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- To characterize the cavitation behavior in megasonic cleaning, the acoustic bubbles that include the growth mechanism, variation of acoustic bubble growth rate in standing wave field and cavitation stability on the surface were observed by using the ultra-high speed camera. There was a considerable transition of the acoustic bubble growth rate directly respected on the increase of megasonic power. It has been shown that the removal of contaminants and the damage of thinfilm by means of the energetic cavitation on surface might be determined by the extent of instability in the acoustic cavitation related to its variation of growth rate. To control the acoustic bubble intensity effectively in megasonic cleaning, the cavitation behavior on the basis of the change of acoustic bubble growth rate as functions of chemical additives such as dissolved gas, surfactant and alcohol was exhaustively investigated and the cavitation intensity was quantitatively evaluated by the test of cleaning performances.
Above all, changes in the cavitation intensity of gases dissolved in water, including H2, N2, and Ar, have been established in studies of acoustic bubble growth rates under ultrasonic fields. Variations in the acoustic properties of dissolved gases in water affect the cavitation intensity at a high frequency (0.83 MHz) due to changes in the rectified diffusion and bubble coalescence rate. It has been proposed that acoustic bubble growth rates rapidly increase when water contains a gas, such as hydrogen faster single bubble growth due to rectified diffusion, and a higher rate of coalescence under Bjerknes forces. The change of acoustic bubble growth rate in rectified diffusion has an effect on the damping constant and diffusivity of gas at the acoustic bubble and liquid interface. It has been suggested that the coalescence reaction of bubbles under Bjerknes forces is a reaction determined by the compressibility and density of dissolved gas in water associated with sound velocity and density in acoustic bubbles. High acoustic bubble growth rates also contribute to enhanced cavitation effects in terms of dissolved gas in water. On the other hand, when Ar gas dissolves into water under ultrasound field, cavitation behavior was reduced remarkably due to its lower acoustic bubble growth rate. It is shown that the change of cavitation intensity in various dissolved gases were verified through cleaning experiments in the single type of cleaning tool such as particle removal and pattern damage based on numerically calculated acoustic bubble growth rates.
We also focused on the characterization and cleaning performance of various alkaline chemicals like choline hydroxide [(CH3)3N(CH2CH2OH)OH] and tetramethyl ammonium hydroxide (TMAH, (CH3)4NOH)) to activate the cleaning ability of solution in megasonic cleaning. The motivation of this study was to reduce the material loss by chemical reaction on surface during the cleaning process. Even though these chemicals have been reported and used for cleaning, there is no extensive and comparative study on cleaning aspects. Dilute alkaline chemicals have been investigated to define not only on an etching of Si substrates but also the variation of interactive forces between the particles and the Si surface in alkaline chemicals. The interaction forces with pH of the solutions were calculated by following the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. We also performed the PRE and PR pattern damage test as a function of alkaline chemical with dissolved gases to find out the optimized cleaning solution for both etch and damage free megasonic cleaning.
A cavitation effect was induced during megasonic cleaning can be controlled by the addition of surfactant. It is a well-known fact that the surfactant in ultrasound field can play an important role in controlling the cavitation intensity in terms of the retardation in bubble coalescence rate. The explanation about the acoustic bubble growth rate, cavitation threshold and their relationship with type of surfactant and its concentration from biomedical and chemical reactions perspective has been reported elsewhere.
It was demonstrated that the change of cavitation effect depends not only on the chain length of the aliphatic alcohols but also on the physical properties such as surface tension and viscosity in alcohols added in solutions. The aliphatic alcohol that isopropyl alcohol (IPA) and Ethanol is one of the surfactant and more volatile than other surfactants. Their cleaning performance with megasonic was evaluated in this study. Its physical property relating to acoustic bubble growth rate was estimated extensively. Also the change of cavitation effect as a function of alcohol concentration was directly confirmed through the PRE and pattern damage test in a single wafer cleaning system. Finally, the effect of gas, alkaline chemical and surfactant in megasonic cleaning solution is fully investigated to better understand and meet the change of cavitation behavior.
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- GRADUATE SCHOOL[S](대학원) > BIONANOTECHNOLOGY(바이오나노학과) > Theses (Ph.D.)
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