Wettability aspects during silicon wafer cleaning in aqueous and organic systems.
Committee ChairRaghavan, Srini
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PublisherThe University of Arizona.
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AbstractAlkaline solutions based on ammonium hydroxide and quaternary ammonium hydroxides such as choline (hydroxyethyl trimethyl ammonium hydroxide) and TMAH (tetramethyl ammonium hydroxide) are used widely in the wet processing of silicon wafers for the control of ionic and particulate impurities. The Wilhelmy plate technique was used in characterizing the ability of alkaline solutions to alter the wettability of wafers. Choline improved the water wettability of wafers, and at concentrations greater than 1000 ppm, rendered the wafers very hydrophilic. Ellipsometric and XPS analyses showed that the exposure of choline-treated surfaces to air resulted in the oxidation of Si to SiO₂. The increase of wettability of wafers in TMAH solutions was due to the roughness introduced by the high etch rate of TMAH solutions. Ammonia solutions without the addition of H₂O₂ did not increase wettability. The addition of H₂O₂ and a non-ionic surfactant to alkaline solutions significantly increased the wettability of wafers, decreased the etch rate, and resulted in smoother surfaces. The use of isopropyl alcohol (IPA) in the drying of wafers has been considered by the semiconductor industry. The addition of IPA to water resulted in a decrease in surface tension at the solution/vapor interface. The surface excess of IPA molecules at the solution/air interface was calculated to have a maximum value of 8.5 x 10⁻¹⁰ moles/cm² at a solution composition of 25% IPA and 75% water. IPA solutions with less than 25% IPA were very effective in removing PSL particles on hydrophilic wafers. Hydrophilic particles such as alumina and glass were difficult to remove from wafers in DI water and IPA solutions, however, hydrophobic particles such as silicon were slightly removable in DI water and IPA solutions. The wettability of particles (θ) and substrate (α) in solutions played important roles in removing particles on substrates. Leenaars' equation for the calculation of magnitude of surface tension force which balance adhesion force, F(A), did not seem to hold for solutions containing less than 25% IPA. Modification to this equation by adding a surface pressure term, π*, was considered in explaining the experimental results.
Degree ProgramMaterials Science and Engineering