Use of ultrasonic cavitation in surface cleaning: A mathematical model to relate cleaning efficiency and surface erosion rate

Ultrasonic cleaning, employing frequencies in the range of 40-200 kHz, is widely used in many industries requiring precision cleanliness in the micrometer to submicron particle size range-e.g., semiconductor wafer fabrication, hard disk drive manufacturing, and integrated circuit assembly. One overriding concern with the use of ultrasonic cleaning for delicate components and assemblies has been the specter of cavitation erosion-surface material loss and other functional degradation due to the impact of shock waves generated by collapsing bubbles and bubble clusters in an oscillating acoustic field. The simultaneous processes of surface cleaning and surface erosion in the presence of a high-frequency ultrasonic field (≥ 58 kHz) are described here mathematically, and the equations are coupled to allow conceptual optimization of parametric settings to maximize cleaning efficiency while minimizing the level of erosion damage. This theoretical analysis is presented for various ultrasonic field conditions (frequency, intensity, etc.), fluid medium properties (viscosity, density), and surface conditions (hardness, smoothness, etc.). The contribution of acoustic streaming to surface cleaning is incorporated in the model, and is shown to have minimal influence on the optimum cluster collapse pressure, but to have a significant effect on the net cleaning efficiency for the surface.