To evaluate the dominant forces for producing bubble motions in a wet cleaning bath for 300 mm diameter silicon wafers, the typical motions of small bubbles generated by the megasonic (MS) wave were studied using a video camera and an optical microscope. The bubbles with diameters of 10–30 and 200–300μm had upward velocities of 0.002–0.003 and 0.02–0.08 m/s, respectively, under the MS wave. The velocity of the 200–300μm diameter bubble depended on three forces, that is, the buoyancy, MS wave, and water flow. The motion of the 10–30μm diameter bubble was mainly driven by the MS wave; the influence of buoyancy was quite low. The path of the bubbles immediately above the wafer support bar could be changed by adjusting the MS wave power.

The influences of a megasonic wave on water and bubble motions in a wet cleaning bath used for 300 mm-diameter silicon wafers were studied. The water motion in the entire bath was observed using a tracer of water-soluble blue-colored ink. The megasonic wave accelerated the water motion in the entire bath, although the water motion under no megasonic wave tended to be localized. The motion of small bubbles generated by the megasonic wave was also traced over the entire wafer surface. The bubble transport rate was shown to be increased by the megasonic wave and water flow.