Sonic radiation is applied to a wafer portion of the planar surface of a rotating, tilted wafer as it is being immersed into a liquid treatment bath. The portion includes the leading outer edge region of the wafer. The area of the wafer portion is significantly less than the total surface area of the planar wafer surface. Power density is minimized. As a result, bubbles are removed from the wafer surface and cavitation in the liquid bath is avoided. In some embodiments, the liquid bath is de-gassed to inhibit bubble formation.