A reactor for heat treatment of a substrate having a process chamber within a substrate enclosing structure, and a support structure configured to position a substrate at a predetermined spacing between the upper part and the bottom part within the process chamber during processing. Streams of gas may lift the substrate from the support structure so that the substrate floats. A plurality of heating elements is associated with at least one of the upper part and the bottom part and are arranged to define heating zones. A controller controls the heating elements individually so that each heating zone is configured to have a predetermined temperature determined by the controller. The heating zones provide for a non-uniform heating laterally across the substrate.