A semiconductor substrate processing apparatus comprises a vacuum chamber in which a semiconductor substrate may be processed, a showerhead module through which process gas from a process gas source is supplied to a processing zone of the vacuum chamber, and a substrate pedestal module. The substrate pedestal module includes a platen, a stem having a side wall defining a cylindrical interior region thereof, a lower surface, and an upper end that supports the platen, and an adapter having a side wall defining a cylindrical interior region thereof and an upper surface that supports the stem. The lower surface of the stem includes a gas inlet in fluid communication with a respective gas passage located in the side wall of the stem and a gas outlet located in an annular gas channel in the upper surface of the adapter. The upper surface of the adapter includes an inner groove located radially inward of the gas outlet and an outer groove located radially outward of the inner groove. The inner groove and the outer groove have respective O-rings therein so as to form a vacuum seals during processing. The platen includes at least one platen gas passage in fluid communication with a respective gas passage in the side wall of the stem through which backside gas can be supplied to a region below a semiconductor substrate when supported on the upper surface of the platen during processing.