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    Modeling and optimization of chemical mechanical polishing of semiconductor materials
    Akbar, Wazir; Ertunç, Özgür; Ertunç, Özgür; Şendur, Polat; Başol, Altuğ; Demir, E.; Nizamoğlu, S.; Department of Mechanical Engineering; Akbar, Wazir
    Chemical mechanical polishing (CMP) is a process of planarization of metal and dielectric surfaces typically in microelectronic devices manufacturing. Planarization is achieved by removing material from the wafer surface due to the synergistic effect of chemical and mechanical actions. In a typical CMP machine the upper part is referred to as the head. It holds the wafer, rotates and applies a force on the wafer against the lower part. The lower part is a rotating platen with an attached relatively softer pad. During CMP slurry is externally provided at a desired flow rate. The slurry is composed of chemicals (oxidizer, chelating agents, corrosion inhibitor and surfactants etc.) and nanometer-sized hard abrasive particles. The chemicals react with surface and modify the surface properties while the abrasive particles indent into the wafer surface and remove material from the wafer surface due to relative motion between the wafer and pad. The combination of chemical and mechanical effects limits our ability to understand the complex material removal mechanism in CMP. Intensive efforts have gone into developing models to explain the complex mechanism of material removal in CMP processes. The material removal rate in chemical mechanical processes have a linear dependency on applied down pressure. However, some experimental studies have reported nonlinear relationship between MRR and applied pressure. The nonlinearity can be attributed to complex interactions among the wafer, pad, abrasive particles, and chemical agents in the slurry. Therefore, in modelling CMP process coupling of both the chemical and mechanical actions is imperative to provide insight into the nonlinear behavior of MRR. Since, treating the chemical effects only as a mere means of softening the wafer surface fails to explain the nonlinear behavior of MRR in silicon dioxide CMP. Here, we present a model that couples micro-contact mechanics with diffusion of slurry into the wafer and predict MRR in CMP of silicon dioxide. The model is validated with experimental results available in the literature. In case of barrier CMP, different materials are simultaneously polished and CMP is expected to achieve a planar surface. The developed model can be utilized to optimize CMP input conditions to achieve similar material removal during barrier CMP. The material removal depends on material properties and the process input parameters. Several studies have investigated the role of slurry chemistry to achieve a certain material removal selectivity of different materials on a patterned wafer. Here we propose a methodology of achieving planar patterned surface of Cu/Mn/MnN system using a model-based optimization for mechanical process parameters. The parameters include applied force, slurry solid concentration, and abrasive particle size. The methodology has been developed via optimization using a genetic algorithm. The proposed methodology suggests that a lower downforce is the key parameter to achieve the desired material removal selectivity and planarity. The first part of the study suggests a low material removal rate (MRR) to achieve a lower standard deviation in MRR. The second part investigates the standard deviation in the thickness removed in the average time needed to remove a known thickness of the materials under consideration. It has been found that the application of lower downforce can also minimize the standard deviation in the thickness removed and a planar patterned surface can be achieved. Another way to achieve uniform material removal of different materials is the formulating of slurry chemistry. During chemical mechanical planarization process for nodes of 10nm and beyond, galvanic corrosion, material removal rate selectivity and defectivity issues need to be addressed. Better understanding of atomic scale chemical and mechanical interactions at the wafer surface to can provide useful insight and guidelines. This study also experimentally investigates the CMP Cu/Mn/MnN system with different slurry formulating. Ex-situ electrochemical analyses of Cu/Mn/MnN system were conducted to evaluate the passivation in different slurry formulations. The slurry formulations used were 0.1 M, 0.2 M, 0.3 M and 0.5 M oxidizer (H2O2) with abrasive solid loading of 2.5 %wt. The results of electrochemical analyses are compared with post CMP surface topography and material removal rate selectivity. It has been found that material removal selectivity of 1:0.8:0.88 can be achieved for Cu/Mn/MnN system, for a slurry composition of 0.3 M H2O2, under 30 N down force and 2.5 %wt. solid concentration of the slurry. The effect of mechanical components such as slurry solid concentration and applied force was also investigated. It was found that for an optimized concentration of oxidizer, more control over material removal selectivity can be achieved by optimizing the mechanical input parameters. The application of lower applied force and lower slurry solid concentration are needed to achieved the objective. This finding is in agreement with the material removal selectivity predicted with model based optimization. The model developed in this work can be used to predict MRR in a typical CMP. Also, the model can be used to optimize the CMP input conditions to achieved a desired MRR. The experimental approach proposed in this work can be beneficial to the barrier layer CMP for advanced interconnect integration schemes for sub-10 nm.