Abstract: A method of processing thin flat articles, particularly semiconductor wafers, utilizing sonic energy. In one aspect, the invention is a method comprising: supporting a substrate in a generally horizontal orientation and transmitting sonic energy to the substrate while flowing liquid onto both sides of the substrate to loosen particles on both sides of the substrate while maintaining said orientation. In another aspect, the invention is a cleaning method comprising: applying cleaning fluid to one side of a thin flat article while supporting the article in a generally horizontal orientation; and applying energy to the other one of the sides with sufficient power to produce vibration on the one side in the area of the cleaning fluid to loosen particles on the one side, while maintaining said orientation.