Thermal processing system

Abstract

A silicon nitride (Si.sub.3 N.sub.4) film and a silicon oxide (SiO.sub.2) film, for example, are successively formed on the surface of semiconductor wafers by the same thermal processing system 3, to form a multilayer insulating structure. A disk trap 5, a valve MV and a water cooled trap 6 are provided in this order in an exhaust gas passage 41 extending from a thermal processing unit 3. A heater 44 is provided to heat the section of the passage 41 upstream of the water-cooled trap 6 as well as the valve MV. Another heater 51 is provided to heat the disk trap 5. A film of Si.sub.3 N.sub.4 is formed on the wafer surface by heating the section of the passage upstream of the water-cooled trap and by heating the disk trap and the valve while cooling the water-cooled trap, to thereby trap a by-product of NH.sub.4 Cl. A film of SiO.sub.2 is then formed on the film of Si.sub.3 O.sub.4 by heating the section of the exhaust passage upstream of the disk trap 5 to trap a by-product of C.sub.x H.sub.y in the disk trap. The valve is closed when processed wafers are transferred out of the thermal processing unit, to prevent reverse flow of a by-product of NH.sub.4 Cl into the thermal processing unit through the water-cooled trap 6.