Surface Tension Measurement of Water Surface Under IPA Vapor and the Impact of Its Concentration Dependence on the Marangoni Convection in Wafer Drying Process
Authors: Miura Shumpei , Yamada Takashi , Ishibashi Tomoatsu , Matsuo Hisanori , Watanabe Katsuhide , Ono Naoki 
Published: February 2020

As higher integration of semiconductor devices has progressed recently, mechano-chemical polishing process is playing a very important role. At the same time, cleaning and drying technologies after the polishing process have been more and more important in order to ensure the surface cleanliness of the wafer products. As a drying process after the cleaning, there is a method of supplying pure water and IPA (Iso-Propyl Alcohol) vapor simultaneously to the surface of the water meniscus region on a rotating wafer (which is called Rotagoni drying method). This technique causes a surface tension difference on the liquid surface and then gives rise to the convection by the Marangoni effect, which is thought to efficiently remove impure particles on the wafer surface. In this method, IPA vapor impinges on the vicinity of the liquid meniscus on the wafer. It is very difficult to measure the surface tension of the liquid surface in that part in situ, and the details about the convection are not well understood. As this technique further advances in near future in order to remove ultra-fine foreign particles on the wafer surface more efficiently, observation and understanding of the Marangoni convection in the meniscus region are necessary for the future progress of the technique. In this study, in order to consider the actual situation of the drying process, not only the measurement of the surface tension of the droplet of the IPA aqueous solution, but also the measurement by exposing pure water droplet to various concentrations of the IPA vapor were performed, and the relationship between the concentration of IPA vapor and the surface tension of the droplet was investigated. The latter situation is much closer to actual drying situation. Specifically, in measurements, a small pure water droplet was placed in a glass container, and the container was filled with a constant concentration of IPA vapor. At the time when it was judged that the IPA adsorption to the liquid surface had reached equilibrium by confirming that the shape of the droplet became unchanged, the image of the droplet shape was taken from the side of the container with digital camera. It is known that the static shape of a droplet which is flattened under the influence of gravity can be analytically expressed by a mathematical equation in consideration of surface tension and mechanical equilibrium. In this study, the surface tension was deduced by assuming that a uniform concentration of IPA and a uniform surface tension over the whole droplet surface. In another project of the authors’ group, physical simulation using a two-dimensional PDMS flow channel was performed to reproduce a situation of impinging IPA vapor to the meniscus region, and PIV (Particle Image Velocimetry) method was applied to obtain the flow velocity of the Marangoni convection. The measurement of IPA concentration on water surface during the actual drying process is very difficult, and moreover, the in-situ measurement of surface tension is almost impossible. From the data of surface tension as a function of IPA concentration and the measured velocity data with a simple analytical flow model, the estimation of surface tension and IPA concentration on the droplet surface will be discussed. END

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