WO2012035000A1

WO2012035000A1 - Method for the removal of f2 and/or of2 from a gas

Info

Publication number: WO2012035000A1
Application number: PCT/EP2011/065833
Authority: WO
Inventors: Johannes Eicher; Francis Feys; Philippe Morelle; Oliviero Diana; Peter M. Predikant; Ercan Uenveren; Holger Pernice; Thomas Schwarze; Wolfgang Kalbreyer; Helge Rau
Original assignee: Solvay Sa
Priority date: 2010-09-15
Filing date: 2011-09-13
Publication date: 2012-03-22
Also published as: KR20130111554A; JP2013539717A; CN103180029A

Abstract

A method for the removal of F₂ and/or OF₂ from gases is described wherein a gas comprising F₂ and/or OF₂ optionally also containing HF is contacted with a liquid comprising a base and a dissolved thiosulfate salt or alkali metal nitrite, preferably the respective sodium compounds. Preferred bases are alkali metal hydroxides and alkali metal carbonates, especially the respective potassium compounds. The method is very suitable to treat gas from industrial processes in which F₂ is applied or formed, e.g. in gases resulting from the manufacture of semiconductors, photovoltaic cells or TFTs. It can be applied, for example, for gases comprising F₂ resulting from emergency events, from the start-up phase of F₂ production, for waste gases from semiconductor etching apparatus including chamber cleaning waste gases and for gases comprising out-of-spec F₂. The method allows the removal of F₂ without formation of a significant level of OF₂ in the treated gas.

Description

Method for the removal of F? and/or OF? from a gas

The present invention which claims benefit of the priority of US provisional patent applications 61/383204 filed September 15, 2010

and 61/383533 filed September 16, 2010 ; European patent application

N° 10177188.9 filed September 16, 2010 ; European patent application

N° 10177216.8 filed September 16, 2010 ; European patent application

N° 10177206.9 filed September 16, 2010 and European patent application 11165500.7 filed May 10, 2011 the content of which patent applications is incorporated herein for all purposes by reference concerns a method for the removal of F₂ and/or OF₂ from a gas. It concerns especially the removal of F₂ and/or OF₂ and to achieve a low content of OF₂ in the treated gas formed during the removal of F₂ from a gas originating from the manufacture semiconductors, photovoltaic cells, thin film transistor (TFT) liquid crystal displays, and micro- electromechanical systems (MEMS). In such manufacturing processes, F₂ can be applied as etching gas, as chamber cleaning gas, often in concentrations from 1 to 50 % by volume in mixtures for example with nitrogen and/or argon, or it can be formed from precursors applied in such processes, e.g. from fluoro substituted organic etchants, or from inorganic fluorides, e.g. SF₆ or F₃.

During the manufacture of semiconductors, photovoltaic cells, thin film transistor (TFT) liquid crystal displays, and micro-electromechanical

systems (MEMS), often consecutive steps of deposition of material and etching of the respective items are performed in suitable chambers ; these processes are often plasma-assisted. During the deposition step, deposits are often not only formed on the item, but also on the walls and other interior parts of the chamber. These deposits are frequently removed by applying an etchant. It was observed that elemental fluorine is a very effective agent both for etching items and for cleaning the chambers to remove undesired deposits. Processes of this kind are for example described in WO 2007/116033 (which describes the use of fluorine and certain mixtures as etchant and chamber cleaning agent), WO 2009/080615 (which describes the manufacture of MEMS), WO 2009/092453 (which describes the manufacture of solar cells), and in unpublished WO patent application PCT/EP2010/066109 which concerns the manufacture of TFTs. The gas withdrawn from the chamber often comprises F₂ molecules. When certain etchants, e.g. fluoro substituted organic compounds, for example, CF₄, or inorganic etchants, e.g. SF₆ or F₃, are subjected, during the etching process or chamber cleaning process, to high temperature or plasma, F radicals form. A part of these F radicals recombine to form F₂. The F₂ used for etching or chamber cleaning is often produced on site by electrolysis of FIF in the presence of conducting salts, especially in the presence of KF which forms adducts with FIF. It is known that during the start-up phase or in the case of anode break, F₂ may be produced the purity of which does not fulfill the conditions of purity necessary for the manufacture of semiconductors, TFTs, photovoltaic cells or MEMS (micro-electromechanical devices) ; such gases may contain high amounts of F₂. Installations for the production and delivery of F₂, be it on site of a manufacturing plant as described above, or be it in an common F₂ producing plant off site, may foresee precautions for an emergency treatment of F₂ comprising gas, for example in the case of an accident or a leakage caused by corrosion in the manufacturing unit, in storage or delivery, or when a danger threats, e.g. an earth quake ; see unpublished EP patent application 10177216.8 filed September 16, 2010. In such an event, the gas containing HF, F₂ or both from the inside of the facility (e.g. an F₂ producing unit according to the skid concept as described in US provisional patent applications 61/383204 filed September 15, 2010 and 61/383533 filed September 16, 2010) may be diluted with ventilation air, resulting in voluminous gas flow with low F₂ concentration.

Consequently, gases comprising F₂ and often HF in broadly varying concentrations and varying gas volume flows originate during the manufacture and use of F₂ or respective precursors in many processes. Since F₂ is a very reactive compound, there is a need for a reliable process to remove F₂ from such gases. Processes for removal of F₂ from gases are already known. Gas-solid processes, for example, contacting the F₂ containing gas with a solid treatment agent, e.g. CaC0₃, have the disadvantage that heat dissipation is a problem, and that layers form on the solid preventing further reaction. Wet processes are also known. When water is used as treatment agent, OF₂ forms unless extensive contact times are provided. Further, explosions were observed. When applying aqueous solutions of potassium hydroxide (KOH), OF₂ is also formed. See H. R. Leech in J. W. Mellor (Editor), Comprehensive treatise of inorganic and theoretical chemistry", supplement II, part I, (Longmans, Green & Co,

London 1956), pages 186 to 197. OF₂ is a very reactive and toxic compound, and its presence in the treated gas is highly undesirable. WO 99/61132 discloses an F₂ abatement process wherein a gas to be treated is contacted with an aqueous composition comprising a reducing agent, e.g. sodium thiosulfate, ammonia or potassium iodide.

Object of the present invention is to provide a process for the removal of F₂ and/or OF₂ from a gas which process is very flexible in view of the concentration of F₂ and/or OF₂ and the volume to be treated per time unit. Another object of the present invention is to provide a process for the removal of F₂ and/or OF₂ and, if present in the gas to be treated, of FIF, from a gas which process is very flexible in view of the concentration of F₂ and/or OF₂ and the volume to be treated per time unit. These objects and other objects are achieved by the present invention.

The invention provides a method for obtaining a gas with a reduced content of F₂ and/or of OF₂ and, if present in the gas to be treated, of FIF, which method includes at least one step of removing F₂ and/or OF₂ and, if present, of FIF, from a gas comprising F₂ and/or OF₂ and optionally HF wherein the gas is contacted with a liquid composition comprising water, dissolved reducing agent selected from the group consisting of alkali metal thiosulfate and alkali metal nitrite and at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate.

According to one embodiment, gases comprising only or essentially OF₂ may be treated. The term "essentially" denotes that the content of OF₂ in the total content of contaminants to be removed is equal to or greater than 90 % by volume, the balance to 100 % by volume is F₂, HF or both. Such gases may be gases leaving a gas treatment apparatus wherein F₂ containing gases are treated with lye in the absence of the reducing agent as additive. Thus, in this embodiment, the treatment may be a post treatment.

According to another embodiment, gases comprising F₂ and OF₂ and optionally HF can be treated. Such gases may originate from a treatment step wherein F₂ containing gases are insufficiently treated with a base or with a combination of a base and the reducing agent as additive.

According to still another embodiment, gases comprising only F₂ and optionally HF are treated. Such gases are especially gases originating from the start-up phase of F₂ manufacture, or the gas may comprise out-of-spec F₂, or from emergency events in such an apparatus ; a preferred apparatus is an apparatus according to the skid concept as described below. In view of this embodiment, the invention will be explained in detail. The term "out of spec" denotes F₂ which has not the desired degree of purity. For example, the content of CF₄ may be too high which could be caused by a broken anode in an electrolysis cell. Such out-of-spec charges of F₂ are usually decomposed because they are not considered suitable for use in the field of semiconductor manufacture.

In the embodiments described above, gases containing F₂ and/or OF₂ are treated which optionally may also contain HF. It is very improbable that in these embodiments, gases should have to be treated which contain only HF but no F₂ and no OF₂. Nevertheless, should that happen, gases comprising only HF could also be treated by the treatment according to the present invention. While the presence of the reducing agent is not necessary in this case, the advantage is that also gases containing only HF can be treated likewise. It may occur that the content of gases to be treated may vary. For example, the gas to be treated may comprise F₂ and HF in the beginning, and later, due to switch-off of the fluorine generator for example, the gas may only comprise HF.

In a preferred embodiment, gases are treated which, when entering the treatment apparatus, contain only F₂ and optionally HF. As mentioned above, the treatment of F₂ with water or caustic aqueous compositions may result in the formation of OF₂. The process of the invention removes formed OF₂ so that at most neglectable amounts of OF₂ are contained in the treated gas when the process of the invention is performed. Thus, a preferred embodiment of the invention provides a method for obtaining a gas with a reduced content of F₂ and, if present, of HF with, if at all, a neglectable content of OF₂, which method includes at least one step of removing F₂ and, if present, of HF, from a gas comprising F₂ and optionally HF wherein the gas is contacted with a liquid composition comprising water, dissolved reducing agent selected from the group consisting of alkali metal thiosulfate and/or an alkali metal nitrite and at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate. The term "if at all, neglectable content" denotes that OF₂ formed during the treatment is removed completely, or, if OF₂ is not removed completely during the treatment, the content of OF₂ in the gas leaving the reactor in which the treatment is performed is equal to or lower than 200 ppm per volume, preferably equal to or lower than 150 ppm per volume.

The gas to be treated and the liquid composition can be contacted using any apparatus known to the expert useful for a liquid-gas reaction. For example, the gas may be passed through the liquid composition in a reactor. It is preferred to provide means which distribute the gas in the form of fine bubbles in the liquid, e.g. a frit made from material resistant to F₂, e.g. from steel or Monel metal, and/or to provide mixing means, e.g. a mixer driven by a rotating shaft.

It is also possible to contact gas and liquid in a counter current in towers, especially towers comprising means for improving the exchange of matter between gas and liquid, e.g. in a tower comprising internals, especially packings, e.g. Raschig rings or Pall rings, or other means to provide a high contact area between gaseous phase and liquid phase. For example, columns with bubble cap trays or Thormann trays are very well suited. In the columns, a high mass transfer occurs between gas and liquid.

Jet scrubbers are also very suitable. For example, one jet scrubber in line with one tower with packings is highly suitable to treat gases having high F₂ content but lower gas flow, while two jet scrubbers installed in line are very suitable for treating gases, having low F₂ content, but high gas flow, such as gases originating from emergencies. Other arrangements of apparatus may, of course, be useful as generally known in the technique of gas-liquid reactions.

Of course, it is possible to apply two or more alkali metal thiosulfate compounds as reducing agent, e.g. sodium thiosulfate and potassium thiosulfate, or two or more alkali metal hydroxides, e.g. sodium hydroxide and potassium hydroxide, two or more alkali metal carbonates, or a combination of one or more alkali metal hydroxides and one or more alkali metal carbonates. Alternatively, or additionally, to the use of alkali metal thiosulfate, alkali metal nitrites may be applied as reducing agents.

It is preferred to apply compounds wherein alkali metal denotes potassium. The term "liquid composition" denotes preferably an aqueous composition which preferably is free of organic solvents.

The term "F₂" denotes elemental fluorine, especially F₂ containing gases originating from the start-up phase from an apparatus electrolytically

producing F₂, or out-of-spec F₂ originating from such an apparatus and F₂, especially diluted F₂, from emergency events in a plant used for the production of F₂.

The liquid composition comprises dissolved thiosulfate and hydroxide and/or carbonate. That does not exclude the additional presence of solid thiosulfate, solid hydroxide and/or solid carbonate.

The liquid composition may comprise further compounds which are reactive towards F₂, for example, ammonia or potassium iodide. Preferably, alkali metal thiosulfate and/or alkali metal nitrite, more preferably, alkali metal thiosulfate, and the at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate are the only compounds (apart from water and undesired impurities in the composition or originating form the gas treated) which react with F₂ and any OF₂ formed. Preferably, the liquid composition is free of ammonia and potassium iodide.

Among alkali metal thiosulfates, sodium thiosulfate and potassium thiosulfate are preferred. Sodium thiosulfate is especially preferred.

Among alkali metal nitrites, sodium nitrite and potassium nitrite are preferred. Potassium nitrite is especially preferred.

Among alkali metal hydroxides, potassium hydroxide and sodium hydroxide are preferred. Potassium hydroxide is especially preferred.

Among alkali metal carbonates, sodium carbonate and potassium carbonate are preferred. Among alkali metal carbonates, potassium carbonate is especially preferred.

Alkali metal hydrogen carbonates, especially sodium hydrogen carbonate, can also be applied.

Alkali metal (hydrogen) carbonates have the disadvantage that carbon dioxide may be formed which may cause foaming. Another disadvantage of carbonates is that, if the thiosulfate content is low due to its consumption in the reaction with fluorine and OF₂, their effectivity to remove F₂ and to decompose formed OF₂, they are not as effective as hydroxides. Accordingly, potassium hydroxide and sodium hydroxide are preferred.

Alkali metal thiosulfate are preferred treatment agents, and especially, sodium thiosulfate.

Liquid compositions comprising sodium thiosulfate and KOH, and liquid compositions comprising sodium thiosulfate and potassium carbonate are especially preferably applied to remove F₂ and essentially any OF₂ formed as byproduct in the gas. Very preferably, liquid compositions are applied which essentially consist of water, sodium thiosulfate and at least one base selected from the group consisting of potassium hydroxide and potassium carbonate. The term "consisting essentially" refers to the active agents in the liquid

compositions, i.e. to the content of water, thiosulfate and hydroxide ; the compositions may further comprise undesired impurities, e.g. contained in the water, the thiosulfate, the hydroxide, or reaction products present in the liquid compositions from the reaction of F₂ with these agents and water. It was found that alkali metal sulfate and alkali metal fluoride forms during the reaction of water, alkali metal hydroxide and alkali metal thiosulfate with F₂ ; consequently, a liquid composition "consisting essentially" from water, alkali metal thiosulfate and alkali metal hydroxide may comprise significant amounts of alkali metal sulfate and alkali metal fluoride.

The concentration of the alkali metal thiosulfate is preferably equal to or higher than 0.1 % by weight, more preferably, equal to or higher than 0.3 % by weight, respective to the total weight of the composition. It is especially preferred that the concentration of thiosulfate is equal to or higher than 1 % by weight.

The concentration of the alkali metal thiosulfate is equal to or lower than the saturation concentration. The liquid composition may even contain undissolved alkali metal thiosulfate. Preferably, the concentration of the alkali metal thiosulfate is equal to or lower than 10 % by weight, more preferably, equal to or lower than 5 % by weight, respective to the total weight of the liquid composition. Liquid compositions preferably comprise from equal to or more than 0.3 % by weight to equal to or lower than 5 % by weight of thiosulfate, more preferably from equal to or more than 0.5 to equal to or lower than 5 % by weight, most preferably, from equal to or more than 1 % by weight to equal to or lower than 5 % by weight of thiosulfate. If an alkali metal nitrite is present instead of an alkali metal thiosulfate, its amounts correspond to those indicated for the thiosulfate.

Generally, the concentration of the alkali metal hydroxide is preferably equal to or higher than 10 % by weight, more preferably, equal to or higher than 15 % by weight, respective to the total weight of the composition.

The concentration of the alkali metal hydroxide, if present, is equal to or lower than the saturation concentration. The liquid composition may even contain undissolved alkali metal hydroxide. Preferably, the concentration of the alkali metal hydroxide is equal to or lower than 45 % by weight, more preferably, equal to or lower than 35 % by weight, respective to the total weight of the liquid composition.

The concentration of the alkali metal carbonate, if present, is preferably equal to or higher than 10 % by weight, more preferably, equal to or higher than 15 % by weight, respective to the total weight of the composition. More preferably, the concentration of the dissolved base, relative to the total weight of the composition, is equal to or greater than 15 % by weight, and equal to or lower than the saturation concentration.

The concentration of the alkali metal carbonate is equal to or lower than the saturation concentration. The liquid composition may even contain undissolved alkali metal carbonate. Preferably, the concentration of the alkali metal carbonate is equal to or lower than 45 % by weight, more preferably, equal to or lower than 35 % by weight, respective to the total weight of the liquid composition.

If alkali metal hydroxide and alkali metal carbonate are both present in the liquid composition, the preferred range of their total content is from 10 % by weight to 45 % by weight, relative to the total weight of the composition.

In a preferred embodiment, the concentration of the alkali metal thiosulfate in from 1 to 5 % by weight, and the concentration of the alkali metal hydroxide is from 10 to 45 % by weight. In a still more preferred embodiment, the concentration of the sodium thiosulfate in from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 10 to 45 % by weight.

In another preferred embodiment, the concentration of the alkali metal thiosulfate in from 1 to 5 % by weight, and the concentration of the alkali metal hydroxide is from 15 to 45 % by weight. In a still more preferred embodiment, the concentration of the sodium thiosulfate in from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 15 to 45 % by weight.

In another preferred embodiment, the concentration of the alkali metal nitrite is from 1 to 5 % by weight, and the concentration of the alkali metal hydroxide is from 10 to 45 % by weight. In a still more preferred embodiment, the concentration of the sodium nitrite is from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 10 to 45 % by weight.

In another preferred embodiment, the concentration of the alkali metal nitrite is from 1 to 5 % by weight, and the concentration of the alkali metal hydroxide is from 15 to 45 % by weight. In a still more preferred embodiment, the concentration of the sodium nitrite is from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 15 to 45 % by weight.

According to one embodiment, a liquid composition is applied wherein the alkali metal thiosulfate, alkali metal hydroxide and alkali metal carbonate is not supplemented during the reaction. In this case, the concentrations given above refer to the respective concentrations when the contact between gas and liquid are started. Since thiosulfate and hydroxide are gradually consumed in the reaction, the respective concentrations will fall below the initial values.

According to another embodiment, alkali metal thiosulfate, alkali metal hydroxide and/or alkali metal carbonate are supplemented to the liquid composition continuously or from time to time to keep the respective

concentration in a preset level. In this case, the respective reagent is preferably supplemented such that its concentration in the liquid composition is equal to or higher than the respective minimum concentration given above. Another alternative is to add alkali metal thiosulfate from a small separate vessel including automation features for feeding the additive to the scrubbing liquid containing alkali metal hydroxide or alkali metal carbonate only when gas detectors signal F₂ alarm, in emergency case or, to be on the safe side, even in case of an HF alarm. The vessel may, for example, be connected to the suction side of the pump which circulates the scrubbing liquid. It is preferred that the pump only operates in a case of emergency, and in this case, the alkali metal thiosulfate is introduced automatically into the circulating scrubbing liquid.

These alternatives are also applicable if an alkali metal nitrite is applied as additive instead of or additionally to alkali metal thiosulfate.

The contact between gas and liquid is preferably performed at a pressure which is equal to or higher than 1 bar (abs.), more preferably, equal to or greater than 1.1 bar (abs). Preferably, the pressure during contact of the liquid and gas is equal to or lower than 3 bar (abs.), more preferably, equal to or lower than 2 bar (abs). A preferred pressure range is from 1 to 3 bar (abs), more preferably 1.1 to 2 bar (abs), especially from 1.1 bar (abs) to 1.5 bar (abs).

The temperature during the contact between gas and liquid is preferably kept in a range from 10°C to 80°C, more preferably, in a range from 20°C to 40°C. It has to be noted that the reaction of F₂ with water, base and thiosulfate is exothermic, and the provision of effective cooling is advantageous. If the temperature exceeds a preset level, e.g. if the temperature of the washing liquid is greater than 80°C, cooling may be necessary, or the treatment may be interrupted.

The concentration of F₂ in the gas to be treated can broadly vary. This is one of the advantages of the method of the present invention.

The method of the invention is suitable to remove the F₂ content from gases which comprise equal to or more than 0.5 % of F₂ by volume and even less than 0.5 % by volume, e.g. the process can be applied even if the content of F₂ is in the ppm range, e.g. if it is equal to or greater than 100 ppm. The method of the invention is also suitable to remove the F₂ content from gases which comprise equal to or less than 100 % by volume.

Thus, the process can be applied to remove F₂ in any concentration, and this flexibility is a big advantage especially for on-site manufacture of fluorine for etching or chamber cleaning purposes.

There are two major sources for gases to be treated in respect of F₂ concentration and volume flow.

One major source is F₂ containing gas from start-up phases and off-spec charges. Gas from this source often has a high F₂ concentration, and the gas volume flow is comparably low. If the gas to be treated originates from the start-up phase of electrolytic cells for F₂ production or from off-spec charges, the F₂ may be very high, e.g. from 70 to 98 % by volume. Preferably, in this embodiment, the concentration of the alkali metal thiosulfate is from 1 to 5 % by weight, the concentration of the alkali metal hydroxide is from 15 to 45 % by weight, and the concentration of the F₂ in the gas to be treated is from 70 to 98 % by volume. In a still more preferred embodiment, the concentration of the sodium thiosulfate is from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 15 to 45 % by weight, and the concentration of the F₂ in the gas to be treated is from 70 to 98 % by volume. The gas volume flow in this embodiment is preferably 1.5 to 15 m³ per hour.

The other major source is gas originating from emergency events. In gas from this source, the F₂ content is often comparably low, and the gas volume flow is comparably high. If the gas to be treated originates from emergency events, it may often be diluted by ventilation air, and the F₂ content may be in a range from 0.5 to 5 % by volume.

Preferably, in this embodiment for the treatment of gas originating from emergency events, the concentration of the alkali metal thiosulfate is from 1 to 5 % by weight, the concentration of the alkali metal hydroxide is from 15 to 45 % by weight, and the concentration of the F₂ in the gas to be treated is from 70 to 98 % by volume. In a still more preferred embodiment, the concentration of the sodium thiosulfate is from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 15 to 45 % by weight, and the concentration of the F₂ in the gas to be treated is from 0.5 to 10 % by volume. The gas volume flow in this embodiment is preferably from 100 to 15.000 m³ per hour. If the gas to be treated originates from charges of out-of-spec F₂, the content of F₂ may be in any range from 0.5 to approximately 100 % by volume, depending on the degree of purification or dilution, the concentration of the alkali metal thiosulfate is preferably from 1 to 5 % by weight, the concentration of the alkali metal hydroxide is preferably from 15 to 45 % by weight. In a still more preferred embodiment, the content of F₂ is in any range from 0.5 to approximately 100 % by volume, depending on the degree of purification or dilution, the concentration of the sodium thiosulfate is from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 15 to 45 % by weight.

If the gas to be treated originates from the start-up phase of electrolytic F₂ manufacture, the content of F₂ is preferably in range from 70 % by volume to approximately 100 % by volume, preferably to 98 % by volume, the

concentration of the alkali metal thiosulfate is preferably from 1 to 5 % by weight, the concentration of the alkali metal hydroxide is preferably from 15 to 45 % by weight. In a still more preferred embodiment, the gas to be treated originates from charges out of spec, the content of F₂ is in a range from 70 % by volume to approximately 100 % by volume, preferably to 98 % by volume, the concentration of the sodium thiosulfate is from 1 to 5 % by weight, and the concentration of the potassium hydroxide is from 15 to 45 % by weight.

Diluted gases originating from an emergency event may arise. In such an emergency event, F₂ may have leaked, or the danger of a potential F₂ leakage exists e.g. when an earthquake is threatening, or in case of the failure of parts of the installation, e.g. broken lines or a valve failure. Installations may provide for the dilution of the F₂ for example with ventilated air in the case of an emergency to reduce the potential hazards of leaked F₂. In such a case, a high volume, often 5.000 to 15.000 m³/h, of diluted F₂ must be treated efficiently and reliably. The method of the invention is very well suited for such a treatment. It was further found by the inventors that solutions of Na₂S₂0₃ in KOH are very stable towards oxidation even after weeks of contact with air which adds another advantage. This safeguards that any pre-fabricated aqueous solutions of alkali metal thiosulfate remain active until an event of emergency occurs.

The inventors found that nitrites, e.g. potassium nitrite, in an aqueous solution of potassium hydroxide, are oxidized very slowly after days in contact with air. Nevertheless, solutions of nitrites in basic solutions are sufficiently stable under practical conditions to be highly acceptable as safe treatment agent. Main factors that influence the degree of F₂ removal and the removal of OF₂ formed are the residence time which is inversely proportional to the gas volume flow, and the concentration of the thiosulfate. Thus, the residence time can be set to a lower or higher level by increasing or lowering the gas volume flow. The residence time of the gas in contact with the liquid treatment composition comprising a certain thiosulfate concentration is set such that F₂ and OF₂ in the gas after treatment are satisfactorily removed. The residence time may be, for example, in the range from 1 second to 10 minutes. A preferred residence time is from equal to or greater than 1 second to equal to or lower than 5 minutes.

The method of the invention can be applied to any source of gas comprising F₂. It can be applied, for example, in any plant which provides F₂ and for any method of producing F₂, e.g. by electrolysis of solutions of KF in FIF, or for processes which set F₂ free from metal fluorides in a higher valency state, for example, for a gas produced when MnF₄ is heated to form F₂ and MnF₃. The process is especially suitable for plants in which F₂ is produced on site to be used for the manufacture of an item selected from the group consisting of semiconductors, photovoltaic cells, TFTs and MEMS, or the cleaning of a chamber used in the manufacture of said item because the method of the invention is very flexible in view of the concentration of F₂ contained, in view of the volume of gas to be treated, and it is very efficient and reliable.

Often, different kinds of F₂ containing gases will have to be treated. A first kind of gases are those which come up regularly in a preset gas flow having a predictable F₂ concentration ; for example, gases from the start-up phase of electrolytic F₂ manufacture or from out-of-spec charges. A second kind of gases come up in an unpredictable manner, e.g. gases comprising diluted F₂ from emergency events.

Accordingly, the gas comprising F₂ preferably originates from the etching or chamber cleaning in a process for the manufacture of semiconductors, MEMS or photovoltaic cells ; more preferably, it originates from an out-of-spec charge of F₂ gas, or it originates from a start-up phase of the electrolytic manufacture of F₂, or it originates from an emergency event.

For the treatment of the first kind of gases (e.g. from the start-up phase of F₂ production, off-spec charges), in a preferred embodiment, 2 or even 3 apparatuses for the treatment of F₂ containing gases are installed in series. Such gases often have a high concentration of F₂ but arise in a comparatively low gas volume flow. Accordingly, an apparatus to treat the first kind of gases includes at least one reactor suitable for the treatment of gases with comparatively low gas flow. Depending on the number of electrolytic cells producing F₂ from HF/KF, the reactor volume may be such that gas volume flows in the range from 1.5 m³/h to 15 m³/h may be treated to remove F₂ and remove or prevent the formation of OF₂ to a satisfactory level. A jet scrubber or a combination of a jet scrubber and a scrubber tower with packings, installed in line, are very suitable as reactors. If desired, when installing 2 or more treatment apparatus in series, the first treatment apparatus could be operated with alkali metal hydroxide in the absence of a reducing agent, for example, with KOH in the absence of thiosulfate, ammonia or alkali metal iodide such as KI. The gas leaving this treatment apparatus may comprise a certain content of OF₂ and may comprise even residual F₂. The gas is then treated in a second apparatus comprising KOH or K₂C0₃, preferably KOH, and thiosulfate, e.g. Na₂S₂0₃. If desired, several apparatus can be installed in series (which provides for a high degree of purification) or in parallel ; this allows continued treatment if an apparatus must be stopped, e.g. for maintenance. Of course, other apparatus known to be applicable in gas-liquid reactions can also be used.

The apparatus for the treatment of the second kind of gases (often originating from emergency events) may be adapted to the fact that such gases often have a low F₂ content but a very high gas volume flow per time unit (for example, 5.000 m³/h and more, e.g.10.000 to 15.000 m³/h and more), and thus, comparatively low residence time. To treat these gases, preferably, the treatment apparatus must be designed to treat such big quantities of gas in a reasonable time, and a sufficiently high volume of the liquid composition is applied which comprises, at the start or permanently, a relatively high content of thiosulfate, preferably in an amount equal to or greater than 1.5 % by weight, more preferably, 2 % by weight of the thiosulfate. The liquid composition in the emergency apparatus may comprise KOH (or another base, e.g. NaOH), preferably in a range from 10 to 35 % by weight, preferably 25 to 35 % by weight. It could be demonstrated that such a liquid composition which may comprise Na₂S₂0₃ in a range from 1 % by weight to 2 % by weight is very suitable for the "oversized" emergency apparatus. While in the technical apparatus used, the risk of plugging caused by the formation and precipitation of K2S0₄ is very low due to the larger pipe dimensions compared with a lab apparatus and also in view of other apparatus ; for example, a jet scrubber is insensitive to plugging compared to a lab scrubber column filled with random packings nevertheless a slightly lower concentration of KOH reduces this risk of plugging. But, if desired, the content of KOH or another base may be higher, e.g. up to 45 % by weight or more.

This emergency apparatus may be considered oversized for the treatment of F₂ containing gases with comparably low gas volume streams and high residence time, but is nevertheless advantageous and even necessary if there is a risk of emergency situations. Fluorine containing gases from emergency situations may be passed directly to and through the emergency scrubber (or other apparatus designed for emergency gas treatment) only. If desired, the emergency apparatus may be connected with a reservoir of a thiosulfate solution to refresh any spent thiosulfate. For example, a reservoir, preferably equipped with automation features comprising thiosulfate in a concentration of equal to or greater than 1.5 % by weight may be connected with the inlet (suction side) of a pump which circulates the liquid composition. In emergency cases, this pump is activated and feeds fresh thiosulfate solution to the emergency apparatus. The term "automation features" denotes equipment to measure the concentration of thiosulfate in the reservoir and means to start the supplementation of additional thiosulfate to the reservoir if the measured concentration is too low.

The apparatus or apparatuses for treatment of the first kind of gases and the second kind of gases may be installed in parallel.

In a very reliable and preferred embodiment, the apparatus or apparatuses for the treatment of F₂ containing the first kind of gases (from the start-up phase and off-spec gases) and an apparatus for the treatment of the second kind of gases (F₂ containing gases from an emergency treatment) are installed in series. Often, two scrubbers (e.g. 2 jet gas scrubbers or a jet gas scrubber and a scrubber tower with internals or packings) for the treatment of the first kind of gases - here, the second scrubber often is already redundant because of the high efficiency in the first scrubber) - and one "oversized" scrubber for the emergency treatment are installed in series. The first kind of gases is passed through all of the three scrubbers while gases from emergency events are passed directly to the scrubber for emergency treatment. In such an installation, the safety level to remove F₂ and OF₂ is very high.

The method of the present invention to treat gases containing F₂ wherein the gases leaving the reactor contain at most traces of OF₂ can be applied to treat such gases from any source including F₂ containing waste gases from chamber cleaning or F₂ containing gases from etching of semiconductors, MEMS, photovoltaic cells or TFTs. It is preferably incorporated into a process wherein F₂ is produced and delivered directly on site. F₂ needed for etching of items as mentioned above or for chamber cleaning is thus not transported via road or rail to the site of use. Such an integrated process of F₂ manufacture and use is a second aspect of the present invention.

A second aspect of the present invention concerns consequently a process for the manufacture of an item selected from the group consisting of

semiconductors, photovoltaic cells, TFTs and MEMS in at least one chamber comprising at least one step selected from the group of a step of etching the item with a F₂ containing gas and a step of cleaning the chamber with an F₂ containing gas suitable for etching or chamber cleaning, wherein the F₂ is manufactured on site by electrolysis of solutions of KF (as electrolyte) in HF, and wherein F₂ containing charges from the start-up phase of the F₂ manufacture or out-of-spec charges of F₂, or F₂ containing gases originating from emergency events are subjected to a treatment step to obtain a gas with a reduced content of F₂ and/or of OF₂ formed during the treatment step wherein the F₂ containing gas is contacted with a liquid composition comprising dissolved alkali metal thiosulfate and/or nitrite and at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate. If HF is present in the gas to be treated, it is removed, too.

Accordingly, a first embodiment of the second aspect of the invention concerns the manufacture of items selected from the group consisting of semiconductors, photovoltaic cells, TFTs and MEMS, comprising a step wherein the item is placed in a chamber, an F₂ containing etching gas is added, the item is etched, optionally supported by applying a plasma, the F₂ containing gas atmosphere is removed from the chamber wherein the removed gas is contacted with a liquid composition comprising dissolved alkali metal thiosulfate and at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate to obtain a gas with a reduced content of F₂. The advantage is among others that, as described above, the level of OF₂ leaving the apparatus is very low, and if present, HF is removed, too.

A second embodiment of the second aspect of the invention concerns the manufacture of items selected from the group consisting of semiconductors, photovoltaic cells, TFTs and MEMS, or for chamber cleaning wherein F₂ is produced by electrolysis of HF comprising dissolved KF, wherein the F₂ produced is intended to be used as etching gas of the items or for chamber cleaning, and wherein the F₂ produced is not suitable to be used because it originates from a start-up phase of the electrolytic cell or it is out-of-spec, and thus is treated according to the method of the invention with a liquid composition comprising dissolved alkali metal thiosulfate and at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate to obtain a gas with a reduced content of F₂ ; also in this embodiment, the advantage is among others that, as described above, the level of OF₂ formation is very low, and that HF is removed, too if it should be contained in the gas. A third embodiment of the second aspect of the invention concerns the manufacture of items selected from the group consisting of semiconductors, photovoltaic cells, TFTs and MEMS, wherein, in a step selected from the group consisting of

• the manufacture of F₂ by electrolysis of FIF/KF solutions including its cleaning and delivery to the point of use,

• the placement of the item in a chamber where an F₂ containing etching gas is added,

• the etching of the item, optionally supported by applying a plasma,

• the removal of the F₂ containing gas atmosphere from the chamber, wherein an emergency event occurs and F₂ containing gas is diluted with air and the diluted gas is contacted with a liquid composition comprising dissolved alkali metal thiosulfate and at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate to obtain a gas with a reduced content of F₂.

The description given above of the invention relates to certain specific embodiments, such as the treatment of F₂ containing gases from the start-up phase, out-of-spec charges of F₂ containing gas, and F₂ containing gases from emergency events which are often diluted. These specific embodiments are preferred.

Nevertheless, the process of the invention can also be applied to treat F₂ containing gases from other sources in the frame of etching and chamber cleaning. For example, if certain organic or inorganic compounds which are substituted by fluorine atoms are applied as etching gas in a treatment chamber or as chamber cleaning gas, especially in plasma supported processes, the gases leaving the chamber may contain F₂. Also such gases can be treated according to the process of the present invention. The method of the invention is especially suitable to treat gas originating from the on-site manufacture of F₂ but it can also be applied to treat gases comprising F₂ originating from the etching or chamber cleaning in a process for the manufacture of semiconductors, MEMS or photovoltaic cells.

In a preferred embodiment of the invention, the method of the invention is performed with treatment means, e.g. a scrubber, incorporated in a F₂ manufacturing unit according to the skid concept. The skid concept for the manufacture of F₂, its purification, storage, if any, delivery to the point of use and amenities is described in detail in US provisional patent

applications 61/383204 filed September 15, 2010 and 61/383533 filed

September 16, 2010 which are mentioned above.

The present invention provides an improved plant suitable to produce fluorine on-site especially for the use as etchant and chamber cleaning agent in the manufacture of semiconductors, photovoltaic cells, thin film transistor liquid crystal displays and micro-electromechanical systems.

The skid concept provides a plant to supply fluorine gas to a tool which applies fluorine gas as reactant to perform chemistry in this tool which apparatus comprises skid mounted modules including at least one skid mounted module selected from the group consisting of

- a skid mounted module comprising at least one storage tank for FIF, denoted as skid 1,

- a skid mounted module comprising at least one electrolytic cell to produce F₂, denoted as skid 2,

- a skid mounted module comprising purification means for purifying F₂, denoted as skid 3,

- a skid mounted module comprising means to deliver fluorine gas to the point of use, denoted as skid 4,

- a skid mounted module comprising cooling water circuits, denoted as skid 5,

- a skid mounted module comprising means to treat waste gas, denoted as skid 6,

- a skid mounted module comprising means for the analysis of F₂, denoted as skid 7, and

- a skid mounted module comprising means to operate the electrolysis cells, denoted as skid 8. The preferred plant of the present invention provides fluorine gas to a tool which applies fluorine gas as reactant to perform chemistry in this tool which apparatus comprises skid mounted modules including

- a skid mounted module comprising at least one storage tank for HF, denoted as skid 1,

- a skid mounted module comprising cooling water circuits, denoted as skid 5,

- a skid mounted module comprising means to treat waste gas, denoted as skid 6,

- a skid mounted module comprising means to operate the electrolysis cells, denoted as skid 8.

The plant preferably also comprises skid modules which may be located close to the skid modules 1 to 8 but may be separated from them, namely

- a skid module 9 which is an electrical sub-station mainly to transform

medium voltage to low voltage, and/or

- a skid module 10 which houses utilities (control room, laboratory, rest room).

Further, the apparatus may comprise means to supply inert gas, e.g. means to supply liquid nitrogen and gaseous nitrogen ; means to supply compressed air and water ; and ancillaries and amenities. At least, skids 1, 2, 3, 4 and 7, preferably all skids, comprise housings for safety reasons.

In skid 1, each hydrogen fluoride storage container has generally a capacity of from 10 to 5000 liters, often 500 liters to 4000 liters, preferably from 500 to 3000 liters. Particular examples of hydrogen fluoride storage containers are tanks approved by RID/ADR - FMDG - of UN T22 or, preferably, UN T20 type. Such tanks are commercially available.

Each HF storage container in skid 1 can be suitably connected to the hydrogen fluoride supply line through a manifold.

Each HF storage container in skid 1 is preferably individually isolatable from the hydrogen fluoride supply line. The HF storage containers in skid 1 can generally be isolated from the hydrogen fluoride supply line by a remotely controlled device, preferably a remotely controlled valve. More preferably each storage container is equipped with a remotely controlled device, preferably a remotely controlled valve, allowing isolating that container from the hydrogen fluoride supply line.

When remotely controlled valves are present, manual valves are suitably installed in addition. The remotely controlled valves allow for example to operate the HF-storage-containers from a remote control-room.

In a preferred embodiment, the HF storage containers comprise an automatic HF level sensor. In particular the HF storage containers can be installed on weighing scales. In that case, preferably, a process control system, in particular an automatic process control system is operable to closes the remotely controlled valve of a first, empty HF container and to open the remotely controlled valve of another second HF-containing hydrogen fluoride storage container. This embodiment is particularly effective to avoid manual handling of HF valves and to ensure a continuous HF supply.

In a preferred aspect, the valves are operable to close automatically in case of abnormal operation state, such as for example a process-interruption in a process-equipment connected to the HF supply line.

In another preferred aspect, the valves are operable to close automatically in case of an HF leakage in skid 1. Such HF leakage can for example be caused by a leakage of optional flange-connections inside the HF storage-container there is the possibility to close these valves via remote control. This avoids in particular the necessity to approach the hydrogen fluoride supply unit in this case.

The skid also contains valves to shut down the supply of HF and nitrogen. Preferably, skid 1 comprises 3 to 10 HF containers ; especially preferably, it comprises 4, 5, 6, 7 or 8 containers. 4 HF containers are suitable for a productivity of 150 tons F₂/year. Tanks with smaller size, especially, if they can be closed via valves separately, improve the safety of the plants. The tanks must be made from or at least lined with material resistant to HF. The walls should be sufficiently thick ; preferably, they have a 10 mm EVIDG code (international Maritime Dangerous Goods code) equivalent thickness.

In a particular embodiment, skid 1 comprises, preferably permanently, at least one HF emergency container. Such HF emergency container is preferably an empty HF storage container as described herein which is preferably connected to the HF supply line. The FIF emergency container is generally operable to receive FIF from a leaking HF storage container. The HF emergency container is suitably kept under pressure of an inert gas or under vacuum.

The tanks are preferably portable so that they can be transported by trucks and/or can be handled by a fork lift.

Skid 1 comprises a ventilation system, and the ambient air is preferably permanently ventilated to a scrubber, especially the "emergency response scrubber" (ERS) for HF and F₂ removal (as described below).

Skid 2 : The skid comprising the electrolytic cell or cells (skid 2) is now described in detail. It contains at least one electrolytic cell. Preferably, it contains at least two electrolytic cells. More preferably, it contains at least 6 electrolytic cells. A skid 2 with 8 electrolytic cells is very suitable. The skid preferably is constructed such that if desired, additional electrolytic cells can be added if the demand for fluorine gas is rising. The cells comprise jackets through which cooling water can be circulated. If desired, skid 2 can be provided in the form of separate sub-skids 2A, 2B and so on. In these sub-skids, a certain number of electrolytic cells are assembled. The separate sub-skids 2A and 2B (and any other sub-skids) are attached together to form one cell room. Often, the cell room will contain 4, 6 or more cells, for example, 8 cells or even more. The advantage of providing several electrolytic cells is that the shut-off of one or even more cells for maintenance or repair can be compensated by raising the output of the other cells. To assemble several sub-skids has the advantage that dimensions can be kept within permissible maximum dimensions for usual road transport. The electrolytic cells are connected to collectors for the F₂ and the H₂ produced. Skid 8 comprises at least one rectifier to supply DC current to the cell anodes. It has to be noted that each cell may comprise 1 or more anodes. Typically, each cell comprises 20 to 30 anodes and even more, up to 70 and more. A cable connects each of the anodes with the rectifier. Each cell cathode is connected through one copper or aluminium bus bar to the rectifier. One rectifier can supply current to several or all the anodes of one or more cells. It is preferred to apply one rectifier per anode. The advantage is that the intensity at each individual anode can be fine tuned depending on the specific anode characteristics, abnormal situations at a specific anode (e.g. overvoltage, short- circuit, or broken anode) can be immediately detected allowing the automatic shutdown of the faulty anode while all other anodes and cells continue to produce F₂. Skid 2 includes a cooling water circuit supplying cooling water to the jackets of the cells.

Skid 2 also comprises settling boxes ; preferably, a settling box for F₂ and a settling box for H₂ are connected with each of the cells. The settling boxes serve to reduce the gas velocity of the F₂ and H₂ produced in the cell to avoid electrolyte dust to be carried over. Preferably, the settling boxes comprise a vibrator and a heating to melt the separated electrolyte dust for easy removal.

The collectors collecting the produced F₂ are connected by a pipe with skid 3, the collectors for produced H₂ are connected by a pipe with a scrubber for H₂ in skid 5B (this will be described in detail below). In a preferred embodiment, skid 2 also comprises a ventilation system to treat accidental releases of F₂ with HF and/or H₂.

The ambient air of skid 2 is ventilated to a scrubber, especially the ERS scrubber for safety reasons (the scrubber is described below).

Skid 3 : it comprises means for the purification of the produced F₂. It comprises a cooler wherein the F₂ is pre-cooled. Skid 3 also comprises an HF washer wherein the pre-cooled F₂ is contacted with HF which is kept at a very low temperature. The HF washer contains a cooling jacket through which a coolant is passed. The coolant may be a liquid coolant as well known by the man skilled in the art or more preferably a mixture of evaporated liquid nitrogen and gaseous nitrogen at defined mixing temperature. Skid 3 further comprises a buffer tank, a compressor, e.g. a diaphragm compressor, an HF condenser operated at low temperature and at least one HF absorber column, preferably containing NaF as absorbent for HF. Preferably, at least two absorber columns are contained in the skid 3. If desired, the absorber columns are redundant so that one set is in absorption mode, the other set can be regenerated. The absorber columns comprise a heating. If desired, a further set of absorber columns may be present in skid 3 or on the site for reloading of absorbent. The HF condenser is connected via pipes to the electrolysis skid 2. Preferably, at least one set of columns is mounted on a wheeled trolley to keep them (re-)movable from the skid.

The HF condenser may be cooled to a temperature where HF condenses to form a liquid or even a solid. It is preferred if it is condensed to form liquid HF. Cooling the trap to a temperature of -60°C to -80°C, preferably to about -70° is very suitable. As cooling medium, well-known cooling liquids operable at the desired low temperature are suitable. It is preferred to apply a N₂ gas which was obtained by mixing liquid N₂ and gaseous N₂ in appropriate amounts. This way of cooling is very reliable. Thus, skid 3 comprises lines to deliver and to withdraw cooling medium.

The ambient air of skid 3 is ventilated to a scrubber, especially the ERS scrubber (described below) for safety reasons.

Skid 4 : this skid serves for storage of fluorine gas and the delivery of fluorine gas to the point of use. Skid 4 comprises filters to remove any remaining entrained solids. Preferably, the skid 4 also contains a filter to remove solids. For example, the F₂ produced in the electrolytic cells may comprise entrained solid electrolyte from the cell, usually, adducts of KF and FIF. The filter is preferably constructed from material resistant to FIF and fluorine ;

stainless steel, copper, Monel metal and especially nickel are especially suitable. Filters made from sintered particles of these metals comprising a pore diameter in the nanometer range to provide semiconductor grade F₂, e.g. with a pore diameter of equal to or less than 5 nm, and more preferably, with a pore diameter of equal to or smaller than 3 nm, are very suitable.

If desired, skid 4 comprises a pre-filter to remove from the F₂ coarser particles with a pore diameter of equal to or less than 1 μπι.

Skid 4 preferably comprises means for the storage of fluorine gas. It may, for example, contain a buffer tank for fluorine gas.

Additionally to, but preferably instead of the buffer tank, skid 4 may comprise a permanent or temporary fluorine gas storage unit in the form of a plurality of hollow bodies to store the F₂. The storage unit is connectable to other skids.

"Permanent fluorine gas storage unit" is understood to denote in particular a fluorine gas storage unit which is integrated into the fluorine plant. For example, the fluorine gas storage unit can be a transportable or preferably a fixed unit which is present in skid 4 throughout operation of the fluorine plant.

Preferably, the permanent fluorine gas storage unit is designed to contain more than 90 wt % more preferably more than 95 wt %, most preferably about

100 wt. % of the fluorine gas relative to the total weight of fluorine gas stored in the plant.

Skid 4 is further able to convey fluorine gas from skid 2 to the point of use. Possible components of skid 4 include but are not limited to supply lines, compressors, mixers and buffer tanks. It may also contain a single-cell FT-IR for fluorine analysis. "Connectable" is understood to denote in particular that the permanent fluorine gas storage unit is equipped to be able to be connected to a component of skid 4. Preferably, the permanent fluorine gas storage unit is equipped to be able to be connected to a fluorine gas supply line. In a preferred aspect, the fluorine gas storage unit is connected to a component of skid 4, in particular a fluorine gas supply line throughout operation of the fluorine gas plant. In a further preferred aspect, the fluorine gas storage unit is directly connected to a component of skid 4.

Suitable equipment for connecting the fluorine gas storage unit connected to a component of skid 4 includes a manifold connected to each hollow body of the fluorine gas storage unit through a line and preferably having a shut-off valve in each line allowing to individually isolate each hollow body and said manifold is further connected to a component of skid 4.

Skid 4 preferably comprises from 4 to 25 hollow bodies, more preferably from 5 to 8 hollow bodies. The hollow bodies are preferably of substantially identical shape and dimensions. Cylindrically shaped hollow bodies (tubes) are preferred. Each hollow body of the fluorine gas storage has preferably a shut-off valve.

The hollow bodies of the fluorine gas storage unit can be suitably fixed together by means of an appropriate frame. Particular frame geometries include triangular, square, and rectangular geometries.

In the fluorine gas plant according to the invention, the fluorine gas storage means are generally able to contain or contains fluorine gas at a pressure of at least 25 psig (about 1.72 barg). Often this pressure is equal to or greater than 35 psig (about 2.4 barg), preferably equal to or greater than 40 psig

(about 2.8 barg). In the plant according to the invention, the fluorine gas storage means is generally able to contain or contains fluorine gas at a pressure of at most 400 psig (about 27.6 barg), preferably, equal to less than 75 psig

(about 5.2 barg). Often, this pressure is equal to or lower than 65 psig

(about 4.5 barg), preferably equal to or lower than 60 psig (about 4.1 barg). It is understood that the hollow bodies of the fluorine gas storage unit are generally able to contain or contain fluorine gas at the aforesaid pressures. It is particularly preferred that the hollow bodies contain fluorine gas at the aforesaid pressures.

In the fluorine gas plant according to the invention the ratio of the molecular F₂ stored in the fluorine storage means to the daily molecular F₂ producing capacity of the fluorine gas plant is generally from 0.1 to 1, preferably from 0.1 to 0.25.

Preferably, each of the hollow bodies can be shut off from the plant separately ; this improves safety. The fluorine leaving skid 4 is transported preferably through double-walled pipes to the point of use. Fluorine gas is transported in the inner tube ; the outer double wall envelope comprises nitrogen. The piping contains a pressure sensor to analyze the nitrogen pressure in the outer double wall envelope. Preferably, the walls of the pipes are thicker than commonly used for transporting gases, i.e. preferably, they are thicker than 1 mm, preferably, thicker than 4 mm ; a wall thickness of equal to or greater than 5 mm is especially preferred ; pipes classified as "schedule 80" are very suitable. This serves to improve the safety. Welded piping with radiographic inspection is very suitable.

The storage container or containers can be mounted on wheels or are transportable by a forklift.

The ambient air of skid 4 is ventilated to a scrubber, especially the ERS scrubber as described below (for safety reasons).

In the fluorine gas plant according to the invention, the point of use can be connected to a further manufacturing plant, for example a chemical plant or, in particular a plant using fluorine gas for surface treatment. The point of use is often connected to a semiconductor manufacturing plant, preferably a

manufacture of photovoltaic devices or flat panel displays.

In a preferred embodiment of the fluorine gas plant according to the invention, skid 4 comprising the fluorine gas storage unit is an enclosed space. The enclosed space generally comprises a fluorine sensor capable to trigger connection of the enclosed space to scrubber skid 6. Suitably, the enclosed space is connected to skid 6 through a suction line connected to a fan which is operable to transport gas from the enclosed space of skid 4 to skid 6.

In a further embodiment, the fluorine gas plant according to the invention further comprises a mixer, preferably a static mixer, said mixer being preferably capable to receive fluorine from skid 4 and to receive inert gas, such as preferably argon and/or nitrogen, from an inert gas supply line.

In an optional embodiment, a pressure control loop adjusts, generally reduces the pressure of fluorine gas supplied to the point of use to a desired value Skid 5 provides for cooling or heating of parts of the plant by means of cooling water. It is preferably located close to the electrolytic skid 2 or sub- skids 2A and 2B or any additional sub-skids 2X, more preferably, it is located above the skids. Skid 5 comprises at least one circuit which serves to heat the electrolyte cells to melt the electrolyte salt when the reaction is started, and to cool the cells when the reaction is running. The circuit is filled with cooling water which may be tap water or distilled water. The circuit includes a buffer tank, a pump which is preferably redundant, and a dry cooler with fans with variable speed drives. The cooling water, during operation, is preferably kept at 75 to 95°C to avoid solidification of the electrolyte in the cells.

Another circuit comprised in skid 5 serves to cool other heat exchangers of the apparatus. It contains a cooling liquid, preferably a mixture of water and ethylene glycol, more preferably, water comprising 40 % by weight of ethylene glycol. Also this circuit comprises a buffer, a pump which preferably is redundant, and a dry cooler. The cooling circuits include detectors to measure the temperature of the cooling water, means to heat the cooling water, e.g.

electric heating, heat exchangers to cool the circulating liquids.

Skid 6 comprises at least one scrubber each for F₂ and H₂. Preferably, the scrubber pumps are redundant. The skids of the plant (especially skids 1, 2, 3, 4 and 7) include a ventilation system to ventilate the ambient air of the skid enclosures permanently through scrubbers in skid 6.

Preferably, scrubber skid 6 comprises an F₂ scrubber for destruction of any F₂ or FIF vent required for safety reasons or maintenance operations. F₂ and HF from the ventilated air from the skids are treated in a scrubber for emergency response (ERS). The scrubbers are preferably jet scrubbers and provide the suction. The scrubbers may be mounted on sub-skids, e.g. a sub- skid 6A which comprises at least one scrubber for emergency response (ERS), a sub-skid 6B which serves to scrub produced H₂ with the purpose to remove HF entrained therein, and a scrubber to remove HF and/or F₂ in waste gas originating from ventilating the ambient atmosphere from skids as explained below.

The capacity of the regular F₂ scrubber (optionally mounted in sub- skid 6B) corresponds at least to the expected amount of F₂ to be removed during regular operation. F₂ is removed by contact with an abatement solution. This scrubber preferably comprises a jet scrubber and a packed column to provide a high contact area between the abatement solution and F₂. Preferably, the gas leaving the regular F₂ scrubber is passed through the back-up scrubber of the ERS scrubber. The ERS scrubber serves as back-up of the regular F₂ scrubber used for removal of F₂ or HF from ventilated air, and for the emergency treatment of ventilated air which contains HF and/or F₂ after a leakage. The capacity of the ERS scrubber (optionally mounted in sub-skid 6A) corresponds preferably at least to the amount of fluorine and HF to be removed in cases for emergency, for example, in the very improbably case of pipe breaking, an accident with one of the tubes containing fluorine or an HF storage tank. It is advisable to select the capacity of the ERS scrubber according to a worst-case scenario ; for example, if HF tanks with 2 m³ capacity and F₂ storage tubes with a capacity of 8 kg F₂ are present, the ERS should be able to abate the respective amounts of HF and F₂ and the plant-holdup. Preferably, the ERS scrubber comprises 2 units for scrubbing to achieve a high destruction and removal efficiency ; redundant pumps are fed by normal and emergency power supply. It preferably comprises a jet scrubber and a packed column to achieve a good contact between the gas to be treated and the abatement solution. The other unit serving for emergency treatment may comprise a packed column, but preferably, it comprises two jet scrubbers in series. The F₂ removal can be performed with agents known to remove F₂. Preferably, a KOH solution or NaOH optionally comprising an alkali metal thiosulfate, e.g. sodium thiosulfate or potassium thiosulfate, or an alkali metal nitrite, e.g. potassium nitrite or sodium nitrite, is used as abatement solution and is pumped through the scrubber or scrubbers and the column, if present, as decomposing agent for F₂. A cooler may be foreseen to cool the KOH solution. Of course, it is an advantage of the concept that one emergency scrubber serves to treat HF and F₂.

The scrubber of skid 6 serving for the HF abatement of the H₂ gas stream may be mounted in sub-skid 6B. Sub-skid 6B includes preferably a jet scrubber operated with aqueous HF solution to reduce the content of HF in the H₂. The concentration of HF may be in a range between 1 and 10 % by weight. The scrubber further comprises a packed column wherein fresh water is given on top of the column to reduce the HF content. Skid 6B also includes a line which allows the dilution of H₂ by nitrogen which was used as cooling medium in skid 3.

The reliability of the scrubbers in skid 6, or in skids 6A and 6B

respectively, is very important. Thus, essential parts like fans or pumps to circulate the abatement solution through the scrubbers may be redundant. From time to time, fresh abatement agent, for example, KOH solution, and/or thiosulfate or its solution, e.g. provided by a truck, is added to the circulating abatement solution.

Skid 6 preferably comprises also one or more retention pits for liquid in case of accidental leakages.

Skid 7 concerns the apparatus used for the analysis of the produced F₂. It is preferably installed near skids 2A and 2B ; very preferably, it is located above the skids 2 A and 2B. Skid 7 is, for example, an analyzer shelter. The ambient atmosphere around it is preferably ventilated to the ERS. The analyzer contains analyzing means suitable to determine the content of the main impurities of the produced F₂. An optional UV spectrometer (which analyses the UV spectrum) and multi-input, multi-cell FT-IR spectrometer (Fourier-Transform Infra-Red spectrum) analyzers are very suitable. Preferably, both raw F₂ taken from the cells and purified F₂ is sent to a multi-cell FT-IR. One channel is analyzed at a given time. The amount of HF, CF₄, C₂F₆, COF₂, SF₆ and S0₂F₂ can, for example, be measured by FT-IR while the content of F₂ may be analyzed by UV. It has been found that UV spectroscopy can be used as a direct measurement tool for fluorine which shows a sharp decrease in fluorine concentration when an anode burn occurs ; an enormous increase of CF₄ is observed at the same time in the FT-IR. Therefore the burn can be detected easily by the sharp decrease of the F₂ concentration during the burn, during which impurities are formed (mainly CF , C₂F₆ and COF₂). The result of this burn (the produced F₂ contains more CF , C₂F₆, COF₂, HF than when working regularly) is not only the alteration of impurities' contents but also a sharp decrease in the content of fluorine monitored by a detector system, especially, in the present invention, by UV spectroscopy. During the measurements using UV spectroscopy, the whole UV spectrum can be used. Preferably, not the whole spectrum but only the absorption at this particular wavelength, particular UV spectroscopy between 200 and 400 nm, more preferred 250 to 330 nm, most preferred between 270 to 290 nm, even at about 280 nm is used for measuring, because it is more or less the maximum of the UV absorption of F₂. The FTIR and UV measurements are also used to control the purity of the purified F₂. Thus, the analysis serves to detect anode burns by measuring the raw F₂ by UV and CF by FT-IR and to document and control the purity of the purified F₂.

The raw F₂ of all cells and the purified F₂ are continuously sampled and analyzed. Current FT-IR can accept up to 9 samples. Thus, the purified F₂ and the raw F₂ of up to 8 electrolysis cells can be analyzed with one multi-cell FTIR of the current generation of apparatus. Alternatively, instead of the FT-IR and optional UV analyzer of skid 7, skid 4 may comprise a single-cell FT-IR.

Skid 8 contains the rectifiers or rectifier racks, the BPCS (basic process control system), the ESD (emergency shutdown system), the F&G (fire and gas system) panel which registers fire alarms and gas alarms, small motor starters, lighting distribution and other means to provide electricity and to control electric means of the plant. Skid 8 is installed near skids 2A and 2B ; preferably, it is located above them. Each electrolytic cell, as mentioned above, usually has a multitude of anodes, e.g. 26. Preferably, each anode is supplied by a rectifier. The rectifiers are preferably assembled in rectifier cabinets installed in air- conditioned enclosure.

It is preferred to provide a slight overpressure in this skid 8 to protect from gas ingress. All cables and cathode bus bars should be carefully sealed.

It is preferred that the rectifier cabinets are bolted on a fixed frame for seismic protection.

Skid 8 comprises walls and a roof. It includes preferably also a fire detection system, especially a VESDA (very early smoke detection apparatus) and a fire extinguishing system operating, e.g. with HFC-227ea or with Inergen^®, a mixture of inert gases (nitrogen, argon and carbon dioxide).

Skid 9 is preferably a pre-fabricated room with concrete walls or similar to a container, made from metal shields. It contains means for connection to electric current and to transform it from medium to low voltage. Preferably, it contains the "electric sub-stations" sub-skids 9A and 9B. Skid 9A preferably contains the MV (medium voltage) cells for incoming and outgoing current and bypass current and the transformers to transform the medium voltage current into low voltage current. It is adaptable to the local network ; for example, the transformers are selected such that they fit to the local voltage which may, for example, be 380 V or 400 V (50 Hz) or 440 V (60 Hz). Skid 9 includes preferably also a fire detection system, especially a VESDA (very early smoke detection apparatus) and a fire extinguishing system operating, e.g. with

HFC-227ea or with Inergen^®, a mixture of inert gases (nitrogen, argon and carbon dioxide).

Skid 9B also is preferably a pre-fabricated concrete room having walls and a roof. This substation houses the low voltage switchgears (LVCS) and a diesel generator. It is interconnected by cables to the skids which need a low voltage power supply, e.g. via a cable trench. It includes preferably also a fire detection system, especially a VESDA (very early smoke detection apparatus) and a fire extinguishing system operating, e.g. with HFC-227ea or with Inergen^®, a mixture of inert gases (nitrogen, argon and carbon dioxide).

Skid 9B preferably also comprises a battery charger station for a forklift. Skid 9B must be interconnected to the process skids, especially skid 2.

Skid 10 comprises utilities for the personal, for example, a control room, a laboratory and a rest room. Preferably, it is divided into sub-skid 10A and sub-skid 10B. Sub-skid 10A contains the control room and the laboratory. The laboratory which may be small includes a fume hood with good ventilation, e.g. up to 500 m³/h and even more, which hood is preferably made from acid- resistant materials and can be used for analytical titrations, a safety cabinet for reagents and samples, a wash basin and a chemical sink wherein chemical waste can be collected, preferably in a drum made from acid resistant material. The laboratory preferably has a gas detector installed in a fresh air intake and a closing mechanism which closes the air intake in case of a gas alarm. Sub- skid 10A preferably is kept under a slight overpressure to prevent gas ingress. Skid 10A preferably comprises air-conditioning.

The control board of the control room is preferably connected online to a remote control board which may be located on another facility. This allows operating several fluorine gas production plants remotely from one single control room.

Sub-skid 10B contains the rest room. It contains installations useful for the control room personal. It preferably includes lockers, a changing room, a toilet, a shower, and cabinets for chemical gowning (gloves, capes etc). The plant safety shower includes eye-shower systems and is preferably is located close to the outside of sub-skid 10B because it must be fed with warm potable water. Skid 10 preferably comprises ventilation and heating.

The method of the invention is performed in the frame of skid 6.

Another aspect of the present invention concerns an aqueous solution containing or consisting of 1 % to 5 % by weight of a treatment agent selected from the group consisting of KN0₂, NaN0₂, Na₂S₂0₃ and K₂S₂0₃, 10 to 45 % by weight of KOH, 10 to 45 % by weight of NaOH, or a mixture of KOH and NaOH the total content of said mixture is 10 to 45 % by weight of the solution.

A preferred aqueous solution contains or consists of 1 % to 5 % by weight of a treatment agent selected from the group consisting of Na₂S₂0₃ and K₂S₂0₃, 10 to 45 % by weight of KOH, 10 to 45 % by weight of NaOH, or a mixture of KOH and NaOH the total content of which mixture is 10 to 45 % by weight of the solution.

A solution comprising or consisting of 1 % to 5 % by weight of Na₂S₂0₃, equal to or more than 50 % by weight of water and equal to or more than 15 % by weight of KOH, is especially preferred.

A solution consisting of 1 % to 5 % by weight of Na₂S₂03, equal to or more than 50 % by weight of water and equal to or more than 15 % by weight of KOH, is most preferred.

The solutions are suitable for F₂ removal and/or OF₂ removal from gases containing them. They can be stored in a separate skid.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The following examples are intended to explain the method of the invention in detail without intending to limit the scope of the invention.

Example 1 : Removal of F₂ using KOH and Na₂S₂0₃ or K₂C0₃ and Na₂S₂0₃ Used apparatus :

An F₂/N₂ gas mixture consisting of 10 % by volume of F₂ and 90 % by volume of N₂ was stored in a pressure bottle. The bottle was connected via a valve and a gas line with a scrubber tower which contained randomly arranged stainless steel packings. A pressure bottle containing pure N₂ was connected via a valve to the gas line ; consequently, the gas mixture comprising 10 % by volume of F₂ could be diluted to any desired F₂ concentration. A scrubber liquid was contained in a storage tank. The storage tank was connected via lines to the bottom and to the top of the scrubber, and the washing liquid was circulated, via a heat exchanger, from the storage tank to the top of the scrubber column, contacted with the gas mixture containing fluorine and nitrogen in a counter current, treated with the washing liquid, and after contact with the washing liquid, collected in the storage tank. The treated gas mixture left the column on the top, was passed through a cool trap to remove any entrained liquid, and was then passed to an FTIR analyzer to determine the residual OF₂ content. The content of F₂ was determined to be very close to 0 (indicated by the mass balance which may be imprecise, though). The gas mixture was passed through a trap containing H₂0 and Na₂S₂0₃ to safeguard the removal of any F₂ still contained in the gas mixture. The storage tank had a volume of 4 liter.

The tests were performed with varying contents of base, thiosulfate, volume per time unit of treated gas/varying residence time and with varying concentrations of F₂ in the gas mixture to be treated. In some of the tests, base or thiosulfate, respectively, was not contained in the washing liquid.

The temperature of the washing liquid in the tower when contacted with F₂ was maintained in a range from 20 to 30°C.

The pressure of the gas mixture to be treated in the scrubber tower was kept constant during the trials between about 1.1 to 1.3 bar (abs.).

Applied base :

• KOH in water with a concentration of about 30 % by weight of KOH

• K₂C0₃ with a concentration of about 20 % by weight.

The influence of the following parameters on the effectivity of F₂ removal and suppression of OF₂ content in the treated gas were tested :

· Concentration of Na₂S₂0₃ : 0 %, 0.5 % by weight, 1 % by weight, 2 % by weight.

• Residence time (in brackets, respective gas flow in liter/h) : 140 sec (25 L(h) ;

70 sec (50 L/h) ; 14 sec (250 L/h) ; 7 sec (500 L/h) (at differing F₂

concentrations in the gas to be treated).

· Liquid load density : (in brackets, m³/(h-m²). A higher liquid load density in the scrubber column means that a greater volume of scrubber liquid entered the top of the column, resulting in better distribution of the liquid and better wetting of the random packing. Two ranges of liquid load density were tested, namely 30 to 50 m³/(h-m²) and 100 to 120 m³/(h-m²).

· F₂ concentration of the gas mixture to be treated at the inlet of the scrubber tower : 0.8 % by volume ; 1 % by volume ; 1.6 % by volume ; 10 % by volume.

• OF₂ concentration given in ppm by volume, as measured at the outlet of the scrubber by FTIR.

· The mass balance shows the effective removal of F₂.

Each trial was operated for a period of about 1 hour to 6 hours. The minimum and maximum for OF₂ values, given in ppmv, at the outlet of the scrubber, indicate the efficiency of the additive Na₂S₂0₃ in the aqueous KOH scrubber solution and the influence of the residence time on the OF₂ destruction. The lower the OF₂ content at the outlet of the scrubber, the better the destruction of this compound. A longer residence time in the scrubber is advantageous for the degree of OF₂ removal.

The respective parameters and resulting concentrations are compiled in table 1. Tests 1, 3, 4, 6 and 8 are included for comparison.

Table 1 : Test results applying varying treatment compositions

(^* It has to be noted that the respective trials are not correlated with the examples given below having the same number. Thus, "trial N° 6" of table 1 has no relation with "Example 6".)

The following conclusions can be taken from the test results :

• Aqueous potassium hydroxide and potassium carbonate, if applied together with thiosulfate, are both effective in the removal of F₂ and in decomposing OF₂. Potassium hydroxide is preferred because no foaming occurs, and in a situation where the thiosulfate is spent, is more effective than the carbonate in decomposing OF₂.

• To remove F₂ from gases with high gas volume flow, e.g. gas originating from emergency situations in which F₂ containing gas is diluted with inert gas, a concentration of thiosulfate in the upper range is advantageous. A liquid composition comprising thiosulfate in a concentration of 2 % by weight is very effective.

• The mass balances of the tests confirm the good results in removing F₂ for defined conditions like constant periods of time for the trials and no plugging of the apparatus used. Mass balance estimations and comparison of the test trial results showed that besides the destruction of OF₂, an F₂ abatement of at least 90 to 95 % was achieved, especially for gas mixtures comprising comparably low F₂ content and short residence time (emergency case).

• Shorter residence times and higher thiosulfate concentrations improve the decomposition of OF₂, as well as a higher liquid load, e.g. 100

to 120 m³/(h-m²) compared with 30 to 50 m³/(h-m²).

Any precipitated K₂S0₄ can be dissolved by water injections or by providing a mixing of the scrubber liquid.

Example 2 : Removal of F₂ using NaOH and Na₂S₂0₃

In the apparatus of example 1, a liquid composition comprising 30 % by weight of NaOH and 2 % by weight of Na₂S₂0₃ is applied as scrubber liquid

(washing liquid). A gas comprising 10 % by volume of F₂, balance to 100 % by volume being N₂, is contacted as described in test 2 of example 1 with the scrubbing liquid in the scrubber tower in counter current. The detected level of OF₂ corresponds to the level of OF₂ as contained in the gas leaving the scrubbing tower in test 2 of example 1.

Example 3 : Removal of F₂ using Na₂C0₃ and Na₂S₂0₃

In the apparatus of example 1, a liquid composition comprising 18 % by weight of Na₂C0₃ and 1 % by weight of Na₂S₂0₃ is applied as scrubber liquid (washing liquid). A gas comprising 10 % by volume of F₂, balance to 100 % by volume being N₂, or 1.6 % by volume of F₂, balance to 100 % by volume being N₂, or 0.8 % by volume of F₂, balance to 100 % by volume being N₂ is contacted as described in test 9 of example 1 with the scrubbing liquid in the scrubber tower in counter current. The detected level of OF₂ corresponds to the level of OF₂ as contained in the gas leaving the scrubbing tower in test 9 of example 1.

Example 4 (comparison example) : Removal of F₂ using Na₂C0₃ (Na₂S₂0₃ absent)

Example 3 is repeated, but no Na₂S₂0₃ is added to the scrubber liquid. The OF₂ content in the gas leaving the scrubber tower corresponds to the OF₂ content of example 8.

Example 5 : Long term stability test of a KOH / Na₂S₂0₃ solution

Example 5.1 : 72-hour test

Pressurized air with a flow of 100 1/h was passed through an aqueous solution contained in 1-liter vessel comprising 1.8 % by weight of Na₂S₂0₃ and about 30 % by weight of KOH for 72 hours. No change in the concentration of Na₂S₂0₃ was observed.

Example 5.2 : 15-day test

Pressurized air with a flow of 100 1/h was passed through an aqueous solution contained in a 1 -liter vessel comprising 4.2 % by weight of Na₂S₂0₃ and about 30 % by weight of KOH for 10 days. No change in the concentration of Na₂S₂0₃ was observed.

Example 5 demonstrates that there exists no risk that a solution comprising KOH and Na₂S₂0₃ could become ineffective by contact with air ; this observation adds a momentum of safety to the process of the invention, especially in an apparatus incorporated in the skid concept where ventilation air from the skids is passed via the scrubber in an emergency case.

Example 6 : Removal of F₂ using KOH and KN0₂

3095g of a scrubber liquid comprising 28.6 % by weight of KOH and 1.02 % by weight of KN0₂ was contacted in the scrubber tower of example 1 for 3.5 h with a gas comprising 10 % by volume of F₂, balance to 100 % by volume being N₂. The gas flow was 25 1/h. Initially, the content of OF₂ in the gas leaving the scrubber tower was approximately 20 ppm. After some time, the level of detected OF₂ in the treated gas began to rise up to a value of 60 ppm.

After said 3.5 h, N₂ was mixed to the gas mixture entering the scrubber tower such that the resulting gas mixture contained 1 % by weight of F₂. The treatment was continued for further 3 h. The level of OF₂ in the gas leaving the scrubber tower was initially determined to be about 5 to 10 ppm and rose slowly to about 20 ppm. This demonstrates that even the almost spent solution is still suitable to treat satisfactorily gases containing lower F₂ concentrations and having low gas flow, such as to be treated in emergency cases. The residual concentration of KN0₂ was determined to be about 0.2 % by weight.

After said 6.5 h, the scrubber solution was once again contacted with a gas comprising 10 F₂ having a gas flow of 25 1/h. The level of OF₂ passing the scrubbing tower increased rapidly and indicated that the scrubber liquid was spent.

This example shows that KN0₂ is a suitable reducing agent. The difference to Na₂S₂0₃ is that the level of OF₂ passing the treatment rises continuously indicating an early consumption of KN0₂ while the level of OF₂ passing the scrubber tower remains on a low level for a long time if Na₂S₂0₃ is applied as reducing agent. It is assumed that a higher starting concentration of KN0₂ or supplementation would improve the suitability of KN0₂ for long term treatments.

Example 7 : Test of the oxidative stability of potassium nitrite/KOH solutions An aqueous solution comprising about 1.5 % by weight of KN0₂ and 30 % by weight of KOH was prepared. For 24 hours, about 100 liters/hour of pressurized air was passed through the solution. 3 % of the initial nitrite concentration, set as 100 %, were oxidized. After 96 hours of this treatment, about 5 % of the nitrite initially present were oxidized.

This test demonstrates that even under the extreme conditions of this example, the nitrite solution is sufficiently stable against oxidation.

Example 8 : Removal of F₂ using a solution comprising 15 % by weight of KOH and 1 % by weight of Na₂S₂0₃

Approximately 2800 g of a solution comprising 15.8 % b y weight of KOH and 1 % by weight of Na₂S₂0₃ were contacted in the scrubber tower of example 1 with a gas consisting of 10 % by volume of F₂ and 90 % by volume of N₂ for 5 hours. The gas flow was 25 liter/h. The maximum content of OF₂ in the gas leaving the scrubber tower was determined by FTIR to be lower than 20 ppm.

The high efficiency of a scrubber liquid comprising only 15 % by weight of KOH demonstrates the high efficiency of the combination of KOH and thiosulfate.

Claims

C L A I M S

1. A method for obtaining a gas with a reduced content of F₂ and/or OF₂ and, if present in the gas to be treated, a reduced content of HF, including at least one step of removing F₂ and/or OF₂ from a gas comprising F₂ and/or OF₂ and optionally FIF wherein the gas is contacted with a liquid composition comprising dissolved alkali metal thiosulfate and/or dissolved alkali metal nitrite and at least one dissolved base selected from the group consisting of alkali metal hydroxide and alkali metal carbonate.

2. The method of claim 1 wherein the gas comprises F₂.

3. The method of claim 1 or claim 2 wherein the gas is contacted with a liquid composition comprising dissolved alkali metal thiosulfate, and wherein the alkali metal thiosulfate is selected from the group consisting of sodium thiosulfate and potassium thiosulfate.

4. The method of anyone of claims 1 to 3 wherein the base is selected from the group consisting of potassium hydroxide and sodium hydroxide.

5. The method of anyone of claims 1 to 3 wherein the liquid composition comprises potassium hydroxide and sodium thiosulfate or potassium carbonate and sodium thiosulfate.

6. The method of anyone of claims 1 to 5 wherein the concentration of the dissolved thiosulfate, relative to the total weight of the composition, is equal to or greater than 0.3 % by weight and equal to or lower than 5 % by weight.

7. The method of anyone of claims 1 to 6 wherein the concentration of the dissolved base, relative to the total weight of the composition, is equal to or greater than 15 % by weight, and equal to or lower than the saturation concentration.

8. The method according to anyone of claims 1 to 7 wherein the gas is contacted with the liquid composition in counter current.

9. The method of anyone of claims 1 to 8 wherein the gas and the composition are contacted under a pressure from 1 to 3 bar (abs), preferably 1.1 to 2 bar (abs).

10. The method of anyone of claims 1 to 9 wherein the gas and the liquid composition are contacted at a temperature in the range from 10 to 80°C, preferably in the range from 20 to 40°C.

11. The method of anyone of claims 1 to 10 wherein the gas comprising F₂ originates from an out-of-spec F₂ gas, or originates from a start-up phase of the electrolytic manufacture of F₂, or it originates from an emergency event.

12. The method of anyone of claims 1 to 11 wherein the gas comprising F₂ originates from an out-of-spec F₂ gas, or originates from a start-up phase of the electrolytic manufacture of F₂ and contains F₂ in an amount of 70 to 98 % by volume, or it originates from an emergency event and contains F₂ in an amount of 1 to 5 % by volume.

13. The method of anyone of claims 1 to 12 wherein the gas obtained after contact with the liquid composition comprises OF₂ in a concentration of equal to or lower than 200 ppm by volume.

14. The method of anyone of claims 1 to 13 wherein the treatment is performed in a treatment means which is incorporated in a skid.

15. A method for the manufacture of an item selected from the group consisting of semiconductors, photovoltaic cells, TFTs and MEMS, or the cleaning of a chamber used in the manufacture of said item which comprises at least one step comprising a method according to anyone of claims 1 to 14.

16. An aqueous solution containing or consisting of 1 % to 5 % by weight of a treatment agent selected from the group consisting of KN0₂, NaN0₂, Na₂S₂0₃ or K₂S₂0₃, 10 to 45 % by weight of KOH, 10 to 45 % by weight of NaOH, or a mixture of KOH and NaOH the total content of said mixture is 10 to 45 % by weight of the solution.

17. The solution of claim 16 comprising or consisting of 1 % to 5 % by weight of Na₂S₂0₃, equal to or more than 50 % by weight of water and equal to or more than 15 % by weight of KOH.