Electromigration in single-crystal aluminum lines with fast diffusion paths made by nanoindentation

Abstract

Segments containing a continuous region of plastic deformation were made in single-crystal aluminum conductor lines having widths near 2 μm using nanoindentation techniques. These segments act as fast diffusion paths similar to the polygranular clusters in a line having a near-bamboo grain structure. Segments of various lengths were created in lines having (111) and (110) planes parallel to the substrate, and these lines were subjected to accelerated electromigration testing with in-situ observations. A critical segment length below which no electromigration damage occurs at a given current density was found which is comparable to that seen in experiments using rectangular aluminum stripes (“Blech experiments”). Subcritical segments placed near one another were observed to interact to generate electromigration damage and failure. Results suggest that defects play a significant role in damage nucleation in near-bamboo lines. Single-crystal lines with (111) orientation have electromigration lifetimes an order of magnitude longer than those having (110) orientation. This difference is attributed to void motion, shape change, and growth processes.

Introduction

The dimensions of the aluminum interconnects used in integrated circuits have continuously decreased as the level of integration has increased. Electromigration-induced failures in these fine metallic lines can limit the reliability of integrated circuits and create a barrier to further miniaturization. As the width of interconnects decreases, changes in diffusion processes, and thus in electromigration failure mechanisms, take place[1]. In a sufficiently wide line, electromigration-induced diffusion occurs primarily along connected grain boundary paths (“polygranular clusters”). However, when the width of an interconnect approaches the average grain size of the film from which the line has been etched, a single “bamboo grain” can span the entire width of the line and diffusion is limited to the interface between the line and its oxide or to the crystal lattice itself. Conductor lines containing both bamboo grains and polygranular clusters are said to have “near-bamboo” grain structure. Because grain boundary diffusion is much faster than interface or lattice diffusion, the boundaries between polygranular clusters and bamboo grains in near-bamboo lines are sites of electromigration flux divergence, which can lead to electromigration damage through accumulation or depletion of material at these sites[1].

Several researchers have proposed that there should be a critical length for polygranular clusters in near-bamboo lines similar to the critical length observed in short isolated rectangular metal stripes1, 2, 3, 4. Blech et al. studied the electromigration behavior of the latter in experiments which have become known as “edge displacement” or “Blech” experiments[5]. The results showed that, at a given current density, there exists a minimum stripe length below which no electromigration damage occurs. This length is often called the “critical” or “Blech” length. This observation has been attributed to the development of normal stress gradients which oppose electromigration through back-flux forces[6]. In a short stripe where the stress gradient is large, the two counteracting forces (electromigration and back-flux forces) become balanced and no further electromigration can occur. In a long stripe, the stress gradient may never be high enough to stop electromigration. At the critical stripe length, L_cr, the two counteracting forces are just in balance. It has been shown[6] that the product of stripe length and current density, jL, must exceed a critical value, (jL)_cr, for electromigration damage to occur. This critical value can be correlated with other materials parameters by[6]

$$\text{(jL)}{}_{\mathrm{<mtext>cr</mtext>}}\text{=}\text{ΩΔ}\text{σ}\text{ρeZ∗}$$

where Ω is the atomic volume, eZ* the effective charge, ρ the resistivity, and Δσ the difference in normal stress between the cathode and anode ends of the stripe.

Polygranular clusters in continuous lines may also show critical length effects because the flux divergences at the ends of these segments are expected to be similar to those which occur at the ends of Blech stripes. If this is the case, then this critical length is one of the most important features associated with the electromigration resistance of near-bamboo lines. Accordingly, a number of groups have attempted to measure this length7, 8, 9, 10, 11. However, it is difficult to unambiguously determine a critical length in a near-bamboo line and, up to now, these efforts have not been successful. Some of these authors, e.g.[10], have concluded that it is likely that no critical length exists.

Difficulties in determining an accurate critical length in conventional near-bamboo lines arise from three sources: First, one must be able to correlate electromigration damage with specific polygranular clusters. Such measurements have generally depended either on simple observations of void–hillock7, 11 or void–precipitate8, 9 spacings on the assumption that these are related to the cluster length, or on complete TEM inspections of the microstructure[10] only after electromigration testing, typically after failure has occurred. Because electromigration voids tend to move away from their sources12, 13, 14, interpretation of such experiments can be difficult. Second, the lengths of polygranular clusters in such lines have been found to have exponential[15] or log–normal[4] distributions, so there can be a wide range of cluster lengths in any given line. A common view3, 4, 9 is that the longest clusters would be the first to generate damage leading to failure under electromigration conditions. Once a line has failed, the electromigration performance of shorter clusters in that line can, of course, not be investigated. Finally, it has been suggested2, 16, 17 that neighboring clusters may interact so as to facilitate electromigration damage. If this is the case, then even lines having no segments longer than the critical length can still fail by electromigration.

We have developed a novel method which avoids these experimental difficulties and allows us to study critical length effects in continuous lines in detail. Using nanoindentation techniques, a row of indentations is made in a single-crystal Al conductor line. The indentations are located closely enough together that the plastically-deformed zones under the indentations overlap, creating a “segment” (as it is called throughout this paper) containing a continuous region of local plastic deformation along the length of the line. We have shown[18] that, under accelerated electromigration testing conditions, diffusivity is enhanced in such a plastically-deformed segment, leading to the same kind of electromigration damage as that produced by a polygranular cluster in a near-bamboo line. We have also shown that the enhanced diffusivity is consistent with dislocation core diffusion along the dislocations induced by the indentations. With this method, we are no longer dependent on the grain structure of the tested material. Accordingly, we have made segments of various lengths and tested them under accelerated electromigration conditions to study critical length effects. Rows of indentations were placed sufficiently far apart so there is no ambiguity in correlating a specific segment with electromigration damage.

In addition, we have used this method to investigate two related effects which could not be investigated using conventional techniques. First, electromigration lifetimes have been reported to be strongly influenced by the texture of the conductor lines19, 20. Therefore, we have studied single-crystal lines with two different orientations; i.e. (110) and (111) planes parallel to the substrate. Second, we have also placed some plastically-deformed segments close to one another to investigate possible interactions.