A substrate processing apparatus removes resist films formed on wafers by holding the wafers in a processing vessel and exposing the wafers to a mixed gaseous fluid of steam and an ozone-containing gas into the processing vessel. The baffle system includes a first enclosure and a second enclosure spaced apart from a wall of the first enclosure to define a first fluid flow passage extending between the second enclosure and the wall. A controller is provided for controlling the composition processor to selectively vary the loss tangent, the permittivity andor the permeability in response to a waveguide attenuator control signal. If the support can be provided, the support is provided; otherwise, the support is excluded.