Silicon boat with protective film, method of manufacture thereof, and silicon wafer heat-treated using silicon boat

#The present invention relates to a heat treatment of a silicon wafer, in particular, a boat used for such a heat treatment and such a heat treatment in which metal contamination of a wafer is suppressed.

#For example, a production process of a single crystal silicon wafer includes a step of heat treatment in which a wafer is placed on a jig called boat consisting of SiC or the like and heat-treated. In this heat treatment step, if the heat treatment is performed in a hydrogen or argon atmosphere and a boat made of SiC is used, both of surfaces of the wafer and the boat are etched, and impurities such as heavy metals contained in the base material of SiC may be transferred to the wafer to cause metal contamination.

#On the other hand, if a boat made of silicon (Si) (also referred to as “Si boat” hereafter) is used, it is completely the same material as that of the wafer and hence a boat of low impurities can be produced. Therefore, contamination level will decrease. However, since the surface of Si boat is etched by hydrogen or argon annealing and as a result, Si atoms are exposed. Since they are active, the surface becomes likely to react with metals. That is, the boat made of Si itself is liable to get metal contamination, and if it once suffers from metal contamination, it is highly likely that it becomes a source of contamination thereafter.

#FIG. 1 is a graph showing Fe concentrations of silicon wafers subjected to an Ar heat treatment using Si boats subjected to different pretreatments.

#Hereafter, embodiments of the present invention will be explained specifically. However, the present invention is not limited to these.

#According to the present invention, there is provided a silicon boat in which a protective film consisting of a thermal oxide film is directly formed on a surface of the boat, as a boat that does not impart metal contamination to a wafer, does not damage the wafer surface and does not generate particles even in a case where the wafer is subjected to a heat treatment at a high temperature by using argon or the like. That is, the boat for heat treatment of a wafer according to the present invention is a protective film consisting of a thermal oxide film is directly formed on a surface of the boat. The expression of “a protective film consisting of a thermal oxide film is directly formed on a surface of the boat” used herein means that a native oxide film is removed and the thermal oxide film is formed on an Si surface of the boat.

#If an Si boat on which surface a native oxide film is formed is used as it is in a high temperature heat treatment of a wafer in an H2 or Ar atmosphere, the native oxide film is removed by reduction or the like, or the native oxide film itself may be often contaminated. Thus, it does not function as a protective film. Therefore, according to the present invention, an Si boat is subjected to a heat treatment in an atmosphere of Ar or the like to remove the native oxide film, and then subjected to a heat treatment in an atmosphere containing oxygen to form a protective film consisting of a thermal oxide film directly on a boat surface. Since the thermal oxide film formed in the manner described above is dense, contains little impurities, and is free from generation of pinholes and so forth. Therefore, it subsequently functions as a good protective film in a high temperature Ar heat treatment and so forth.