METAL OXIDATION APPARATUS

Abstract: Metal oxidn. appts. for forming a passivation film on the surface of a bent pipe of a metal to be oxidised has a bent portion such as a stainless steel pipe. The appts. includes an oxidn. furnace, a gas inlet for introducing a gas into the oxidn. furnace, an exhaust port for discharging the gas from inside the furnace, a heater for heating the furnace to a predetermined temp. and a holder for fixing a tubular metal to be oxidised having a bent portion inside the furnace, the holder serving also as a connection joints. The inlet comes into contact with one of the ends of the bent metal pipe and the exhaust comes into contact with the other end. The bent metal pipe to be oxidised is heated and oxidised in a dry oxidising atmos. while a gas is being passed through the inside of the bent metal pipe to be oxidised.

Description: The present invention relates to the apparatus for the oxidation treatment of metal, and particularly to the oxidation treatment apparatus for the passivation of metal tubular parts having the curved portion, which are to be used for ultra-high clean gas piping system or ultra-high vacuum equipment. In recent years, the technique to attain ultra-high vacuum or the technique to produce ultra-high clean reduced pressure atmosphere by introducing the gas at low flow rate into vacuum chamber is becoming increasingly important. These techniques are applied for the research of material characteristics, for the formation of various types of thin film, for the manufacture of semiconductor devices, etc. As the result, higher degree of vacuum is attained, while there is a strong demand on the reduced atmosphere, where the intermingling of impure elements and impure molecules could be reduced to utmost extent. For example, in the manufacture of semiconductor devices, the dimensions of unit elements are being reduced year by year to attain higher integration of integrated circuit. Fervent research and development activities are carried out for the practical application of semiconductor devices having the dimension of 1 µm to submicrometers or 0.5 µm or less. Such semiconductor devices are manufactured by repeating the process to form thin film and the etching process of the film thus formed into the specified circuit pattern. Usually, such processes are performed in ultra-high vacuum conditions or in reduced pressure atmosphere with the specified gas by placing the silicon wafers into vacuum chamber. If the impurities are intermingled during these processes, the quality of thin film may be reduced or the precision fabrication may not be achieved. This is the reason why ultra-high vacuum and ultra-high clean reduced pressure atmosphere is wanted. One of major reasons to hinder the actualization of ultra-high vacuum and ultra-high clean reduced pressure atmosphere has been the gas released from the surface of stainless steel widely used for the chamber of gas pipe. Above all, the source of the worst contamination has been the moisture adsorbed on the surface, which comes apart under vacuum or reduced pressure atmosphere. Fig. 9 is a graphic representation showing the relation between total leakage of the system, including the gas piping system and reaction chamber in each apparatus (the sum of gas quantity released from inner surface of piping system and reaction chamber with the external leakage), and gas contamination. It is assumed that original gas does not contain the impurities. The lines in the diagram indicate the results when the values are changed with gas flow rate as parameter. Naturally, the lower the gas flow rate is, the more the concentration of the impurities increases as the influence of the released gas from inner surface becomes conspicuous. In the semiconductor manufacturing process, there is a trend to increasingly reduce the gas flow rate in order to attain the process of higher accuracy by opening and filling the holes with high aspect ratio. For example, it is now normal to use the flow rate of several tens of cm³/min or less for the process to manufacture ULSI of submicron order. Suppose that the flow rate is 10 cm³/min and that the total leakage of the system is 0.13 to 0.00013 Pa·l/s (10⁻³ to 10⁻⁶ Torr l/s) as the apparatus currently in use, the purity of the gas is 1% to 10 ppm, and this is far from the high clean process. The present inventors have invented the ultra-high clean gas supply system and have succeeded in reducing the leakage from outside the system to less than 1.3 x 10⁻⁹ Pa·l/s (1x10⁻¹¹ Torr l/sec) which is the detection limit of the detectors presently in use. However, the concentration of the impurities in the reduced pressure atmosphere could not be reduced due to the leakage from inside the system or due to the components of the released gas from the surface of said stainless steel. The minimum value of the surface released gas quantity as obtained by the surface treatment in the ultra-high vacuum technique at present is 1.3 x 10⁻⁹ Pa·l/cm²s 1 x 10⁻¹¹ Torr l/sec cm²) in case of stainless steel. Suppose that the surface area exposed to the interior of the chamber is estimated to the minimum, e.g. to 1 m², the total leakage is 1.3 x 10⁻⁵ Pa·l/s (1 x 10⁻⁷ Torr l/sec. This means that only the gas with purity of about 1 ppm can be obtained to the gas flow rate of 10 cm³/min. The purity is doubtlessly decreased when gas flow rate is lowered further. In order to decrease the released gas from inner surface of the chamber to 1.3 x 10⁻⁹ Pa·l/s (1 x 10⁻¹¹ Torr l/sec), i.e. to the same level as the external leakage of total system, it is necessary to set the released gas from the surface of stainless steel to less than 1.3 x 10⁻¹³ Pa·l/cm²s (1 x 10⁻¹⁵ Torr l/sec cm²). As the result, there is strong demand on the better processing technique for the surface of stainless steel to have lower gas release. In the semiconductor manufacturing process, a wide variety of gas is used from the relatively stable common gas (such as O₂, N₂, Ar, H₂, He) to special gas having higher reactivity, corrosive property and toxicity. As the material for the piping and chamber for these gases, stainless steel is normally used because of its higher reactivity, corrosion resistance, high strength, easy secondary fabrication, weldability and easy polishing of inner surface. Stainless steel shows excellent corrosion-resistant property in a dried gas atmosphere. Among the special gases, however, there are boron trichloride (BCl₃) or boron trifluoride (BF₃), which generate high corrosive property by generating hydrochloric acid or hydrofluoric acid through hydrolysis when moisture exists in the atmosphere. Thus, stainless steel is easily corroded when moisture exists in the gas atmosphere containing BCl₃ or BF₃. Therefore, anti-corrosion processing is indispensable after surface polishing of stainless steel. As the anti-corrosion processing, there are the methods, one of which is Ni-W-P coating (clean escorting method) to coat the highly corrosion-resistant metal on stainless steel. There are some problems with this method because cracking and pinholes often occur and the adsorbed moisture on inner surface or the residual solution components increase because the wet type metalplating is employed. There is also another anti-corrosion processing method by passivation treatment to produce the oxide film on metal surface. Stainless steel is passivated when it is immersed in the solution containing sufficient quantity of oxidizer. In this method, stainless steel is usually immersed in the nitric acid solution at normal temperature or a little higher and the passivation treatment is performed. However, this method is also of wet type, and the residues of moisture and the processing solution remain on inner surface of the piping and the chamber. In the methods as described above, the existence of moisture adsorbed on inner surface gives severe damage to stainless steel when the gas of chlorine type or fluorine type is introduced. Therefore, it is very important for the ultra-high vacuum technique or in the semiconductor manufacturing process to fabricate the chamber or the gas piping system with stainless steel having the passivated film, which is not easily damaged by corrosive gas and which occludes or adsorbs less moisture. For example, in the passivation treatment of stainless steel pipe, the passivated film having excellent degassing property is obtained when heating and oxidation are performed in a highly clean atmosphere with moisture content of less than 10 ppb. Fig. 10 summarizes the changes of moisture contained in the purge gas when the stainless steel pipes with different internal process conditions are purged at normal temperature. In the experiment, argon gas was passed at the flow rate of 1.2 ℓ/min through a stainless steel pipe of 9.55mm (3/8") with total length of 2 m, and the moisture content in argon gas at the outlet was determined by APIMS (atmospheric pressure ionization mass spectrometer). The stainless steel pipes under test are divided into three types: (A) Stainless steel pipe with inner surface processed by electrolytic polishing; (B) Stainless steel pipe with inner surface processed by passivation treatment with nitric acid after electrolytic polishing; (C) Stainless steel pipe, on which the passivated film is formed by heating oxidation in highly clean and dry atmosphere after electrolytic polishing. In Fig. 10, these are represented by the curves A, B and C. The experiment was performed after leaving each of these stainless steel pipes in a clean room maintained at relative humidity of 50% and at temperature of 20°C for about one week. As it is evident from the curves A and B, a large quantity of moisture was detected from the electropolished pipe (A) and the electropolished pipe with passivation treatment with nitric acid (B). After gas was passed for about one hour, moisture of 68 ppb was detected in A and 36 ppb in B. Moisture content did not decrease after 2 hours, showing 41 ppb and 27 ppb in A and B respectively. In contrast, moisture content decreased to 7 ppb within 5 minutes after the gas was passed through the pipe (C) with the passivation film formed in high clean and dry atmosphere. After 15 minutes, it decreased to less than the background level of 3 ppb. Thus, it was demonstrated that (C) has excellent degassing property to adsorption gas. However, in order to attain ultra-high clean oxidation atmosphere with moisture content of less than 10 ppb to produce the stainless steel pipe similar to (C) in Fig. 10, it is essential to have high-grade condition control. This involves higher cost and lower production efficiency and is not suitable for mass production. In other words, it is impossible to attain ultra-high clean oxidation atmosphere by the metal oxidation apparatus and metal oxidation method as conventionally employed. Particularly, in the stainless steel pipe or the stainless steel pipe having the curved portion with smaller inner diameter such as 6.35mm, 9.5mm and 12.7mm (1/4", 3/8" and 1/2"), gas is very likely to stagnate, and oxidation treatment is performed with the inside of the stainless steel pipe exposed to atmospheric air, resulting in contamination. This makes it impossible to form the passivation film of good quality having superb corrosion-resistant property and with lesser moisture occlusion and adsorption. Because outer surface of stainless steel pipe is not directly related with the supply of ultra-high purity gas, the surface becomes contaminated after oxidation treatment due to roughness and dirtiness of the surface. The oxidation of outer surface of stainless steel pipe results in the problems such as poor external appearance or the generation of particles when pipes are installed in a clean room. Therefore, there have been strong demands on the establishment of the mass production technique for the passivation treatment for stainless steel pipe in order that the passivation film is formed to provide inner surface with excellent corrosion-resistant property and to occlude or adsorb the moisture in lesser content and that outer surface is not oxidized. The object of the present invention is to solve these problems by offering metal oxidation treatment apparatus by which the contamination caused by the released gas or the impurities such as moisture from the oxidised surface of stainless steel pipe having the curved portion is reduced and the stainless steel pipe for ultra-high vacuum and ultra-high clean reduced pressure apparatus and for gas supply system having excellent corrosion-resistant property can be produced in large quantity. Another object of this invention is to offer metal oxidation treatment apparatus capable of self-cleaning and self maintenance in addition to the above object. JP-A-61-281864 discloses a method and an apparatus for oxidising a steel strip in order to obtain an oxidised layer by providing a furnace having an oxidising gas inlet and a gas discharge outlet, and heating facilities. US-A-4636266 describes the passivation of the inner walls of stainless steel pipes by subjecting these walls to an oxidising atmosphere at elevated temperatures. According to the invention, there is provided a metal oxidation treatment apparatus for forming a passivation film on the inner surface of a metal pipe having a curved portion, the apparatus comprising: an oxidation furnace, a first gas inlet for introducing a gas into the oxidation furnace, a first discharge outlet for discharging gas from the oxidation furnace, and a heater for heating the oxidation furnace; characterised by a pair of holders specially arranged for holding the pipe, at least one of the pair of holders having a groove therein to hold the pipe, the pair of holders including a pipe insert portion, and each a passage which communicates with the first gas inlet, the discharge outlet and the respective pipe insert portions such that in use oxidation of the inner surface of said pipe is performed by passing the gas into the interior of the curved metal pipe. Preferably, there is a second gas inlet for introducing a purge gas into the oxidation furnace, the second gas inlet being arranged not to in communication with the interior of the curved metal pipe; and a second discharge outlet for discharging the purge gas from the oxidation furnace, the second discharge outlet not being in communication with the interior of the curved metal pipe, such that the exterior of the curved metal pipe in use is prevented from oxidation. In the present invention, stress is given to the efficient exclusion of the impurities such as moisture from the oxidation atmosphere when the oxidation furnace is closed, and the new gas is permanently introduced into the oxidation furnace and the gas is discharged from inside the oxidation furnace. Specifically, when the oxidation treatment is conducted in the interior of the oxidized curved metal pipe such as stainless steel pipe with smaller inner diameter and with the curved portion, where gas is difficult to flow, the gas inlet and outlet are arranged in such manner that they come into contact with both ends of the pipe. Thus, gas is introduced at one end while the oxidation atmosphere gas is forcibly passed through the curved pipe by permanently discharging the gas at the other end. The impurities such as moisture separated from the surface of the oxidized curved metal pipe in the oxidation furnace is discharged from the oxidation furnace, and the oxidized curved metal pipe is heated and oxidized in a dry oxidation atmosphere. This makes it possible to decrease the moisture content in the oxidation atmosphere to lower than the desired value (e.g. less than 10 ppb) and to form good passivation film on the surface of the oxidized metal. To prevent the oxidation of outer surface of the curved pipe, it is possible to form the passivation film only on inner surface of the curved pipe without oxidizing outer surface thereof by passing the inert gas to outside the curved pipe in the oxidation furnace. To obtain such effect more positively, it is advised to increase the pressure of inert gas outside the curved pipe higher than the pressure of the oxidation atmosphere gas inside the curved pipe. This suppresses the flow of the gas from inside to outside of the curved pipe and prevents the leaking of the oxidation atmosphere gas to outside the curved pipe. Giving attention to the contamination before the oxidation furnace is closed, it was attempted in this invention to prevent the intermingling of the impurities such as moisture in the oxidation furnace when the oxidation furnace is opened. When the oxidation furnace is opened and the oxidized curved metal pipe is arranged or fixed in the oxidation furnace, it is very effective, for preventing the exposure of the interior of oxidation furnace and the oxidized curved metal pipe to the atmospheric air containing the impurities, to provide the opening on the side of discharge outlet of the oxidation furnace, to introduce the purge gas permanently from the inlet and to build up gas flow, which passes from inside the oxidation furnace to the opening. This makes it possible to prevent the atmospheric air from entering into the opened oxidation furnace and to reduce the time required for decreasing the moisture content in the oxidation atmosphere to lower than the desired value (e.g. less than 10 ppb). It is also important to obtain the better effect to provide the supply system for the introduced gas with the function to permanently supply high purity gas. Particularly, incase two gas lines such as purge gas line and oxidation atmosphere gas line are connected with the inlet, contamination often occurs within the system with impurities such as moisture when switched over from purge gas to oxidation atmosphere gas or from oxidation atmosphere gas to purge gas. This is mainly caused by the contamination with the released gas, mostly the moisture from inner wall of the pipe when the supply gas (e.g. O₂ as oxidation atmosphere gas) is stopped. When the metal is to be heated and oxidized in the oxidation atmosphere, after the oxidized curved metal pipe is arranged or fixed in the oxidation furnace, the baking and the purge are performed for the oxidation furnace and the oxidized curved metal pipe. Baking is performed at the same temperature as the oxidation temperature until the moisture content in the discharge gas becomes sufficiently low (e.g. less than 10 ppb). After the baking and the purge by the purge gas are completed, the gas to be supplied into the oxidized curved metal pipe is switched over to the oxidation atmosphere gas (such as O₂) to start the oxidation treatment (passivation treatment). If the impurities, mostly moisture, are intermingled in the system during the switch-over of gas, heating and oxidation are performed in the atmosphere containing moisture. Therefore, it is necessary to decrease the temperature inside the oxidation furnace to room temperature for once, to purge the oxidation atmosphere gas when oxidation is not proceeding within the oxidation furnace and to perform the oxidation by increasing the temperature of oxidation furnace after the contaminants are completely removed. However, the time as long as 12 ∼ 24 hours is required for the treatment by decreasing temperature, and it is desirable to have the system, which is capable to reduce the contamination within the system as practical as possible when gas is switched over in order to shorten the oxidation time. For this reason, a system is proposed, in which the inert gas supply line and the oxidation atmosphere gas supply line are switched over by mono-block valve, formed by integrating four valves to minimize dead space, and, of the inert gas supply line and the oxidation atmosphere gas supply line, the supply line not supplying gas to oxidation furnace is always discharged, preventing thereby the stagnation of gas and supplying ultra-high pure gas. This system makes it possible to maintain ultra-high purity of the supplied gas in stable and satisfactory conditions, to switch over the gas very easily and to eliminate the intermingling and the influence of the impurities during switch-over even when oxidation furnace is at high temperature. Specifically, this can be maintained if the moisture content of the atmosphere in the oxidation furnace is set to lower than the desired value (e.g. less than 10 ppb) for once, gas can be switched over without decreasing the temperature of oxidation furnace or performing long-time purge with gas in the oxidation furnace. Further, by installing the heater in the gas supply system, it is possible to heat the introduced gas to the temperature equal to that of the oxidation atmosphere in oxidation furnace, to maintain the temperature of the oxidation atmosphere, to perform positive temperature control in the oxidation furnace and to improve the oxidation efficiency. Thus, it is possible to create an even passivation film on the surface of the oxidised curved metal pipe, to reduce the impurities caused by the released gas from the surface, and to provide a metal oxidation apparatus to offer the parts for ultra-high vacuum and ultra-high clean reduced pressure apparatus and gas supply piping system having excellent anti-corrosion property against the reactants and corrosive gases. In the following, an embodiment of the present invention will be described in connection with the drawings. Fig. 1 is a schematical drawing of an embodiment for the oxidation treatment of an elbow according to the invention. In Fig. 1, 101 represents an elbow, i.e. an oxidized metal pipe having the curved portion, which is usually a pipe of SUS 316L of 6.35mm, 9.5mm and 12.7mm (1/4", 3/8" or 1/2") in diameter with electropolished inner surface. Normally, 20 - 100 pieces of this pipe in regular size are used. Naturally, the pipe may have the diameter other than above. 102 shows an oxidation furnace. This may be made of quartz pipe, but it is desirable to fabricate it with stainless steel with inner surface processed by electropolishing and passivation treatment if consideration is given to thermal expansion and gastightness of the elbow 101 when heating oxidation is performed. 103 and 104 are the holders, concurrently used as gaskets, to give airtightness to the elbow 101 to pass the gas. To provide airtightness when it is inserted into the elbow and heated, it is desirable to fabricate them with the material (such as nickel alloy) having lower thermal expansion coefficient than stainless steel, with easier internal treatment and with less influence from the released gas. Also, the holder 103 is provided with a guide to fix the elbow in upward position. A schematical drawing of the holder 103 is shown in Fig. 7 (a) and (b). Fig. 7 (a) is a view of the holder 103 from above, and a holder to accommodate 34 elbows is given in this example. 701 is a grooved guide to fix the elbow, and 702 is an elbow insert. Fig. 7 (b) is a view of the holder 103 from lateral side. Its left half is a perspective side view, and the right half is a cross-sectional view along the centerline. One end of the elbow is inserted into the elbow insert 702 and a gas inlet 703 is provided to come into contact with one end of the elbow. In the following, description will be given in connection with Fig. 1. 105 and 106 are the flanges and are shaped in such manner that the gas flows evenly in relation to each elbow. 107 is a gas inlet pipe to supply the purge gas (such as Ar) and the oxidation atmosphere gas (such as O₂) into each of the elbows, 108 is an inlet pipe for the purge gas to supply the inert gas (such as Ar) to prevent the contamination through oxidation of outer surface of elbow by providing outer surface of the elbow, and 114 and 115 are the discharge lines of the gas flowing inside and outside each of the elbows. The gas inlet pipes 107 and 108 and the discharge lines 114 and 115 are made of SUS 316L pipes with electropolished inner surface with pipe diameter of 9.5mm, 12.7mm (3/8", 1/2")etc. The opening from gas inlet pipe 107 into the oxidation furnace 102 serves as the inlet, and the opening from the gas inlet pipe 108 into the oxidation furnace 102 is the other inlet. The opening from the discharge line 114 into the oxidation furnace 102 is the discharge inlet, and the opening from the discharge line 115 into the oxidation furnace 102 is the other discharge outlet. 118 represents a float type flowmeter, and 109 and 110 are the mass flow controllers, which regulate the flow rate of the gas flowing in the oxidation furnace 102 and calculate the gas quantity flowing from 109, 110 and 118 to the elbow 101. Of course, a mass flow controller may be used as 118, and the float type flowmeters with needle valve may be used as 109 and 110, but it is desirable to use the mass flow controllers for 109 and 110 in order to keep the atmosphere in the oxidation furnace 102 in highly clean conditions. Although the flowmeter 118 is furnished on the discharge line 115, it may be furnished on the discharge line 114 or on both of the discharge lines 114 and 115. 116 and 117 are MCG (metal C-ring type) joints to separate the discharge lines 114 and 115 when the flange 106 is removed. It is preferrable to use MCG joints in order to exclude external leakage and particles. 119 is a heater, and it is advisable to use a two-piece type electric furnace with longitudinal wiring from the viewpoints of maneuverability and the equalization of the oxidation treatment temperature. 120 and 121 are the heat insulating material to prevent the heat radiation toward longitudinal direction of the electric furnace and to equalize the temperature within the oxidation furnace 102 as practical as possible. 111, 112 and 113 are the heaters to heat the gas entering the oxidation furnace 102 up to the oxidation temperature. 122 is a guide to fix the position of the elbow 101 so that the end of the elbow 101 is easily inserted into the holder 104. 123, 124, 125, 126 and 127 are the packings to seal the oxidation furnace 102 with the flanges 105 and 106, and it is desirable to use the material having elasticity even at more than 500°C (such as nickel alloy) from the viewpoint of the heating and oxidation temperature. Next, the functions and operating procedure of this apparatus will be described in connection with the drawings. Fig. 2 shows the condition where the oxidation furnace 102 is opened and the elbow is not yet accommodated. In the passivation treatment technique, it is necessary to open it in an atmosphere as clean as possible because the cleanness of the atmosphere gives strong influence on the thickness and quality of the passivation film. For this reason, the condition of Fig. 2 is maintained in as short time as possible to minimize the contamination inside the oxidation furnace 102 by atmosphere air. In this embodiment, the side having the flange 106 is opened. The side to be opened may be the side having the flange 105, whereas it is most preferrable from the viewpoint of the contamination due to atmospheric air as described above to continuously pass the purge gas (such as Ar) from the side of 105 while the flange to be opened is provided on the side of 106 and to prevent the intermingling of the atmospheric components into the oxidation furnace 102. Fig. 3 shows the condition where the elbow 101 is accommodated in the oxidation furnace 102 after the condition of Fig. 2. The elbow 101 is inserted along the guide (the guide 701 as shown in Fig. 7 (a)) of the holder 103, and it is set into the elbow insert (the elbow insert 702 as shown in Fig.7 (a) and (b)) of the holder 103. In this case, the intermingling of the atmospheric components should be minimized as practical as possible in similar manner as in Fig. 2. Also, to prevent the generation of the particles, gas should be passed from the gas inlet pipes 107 and 108. Further, a gas guide 122 is placed at the center and fixed. Fig. 4 gives the condition, where, after the condition of Fig. 3, the holder 104 and the flange 106 are mounted on the oxidation furnace 102, where the elbow 101 is set. Fig. 5 shows the condition, where, after the condition of Fig. 4, the discharge lines 114 and 115 are connected with the the joints 116 and 117 respectively. Under this condition, the purge gas (such as Ar) is passed into the elbow 101 and the oxidation furnace 102, and the atmosphere inside the oxidation furnace 102 contaminated by atmospheric air is replaced by inert gas atmosphere. The flow rate of the purge gas naturally differs according to the number of the elbow processable at one time and to the size of oxidation furnace 102. For example, purging is performed with a large quantity of gas for 2 ∼ 4 hours at flow rate of 2 ∼ 10 m/sec to eliminate the contaminants, mostly moisture, inside the oxidation furnace 102. Fig. 6 shows the condition where, after the condition of Fig. 5, the heater 119 and heat insulating material 121 are set. Under this condition, baking and purge of the oxidation furnace 102 and the elbow 101 are performed. Baking is performed at the same temperature as oxidation temperature (e.g. 400 ∼ 550°C) until the moisture content in the gas at the outlet is reduced to less than 5 ppb. In this case, the heaters 111, 112 and 113 of the gas inlet pipe are also heated simultaneously, and the temperature of the gas introduced into oxidation furnace 102 is set to the oxidation temperature (e.g. 400 ∼ 550°C) in order to prevent the temperature decrease inside the oxidation furnace 102 due to the introduction of gas. After baking and purge by the purge gas are completed, the gas supplied into the elbow 101 is switched over to the oxidation atmosphere gas (such as O₂), and oxidation (passivation treatment) is started. During the switch-over of gas, the contaminants, mostly moisture, enters the system. For this reason, it is necessary to decrease the temperature in the oxidation furnace 102 to the room temperature for once, to switch over the gas from the purge gas to the oxidation atmosphere gas (such as O₂) and to perform oxidation by increasing the temperature of oxidation furnace 102 after purging the oxidation atmosphere gas and completely removing the contaminants while oxidation reaction is still not advanced in the oxidation furnace 102. However, the time as long as 12 ∼ 24 hours is required for decreasing the temperature. Therefore, it is necessary to reduce the oxidation time by providing the piping system to minimize the contamination of the system during gas switch-over and by switching over the gas while the oxidation furnace 102 is at high temperature. The contamination of the system, mostly by moisture, during the gas switch-over from the purge gas to the oxidation atmosphere gas or from the oxidation atmosphere gas to the purge gas is caused by the contamination by the released gas, mostly moisture, from inner wall of the pipe because the supplied gas (such as O₂) is stagnated there. Consequently, it is desirable to set up a system where the oxidation atmosphere gas and the purge gas can be always purged and to reduce the contamination in the system during gas switch-over. Fig. 8 shows an example of the piping system to prevent the system contamination during gas switch-over. 107 and 109 correspond to the mass flow controller and gas supply pipe as shown in Fig. 1. 801 shows a supply line of oxidation atmosphere gas (such as O₂) and 802 a supply line of the purge gas (such as Ar). The material differs according to the number of stainless steel pipes to be oxidized or to the size of the oxidation furnace 102. It is usually made of SUS316L of 9.5mm or 12.7mm (3/8" or 1/2")with electropolished inner surface. 803, 804, 805 and 806 represent stop valves. They are a mono-block valve, in which 4 valves are integrated to minimize the dead space. 807 and 808 are the spiral pipes to prevent the intermingling due to reverse diffusion of atmospheric components from the discharge outlet, and 809 and 810 are the float type flowmeters with needle valves. Naturally, the float type flowmeter with separated needle valve or the mass flow controller may be used as 809 or 810. 811 and 812 are the discharge lines, where the gas is discharged after adequate discharge treatment. 813 is an atmosphere gas supply line to supply the gas to oxidation furnace 102 shown in Fig. 1. Next, description will be given on the operation of the piping system of Fig. 8. When purging is performed inside the oxidation furnace, the valves 803 and 806 are closed and 804 is opened, and the purge gas is supplied from 802 through the gas inlet pipe 107 and the mass flow controller 109 to the gas supply line 813. In this case, the valve 805 is opened to purge the oxidation atmosphere gas from the gas supply line 801 through the spiral pipe 807 and the float type flowmeter with needle valve 809 to the discharge line 811. When the purging in the oxidation furnace is completed, the valves 804 and 805 are closed and 803 is opened, and oxidation atmosphere gas is supplied to the atmosphere gas supply line 813. In this case, the valve 806 is opened, and the purge gas is purged to the discharge line 812. The contamination in the system is caused mostly by the moisture when the switch-over is performed from the purge gas to the oxidation atmosphere gas or from the oxidation atmosphere gas to the purge gas. This contamination is mainly caused by the contamination of the gas to be supplied (such as O₂) with the released gas, mostly the moisture, from inner wall of the pipe because the gas is stagnated. Therefore, it is desirable to provide a system, which can permanently purge the oxidation atmosphere gas and the purge gas in order to minimize the contamination of the system even when the gas is switched over. Also, when oxidation atmosphere gas is supplied into oxidation furnace 102 in Fig. 6, it is desirable not to release the oxidation atmosphere gas out of the holders 103 and 104 by decreasing the supply pressure of the oxidation atmosphere gas (for example O₂ supplied from the gas supply piping line 118) flowing inside the pipe to lower than the pressure of inert gas (for example Ar supplied from the gas supply piping line for purge 119) flowing outside the elbow 101 by 9.8 to 29.4 kPa (0.1 to 0.3kg/cm²), to prevent the oxidation and contamination of outer surface of elbow 101. However, if there is no need to protect the outer surface of elbow from oxidation or contamination, it is naturally unnecessary to give differential pressure to the gases flowing inside and outside the elbow and to provide inert atmosphere outside the elbow. When moisture content in the gas discharged from the outlet was measured in this embodiment, the stabilized value of less than 10 ppb was obtained during oxidation treatment. Particularly, the time to attain the value of less than 10 ppb could be reduced in the equipment configuration of Fig. 7. In the piping system of Fig. 8, the value of less than 10 ppb could be maintained even during gas switch-over. Further, after the stainless steel pipe of 9.5mm (3/8") with total length of 2m as obtained by the present embodiment was left for about one week in a clean room maintained at relative humidity of 50% and at temperature of 20°C, argon gas was passed through at flow rate of 1.2 ℓ/min, and moisture content in argon gas at the outlet was measured by APIMS (atmospheric pressure ionization mass spectrometer). As shown by C in the graph of Fig. 10, the value dropped to 7 ppb within 5 minutes after gas was passed and to less than the background level of 3 ppb after 15 minutes. This reveals that the elbow obtained by the embodiment of this invention has excellent degassing property to the adsorbed gas and that the heating oxidation was performed in ultra-high clean atmosphere containing moisture of less than 10 ppb. As described above, the embodiment according to the invention can provide ultra-high clean oxidation atmosphere with moisture content of less than 10 ppb, which the conventional metal oxidation apparatus and metal oxidation method could not actualize, and this is done at low cost and with better production efficiency. In the embodiment above, description was given to the apparatus of Fig. 1 for the passivation treatment of the elbow of stainless steel pipe, whereas it is obvious that the invention is applicable not only to the passivation treatment of elbow but also to the treatment of the metals with different material and shape, e.g. the pipes with the curved portion, valves, etc. of Ni, Aℓ, etc. or to the passivation treatment of the parts of highly clean reduced pressure apparatus. The position, the number and the angle of the curved portion may be selected as desired, and the gas inlet and the discharge outlet can be furnished at the appropriate position depending upon the shape of the metal pipes to be oxidized. Also, the apparatus of this embodiment is shown with the oxidation furnace 102 of vertical type to facilitate the positioning of the elbow for the oxidation treatment, while it may be of horizontal type. The following effects can be obtained by this invention: (1) The invention makes it possible to efficiently eliminate the moisture from the oxidation atmosphere, to perform the heating oxidation for the oxidized metal such as narrow elbow and the like in ultra-high and dry oxidation atmosphere containing very few impurities such as moisture, and to form the passivation film with less released gas containing moisture on the surface of said oxidized metal in easier and efficient manner. (2) In addition to the effects offered by (1) above, it is possible to form the passivation film only on inner surface of the tubular oxidized metal having the curved portion such as elbow and to prevent the oxidation of outer surface. This prevents the roughening and the contamination of outer surface after the oxidation treatment and eliminates the generation of particles when pipe is installed in a clean room. (3) In addition to the effects offered by (1) and (2) above, the invention makes it possible to efficiently prevent the contamination by moisture from atmospheric air when tubular oxidized metal having the curved portion is installed or fixed in the oxidation furnace, to shorten the time until the ultra-high clean and dry oxidation atmosphere is attained, and to form the passivation film in more efficient and satisfactory manner. (4) In addition to the effects offered by (1) to (3) above, the invention makes it possible to perfectly prevent the contamination of the system, mostly from moisture, when the switch-over takes place from the purge gas to the oxidation atmosphere gas or from the oxidation atmosphere gas to the purge gas, and to maintain the ultra-high clean atmosphere at all times, even when the gas is switched over. Therefore, it is possible not only to form the passivation film in satisfactory manner but also to simplify the operation and to eliminate the temperature decrease process during the gas switch-over. Further, it is possible to shorten the time required for the process, and to achieve extensive low cost production by saving energy because no re-heating of the oxidation furnace is required. (5) In addition to the effects offered by (1) to (4) above, the gas is supplied by heating it to that of the oxidation atmosphere. This makes it possible to maintain the oxidation temperature at constant level and to facilitate the control of the processing condition and to improve the oxidation treatment efficiency. As described in (1) to (5) above, the invention makes it possible to actualize mass production of the metal parts such as elbow and the like of stainless steel having the passivation film with very few gas release and having excellent anti-corrosive property. With the elbow and the like thus obtained, it is now possible to provide the system, which can supply ultra-high purity gas to the process equipment within short time, in easier manner and at low cost.

Claims: A metal oxidation treatment apparatus for forming a passivation film on the inner surface of a metal pipe having a curved portion (101), the apparatus comprising: an oxidation furnace (102), a first gas inlet (107) for introducing a gas into the oxidation furnace, a first discharge outlet (114) for discharging gas from the oxidation furnace (102), and a heater (119) for heating the oxidation furnace (102); characterised by a pair of holders (103,104) specially arranged for holding the pipe (101), at least one of the pair of holders (103) having a groove (701) therein to hold the pipe (101), the pair of holders (103,104) including a pipe insert (702) portion, and each a passage which communicates with the first gas inlet (107), the discharge outlet (114) and the respective pipe insert portions (702) such that in use oxidation of the inner surface of said pipe (101) is performed by passing the gas into the interior of the curved metal pipe (101). A metal oxidation treatment apparatus as claimed in Claim 1, characterised by: a second gas inlet (108) for introducing a purge gas into the oxidation furnace (102), the second gas inlet (108) being arranged not to be in communication with the interior of the curved metal pipe (101); and a second discharge outlet (115) for discharging the purge gas from the oxidation furnace (102), the second discharge outlet not being in communication with the interior of the curved metal pipe (101), such that the exterior of the curved metal pipe (101) in use is prevented from oxidation.

Inventors: MIZOKAMI SATOSHI SAIWAI-GARDEN, NAKAHARA FUMIO SANKEN-HAITSU-K, OHMI TADAHIRO, SUGIYAMA KAZUHIKO SANRAIFU-NAG, MIZOKAMI, SATOSHI SAIWAI-GARDENMANSION 212, NAKAHARA, FUMIO SANKEN-HAITSU-KOIWA 101, OHMI, TADAHIRO, SUGIYAMA, KAZUHIKO SANRAIFU-NAGAMACHI 303
Applicants: OHMI TADAHIRO, OSAKA OXYGEN IND, OHMI, TADAHIRO, OSAKA SANSO KOGYO KABUSHIKI-KAISYA
Application year: 1989
Main classification: C23C8
IPC codes: C23C8