of continuum mechanics are recovered as a subset of the general equations. All constitutive relations can be constructed by a general and systematic procedure.

A systematic analysis is presented of the mesoscopic mechanisms that govern the structural evolution of polycrystalline conductors under the action of applied electric fields. A recently derived set of general dynamical mesoscopic equations is specialized to study the behavior of microstructural building blocks. Scaling analysis is presented for the dynamics of the corresponding transport and micromechanical processes associated with a wide range of time scales. Results of numerical simulations are presented for vacancy migration along grain boundaries, evolution of current-induced stresses, surface diffusion, and void stability and growth. The calculated dependence of characteristic times on current density is discussed.

The solution to the finite electromigration boundary value problem has been obtained for the case of a blocking boundary downstream and a constant vacancy concentration upstream. The results are similar to the solution to the semi-infinite problem [1] except in the vicinity of the steady state. Consequences of achieving a “sub-threshold” steady state profile are discussed.

Three conditions of pure aluminum films were deposited, patterned into electromigration test structures, and annealed at 400°C for I hour. The grain size distribution was essentially the same for all conditions, but the textures were substantially different. Electromigration failure distributions were developed at 225°C and 1 MA/cm2 for line widths of both 1.8 μm and 0.5 μm. Three conditions were tested at the wider line width while the strongest and weakest textures were evaluated in the narrow line width. Texture exerts a dominant effect at the wider line width where the lines are polycrystalline. The values of t50 increase as texture becomes stronger although σ varies from <0.5 for strong textures to >1 for a bimodal failure distribution in the weakest texture. In the narrow lines, the texture effect is substantially reduced, and the failure distributions are bimodal with a few early fails followed by a monomodal distribution characterized by a low cr. The electromigration behavior is discussed in terms of both texture and the line width to grain size ratio.

Transgranular slit-like voiding has been observed to cause fatal defects in electromigration- or stress migration-damaged Al submicron interconnects. To study the morphology and crystallographic characteristics of transgranular voids, we have tested single crystalline Al lines with various textures and in-plane orientations. Lines with (100), (110), and (111) textures were found to have triangular erosion voids along their edges. These voids were generally bound by {111} facets. While triangular erosion voids are frequently found in the lines, not all lead to failure of the lines. Some lines fail due to narrow, slit-like voids which are bound by {111} planes and lie along either <110> or <112> in-plane directions, depending on the orientations of the crystals relative to the orientations of the lines. Some of the slit-like voids are initiated along the erosion void facets. Similar void morphologies are observed in both unpassivated and passivated lines. Some of the lines have very large voids at the negative pad implying that voids are mobile. Through continued studies of the failure rates and failure morphologies of single crystalline lines, the mechanisms of transgranular failure of polycrystalline lines should be clarified.

The microstructural evolution of unpatterned Al-2wt.%Cu thin films has been examined to elucidate the mechanism by which copper improves electromigration resistance. After annealing at 425°C and cooling to room temperature at a rate of approximately 1°C/min., the microstructure of the Al-Cu films consisted of 1 μm aluminum grains with θ-phase Al2Cu precipitates at grain boundaries and triple points. The grain size and precipitation distribution did not change with subsequent isothermal heat treatments in the temperature range of 200° to 400°C. Al-Cu thin films annealed at 400°C, a temperature just below the Al/Al+θ solvus, exhibited microstructures in which the aluminum grain boundaries were depleted in copper except for the presence of the pre-existing large, widely dispersed AI2Cu precipitates. Al-Cu thin films annealed at 200° to 300°C were enriched with copper at the aluminum grain boundaries. The large, widely dispersed Al2Cu precipitates remained after the lower temperature anneals. From these results, it is proposed that the presence of copper in aluminum thin films improves electromigration resistance due to the precipitation of a thin film of Al2Cu, or a substoichiometric precursor, along the grain boundaries. The grain boundary phase retards grain boundary diffusion in the thin films, thereby reducing total mass transport and improving electromigration resistance.

Unpassivated 1.4 mm long lines of Al-4wt.%Cu metallization have been successively imaged (by scanning electron microscopy) and electromigration stressed until failure. The resistance of lines, evolution of line microstructure and the development of electromigration damage are thus discontinuously recorded through the accelerated life-testing (260°C, 2 × 1010 A m-2). Correlations are made among microstructure evolution, electromigration damage development and line resistance. The probable mechanisms of damage development are discussed.

Copper thin films were deposited by sputtering, electron beam evaporation, and electroless plating onto nitride membranes for TEM analysis. The samples were heat treated in-situ from room temperature to 600 °C for structural and chemical analysis. The as-deposited and heat treated microstructures were investigated. Orientation changes with heat treatment and reactions among the sample layers were analyzed by electron diffraction. This work provides baseline information for a study of the thermal evolution of copper lines.

The reliability of interconnects is strongly affected by their thermal history, including the deposition temperature, the temperature at which patterned films are passivated, and the temperature and cooling rates of the final packaging processes. The thermal history affects the stress, the grain structure, and the distribution of alloy additions, such as Cu. All of these affect the reliability of interconnects, especially as it is limited by electromigration. The postpatterning thermal history is especially important. For example, it is post-patterning annealing which leads to the evolution of electromigration resistant bamboo-microstructures. In Al-Cu alloy interconnects, pre- and post-patterning anneals can also affect the distribution of Cu in solution and in precipitates. This too can affect interconnect reliability. Experiments and modeling concerned with these effects of thermal history will be reviewed and discussed.

The microstructure of 1 μim thick Al films containing 0.5 and 2%Cu (weight percent), 0.3%Pd, and 0.3%Pd-0.3%Nb were investigated by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS) as a function of isochronal and isothermal anneals. The grain size, grain size distribution, and precipitate morphology of these films was measured from 200 to 500ºC, with the activation energy for grain growth (Ea) determined for I h anneals at 200, 300, 400 and 500ºC. Normal grain growth was recorded for the A1Cu films annealed at temperatures ≤400ºC; however secondary grain growth occurred in the AI-2Cu film annealed for I h at 500ºC, with grains as large as 16 μm in diameter observed. Grain growth in the AI-0.3Pd films resulted in strongly bi-modal grain size distributions, with the onset ofsignificant grain growth retarded for I h anneals at temperatures ≤300ºC.The addition of Nb to the AI-0.3Pd film resulted in monomodal grain size distributions over the entire temperature range. The role of crystallographic texture on grain growth in thin films is discussed.

The effect of solutes and refractory-metal laminates on the electromigration resistance of Al films is consistent with the effect of those variables on Al grain growth during deposition and/or post-patterning anneal.

It 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.

Strongly non uniform Fe depth distributions have been observed in AI(0.13 at% Fe) thin films deposited at temperatures of 350ºC and above. The concentration of Fe is uniform in depth at a deposition temperature of 300ºC but is increasingly enhanced toward the substrate interface at 450ºC. Subsequent annealing produces only a slight redistribution of Fe. The Fe is primarily present as precipitates smaller than 100 nm. A model of grain boundary diffusion of Fe and precipitate formation and grain growth is proposed to explain the observed behavior.

The Cu distribution in AI(Cu) thin films has been examined in blanket and patterned samples as a function of annealing. The Cu concentration in the Al grains, measured at room temperature, closely follows the solubility at the anneal temperature when a high cooling rate is used (-100 or -200°C/min) from the annealing temperature. With fine lines, the room temperature Cu concentration follows the solubility at the anneal temperature only at low anneal temperatures, ≤350°C. With higher temperatureannealing, >400°C, the room temperature Cu concentration in the Al grains was substantially less than the solubility at the anneal temperature. These differences are attributed to the smaller grain size in fine lines, which reduces the distance to grain boundaries. With blanket films, the Θ-phase (Al2Cu) precipitate morphology depends on the Al grain size and annealing temperature. With small Al grains (100-300 nm), the Θ particles are small (100-200 nm) and round whereas with large Al grains (0.5-2μm), the precipitates are long and irregularly shaped. The morphology of the Θ precipitates is constrained by the Al grain size. With fine lines, having a bamboo structure, the Θ precipitates are “wedge” shaped along grain boundaries or span the width of the line. The electromigration lifetime was found to depend strongly on the heat treatment used, an increase of 3X in lifetime was obtained for samples rapidly cooled from a temperature above the solvus curve as compared with samples subjected to additional aging at low temperatures. This is believed to be dueto the different concentrations of Cu in solution and to the size and distribution of Θparticles in the patterned lines.

A laser based non-destructive technique has been used to study the morphology of sputterdeposited aluminum alloy films. The data emanating from the Therma-wave Imager that makes use of this principle, has been correlated with reflectivity, grain size and micro-roughness of the film. In addition, through the use of a case study, this paper demonstrates the utility of this application as an in-line monitor in an integrated circuit fabrication line.

Well known principles governing metallic corrosion are summarized in this paper. Severalelectrochemical techniques that are suitable for evaluation of corrosion reactions and mechanisms are discussed. The challenge of minimizing metallic degradation in processing, in storage and during operation is discussed with examples drawn from thin films used inthe electronics industry.