A gas heating device and a processing system for use therein are described for depositing a thin film on a substrate using a vapor deposition process. The gas heating device includes a heating element array having a plurality of heating element zones configured to receive a flow of a film forming composition across or through said plurality of heating element zones in order to cause pyrolysis of one or more constituents of the film forming composition when heated. Additionally, the processing system may include a substrate holder configured to support a substrate. The substrate holder may include a backside gas supply system configured to supply a heat transfer gas to a backside of said substrate, wherein the backside gas supply system is configured to independently supply the heat transfer gas to multiple zones at the backside of the substrate.