Synergy between chemical dissolution and mechanical abrasion during chemical mechanical polishing of copper

Copper has been chosen as an interconnecting material in integrated circuits (IC) because of its low resistivity and high electromigration resistance compared to aluminum and aluminum alloy interconnects. It has been shown recently that the patterning difficulty of copper is resolved by the dual damascene technique, combined with a chemical/mechanical planarization (CMP) process. CMP is becoming a very promising mainstream semiconductor processing method because of its demonstrated ability to achieve better local and global planarization for various materials, such as silicon oxide, tungsten and copper. However, the CMP process is influenced by a set of factors, which lead to a poor understanding of the material removal mechanisms involved during the CMP. The absence of reliable physically based models inhibits the migratability of the lab scale experiments to industrial practice. This work addresses the synergistic role of chemical dissolution rate (CDR) and mechanical abrasion rate (MAR) on the material removal mechanisms during CMP process. Initial in situ wear test in a chemically active slurry showed an increased material removal rate (MRR) relative to dry wear test. To understand the synergistic effects between CDR and MAR, two plausible mechanisms of material removal; (i) chemical dissolution enhances mechanical abrasion and (ii) mechanical abrasion accelerates chemical dissolution, were investigated. In addition, a phenomenological material detachment mechanism based on scratch intersections to understand the role of consumables (e.g. slurry particle concentration and size, pad surface morphology and stiffness) and the process parameters (e.g. applied pressure and relative sliding velocity) on the MRR was formulated. The proposed models, through their mechanistic description, will facilitate an exploration of the design space and identification of realistic CMP process domains. Moreover, it would enable understanding the root causes of defect generation mechanism and render remedies for yield improvements. For the mechanism of mechanical abrasion enhanced by chemical dissolution, it is proposed that a soft layer of chemical products is formed on top of the polished surface due to chemical reaction at a rate much faster than the mechanical abrasion rate. This is then followed by a gentle mechanical abrasion of that soft layer. A combined experimental and modeling technique was devised to understand the mechanical properties of the soft layer formed due to chemical exposure in the CMP. The developed experimental approach, in collaboration with Hysitron Inc., utilized nano-scratch and a nano-dynamic mechanical analysis (DMA) tests under dry (without chemical exposure) or wet conditions (with chemical exposure). The methodology combines the limit analysis solution of surface plowing under a spherical traveling indenter to analyze the nano-scratch experimental measurements, in order to deduce the ratio of the film (soft layer) yield strength to the substrate yield strength as well as the film thickness. A nano-dynamic mechanical analysis (DMA) test was performed. The measured apparent modulus and hardness from a set of DMA tests at different indentation depth through the composite soft layer-substrate structure were assessed in view of a detailed finite element simulation for the single-layer structure to deconvolute the ratio of film elastic modulus to substrate modulus. It was found that, for pure copper treated with ammonium hydroxide solution, the yield strength of the film is about 50% of the substrate yield strength, and the film modulus is about 20% of the substrate modulus. The film thickness was found to be on the order of few nanometers, and increases with the exposure time. For the mechanism of mechanical abrasion accelerating chemical dissolution, the wear depth and surface topography evolution were investigated. We made use of the established fact of stress-enhanced chemical dissolution, wherein a flat surface of a stressed solid is configurationally unstable under chemical dissolution. The roughness with wavelength above a critical value grows while roughness of lower wavelength decays during dissolution. Here, a nano-wear experiment was performed to induce the local variation of the residual stress level, followed by chemical dissolution to investigate the variation of the wear depth and the evolution of surface topography due to dissolution. It is found that the residual stress caused by the mechanical wear enhances the chemical dissolution rate, as manifested by an increase in wear depth. The wavelength selectivity for stressed solid under etching could be beneficial to achieve a global and local smooth surface, since the mechanical polishing helps smooth the long wavelength and the chemical etching helps smooth the short wavelength. The developed understanding from these experiments can be used in future studies to control the relative rates of CDR and MAR as well as investigating the various processinduced defects, like dishing, erosion, scratches, corrosion. The results can also be used to optimize process parameters, including: (i) particle shape, size and concentration; (ii) adapting slurry chemistry for required rates of chemical dissolution and mechanical abrasion; and (iii) selecting pads with the proper surface morphology and stiffness.