(Dr. Steven Danyluk, advisor)
"Mechanical Interactions at the Interface of Chemical Mechanical Polishing"
This thesis addresses the mechanical aspects of sliding a 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 becomes an indispensable process for microelectronics fabrication. The manufacturability of current CMP process is limited by the poor understanding of the mechanisms, and fundamental studies are included in this thesis to improve CMP performance at low cost.
The material removal and planarization mechanisms were correlated to mechanical interactions at a polishing interface, and a consistent chemistry was assumed. A “nano-film” theory was established based on the pad asperity-scale lubrication effect, and the material removal was associated with the thickness of the “nano-film” as well as abrasive particle size. According to this theory, material removal should be proportional to total “contact” area and thus applied load for given abrasive density. Consequently, the polishing uniformity/planarization across a wafer was associated with the contact stress distribution, which is the combination of “dry” contact and fluid pressure.
Simulated and polishing experiments were designed to justify the theories and models included in this thesis. Interfacial fluid pressure tends to build up at the interface in most cases, and a large portion of the interface at the leading edge exhibits a sub-ambient fluid pressure, while a small trailing region may have a positive pressure. The magnitude and distribution of this fluid pressure varies with most process variables, and the average fluid pressure is of the same order of the applied normal load. Polishing experiments on SiO2 thin film show good agreement with the predictions of the theories. Static contact image processing showed promising potential in analyzing rough contacts.
Both analytical and numerical models were developed to estimate the interfacial fluid pressure and the effective contact stress distributions. Some modeling examples were included with real CMP process parameters, and reasonable results were obtained.