Study of Bubble Activity in a Megasonic Field Using an Electrochemical Technique

In the megasonic cleaning of wafers, size and motion of cavitating bubbles and fluid flow due to acoustic streaming play a very important role. In this paper, chronoamperometric technique has been used to seek information on acoustic streaming and bubble activity in a 1 MHz sound field. Specifically, current transients during reduction of potassium ferricyanide were recorded. Data collected at 1-6 MHz sampling rate using a 25 μm platinum electrode show current “peaks” indicative of the approach of oscillating bubbles to the electrode and current “valleys” due to blocking of the electrode by bubbles. Acoustic streaming velocity ( ~ 1.5 cm/s) and bubble size (maximum radius of ~ 1 μm for oscillating bubbles) have been estimated from local current transients caused by bubble activity near the electrode.