Study On Fluid Pressure And Friction Characteristics In CMP

Chemical Mechanical Polishing (CMP) is widely used in integrated circuit (IC) manufacturing as the only method to achieve local and global flatness. Because the CMP process is very complicated, there are many factors that affect the performance of CMP. The fluid pressure and friction are two key factors which affect the material removal rate (MRR) and within wafer non-uniformity (WIWNU). Furthermore, with the reduced feature size and increased wafer diameter in the future IC manufacturing, the influence of the two factors become more significant. Thus, a further study on fluid pressure and friction characteristic in CMP is important for realizing the goal of high efficiency, ultra-flat, non-damage surface.There are many disadvantages to explore mechanism and process optimization of CMP by the traditional methods such as taking experiments. Thus, a theoretical model of CMP based on mixed elastohydrodynamic lubrication (EHL) is established and used in the study of the fluid pressure and friction characteristic in the CMP process. The main content and results of the research in this paper are as follows:(1) A mixed EHL model considering the roughness of polishing pad, the deformation of asperities, and the balance of wafer is established, in which different models of polishing pad are taken into account. The simulation results show that the main difference of the fluid pressure and the deformation of the polishing pad is mainly at the periphery region of the wafer. The layered elastic model is more suitable for predicting the situation at the periphery region than the Winkler elastic foundation model. Negative fluid pressure dominate for the flat wafer because the wafer leans towards the leading edge during CMP due to friction. The simulation results provide a theoretical support for the related experimental results.(2) A software framework for the simulation based on multi-layer subroutine technique is established by using mixed programming with C++and FORTRAN. And the software framework is easy to use and expand.(3) The influence of the original surface profile of the wafer in CMP is studied. The results show that the fluid pressure of the concave wafer tends to be negative, on the contrary, the convex wafer tends to be positive, which is consistent with the related experimental results.(4) The influence of the retaining ring in CMP is studied. It is found that the retaining ring obviously obstructs the slurry flowing into the wafer-pad interface. Compared with the first generation carrier, the second generation carrier is more controllable to realize the uniformity of the MRR. Moreover, effects of the process parameters, for example, the relative velocity and the polishing pressure on fluid pressure and friction of wafer-pad interface are studied.(5) The tribological behavior in CMP is studied. The phenomenon that the coefficient of friction decrease with the increase of the rotational speed is confirmed. In order to explain the contradiction between experimental results and EHL simulation results, a lubrication model of a single asperity using COMSOL is established, and the distribution of fluid pressure can be obtained. The results show that the average fluid pressure of the contact region is positive.