Localized Measurement of Strains in Damascene Copper Interconnects by Convergent-Beam Electron Diffraction
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Localized Measurement of Strains in Damascene Copper Interconnects by Convergent-Beam Electron Diffraction Julie A. Nucci1, Robert R. Keller2, Stephan Krämer1, Cynthia A. Volkert1, and Mihal E. Gross3 1 Max-Planck-Institut für Metallforschung, Seestraße 92, D-70174 Stuttgart, GERMANY 2 N.I.S.T. Materials Reliability Division, 325 Broadway, Boulder, CO 80303, U.S.A. 3 Bell Labs, Lucent Technologies, Murray Hill, NJ 07974, U.S.A.
ABSTRACT Convergent beam electron diffraction (CBED) was used to measure localized lattice strains in damascene copper interconnects. This method provides data from areas of approximate diameter 20 nm, enabling evaluation of strain states within individual grains. Lattice parameters were determined by measuring the deficient higher order Laue zone (HOLZ) line positions in experimental zone axis patterns and subsequently comparing them to kinematical and dynamical simulations. Quantitative comparison was accomplished using a least squares analysis of distances between line intersections. Deposition-induced strains between 0.06% and 0.14% were measured in 2.0 µm wide lines. The uncertainty in strain determination was approximately 0.02%, as limited by the precision in HOLZ line detection. In addition to enabling localized analysis of strain states, another advantage of using CBED is that the microstructure can be fully evaluated. Used in conjunction with global methods such as X-ray diffraction, CBED may provide unique insight into localized failure phenomena such as electromigration void formation in damascene copper.
INTRODUCTION Several methods are used to measure strains, and hence to infer stresses, in thin films. X-ray diffraction and wafer curvature, which are the most commonly used methods, provide averaged information and result in the determination of a global strain state. Data from such measurements is often correlated to average microstructure, such as texture strength or median grain size. As a result of their global nature, insight into localized phenomena that occur within interconnects is difficult to realize using these approaches. Examples of such localized behavior include electromigration or stress-induced void formation, grain to grain variation in thermal stresses due to elastic anisotropy, and dislocation clustering during plastic deformation in interconnects. A method being developed for local strain measurement in interconnects is X-ray microdiffraction. Focussed beams of approximate diameter 1 µm [1] were recently produced using this technique. This is sufficient for obtaining strain information from larger individual grains. The main advantage of this technique is that no special specimen preparation techniques are needed; the measured sample strain state represents that of the as-fabricated structure. A disadvantage of this technique is that synchrotron radiation is required to produce such a finely focussed X-ray beam. While it is possible to extract local texture information using this
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