A chemical mechanical polishing system includes a platen to support a polishing pad, a carrier head to hold a substrate in contact with the polishing pad, a motor to generate relative motion, a steam generator, and an arm extending over the platen with and at least one opening to deliver steam from the steam generator onto the polishing pad. The steam generator includes a canister, a barrier in the canister dividing the canister into a lower chamber having a water inlet and an upper chamber having a steam outlet, a heating element configured to apply heat to a portion of lower chamber, and a nucleation surface positioned in the lower chamber. The barrier has apertures for steam to pass from the lower chamber to the upper chamber and allows for condensation to pass from the upper chamber to the lower chamber.