Formation and control of liquid droplets on a rotating wafer*

We experimentally investigate the formation of liquid droplets generated from a thin liquid (DI water) film on a rotating wafer. The primary droplets propelled radially from ligaments subsequently collide with a chamber wall, leading to the formation of secondary droplets. Since the secondary droplets can cause defects in various semiconductor processes, it is important to have a way to control. Using a high-speed shadowgraphy, we capture and analyze the droplets generated at the wafer edge and their subsequent collisions with the wall. Through image processing, we examine the trajectory and size of the primary droplets formed at the wafer edge, as well as the mechanisms, size, and trajectories of the resulting secondary droplets. While varying the rotational speed of a wafer in the range of 200–500 rpm (fixed flow rate of 1 LPM), we conduct experiments on various walls with contact angles ranging from 20° to 160° (i.e., different wettability). The larger the contact angle with the wall, such as with a superhydrophobic surface, the larger the secondary droplets that are formed. Additionally, these droplets make smaller angles with the wall in their flight direction. Conversely, for surfaces like glass with a small contact angle, very small secondary droplets are formed and exhibit very sporadic bounces.