A cover member for a semiconductor processing thermal reactor. The cover member has a central quartz window portion and an outer flange portion. The central window portion has either an inward bow defining a concave outside surface, or an outward bow defining a convex outside surface. The centerline of the central window portion has a radius of curvature which when extended through the flange portion divides the flange portion into an upper flange section and a lower flange section. The upper and lower flange portions having substantially equal masses.