The Effects of the Mechanical Properties of the Confinement Material on Electromigration in Metallic Interconnects
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The Effects of the Mechanical Properties of the Confinement Material on Electromigration in Metallic Interconnects Stefan P. Hau-Riege and Carl V. Thompson Department of Materials Science and Engineering, M.I.T., Cambridge, Massachusetts 02139 ABSTRACT New low-dielectric-constant inter-level dielectrics are being investigated as alternatives to SiO2 for future integrated circuits. In general, these materials have very different mechanical properties from SiO2. In the standard model, electromigration-induced stress evolution caused by changes in the number of available lattice sites in interconnects is described by an effective elastic modulus, B. Finite element calculations have been carried out to obtain B as a function of differences in the modulus, E, of interlevel dielectrics, for several stress-free homogeneous dilational strain configurations, for several line aspect ratios, and for different metallization schemes. In contradiction to earlier models, we find that for Cu-based metallization schemes with liners, a decrease in E by nearly two orders of magnitude has a relatively small effect on B, changing it by less than a factor of 2. However, B, and therefore the reliability of Cu interconnects can be strongly dependent on the modulus and thickness of the liner material. INTRODUCTION Electromigration continues to be one of the most important reliability issues for integrated circuit metallization systems [1]. Electromigration is electronic-current-induced atomic diffusion due to momentum transfer from flowing electrons to host atoms. As atoms electromigrate, volumes in which atoms accumulate develop more compressive stresses, while volumes from which atoms are depleted develop more tensile stresses. Electromigration-induced stress evolution in interconnects has been successfully described by the Korhonen model [2]. In this model, the relationship between a change in the number of available lattice sites per unit volume, dC, and a change in the hydrostatic stress, dσ, is described through dC dσ , =− C B
(1)
where B is an effective elastic modulus. B has been calculated analytically for an elliptical aluminum interconnect embedded in an infinite SiO2 or Si matrix [2], based on Eshelby’s theory of inclusions [3]. The Korhonen model was originally developed for SiO2-embedded Al-based metallization systems for which grain boundaries provide the fastest diffusion paths for electromigration [4]. However, the increase in the ratio of the wiring delay to the total intrinsic transistor delay has provided the motivation for the IC industry to move from aluminum-based interconnects embedded in SiO2 to copper-based metallization systems with inter-level dielectrics (ILD) having lower dielectric constants, k, than SiO2 [5]. Low-k ILD’s are often polymer-based, and are mechanically much softer than SiO2 [6], having Young’s moduli that are more than an order of magnitude smaller than that of SiO2. A change of the mechanical properties of the ILD alters the degree to which electromigration-induced stresses build up in
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