Jan 30 2019
Semiconductor manufacturing and numerous other industrial tasks involve cleaning processes, and from health and environmental standpoints, it has become extremely necessary to use physical cleaning methods such as underwater ultrasound or liquid jets rather than toxic chemicals.
Astonishingly, scientists who are experts in mechanical engineering have not given much attention to physical cleaning issues. But currently, the underlying physics of what occurs when to-be-cleaned surfaces are struck by liquid jet collisions has been revealed by mechanical engineers specializing in the mechanism of fluid motion at Keio University, Japan.
The engineers accomplished this by using a fluid dynamics simulation to investigate the effect of water droplets against a dry/wet rigid wall as an official example. They describe their efforts in the journal Physics of Fluids, from AIP Publishing.
In semiconductor manufacturing, smaller-sized contaminant particles need to be removed from silicon wafers as the devices become further miniaturized. High-speed droplet impact is favored when it comes to removing particles of very small sizes—on the order of 10 nanometers—but it can cause surface erosion.
Keita Ando, Assistant Professor of Mechanical Engineering, Keio University
Therefore, it is essential to consider the effects of compressibility as well as viscosity of the fluid on the impact dynamics. “The fluid is viscous, so it produces a mechanical friction that plays an essential role in particle removal,” he said. “In addition, the fluid is compressible, which means that it produces a water-hammer shock at the impact that may end up causing surface damage.”
Computational fluid dynamics (CFD) accounting for both compressibility and viscosity are a huge challenge, so the engineers carried out the first known viscous and compressible flow simulation to judiciously analyze high-speed droplet impact dynamics. “The phenomena are small-sized and very fast; it is very tough to resolve them with current experimental techniques,” said Ando.
Why is this simulation method so important? It enables recognizing a trade-off connection between efficient cleaning performance—also called particle removal efficiency—and damage-free cleaning.
Our approach can be used to quantify friction force and water-hammer impact pressure. These are useful for exploring optimal values of the impact speed, for example, given contaminant particle properties including size. The direct impact of water droplets against dry surfaces produces high friction and impact pressure, which implies efficient but erosive cleaning.
Keita Ando, Assistant Professor of Mechanical Engineering, Keio University
In their simulation, the researchers examined the case by primarily adding a film of water that covers the cleaning surface. Their results reveal that this film can mitigate the droplet impact, which is vital for less erosive cleaning.
Environmentally friendly cleaning techniques—including water jet and underwater ultrasound—will play a more important role in the future. Their technological advance has always been hindered by a lack of understanding of the underlying physics, which can now be understood. In addition, beyond experimental research, the CFD approach is an essential tool when it comes to the quantification of fluid flow.
Keita Ando, Assistant Professor of Mechanical Engineering, Keio University