1. Field of the Invention
The present invention relates to a silicon/silicon carbide composite and a process for manufacturing the same and more particularly to a silicon/silicon carbide composite which is suitably used for a semiconductor heat treatment member such as a dummy wafer or the like and a process for manufacturing the silicon/silicon carbide composite using cellulose fibers as material.
2. Description of the Related Art
Because the silicon carbide material has characteristics of having high quality in hardness, heat resistance and electric resistance, it is used for an abrasive, a fire resistant material and a heating element or the like.
However, because of such characteristic of high quality in hardness as described above, the silicon carbide material has a defect that its moldability is poor to such an extent that molded products are found fragile.
Conventionally, a reaction sintering method involving use of silicon carbide powder as its starting material and a carbon silicification method using a carbon molding as its base material are subjected to a silicification step or the like for the manufacture of said silicon carbide material.
The reaction sintering method is a method for manufacturing the silicon carbide material by using the silicon carbide powder as its material. After a binder or the like is added to said silicon carbide powder to obtain a molding, said molding is then calcinated at a temperature of 700° C. through 900° C. in a non-oxidizing atmosphere. Then, a fused silicon is permeated thereinto before a reaction sintering is conducted.
On the other hand, a process known as the carbon silicification method is generally disclosed in Japanese Patent Examined Publication 02-56307 or Japanese Patent Unexamined Publication 63-242969; fused silicon or silicon monoxide gas is permeated into a porous molding made of carbon material such as graphite or pitch or the like, where the carbon is silicified with a reaction ofC+Si→SiC or 2C+SiO→SiC+COor the like.
However, it was found in said reaction sintering method that the silicon carbide powder is too expensive and the purity of the silicon carbide material as a product is too dependent on the purity of said raw material. Therefore, such expensive high purity silicon carbide powder makes the reaction sintering method unfeasible in view of the cost of manufacturing the silicon carbide material of high purity.
Also, it has a problem that if molding is conducted after calcinations, the hardness increases due to carbonization of the binder to affect the pre-sintering workability and increase the cost.
Further, micro-grit with a particle diameter of several μm and coarse grit with a particle diameter of tens to hundreds μm are normally mixed for making higher density using the silicon carbide powder for the raw material. However, said coarse grit is easily lost at the time of molding, thus worsening the workability thereof.
On the other hand, it is difficult to silicify such dense carbon material completely by said carbon silicification method because the speed of said reaction is too low. Most of the time, only the surface of the carbon material is silicified resulting in a silicon carbide layer being formed thereon. Therefore, it has been made into a product only in the form of composite material of carbon/silicon carbide.
Also, a method of adding resin to the carbon material for molding is already known in said carbon silicification process from the viewpoint of size stability and molding easiness. For example, Japanese Patent Examined Publication 49-39887 already discloses a method for preparing a silicon carbide material by carbonizing a molding made of a cellulose sheet such as a sheet of paper treated with the resin.
However, with this method, it is difficult to silicify completely the obtained carbon body as deep as the inside thereof because of the high density and elaborate structure thereof with the result that only the silicon carbide in the form of a thin multiplayer structure is obtained.
In this connection, the dummy wafer, wafer boat, reactor core tube or other semiconductor heat treatment members used in the reactor for various semiconductor wafer heat treatments is in general manufactured from a material composed of silicon, a material composed of CVD silicon carbide alone, or a material prepared by forming a CVD silicon carbide film on the surface of a silicon/silicon carbide composite (having silicon contents of 15 to 20 weight %) obtained by adding a binder to two or three types of silicon carbide powder having different mean particle sizes then kneading, corning, forming, calcinating and sintering the same through reaction with fused silicon.
In said semiconductor heat treatments, the CVD film formation on the wafer to be treated simultaneously causes the formation of a CVD film also on the surface of the semiconductor heat treatment member such as a dummy wafer or the like. If such treatments are repeated, the CVD film formed on the surface of said member will become thicker to such an extent the CVD film becomes so brittle due to the difference of the thermal expansion coefficients between said member and the CVD film thereon that an exfoliation thereof can take place during the heat treatments. Such exfoliated CVD film can be broken into particles, which will scatter and fly within the reactor to cause the contamination thereof, lowering the yield. If worse, the member itself can warp or be broken.
Given the situation, the semiconductor heat treatment member itself must be subjected to acid washing after a predetermined service time such that the CVD film on the surface thereof is able to be removed for reuse.
However, semiconductor heat treatment members made of the conventional silicon material is so fragile, limited in the mechanical strength and subject to breakage that an extra care is needed in handling thereof. Moreover, it is so difficult to remove the silicon film alone by means of acid washing in the case of the heat treatment for forming the silicon film thereon that the member itself will be corroded if actually subjected thereto to shorten the service life thereof.
In order to reduce said corrosion of the member caused by the acid, it was proposed to form a silicon carbide film on the surface of the member. However, it was found extremely difficult to form a uniform silicon carbide film without micro cracks being formed on the silicon material surface due to the difference of thermal expansion coefficient between the silicon member and the silicon carbide film.
On the other hand, the silicon/silicon carbide composite obtained from said silicon carbide powder can enjoy a longer service life because the CVD silicon carbide film formed on the surface thereof helps reduce the corrosion of the member caused by the acid washing.
However, the silicon carbide of 80 to 85 weight % contained in the silicon/silicon carbide composite can produce particles after the exfoliation of the silicon film as a result of the difference between the thermal expansion coefficients of the member and the silicon film.
On the other hand, the member made of the CVD silicon carbide material alone has the similar tendency as the silicon/silicon carbide composite produced from said silicon carbide power. Moreover, said member is in general prepared by forming a CVD silicon carbide film on the surface of a carbon substrate and, thereafter, burning out said carbon substrate with the result that the difference between thermal expansion coefficients of the carbon substrate and the CVD silicon carbide film can cause warps thereof and a resultant breakage thereof to make the manufacture of a large product therefrom difficult.