Physical Simulation and Visualization of the Marangoni Convection inside Meniscus Region under IPA Vapor in Wafer Drying Process
Authors: Ono Naoki , Yamada Takashi , Miura Shumpei , Ishibashi Tomoatsu , Matsuo Hisanori , Watanabe Katsuhide 
Published: February 2020

With the recent rapid progress of higher integration of semiconductor devices, mechano-chemical polishing process is playing a very important role. Along with this trend, cleaning and drying processes after the mechano-chemical process, have been regarded as more and more important technologies for ensuring surface cleanliness of the wafers. As a drying process after cleaning, there is a method of flowing pure water to a rotating wafer, and simultaneously supplying IPA (Iso-Propyl Alcohol) vapor to the meniscus region of the liquid layer (this is known as Rotagoni drying). This method causes a surface tension difference on the liquid surface and then drives fluid convection by the Marangoni effect, which is thought to efficiently remove impure particles on the wafer surface. During the impingement of IPA vapor to the meniscus region, in-situ observation of Marangoni convection is so difficult that details of the flow velocity and flow pattern are not well understood. In order to further improve this method to efficiently remove ultrafine foreign particles on wafer surface in future, the authors believe that the observation and the understanding of the Marangoni convection are indispensable. In this study, as a first step to observe the flow, rather than treating the complicated three-dimensional flow on actual rotating wafer from the beginning, physical simulation by creating a simplified two-dimensional flow was performed. Using photoetching method and PDMS material as in the fabrication of MEMS devices, a two-dimensional flow channel with a depth of about 600 mm and a width of about 1000 mm was created, and pure water and IPA vapor (mixture of nitrogen and IPA in which concentration was controlled.) was supplied to reproduce the situation where IPA vapor collided with pure water in the meniscus region. Pure water was provided from one direction of the T-shaped flow channel by a syringe pump, and IPA vapor was supplied from the other direction. By setting a minute step for locking the position of the liquid, the stable fluid meniscus in an oblique direction to the channel width was realized in the channel. The Marangoni convection was generated in the meniscus where the IPA impinged, and the flow was visualized by utilizing PIV (Particle Image Velocimetry) method with tracer particles. From the observations, the flow velocities and the flow patterns were successfully obtained from the PIV image analysis. It was found that a rapid convection of 20 mm/s occurred at the maximum velocity when IPA vapor was introduced. This was about ten times of the velocity when only nitrogen gas was provided. In the case with only nitrogen gas, the liquid convection was generated only by the shear stress to the liquid surface caused by the gas flow. However, the Marangoni convection caused by the IPA vapor was about ten times larger than the flow velocity with only nitrogen, of which difference was unexpectedly large. Moreover, the direction of the Marangoni convection was not as simple as anticipated, which could change depending on the impinging position of the IPA vapor. In an experimental condition of the present study, the Marangoni convection was directed from the gas-liquid interface to the triple point on the wall. It is thought that these flow velocities and directions would be strongly related to the removal of foreign particles on the wafer surface. Videos taken in the observation will be also shown in the presentation. END

Figure 1