Disruptive bubble behaviour leading to microstructure damage in an ultrasonic field

Bubble oscillations play a crucial role in ultrasonic cleaning, a process by which micro- and nanoscale contaminant particles are removed from solid surfaces, such as semiconductor wafers, photomasks and membranes. Although it is well known that the ultrasonic cleaning may damage the functional patterns of ever-shrinking size in current manufacturing technology while removing dust and debris, the mechanisms leading to such damage have been elusive. Here we report observations of the dynamics of bubbles that yield microstructure damage under a continuous ultrasonic field via high-speed imaging. We find that the bubble behaviour can be classified into four types, namely volume oscillation, shape oscillation, splitting and chaotic oscillation, depending on the acoustic pressure and bubble size. This allows us to construct a regime map that can predict the bubble behaviour near a wall based on the experimental parameters. Our visualization experiments reveal that damage of microwalls and microcantilevers arises due to either splitting small bubbles or chaotically oscillating large bubbles in the ultrasonic field, with the forces generated by them quantitatively measured.