Mechanical interactions at the interface of chemical mechanical polishing

This thesis addresses the mechanical aspects of sliding hard surface over a soft polymer pad in the presence of a fluid. This process is termed Chemical Mechanical Polishing (CMP) in semiconductor manufacturing. With the increasing integration density, especially the emergence of new materials and technologies, CMP has become an indispensable process for microelectronics fabrication. The manufacturability of current CMP processes however is limited by the poor understanding of the mechanisms, and fundamental studies are included in this thesis to improve CMP performance.; The major contributions of this thesis to the existing CMP theories are to correlate the material removal and planarization mechanisms with mechanical interactions at a polishing interface. For the first time, a “nano-film” is suggested to exist between the load-supporting asperities and a target surface, and the material removal is associated with the thickness of the nano-film as well as the abrasive particle size. Accordingly, the material removal rate is related to the “contact” area and thus to the applied load for a given abrasive density. The polishing uniformity or planarization across a wafer is associated with the contact stress distribution. Since the contact stress is a result of both applied load and the interfacial fluid pressure, the latter was studied extensively. It was found that a subambient fluid pressure exists at the polishing interfaces [1], and the magnitude and distribution of this fluid pressure varies with process variables. Both an exact analysis and numerical models were developed to understand and predict the fluid pressure, and a finite element analysis is used to determine the resulting contact stress.; Experiments were designed to justify the theories and models. The friction measurements show agreement with the existence of the nanofilm, and the results from polishing experiments are consistent with an estimation of the nano-film thickness. The polishing profile across the 100mm diameter wafers follows the calculated contact stress distribution, and the interfacial fluid pressure has a strong effect on it. Consequently, global planarization should be achievable by designing a “flat” contact stress distribution.