Ozone generating apparatus

Abstract: An ozone generating apparatus for producing highly pure ozone gas which can be used in the semiconductor manufacturing process. The ozone generating apparatus comprises a high voltage source, an ozone generating cell which generates ozone gas by supplying a material gas while applying a high voltage from the high voltage source, and a passage for delivering the generated ozone gas from the ozone generating cell to a desired location. The passage comprises a material which has a passivation film formed by a passivation treatment in a dry process. The oxide passivation film comprises chromium oxide film, iron oxide film or a composite film of chromium oxide and iron oxide.

Description: The present invention relates to an ozone generating apparatus for producing highly pure ozone gas which can be used in a process such as a semiconductor manufacturing process. Ozone generating apparatuses for producing highly pure ozone gas have conventionally employed stainless steel as the material for the portions of the apparatus which come in contact with the generated ozone gas. In the case of semiconductor manufacturing processes which are highly sensitive to the generation of particles or gases, stainless steel which has been treated by an electrolytic polishing has been used. As ozone generating apparatuses, there have heretofore been apparatuses which utilize an electric discharge. In the case of using highly pure oxygen gas as material gas, the concentration of ozone is lowered gradually. In order to prevent such lowering of the concentration of ozone, a small amount of nitrogen gas is mixed with the oxygen gas. When stainless steel is employed as the material for the portions of the apparatus which come in contact with the ozone gas, the generated ozone gas contains a trace amount of chromium (Cr) compounds. It is considered that these chromium compounds are generated in the following manner. Namely, when oxygen gas (O 2 ) to which nitrogen gas (N 2 ) has been added is employed as the material gas, NOx (nitrogen oxides) is generated from the nitrogen gas in the ozone generating cell. The NOx reacts with an extremely trace amount of moisture which has been adsorbed by the surface of the stainless steel to form nitric acid. It is considered that this nitric acid then reacts with the stainless steel to form Cr compounds. Further, while the amount of Cr compounds will decline with time if a continuous flow of ozone gas is provided, the amount of time required to reduce the amount of Cr compounds to a level which is not problematic from a practical standpoint is considerably long in the case of regular stainless steel or stainless steel which has been treated by an electrolytic polishing. Further, even if the amount of Cr compounds is reduced once, they may again be generated when the stainless steel comes in contact with the atmosphere during an inspection or overhauling of the pipelines. Thus, a lengthy preparatory operation must be carried out to reduce the amount of Cr compounds. Moreover, SOx (sulfur oxide) or hydrogen fluoride (HF) may be generated in the case where carbon tetrafluoride (CF 4 ), sulfur hexafluoride (SF 6 ), or nitrogen trifluoride (NF 3 ) has been is added to the oxygen gas, resulting in the generation of Cr compounds. Contamination of ozone gas by a trace amount of Cr compounds has not caused a problem in the semiconductor manufacturing process in which the ozone gas has been employed. As the applications for ozone gas have grown, the Cr compounds in the ozone gas used in the semiconductor manufacturing process have become a problem, however, there has not been an effective means of solution. The Document EP-A-0 567 114 describes a discharge reactor for generating a discharge in a space between a dielectric and a high voltage electrode or grounded electrode, the dielectric being located between both electrodes. The reactor may be used as a ozonizer for producing ozone from oxygen gas which is supplied into the discharge space. Dielectrics made of conventional materials such as ceramics are described as having material defects which lead to a deterioration or failure of the dielectric, and as containing impurities which are released into the reaction product. Furthermore, it is described that the electrodes are exposed to concentrated ozone and a high density plasma, which leads to a consumption of the electrodes and a contamination of the product gas with electrode material usually in the form of oxides. In order to provide a discharge reactor which exhibits a constant performance over a prolonged time and which yields cleaner ozone containing reduced amounts of Na, K, Mg, Fe, Cu and Ni and is fit for use in semiconductor fabrication, the dielectric is formed of highly pure quartz glass or crystallized aluminium oxide or fired aluminium oxide. The electrodes exposed to the discharge are made of highly pure aluminium, and the area of the electrodes exposed to discharge is coated with a hard anodic oxidation film. It is therefore the object of the present invention to provide an ozone generating apparatus including a passage for delivering the generated ozone gas from an ozone generating cell to a desired location, the material of the passage comprising stainless steel, wherein the time required for the amount of Cr compounds in a continuous flow of ozone gas to decline is greatly reduced. According to the present invention this object is achieved by a discharge reactor having the features of claim 1. According to the present invention, there is provided an ozone generating apparatus comprising: a high voltage source; an ozone generating cell which generates ozone gas from a supplied material gas, while a high voltage is applied from the high voltage source; and a passage for delivering the generated ozone gas from the ozone generating cell to a desired location, the passage comprising a material having a passivation film formed by a passivation treatment in a dry process. The material for the passage may comprise stainless steel, and the oxide passivation film may comprise chromium oxide film, iron oxide film or a composite film of chromium oxide and iron oxide. Further, a material employing a combination of stainless steel and another material such as Teflon (registered trademark) which does not contain Cr may also be used without altering the efficacy of the apparatus of the present invention. As explained above, moisture adsorbed by the surface of the pipe arrangement or contained in the surface layer is considered to be the cause of generation of Cr compounds. In the case of electrolytically polished stainless steel, because the electrolytic polishing treatment is a wet process, moisture is contained in the naturally grown oxide film layer on the surface and in the base metal underneath this oxide film layer. Further, because the conventional passivation treatment using a nitric acid solution is also a wet process, moisture is contained in the surface of the pipe arrangement, thus requiring a lengthy purge with ozone gas for its removal. In contrast, in the case of a material having a passivation film formed by a passivation treatment in a dry process, the amount of moisture contained in the film is extremely small, and the moisture adsorbed by the surface can be easily removed through purging because of its low activation energy. According to the present invention, the passage of the ozone gas delivery route from the ozone generating cell which comes in contact with the ozone gas comprises stainless steel which has a passivation film formed by a passivation treatment in a dry process. Therefore, the generation of Cr compounds along the ozone gas delivery route can be greatly reduced in a short period of time. The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. FIG. 1 is a schematic view of an ozone generating apparatus according to an embodiment of the present invention; FIG. 2 is a schematic view of an apparatus for measuring the amount of Cr contamination; and FIG. 3 is a graph showing the results of measuring the amount of Cr contamination. An ozone generating apparatus according to a preferred embodiment of the present invention will be described below in detail with reference to FIGS. 1 through 3. FIG. 1 shows a whole structure of the ozone generating apparatus. As shown in FIG. 1, an ozone generating apparatus 10 comprises an ozone generating cell 11, a material gas supply system 12, a gas filter 15, a high frequency high voltage electric source 16, a controller 17 and a chiller unit 18. The ozone generating cell 11 has a cell or a plurality of cells therein. Each of the cells is connected to a pressure regulating valve 14 by a pipe 22. The pressure regulating valve 14 is connected to the gas filter 15 by a pipe 23. The gas filter 15 is connected to an ozone gas outlet 25 by a pipe 24. Further, a coupler 22-1 is provided between the ozone generating cell 11 and the pipe 22. The pipe 22, the coupler 22-1, the pressure regulating valve 14, the pipe 23, the gas filter 15 and the pipe 24 constitute a passage of an ozone gas delivery route for delivering ozone gas generated in the ozone generating cell 11 to a desired location. The passage which comes in contact with the ozone gas comprises stainless steel which has a passivation film formed by a passivation treatment in a dry process. The material gas supply system 12 supplies material gas supplied from a material gas inlet 19 to the ozone generating cell 11 at a constant flow rate. Further, the high frequency high voltage electric source 16 serves to apply a high frequency high voltage to electrodes in the ozone generating cell 11 under the control of the controller 17. The pressure regulating valve 14 serves to regulate the pressure in the ozone generating cell 11 to a predetermined value under the control of the controller 17. The gas filter 15 serves to remove particles which are contained in the ozone gas supplied from the ozone generating cell 11. Deionized water for cooling the ozone generating cell 11 is circulated between the chiller unit 18 and the ozone generating cell 11. The cooling water flows in from a cooling water inlet 20 and flows out from a cooling water outlet 21. The chiller unit 18 is designed so that the deionized water is cooled by the cooling water and supplied to the ozone generating cell 11. As a result, the heat generated by electric discharges such as silent discharges which occur in the ozone generating cell 11 is removed. In the ozone generating apparatus thus constructed, a high frequency high voltage is applied to the ozone generating cell 11 by the high frequency high voltage electric source 16 under the control of controller 17, and material gas is supplied to the ozone generating cell 11 from the material gas supply system 12. The ozone gas generated in the ozone generating cell 11 passes through the pipe 22, and flows into the gas filter 15 via the pressure regulating valve 14. Particles or the like contained in the ozone gas are removed by the gas filter 15. Thereafter, the ozone gas passes through the pipe 24 and the ozone gas outlet 25, and is supplied to a certain process such as the semiconductor manufacturing process. The passage on the downstream side of the ozone generating cell 11, consisting of the pipe 22, the coupler 22-1, the pressure regulating valve 14, the pipe 23, the gas filter 15 and the pipe 24, which comes in contact with the ozone gas comprises stainless steel having a passivation film formed by a passivation treatment in a dry process. If the ozone gas generated in the ozone generating cell 11 does not contain Cr compounds, then the amount of the Cr compounds discharged from the ozone gas outlet 25 can be greatly reduced in a short period of time. Thus, the ozone gas which is not contaminated by Cr compounds can be delivered to the process such as the semiconductor manufacturing process. In a case where the pipe 22, the coupler 22-1, the pressure regulating valve 14, the pipe 23, the gas filter 15 and the pipe 24 in the ozone generating apparatus 10 comprised the materials described below, the amount of Cr compounds in the ozone gas discharged from the ozone gas outlet 25 was equal to or less than 1 × 10 10 atms/cm 2 as measured by the method described below. Structural Material: The coupler 22-1, the pressure regulating valve 14, the pipes 22, 23, 24 and the casing of the gas filter 15 are made of stainless steel (SUS 316) which has a passivation film formed by a passivation treatment in a dry state. The interior of the gas filter is made of Teflon (registered trademark; PTFE and/or PFA). As a passivation treatment in a dry process, there is a method in which electrolytically polished SUS 316L is baked at a high temperature in an inert gas environment, and then oxidized with highly concentrated ozone gas, thereby forming a passivation film of chromium and iron oxides on the surface of the stainless steel. There is also a method, commonly known as the CRP or dry gold method, in which electrolytically polished SUS 316L is baked at a high temperature in an inert gas environment in the same way as the above, and then oxidized with an oxidizing gas, thereby forming a passivation film of chromium oxide or a composite passivation film of chromium and iron oxides. In the oxide passivation films obtained by the above methods, the amount of moisture contained in the film and in the base metal underneath the film is extremely small. FIG. 2 shows an apparatus for measuring the amount of Cr contamination in the cases where different materials are employed in the passage on the downstream side of the ozone generating cell. In FIG. 2, an ozone generating apparatus 33 incorporating an ozone generating cell 34 is provided. Oxygen and nitrogen gases are supplied to the ozone generating cell 34, having electrodes to which a high frequency high voltage is applied, at a constant flow rate from an oxygen gas mass flow controller 31 and a nitrogen gas mass flow controller 32, respectively. The ozone gas generated by the ozone generating cell 34 is introduced into a chamber 37 made of Teflon via Teflon pipes 35-1,35-2, a sample pipe 36 and a Teflon pipe 35-3. The ozone gas in the chamber 37 is then discharged through a Teflon pipe 35-4 and an ozone decomposition tower 39. A silicon wafer 38 is placed in the chamber 37. Cr compounds attached to the surface of the silicon wafer 38 are analyzed by total reflectance fluorescent X-ray spectroscopy (not shown) to measure the amount of Cr contamination. The Cr contamination was measured under the following conditions. Testing Conditions: ozone gas concentration: 8 vol% flow rate of material gas: 10 Nl/min pressure in the ozone generating cell: atmospheric pressure material gas: oxygen containing 0.8 vol% nitrogen purity of material gas: 99.9995% for both O 2 and N 2 ozone gas supplying duration: 10 min diameter of silicon wafer 38: 15.24 cm (6 inches) Test Results: The test results are shown in the graph of FIG. 3. In the case of ordinary SUS 316L EP pipe (electrolytically polished SUS 316L pipe), 120 hours were required to reduce the level of Cr contamination to the order of 1 × 10 10 atoms/cm 2 . In contrast, only 40 hours were required to reduce the level of Cr contamination to the same order of 1 × 10 10 atoms/cm 2 in the case where SUS 316L which has a passivation film formed by a passivation treatment in a dry process was employed. Further, oxygen gas to which nitrogen gas is added, or oxygen gas to which NO X such as NO, NO 2 or N 2 O 5 is added may be used as the material gas. Also, oxygen gas to which one or more of F 2 , SF 6 , CF 6 and NF 3 is added may be used as the material gas. By using stainless steel which has a passivation film formed by a passivation treatment in a dry process for the material of the passage which extends from the ozone gas outlet 25 to the process where the ozone gas generated by the ozone gas generating apparatus 10 is utilized, it is of course possible to reduce the amount of Cr compounds greatly in a short period of time. It should be noted that FIG. 1 shows a structural example of the ozone generating apparatus of the present invention, but any other type of ozone generating apparatus may be used. The essence of the present invention is that the stainless steel which has a passivation film formed by a passivation treatment in a dry process is used for at least the passage of the ozone gas delivery route that comes in contact with the ozone gas supplied from the ozone generating cell in which highly pure ozone gas is generated. Accordingly, the ozone generating mechanism may of course have any type of structure. As is apparent from the above description, according to the present invention, the stainless steel which has a passivation film formed by a passivation treatment in a dry process is used for at least the passage of the ozone delivery route which comes in contact with the ozone gas. As a result, the amount of the Cr compounds along the passage of the ozone delivery route from the ozone generating cell to a desired location such as a semiconductor manufacturing apparatus can be greatly reduced in a short period of time. Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Claims: An ozone generating apparatus (10) comprising: a high voltage source; an ozone generating cell (11) which generates ozone gas from a supplied material gas comprising oxygen gas to which a small amount of nitrogen gas has been added, while a high voltage is applied from said high voltage source (16); and a passage (22, 22-1, 14, 23, 15, 24) for delivering the generated ozone gas from said ozone generating cell to a desired location, said passage comprising stainless steel having a passivation film comprising a chromium oxide film, an iron oxide film or a composite film of chromium oxide and iron oxide formed by a passivation treatment in a dry process. An ozone generating apparatus according to claim 1, wherein said passivation treatment has been performed by baking electrolytically polished stainless steel at a high temperature in an inert gas environment, and oxidizing the baked stainless steel with highly concentrated ozone gas. An ozone generating apparatus according to claim 1, wherein said passivation treatment has been performed by baking electrolytically polished stainless steel at a high temperature in an inert gas environment, and oxidizing the baked stainless steel with an oxidizing gas.

Inventors: HARADA MINORU, KAJI NARUHIKO, NAKANO YUTAKA, NAKATA REMPEI, SHINJO RYOICHI, TSUJIMURA MANABU, HARADA, MINORU, KAJI, NARUHIKO, NAKANO, YUTAKA, NAKATA, REMPEI, SHINJO, RYOICHI, TSUJIMURA, MANABU
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1996
Main classification: C01B13
IPC codes: C01B13