Electromigration behavior and reliability of bamboo Al(Cu) interconnects for integrated circuits

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999.Includes bibliographical references (leaves 103-108).by V.T. Srikar.Ph.D

Analysis of critical-length data from electromigration failure studies

An accurate estimation of the Blech length, the critical line length below which interconnect lines are immortal, is vital as it allows EDA tools to reduce their workload. In lines longer than the Blech length, either a void will inevitably nucleate and grow until the line fails, or the line will rupture. The majority of failure analyses reveal voiding as the failure mechanism however recent analysis suggest Blech length failures are characterised by simultaneous [6] voiding and rupture, and a non-zero steady-state drift velocity. This paper provides an alternative interpretation of results

Effects of scaling on microstructure evolution of Cu nanolines and impact on electromigration reliability

textScaling can significantly degrade the electromigration (EM) lifetime for Cu interconnects, raising serious reliability concerns. Different methods have emerged to enhance the EM resistance of Cu by suppressing the interface diffusion (the historically fastest diffusion path), notably using CoWP metal cap and Mn alloying. With further scaling of Cu interconnects, EM reliability becomes increasingly complex due to changes in Cu microstructure. In ultra-fine Cu lines a large population of small grains mix with bamboo-type grains, resulting in an additional contribution of grain boundary diffusion to EM degradation. With the interface diffusion suppressed by CoWP or Mn alloying, the grain structure effect becomes even more important. The objective of this study is to investigate the EM reliability of ultra-fine Cu interconnects, focusing on the scaling effect on grain structure and mass transport. First, the detailed microstructure information of Cu interconnects down to the 22 nm node was analyzed using a transmission electron microscope (TEM)-based high resolution diffraction technique. A dominant sidewall growth of {111} grains was observed for 70 nm Cu lines (45 nm node), reflecting the importance of interfacial energy in controlling grain growth. The strength of the {111} texture was found to significantly increase as line width was reduced to 40 nm (22 nm node), while the length fraction of coherent twin boundaries was reduced to ~1%. Secondly, the results from microstructure together with the deduced interfacial and grain boundary diffusivities were used to identify flux divergent sites for void formation and to analyze the grain structure effect on EM statistics using a microstructure-based kinetic model. Finally, based on the analysis of Cu grain structure evolution with downscaling, the scaling behavior of EM drift velocity was investigated for Cu interconnects with CoWP capping and Mn alloying. This enables us to project the EM lifetime and statistics for future technology nodes. The Mn alloying effect on mass transport in combination of grain structure control was found to provide an effective means to improve EM reliability especially with further scaling. In summary, this study establishes a correlation between the microstructure of Cu nanolines, void formation kinetics, and EM statistics.Mechanical Engineerin

Scaling and process effect on electromigration reliability for Cu/low k interconnects

textThe microelectronics industry has been managing the RC delay problem arising from aggressive line scaling, by replacing aluminum (Al) by copper (Cu) and oxide dielectric by low-k dielectric. Electromigration (EM) turned out to be a serious reliability problem for Cu interconnects due to the implementation of mechanically weaker low-k dielectrics. In addition, line width and via size scaling resulted in the need of a novel diffusion barrier, which should be uniform and thin. The objective of this dissertation is to investigate the impacts of Ta barrier process, such as barrier-first and pre-clean first, and scaling of barrier and line/via on EM reliability of Cu/low-k interconnects. For this purpose, EM statistical test structures, having different number of line segments, line width, and via width, were designed. The EM test structures were fabricated by a dualdamascene process with two metal layers (M1/Via/M2), which were then packaged for EM tests. The package-level EM tests were performed in a specially designed vacuum chamber with pure nitrogen environment. The novel barrier deposition process, called barrier-first, showed a higher (jL)[subscript c] product and prolonged EM lifetime, compared with the conventional Ta barrier deposition process, known as pre-clean first. This can be attributed to the improved uniformity and thickness of the Ta layer on the via and trench, as confirmed by TEM. As for the barrier thickness effect, the (jL)c product decreased with decreasing thickness, due to reduced Cu confinement. A direct correlation between via size and EM reliability was found; namely, EM lifetime and statistics degraded with via size. This can be attributed to the fact that critical void length to cause open circuit is about the size of via width. To investigate further line scaling effect on EM reliability, SiON (siliconoxynitride) trenchfilling process was introduced to fabricate 60-nm lines, corresponding to 45-nm technology, using a conventional, wider line lithograph technology. The EM lifetime of 60-nm fine lines with SiON filling was longer than that of a standard damascene structure, which can be attributed to a distinct via/metal-1 configuration in reducing process-induced defects at the via/metal-1 interface.Materials Science and Engineerin

Electromigration time-to-failure analysis using a lumped element model

This thesis presents a theoretical and computer simulation of electromigration behaviour in the Integrated Circuit (IC) interconnection, with a particular emphasis on the analysis of the time-to-failure (TTF) produced through the Lumped Element model. The current and most accepted physical model for electromigration is the Stress Evolution Model which forms the basis for the development of the current Lumped Element Model. For early failures, and ignoring transport through the grain bulk, the problem reduces to that of solving the equations for stress evolution equation on the complex grain boundary networks which make the cluster sections of the near-bamboo interconnect. The present research attempts to show that the stress evolution in a grain boundary cluster network mimics the time development of the voltage on an equivalent, lumped CRC electrical network. [Continues.

New methodologies for interconnect reliability assessments of integrated circuits

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000.Includes bibliographical references (leaves 245-251).The stringent performance and reliability demands that will accompany the development of next-generation circuits and new metallization technologies will require new and more accurate means of assessing interconnect reliability. Reliability assessments based on conventional methodologies are flawed in a number of very important ways, including the disregard of the effects of complex interconnect geometries on reliability. New models, simulations and experimental methodologies are required for the development of tools for circuit-level and process-sensitive reliability assessments. Most modeling and experimental characterization of interconnect reliability has focused on simple straight lines terminating at pads or vias. However, laid-out integrated circuits usually have many interconnects with junctions and wide-to-narrow transitions. In carrying out circuit-level reliability assessments it is important to be able to assess the reliability of these more complex shapes, generally referred to as "trees". An interconnect tree consists of continuously connected high-conductivity metal within one layer of metallization. Trees terminate at diffusion barriers at vias and contacts, and, in the general case, can have more than one terminating branch when the tree includes junctions. We have extended the understanding of "immortality" demonstrated and analyzed for straight stud-to-stud lines, to trees of arbitrary complexity. We verified the concept of immortality in interconnect trees through experiments on simple tree structures. This leads to a hierarchical approach for identifying immortal trees for specific circuit layouts and models for operation. We suggest a computationally efficient and flexible strategy for assessment of the reliability of entire integrated circuits. The proposed hierarchical reliability analysis can provide reliability assessments during the design and layout process (Reliability Computer Aided Design, RCAD). Design rules are suggested based on calculations of the electromigration-induced development of inhomogeneous steadystate mechanical stress states. Failure of interconnects by void nucleation in single-layermetallization, as well as failure by void growth in the presence of refractory metal shunt layers are taken into account. The proposed methodology identifies a large fraction of interconnect trees in a typical design as immune to electromigration-induced failure. To complete a circuit-level-reliability analysis, it is also necessary to estimate the lifetimes of the mortal trees. We have developed simulation tools that allow modeling of stress evolution and failure in arbitrarily complex trees. We have demonstrated the validity of these models and simulations through comparisons with experiments on simple trees, such as "L"- and "T"-shaped trees with different current configurations. Because analyses made using simulations are computationally intensive, simulations should be used for analysis of the least reliable trees. The reliability of the majority of the mortal trees can be assessed using a conservative default model based on nodal reliability analyses for the assessment of electromigration-limited reliability of interconnect trees. The lifetimes of the nodes are calculated by estimating the times for void nucleation, void growth to failure, and formation of extrusions. The differences between straight stud-to-stud lines and interconnect trees are studied by investigating the effects of passive and active reservoirs on electromigration. Models and simulations were validated through comparisons with experiments on simple tree structures, such as lines broken into two limbs with different currents in each limb. Models, simulations and experimental results on the reliability of interconnect trees are shown to yield mutually consistent results. Taken together, the results from this research have provided the basis for the development of the first RCAD tool capable of accurate circuit-level, processing sensitive and layout-specific reliability analyses.by Stefan P. Hau-Riege.Ph.D

Multiscale microstructures and microstructural effects on the reliability of microbumps in three-dimensional integration

The dimensions of microbumps in three-dimensional integration reach microscopic scales and thus necessitate a study of the multiscale microstructures in microbumps. Here, we present simulated mesoscale and atomic-scale microstructures of microbumps using phase field and phase field crystal models. Coupled microstructure, mechanical stress, and electromigration modeling was performed to highlight the microstructural effects on the reliability of microbumps. The results suggest that the size and geometry of microbumps can influence both the mesoscale and atomic-scale microstructural formation during solidification. An external stress imposed on the microbump can cause ordered phase growth along the boundaries of the microbump. Mesoscale microstructures formed in the microbumps from solidification, solid state phase separation, and coarsening processes suggest that the microstructures in smaller microbumps are more heterogeneous. Due to the differences in microstructures, the von Mises stress distributions in microbumps of different sizes and geometries vary. In addition, a combined effect resulting from the connectivity of the phase morphology and the amount of interface present in the mesoscale microstructure can influence the electromigration reliability of microbumps

Evolving microstructure: Mechanisms of electromigration in stressed aluminum-copper and copper films

We report on a collective body of work wherein we have studied the mass transport phenomena which are likely to be operative during stress driven changes in microstructure arising from electromigration and stress voiding. Our goal is to understand such microstructural evolution leading to failure of the metal lines or interconnects associated with integrated electronic circuits or chips. This work, when complete, will lead to improved electronics performance and reliability and faster product development arising from accurate and predictive models of wearout phenomena. We report on the role of thermal induced strain leading to hole and hillock formation, the influence of grains structure on the reliability of Al- based interconnects, and the observation of counter-current electromigration of Ua in Al grain boundaries

Multiscale microstructures and microstructural effects on the reliability of microbumps in three-dimensional integration

The dimensions of microbumps in three-dimensional integration reach microscopic scales and thus necessitate a study of the multiscale microstructures in microbumps. Here, we present simulated mesoscale and atomic-scale microstructures of microbumps using phase field and phase field crystal models. Coupled microstructure, mechanical stress, and electromigration modeling was performed to highlight the microstructural effects on the reliability of microbumps. The results suggest that the size and geometry of microbumps can influence both the mesoscale and atomic-scale microstructural formation during solidification. An external stress imposed on the microbump can cause ordered phase growth along the boundaries of the microbump. Mesoscale microstructures formed in the microbumps from solidification, solid state phase separation, and coarsening processes suggest that the microstructures in smaller microbumps are more heterogeneous. Due to the differences in microstructures, the von Mises stress distributions in microbumps of different sizes and geometries vary. In addition, a combined effect resulting from the connectivity of the phase morphology and the amount of interface present in the mesoscale microstructure can influence the electromigration reliability of microbumps