A method for applying a digital watermark to a content signal is disclosed. The mobile service data is subjected to an additional coding process and the coded mobile service data is transmitted. Thereafter, a second gas including a nitrogen-containing compound is introduced into the vacuum chamber, and the semiconductor substrate is irradiated with the second gas plasma. In use, a slurry of fluid and particles enters the cavity through the inlet, the particles are caused to separate from the fluid by action of the centrifugal force, and the separated particles and fluid leave the cavity via the outlet with the particles tending to be closer to the sleeve than the shaft.