A semiconductor substrate processing apparatus includes a vacuum chamber having a processing zone in which a semiconductor substrate may be processed, a process gas source in fluid communication with the vacuum chamber for supplying a process gas into the vacuum chamber, a showerhead module through which process gas from the process gas source is supplied to the processing zone of the vacuum chamber, and a substrate pedestal module. The substrate pedestal module includes a platen made of ceramic material having an upper surface configured to support a semiconductor substrate thereon during processing, a stem made of ceramic material having an upper stem flange that supports the platen, and a backside gas tube made of ceramic material that is located in an interior of the stem. The backside gas tube includes an upper gas tube flange that is located between a lower surface of the platen and an upper surface of the upper stem flange wherein the backside gas tube is in fluid communication with at least one backside gas passage of the platen and the backside gas tube is configured to supply a backside gas to a region below a lower surface of a semiconductor substrate that is to be supported on the upper surface of the platen during processing.