US Patent 7789964 Method for cleaning a surface of a photomask

A method of cleaning a surface of a photomask by removing contaminants from its surface that includes placing the photomask in a vessel, which is held under an elevated pressure and feeding a supercritical fluid, in particular, CO2 in a supercritical state, to the vessel. An additive, such as alcohol, water ketones, esters, surfactants, and organic solvents, can be added to the fluid. The vessel can be held under a pressure higher than the critical pressure of the fluid and at a temperature higher than the critical temperature of the fluid.