Browsing by Author "Hussain, Yazan Ahed"

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    Stiction Reduction Agents Studies Using QCM
    (2003-11-28) Hussain, Yazan Ahed; Jacqueline Krim, Committee Member; Christine Grant, Committee Chair; Saad Khan, Committee Member
    The problem of stiction in microelectromechanical systems (MEMS) is highly limiting their fabrication and functionality. The problem occurs during the fabrication, release stiction, as well as during the use of the devices, in-use stiction. Anti-stiction agents are currently an active area of research in the MEMS field to address this issue. These agents are primarily deposited in the form of self-assembled monolayers (SAM's) on the substrate to change its surface properties; to reduce what is known as the stiction problem. One commonly used SAM is the octadecyltrichlorosilane (OTS). On silicon substrates, the most used material in MEMS fabrication, OTS has shown high effectiveness in reducing devices stiction. The use of a quartz crystal microbalance (QCM) as an analytical technique for studying the OTS anti-stiction agent is presented in this work. Using the QCM as the primary analytical tool, we were able to extract comprehensive information about the formed SAM. The dependence of SAM deposition on the bulk phase concentration of the deposit solution is shown. A rough estimation of the adsorption kinetics' rate constants were calculated, and the equilibrium constant was determined from their values. The equilibrium constant shows the high favorability of OTS deposition on silicon substrate compared to the reverse desorption process. The complex nature of the OTS SAM and its formation mechanism were also shown. These conclusions were made based upon comparison between the more robust SAM system of thiols on gold and the OTS on silicon. Finally, the interaction between the OTS and a vapor-phase lubricant (tertiary-butyl phenyl phosphate, TBPP), for friction reduction, was studied. Preliminary QCM results show a change in the adsorption of lubricant on bare silicon compared to OTS coated silicon. In addition, the lubricant film is believed to have higher slippage when OTS was present as an underlayer.
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    Supercritical CO2 Aided Processing of Thin Polymer Films Studied Using the Quartz Crystal Microbalance
    (2006-11-22) Hussain, Yazan Ahed; Christine Grant, Committee Chair; Ruben Carbonell, Committee Member; Saad Khan, Committee Member; Richard Spontak, Committee Member
    Fundamental and applied aspects of the interactions between carbon dioxide (CO₂) and different polymer systems were investigated to demonstrate the effect and performance of CO₂ during polymer processing. From a fundamental perspective, the sorption of CO₂ into a non-soluble polymer and its dependence on the different system variables were examined. Another fundamental study investigated the dissolution of a fluorinated polymer in CO₂ at different conditions. Finally, the application of supercritical CO₂ for the impregnation of additives into two different polymers was evaluated. In all these studies, the quartz crystal microbalance (QCM) was used as the primary analytical technique. In the first part of this work, the sorption of CO₂ into poly(methyl methacrylate), PMMA, was investigated. The effect of several parameters, including pressure, temperature, film thickness, and polymer state, on the equilibrium and kinetics of the sorption process was studied. The uptake isotherms of CO₂ into PMMA were estimated from the QCM frequency change. This uptake was found to decrease with temperature and to depend on the film thickness. The presence of hysteresis in the sorption-desorpotion isotherms clearly marked the glass transition which was found to be in good agreement with previously reported values. This glass transition also affected the sorption kinetic. In the glassy state, two-stage sorption curves were observed, whereas in the rubbery stage, Fickian diffusion was evident. The results from this study were utilized to examine the reliability of Sauerbrey equation for mass calculation. By measuring the change in QCM resistance, it was found that both the thickness and the amount of CO₂ dissolved in the polymer can affect the QCM response. However, it was demonstrated that Sauerbrey equation was still applicable for films up to ˜1 μm thick. In the next part, the dissolution of poly(dihydroperfluorooctyl methacrylate-r-tetrahydropyranyl methacrylate); PFOMA, a copolymer was studied. The dissolution process consisted of two stages: CO₂ sorption and polymer dissolution. The measured frequency was utilized to determine mass changes for both processes. In the sorption stage, the solubility of CO₂ into PFOMA was measured at different temperatures and pressures. The solubility was found to depend on both the CO₂ density and the temperature. Polymer dissolution started at pressures between 1100 and 1600 psi, depending on the temperature. The dissolution rate was found to increase as the CO₂ density increases, but has a possible dependence on the temperature. Finally, the fraction of undissolved polymer after 1 hour of CO₂ exposure was estimated. This fraction increased linearly from 20 to more than 90% with CO₂ density. The last part in this work examined the impregnation of ibuprofen (IBU) into two biocompatible polymers: PMMA and poly(vinyl pyrrolidone), PVP. For PMMA, the amount of impregnated IBU decreased as the CO₂ density increased. The solubility parameter approach provided a possible explanation for this behavior based on the interactions among PMMA, IBU, and CO₂. High partitioning coefficients of IBU between PMMA and CO₂ were estimated, indicating a thermodynamically driven impregnation mechanism. A linear increase in the IBU uptake with the initial polymer mass was observed. This behavior could indicate uniform distribution of IBU in the polymer sample. The impregnation rate was found to have a strong dependence on the temperature. Pressure, on the other hand, did not seem to have significant effect. For the impregnation of IBU into PVP, the frequency response was significantly larger than the PMMA case. This unusual behavior can indicate that the PVP films physical properties (e.g., viscoelastic nature of the film or in the film-substrate adhesion) are affected by IBU which might add a non-gravimetric contribution to the frequency change.