Methods for cleaning substrates are described including cleaning substrates having hardmask masks and polymer films, such part of semiconductor fabrication. Cleaning methods include ultraviolet (UV) light exposure of process gas mixtures and liquid cleaning chemistries. A substrate and/or process fluids are exposed to ultraviolet radiation. A process gas mixture being irradiated can include an oxidizing gas mixture (air, clean dry air, oxygen, peroxygen, etc.). Reducing gas mixtures, having hydrogen, can also be irradiated. Reactive species from irradiated gas mixtures are exposed to the substrate to chemically modify film properties, such as by facilitating a subsequent liquid cleaning step. Liquid cleaning chemistries on a substrate surface can also be irradiated. Such cleaning techniques enable shorter cleaning times, lower processing temperatures, and reduced damage to underlying or intermediate layers such as dielectric layers.