System and method for on-site generation and distribution of fluorine for fabrication processes

Abstract

An system and method for on-site generation and distribution of fluorine for semiconductor processes are disclosed. In one aspect, a system for providing fluorine in association with a fabrication process includes a fluorine generator operable to provide fluorine having a concentration and a distributor operable to transfer a volume of fluorine to a selective processing tool. A disclosed method for providing fluorine for a fabrication process includes generating fluorine at a location associated with a process tool and distributing the fluorine to the process tool to be used in association with the fabrication process.

Description

RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/333,405, filed Nov. 26, 2001, entitled “System and Method for Generating a Non-Ozone Depleting Material,” which is hereby fully incorporated by reference and is related to United States Patent Application No. 10/038,745, filed Jan. 2, 2002, entitled “Method and System for On-Site Generation and Distribution of a Process Gas,” which is hereby fully incorporated by reference.

TECHNICAL FIELD

[0002] The present invention generally relates to process materials, and more particularly, to systems and methods for on-site generation and distribution of fluorine for fabrication processes.

BACKGROUND

[0003] The increasing demand for efficient fabrication processes which may be used to advance technology while reducing fabrication cost has driven fabrication processes in many directions. Some conventional processes use exclusive materials during or post a fabrication process to produce a desired result. For example, nitrogen triflouride NF3 gas may be used in several processes to remove undesirable contaminants associated with deposition processes. Some conventional fabrication deposition processes include depositing layers of materials through either Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD).

[0004] CVD processes may include several different processes such as atmospheric pressure deposition, Low Pressure Chemical Vapor Deposition (LPCVD), plasma Enhanced Chemical Vapor Deposition (PECVD), Vapor Phase Epitaxy (VPE), and Metalorganic Chemical Vapor Deposition (MOCVD). In general, during a CVD process chemical containing atoms of a material to be deposited on a substrate reacts with another reactant chemical leaving the product of the chemical reaction to be deposited on the wafer while unwanted by-products of the reaction may be removed through subsequent steps such as introducing an active gas such as NF3 into a chamber to react with the by-products. Chemical vapor deposition may be conducted in a controlled environment known as a reaction chamber. Generally, the relative concentration of both a carrier chemical and the reactive chemical can be varied depending on the desired outcome. Likewise, the temperature and pressure inside of the reaction chamber may be varied, and the time in which a substrate is exposed to the chemical solution can be carefully controlled, in order to achieve optimal conditions for a reaction given a desired thickness of the deposit material.

[0005] In a PVD process, a variety of techniques may be used to physically deposit material on the outer surface of the wafer. For example, an evaporation PVD process may include heating a source material from a solid to a gaseous state with the result that some of the source material is deposited adhesivley to the surface of a substrate. During a sputtering PVD process, a thin film of material may be deposited on the exterior surface of a wafer by bombarding a target material with radio-frequency excited ions which knock atoms from the target material. The dislodged atoms are embedded on the surface of the wafer to provide a layer of the material. With both evaporation and sputtering, process condition may be varied to accomplish a desired thickness in the material of the layer being deposited. Upon depositing a layer at a desired thickness, subsequent cleaning of residual material may be achieved by introducing a material such as NF3 into the chamber to remove contaminants for current or subsequent processing.

[0006] Another fabrication process which may use gaseous materials such as NF3 may include etch processes. During an etch process, external material of a substrate that is not covered by a protective layer, such as photoresist, may be removed. As such, portions of an exterior surface of a substrate covered with a protective layer do not react and are therefore maintained whereas portions of the exterior surface that are not covered by the protective layer are removed by the etching. In one form, a plasma etch may be used to remove material from a substrate. Plasma etching includes using gases and plasma energy to cause a chemical reaction with the surface of the exposed substrate to etch the desired material. In general, plasma etching uses a plasma chamber, a vacuum system, a gas supply and a power supply. Essentially, a substrate may be introduced into a plasma chamber at a specific pressure and having a specific solution of gas. The power supply creates a radio frequency field through electrodes placed within the chamber. The field energizes the reactive gas mixture to a plasma state containing energized ions. The etching ions attack or etch the surface from one direction, resulting in vertical etch profiles of the substrate.

[0007] Although these processes are well known in the art, desirable advancements in new processes and materials warrant exploring new innovations for fabricating devices.

SUMMARY

[0008] Methods and systems are provided for generating and distributing fluorine for fabrication processes.

[0009] A system for providing fluorine in association with a fabrication process is disclosed. The system includes a fluorine generator operable to provide fluorine having a concentration. The system further includes a distributor operable to transfer a volume of fluorine to a selective processing tool.

[0010] A method for providing fluorine for a fabrication process is disclosed. The method includes generating fluorine at a location associated with a process tool and distributing the fluorine to the process tool to be used in association with the fabrication process.

[0011] A fabrication process system for fabricating a device is disclosed. The system includes a processing environment operable to process a substrate associated with the device and a fluorine generator coupled to the processing environment. The fluorine generator is operable to provide fluorine in association with fabricating the device.

[0012] A method for fabricating a semiconductor device is disclosed. The method includes providing a substrate within a process environment operable to process the device and processing the substrate in association with a distributed fluorine generated at a location associated with the process environment.

[0013] A semiconductor device fabricated in association with on-site generated fluorine is disclosed. The device includes a substrate operable to provide the device having a plurality of process layers. A fabrication process associated with fabricating the device includes utilizing an on-site generated fluorine in association with fabricating the plurality of process layers.

[0014] It is a technical advantage to provide a safe generation and distribution system for hazardous materials such as fluoride.

[0015] It is a further technical advantage to utilize fluorine having a desirable concentration for processes operable to use fluorine.

[0016] It is another technical advantage to provide an on-site fluorine generator which may be located proximal, distal or integrated as a part of a processing tool.

[0017] It is another technical advantage to provide a semiconductor process operable to exploit desirable characteristics of fluorine.

[0018] It is a further technical advantage to provide a fluorine distribution system operable to distribute fluorine to a plurality of process tools.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings provided in Exhibit A hereto, in which like reference numbers indicate like features, and wherein:

[0020] FIG. 1 illustrates a system for on-site generation and distribution of fluorine according to one embodiment of the present invention;

[0021] FIG. 2 illustrates a method for generating and distributing fluorine for a fabrication process according to one embodiment of the present invention;

[0022] FIG. 3 illustrates a process tool having an integrated fluorine generation and distribution system according to one embodiment of the present invention; and

[0023] FIG. 4 illustrates a method for fabricating a semiconductor device utilizing fluorine according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Preferred embodiments and their advantages are best understood by reference to FIGS. 1 through 4, wherein like numbers are used to indicate like and corresponding parts.

[0025] The conceptual groundwork involves providing safe delivery of hazardous materials for fabrication processes. According to one aspect, an on-site generator operable to provide fluorine (F2) is provided. The on-site fluorine generator may be used in association with a distribution system for providing desirable quantities of fluorine to a fabrication process system or tool. In one embodiment, an electrolytic process may be used for on-site generation of fluorine. A distribution system may be coupled to the fluorine generator and operable to distribute desirable quantities and concentrations of fluorine to one or more process tools. As such, fluorine may be used during a fabrication process for the fabrication of devices such as microelectronic devices, integrated microelectronic circuits, ceramic substrate based devices, flat panel displays or other devices which may be fabricated using the present invention.

[0026] For example, desirable concentrations and volumes of fluorine may be used as aggressive agents during a semiconductor process which may be advantageous over conventional chemicals or gas compositions. Several embodiments may be realized by the present invention which may include utilizing fluorine as an aggressive agent such that fluorine may reduce processing time associated with fabricating a semiconductor device. Such processes may include etching a substrate, cleaning a deposition chamber, or other processes operable to use fluorine in association with a semiconductor process. In this manner, the overall processing time associated with fabricating a semiconductor device may be reduced thereby reducing the total cost for fabricating the semiconductor device.

[0027] FIG. 1 illustrates a system for on-site generation and distribution of fluorine. The system, illustrated generally at 100, includes an on-site fluorine generator 101 fluidly coupled to a first distribution line 102 and a second distribution line 104 operable to distribute variable amounts of fluorine within a fabrication facility. Distribution lines illustrated in FIG. 1 may include associated plumbing and fluid transfer devices such as pumps, valves, etc. configured to fluidly communicate fluorine. For example, first distribution line 102 may be a double-lined distribution line operable to fluidly communicate hazardous materials. As such, desirable quantities of hazardous materials such as fluorine, may be safely distributed to a process tool, system or cell. In one embodiment, system 100 may be configured to be located proximal or distal to a plurality of process tools which may be operable to utilize fluorine. A first process tool 103 may be coupled to on-site generator 101 via first distribution line 102. On-site fluorine generator 101 may further be coupled to second process tool 110 via second distribution line 104 and single tool distribution line 105.

[0028] On-site fluorine generator 101 may also be coupled to a multi-port distribution line 106 via second distribution line 104. Multi-port distribution line 106 may be coupled to several process cells operable to utilize various amounts of fluorine for various fabrication processes. For example, multi-port distribution line 106 may be coupled to a first process cell 111 having a first process tool 114, a second process tool 115, and a third process tool 116. As such, multi-port distribution line 106 may be operable to distribute variable quantities of fluorine to first process cell 111 as required.

[0029] Multi-port distribution line 106 may also be coupled to a second process cell 112 which may include a first process tool 117 and a second process tool 118 operable to use fluorine during a fabrication process. First process tool 117 and second process tool 118 may be coupled in a parallel configuration and may be operable as identical or different tools. For example, second process cell may be operable as a deposition processing cell having a plurality of deposition processing tools. As such, on-site fluorine generator 101 may provide second process cell 112 desirable amounts of fluorine for deposition processing.

[0030] Multi-port distribution line 106 may further be coupled to a third process cell 113 which may include a first process tool 119 and a second process tool 120 serially connected to first process tool 119. During use, multi-port distribution line 106 may be operable to provide fluorine as needed during a fabrication process.

[0031] During use, system 100 may be operable to distribute desirable quantities of fluorine to a plurality of systems, process tools, process cells, etc. In one embodiment, on-site fluorine generator 101 may be operable as an electrolytic fluorine generator and operable to produce large volumes of fluorine. An electrolytic generator may include a plurality of cells operable to produce specified volumes of fluorine. Each such cell may include an electrolyte having a mixture of predetermined chemicals operable to facilitate producing fluorine. Such an embodiment may include utilizing a mixture of potassium fluoride and hydrogen fluoride within a tank fitted with a steam heating coil, and associated plumbing such as a submersible pump, and a feed pipe for adding hydrogen fluoride to the electrolyte within the tank. On-site fluorine generator 101 may also include a cooling system operable to maintain on-site generator 101 at a constant temperature. For example, cooling of an electrolyte may be performed by using cooling tubes placed in the electrolyte of an electrolytic cell and/or by cooling the outer walls of the electrolytic cell. As such, a constant temperature may be maintained and fluorine may be obtained and distributed accordingly.

[0032] FIG. 2 illustrates a method for generating and distributing fluorine for a fabrication process. The method may be used in association with the system illustrated in FIG. 1 or other systems operable to generate and distribute fluorine for fabrication processes.

[0033] The method begins generally at step 200. At step 201, an onsite generator produces fluorine utilizing a fluorine generation process. The on-site generator may be located distal or proximal to process equipment as a facility may allow for, and may operable to produce variable amounts and concentrations of fluorine using an electrolyte process as described above or other fluorine generating processes. For example, an on-site generator may include several electrolyte cells with each electrolyte cell producing a volume of fluorine. As such, one or more of the cells may be used to provide desirable volumes of fluorine to one or more process tools.

[0034] Upon generating the fluorine, the method proceeds step 202 where the method distributes the fluorine to one or more process tools. For example, a distribution system may be coupled to plural process tools and operable to fluidly communicate desirable amounts of fluorine to one or more of the process tools. As such, an on-site generator operable to produce large quantities of fluorine may distribute the fluorine to a plurality of process tools operable to be used within a fabrication facility.

[0035] Upon distributing the fluorine to one or more process tools, the method proceeds to step 203 where a process tool uses the fluorine during a fabrication process. In one embodiment, a process tool which in one instance may be operable to use NF3 may be operable to use fluorine during processing. For example, a vapor deposition tool may use NF3 during a cleaning step to remove undesirable contaminants during or after deposition of, for example, a conductive thin film. As such, the method may be operable to provide a desirable amount of fluorine within a process tool's process chamber during or after depositing a thin film onto a substrate. For example, a single wafer thin film process tool may include a reaction chamber operable to deposit a thin film on a substrate. As such, contaminants from a variety of species associated with the deposition process may be residual within the reaction chamber. Fluorine may then be introduced into the reaction chamber to clean or remove contaminants within the reaction chamber (e.g., walls, handler, etc.) As such, fluorine may reduce contaminants associated with a thin film process while providing a relatively contamination free environment within the reaction chamber for current or subsequent processing.

[0036] Upon utilizing the fluorine gas, the method proceeds to step 204 where the method ends. In this manner, this fabrication process advantageously utilizes fluorine which has been generated and distributed on-site, proximal, or distal to a process tool operable to utilize fluorine.

[0037] In one embodiment, the method may be modified to use an accumulator associated with a process tool for storing the distributed fluorine. As such, an on-site generator may produce fluorine and distribute fluorine to an accumulator associated with a process tool. The method may also monitor an accumulator for certain volume levels and replenish the level of fluorine stored within the accumulator upon the accumulator depleting to a level.

[0038] In another embodiment, the method may be modified to purge a chamber associated with the process tool of undesirable residual gas and subsequent processing. For example, a process tool may introduce fluorine into a chamber in addition to other elements as a part of a fabrication process. The chamber may then be purged and additional processing of a device may occur. As such, the method may be modified to purge a chamber, fabricate a device, and utilize fluorine as required.

[0039] In another embodiment, the method may be modified to recycle the used fluorine gas. As such, a recycle system may be operable to receive the used fluorine and recycle the fluorine gas such that unwanted contaminants within the fluorine gas may be removed and the fluorine may be reused for subsequent processing. The recycled fluorine may then be used in association with a distribution system operable to distribute fluorine for a fabrication process.

[0040] FIG. 3 illustrates a process tool having an integrated fluorine generation and distribution system. The process tool, illustrated generally at 300, includes a fluorine generator 301 operable to generate fluorine for use in association with a fabrication process. Generator 301 may include an accumulator 302 fluidly coupled to a process chamber 303 operable to fabricate a device such as a semiconductor device. For example, system 300 may be operable as a semiconductor process tool and may be realized as one of several different types of semiconductor process tools. In one embodiment, system 300 may be configured as an etch tool operable to etch a substrate using fluorine as part of an active species. As such, fluorine may react with regions of a substrate to provide etched locations of the substrate.

[0041] In another embodiment, system 300 may be configured as deposition process tool operable to deposit a thin layer of material (e.g., conductive layer, barrier layer, etc.) on a substrate. As such, fluorine may be introduced during or post deposition of a substrate to remove undesirable contaminants from a process chamber associated with system 300. For example, system 300 may be operable as a process tool, and may be configured to use fluorine in the place of, or in addition to, NF3. As such, fluorine may be used during a semiconductor process to remove undesirable contaminants, metals, compounds, by-products, etc. which may be residual from a deposition process.

[0042] FIG. 4 illustrates a method for fabricating a semiconductor device utilizing. The method may be used in association with a system such as system 300 illustrated in FIG. 3 or other systems operable to use the method illustrated in FIG. 4.

[0043] The method begins generally at step 400. At step 401 fluorine may be generated on-site, proximal, or distal to a semiconductor process tool. For example, a fluorine generator may be located in a processing bay or utility bay associated with a semiconductor fabrication facility and located proximal to a process area. As such, the on-site generated fluorine may be generated proximal to the process area for use during processing of a semiconductor device. In another embodiment, a process tool may be configured to generate fluorine as an integral part of the process tool. For example, a process tool may include a fluorine generator operable to produce desirable quantities of fluorine and use the generated fluorine accordingly. As such, distribution of fluorine may be minimized thereby reducing potential mishaps which may be encountered by distributing or handling hazardous materials such as fluorine.

[0044] Upon generating fluorine, the method proceeds to step 403 where one or more substrates may be processed using the fluorine. As mentioned above, the process may vary depending on desired result.

[0045] For example, a process may include etching a substrate or cleaning a chamber prior, during, or post a deposition process. One example of using fluorine may include using a fluorine as a substitute or in association with NF3. As such, fluorine may be advantageously used as an aggressive agent operable to remove undesirable contaminants, metals, by-products, etc. associated with deposition processes thereby reducing cycle time associated with fabricating a semiconductor device. Upon processing the substrate, the method proceeds to step 404 where the method ends.

[0046] The method illustrated in FIG. 4 provides advantages over conventional semiconductor processes and may be applicable to other industries outside of the semiconductor industry. As such, the method of FIG. 4 should not be viewed in a limiting sense, but to illustrate one method of generating fluorine on-site and distributing fluorine to process tools or equipment operable to use fluorine.

[0047] Although the disclosed embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments without departing from their spirit and scope.

[0048] Plasma Etch Process

[0049] Upon each semiconductor substrate within two series of semiconductor substrates was formed an integrated circuit microelectronics fabrication structure comprising a series of patterned aluminum containing conductor layer bond pads having formed thereupon a composite silicon oxide/silicon nitride passivation layer. The composite silicon oxide/silicon nitride passivation layer employed a silicon oxide layer of thickness about 2000 angstroms formed through a plasma enhanced chemical vapor deposition (PECVD) method employing silane as a silicon source material having formed thereupon a silicon nitride layer of thickness about 7000 angstroms similarly formed through a plasma enhanced chemical vapor deposition (PECVD) method. In turn each composite silicon oxide/silicon nitride passivation layer had formed thereupon a patterned photoresist layer exposing portions of the composite silicon oxide/silicon nitride passivation layer through which vias were to be formed accessing the patterned aluminum containing conductor layer bond pads. The patterned aluminum containing conductor layer bond pads, the composite silicon oxide/silicon nitride passivation layers and the patterned photoresist layers were formed through methods as are otherwise conventional in the art of integrated circuit microelectronics fabrication.

[0050] Within each of the semiconductor substrates, the patterned aluminum containing conductor layer bond pads were formed of a thickness about 8000 angstroms and of areal dimensions about 15 microns by about 15 microns over the semiconductor substrate. As noted above, the composite silicon oxide/silicon nitride passivation layers were each formed to a thickness about 9000 angstroms. The patterned photoresist layers were each formed to a thickness of from about 35000 angstroms from a JSR positive photoresist material available from JSR Company, Japan.

[0051] The composite silicon oxide/silicon nitride passivation layers upon a first series of the two series of semiconductor substrates was then etched within a fluorine containing etching plasma and is conventional in the art, employing an etchant gas composition comprising carbon tetrafluoride, trifluoromethane, argon and sulfur hexafluoride, to form a patterned composite silicon oxide/silicon nitride passivation layer defining terminal vias of areal dimensions of about 15 microns by about 15 microns which exposed the patterned aluminum containing bond pad layers. Formed upon the terminal via sidewalls were fluorocarbon polymer residue layers. The fluorine containing etching plasma was formed within an Applied Materials MxP+brand tool which also employed: (1) a reactor chamber pressure of from about 150 mtorr; (2) a source radio frequency power of about 0 watts at a source radio frequency of 13.56 MHZ (without a bias power); (3) a semiconductor substrate temperature of about 250 degrees centigrade; and (4) an oxygen flow rate of about 8000 standard cubic centimeters per minute (sccm).

[0052] Finally, the fluoropolymer residue layers which remained within the terminal vias through the composite silicone oxide/silicone nitride passivation layers were stripped from the semiconductor substrates through immersion within a stripping solvent comprised of monoethanolamine available as ACT or EKC stripper from Ashland Chemical Technology, Inc. or EKC Technology Inc.

[0053] Plasma Cleaning Process

[0054] In a more specific exemplary process, a gas capable of reacting with the deposits to be removed is flowed into the space to be cleaned, e.g., the vacuum deposition chamber. Then a plasma is ignited in the space while the gas is flowing and reacting with the deposits on the exposed surfaces.

[0055] The gas employed in the etching process typically is gaseous source of a halogen, such as fluorine or chlorine. Preferably the gas is a gaseous source of fluorine, such as NF3, SF6, CF4 or C2F6. NF3 is particularly preferred. Other gases which are employed in plasma etching processes are also contemplated as being useful according to the present invention. A mixture of gases can also be employed. An inert or non-reactive diluent gas, such as argon, neon or helium can also be combined with the gas or mixture of gases.

[0056] The appropriate flowrate of the gas (or gases) reactive with the deposits to be removed, and temperature and pressure conditions within the vacuum deposition chamber or other space, can readily be determined by one of ordinary skill in the art, taking into account the volume of space from which deposits are to be removed, the quantity of deposits to be removed, among other conventional factors. Typical process parameters are set forth in Chang, U.S. Pat. No. 5,207,836, the entirety of which is incorporated by reference.

[0057] If desired, an optional additional step of removing any residues, such as fluorine residues, remaining from the plasma etching step can be carried out after the etching step. Processes including such additional removal steps are described, for example, in Chang, U.S. Pat. No. 5,207,836, the entirety of which is incorporated by reference.

[0058] A. Deposition

[0059] A silicon wafer is introduced into the vacuum deposition chamber of the Precision 5000 xZ apparatus and heated to the selected processing temperature of 475° C. After conventional pre-nucleation with WF6 and Si4, chamber purge pressurization and stabilization of the wafer on the heater plate, tungsten deposition is carried out using WF6 (flowrate 95 sccm) under a pressure of 90 Torr. The wafer is then removed, the chamber is purged and pumped (Ar/N2/H2 purge), and the deposition process is repeated until 25 silicon wafers have been processed.

[0060] B. NF3 Plasma Clean

[0061] With the heater plate maintained at the selected processing temperature of 475° C., an aluminum nitride covering wafer is introduced into the vacuum deposition chamber and heated over a period of 23 seconds. Concurrently or subsequently, NF3 is introduced into the chamber at 150 sccm and a base pressure of 300 mT. After the 23 second heating, the covering wafer is placed on the heater plate such that the area covered by the silicon wafer is protected. Once the covering wafer is in place, a 600 watt plasma is ignited and NF3 plasma etching is carried out for 227.5 seconds with plasma power reduced to 200 watts at 224 seconds. After two purge/pump cycles (30 second Ar/N2/H2 purge, 3 second pump per cycle, the deposition procedure is repeated.

Claims

1. A method for cleaning a process chamber for semiconductor and/or flat panel display manufacturing, comprising the steps of: converting a feed gas to a cleaning gas in a remote location, wherein said feed gas does not clean the process chamber; and delivering said cleaning gas to the process chamber.

2. The method of claim 1, further comprising the step of: activating said cleaning gas outside the chamber prior to delivering said cleaning gas to the process chamber.

3. The method of claim 2, wherein said step of activating is performed through a means selected from the group consisting of a remote plasma source, a heat source, and an electrical source.

4. The method of claim 3, wherein said remote plasma source is selected from the group consisting of a microwave energy source and a radiofrequency energy source.

5. The method of claim 1, wherein said feed gas is HF.

6. The method of claim 5, wherein the cleaning gas is F.sub.2.

7. The method of claim 6, wherein the conversion is done by electrolysis.

8. A method for cleaning a process chamber for semiconductor and/or flat panel display manufacturing, comprising the steps of: converting a feed gas to a cleaning gas in a remote location, wherein the resulting gas is a mixture of the feed gas and the cleaning gas; transferring the resulting gas mixture to a trap, wherein the feed gas is converted into a liquid form, and the cleaning gas remains in a gaseous form; and delivering said cleaning gas to the process chamber.

9. The method of claim 8, prior to said step of delivering the cleaning gas to the process chamber, further comprising the step of: pumping the cleaning gas into a storage unit.

10. The method of claim 9, after the step of pumping the cleaning gas into a storage unit, further comprising the step of: activating the cleaning gas outside the chamber before delivering the cleaning gas to the chamber.

11. The method of claim 10, wherein said step of activating is performed through a means selected from the group consisting of a remote plasma source, a heat source, and an electrical source.

12. The method of claim 11, wherein said remote plasma source is is selected from the group consisting of a microwave energy source and a radiofrequency energy source.

13. The method of claim 8, wherein said feed gas is HF.

14. The method of claim 13, wherein the cleaning gas is F.sub.2.

15. The method of claim 14, wherein the conversion is done by electrolysis.