A thermal processing system, for example, of hot wall type, may employ a film forming process such as CVD (Chemical Vapor Deposition) method, in which a plurality of wafers held on a wafer boat as substrates, are carried into a reaction tube, and a processing gas is supplied under a reduced pressure. Films are then formed on the wafers by heating the wafers or the processing gas by means of a heater placed at the circumference of the reaction tube.
The wafers carried into the reaction tube are maintained in parallel on the wafer boat to have a gap between each other. Thus, films may be formed by the processing gas not only on the front surfaces of the wafers on which semiconductor devices are formed, but also on the back surfaces of the wafers. The films formed on the back surfaces of the wafers need to be removed because they may cause a deformation of the wafers in a thermal processing.
There has been a conventionally known method that removes particles adhered to a back surface of a wafer, by supplying an organic solvent to the back surface of the wafer rotating about its vertical axis, while the wafer is gripped at the peripheral end and held substantially horizontally to expose its back surface. See, for example, page 2, right column, line 4 from the bottom to page 3, left column, line 15, and FIGS. 1 and 2, of Japanese Patent Laid-Open Publication No. HEI 03-030426. In the following description, a front surface and a back surface of a wafer will be referred to as an upper surface and a lower surface of the wafer, respectively.
In the conventional method as mentioned above, upon supplying a chemical liquid to the lower surface of the wafer, the chemical liquid may spread throughout the lower surface of the wafer due to the centrifugal force resulting from the rotation of the wafer. Then, the chemical liquid is dispersed from the periphery of the wafer after it reaches the distal end of the wafer, and discharged out. However, there is concern that some of the chemical liquid, dispersed away from the periphery of the wafer, may be vaporized to be gas or mist, which rises up and circulates to turn back and adhere to the upper surface of the wafer. As a result, there is a problem that the film formed on the upper surface of the wafer may be partially etched out to damage the semiconductor device thereon.