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. / text

Megasonic cleaning is routinely used in the semiconductor industry to remove particulate contaminants from wafer and mask surfaces. Cleaning is achieved through proper choice of chemical solutions, power density and frequency of acoustic field. Considerable work has been done to increase understanding of particle removal mechanisms in megasonic cleaning using different solution chemistries with varying ionic strengths. However, to date, the focus of all these studies of particle removal has been either cavitation or acoustic streaming.The propagation of sound waves through a colloidal dispersion containing ions is known to result in the generation of two types of oscillating electric potentials, namely, Ionic Vibration Potential (IVP) and Colloid Vibration Potential (CVP). These potentials and their associated electric fields can exert forces on charged particles adhered to a surface, resulting in their removal. In addition, the pressure amplitude of the sound wave is also altered in solutions of higher ionic strengths, which can affect the cavitation process and further aid in the removal of particles from surfaces. To test the two hypotheses, investigations have been conducted on the feasibility of removal of charged particles from silicon wafers in electrolyte solutions of different ionic strengths irradiated with a megasonic field of different power densities. Cleaning experiments have been performed using potassium chloride (KCl) as a model electrolyte and silica particles as model contaminant particles. The cleaning performance in KCl solution has been compared to that in other electrolytes solutions such as sodium chloride, cesium chloride and lithium chloride. In order to characterize the cavitation events in KCl solutions, acoustic pressure and sonoluminescence measurements have been performed using hydrophone and cavitation probe respectively. The results indicate that particle removal efficiency (PRE) increases with KCl concentration and transducer power density and much lower power densities are required at higher KCl concentration for a comparable level of cleaning. Further, cleaning performances in NaCl and CsCl were found to be superior to those in KCl and LiCl solutions. Theoretical computations show that the removal forces due to CVP are much larger in magnitude than those due to IVP and are comparable to van der Waals forces.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references and index.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Danyluk, Steven; Committee Member: Melkote, Shreyes; Committee Member: Rohatgi, Ajeet.

Thesis (M.S.)--Rochester Institute of Technology, 2008. / Typescript. Includes bibliographical references (leaves 73-75).

Thesis, (Ph. D.)--Oregon Graduate Institute of Science and Technology, 1997.

Thesis (B.S.)--Rochester Institute of Technology, 1985. / Typescript. Includes bibliographical references (leaves 31-32).

Over the last five years, a dramatic expansion of renewable energy from Photovoltaic (PV) solar cells has been witnessed. This expansion is due in part to wafer based silicon solar cells. Crystalline silicon solar cells currently dominate the PV market because of their low cost per watt of electricity production. In order for silicon solar cells to continue to govern the market, efficiency improvements and cost reductions must be made. This work focuses on both cost reduction and efficiency improvements, for wafer based silicon solar cells. The main aim of the work was to produce a thin monocrystalline wafer based silicon solar cell. A large proportion of the cost of conventional monocrystalline solar cells is related to the use of high purity silicon substrates. By producing a cell that uses less silicon, significant cost savings can be made. Conventional wafering techniques used in industry are reaching their limit for thin wafer production. The method adopted in this work uses a simple silicon exfoliation technique capable of producing ultrathin silicon foils. A fully operational solar cell was fabricated from a 40mum exfoliated silicon foil. The thin wafer based silicon solar cell was more than four times thinner than a commercially produced equivalent. The work investigated a variety of principles related to the exfoliation and the suitability of the technique for thin photovoltaic devices. By using a thin exfoliated substrate, conventional anti-reflective (AR) suppressing processes could prove problematic. Experiments were conducted into finding an alternative technique to match the performance of the conventional AR process. The formation of porous silicon (PSi) on the surface of a silicon substrate was found not only to match the commercial process, but to exceed it. With a porous silicon layer, reflectivity was suppressed to just 6.68%. The technique could be applied to both thin silicon solar cells and conventional thicker wafer based cells. The reflectivity suppressive layer could be fabricated in a single simple processing step. Investigation was also focused upon the top contact for silicon solar cells. As the top of the cell is responsible for current collection and light absorption, large electrical contacts shade the cell resulting in a decrease in efficiency. Silver nanowires (AgNWs) were successfully analysed and deposited onto standard silicon solar cell top contacts as an enhancement coating. Such a coating was found to improve the collection ability of the top contact without causing a significant increase in shading loss. The use of an optimised AgNW coating can increase cell efficiency by as much as 37%.

<p> Solar cells were made by diffusing impurities into the rough or backside of commercially available silicon wafers to form a junction. The properties of these solar cells were compared to solar cells made by diffusing impurities into the polished surface of similar silicon wafers. The processing steps involved in preparing each type of solar cell were identical. </p> / Thesis / Master of Engineering (MEngr)

<p> Solar Cells of at least 10% conversion efficiency were fabricated from silicon wafers of one inch diameter and the same processing procedure was applied to wafers of three inch diameter. Four of the three inch diameter solar cells were affixed to a galvanized steel plate and hooked in a parallel configuration to make a solar cell panel. A piece of special plastic was placed over the solar cells on the panel and hermetically sealed to protect the solar cells from the environment which in time would degrade the performance of the solar cells. </p> / Thesis / Master of Engineering (MEngr)