WO2009074561A1

WO2009074561A1 - Method for recovery of fluorine

Info

Publication number: WO2009074561A1
Application number: PCT/EP2008/067086
Authority: WO
Inventors: Marcello Riva
Original assignee: Solvay Fluor Gmbh
Priority date: 2007-12-11
Filing date: 2008-12-09
Publication date: 2009-06-18
Also published as: TW200932340A

Abstract

Process for the removal or recovery of fluorine from fluorine containing waste gases Waste gases, especially those from pro cesses used in the semiconductor industry, e.g. from etching silicon wafers or clean ing processing chambers with elemental fluorine, are contacted with a metal or metal compound which reacts with the fluorine contained in the waste gas mi xtures forming a metal fluoride which comprises reversibly bound fluorine. Pref erably, a manganese fluoride is applied which forms manganese tetrafluoride which, in turn, can be heated to split off elemental fluorine. The process allows the recovery of elem ental fluorine in a pure form suitable for reuse. No figure

Description

Method for recovery of fluorine

The invention concerns a process for the removal and the recovery of elemental fluorine from waste gas.

Elemental fluorine is applied for many purposes. For example, it can be used for the fluorination of plastic material or in the semiconductor industry. For example, it can be used as etching agent in plasma or thermal treatment chambers used for the production of semiconductors, or as cleaning agent for treatment chambers used for semiconductor manufacture. Often, it is diluted by inert gases, e.g. nitrogen. In many applications, it is not completely consumed during its use. Sometimes, it is formed in plasma treatment processes from other etchants, e.g., when NF₃ is applied. Often, due to impurities in the resultant gas mixture, or due to its fluctuating concentration, it cannot be reused. It cannot be released immediately into the atmosphere for obvious reasons, and thus is passed through cleaning installations where it is reacted chemically, usually to transfer it into fluoride which then can be absorbed, for example, with calcium compounds to form calcium fluoride. In principle, this is a waste of valuable material.

US patent 6,955,707 discloses a method for recovery of elemental fluorine without the need to convert it into fluoride ions. Fluorine and other impurities, removed with the exhaust gas from a chamber cleaning unit, are contacted with an adsorbent which adsorbs the impurities but not the fluorine. The thus purified fluorine can be reused.

It appears to be a disadvantage of that process that it will not be easy to provide adsorbents for all impurities comprised in the exhaust gas; especially considering the fact that depending of the nature of the items to be treated, the constitution of the exhaust gas may differ. Problem of the present invention is to provide a process for removal and recovery of elemental fluorine from waste gases which also can be applied to waste gases with changing constituents and which allows the recovery of purified fluorine.

This object and other objects of the present invention are achieved by the process as outlined in the claims.

The process according to the present invention for removal of fluorine from waste gas mixtures containing elemental fluorine together with at least one other constituent comprises at least one step of contacting the fluorine containing waste gas mixture with a metal or a metal fluoride in which step a metal fluoride which comprises reversibly bound fluorine is formed. The fluorine thus is removed from the waste gas as fluoride and is reversibly bound to the cation. Thus, by releasing it, in at least one additional step, from the fluoride in which it is reversibly bound, fluorine can be recovered. Often, fluorine is released from that fluoride by a heat treatment and, if desired, additionally a vacuum. By the heat treatment, elemental fluorine is formed, and a metal fluoride with a lower degree of fluorination which often is suitable to be contacted with further waste gas to remove and, if desired, recover further fluorine. The fluorine released can be compressed and filled into storage containers, or it can, optionally after mixing it with other active or inert gases, be returned to a desired use. For example, fluorine can be removed from waste gases of the semiconductor industry, can be recovered from the sorbent by releasing it via a thermal treatment (heating) and filling it into a storage tank, optionally together with nitrogen, argon or other useful gases, for later use; or it can be mixed with nitrogen and/or argon, and the mixture can be re-used immediately as etching gas mixture.

It is apparent for the expert that the metal fluoride comprising reversibly bound fluorine is higher fluorinated than the metal fluoride applied as starting material. It can be released from the formed higher-fluorinated metal compound to which it is bound reversibly as elemental fluorine at any desired time. Ideally, the starting metal fluoride is formed hereby simultaneously. For example, the formed higher fluorinated metal fluoride can be removed from the reactor and transported to a site of use to release there the elemental fluorine. Alternatively, the contact between the waste gas mixture and the metal or metal fluoride can be stopped, and immediately thereafter, elemental fluoride can be released. The elemental fluorine can be recycled into the process from which it originated, or it can be stored for later use. The metal fluoride which forms upon fluorine release can then again be applied for fluorine removal from the waste gas mixture.

The term "reversibly bound fluorine" means that metal fluoride compounds are formed in which the fluorine is chemically bound, but can be released again as elemental fluorine, usually, but not necessarily only, upon heating. The metal compound is in a lower oxidation state (if a metal is applied, its oxidation state, of course, is 0) when contacted with the fluorine containing waste gas mixture. The metal or metal compound is oxidized during contact with the elemental fluorine and thereby chemically binds it (thus, a higher fluorinated compound is formed). The other components of the waste gas do not react with the metal or metal compound and pass the metal or metal compound; they can be removed from the remaining waste gas mixture by suitable other adsorbents, washers, by condensation or other means. The heating is performed at a temperature which is high enough to release a desired amount of elemental fluorine per time unit. The suitable treatment temperature can be determined, if necessary, by simple tests. If manganese difluoride or manganese trifluoride is used as sorbent, and manganese tetrafluoride is the formed metal fluoride which contains reversibly bound fluorine, then fluorine is released at a temperature of, preferably, equal to or higher than 350 ⁰C up to 450 ⁰C or even higher.

Suitable metals or metal compounds are those capable of forming a fluoride compound reversibly in a lower and a higher oxidation state and can be oxidized with fluorine to form metal fluorides in a higher oxidation state, but can also split off elemental fluorine and are thereby reduced. Preferred compounds of this type are selected from silver compounds, bismuth compounds, iron compounds, nickel compounds, cobalt compounds, copper compounds and manganese compounds, especially their fluorides, or their mixtures or adducts with alkali metal fluorides. Even the metals can be successfully applied. For example, Mn powder could be used as a starting material to react with the elemental fluorine in the fluorine-containing gas mixture. It is known that Mn powder reacts with fluorine, see H. Roesky and O. Glemser, Angew. Chem. (1963), pages 920 and 921. Nevertheless, it is preferred to apply metal fluoride salts as starting material. It is known that manganese difluoride and manganese trifluoride react with fluorine to form manganese tetrafluoride. Principally, any methods for reacting manganese fluorides with fluorine to yield manganese tetrafluoride known in the art can be applied in the process according to the present invention to reversibly remove fluorine from waste gas mixtures. The reaction can be performed under variable conditions in view of reaction temperature, pressure, and duration, or particle size of the solid.

For example, the reaction between manganese difluoride or trifluoride and fluorine-containing waste gas can be performed at 550 ⁰C. The formed manganese tetrafluoride sublimates. See R. Hoppe et al., Ann. 658 (1962), pages 1 to 5. It can be prepared from manganese difluoride and fluorine under UV light, see Z. Mazej, J. Fluorine Chem. 114 (2002), pages 75 to 80. Alternatively, - A -

a reaction between manganese difluoride and fluorine to be removed from the waste gas mixture could be performed in anhydrous HF in the presence of UV light. It can be removed by the fluidized bed reaction with elemental manganese as described by Roesky and Glemser. From the manganese tetrafluoride formed, elemental fluorine can be released (split off) by heating, cf. WO 2006/033474. As a result, manganese trifluoride is formed. Manganese trifluoride thus formed can also be used as starting material in the process of the present invention and can fluorinated again to manganese tetrafluoride. As described in WO 2006/033480, the solid/gas fluorination reaction proceeds on the surface of the starting material and is accompanied by the sintering of the particles which obstructs the penetration of fluorine into the particles. Consequently, the stoechiometric ratio of manganese to fluorine of 1 : 4 is difficult to achieve. Thus, in the context of the present invention, the term "manganese tetrafluoride" denotes manganese fluoride compositions wherein the atomic ratio between fluorine and manganese is equal to or greater than 3.75. Said international patent application discloses a process in which the solid/gas reaction described before is performed under heating and pressure while continuously or discontinuously, the particles are comminuted (crushed or ground) to improve contact between solid and gas. A ball mill can be used to crush the particles. According to EP 1580163, the salts from which elemental fluorine is split off should be used in the form of pellets.

In a preferred embodiment of the invention which will be described later, the manganese fluoride particles are continuously or semi-continuously agitated so that permanently, "fresh" surfaces are produced, essentially without crushing the particles, and also without significant agglomeration taking place. This embodiment will be described below.

While the invention can be performed by using any of the starting materials mentioned above, the use of nickel fluoride and manganese fluoride is preferred. Manganese fluoride is especially preferred, and in view of this especially preferred embodiment, the process according to the present invention will now be explained in detail.

The contact between manganese fluoride and the waste gas mixture can be performed as described above. A preferred embodiment will now be described and concerns a very advantageous way of contacting the waste gas mixtures with manganese fluoride. This embodiment of the present invention provides for removal or recovery of elemental fluorine comprised in a waste gas mixture by the formation of manganese tetrafluoride by reacting solid manganese difluoride and/or manganese trifluoride particles with the elemental fluorine in the mixture wherein during the reaction, particle surfaces are rendered "fresh". This is achieved essentially without comminuting particles. In the context of the present invention, the term "essentially without comminuting particles" means that the particles are not intentionally crushed or milled, and the average particle size does not change significantly, be it to a larger size (e.g. by agglomeration), be it to a smaller size. Preferably, the average particle size of the fluorinated particles compared to the average particle size before fluorination lies in a range of 1.5 : 1 to 1 : 1.5. To achieve this, the particles are treated preferably by mechanical means which prevent them to agglomerate and which provide fresh surfaces for contact with fluorine. Means are for example movable components inside the reactor which, when moving, agitate the particles therein and thus prevent them to agglomerate and, by mechanical impact, render "fresh" surfaces of the particles. The term "fresh" means that coatings of manganese tetrafluoride on the surface are at least partially removed or made porous so that further fluorine can diffuse more easily into the particle and react with the fresh surface or unreacted manganese fluoride. While movable mechanical means are the preferred embodiment, other means are considered to be applicable, e.g. irradiation with ultrasound. Alternatively, a reactor with fixed components such as metal plates might be rotated at sufficient speed so that the impact of the particles hitting the plates inside the reactor provides fresh surfaces. The impact between the particles and the movable or fixed means is high enough to prevent agglomeration, sintering and vitrification and to achieve a good degree of fluorination, but is not so powerful or intense that the particles crush.

Preferred mechanical means are means used for mixing, preferably stirrers or mixers with helical means, e.g. helical stirrers or agitators, and especially preferably mixer screws. Helical stirrers, and especially mixer screws, are very advantageous because the manganese fluoride is not only agitated horizontally, but also vertically which appears to have a positive effect on the effectivity of the contact with elemental fluorine. Manganese difluoride or manganese trifluoride (described as "manganese fluoride" to make a distinction to "manganese tetrafluoride") can be applied as preferred starting material. Sometimes, in manganese fluoride, at least concerning manganese trifluoride, the molar ratio between manganese and fluorine is not always stoechiometric, especially when manganese fluoride is used which has been obtained from splitting off fluorine from manganese tetrafluoride. Such manganese fluoride may contain manganese trifluoride, residual manganese tetrafluoride, even some manganese difluoride. Any non-stoechiometric manganese fluoride which can react with elemental fluorine to form manganese tetrafluoride is suitable as starting material. The starting material can thus be characterized as MnF_x with 2 < x < 4. Preferably, x is equal to or lower than 3.

Manganese difluoride may be used as a starting material. It is obtainable by the reaction manganese (II) salts, for example, manganese dichloride, manganese oxide or manganese carbonate with HF and subsequent drying in an oven (preferably an evacuated oven). Manganese carbonate is the preferred starting material for manganese difluoride. If the starting material contains water, it is preferred to dry it before performing the reaction of the present invention, for example, by heating it in an oven, e.g. an evacuated oven or under passing of inert gas through it, to a temperature up to 400 ⁰C

Manganese trifluoride is also suitable as agent for the removal of elemental fluorine of waste gas mixtures. It can be obtained by reaction of manganese difluoride with elemental fluorine. Another source for manganese trifluoride is, as mentioned above, the manganese fluoride residue which is obtained when manganese tetrafluoride is heated to split off elemental fluorine.

The amount of fluorine which can be consumed is dependant from the degree of fluorination of the starting material. The reaction equation for manganese difluoride is MnF₂ + F₂ -> MnF₄ (I)

The reaction equation for manganese trifluoride is MnF₃ + V₂ F2 -> MnF₄ (II)

Preferably, the reaction between fluorine in the waste gas mixture and the manganese fluoride is performed until the manganese fluoride starting material has reacted to manganese tetrafluoride. Preferably, the amount of fluorine corresponds approximately to that amount needed to fluorinate the manganese fluoride starting material to form manganese tetrafluoride. Preferably, the molar ratio of elemental fluorine needed to convert the manganese fluoride into manganese tetrafluoride is equal to or greater than 0.9 : 1. It is preferably equal to or lower than 1.1 :1. For safety considerations, it is preferably equal to or lower than 1 : 1. To prevent any waste of fluorine, the contact, as described above, is stopped when elemental fluorine is detected in the waste gas which leaves the reactor.

The particle size of the starting material is variable. Particles with a size of equal to or greater than 0.1 μm are suitable. Particles with a size equal to or lower than 5 millimeters are suitable. Preferably, the particle size is equal to or greater than 1 μm. Preferably, the particle size is equal to or lower than 0.5 mm. More preferably, the particle size is equal to or lower than 200 μm. Of course, insignificant amounts, e.g. up to 5 % by weight of the particles, may lie outside the respective preferred ranges. The reaction temperature for the fluorination reaction is variable.

Preferably, it is equal to or higher than 160 ⁰C, especially it is equal to or higher than 180 ⁰C. Preferably, it is equal to or lower than 330 ⁰C, especially it is equal to or lower than 320 ⁰C.

The pressure during fluorination in the reactor is preferably equal to or higher than 2 bar (abs.). Preferably, it is equal to or higher than 3 bar (abs.).

Preferably, it is lower than 20 bar (abs.). More preferably, it is lower than 10 bar (abs.). Especially preferably, it is equal to or lower than 8 bar (abs.). Still more preferably, it is lower than 7 bar (abs.). A highly preferred range is 4 to 6.5 bar (abs.). The manganese fluoride is preferably used for the removal of fluorine from the waste gas until the desired degree of its fluorination is achieved. The higher fluorinated manganese fluoride obtained during the reaction with fluoride is a valuable material because upon heating, purified elemental fluorine can be released. The effectiveness of the fluorinated manganese fluoride is the higher the higher the degree of fluorination is. Accordingly, the fluorination reaction is preferably performed until manganese fluoride of formula MnF_x is obtained wherein x is equal to or higher than 3.75, preferably equal to or higher than 3.9. The mechanical impact on the particles to render fresh surfaces is exerted at least during a part of the reaction. For example, it can be exerted intermittently. Preferably, it is exerted during at least 50% of the fluorination reaction time. More preferably, it is exerted during at least 70 %, especially preferably during at least 90 % of the fluorination reaction time. It is of course possible that it is exerted during 95% or more of the fluorination reaction time, and even up to 100% of the reaction time. The reaction between elemental fluorine comprised in the waste gas mixture can be performed in one single step, be it using manganese difluoride or manganese trifluoride as a starting material. It is possible to interrupt the reaction, for example, to analyze the degree of fluorination, or during times when no waste gas mixture needs to be purified.

A very suitable method of determination the end point of fluorination is to control the breakthrough of fluorine. As long as all fluorine is absorbed, or the desired degree of fluorine removal is achieved the manganese fluoride used for fluorine removal is suitable as fluorine-removing agent. As soon as an undesired breakthrough of fluorine is observed, fresh manganese fluoride must be provided, or the reaction must be stopped. If desired, two or more reactors with manganese fluoride can be provided which are arranged parallel to each other. A first reactor or a group of first reactors for fluorine removal are operated; when the desired degree of fluorination of the manganese fluoride is achieved, or when the desired degree of purity of the waste gas mixture is no longer achieved, the gas mixture to be purified can be contacted with a second reactor or group of reactors. This allows a continuous process.

The manganese tetrafluoride from the switched off reactor or reactors can be either removed and applied elsewhere for production of purified elemental fluorine, simply by heating it, or it can be heated directly in the reactor so that purified elemental fluorine is released and can be either stored or recycled, optionally after dilution with inert gases or after including any desirable additives, to a process for further use. After fluorine release, the formed manganese fluoride can be used again for absorbing fluorine from waste gas mixtures.

Consequently, the process of the present invention simultaneously provides a treated waste gas mixture which is depleted in elemental fluorine, and manganese tetrafluoride which is especially suitable, as described above, as a carrier for elemental fluorine which can be released in pure form by heating the manganese tetrafluoride produced. Expediently, when the starting material has been converted to manganese tetrafluoride, the contact with the waste gas mixture is stopped and the reactor containing the manganese tetrafluoride (or a container to which the manganese tetrafluoride was transferred) is evacuated. If the manganese tetrafluoride is heated purified elemental fluorine is produced.

The elemental fluorine which can be split off from the formed manganese tetrafluoride can be stored or used for any purpose. In a preferred embodiment of the process according to the present invention, purified elemental fluorine is recycled to the process from which the waste gas mixture originates. Thus, a preferred embodiment of the present invention concerns a process wherein a waste gas mixture comprising elemental fluorine is contacted with manganese difluoride and/or manganese trifluoride, the elemental fluorine is removed from the waste gas mixture and manganese tetrafluoride is produced, then elemental fluorine is released from the manganese tetrafluoride, the elemental fluorine formed is recycled to the process where it originates from, and the manganese trifluoride formed is recycled to absorb elemental fluorine from the waste gas mixture.

The process can generally be used for fluorine containing waste gases. Expediently, the waste gas does not contain compounds which react in an undesired manner with manganese difluoride, trifluoride or tetrafluoride. For example, the process can be applied to remove fluorine from fluorine-containing waste gas originating from surface fluorination of plastics, e.g. in the production of fuel tanks for vehicles. Preferably, waste gas mixtures from the semiconductor industry are treated according to the process of the present invention. It is well known that elemental fluorine, optionally in admixture with inert gas, e.g. nitrogen or argon, can be used during preparation of semiconductors. A photo mask is applied to a base material, e.g. a silicon wafer, and exposed in a desired pattern. Unexposed parts of the photo mask are removed. The uncovered silicon surface is then etched with fluorine or fluorine containing gas. The resulting waste gas mixture comprises fluorine, inert gas, and silicon fluoride and can be treated according to the process of the present invention.

For example, the waste gas mixture may additionally comprise oxygen, nitrogen, air, noble gases, for example, helium or argon, compounds comprising carbon and fluorine (optionally also hydrogen), for example, perfluorocarbons or hydrofluorocarbons, hydrogen fluoride, nitrogen trifluoride, sulfur hexafluoride, carbon dioxide, water (preferably, at most in the ppm range, for example, 1 to 50 ppm), silicon fluorides or silicon oxyfluorides. The waste gas mixture may also comprise reaction products which are a result of the termally or plasma- induced reaction of fluorine or its precursors, e.g. NF₃ or SF₆, with semiconductor material or flat panel material, for example, reaction products of fluorine with silicon, silicon oxide, metals, e.g. tantal or tungsten, gallium compounds, for example, GaAs, or with residues, e.g. carbonfluoride polymers, originating from contaminations on the walls of semiconductor treatment apparatus, for example, CVD chambers or etching apparatus, or with construction material of that apparatus. Consequently, the term "waste gas" denotes in the context of the present invention preferably a gas mixture which contains or consists of fluorine and at least one other gaseous constituent, which gas mixture was removed from a reactor in which a fluorine containing reactant gas mixture was used to react a part of the fluorine content with a substrate, thereby forming a waste gas depleted in fluorine, but still containing fluorine. The fluorine consuming reaction is preferably a process applied in the semiconductor industry, e.g. an etching process wherein silicon or silicon oxide is etched during the manufacture of semiconductors, or a process for the manufacture of MEMS (microelectromechanical systems). Preferably, the waste gas of the present invention contains equal to or less than 25 % by volume of elemental fluorine, preferably equal to or less than 20 % by volume. The remainder to 100 % by volume is formed by one or more other constituents present in reactant gas mixtures. Preferably, the waste gas mixture contains or consists of at least one other constituent from the group consisting of helium, nitrogen, argon, xenon, oxygen, air, hydrogen fluoride, silicon fluoride, volatile phosphor fluorides, carbon fluorides, polymeric fluorocarbons, volatile metal fluorides or oxyfluorides, e.g. tungsten fluoride, tantalum fluoride or gallium fluoride, SF₆, SO₂F₂, COF₂, CO₂, hypofluorites, CF₃C(O)F and NF₃. The term "carbon fluorides" includes saturated and unsaturated linear and branched aliphatic fluorocarbons and hydrofluorocarbons, especially those with 1 to 8 carbon atoms, e.g. CF₄, C₂F₆, C3F8, C2F4, perfluoroethene, perfluoropropene, perfluorobutene, or perfluorobutadiene. More preferably, the waste gas mixture contains or consists of elemental fluorine, at least one other constituent selected from the group consisting of nitrogen and argon and at least one additional constituent selected from the group consisting of helium, xenon, oxygen, air, hydrogen fluoride, silicon fluoride, volatile phosphor fluorides, carbon fluorides, SF₆, SO₂F₂, COF₂, CO₂, hypofluorites, CF₃C(O)F and NF₃.

The fluorine in the waste gas can originate from elemental fluorine supplied to the reactor or from fluorine precursors, and consequently, an embodiment of the process of the present invention provides for the treatment of fluorine containing waste gases wherein the fluorine was formed from a fluorine precursor, preferably in a process wherein the fluorine precursor is applied as etching gas, especially for the production of semiconductors, flat panels or for chamber cleaning. It is known, for example, that NF₃ forms fluorine under plasma conditions. For the expert, it is evident that elemental fluorine (and possibly other impurities in the gas mixture, e.g. HF, if comprised) is a very aggressive substance. Accordingly, it is preferred to use fluorine-resistant materials for those parts of the apparatus which come into contact with it. Useful materials are known, for example, nickel, nickel alloys, Monel metal or Hastelloy type metal. It also possible to use apparatus coated with or made totally or partially from fluorine -resistant materials. Suitable polymers are for example fluorinated polymers, e.g., polytetrafluoroethylene or its copolymers with fluorinated propylene. The process according to the present invention has several advantages over the processes of the prior art. It allows the removal of elemental fluorine from waste gas mixtures allowing the reuse of it. In a preferred embodiment it provides for the simultaneous manufacture of manganese tetrafluoride at comparably low pressure and comparably low temperature. Contrary to other processes, fluorine is upgraded to a purified product.

The following examples are intended to explain the invention in further detail without intention to limit it. Examples Apparatus: The purification can be performed in a nickel reactor with an internal volume of 380 ml. It is provided with a magnet coupled mixer screw made from nickel. When rotated, it acts as a stirrer causing the particles to ascend within the diameter of the screw and to descend along the outer side of it. The mixer screw rotates at 30 rpm. The reactor further comprises an inlet line for nitrogen and an inlet line for the waste gas mixture and an outlet line connected, via a scrubber, to an exhaust line. The reactor also contains an external heating system. Example 1 : Purification of a waste gas from semiconductor etching 1.1. Preparation of manganese difluoride absorbent: Manganese carbonate was reacted with aqueous HF to form manganese difluoride. The reaction product was filtered off, dried in an oven at 180⁰C at reduced pressure for 12 hours and then ground. The water content amounted to around 0.5 to 1 % by weight. The identity of the substance was confirmed by XRD (X ray diffraction) spectroscopy.

25O g of dried manganese difluoride are introduced into the reactor and the mixer screw is switched on to rotate at 30 rpm. The reactor is evacuated, heated to 400 ⁰C by means of the external heating and purged with 5 1/h of inert gas for 20 hours. The weight loss due to the removal of water amounts to 1.2 % by weight.

1.2. Purification of the waste gas in a batch process Semiconductor parts based on Si are etched in a plasma reactor using a mixture comprising 20% by volume of elemental fluorine and 80% by weight of argon. The waste gas mixture leaving the plasma reactor comprises fluorine, argon and silicon fluoride compounds. The waste gas is passed through the reactor comprising manganese fluoride prepared like described in example 1.1. During the reaction, the manganese fluoride is maintained at a temperature of 305 ⁰C. The pressure of the fluorine containing waste gas mixture in the supply line is regulated to be about 5.5 to 6 bars (abs.). The pressure in the reactor is lower because fluorine is absorbed. Thus, the content of fluorine in the waste gas mixture is reduced. After the absorption of fluorine has stopped, gaseous compounds in the reactor are removed. The gas leaving the reactor consists essentially of argon and of silicon fluoride compounds and small amounts of fluorine. The residual waste gas mixture is passed through an alkaline washer (NaOH solution, for example) whereby silicon compounds and fluorine are removed. The remaining argon can be further purified and reused. For example, possible fluorine residues may be removed by scrubbing with potassium iodide solution.

The elemental fluorine contained in the waste gas mixture is absorbed by the manganese fluoride which forms manganese tetrafluoride. To recover the fluorine, the manganese tetrafluoride formed is heated to 400 ⁰C. Purified fluorine is now split off and can be recycled, after mixing with argon, to the etching chamber.

While the reactor is shut off during evacuation and fluorine split-off, the waste gas mixture can be passed into a second reactor containing manganese fluoride.

In this manner, a batch- wise or semi-continuous operation of the waste gas treatment is possible.

Example 2: Continuous purification of a waste gas mixture which contains fluorine, argon and nitrogen

An etch gas consists of 2.6 vol.-% F₂, 10.4 vol.-% N₂ and 91 vol.-% argon and is applied to etch a silicon wafer in a commercial etching chamber. The waste gas mixture leaving the etching chamber contains fluorine, nitrogen, argon, and silicon fluorides. It is passed consecutively through four or if desired more reactors containing manganese fluoride. The number of reactors can be selected to reduce the fluorine content to a desired level or to shorten the residence time in the reactors. The reaction conditions concerning temperature and pressure are comparable to those of example 1.2. The gas mixture leaving the last reactor has a reduced content of fluorine and is passed through an alkaline water scrubber. A mixture of nitrogen and argon leaves the scrubber. If fluorine should still be contained, it can additionally be passed through an aqueous potassium iodide solution. The argon/nitrogen mixture can be recycled to etching chamber; if needed, the ratio of nitrogen to argon can be adjusted by adding one of them. Optionally, also fluorine can be added; alternatively, the nitrogen/argon mixture and fluorine can be supplied to the etching chamber separately.

Example 3: Purification of a waste gas mixture comprising fluorine, nitrogen trifluoride, and argon A mixture of nitrogen trifluoride and argon is used as etching gas in a plasma-operated etching chamber. The waste gas leaving the etching chamber comprises fluorine, nitrogen fluoride, argon and silicon fluorides. The waste gas is contacted with the manganese fluoride as describe in example 1. The gas mixture leaving the reactor is passed through an alkaline washer. The mixture of nitrogen fluoride and argon which leaves the washer can be further purified (e.g., dried) and can then be recycled to the etching chamber, as well as the fluorine split off form the manganese tetrafluoride formed in the reactor during absorption of fluorine from the waste gas by the manganese fluoride. Example 4: Purification of the waste gas mixture in a reactor containing a ball mill for crushing particles

A reactor is used like the one described in figure 4 of WO 2006/033480. The cylindrical reactor comprises heating elements and balls for crushing particles. It rotates along a horizontal axis. During rotation, the balls roll along the inner wall and during this movement, they crush agglomerates formed from the manganese fluoride particles during fluorination.

A waste gas mixture comprising fluorine, argon and other impurities is passed through the rotating reactor. Fluorine is absorbed; nitrogen and other impurities leave the reactor and are passed through an alkaline washer where also HF and any residual fluorine is absorbed. The remaining nitrogen can be passed through an exhaust into the atmosphere. The fluorine absorbed by the manganese fluoride can be released by heating and reused. The washer solution which comprises alkali metal fluoride can be dried and dumped, or the fluoride values can be recovered, e.g. by reaction with sulfuric acid, thereby liberating HF which is a valuable starting material.

Example 4: Purification of waste gas mixtures from chamber cleaning A chamber used for etching semiconductor parts using fluorocarbon etchants in a plasma is contaminated with fluorocarbon polymeric residues on the inner walls and parts inside the chamber. Such polymeric residues must be removed from time to time. To remove them, fluorine or a mixture of fluorine with nitrogen, argon, or a mixture thereof can be subjected to a plasma treatment, for example, a treatment with a remote plasma. Alternatively, fluorine is reacted thermally with the residues. The resulting waste gas comprises fluorine und gaseous fluorocarbon compounds and, depending on the nature of the gas used for purification, nitrogen, argon or their mixtures.

The waste gas mixture from chamber cleaning is passed consecutively through several reactors containing manganese fluoride like described in example 2. The fluorine absorbed by manganese fluoride can be recovered by heating the manganese tetrafluoride formed. Any fluorine leaving the reactors is removed in an alkaline washer. Any nitrogen and/or argon leaving the reactor or washer can be either disposed off into the atmosphere or recycled to the chamber cleaning process.

Claims

C L A I M S

1. A process for removal of fluorine from waste gas mixtures containing elemental fluorine together with at least one other constituent which process comprises at least one step of contacting the fluorine containing waste gas mixture with a metal or a metal fluoride in which step a metal fluoride which comprises reversibly bound fluorine is formed.

2. The process according to claim 1 for recovery of fluorine from waste gas mixtures comprising at least one additional step wherein the fluorine removed from the waste gas in the at least one step of contacting the fluorine containing waste gas mixture with a metal or a metal fluoride is released from the metal fluoride binding it reversibly, and is recovered.

3. The process according to claim 1 wherein a metal fluoride is selected from the fluorides of nickel or manganese, optionally in the presence of alkali metal fluoride.

4. The process according to claim 1 wherein manganese fluoride is applied as metal fluoride and manganese tetrafluoride is formed as metal fluoride which comprises reversibly bound fluorine.

5. The process according to claim 1 wherein during the contact of the metal fluoride with the waste gas mixture, particle surfaces are rendered fresh.

6. The process according to claim 5 wherein the mechanical impact is achieved by a helical stirrer or a mixer screw.

7. The process according to claim 1 wherein the contact is performed at a temperature equal to or higher than 160 ⁰C, preferably equal to or higher than 180 ⁰C.

8. The process according to claim 1 wherein the contact is performed at a temperature equal to or lower than 330 ⁰C, preferably equal to or lower than

320 ⁰C.

9. The process according to claim 1 wherein the reaction is performed under a pressure equal to or higher than 2 bar (abs.), preferably equal to or higher than 3 bar (abs.).

10. The process according to claim 1 wherein the reaction is performed under a pressure equal to or lower than 10 bar (abs), preferably equal to or lower than 8 bar (abs.).

11. The process according to claim 2 wherein the metal or metal fluoride, preferably manganese difluoride and/or manganese trifluoride, is contacted in a step a) with a waste gas mixture containing elemental fluorine to form manganese tetrafluoride, and in a subsequent step b) which is performed separately from step a), elemental fluorine is released from the manganese tetrafluoride prepared in step a), and manganese trifluoride is formed.

12. The process according to claim 11 wherein the manganese trifluoride formed in step b) is subjected to another step a) to form manganese tetrafluoride.

13. The process according to claim 1 wherein fluorine containing waste gas mixtures from the semiconductor industry are treated, especially waste gas mixtures from etching or chamber cleaning.

14. The process according to claim 1 wherein the content of fluorine in the waster gas is equal to or lower than 25 % by volume.

15. The process of claim 1 wherein the waste gas mixture contains elemental fluorine and at least one other constituent selected from the group consisting of helium, nitrogen, argon, xenon, oxygen, air, hydrogen fluoride, silicon fluoride, volatile phosphor fluorides, carbon fluorides, volatile metal fluorides or oxyfluorides, SF₆, SO₂F₂, COF₂, CO₂, hypofluorites, CF₃C(O)F and NF₃

16. The process of claim 15 wherein the waste gas mixture contains elemental fluorine and at least one other constituent selected from the group consisting of nitrogen and argon and at least one further constituent selected from the group consisting of helium, xenon, oxygen, air, hydrogen fluoride, silicon fluoride, volatile phosphor fluorides, carbon fluorides, volatile metal fluorides or oxyfluorides, SF₆, SO₂F₂, COF₂, CO₂, hypofluorites, CF₃C(O)F and NF₃.