Thermal Failure Mechanism And Voltammetry Metrology For Cu/barrier/low K Integration
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The present study investigates the thermal stability of Cu/barrier/porous low κ (PLK) integration first, and then a new characterization method based on voltammetry is developed to characterize quality of diffusion barrier and pore structure in low κ materials and its thermal stability. The study of thermal stability reveals that Cu/barrier/PLK interconnect fails by Cu out-diffusion through the defects of diffusion barrier into the PLK structure, which is driven by oxidation and stress gradient within the interconnect structure. The failure appears to be triggered by defects in diffusion barrier, however, such defects are not effectively detected by TEM observation due to their small and localized nature. This motivates the development of a new method to characterize defects of diffusion barrier and pore structure of low κ materials in as-processed Cu/barrier/PLK interconnect. Firstly, a cyclic voltammetry-based method is developed to detect the quality of diffusion barrier by monitoring the current resulting from an applied voltage on the established cell. It utilizes a fact that electrolyte solution is able to infiltrate into the low k layer between two interconnects and creates a situation essentially the same as two-electrode electrolytic cell. When the barrier is intact (defect-free), the I-V shows simple hysteresis without the presence of current peaks. On the other hand, when the barrier is defective, Cu is exposed to electrolyte and current peak is present in the I-V curve due to Cu redox reactions. The application of the developed method on an extensive number of real interconnects provides sufficient evidence that the method is simple, fast, and accurate in detecting the defective barrier. Furthermore, it has a potential to quantify defect density based on the intensity of the current peak and the integration areas within the I-V curves. Secondly, a step voltammetry-based method is developed to characterize pore structure by measuring the effective ions diffusivity. The study produces the identical activation energy and diffusivity results for bulk solution which are in good agreement with references, and reveals that electrolyte ions migrate in dense low κ (DLK) and PLK with different mechanism. The application of the method reveals that pores in low κ materials are not thermally stable but can either collapse or coalesce depending on the stress conditions.