An apparatus and method for substrate processing, specifically including cleaning and/or photoresist stripping, in non-immersion type megasonic processing tools. In one embodiment, the invention utilizes the concept of dissolving a gas into a liquid at or near the point of use with a gasifier, such as a membrane contactor, during the processing of the substrate, thus eliminating the need for pre-made liquid/gas processing mixtures that are typically stored in auxiliary tanks. In one aspect, the invention is an apparatus comprising: a process chamber having a support for supporting a substrate; a source of liquid; a supply line coupling said source of liquid to said process chamber; a gasifier operatively coupled to said supply line, said gasifier causing a gas to be dissolved into said liquid to form a mixture of said liquid and said gas; means for applying a film of said mixture to one side of said substrate while on said support, said first means being in fluid association with said supply line; and a transmitter configured to apply sonic energy to said substrate. The method comprises, in one aspect: supporting a substrate in a process chamber; supplying a liquid to said process chamber from a source of said liquid via a supply line; dissolving a gas into said liquid with a gasifer operatively coupled to said supply line to form a mixture of said liquid and said gas; applying a film of said mixture to one side of said substrate; and applying sonic energy to said substrate while said mixture is being applied. When used to remove photoresist from substrates, the fluid will preferably be deionized water and the gas will be ozone gas.