A method of treating or processing at least one substrate/workpiece in a plasma comprises steps of: (a) providing an apparatus comprising a chamber defining an interior space; (b) mounting/positioning at least one substrate/workpiece in the interior space; (c) injecting gas(es) into the interior space by means of an electrically isolated gas supply means having at least one outlet orifice; (d) generating a plasma in the interior space; (e) applying a bias potential to the gas supply means to suppress plasma formation at the at least one outlet orifice; and (f) treating/processing the at least one substrate/workpiece in the plasma.