Abstract: In the heat treating apparatus disclosed herein, semiconductor wafers are baked on the circular hot plate while being sheathed with a uniform vapor flow. The preferred vapor is a mixture of a relatively large volume of nitrogen carrying a relatively small volume of HMDS. After passing over the wafer, the vapor sheath is drawn, through an annular gap, into an exhaust chamber which surrounds and underlies the hot plate thereby avoiding heat and vapor loss into the other portions of the semiconductor fabrication line within which the baking apparatus is typically incorporated.