A thermal reactor system for semiconductor processing incorporates a reaction vessel with a rectangular quartz tube with reinforcing parallel quartz gussets. The gussets enable sub-ambient pressure processing, while the rectangular tube maximizes reactant gas flow uniformity over a wafer being processed. The gussets facilitate effective cooling, while minimally impairing heating of the wafer by allowing minimal wall thickness. The thermal reactor system further includes a gas source for supplying reactant gas and an exhaust handling system for removing spent gases from and establishing a reduced pressure within the reaction vessel. An array of infrared lamps is used to radiate energy through the quartz tube; the lamps are arranged in a staggered relation relative to the quartz gussets to minimize shadowing. In addition, other non-cylindrical gusseted vessel geometries are disclosed which provide for improved sub-ambient pressure thermal processing of semiconductor wafers.