Simulations of removal of molecular contaminants from silicon wafer surface
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With the decrease in feature size in semiconductor manufacturing, molecular contamination problems are increased significantly. In order to optimize the yields in wafer fabrication units there is a need for process modeling that addresses the details of wafer contamination. Wafer contamination and cleaning is a complex process that covers various length and time scale events and phenomena. At the largest scales, there is the availability and transport of specific species within the fabrication unit and subsequent contamination of the wafer surface either through processing steps or through simple ambient transport processes. To limit wafer contaminant levels and/or to decontaminate them, wafers in the semiconductor fabrication unit are often cleaned and transported in a closed enclosure called Front Opening Unified Pod (FOUP) and purged with an inert gas like nitrogen. For the FOUP geometry, I analyze the large scale process modeling approaches to cleaning wafers. At smaller scales, the specific molecular configuration of the contaminant species impacts the kinetic chemical-physical cleaning mechanisms. To determine, from a fundamental perspective, the mechanisms contributing to wafer cleaning requires different scale tools from transport tools aimed at characterizing equipment scale (e.g., FOUP) contamination issues. I use molecular dynamics models and optimization techniques to infer physicochemical rates for molecular desorption on wafer surfaces. This dissertation considers these problems from a common perspective. The objective of this study has been to characterize the multi-scale problem of wafer cleaning with the objective of developing appropriate tools and models at different scales to best predict the dynamics of contaminant removal from wafer surfaces. A standardized method has been presented to extract kinetic rate parameters using molecular dynamics simulation (smaller-scale) and optimization for use in a larger-scale model of wafer decontamination using computational fluid dynamics (CFD). Also, by using available experimental data and CFD analysis an optimized FOUP purging recipe for better decontamination is presented and the relative magnitude of the time scales associated with surface kinetics and FOUP purging have been estimated.