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Products Specification

Stainless steel for high-purity gases

 

Stainless steel for high-purity gases

In the field of the manufacturing of semiconductors or liquid crystal displays, the degree of the integration of devices has increased in recent years.


   

In the manufacturing of a device called VLSI, a fine pattern of 1 micron or less is required. In such a manufacturing process, fine dust or an extremely small amount of gas impurities are deposited to or adsorbed by a wiring pattern to cause a circuit failure. It is therefore necessary that both a reaction gas and a carrier gas used have high purity; that is, only a few particles and gas impurities can be present in these gases. For this reason, a pipe or a member used for such gases that have high-purity is required that the inner surface thereof discharges as contaminants only minimum amounts of particles and gases. Besides inert gases such as nitrogen and argon, many gases called speciality gases are also used as gases for manufacturing semiconductors. Examples of the speciality gases include corrosive gases such as chlorine, hydrogen chloride and hydrogen bromide, and chemically-unstable gases such as silane. For the former gases is required corrosion resistance, and for the latter gases is required non-catalytic property (the property of preventing the decomposition of silane gas or the like to produce particles, which is caused due to the catalytic property of the inner surface of a pipe).


   

Heretofore, in order to reduce the deposition or adsorption of dust or water, the inner surface of the pipe or the member for gases used for manufacturing semiconductors has been smoothed until the roughness thereof in Rmax becomes 1 micron or less. Cold drawing, mechanical polishing, chemical polishing, electropolishing, or the combination thereof can be mentioned as the method for smoothing the inner surface of the pipe or the member. However, a highly-smoothed material having an Rmax of 1 micron or less is chiefly obtained by means of electropolishing. The pipe or the like whose inner surface is smoothed is then washed with high-purity water, and dried by a high-purity gas to obtain a final product.


   

Welding is generally adopted when a pipe line is laid. This is because welding can ensure high strength and good airtightness to the pipe line. In the laying of a pipe line by welding, usually a high-purity inert gas, typically argon gas is allowed to run as a shielding gas through a pipe whose inner surface will come into contact with a high-purity gas, in order to avoid, as much as possible, contamination and oxidation of a part which is heated to high temperatures. Further, after the pipe line is laid, the pipes are purged with high-purity argon or nitrogen gas to remove those particles which are still remaining in the pipes. It takes several days to several weeks for this purging operation when the pipe line is long and complicated, such as a plant pipe line. Recently, decrease in the cost of the construction of a semiconductor-manufacturing plant and the early operation of the plant have been strongly demanded. To meet these demands, it is now required to shorten the purging time.


   

Besides the aforementioned properties, the pipe and the member for high purity gases are required to have weldability; the joint area thereof to which mechanical sealing is applied is required to have abrasion resistance; and when parts such as joints are produced by machining, machinability is required.


   

On the other hand, it has been known that corrosion resistance to and non-catalytic property against speciality gases, which are required for the pipe or the like for gases used for manufacturing semiconductors, can be improved by forming a Cr oxide layer on the surface of stainless steel by heating the steel under such an atmosphere in that the partial pressure of oxygen is controlled (see "Special Technique for Non-Corrosive, Non-Catalytic Cr2 O3 Stainless Steel Pipes", The 24th VLSI Ultra-Clean Technology Workshop held by Ultra Clean Society, pp. 55-67, Jun. 5, 1993). It is noted that the objective material for the pipes reported in this literature is assumed to be SUS 316L stainless steel.


   

The above demand of corrosion resistance and non-catalytic property is made not only for a pipe line for gases. The same demand is also made for stainless steels which are used for various types of apparatus for manufacturing semiconductors, in which a wafer is finely processed. Austenitic stainless steels, in particular, type SUS 316L is mainly used as a material for the pipes and the members of such apparatus.


   

Japanese Laid-Open Patent Publication No. 161145/1988 discloses non-standard high-cleanness austenitic stainless steels which are used for steel pipes arranged in a clean room. Non-metallic inclusions are reduced by limiting Mn, Si, Al and O (oxygen) contents so as to decrease the production of the previously-mentioned particles from the inner surface of the pipes.


   

Further, Japanese Laid-Open Patent Publication No. 198463/1989 discloses stainless steel members for an apparatus used for manufacturing semiconductors. These members are produced in such a manner in that stainless steel after subjected to electropolishing is heated in an oxidizing gas which is under the specific conditions to form thereon an oxide layer having a thickness of 100 to 500 angstrom, in which the proportion of the number of Ni atoms in the outer part of the layer and that of the numbers of Cr atoms in the inner part of the layer are in respective predetermined ranges.


   

Furthermore, Japanese Laid-Open Patent Publication No. 59524/1993 discloses stainless steel members for an ultra-high vacuum apparatus, which are obtained by forming a Cr2 O3 layer having a thickness of 20 to 150 angstrom on the surface layer of stainless steel in which Cr and Mo contents are in a specific relation. It is described that this layer can be obtained, for example, by heating the stainless steel at 250 to 550�� C. under such an atmosphere in that the partial pressure of oxygen is 5 Pa (50 ppm) or less.


   

It can be expected that non-dusting characteristics under steady state conditions, which are indispensable for a stainless steel pipe for high-purity gases, are obtained by smoothing the inner surface of the pipe, and by reducing non-metallic inclusions as described in Japanese Laid-Open Patent Publication No. 161145/1988. However, when pipes or members are laid by welding, the welds thereof produce a large amount of dust. This is an essential problem for a pipe line for high-purity gases, for which the characteristics of producing no dust or only a few dust particles are important.


   

Regarding the dust which is produced when the pipes or members are welded, the particles remaining therein are removed by means of purging after they are laid as described previously. However, to purge a complicated pipe line in a whole plant creates two problems from the viewpoints of decreasing the cost of plant construction and of the necessitating the early operation of the plant. These problems cannot be successfully solved by the conventionally adopted methods, such as the smoothing of the surface of stainless steel, and the simple reduction of non-metallic inclusions contained in steel.


   

Further, the previously-described corrosion resistance and non-catalytic property against speciality gases can be improved by forming a Cr oxide layer on the surface of stainless steel. When the method for producing a pipe or a member for gases used for manufacturing semiconductors is taken into consideration, the treatment for forming a Cr oxide layer should be carried out after the surface of the stainless steel which will come into contact with a gas is smoothed by means of electropolishing. However, since the diffusion of Cr is slow in conventional austenitic stainless steel, it is not easy to form on the steel a Cr oxide layer which can sufficiently show the above properties even when the steel is subjected to the oxidation treatment after it is smoothed by electropolishing. This problem cannot be solved even by reducing the amount of non-metallic inclusions.