Abstract:
A method of reducing metallic impurities in an ozone gas piping means, which entails producing ozone gas in an ozone-generating means and piping the produced ozone gas to stainless steel piping means, wherein the ozone gas is produced from a raw material oxygen gas containing at least one additional gas selected from the group consisting of nitrogen, helium, carbon dioxide, carbon monoxide and ozone; the additional gas being used in an amount of 0.025% by volume or more in total, with the proviso that nitrogen is not used as or in said additional gas in an amount of more than 1.0% by volume.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of reducing metallic impurities in ozone gas piping. 
     2. Description of the Background 
     In semiconductor manufacture ozone gas is now used in forming film oxides, ashing resists, and washing silicon wafers. So-called high purity oxygen of 99.95% or more, produced by silent electric discharge as raw material gas, is normally used, because ozone gas of high density and reduced impurities is required for semiconductor manufacture. The high purity/high density ozone gas generated is fed to the site of use by piping made of stainless steel, such as SUS316L for example, for preventing pollution thereof. 
     However, the density of the ozone gas is reduced as time passes when high purity oxygen is used as a raw material gas. In an attempt to solve this problem, a microcatalytic gas, of high purity, has been added to high purity oxygen in certain amounts (Japanese Patent Application Laid-Open Tokkaihei No. 1-282104, Tokkaihei Nol 1-298003, Tokkaihei No. 3-2189051. Nitrogen gas of high purity (99.99% or more) as a catalyst gas is often used in semiconductor manufacture because it is easily available. 
     However, it has recently been determined that when high purity/high density ozone gas produced using high purity oxygen is fed to the site of use, the ozone gas obtained at the feeding location includes metal impurities, thus presenting the possibility that the metallic impurities will deleteriously affect semiconductor manufacture. It is considered that the metallic impurities detected in the supplying location are produced from the feeding piping made of stainless steel, because ozone gas produced by an ozone generator includes little, if any, metallic impurities. However, use of higher quality piping material is impractical due to the increased cost thereof. 
     Thus, a need exists for a method of reducing metallic impurities in ozone gas piping without the attendent higher cost which accompanies the use of higher quality piping. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a means for reducing metallic impurities in ozone gas piping without using more expensive piping materials. 
     The above object and others are provided by a method of reducing metallic impurities in ozone gas piping, which entails producing ozone gas in an ozone-generating means and piping the produced ozone gas to stainless steel piping means, wherein the ozone gas is produced from a raw material gas containing high density/high purity oxygen gas and at least one additional gas selected from the group consisting of nitrogen, helium, carbon dioxide, carbon monoxide and argon; the additional gas being used in an amount of about 0.025% by volume or more in total, with the proviso that nitrogen is not used as or in the additional gas in an amount of more than about 1.0% by volume. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a film making apparatus using the present invention. 
     FIGS. 2A and 2B illustrate the use of the present invention with a discharge cell. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a method of reducing the amount of metallic impurities from ozone gas supply piping which contaminate ozone in the supply piping. The problem of metallic impurities arises from feeding ozone gas, produced by an ozone generator using high purity oxygen, through the stainless steel piping. 
     The present invention is predicated, in part, upon the surprising discovery that this problem can be allevated by using a certain amount of nitrogen gas as a catalyst in the high purity oxygen, thereby preventing the reduction in density of the ozone gas as time passes. 
     In particular, when nitrogen gas is present in the raw material high purity oxygen, nitrogen oxide is produced as a by-product in the product ozone gas to deteriorate or corrode the inner face of the stainless steel piping. As a result, the metallic impurities originating from the stainless steel piping are separated at the site of use. 
     It has been considered that the nitrogen oxide is not harmful enough to corrode the stainless steel piping, because the dew point of the oxygen in the high purity oxygen used in semiconductor manufacture is sufficiently low. As the dew point of the raw material gas is lowered when the ozone is produced with the use of an ozone producing machine, nitrogen oxide production as a by-product is prevented. 
     It has been further discovered that the metallic impurity amount may be controlled to be negligible even if a sufficient amount of other catalytic gas is added as long as the nitrogen density in the raw material gas is restricted to be about 1.0% by vol. or lower, and, preferably, 0.25% by vol. or lower. 
     The present invention, thus, relates to a method of reducing metallic impurities in ozone gas piping, whereby one or more of nitrogen, helium, argon, carbon dioxide and/or carbon monoxide are added in an amount of about 0.025% by vol. or more in total to the high purity oxygen gas, which is used as a raw material gas for production of ozone from an ozone generator. Notably, the nitrogen gas density in the raw material is limited to 1.0% by vol. or lower. 
     In the present method of reducing metallic impurities, the catalytic gas is added with the object of preventing reduction in ozone density with the passage of time. One or more of nitrogen, helium, argon, carbon dioxide and/or carbon monoxide are used as the catalytic gas. Even when other catalytic gases are used, a nitrogen gas density of 1.0% by vol. or lower, and preferably, 0.5% by vol. or lower, and more preferably, 0.25% by vol. or lower is used. 
     The catalytic gas is used in a total amount of no more than about 10% by volume based upon the total gas volume. However, helium, argon, carbon dioxide and/or carbon monoxide are preferably used in an amount of up to about 1.0% by vol. or even more in total as a sufficient adding effect is obtained even when small amounts of 1.0% by vol. or lower are used. Therefore, helium, argon, carbon dioxide and carbon monoxide are tolerable for metallic oxide reduction when used in an amount of about 1.0% by vol. or more in total amount, although the nitrogen gas used is preferably not added in an amount of more than about 1.0% by volume. 
     Also, the effect of controlling economy in the zone density is larger with nitrogen gas. Thus, it is effective to mix a small amount of nitrogen gas into one or more of helium gas, argon gas, carbon dioxide gas and carbon monoxide gas to form a mixture of the additional gases. 
     The present method of reducing metallic impurities can be applied, particularly, to an ozone gas feeding system which is used in semiconductor manufacture, and can also be applied to an ozone gas feeding system in the manufacture of liquid crystal display. The purity of the high purity oxygen is preferably about 99.99% or more, whereas the purity of the catalytic gas is preferably about 99.99% or more. 
     The present invention will now be further illustrated by reference to certain examples which are provided solely for purposes of illustration and are not intended to be limitative. 
     Example 1 
     FIG. 1 is a model view of a film making apparatus showing the application example of the present invention. This is a so-called TEOS-CVD. The silicon basic plate 1 is heated by heater 2, while the raw material gas for making film is sprayed on the surface of the silicon basic plate 1 by nozzle 3 of a branch plate type. 
     The raw material gases for film manufacturing use are TEOS and ozone gas. TEOS, or Si(OC 2  H 5 ) 4 , is heated as a liquid and evaporated, and is fed, together with the nitrogen as a carrier gas, to nozzle 3 through stainless steel piping 4. Reference numeral 7 denotes a heater. In the ozone gas, high purity oxygen gas is used as the raw material gas, and nitrogen gas as the catalytic gas, with both being fed to ozone producing machine 5. The ozone gas produced is fed to nozzle 3 through stainless steel pipe 6. The silicon oxygen film is formed on the surface by the spraying operation of the respective gases on the surface of silicon basic plate 1 from nozzle 3. 
     At this time, metallic oxide orginating from stainless steel piping 6 for feeding oxygen gas becomes problematic. When the catalytic gas used is nitrogen, the density in the raw material gas is from about 0.025% by vol. to 1.0% by vol., and preferably about 0.5% by vol. or lower and even more preferably about 0.25% by vol. or lower. When the catalytic gas used is helium gas, the density used is from about 0.025 to 10% by vol. When the catalytic gas used is argon gas, carbon dioxide gas, or carbon monoxide gas, an amount of about 0.025 to 10% by vol. When a mixture of these gases is used, the total amount used is from about 0.025 to 10% by vol. The nitrogen gas amount used is about 1.0% by vol. or lower, preferably 0.5% by vol. or lower, and even more preferably, 0.25% by vol. or lower. 
     The present invention will now be further described with reference to certain Examples and comparative Examples. The examples herein are provided solely for purposes of ilustration and are not intended to be limitative. 
     EXAMPLES 
     Ozone gas produced by an ozone generator (Table 1) of a silent electric discharge type is fed through stainless steel piping. The metallic impurity amount, and the ozone density of the feeding gas were measured. 
     As a raw material gas, the following gas mixture was used: high purity oxygen gas of 99.99% or more in purity, nitrogen gas of 99.999% or more in purity in an amount of 0.020 to 1.5% by vol., argon gas of 99.999% or more in purity in an amount of 0.020 to 12% by vol., helium gas of 99.999% or more in purity in an amount of 0.018 to 11% by vol., or carbon dioxide gas of 99.999% or more in purity in an amount of 0.010 to 15% by vol., carbon monoxide gas of 99.999% or more in purity in an amount of 0.020 to 14% by vol., nitrogen gas of 0.2% by vol. and argon gas of 0.28 to 3.0% by vol. mixed were used, and nitrogen gas of 0.2% by vol. and helium gas of 0.6 to 5.3% by vol. mixed were used. 
     SUS 316L-EP of 1/1 inch (6.35 mm) in outer diameter, 1 mm in thickness, 1 m in length was used as stainless steel piping. Composition (standard value) of the SUS 316L (JIS G3459) is shown in Table 2. &#34;EP&#34; denotes electrical polishing, and refers to electrolytic polishing. 
     Quantitative analysis of the metallic impurities was conducted by spraying on the surface of silicon wafer ozone gas of ozone density of 120 g/m 3  for 15 minutes×6 liter/min., decomposing the surface with fluorine acid steam by a vapor phase decomposing method (VPD: Vapor Phase Decomposition), and analyzing the recovered decomposition liquid by ICP-MS (Induction Coupled Plasma Mass Spectroscopy.) 
     The measurement of nitrogen oxide, conducted to confirm whether the reduction in metallic impurities was caused by the influence of nitrogen oxide, was conducted by introducing ozone gas of 120 g/m 3  to the air with pure water contained therein for 12 hours×6 liter/min. to absorb or collect the nitrogen oxide in the ozone gas, and then analyzing the solution by ion chromatography. 
     The measurement of the ozone density entailed observing the stability of the ozone density using an infrared ray extinction ozone monitor. 
     The measured results are shown in Tables 3 through 8. The tolerance of the metallic impurities with respect to the gas to be used in a semiconductor manufacturing step is normally 10 11  atoms/cm 2  or lower on the silicon wafer. The reduction of metallic impurities, the ozone density stability, and the economy were evaluated by the following standards. 
     Reduction of Metallic Impurities 
     Evaluation by the largest amount of metallic elements: 
     X ; about 10 12  atoms/cm 2   
     Δ ; about 10 11  atoms/cm 2   
     ∘ ; about 10 10  atoms/cm 2   
     ⊚; about 10 4  atoms/cm 2   
     Ozone Density Stability 
     Evaluation by ozone density variation amount in continuous one hour: 
     X; 120 g/M 3  ±10% or more 
     ∘; 120 g/m 3  ±10% or lower 
     Economy 
     Evaluation in the additional amount of catalytic gas: 
     X; 10% or more 
     Δ; 1% or more 10% or lower 
     ∘; 1% or lower 
     As shown in Table 3, the amount of each of Cr, Fe and Ni can each be reduced to 10 11  atoms/cm 2  or lower by using a nitrogen gas density of 1.0% by vol. or lower. However, the stability of the ozone density is reduced when a nitrogen gas density of 0.025% by vol. or lower is used. 
     When the catalytic gas used is argon, helium, carbon dioxide, and/or carbon monoxide, the production of metallic impurities is negligible, and the effect of the catalytic gas is independent of the gas density thereof. 10% by vol. or lower is sufficient. In fact, no difficulty is noted even if the amount of catalytic gas used is 1.0% by vol. or lower. Therefore, 1.0% by vol. or lower is acceptable. However, the stability of the ozone density is reduced when the gas density thereof becomes 0.025% by vol. or lower. 
     As is apparent from Table 6, the nitrogen gas used can be mixed with other gases if the amount of nitrogen gas is small. The other gas, even in this case, need not be added in an amount of 1.0% by vol. or more. 
     
                       TABLE 1______________________________________The Specifications of Ozone Producing Machine______________________________________Discharge Cell       Construction  Plate type (See FIG. 2)       Electrode     Ceramic coated electrode       Cooling system                     Air and water cooled       Amount of     6 cells       discharge cellsElectric    Discharge voltage                     ˜10 kv (less than 10 kv)Specifications       Discharge     500 through 1500 Hz       frequency       Discharge power                     0.5 through 1.5 w/cm.sup.2       density       Power consumption                     ˜200 w/cell______________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Element C   Si  Mn  P    S    Ni     Cr     Mo    Fe__________________________________________________________________________Component &lt;0.03%     &lt;1.00%         &lt;2.00%             &lt;0.040%                  &lt;0.030%                       12.00˜16.00%                              16.00˜18.00%                                     2.00˜3.00%                                           residueRatio__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________At the nitrogen addition time            Nitrogen            Oxide    Metallic Impurity AmountAdditional Gas   Amount   (10.sup.10 atoms/cm.sup.2)                                 Ozone       Addi.            (ng/l)          Reduction                                 Density                                      EconomyItem  No   Kind       Amount            NO.sub.3 --                 Cr  Fe  Ni Effect                                 Stability                                      (additional gas)__________________________________________________________________________Cornparative 1 nitrogen       1.5% 620000                 310 120 0.61                            x    ◯                                      ΔExample 2 nitrogen       1.0% 240000                 300 110 0.55                            x    ◯                                      ◯Present 3 nitrogen       0.9% 95000                 82  41  0.48    ◯                                      ◯Invention                        Δ 4 nitrogen       0.60%            76000                 19  11  0.48                            Δ                                 ◯                                      ◯ 5 nitrogen       0.50%            58000                 9.3 6.7 0.52                            ◯                                 ◯                                      ◯ 6 nitrogen       0.20%            17000                 0.59                     0.82                         0.47                            ⊚                                 ◯                                      ◯ 7 nitrogen        0.025%             5900                 0.45                     0.78                         0.53                            ⊚                                 ◯                                      ◯Comparative 8 nitrogen        0.020%             5200                 0.52                     0.95                         0.61                            ⊚                                 x    ◯Example 9 no addition             300 0.44                     0.82                         0.49                            ⊚                                 x    ◯__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________At the argon addition time            Nitrogen            Oxide    Metallic Impurity AmountAdditional Gas   Amount   (10.sup.10 atoms/cm.sup.2)                                 Ozone       Addi.            (ng/l)          Reduction                                 Density                                      EconomyItem  No   Kind       Amount            NO.sub.3 --                 Cr  Fe  Ni Effect                                 Stability                                      (additional gas)__________________________________________________________________________Comparative 1 argon       12%  250  0.62                     0.84                         0.54                            ⊚                                 ◯                                      xExample 2 argon       9.0% 320  0.46                     0.49                         0.85                            ⊚                                 ◯                                      ΔExamples 3 argon       4.5% 430  0.76                     0.63                         0.33                            ⊚                                 ◯                                      Δ(Present 4 argon       1.0% 280  0.48                     0.76                         0.76                            ⊚                                 ◯                                      ΔInvention) 5 argon        0.90%            390  0.71                     0.65                         0.62                            ⊚                                 ◯                                      ◯ 6 argon        0.028%            230  0.56                     0.81                         0.66                            ⊚                                 ◯                                      ◯Cornparative 7 argon        0.020%            180  0.45                     0.84                         0.81                            ⊚                                 x    ◯Example 8 no addition            300  0.44                     0.82                         0.49                            ⊚                                 x    ◯__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________At the helium addition time            Nitrogen            Oxide    Metallic Impurity AmountAdditional Gas   Amount   (10.sup.10 atoms/cm.sup.2)                                 Ozone       Addi.            (ng/l)          Reduction                                 Density                                      EconomyItem  No   Kind       Amount            NO.sub.3 --                 Cr  Fe  Ni Effect                                 Stability                                      (additional gas)__________________________________________________________________________Comparative 1 helium       11%  290  0.75                     0.46                         0.79                            ⊚                                 ◯                                      xExample 2 helium       8.8% 350  0.62                     0.52                         0.32                            ⊚                                 ◯                                      ΔExamples 3 helium       5.3% 270  0.48                     0.75                         0.62                            ⊚                                 ◯                                      Δ(Present 4 helium       1.0% 490  0.92                     0.95                         0.45                            ⊚                                 ◯                                      ΔInvention) 5 helium        0.88%            470  0.83                     0.61                         0.43                            ⊚                                 ◯                                      ◯ 6 helium        0.027%            260  0.60                     0.84                         0.32                            ⊚                                 ◯                                      ◯Comparative 7 helium        0.018%            410  0.36                     0.54                         0.80                            ⊚                                 x    ◯Example 8 no addition            300  0.44                     0.82                         0.49                            ⊚                                 x    ◯__________________________________________________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________At the mixed gas addition time              Nitrogen              Oxide                   Metallic Impurity Amount              Amount                   (10.sup.10 atoms/cm.sup.2)                                 OzoneAdditional Gas     (ng/l)        Reduction                                 Density                                     EconomyItem No    %     % NO.sub.3 --                   Cr Fe Ni Effect                                 Stability                                     (additional gas)__________________________________________________________________________Example1 nitrogen      0.2        argon            9.0              19000                   0.75                      0.82                         0.48                            ⊚                                 ◯                                     Δ2 nitrogen      0.2        argon            4.5              21000                   0.51                      0.96                         0.39                            ⊚                                 ◯                                     Δ3 nitrogen      0.2        argon            1.0              15000                   0.55                      0.74                         0.51                            ⊚                                 ◯                                     Δ4 nitrogen      0.2        argon            0.2              22000                   0.81                      0.61                         0.73                            ⊚                                 ◯                                     ◯5 nitrogen      0.2        helium            5.3              18000                   0.86                      0.81                         0.46                            ⊚                                 ◯                                     Δ6 nitrogen      0.2        helium            0.6              19000                   0.89                      0.72                         0.49                            ⊚                                 ◯                                     ◯__________________________________________________________________________ 
    
     
                                           TABLE 7__________________________________________________________________________At the carbon dioxide addition time            Nitrogen            Oxide    Metallic Impurity AmountAdditional Gas   Amount   (10.sup.10 atoms/cm.sup.2)                                 Ozone       Addi.            (ng/l)          Reduction                                 Density                                      EconomyItem  No   Kind       Amount            NO.sub.3 --                 Cr  Fe  Ni Effect                                 Stability                                      (additional gas)__________________________________________________________________________Comparative 1 carbon       15%  150  0.32                     0.82                         0.34                            ⊚                                 ◯                                      xExample dioxide 2 carbon       7.5% 260  0.75                     0.33                         0.71                            ⊚                                 ◯                                      Δ   dioxideExamples 3 carbon       5.0% 180  0.60                     0.64                         0.75                            ⊚                                 ◯                                      Δ(Present   dioxideInvention) 4 carbon       1.0% 170  0.46                     0.39                         0.66                            ⊚                                 ◯                                      Δ   dioxide 5 carbon        0.75%            210  0.31                     0.41                         0.52                            ⊚                                 ◯                                      ◯   dioxide 6 carbon        0.025%            210  0.42                     0.51                         0.63                            ⊚                                 ◯                                      ◯   dioxideComparative 7 carbon        0.010%            190  0.33                     0.39                         0.82                            ⊚                                 x    ◯Example dioxide 8 no addition            300  0.44                     0.82                         0.49                            ⊚                                 x    ◯__________________________________________________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________At the carbon monoxide addition time            Nitrogen            Oxide    Metallic Impurity AmountAdditional Gas   Amount   (10.sup.10 atoms/cm.sup.2)                                 Ozone        Addi.             (ng/l)         Reduction                                 Density                                      EconomyItem  No   Kind Amount             NO.sub.3 --                  Cr Fe  Ni Effect                                 Stability                                      (additional gas)__________________________________________________________________________Comparative 1 carbon        14%  190  0.91                     0.81                         0.96                            ⊚                                 ◯                                      xExample monoxide 2 carbon        8.0% 210  0.31                     0.40                         0.21                            ⊚                                 ◯                                      Δ   monoxideExamples 3 carbon        5.0% 220  0.41                     0.55                         0.62                            ⊚                                 ◯                                      Δ(Present   monoxideInvention) 4 carbon        1.0% 150  0.31                     0.42                         0.61                            ⊚                                 ◯                                      Δ   monoxide 5 carbon         0.80%             220  0.71                     0.79                         0.86                            ⊚                                 ◯                                      ◯   monoxide 6 carbon         0.030%             160  0.92                     0.71                         0.86                            ⊚                                 ◯                                      ◯   monoxideComparative 7 carbon         0.020%             150  0.66                     0.72                         0.60                            ⊚                                 x    ◯Example monoxide 8 no addition             300  0.44                     0.82                         0.49                            ⊚                                 x    ◯__________________________________________________________________________ 
    
     Having described the present invention, it will now be apparent to those skilled in the art that many changes and modifications may be made to the above-described embodiments, without departing from the point of the present invention.