This application claims priority to European application No. 10157894.6 filed March 26 ^th , 2010, the whole content of this application being incorporated herein by reference for all purposes.

The invention concerns a method of treating waste gases from the manufacture of electronic devices, especially semiconductors, photovoltaic cells, micro-electromechanical systems ("MEMS"), TFTs (thin film transistors, often used for liquid crystal displays) and the like.

For the manufacture of semiconductors, photovoltaic cells, MEMS and TFTs, several consecutive steps of deposition of layers and etching a part of them away are necessary.

Deposition of layers is often performed according to the CVD method including the decomposition of gases or vapors in deposition chambers.

A lot of different gases and vapors are applied as agents for deposition or etching, e.g. silanes, for example, SiH ₄ and Si ₂ H ₆ , boranes, e.g. B ₂ H ₆ , GeH ₄ , phosphane, alkoxysilanes, e.g. tetramethoxyorthosilicate ("TMOS"),

tetraethylorthosilicate ("TEOS"), alkylphosphines, alkylboranes, ammonia, carbon monoxide, halogen compounds, e.g. boron trichloride, WF ₆ ,

chlorofluorocarbons, and others. Deposition is often enhanced by plasma energy.

Etching is often performed using halogens or halogen compounds, e.g. chlorine, fluorine, hydrochlorof uorocarbons, chlorofluorocarbons, COF ₂ , hydro f uoro carbons and perf uorocarbons, e.g. fluoroform, CF ₄ and C ₂ F ₆ , hexafluorobutadiene, or nonmetal fluorides, e.g. SF ₆ and NF ₃ . Some of these compounds, e.g. SF ₆ , F ₂ , COF ₂ , are also used to clean plasma chambers from undesired deposits.

Elemental fluorine is usually produced electrolytically from HF in the presence of molten adducts of KF and HF with the approximate

formula KF-2HF. It can be observed that elemental fluorine is not necessarily produced in constant high quality. Sometimes, charges are obtained which are not suitable for applications sensitive to impurities. For example, the content in (CF ₄ due to the reaction of fluorine with carbon electrodes) may be too high, or the oxygen or nitrogen content may be too high, e.g. by a leak through which air did intrude into the reactor system, and the fluorine is rejected for use in the manufacture of electronic devices. While it might be possible to remove such impurities, it is often not economically feasible, and thus, the impure fluorine is treated as waste gas.

Thus, the processes mentioned above produce waste gases comprising unreacted agents and their decomposition products, or impure fluorine. The waste gases must be transformed to environmentally acceptable products before they can be dumped. A well known way to render the waste gases harmless is to burn them.

EP-A-0155061 discloses a process wherein hydrogen as a by-product of F ₂ production is catalytically oxidized.

US 2007/0074743 discloses a process wherein hydrogen as by-product from F ₂ production is used to treat chambers to remove residual fluorine.

GB 2353034A discloses a method of scrubbing halogens, e.g. F ₂ , and halogen compounds with inorganic hydrides. H ₂ is mentioned but considered as dangerous to handle.

US 5,603,905 discloses a process for the disposal of global warming, air- polluting compounds such as C ₂ F ₆ and NF ₃ .

EP-A-0 262561 describes a burner for waste gases from the semiconductor manufacture. The waste gas, oxygen and a burner gas ("LPG", liquefied petroleum gas which is propane and/or butane) are delivered in a specific manner to a burner. The waste gas in then burned.

US patent 7,112,060 discloses a waste gas treating burner for

decomposition of silane, fluoroform, tetrafluoromethane and hexafluoro ethane in the presence of oxygen or an oxygen containing gas and a fuel gas, for example, hydrogen, town gas, LPG or the like. It is also known that hydrogen can be used as a fuel for decomposition of waste gases from semiconductor plants.

Elemental fluorine (F ₂ ) was identified as an especially suitable etching gas in the manufacture of electronic devices, especially semiconductors, TFTs, MEMS, and solar cells. F ₂ is difficult to handle and must be treated very cautiously ; the transport of F ₂ in greater amounts via lorry or railway presents some risk in case of an accident. Thus, facilities with a greater F ₂ demand are provided with F ₂ produced directly on site.

Problem of the present invention is to provide an improved process to treat waste gas from the manufacture of electronic devices, especially semiconductors, TFTs, MEMS, and solar cells, with the purpose to convert the contaminants contained therein into products which can be dumped, easily removed from the waste gas or even released to the atmosphere.

Consequently, the invention concerns a method for the treatment of waste gas from the manufacture of electronic devices, especially of semiconductors, TFTs, MEMS, and solar cells, in which F ₂ is applied as reactive gas and is electrolytically produced together with H ₂ on site and wherein the treatment of the waste gas includes a step of waste gas heat treatment in the presence of a fuel wherein at least a part of the fuel is constituted by the H ₂ electrolytically produced in an F ₂ generating unit on site together with F ₂ . The term "waste gas" includes impure fluorine which, due to a high degree of impurities, is not suitable for its application as etchant or chamber cleaning agent in the manufacture of electronic devices. The occurrence of impurities in F ₂ may be caused by troubles in the electrolytic cells, e.g. by a broken anode. Another source for impure F ₂ is product produced in the start-up phase of the electrolytic cells. The term "on site" means that the apparatus for the generation of elemental fluorine is located close to the chamber or tool for the manufacture of the electronic device, in which it is intended to be applied as etching agent or chamber cleaning agent. The distance is preferably equal to or shorter than 500 m, more preferably, equal to or shorter than 100 m. The distance can even be shorter, e.g. equal to or lower than 50 m, and it is possible that the apparatus for fluorine generation is located in close proximity of the tool or chamber, e.g. in a distance equal to or shorter than 10 m. Especially preferably, the apparatus for fluorine manufacture is in fluid connection to the tool in which it is applied. Thus, by a fluid connection, it is avoided to transport the fluorine in tanks or bottles ; the fluorine is directly delivered through a pipe from the generator to the tool, optionally via means for its purification, e.g. e layer of an adsorbent for HF, e.g. NaF, a filter for removal of solids, e.g. a Monel or nickel frit, or a storage tank.

In the F ₂ generating unit, F ₂ is produced electrolytically from a precursor. The precursor is selected such that, in a liquid or molten state, it releases fluorine if a voltage of at least 2.8 V is applied. Usually, the voltage is about 8 to 10 V.

KF-2HF can be electrolyzed in molten form to produce H ₂ and F ₂ according to equation (1) :

KF-2HF -> KF + H ₂ + F ₂ (1) HF in a molten HF adduct of KF having the formula KF (1.8 to 2.3)HF is the preferred starting material. F ₂ is electrolytically formed according to the equation (2) by applying a voltage and passing electric current through the molten salt :

2HF -> H ₂ + F ₂ (2) As can be realized, one mol of hydrogen is produced together with one mol of fluorine.

Prior to its use as fuel, the H ₂ is treated to remove entrained HF and/or entrained solids, especially to remove solidified electrolyte salt. It is preferred that solids are removed before using the H ₂ as fuel, for example by a filter, e.g. a filter made from HF resistant materials, e.g. stainless steel, Monel metal or nickel, or by a separator for solids, e.g. a cyclone. Alternatively, or additionally, the H ₂ can be contacted with liquid HF to remove solids, e.g. in a jet gas scrubber. The HF can also be removed from the H ₂ by contacting the impure H ₂ with solid adsorbents, e.g. A1 ₂ 0 ₃ , or water or caustic water, e.g. water comprising KOH or NaOH, e.g. in a scrubber, or by means of a low temperature treatment ; this can be performed, for example, in a cooled trap.

It is preferred to remove entrained solids, especially entrained solidified electrolyte salt, by contact with HF, especially in a jet scrubber. Entrained HF is preferably removed by contact with NaF, e.g. in an NaF tower.

It is an advantage of the process of the invention that H ₂ which is a by- product of the on-site fluorine manufacture and which otherwise would have to be destroyed can be applied in a useful manner to supplement other fuel, especially town gas, liquefied petroleum gas and H ₂ from other sources. H ₂ has high energy content and reduces effectively the amount of other fuel.

Metal hydrides, especially silanes such as SiH ₄ , Si ₂ H ₆ , alkyl silanes, e.g. tetramethyl silane, and germanes are applied to deposit layers of silicon and germanium. Ester derivatives of boron, silicon and phosphorous, e.g. trialkyl phosphites, trialkyl phosphates, alkoxysilanes, e.g. tetramethoxysilane and tetraethoxysilane, are used to deposit boron oxide, phosphorous oxide and silicon oxide layers of for doping layers with these oxides. Some nonmetal hydrides, especially phosphane and borane, and silicon, boron or phosphor compounds with organic substitutes, e.g., tetramethyl silane or tetraethyl silane,

trimethylphosphine and trimethylborane, are useful to dope layers. HBr, Cl ₂ , saturated and unsaturated perfluorocarbons, e.g. CF ₄ , C ₂ F ₆ , hexafluorobutadiene, saturated or unsaturated hydrochlorofluorocarbons, saturated or unsaturated chlorofluorocarbons, saturated or unsaturated hydrofluorocarbons,

e.g. fluoroform, NF ₃ , SF ₆ , carbonyl fluoride and F ₂ , optionally together with additives, e.g. nitrogen, helium or argon, are used as etching agents. The reagents are decomposed in the plasma chamber (or thermal chamber) when they are applied to form deposits or to etch deposited material. The formed decomposition products may also be contained in the waste gas and are burned to products which then can be easily removed by washing or other treatments.

In the following table, some chemicals which are applied for deposition, etching or chamber cleaning and possible compounds which are to be decomposed or removed from the resulting waste gas are compiled.

Often, oxygen or a gas which contains oxygen, especially air, is delivered to the burner in which the waste gases are treated. If impure fluorine is reacted with the hydrogen, no oxygen is needed. The formed HF can be used to supply the fluorine generator.

For many contaminants, a high temperature in the burner is desirable. Preferably, the temperature in the burner is equal to or greater than 900°C.

Suitable burners are described in EP-A-0 262561 and US-A 7,112,060.

Treatment chambers in which water plasma is generated are also suitable. Such plasma treatment chambers are described in DE 195 18 208 C2

and DE 199 53 928 Al . In this apparatus, H ₂ serves as hydrogen source.

Such apparatus contain a plasma chamber wherein an extremely hot gas, a plasma gas, (e.g. at about 10.000°C) is generated by an arc. The plasma gas and the waste gas to be treated are introduced into a mixing chamber. Upon contact with the plasma gas, the contaminants in the waste gas are pyrolyzed to products which can be removed easily from the waste gas. A preferred plasma gas is water ; this can be produced in situ from the H ₂ produced in the F ₂ generating unit and oxygen delivered into the chamber. The contaminants are chemically transformed to metal or nonmetal oxides, carbon dioxide, HC1, HF and other compounds.

Consequently, the method of the invention for the treatment of waste gas from the manufacture of electronic devices is especially suitable for the treatment of waste gas containing at least one contaminant selected from the group consisting of metal hydrides, non-metal hydrides, alkyls or alkyl esters of elements of the 3 ^rd , 4 ^th or 5 ^th main group of the periodic system of the elements, organometallic compounds, metal amides, e.g. tetrakis dimethylamino titanium, halogens, e.g impure fluorine unsuitable for its use as etchant or chamber cleaning gas in the tools on site, halogensubstituted compounds, hydrocarbons and carbon monoxide. Alkyl preferably denotes a saturated or unsaturated CI to C5 alkyl group, more preferably a saturated or unsaturated CI to C3 alkyl group ; or a saturated or unsaturated CI to C5 alkyl group, or more preferably a saturated or unsaturated CI to C3 alkyl group which is substituted by at least one halogen atom, preferably by at least one fluorine atom.

If desired, HF can be separated from the treated waste gas and can be returned to the F ₂ generating unit.

The waste gas preferably comprises at least one contaminant selected from the group consisting of halogenated non-metal compounds, halogenated metal compounds, halogenated carbon compounds, halogenated hydrocarbon compounds, silicium hydrides, germanium hydrides, boron hydrides, phosphane, tetraalkyl silicates, tetraalkyl silanes, disilanes, polysilanes, trialkyl boranes, trialkyl phosphines, trialkylphosphites, trialkyl phosphates, triethyl borane, CO, WF ₆ , BC1 ₃ , CF ₄ , C ₂ F ₆ , F ₂ , SF ₆ , Cl ₂ , NF ₃ , HBr, wherein the term "alkyl" denotes saturated or unsaturated CI - C5 groups, optionally substituted by at least 1 halogen atom, and their decomposition products, and decomposition products of photoresists. Decomposition products are the result of plasmatic reactions in the chamber during deposition or etching ; they are for example partially oxidized compounds, e.g. COF ₂ , and fluorinated decomposition products, e.g., fluorocarbons, SiF ₄ , and S0 ₂ F ₂ .

According to one embodiment, the waste gas originates from a deposition step in a deposition chamber, e.g. a CVD chamber, or from an etching step in an etching chamber. According to another embodiment, the waste gas originates from a chamber cleaning step. According to another embodiment which is preferred the waste gas originates from the start-up phase of the electrolytic cells. According to still another embodiment which is preferred the waste gas is a charge of F ₂ comprising impurities and thus considered as not suitable for the intended use, especially not suitable for the application in the manufacture of electronic items. In the latter embodiments, the content of F ₂ may be quite high. For example, F ₂ from the start-up phase may comprise 70 to 98 % by volume of F ₂ . In charges considered unsuitable for application in the manufacture of electronic items such as photovoltaic cells, TFTs or semiconductors, the content of F ₂ may be equal to or greater than 5 % by volume, and preferably equal to or greater than 10 % by volume. It may be up to almost 100 % by volume, depending on the degree of purification or dilution (e.g. dilution with N ₂ , neon, argon or other compounds). Thus, a preferred range of F ₂ concentration is 5 to almost 100 % by volume, e.g. 5 to 99.5 % by volume.

According to one embodiment of the invention, the waste gas to be treated originates from a deposition step, usually performed in a CVD chamber. In waste gas of that type, residues of the reactive chemicals used, often metal hydrides, non-metal hydrides, organometallic compounds, alkyl compounds or alkyl ester compounds of elements of the 3 ^rd , 4 ^th and 5 ^th main group of the periodic system of the elements, halogen-substituted metal compounds and halogenated non-metal compounds, for example WF ₆ , halogens, hydrocarbons, and carbon monoxide, metal amides, e.g. tetrakis dialkylamino titanium, are contained, as well as their decomposition products. Metal and non-metal hydrides, for example, NH ₃ , and especially silanes such as SiH ₄ , Si ₂ H ₆ , germanes and alkyl silanes, e.g. tetramethyl silane, are applied to deposit layers of silicon and germanium. Some compounds mainly used for doping, e.g. nonmetal hydrides, especially phosphane and borane, ester derivatives, e.g. trialkyl phosphites and trialkyl phosphates, and silicon, halogen compounds, for example, BC1 ₃ , boron or phosphor compounds with organic substitutes, e.g., trimethylphosphine and trimethylborane, and decomposition products of these compounds can be present as contaminants. The term alkyl preferably denotes a saturated or unsaturated CI to C5 alkyl group, preferably a saturated or unsaturated CI to C3 alkyl group ; or a saturated or unsaturated CI to C5 alkyl group which is substituted by at least one halogen atom, preferably by at least one fluorine atom, and especially a saturated or unsaturated CI to C3 alkyl group which is substituted by at least one halogen atom, preferably by at least one fluorine atom.

According to another embodiment, the waste gas originates from an etching step, be it thermal etching or plasma-assisted etching. In such an etching step, often deposited material is removed by converting a part of the layer to volatile fluorinated compounds. Predominant etching agents are F ₂ , COF ₂ , SF ₆ , NF ₃ , and halogenated carbon compounds and halo hydro carbon compounds, especially saturated or unsaturated hydro fluorocarbons, perfluorocarbons, chlorofluorocarbons, hydro chloro fluorocarbons and their mixtures with additive gases, e.g. oxygen, nitrogen, helium or argon. The waste gas comprises unreacted etching agent and decomposition products of the etching agents and the decomposed deposited material, for example, HF, carbonyl fluoride, silicon fluoride, volatile metal fluorides, and fluorinated carbon compounds. The concrete composition of the contaminants depends on the etched layer. For example, if layers of silicon, silicon oxides or silicon nitrides are etched, the contaminants will comprise silicon fluorides. If photoresist layers are etched, the contaminants will comprise fluoro substituted decomposition products of the photoresist. Often, photoresists are manufactured from polymethyl methacrylate, polymethyl glutarimides or phenyl formaldehyde resins. Thus, the

decomposition products will be composed of volatile compounds comprising essentially carbon, fluorine, hydrogen and oxygen.

Preferred sources of F ₂ to be reacted with the H ₂ is obtained from the startup phase of the electrolytic F ₂ generation from KF/HF electrolytes and from out- of-spec charges of F ₂ electro lytically produced from KF/HF electrolytes.

The treated waste gas is then preferably subjected to at least one subsequent treatment step (post-treatment step). In this subsequent treatment step or post-treatment step, the treated waste gas is contacted with a washer, an absorbent or an adsorbent to remove the volatile compounds and solid contents in the treated waste gas stream. Water is very suitable as washer ; if desired, the washer contains alkaline compounds which support the removal of acidic constituents in the treated waste gas. For example, if it is intended to contact the treated waste gas with alkaline water, water comprising sodium lye, potassium lye, sodium lye or sodium hydrogen carbonate is very suitable. Water soluble compounds, e.g. HF, fluoro substituted phosphor compounds, boron compounds and fluorosilicon compounds, and any solids, are retained in the water. Thus, the at least one post treatment step is preferably selected from the steps of passing the gas stream through a water washer, through an alkaline water washer, and contacting the gas stream with an adsorbent or absorbent. Any HF formed may also be recovered by passing the treated gas through at least one trap kept at a low temperature, e.g. through at least one trap having a temperature of -60°C or lower. For example, the trap may have a temperature of equal to or higher than -80°C.

Washer liquid and waste gas can be contacted in a counter current, for example, in towers which contain packings like Pall or Raschig rings to improve the contact. The washer liquid can be dried, and the neutralized dry mass obtained can be dumped.

Alternatively or additionally, the treated waste gas can be contacted with an adsorbent, for example, activated carbon, silica gel or aluminium oxide.

The provision of an integrated process for the manufacture of elemental fluorine by electrolysis for the application in the manufacture of electronic devices, especially in semiconductor, solar panel, TFT (for liquid crystal displays or flat panel displays) and MEMS industry and for the treatment of waste gases is very advantageous since H ₂ as a by-product of the F ₂ manufacture is used as fuel for the treatment of waste gas produced during the use of the elemental fluorine simultaneously produced with the H ₂ .

In still another embodiment of the invention which is especially preferred generated fluorine that is unsuitable for application in the manufacture of electronic devices is reacted with the hydrogen produced in the HF electrolysis process. H ₂ and F ₂ are mixed and burned to form HF which is cooled, condensed and can be used as such or, in a preferred embodiment, is returned to the fluorine generator to supply electrolyzed HF. Also this embodiment is advantageous because it allows the recycling of both H ₂ and (impure) F ₂ formed. In this embodiment, the F ₂ and the H ₂ are treated to remove HF and/or entrained solids, especially entrained electrolyte salt, before they are reacted. The entrained solids are removed preferably by contact with liquid HF. The temperature of the liquid HF during its purifying contact with F ₂ and H ₂ , respectively, is equal to or higher than the melting point of the HF at the respective pressure in the scrubber.

Preferably, it is equal to or higher than -83°C, more preferably, it is equal to or higher than -82°C. It is preferably equal to or lower than -40°C. The

temperature of the liquid HF is preferably in the range between -60°C and -82°C. The pressure is selected such that the hydrogen fluoride remains in the liquid state. Preferably, the pressure is equal to higher than 0.5 bar (abs.). Preferably, it is equal to or lower than 20 bar (abs). For technical reasons, the contact between F ₂ and H ₂ , respectively, and HF is preferably performed at ambient pressure or slightly above ambient pressure, for example, at ambient pressure up to about 1.5 bar (abs). Good results are achieved in a technically very feasible manner at ambient pressure (about 1 bar abs).

According to still another embodiment, the F ₂ and, if desired, the H ₂ originate from the start-up phase of the electrolytic process for the manufacture of F ₂ and H ₂ . In the start-up phase, the F ₂ produced often does not fulfill the specification of F ₂ used for the etching of electronic parts or for chamber cleaning. According to the process of the invention, the F ₂ from the start-phase is reacted with H ₂ which amy also originate from the start-up phase. It is preferred in this embodiment, too that F ₂ and H ₂ , respectively, are treated to remove entrained solids and/or HF before they are reacted to form HF. In this embodiment, the F ₂ and the H ₂ are treated to remove HF and/or entrained solids, especially entrained electrolyte salt, before they are reacted. Also here, the entrained solids are removed preferably by contact with liquid HF. The temperature of the liquid HF during its purifying contact with F ₂ and H ₂ , respectively, is equal to or higher than the melting point of the HF at the respective pressure in the scrubber. Preferably, it is equal to or higher than -83°C, more preferably, it is equal to or higher than -82°C. It is preferably equal to or lower than -40°C. The temperature of the liquid HF is preferably in the range between -60°C and -82°C. The pressure is selected such that the hydrogen fluoride remains in the liquid state. Preferably, the pressure is equal to higher than 0.5 bar (abs.). Preferably, it is equal to or lower than 20 bar (abs). For technical reasons, the contact between F ₂ and H ₂ , respectively, and HF is preferably performed at ambient pressure or slightly above ambient pressure, for example, at ambient pressure up to about 1.5 bar (abs). Good results are achieved in a technically very feasible manner at ambient pressure (about 1 bar abs).

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 explain the invention further without intention to limit it. General : The method is performed in a semiconductor manufacturing unit. Elemental fluorine (and, as co-product, H ₂ ) is produced electrolytically in an on-site F ₂ producing unit and delivered to the semiconductor manufacturing unit. In the semiconductor manufacturing unit, chambers contaminated by silicon- oxygen deposits are cleaned using the fluorine gas ; the chamber is regularly evacuated, and gaseous constituents are withdrawn from the chamber and forwarded through a waste gas line to a burner which has the features of the burner described in EP-A-0 262561. The burner is operated using a fuel containing liquefied petroleum gas (essentially propane and/or butane) and H ₂ from the on-site F ₂ manufacturing unit. Treated waste gases are passed through a water scrubber.

Example 1 : Electrolytically produced H ₂ as burner fuel for semiconductor waste gas

Fluorine and hydrogen are produced by electrolyzing HF in the presence of HF adducts of KF as electrolyte salt at a temperature of about 80 to 100°C in a metallic vessel which serves as electrode. Appropriate voltage (about 8 to 10 V) is applied, and the electrolysis is started. F ₂ and H ₂ are collected in the respective compartments of the apparatus. F ₂ is delivered to the semiconductor plant, and H ₂ is delivered to the burner together with LPG fuel.

In the semiconductor plant, F ₂ is applied as chamber cleaning gas to remove deposits comprising carbon- fluoride polymers and silicon. In the burner, the waste gas containing F ₂ , SiF ₄ , volatile fiuoro carbons and Si0 ₂ is contacted with oxygen and the fuel. The resulting treated waste gas comprises essentially only HF, Si0 ₂ , water and C0 ₂ from the volatile carbon compounds and from burnt fuel. The treated waste gas is passed through a water scrubber comprising NaOH. HF is reacted to NaF, C0 ₂ is absorbed, mostly as NaHC0 ₃ , and Si0 ₂ is removed from the treated waste gas in solid form. The gas stream leaving the washer can be passed to the environment.

Example 2 : Recycling of impure F ₂

Fluorine and hydrogen are produced by electrolyzing HF in the presence of HF adducts of KF as electrolyte salt at a temperature of about 80 to 100°C in a metallic vessel which serves as electrode. Appropriate voltage (8 to 10 V) is applied, and the electrolysis is started. F ₂ and H ₂ are collected in the respective compartments of the apparatus. Analysis of the produced F ₂ reveals that it contains too much CF ₄ to be suitable for being used in the semiconductor manufacture. Consequently, H ₂ and F ₂ produced are mixed and burned to form HF which is condensed and returned to the metallic vessel for electrolysis. Example 3 : Recycling of impure F ₂

Fluorine and hydrogen are produced by electrolyzing HF in the presence of HF adducts of KF as electrolyte salt at a temperature of about 80 to 100°C in a metallic vessel which serves as electrode. Appropriate voltage (8 to 10 V) is applied, and the electrolysis is started. F ₂ and H ₂ are collected in the respective compartments of the apparatus. Analysis of the produced F ₂ reveals that it contains too much CF ₄ to be suitable for being used in the semiconductor manufacture.

F ₂ and H ₂ are passed separately through a jet scrubber operated with liquid HF having a temperature of approximately -80°C. The F ₂ and H ₂ leaving the jet scrubbers are passed through a trap cooled to -80°C to condense out entrained HF. If desired, the F ₂ and H ₂ , respectively, leaving the cooled trap may be passed through a second cooled trap and/or a NaF tower to remove residual HF. Then, H ₂ and F ₂ are mixed and burned to form HF which is condensed and returned to the metallic vessel for electrolysis.

Example 4 : Recycling of impure F ₂ from the start-up phase

An electrolyte salt having the approximate composition of KF-2HF is introduced into the cells of an on-site electrolysis apparatus for the production of F ₂ , generating H ₂ as side product. The electrolysis is started, and HF is supplied to the cells. Fluorine and hydrogen are produced by electrolyzing HF in the presence of HF adducts of KF as electrolyte salt at a temperature of about 80 to 100°C in a metallic vessel which serves as electrode. Appropriate voltage (8 to 10 V) is applied, and the electrolysis is started. F ₂ and H ₂ are collected in the respective compartments of the apparatus. Analysis of the produced F ₂ reveals that it contains too much CF ₄ to be suitable for being used in the semiconductor manufacture.

F ₂ and H ₂ are passed separately throuh a jet scrubber operated with liquid HF having a temperature of approximately -80°C. The F ₂ and H ₂ leaving the jet scrubbers are passed through a trap cooled to -80°C to condense out entrained HF. If desired, the F ₂ and H ₂ , respectively, leaving the cooled trap may be passed through a second cooled trap and/or a NaF tower to remove residual HF. Then, H ₂ and F ₂ are mixed and burned to form HF which is condensed and returned to the metallic vessel for electrolysis.