Unexpected Mode of Plastic Deformation in Cu Damascene Lines Undergoing Electromigration
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Unexpected Mode of Plastic Deformation in Cu Damascene Lines Undergoing Electromigration Arief S. Budiman1, N. Tamura2, B. C. Valek2, K. Gadre3, J. Maiz3, R. Spolenak4, W. A. Caldwell2, W. D. Nix1 and J. R. Patel1,2 1
Department of Materials Science & Engineering, Stanford University, Stanford, California 94305; 2 Advanced Light Source (ALS), Lawrence Berkeley National Laboratory (LBNL), 1 Cyclotron Rd., Berkeley, California 94720; 3 Intel Corporation, Hillsboro, Oregon 97124; 4 Max-Planck-Institut fur Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany. ABSTRACT An unexpected mode of plastic deformation was observed in damascene Cu interconnect test structure during an in-situ electromigration experiment and before the onset of visible microstructural damages (void, hillock formation). We show here, using a synchrotron technique of white beam X-ray microdiffraction, that the extent of this electromigration-induced plasticity is dependent on the line width. The grain texture of the line might also play an important role. In wide lines, plastic deformation manifests itself as grain bending and the formation of subgrain structures, while only grain rotation is observed in the narrower lines. This early stage behavior can have a direct bearing on the final failure stage of electromigration. INTRODUCTION Copper is a much better conductor than Aluminum but the difficulty in processing Cu to obtain patterned interconnect lines has long prevented its use in the semiconductor industry. With the advent of the damascene technique, Cu is now quickly replacing Al as the major metallization layer materials. Electromigration, the diffusion of the constitutive atoms of the metal lines under the influence of a high current density, still constitutes a major reliability challenge for the microelectronics industry.1 Several solutions such as the use of “shunt” layers, were devised to alleviate or completely stop this phenomenon to take place in lines with the current micron and submicron dimensions. However, with the ever decreasing size of interconnect widths, current densities will rise to new high levels and the technologies of today will not be sufficient to prevent the occurrence of electromigration damage. A thorough understanding of the mechanisms leading to electromigration damage is therefore needed.2-4 Recently, a very early stage of plastic deformation and microstructural evolution during an electromigration test was detected in Al(Cu) interconnect lines, long before any macroscopic damages become visible, by using a synchrotron technique involving white beam X-ray microdiffraction.5 In particular it was observed that during in-situ electromigration a gradient of plastic deformation evolves along the line which results in bending and in polygonization of the largest grains between the cathode and the anode end. Smaller grains do not readily deform but do rotate as electromigration proceeds. Plastic deformation is initiated at the cathode end and
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gradually progresses toward the anode end whil
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