A vertical diffusion furnace is provided to prevent aluminum powder generated in forming Al2O3 from being blown to a wafer loaded into a boat by mounting a flange cover on a flange of a vertical diffusion furnace. A vertical diffusion furnace includes a flange(22) for mounting a single tube(12) to avoid diffusion of powder. A flange cover(28) is mounted on the flange so that aluminum powder is prevented from being diffused to a wafer loaded into the single tube during the fabricating process. The flange cover is made of quartz.
