A deposition system in accordance with one embodiment of the present invention includes a process chamber, a stationary pedestal for supporting a substrate in the process chamber, and a moveable shield forming at least a portion of an enclosure defining the process chamber. Motion of the shield with respect to the stationary pedestal controls a variable gas conductance path for gases flowing through the process chamber thereby modulating the pressure of the process chamber with respect to an external volume. The moveable shield in accordance with an embodiment of the present invention may include several gas channel openings for introducing various process gases into the process chamber. In some embodiments, the moveable shield may alternatively or additionally include an interior cooling or heating channel for temperature control.