| The surface roughness of mould cavity exerts tremendous effect on device’s external appearance and performance. The surface quality requirement demanded by advanced mould steel is becoming more and more strict, and the current polishing techniques have displayed great limitations and are hard to satisfy the harsh requirement. Exploring a kind of polishing method to obtain excellent global planarization and ultra-smooth surface of advanced mould steel is quite urgent and of great realistic significance. Regarded as the only polishing technique that can achieve excellent global planarization, chemical mechanical polishing (CMP) has been widely applied in Ultra Large Scale Circuit (ULSI) and the polishing material can acquire prefect polish performance. S136 steel, as a kind of advance mould steel applied most extensive, was taken an example of adopting CMP how to acquire premium polishing characteristic. The objective of this paper was to explore the optimal CMP process parameters for S136 steel and analysis its material removal mechanism and chemical reaction process.X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the polishing performance of S136 steel. Root mean square roughness (RMS) and material removal rate (MRR) were employed to evaluate the polishing level. The most important parameters affecting polishing performance including acid-base property of slurry, down pressure, platen speed, slurry pH and abrasive category were investigated in this paper. Single factor method was adopted to confirm the approximate distribution level of above referenced parameters. The results demonstrated that the platen speed and down pressure displayed similar tendency that the MRR increased with respect to the increasing of speed and pressure. When the speed and pressure was relatively small, the relationship of MRR with respect to speed or pressure was in accordance with a modified Preston equation and the error of MRR between experimental data and calculated data by modified Preston equation was within ±2%. With a further raise of table speed and down pressure, the modified Preston equation is not suitable any more. The acid-base property of slurry imposed great influence on polish process and S136 steel can obtain a better polishing quality in acid slurry system. In terms of abrasive category, the maximum of MRR and minimum of RMS can not be acquired simultaneously on condition that the abrasive category is same. After comprehensive consideration, SiO2 particle was selected as the optimal abrasive used in this paper.After the approximate distribution level was confirmed by single factor method, we adopted orthogonal design method to propose the optimized process parameters and validated the reliability of optimized process parameters by MRR and RMS index, XRD, SEM and AFM analysis. The results displayed that the ultra-smooth surface with RMS roughness 2.87 nm was obtained under optimized condition with MRR as high as 173.26nm/min. Compared with other process parameters group, the optimal parameters group simultaneously achieved maximum MRR and minimum RMS, which illustrated the reliability and effectiveness of optimal process parameters proposed by orthogonal design method. And then we analyzed the influence degree that every parameter referenced above imposed to S136 CMP by index relationship figure.Finally, the material removal mechanism and chemical reaction process of S136 CMP were discussed elaborately in this paper. We also analyzed the form reason of polishing defect such as surface hole and scratch. For the CMP process of S136 steel, we draw conclusion that the most critical condition was how to obtain balance between chemical reaction and mechanical effect, the essence of varying all parameters lie in the change of parameter could affect the balance degree of chemical reaction and mechanical effect. |
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