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Cleaning tests with ultrasound-superposed water jets are performed to explore the optimal injection distance from the jet nozzle to a glass plate spin-coated with small silica particles (as a cleaning sample). The cleaning performance is evaluated based on particle removal efficiency (PRE) that is calculated using the haze method. Visualization of the water jet and liquid film flow over the cleaning target shows that the jet flow with short injection distance tends to be in a steady state, while the water jet shape instability grows for long injection distance, leading to atomization of the jet. The cleaning tests with varying the injection distance suggest that there exists an optimal injection distance at which the PRE becomes maximal.

Visualization experiments were performed to examine whether acoustic bubbles play a role in ultrasonic water flow cleaning, as in convention cleaning with ultrasonic baths. Schlieren visualization confirmed the standing-wave-like acoustic field in ultrasonic water flow that collides with a glass surface. Backlight visualization showed that cavitation bubbles appear in the water flow spreading over the glass surface. These bubbles are found to oscillate in volume and move inside film flow and thus expected to play a role as active cleaning agents.

In cleaning and drying processes after chemical mechanical polishing process, there is a method of flowing pure water to a rotating wafer, and simultaneously supplying IPA (Iso-Propyl Alcohol) vapor to the meniscus region (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 technically difficult. In this study, physical simulation by creating a simplified two-dimensional flow was performed. Using MEMS device technique, a two-dimensional flow channel was created, and pure water and IPA vapor was supplied to reproduce the situation, and the flow was visualized by utilizing PIV (Particle Image Velocimetry) method. The flow velocities and the flow patterns were successfully obtained from the image analysis. The surface flow with IPA vapor was in the reversed direction of that with only nitrogen gas, and the velocity became larger.

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