WO2005031039A2

WO2005031039A2 - Fluorine gas production unit

Info

Publication number: WO2005031039A2
Application number: PCT/IB2004/003030
Authority: WO
Inventors: Minoru Ino; Yukinobu Nishikawa; Fumihiro Honma
Original assignee: L'air Liquide, Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2003-09-24
Filing date: 2004-09-16
Publication date: 2005-04-07
Also published as: WO2005031039A3; JP2005097667A; TW200512317A

Abstract

There is provided a fluorine gas production unit capable of restraining damage in an abnormal situation in minimum. A fluorine gas production unit (30) comprises an electrolytic cell (32), wherein product gas consisting of fluorine gas as a main component is generated in the first gas phase portion on the anode side thereof and by-product gas consisting of hydrogen gas as a main component is generated in the second gas phase portion on the cathode side thereof. First and second pipes (52, 72) are disposed to derive the product gas and by-product gas. In the second pipe (72), there is disposed a deriving flow rate control section (74) for controlling the deriving flow rate of the byproduct gas. A first dilution pipe (82) for introducing inert gas is disposed so that the hydrogen concentration in gas in the second gas phase portion on the cathode side gets 70 ≈10%. A second dilution pipe (83) for introducing inert gas into the second pipe (72) downstream of the deriving flow rate control section (74) is disposed so that the hydrogen concentration in gas coming out of the second pipe (72) gets less than 4%.

Description

FLUORINE GAS PRODUCTION UNIT The present invention relates to a fluorine gas production unit and especially to a fluorine gas production unit, which is disposed in the gas supply system of a semiconductor treatment system. "Semiconductor treatment" used here means various kinds of treatments performed in order to form a semiconductor layer, an insulation layer and a conductive layer in predetermined patterns on substrates to be treated such as semiconductor wafers or LCD substances, thereby manufacturing structures including semiconductor devices and wirings or electrodes connected to the semiconductor devices on the same substrates to be treated. In the manufacture of semiconductor devices, various semiconductor treatments, for example, film-forming, etching and diffusion are applied to substrates to be treated such as semiconductor wafers or LCD substrates. In a semiconductor treatment system for carrying out such treatments, fluorine-series gases are utilized as treatment gases for various uses, for example, in a case where a silicon film or silicon oxide film is etched or a case where a treatment chamber is cleaned up. Although fluorine gas is remarked as new etching gas or cleaning gas, the production of fluorine is not generally carried out in a site where semiconductor devices are manufactured because problems in the points of view of security and reliability are not completely solved.- In a gas-producing factory, on the other hand, a unit using an electrolytic cell is known as a unit, for producing fluorine gas. In such an electrolytic cell, hydrogen fluoride is electrolyzed in an electrolytic bath comprising a molten salt containing hydrogen fluoride. By virtue of this electrolyzing, product gas^' consisting of fluoride gas as a main component is generated on the anode side and by- product gas consisting of hydrogen gas as a main component is generated on the cathode side. Fluorine gas is a very active oxidizing agent (having combustion- supporting property), and the reaction of fluorine gas with hydrogen gas through their contact is very dangerous. Therefore, there have been made various devices for preventing the product gas on the anode side and the by-product gas on the cathode side from mixing and contacting. For instance, Patent Document 1 and Patent Document 2 disclose a fluorine gas production unit for. producing fluorine gas in a gas-producing factory. In the unit disclosed in the official gazettes thereof, the inside of an electrolytic cell is partitioned to a central anode room and a surrounding cathode room by a partition plate (skirt) extending from above into a molten salt. A pair of probes is disposed in the anode room so as to terminate at different heights. The pair of said probes functions a level gauge for carrying out the on/off control of an electric current to be supplied between the anode and cathode. In this unit, namely, the mixed contact of gas on the anode side and gas on the cathode side is prevented by the partition plate and the generation of fluorine gas is controlled with detecting the level of the molten salt. In a case of this unit, however, the liquid level is detected only at two height positions and the occurrence of some variation in the liquid level can not be therefore avoided. Fig.4 is a schematic view showing another conventional fluorine gas production unit. This unit has an improved mechanism for controlling the supply of fluorine gas from an electrolytic cell. In a case of this unit, fluorine gas generated on the side of the anode 114 of an electrolytic cell 1 12 is continuously supplied to an intermediate capacitor 116 through a pipe 120 and temporarily stored there. When the capacitor 116 has achieved a certain constant pressure, a shut-off valve 122 disposed between a compressor 1 18 and the capacitor 116 is temporarily opened so that a certain predetermined amount of fluorine gas is sucked into the compressor 118. By repeating the operation of the shut-off valve 122 several times, the pressure of a secondary buffer tank 124 is elevated up to a predetermined value. In a case of the unit shown in Fig.4, such a valve operation is carried out, which comprises opening the valve 122 at an upper limit of the pressure of the capacitor 116 and closing the valve 122 at a lower limit thereof. Therefore, large pressure variation occurs in the pipe 120 for fluorine gas, whereby the molten salt in a mist form is accumulated at the inlet of the pipe 120 and this accumulation of the molten salt causes the blockage of the pipe 120. Furthermore, it is expected that the pressure of a discharge system varies depending on the installing environment. For example, when the suction capacity of an exhaust system is higher, the pressure on the cathode side naturally gets negative and the level of the molten salt on the cathode side rises accompanying with this negative pressure. In this case, the liquid level varies vastly and the molten salt is dragged into the side of a pipe 121 for hydrogen gas, wherein this dragging of the molten salt causes the blockage of the pipe 121. In a case of a conventional fluorine gas production unit, as mentioned above, there are several problems in the points of view of security and reliability. If such problems are not solved, it will be practically difficult to use a fluorine gas production unit as the unit is incorporated in an automated production system, for example a semiconductor device manufacture system. Although any device is made so as to prevent the mixed contact of gas on the anode side and gas on the cathode side in a normal operating state, as mentioned below, in a conventional production unit, the inventors have found such problem that a countermeasure for meeting a case where the operation of the unit is stopped by any abnormal situation or a case where the mixed contact of both the gases occurs by any

chance is not sufficient. [Patent Document 1] the official gazette of Japanese Patent Application Lid-open (KOHYO) No. 505,853/1997, [Patent Document 2] the official gazette of Japanese Patent Application Lid-open No. 339,090/2002. In a certain point of view, the present invention is aimed at providing a fluorine gas production unit capable of restraining damage in an abnormal situation in minimum. In another point of view, the present invention is aimed at providing a fluorine gas production unit capable of working with high security and reliability even when it is operated for a longer period of time. In a further point of view, the present invention is aimed at providing a unit optimum for producing fluorine gas on site and on demand. The term "on site" used here means a combination of a fluorine gas production unit with a predetermined main treatment unit, for example a main treatment unit of a semiconductor treatment system. And, the term "on demand" means a possibility of supplying gas at good timing according to a demand from the side of the main treatment unit and under regulation of a required composition. According to the first aspect of the present invention, a fluorine gas production unit for producing fluorine gas comprises: an electrolytic cell, where hydrogen fluoride is electrolyzed in an electrolytic bath comprising a molten salt containing hydrogen fluoride, thereby generating product gas consisting of fluorine gas as a main component in the first gas phase portion on the anode side thereof and generating by-product gas consisting of hydrogen gas as a main component in the second gas phase portion on the cathode side thereof; a first pipe for deriving the product gas from the first gas phase portion; a second pipe for deriving the by-product gas from the second gas phase portion; a deriving flow rate control section disposed in the second pipe, for controlling the deriving flow rate of the by-product gas; a first dilution pipe for introducing inert gas into the second gas phase portion so that the hydrogen concentration in gas in the second gas phase portion gets

70-10%; and

a second dilution pipe for introducing inert gas into the second pipe downstream of the deriving flow rate control section so that the hydrogen concentration in gas coming out of the second pipe gets less than 4%. According to the second aspect of the present invention, a unit of the first aspect comprises: a pressure control mechanism for substantially equalizing the pressures in the first and second gas phase portions to each other, wherein the deriving flow rate control section is used as part of the pressure control mechanism. In a unit of the second aspect, according to the third aspect of the present invention, the pressure control mechanism comprises: a first pressure gauge for continuously measuring the pressure of the first gas phase portion; a second pressure gauge for continuously measuring the pressure of the second gas phase portion; a first flow rate control valve disposed in the first pipe; a second flow rate control valve disposed in the second pipe as the deriving flow rate control section; a first control member for regulating the open degree of the first flow rate control valve on the basis of the measurement results of the first pressure gauge so that the pressure of the first gas phase portion is maintained at a first set value; and a second control member for regulating the open degree of the second flow rate control valve on the basis of the measurement results of the second pressure gauge so that the pressure of the second gas phase portion is maintained at a second set value that is substantially equal to the first set value. According to the fourth aspect of the present invention, a unit of the second or third aspect further comprises: an introducing flow rate control section disposed in the first dilution pipe for controlling the introducing flow rate of inert gas. According to the fifth aspect of the present invention, a unit of anyone of the first to fourth aspects further comprises: a means for setting a supply time for supplying inert gas from the first dilution pipe into the second gas phase portion. In a unit of anyone of the first to fifth aspects, according to the sixth aspect of the present invention, both the inert gases introduced from the first and second dilution pipes are nitrogen gas. According to the seventh aspect of the present invention, a unit of anyone of the first to sixth aspects further comprises: a main control section for controlling the action of the same unit, wherein the main control section is set, for a predetermined abnormal situation, so as to perform a trouble-meeting mode for continuing the introduction of inert gas from the first and second dilution pipes. According to the eighth aspect of the present invention, a unit of the seventh aspect further comprises: a purge pipe for introducing, in the trouble- meeting mode, inert gas into the first gas phase portion. According to the fifth aspect of the present invention, a unit of the seventh or eighth aspect further comprises: a back-up pipe and a selector valve for causing, in the trouble-meeting mode, the first and second dilution pipes to selectively communicate with a back-up gas source. In a unit of anyone of the seventh to ninth aspects, according to the tenth aspect of the present invention, the main control section performs, in the interception of an external power source, the trouble-meeting mode by the supply of electric power from a back-up power source. In an embodiment of the present invention, furthermore, inventions of various stages are contained and various inventions may be extracted by a proper combination in a plurality of disclosed, constitutional requisites. For instance, in a case where an invention is extracted by omission of some constitutional requisites from all the constitutional requisites given in the embodiment, the omitted portions will be properly supplemented by well-known usual arts when the extracted invention is performed. By lowering the hydrogen concentration in gas in the second gas phase portion on the cathode side, the pressure generated through the mixed contacting reaction of gas on the anode side and gas on the cathode side is lowered. Since

load on the electrolytic cell is therefore lightened, damage in an abnormal situation can be restrained in minimum. By substantially equalizing the pressures in the first and second gas phase portions on the anode side and cathode side to each other, variation in the liquid level in the electrolytic cell is caused to nearly disappear. It becomes therefore possible for the fluorine gas production unit to work with high security and reliability even when it is operated for a longer period of time. By continuing, in the trouble-meeting mode, the introduction of inert gas to the second gas phase portion on the cathode side, the hydrogen concentration in gas on the cathode side is lowered with a lapse of time in occurrence of an abnormal situation. Even if gas on the anode side and gas on the cathode side are mixed and contacted with each other, the pressure generated through the mixed contacting reaction of them is lowered and damage in an abnormal situation can be therefore restrained in minimum. In the course of developing the present invention, the inventors have

studied problems in the points of view of security and reliability in conventional fluorine gas production units. As a result, the inventors^' have obtained such findings as mentioned below. In a conventional unit, there have been made various devices for preventing the mixed contact of gas on the anode side and gas on the cathode side (i.e. the mixed contact of fluorine gas and hydrogen gas) under a normal operating state (that is a state where gas generated from the anode side is recovered as a product). In a case where an abnormal situation such as vast variation in the liquid level or malfunction of a power source or gas source has occurred in practice, however, a disposition of usually intercepting the supply of electric power to the whole of a unit to stop the generation of fluorine gas is merely taken in the conventional unit. Here, the mixed contact of gas on the anode side and gas on the cathode side shall not be generated, as a principle, by stopping the operation of the unit. In a case where the operation of the unit has been urgently stopped by an abnormal situation, however, there is such a possibility that pressure variation occurs in the electrolytic cell depending on various factors and the mixed contact of gas on the anode side and gas on the cathode side is generated. When a time elapses after the supply of electric power to the electrolytic cell 32 has been intercepted so that the heater gets turned off, the solidification of the molten salt in the electrolytic cell progresses. Owing to the solidification of the molten salt, the communication of gas occurs between the anode side and the cathode side (it is

confirmed that this communication amount is 1 x 10^"2 ~ 1 x10 Pa.m3/sec). When a

time elapses after the operation of the unit has been urgently stopped, namely, there is a possibility that the mixed contact of gas on the anode side and gas on the cathode side occurs. In a case where the mixed contact of gas on the anode side and gas on the cathode side has occurred by any chance, in the conventional unit, a countermeasure against the reaction of both the explosive gases depends only on the mechanical strength of the electrolytic cell (In general, an electrolytic cell has a structure unendurable against high pressures). An embodiment of the present invention constructed on the basis of such findings as mentioned above will be described below referring to the drawings. In the following descriptions, constitutional elements having almost the equal function and construction will be given the same numerals and duplicate descriptions will be carried out where necessary. Fig.1 is a schematic view showing a semiconductor treatment system, in which a fluorine gas production unit according to the embodiment of the present invention is incorporated. This semiconductor treatment system has a semiconductor treatment unit 10 for subjecting substrates to be treated such as semiconductor wafers or LCD substrates to a treatment such as film-forming, etching or diffusion. The semiconductor treatment unit 10 has a treatment chamber 12 for accommodating substrates to be treated and subjecting them to semiconductor treatments. In the treatment chamber 12 is disposed a carrier base also used as a bottom electrode (a supporting member) 14 for carrying the substrates to be treated. Also in the treatment chamber 12 is disposed a top electrode 16 in opposition to the carrier base 14. By applying between both the electrodes 14, 16 RF (radio-frequency) power from a RF power source 15, a RF electric field for converting treatment gas to plasma is formed in the treatment chamber 12. To the lower portion of the treatment chamber 12 is connected an exhaust system 18 for exhausting the inside thereof and setting the same inside in vacuum. And, to the upper portion of the treatment chamber 12 is connected a gas supply system 20 for supplying the treatment gas thereto. In addition, the fluorine gas production unit according to the embodiment of the present invention can be used as a gas supply system of a semiconductor treatment unit of the type having a remote plasma chamber attached to a treatment chamber. And, the fluorine gas production unit according to the embodiment of the present invention is also applicable to a case where cleaning gas or the like is supplied to a semiconductor treatment unit utilizing no plasma, for example a thermal CVD unit.

In the gas supply system 20 is disposed a flow management section 22 for selectively switching over, for example, treatment gas for carrying out the semiconductor treatment or treatment gas for cleaning the inside of the treatment chamber 12 as optional gas to be supplied to the treatment chamber 12, and for supplying the same gas at a predetermined flow rate. To the flow management section 22 is connected a gas storage section 24 having plural gas sources for storing various active gas and inert gas. Also to the flow management section 22 is connected a gas production section 26 for producing fluorine-series treatment gas through a reaction treatment. To the flow management section 22 and gas production section 26 is detachably connected the fluorine gas production unit 30 according to the embodiment of the present invention. The production unit 30 is namely used in order to directly supply fluorine gas, to the flow management section 22 or to supply a fluorine gas feed material to the gas production section 26 (a valve for switching use is not shown). In the gas production section 26, interhalogenous fluorine compound gas is produced by reacting, for example, a fluorine gas feed material with another halogen gas such as chlorine. The fluorine gas production unit 30 has a gas-tight electrolytic cell 32, in which an electrolytic bath consisting of a molten salt containing hydrogen fluoride is accommodated. The action of the whole of the production unit 30 including the electrolytic cell 32 is controlled by a main control section 40. The production unit 30 is usually operated by a main power source installed in a semiconductor- manufacturing factory. In order to meet any abnormal situation of a power source system such as power failure, in addition, the production unit 30 is set so as to utilize a back-up power source BB. The back-up power source BB may be one exclusive for the production unit 30 or one installed in a semiconductor- manufacturing factory. The molten salt in the electrolytic cell 32 comprises a mixture (KF/2HF) of potassium fluoride (KF) and hydrogen fluoride (HF) or a mixture of Fremy's salt and hydrogen fluoride. The electrolytic cell 32 is divided to an anode room 34 and a cathode room 36 by a partition plate (skirt) 35 extending from above into the molten salt. In the anode room 34 and cathode room 36, a carbon electrode (anode) 42 and a nickel electrode (cathode) 44 are immersed in the molten salt, respectively. On the electrolytic cell 32 is attached an electric current source 38 for supplying an electric current between the anode 42 and cathode 44. In the cathode room 36, a feed material supply pipe 31 is disposed as it is immersed in the molten salt, in order to supply hydrogen fluoride gas, that is a consumable feed material, into the molten salt. In the anode room 34 and cathode room 36 are disposed level sensors 37a, 37b for detecting the level of the molten salt. Although the level sensors 37a, 37b each are shown by one line in Fig.1 , in addition, the level sensors 37a, 37b each comprise a combination of plural sensors so that a plurality of the liquid levels can be detected. During the electrolyzing treatment, the electrolytic cell 32 is heated and

warmed at 80 = 90°C by a heater 33 attached thereon. By electrolyzing hydrogen

fluoride in the electrolytic bath, product gas consisting of fluorine gas (F2) as a main component is generated in the gas phase portion of the anode room 34 and by-product gas consisting of hydrogen gas as a main component is generated in the gas phase portion of the cathode room 36. In each of the product gas and by- product gas, hydrogen fluoride gas is mixed (for example, 5%) only by a rate of the vapor pressure of hydrogen fluoride gas in the molten salt of the feed material. In the anode room 34 and cathode room 36 are disposed first and second pressure gauges 46, 48 for continuously measuring the pressures of the respective gas phase portions thereof. To the anode room 34 is connected a first pipe 52 for deriving the product gas therefrom and sending the same product gas to the flow management section 22 and gas production section 26 of the gas feed system 20. In the first pipe 52, there are disposed a first flow rate control valve 54, adsorption cartridge 56, mini buffer tank 58, compressor (suction means) 62 and main buffer tank 64 in turn from the upstream side thereof. Owing to the fact that the first pipe 52 is sucked by the compressor 62, the product gas generated in the anode room 34 is forcibly derived from the anode room 34 and stored in the main buffer tank 64. In addition, the reference numeral 66 in Fig.1 represents a line filter. Several percents (for example, 5%) of hydrogen fluoride are mixed in the product gas, as mentioned above. This hydrogen fluoride is removed when the product gas passes through the adsorption cartridge 56. An adsorbent for catching hydrogen fluoride by adsorption is therefore charged in the cartridge 56. The adsorbent comprises, due to consideration of its handling and pressure loss, a plurality of pellets filled in the cartridge 56. The adsorbent is an adsorbent, whose adsorbing capacity is changed depending on temperatures, such as sodium fluoride (NaF). On the periphery of the cartridge 56 is disposed a temperature- regulating jacket (heater) 57 for regulating the temperature of the cartridge 56. On the main buffer tank 64 is disposed a pressure gauge 65, whereby the pressure in the same tank 64 is continuously measured. This measurement result is transmitted to a control member 39 attached on the electric current source 38. The control member 39 turns on/off the electric current source 38 on the basis of the measurement result transmitted thereto to control the supply of an electric current to the electrolytic cell 32. When the pressure in the tank 64 has been decreased to a certain pressure, namely, the electric current source 38 is turned on to start the production of fluorine gas. And, when it has been increased to another certain pressure, the electric current source 38 is turned off to stop the production of fluorine gas. By virtue of the on/off control of the electric current source 38, the electrolyzing can be stopped while no difference in the level of the molten salt is provided between the anode room 34 and cathode room 36 in the electrolytic cell 32. In addition, the pressure in the tank 64 is set, for example, at the atmospheric pressure D^N the atmospheric pressure + 0.18Mpa. To the cathode room 36, on the other hand, is connected a second pipe 72 for deriving the by-product gas. The second pipe 72 is detachably connected to, for example, a pipe of an exhaust system (suction means) 78 of a semiconductor- manufacturing factory. In the second pipe 72 are disposed a second flow rate control valve 74 and a harmful substance removal section 76. Owing to the fact that the second pipe 72 is sucked by the exhaust system 78, the by-product gas generated in the cathode room 36 is forcibly derived from the cathode room 36. The same by-product gas is sent to the exhaust system 78 after it has passed through the harmful substance removal section 76. During the electrolyzing treatment, as has been described or mentioned below, a pressure balance between the anode room 34 and cathode room 36 is lost depending on various factors so that variation in the liquid level easily occurs in the electrolytic cell 32. Even in other periods except for the electrolyzing treatment, for instance, mainly just after a gas switching step such as a step of purging in the electrolytic cell 32 by nitrogen gas or a nitrogen gas purging step

after the completion of the supply of feed material hydrogen fluoride gas, variation in the liquid level easily occurs in the electrolytic cell 32. This variation causes to damage the security and reliability of the fluorine gas production unit. In the fluorine gas production unit shown in Fig.1 , on the contrary, the pressures in the respective gas phase portions of the anode room 34 and cathode room 36 are continuously measured by the first and second pressure gauges 46, 48. These measurement results are transmitted to first and second control members 55, 75 which are attached on the first and second flow rate control valves 54, 74, respectively. The first and second control members 55, 75 regulate the open degrees of the first and second flow rate control valves 54, 74 on the basis of the measurement results transmitted thereto so that the pressures of the respective gas phase portions of the anode room 34 and cathode room 36 are maintained at first and second set values that are substantially equal to each other. Namely, the open degrees of the first and second flow rate control valves 54, 74 are continuously regulated under control of the first and second control members 55, 75 attached thereon, respectively. Since the pressures of the anode room 34 and cathode room 36 are independently measured and controlled all the time, respectively, as mentioned above, the level states of the molten salt in the anode room 34 and cathode room 36 are uniformly maintained. Owing to this construction, in the other words, the electrolytic cell 32 is protected from any bad influences which are caused by the generating state of fluorine, the state of the first and second pipes 52, 72, the working state of the compressor 62 or the exhaust system 78 of a semiconductor- manufacturing factory, or other variations in environment. Therefore, damage given on expensive electrodes or the anode effect can be avoided and the electrolyzing treatment can be advanced in safety and with no sudden stoppage of the electrolyzing. There is no need of frequently carrying out maintenance because the molten salt is not solidified at the inlets of the first and second pipes 52, 72 and the blockage of these pipes is not caused. In addition, the first and second set values of the respective gas phase portions of the aforementioned anode room 34 and cathode room 36 are set

preferably at the atmospheric pressure ~ 820Torr or more preferably at the

atmospheric pressure = 770Torr. In order to stabilize the pressures of the anode

room 34 and cathode room 36, it is necessary for the first and second flow rate control valves 54, 74 that their opening degree is continuously adjustable with good responsibility. From this point of view, a piezo valve is desirably used as the first and second flow rate control valves 54, 74. Fig.2 is a schematic view showing a supply pipe system, partially taken out, for inert gas such as nitrogen gas (N2) in the fluorine gas production unit shown in

Fig.1. To the cathode room 36 is connected a first dilution pipe 82 in order to introduce inert gas such as nitrogen gas into the gas phase portion of the cathode room 36. The first dilution pipe 82 is detachably connected to, for example, an inert gas source 80 of a semiconductor-manufacturing factory. In the first dilution pipe 82 is disposed a flow rate control section 84 such as a mass flow controller. During the normal operation of the production unit 30, inert gas is introduced, under control of the main control section 40, from the first dilution pipe 82 into the gas phase portion of the cathode room 36 at a controlled flow rate (controlled by the flow rate control section 84). By virtue of this introduction of inert gas, the hydrogen concentration in gas in the gas phase portion of the cathode room 36 is reduced to such a predetermined value as mentioned below. Since nitrogen gas is not dissolved in the molten salt, in addition, it is directly mixed into hydrogen gas and thereafter discharged to the second pipe 72. Also to the second pipe 72 is connected a second dilution pipe 83 in order to introduce inert gas such as nitrogen gas into the same pipe 72. The second dilution pipe 83 is also detachably connected to, for example, the inert gas source 80 of a semiconductor-manufacturing factory. The second dilution pipe 83 is connected to the second pipe 72 by way of a vacuum generator 75 between the second flow rate control valve 74 and the harmful substance removal section 76. Owing to the fact that gas flows through the second pipe 72, suction force is generated in the vacuum generator 75 and the inert gas from the second dilution pipe 83 is drawn in a large amount into the second pipe 72 by this suction force. By virtue of this drawing of inert gas, the hydrogen concentration in gas flowing through the second pipe 72 is reduced in a large extent. By inert gas from the first dilution pipe 82, the hydrogen concentration in

gas in the gas phase portion of the cathode room 36 is reduced to about 70 ~

about 10%, preferably about 40 ~ about 15%. This reduction of the hydrogen

concentration is adopted as a countermeasure against the final situation where gas on the side of the anode room 34 and gas on the side of the cathode room 36 are mixed and contacted with each other in the electrolytic cell 32. Since the pressure generated through the mixed contacting reaction of both the gases is lowered by reducing the hydrogen concentration, the electrolytic cell 32 can be prevented from being destroyed. In addition, the supply of inert gas from the first dilution pipe 82 is not necessarily continuous. For instance, it may be devised that a supply time for inert gas is set by the main control section 40 and the supply of inert gas only for this supply time is repeatedly carried out at a predetermined interval. By inert gas from the second dilution pipe 82, on the other hand, the hydrogen concentration in gas (coming out of the second pipe 72) sent to the exhaust system 78 through the harmful substance removal section 76 by the second pipe 72 is reduced so as to be less than 4%. The concentration 4% of hydrogen gas is a concentration of the explosive limit of hydrogen gas. Owing to this reduction of the concentration of hydrogen gas, namely, such a danger that gas coming out of the second pipe 72 causes an explosive reaction can be avoided Fig.3 is a graph showing a relation between the supplying flow rate (seem) of dilution gas (inert gas) to the cathode room 36 and the pressure (bar) generated through the mixed contacting reaction of fluorine gas and hydrogen gas. Data shown in Fig.3 were obtained under such a condition that the generation amount of hydrogen gas was 650sccm and the dilution gas was nitrogen gas. However, it is conceived that similar results are obtained even if the dilution gas is another inert gas. As shown in Fig.3, the generated pressure decreases like a quadratic function as the flow rate of the dilution gas increases. Up to a point where the flow

rate of the dilution gas is δOOsccm (corresponding to a hydrogen concentration of about 55%), the generated pressure decreases approximately linearly with the increase of the flow rate of the dilution gas. Over a point where the flow rate of the dilution gas is more than 5,000sccm (corresponding to a hydrogen concentration of about 10%), on the other hand, the generated pressure does not decrease vastly even if the flow rate of the dilution gas increases. Judging from the relation between the supply amount of the dilution gas and the lowering effect of the generated pressure, namely, it will be concluded that the flow rate of the dilution gas is preferably made to be up to 5,000sccm (corresponding to a hydrogen concentration of about 10%). The amount of inert gas supplied from the first dilution pipe 82 to the gas phase portion of the cathode room 36 is determined taking into consideration several important factors in addition to the aforementioned supply amount versus effect relation. One of the factors is the mechanical strength of the electrolytic cell 32 endurable against the generated pressure (bar). At present, the electrolytic cell 32 which is generally used can endure a generated pressure as high as 25bar. From, this point of view, the supply amount of inert gas (the flow rate of dilution gas) must be at least about 250sccm (corresponding to a hydrogen concentration of about 70%), and the larger the supply amount is, the better it is. Another factor is a pressure balance between the gas phase portion of the anode room 34 of the electrolytic cell 32 and the gas phase portion of the cathode room 36 thereof. By the pressure control mechanism including the first and second pressure gauges 46, 48 and the first and second flow rate control valves 54, 74, as mentioned above, the pressures in the respective gas phase portions of the anode room 34 and cathode room 36 are continuously controlled so as to be substantially equal to each other. Since gas in the same volume is essentially generated in the anode room 34 and cathode room 36, it is relatively easy to maintain the pressure balance between both the rooms 34, 36 when inert gas is not supplied to the cathode room 36. When inert gas is supplied to the cathode room 36, however, the pressure balance between the anode room 34 and the cathode room 36 is easily lost in proportion to the supply of inert gas. From this point of view, therefore, it is undesirable to make the supply amount of inert gas (the flow rate of dilution gas) too larger. Taking into consideration such various factors as mentioned above, the

supply amount of inert gas from the first dilution pipe 82 is set at about 250 ~

about 5,000sccm and desirably about 1 ,000 ~ about 4,000sccm under a condition

that the generation amount of hydrogen gas is 650sccm. If this supply amount is calculated as the hydrogen concentration in gas in the gas phase portion of the

cathode room 36, it become about 70 ~ about 10% and desirably about 40 ~ about

15%. In order to introduce inert gas such as nitrogen gas, to the gas phase portion of the anode room 34, now turning back to Fig.2, a purge pipe 86 is connected to the anode room 34. The purge pipe 86 is detachably connected to, for example, the inert gas source 80 of a semiconductor-manufacturing factory. In the purge pipe 86 is disposed a flow rate control section 88 such as a mass flow controller. Differently from the first and second dilution pipes 82, 83, inert gas from the purge pipe 86 is not introduced into the gas phase portion of the anode room 34 during the normal operation of the production unit 30. Inert gas from the purge pipe 86 will be introduced to the gas phase portion of the anode room 34 when a trouble-meeting mode which will be hereinafter mentioned is performed. In order to meet an abnormal situation of the inert gas source 80 which is caused by any malfunction of the power source system, the fluorine gas production unit 30 is set so as to utilize a back-up inert gas source GB. The backup inert gas source GB may be one exclusive for the production unit 30 or one installed in a semiconductor-manufacturing factory. The first and second dilution pipes 82, 83 and the purge pipe 86 are connected to the back-up inert gas source GB by way of a back-up pipe 92, 93, 96, respectively. The back-up pipes 92, 93, 96 each have a selector valve 94, 95, 97 disposed therein. In the next place, there will be described the trouble-meeting mode carried out under control of the main control section 40. In a case where vast variation in the liquid level in the electrolytic cell 32 has been detected by the level gauges 37a, 37b, for instance, the detection of this vast variation is transmitted to the main control section 40. The main control section 40 produces alarm to notify the occurrence of an abnormal situation in the electrolytic cell 32 to an operator and starts to count the lapse of time from the occurrence of this abnormal situation by a timer built therein. When the operator does not meet this abnormal situation, the main control section 40 performs the trouble-meeting mode after the lapse of a predetermined period of time. In the trouble-meeting mode, the supply of hydrogen fluoride gas that is a consumable feed material from the feed material supply pipe 31 and the supply of electricity to the most portions of the production unit 30 are intercepted. In this time, however, the introduction of inert gas from the first and second dilution pipes 82, 83 into the gas phase portion of the cathode room 36 and the second pipe 72 is continued and the introduction of inert gas from the purge pipe 86 into the gas phase portion of the anode room 34 is started up. In this time, the flow rate control sections 84, 88 of the first dilution pipe 82 and purge pipe 86 are namely maintained in a working state. And, a pressure control mechanism including the first and second pressure gauges 46, 48 and the first and second flow rate control valves 54, 74 is also maintained in a working state. Furthermore, the sector valve 68 of the bypass pipe 67 connected to the first pipe 52 is opened while the sector valve 53 for a flow passage to the adsorption cartridge 56 and compressor 62 of the first pipe 52 is closed. By virtue of this valve operation, a purge route where inert gas flows from the purge pipe 86 to the bypass pipe 67 through the anode room 34 is formed on the anode side. And, a purge route where inert gas flows from the first dilution pipe 82 to the second pipe 72 through the cathode room 36 is formed on the cathode side. In addition, the bypass pipe 67 is detachably connected to, for example, the pipe of the exhaust system 78 of a semiconductor-manufacturing factory by way of the harmful substance removal section 69. After the operation of the production unit 30 has been stopped, according to the aforementioned trouble-meeting mode, the fluorine concentration in gas in the gas phase portion of the anode room 34 and the hydrogen concentration in gas in the gas phase portion of the cathode room 36 lower with the lapse of time. When a time elapses after the supply of electric power to the electrolytic cell 32 has been intercepted so that the heater 33 gets turned off, as mentioned above, the solidification of the molten salt in the electrolytic cell 32 progresses. Owing to the solidification of the molten salt, there is a possibility that gas is permitted to communicate between the anode side and the cathode side so that the mixed contact of gas on the anode side and gas on the cathode side occurs. Even in this case, however, an explosive mixed contacting reaction does not occur because the fluorine concentration in gas on the anode side and the hydrogen concentration in gas on the cathode side get sufficiently lower. In a case where the stoppage of the supply of power from the main power source caused by a power failure or the like has been detected, the back-up power source BB is started to operate and this information is transmitted to the main control section 40. The main control section 40 produces alarm to notify the occurrence of an abnormal situation in the main power source to an operator and starts to count the lapse of time from the occurrence of this abnormal situation by the timer built therein. When the power source is not retrieved even by the operator's countermeasure against this abnormal situation, the main control section 40 performs the aforementioned trouble-meeting mode by using an electric power supplied from the back-up power source BB after the lapse of a predetermined period of time. Although the fluorine gas production unit 30 is detachably incorporated, in the aforementioned embodiment, in the semiconductor treatment system, it may be fixedly installed in the same system. As for several members in the fluorine gas production unit 30 such as the back-up inert gas source GB, back-up power source BB, compressor 62, main buffer tank 64 and harmful substance removal sections 69, 76, there may be used those placed on the side of a semiconductor- manufacturing factory. Although fluorine gas is selectively supplied to the flow management section 22 or gas production section 26, this gas may be directly supplied to the treatment chamber 12 separately from other treatment gases. In addition, the gas production section 26 may be constructed so as to produce another fluorine-series treatment gas, not produce interhalogenous fluorine compounds. In the category of the idea of the present invention, furthermore, it is possible for these skilled in the art to think of various variants and modifications, and it will be understood that these variants and modification also belong to the scope of the present invention. According to the present invention, a fluorine gas production unit can be provided, which is optimal for producing fluorine gas on site and on demand. Fig.1 is a schematic view showing a semiconductor treatment system, in which the fluorine gas production unit according to the embodiment of the present invention is incorporated; Fig.2 is a schematic view showing a supply pipe system, partially taken out, for inert gas such as nitrogen gas (N2) in the fluorine gas production unit shown in

Fig.1 ; Fig.3 is a graph showing a relation between the supplying flow rate (seem) of dilution gas (inert gas) to the cathode room and the pressure (bar) generated through the mixed contacting reaction of fluorine gas and hydrogen gas; and Fig.4 is a schematic view showing a conventional fluorine gas production unit.

Claims

1. A fluorine gas production unit for producing fluorine gas comprising: an electrolytic cell, where hydrogen fluoride is electrolyzed in an electrolytic bath comprising a molten salt containing hydrogen fluoride, thereby generating product gas consisting of fluorine gas as a main component in the first gas phase portion on the anode side thereof and generating by-product gas consisting of hydrogen gas as a main component in the second gas phase portion on the cathode side thereof; a first pipe for deriving the product gas from the first gas phase portion; a second pipe for deriving the by-product gas from the second gas phase portion; a deriving flow rate control section disposed in the second pipe for controlling the deriving flow rate of the by-product gas; a first dilution pipe for introducing inert gas into the second gas phase portion so that the hydrogen concentration in gas in the second gas phase portion

gets 70=10%; and a second dilution pipe for introducing inert gas into the second pipe downstream of the deriving flow rate control section so that the hydrogen concentration in gas coming out of the second pipe gets less than 4%.

2. A fluorine gas production unit according to claim 1 , which comprises: a pressure control mechanism for substantially equalizing the pressures in the first and second gas phase portions to each other, wherein the deriving flow rate control section is used as part of the pressure control mechanism.

3. A fluorine gas production unit according to claim 2, in which the pressure control mechanism comprises: a first pressure gauge for continuously measuring the pressure of the first gas phase portion; a second pressure gauge for continuously measuring the pressure of the second gas phase portion; a first flow rate control valve disposed in the first pipe; a second flow rate control valve disposed in the second pipe as the deriving flow rate control section; a first control member for regulating the open degree of the first flow rate control valve on the basis of the measurement results of the first pressure gauge so that the pressure of the first gas phase portion is maintained at a first set value;

and a second control member for regulating the open degree of the second flow rate control valve on the basis of the measurement results of the second pressure gauge so that the pressure of the second gas phase portion is maintained at a second set value that is substantially equal to the first set value.

4. A fluorine gas production unit according to claim 2 or 3, which further comprises: an introducing flow rate control section disposed in the first dilution pipe for controlling the introducing flow rate of inert gas.

5. A fluorine gas production unit according to anyone of claims 1 to 4, which further comprises: a means for setting a supply time for supplying inert gas from the first dilution pipe into the second gas phase portion.

6. A fluorine gas production unit according to anyone of claims 1 to 5, in which both the inert gases introduced from the first and second dilution pipes are nitrogen gas.

7. A fluorine gas production unit according to anyone of claims 1 to 6, which further comprises: a main control section for controlling the action of the same unit, wherein the main control section is set, for a predetermined abnormal situation, so as to perform a trouble-meeting mode for continuing the introduction of inert gas from the first and second dilution pipes.

8. A fluorine gas production unit according to claim 7, which further comprises: a purge pipe for introducing, in the trouble-meeting mode, inert gas into the first gas phase portion.

9. A fluorine gas production unit according to claim 7 or 8, which further comprises: a back-up pipe and a selector valve for causing, in the trouble-meeting mode, the first and second dilution pipes to selectively communicate with a back- up gas source.

10. A fluorine gas production unit according to anyone of claims 7 to 9, in which the main control section performs, in the interception of an external power source, the trouble-meeting mode by the supply of electric power from a back-up power source.