Implementations of the present disclosure provide apparatus and method for improving gas distribution during thermal processing. One implementation of the present disclosure provides an apparatus for processing a substrate comprising a chamber body defining a processing volume, a substrate support disposed in the processing volume, wherein the substrate support has a substrate supporting surface, a gas source assembly coupled to an inlet of the chamber body, an exhaust assembly coupled to an outlet of the chamber body, and a side gas assembly coupled to a sidewall of the chamber body, wherein the side gas assembly comprises a gas inlet pointed in a direction that is tangential to the edge of the substrate supporting surface, and wherein the gas inlet, the inlet of the chamber body, and the outlet of the chamber body are angularly offset at about 90° with respect to each other, and the gas inlet, the inlet of the chamber body, and the outlet of the chamber body are intersected by a common plane.