Dielectric film surface repair using reducing plasma

Abstract

A method for forming an EUV photoresist hard mask is provided. The method comprises treating a metal-rich layer on a substrate with a reducing plasma to form a metallic surface on the metal-rich layer, wherein the metal-rich layer has an upper layer comprising a metal oxide layer. The metal-rich layer comprises one or more of tin (Sn), indium (In), gallium (Ga), zinc (Zn), tellurium (Te), antimony (Sb), nickel (Ni), titanium (Ti), aluminum (Al), tantalum (Ta), bismuth (Bi), and lead (Pb).

Description

Technical Field

[0001] The present disclosure generally relates to a method of forming an EUV hard mask. In particular, the present disclosure relates to a method for forming an EUV hard mask containing a high-Z metal.

Background Art

[0002] Ensuring submicron and smaller features is one of the major requirements for very large scale integration (VLSI) and ultra very large scale integration (ULSI) of semiconductor devices. However, with the continuous miniaturization of circuit technology, the sizes and pitch dimensions of circuit features such as interconnects impose additional requirements on processing capabilities. The multi-level interconnects that are at the core of this technology require accurate imaging and placement of high aspect ratio features. Reliable formation of these interconnects is required for further increases in device and interconnect density.

[0003] One process used to form various interconnects and other semiconductor features is EUV (extreme ultraviolet) lithography. Conventional EUV patterning uses a multi-layer stack in which the photoresist is patterned on top of a hard mask. Common hard mask materials are spin-on silicon anti-reflection coating (SiARC) and deposited silicon oxynitride (SiON). SiARC incorporates an organic component into the silicon backbone and maintains sufficient etch selectivity with respect to the photoresist and the underlying stack. Scaling the thickness of the SiARC backbone can be difficult, and spin coating limits the minimum thickness that can be achieved without excessive defects. SiON hard masks use an organic adhesion layer (OAL) for improved resist adhesion. The OAL prevents poisoning by nitrogen and can be reworked.

[0004] Several metal and metal oxide materials, such as high-Z materials, have been tested as EUV hard masks (HMs). Due to their chemical properties, high-Z films tend to react with oxygen and oxidize in air over time. Currently, there is no efficient way to prevent this spontaneous surface oxidation, nor is there a feasible way to repair the surface of high-Z films. This problem makes it difficult to control dose-to-size for EUV exposure, as well as to predict corresponding EUV lithography performance, such as critical dimension (CD) and linewidth roughness (LWR).

[0005] Therefore, there is a continuing need in this field for methods to repair high-Z surfaces. [Overview of the project]

[0006] One or more embodiments of the present disclosure relate to a method for forming an EUV photoresist hard mask. The method includes treating a metal-rich layer on a substrate with a reducing plasma to form a metal surface on the metal-rich layer, wherein the metal-rich layer has an upper portion comprising a metal oxide layer.

[0007] Additional embodiments of the present disclosure relate to methods for forming EUV photoresist hard masks. In one or more embodiments, a method for forming an EUV photoresist hard mask includes forming a metal-rich layer on a substrate, wherein the metal-rich layer has a thickness in the range of 10 Å to 50 Å and has an upper portion comprising a metal oxide layer having a thickness in the range of 20 Å to 100 Å, and treating the metal-rich layer with a reducing plasma to form a metal surface on the metal-rich layer.

[0008] To allow for a more detailed understanding of the features described above, a more detailed description of the disclosure, which is briefly summarized above, may be given by reference to embodiments, some of which are illustrated in the accompanying drawings. However, it should be noted that the accompanying drawings only illustrate typical embodiments of the disclosure and should therefore not be considered limiting, as the disclosure may acknowledge other equally effective embodiments. Embodiments described herein are illustrated, not limiting, but as examples, in the drawings, where similar reference numerals point to similar elements. [Brief explanation of the drawing]

[0009] [Figure 1] This is a process flow diagram of a method for depositing a film on a substrate according to one or more embodiments. [Figure 2A] This is a cross-sectional view of a substrate according to one or more embodiments. [Figure 2B] This is a cross-sectional view of a substrate according to one or more embodiments. [Figure 2C] This is a cross-sectional view of a substrate according to one or more embodiments. [Figure 2D] This is a cross-sectional view of a substrate according to one or more embodiments. [Modes for carrying out the invention]

[0010] To facilitate understanding, the same reference numerals are used to designate identical elements common to the figures, where possible. The figures are not drawn to a fixed scale and may be simplified for clarity. Elements and features of one embodiment may be usefully incorporated into other embodiments without further description.

[0011] Before describing several exemplary embodiments of the present invention, it should be understood that the present invention is not limited to the details of the configuration or process steps described below. Other embodiments of the present invention are possible and can be practiced or carried out in various ways.

[0012] As used herein, the term “approximately” means roughly or nearly, and in contexts where a number or range of numbers is given, it means a variation of ±15% or less of the number. For example, values ​​that differ by ±14%, ±10%, ±5%, ±2%, or ±1% satisfy the definition of approximately.

[0013] As used herein and in the appended claims, the terms “substrate” or “wafer” refer to a surface or portion of a surface on which a process acts. Those skilled in the art will also understand that a reference to a substrate may refer to only a portion of a substrate unless the context otherwise explicitly indicates. Additionally, a reference to depositing on a substrate may mean both a bare substrate and a substrate having one or more films or features deposited or formed thereon.

[0014] "Substrate" or "substrate surface" refers to any substrate or material surface formed on a substrate against which a film treatment is performed during the manufacturing process. For example, substrate surfaces against which a treatment may be performed include, depending on the application, materials such as silicon, silicon oxide, strained silicon, silicon-on-insulator (SOI), carbon-doped silicon oxide, amorphous silicon, doped silicon, germanium, gallium arsenide, glass, sapphire, as well as any other materials such as metals, metal nitrides, metal alloys, and other conductive materials. A substrate includes, but is not limited to, a semiconductor wafer. A substrate may undergo pretreatment processes to polish, etch, reduce, oxidize, hydroxylate, anneal, UV cure, electron beam cure, and / or sinter the substrate surface. In addition to film treatment directly on the substrate surface itself, in the present invention, any of the film treatment steps disclosed may be performed on an underlying layer formed on the substrate as more specifically disclosed below, and the term "substrate surface" is intended to include the underlying layer as the context indicates. As a result, for example, if a film / layer or partial film / layer is deposited on the substrate surface, the exposed surface of the newly deposited film / layer becomes the substrate surface.

[0015] The term "on" indicates direct contact between elements. The term "directly on" indicates direct contact between elements without an intervening element.

[0016] In this specification, "extreme UV," "EUV," etc., refer to radiation in the approximate range of 10 nm to 124 nm. In some embodiments, EUV radiation (also called EUV light) in the range of 10 nm to 15 nm is used. In one or more embodiments, EUV light with a wavelength of approximately 13.5 nm is used.

[0017] As used herein and in the appended claims, terms such as “precursor,” “reactant,” and “reactive gas” are used interchangeably to refer to any gaseous nuclide that can react with a substrate surface.

[0018] As used herein and in the appended claims, terms such as “reactive compound,” “reactive gas,” “reactive nuclide,” “precursor,” and “process gas” are used interchangeably to mean a substance having a nuclide capable of reacting with a substrate or material on a substrate in a surface reaction (e.g., chemisorption, oxidation, reduction, or cycloaddition). The substrate, or portion of the substrate, is successively exposed to two or more reactive compounds introduced into the reaction zone of a processing chamber.

[0019] Several metal and metal oxide materials, such as high-Z materials, have been tested as EUV dielectric hard masks (HMs). The term “dielectric” as used herein refers to electrically insulating materials that can be polarized by an applied electric field. Due to their chemical properties, high-Z films tend to react with oxygen and oxidize in air over time. Currently, there is no efficient way to prevent this spontaneous surface oxidation, nor a feasible way to repair the high-Z film surface. This problem makes it difficult to control dose-to-size (DtS) for EUV exposure, as well as to predict corresponding EUV lithography performance, such as critical dimension (CD) and linewidth roughness (LWR). High-Z materials beneath metal oxide photoresists can reduce the dose-to-size (DtS) in EUV processing. More specifically, second electrons induced by EUV light from high-Z films can significantly reduce the actual EUV dose required for lithography, resulting in increased throughput.

[0020] One or more embodiments are directed to a method of preventing spontaneous oxidation and repairing a high-Z surface. By use of a reducing plasma treatment, such as H* radicals in an advanced process chamber (APC), a metal-rich surface where the metallic state is dominant is advantageously and effectively repaired, resulting in a predictable high-Z metal composition, leading to a more reproducible EUV lithographic process and addressing concerns of EUV performance shifts such as dose vs. size, target CD, and LWR. In one or more embodiments, application of a reducing plasma to a naturally oxidized high-Z film can advantageously repair the metal-rich surface and result in reproducible EUV lithographic performance.

[0021] Embodiments of the present disclosure are described through figures that illustrate devices (e.g., transistors) and processes for forming transistors according to one or more embodiments of the present disclosure. The processes shown are merely exemplary contemplated usages of the disclosed processes, and those skilled in the art will recognize that the disclosed processes are not limited to the illustrated applications.

[0022] One or more embodiments of the present disclosure are described with reference to the figures. In one or more embodiments, a method of forming an EUV photoresist hard mask includes treating a metal-rich layer with a reducing plasma to form a metal surface on the metal-rich layer on a substrate, the metal-rich layer including treating the metal-rich layer having an upper portion comprising a metal oxide layer.

[0023] Referring to FIGS. 1 and 2A, method 10 begins in operation 12 by providing a substrate 102. As used herein and in the appended claims, the term "provided" means that the substrate 102 or substrate surface is made available for processing (e.g., placed in a processing chamber). Substrate 102 can be any substrate suitable for forming an EUV photoresist hard mask.

[0024] In operation 14, a metal-rich layer 104 is formed on a substrate 102. In some embodiments, the metal-rich layer comprises a high-Z metal. As used herein, the term "high-Z" refers to a chemical element having protons with a high atomic number (Z) in the atomic nucleus, e.g., a metal. In some embodiments, the high-Z metal has an atomic number greater than 50.

[0025] In one or more embodiments, the metal-rich layer 104 comprises one or more of tin (Sn), indium (In), gallium (Ga), zinc (Zn), tellurium (Te), antimony (Sb), nickel (Ni), titanium (Ti), aluminum (Al), tantalum (Ta), bismuth (Bi), and lead (Pb). In a more specific embodiment, the metal-rich layer comprises tin (Sn).

[0026] The metal-rich layer 104 can be formed by any suitable means known to those skilled in the art. In some embodiments, the metal-rich layer 104 can be deposited by one or more of physical vapor deposition, chemical vapor deposition, or atomic layer deposition.

[0027] In one or more embodiments, the metal-rich layer 104 can have any suitable thickness. In one embodiment, the metal-rich layer 104 has a first thickness within the range from 10 Å to 100 Å.

[0028] Referring to Figure 2B, the metal-rich layer 104 is oxidized in air, forming a metal oxide layer 106 on the upper surface of the metal-rich layer 104. The metal oxide layer 106 contains oxides of the metal-rich layer 104. Thus, the metal oxide layer 106 contains high-Z metal oxides. In one or more embodiments, the metal oxide layer 106 contains one or more of the following: tin oxide (SnOx), indium oxide (InOx), gallium oxide (GaOx), zinc oxide (ZnOx), tellurium oxide (TeOx), antimony oxide (SbOx), nickel oxide (NiOx), titanium oxide (TiOx), aluminum oxide (AlOx), tantalum oxide (TaOx), bismuth oxide (BiOx), and lead oxide (PbOx). In a more specific embodiment, the metal oxide layer 106 contains tin oxide (SnOx).

[0029] In one or more embodiments, when the metal oxide layer 106 is formed on the metal-rich layer 104, the thickness of the metal-rich layer 104 is reduced to a range of 10 Å to 50 Å. In one or more embodiments, the metal oxide layer 106 has a thickness in the range of 20 Å to 100 Å.

[0030] Referring to Figures 1, 2C, and 2D, in operation 16, the metal-rich layer 104 and the metal oxide layer 106 are treated with a reducing plasma 108. Treatment with the reducing plasma 108 repairs the metal surface 110 on the metal-rich layer 104.

[0031] In some embodiments, the metal surface 110 is substantially more metal-rich than the metal-rich layer 104. As used herein, the term “substantially more” means that there is more than 1%, including more than 2%, more than 3%, more than 4%, and more than 5% of the metal in the metal-rich layer 104.

[0032] In one or more embodiments, the metal surface 110 formed on the metal-rich layer 104 has a thickness in the range of 10 Å to 50 Å. In one or more embodiments, treating the metal-rich layer 104 with a reducing plasma 108 reduces the thickness of the metal oxide layer 106 by 10 Å to 50 Å. In one or more embodiments, after treatment with the reducing plasma 108, the metal oxide layer 106 and the metal surface 110 have a combined thickness in the range of 20 Å to 100 Å.

[0033] In some embodiments, seven days after treatment of the high-Z layer 104, the metal content of the metal surface is at least 40%. When treatment with the reducing plasma 108 is not performed, the metal content of the high-Z layer 104 is approximately 35%. Thus, treatment with the reducing plasma can restore the metal content of the high-Z layer while minimizing changes in the thickness profile of the high-Z layer 104.

[0034] In some embodiments, the reducing plasma 108 contains one or more of hydrogen (H2) and helium (He). In one or more embodiments, the reducing plasma 108 contains at least 1% hydrogen, or at least 2% hydrogen, or at least 3% hydrogen, or at least 4% hydrogen, or at least 5% hydrogen, or at least 6% hydrogen, or at least 7% hydrogen, or at least 8% hydrogen, or at least 9% hydrogen, or at least 10% hydrogen, or at least 11% hydrogen, or at least 12% hydrogen, or at least 13% hydrogen, or at least 14% hydrogen, or at least 15% hydrogen, or at least 16% hydrogen, or at least 17% hydrogen, or at least 18% hydrogen, or at least 19% hydrogen. In one or more embodiments, the reducing plasma 108 contains hydrogen in the range of 1% to 20%. In one or more embodiments, helium (He) constitutes the remainder of the plasma.

[0035] In one or more embodiments, the reducing plasma may have any preferred flow rate. In one or more embodiments, the reducing plasma has a flow rate in the range of 1 sccm to 1000 sccm, or in the range of 1 sccm to 500 sccm, or in the range of 1 sccm to 400 sccm, or in the range of 1 sccm to 300 sccm, or in the range of 1 sccm to 200 sccm, or in the range of 1 sccm to 150 sccm, or in the range of 1 sccm to 50 sccm, or in the range of 1 sccm to 40 sccm, or in the range of 1 sccm to 30 sccm, or in the range of 1 sccm to 20 sccm, or in the range of 1 sccm to 10 sccm.

[0036] In one or more embodiments, the plasma treatment may be performed at any preferred pressure. In one or more embodiments, the device 100 is treated with plasma at a pressure in the range of 0.2 mTorr to less than 500 mTorr, or in the range of 0.2 mTorr to 400 mTorr, or in the range of 0.2 mTorr to 300 mTorr, or in the range of 0.2 mTorr to 250 mTorr, or in the range of 10 mTorr to 200 mTorr, or in the range of 10 mTorr to 100 mTorr. In some embodiments, the pressure is greater than 50 mTorr, or greater than 60 mTorr, or greater than 70 mTorr, or greater than 80 mTorr, or greater than 90 mTorr, or greater than 100 mTorr.

[0037] In one or more embodiments, the plasma treatment may be performed for any preferred period of time. In one or more embodiments, the device 100 is treated with plasma for a period of time ranging from 2 seconds to 10 minutes, or from 2 seconds to 5 minutes, or from 2 seconds to 4.5 minutes, or from 2 seconds to 3 minutes, or from 2 seconds to 2 minutes, or from 2 seconds to 1 minute.

[0038] In one or more embodiments, the plasma treatment may be performed at any preferred temperature. In one or more embodiments, the plasma treatment is performed at a temperature in the range of 10°C to 400°C, including the range of 20°C to 200°C. In other embodiments, the plasma treatment is performed at ambient temperature or at room temperature.

[0039] In some embodiments, the plasma gas is flowed into the processing chamber and then ignited to form a direct plasma. In some embodiments, the plasma gas is ignited outside the processing chamber to form a remote plasma.

[0040] In some embodiments, the plasma is inductively coupled plasma (ICP). In some embodiments, the plasma is conductively coupled plasma (CCP). In some embodiments, the plasma is microwave plasma. In some embodiments, the plasma is generated by flowing a plasma gas over a heat ray.

[0041] In one or more embodiments, the plasma treatment may be performed at any suitable power level. In one or more embodiments, the power level is in the range of 10W to 2000W, or 100W to 1500W, or 100W to 1000W, or 100W to 750W.

[0042] Several well-known cluster tools that may be adapted for this disclosure are Olympia®, Continuum®, and Trillium®, all available from Applied Materials, Inc. in Santa Clara, California. However, the exact configuration and combination of chambers may be modified for the purpose of performing specific steps of the processes described herein. Other processing chambers that may be used include, but are not limited to, periodic layer deposition (CLD), atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma treatment, etching, pre-cleaning, chemical cleaning, heat treatment such as RTP, plasma nitriding, degassing, hydroxylation, and other substrate processes. By performing the process in a chamber on a cluster tool, surface contamination of the substrate by impurities in the atmosphere can be avoided, and oxidation will not occur before the next film is deposited.

[0043] According to one or more embodiments, the substrate is continuously under vacuum or “load-lock” conditions and is not exposed to ambient air when moved from one chamber to the next. The transfer chamber is thus under vacuum and “pumped down” under vacuum pressure. An inert gas may be present in the processing chamber or transfer chamber. In some embodiments, the inert gas is used as a purge gas to remove some or all of the reactants (e.g., one reactant). According to one or more embodiments, the purge gas is injected at the outlet of the deposition chamber to prevent reactants (e.g., one reactant) from moving from the deposition chamber to the transfer chamber and / or additional processing chambers. This causes the flow of the inert gas to form a curtain at the chamber outlet.

[0044] Substrates may be processed in a single-substrate deposition chamber where one substrate is loaded, processed, and unloaded before another substrate is processed. Substrates may also be processed continuously, as with a conveyor system where multiple substrates are individually loaded into a first part of the chamber, moved through the chamber, and unloaded from a second part of the chamber. The shape of the chamber and associated conveyor system can form a straight or curved path. Additionally, the processing chamber may be a carousel where multiple substrates are moved around a central axis and undergo processes such as deposition, etching, annealing, and washing along the entire carousel path.

[0045] During processing, the substrate may be heated or cooled. Such heating or cooling may be carried out by any preferred means, but are not limited to, changing the temperature of the substrate support and flowing a heated or cooled gas over the substrate surface. In some embodiments, the substrate support includes a heater / cooler which can be controlled to electrically change the substrate temperature. In one or more embodiments, the employed gas (either a reactive or inert gas) is heated or cooled to locally change the substrate temperature. In some embodiments, the heater / cooler is placed in a chamber adjacent to the substrate surface to convectively change the substrate temperature.

[0046] The substrate may also remain stationary or rotate during processing. A rotating substrate may rotate (around the substrate axis) continuously or in discrete steps. For example, the substrate may rotate throughout the entire process, or it may rotate only slightly between exposures to different reactive or purging gases. Rotating the substrate during processing (either continuously or in steps) may help result in more uniform deposition or etching, for example, by minimizing the effects of local variations in gas flow geometry.

[0047] In the context describing the materials and methods discussed herein (in particular in the context of the following claims), the use of the terms “a,” “an,” and “the,” as well as similar demonstrative pronouns, should be interpreted as encompassing both singular and plural forms unless otherwise indicated herein or explicitly refuted by the context. The representation of ranges of values ​​herein is intended only as abbreviations for each distinct value falling within the range, unless otherwise indicated herein, and each distinct value is incorporated herein as if it were represented individually. All methods described herein may be performed in any preferred order unless otherwise indicated herein or explicitly refuted by the context. The use of any and all example or illustrative words provided herein (e.g., “such as”) is intended only to better illustrate the materials and methods and does not impose any limitation on their scope unless otherwise claimed. Nothing in this specification should be construed as referring to any non-claimed element as essential to the practice of the disclosed materials and methods.

[0048] Any reference throughout this Specification to “one embodiment,” “some embodiments,” “one or more embodiments,” or “embodiments” means that any particular feature, structure, material, or property described in relation to an embodiment is included in at least one embodiment of this disclosure. Thus, any occurrence of phrases such as “in one or more embodiments,” “in some embodiments,” “in one embodiment,” or “in an embodiment” in various places throughout this Specification does not necessarily refer to the same embodiment of this disclosure. Furthermore, any particular feature, structure, material, or property may be combined in any preferred manner in one or more embodiments.

[0049] While the disclosures herein have been described with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the disclosure. It will be apparent to those skilled in the art that various modifications and variations can be made to the methods and apparatus of the disclosure without departing from the spirit and scope of the disclosure. Accordingly, the disclosure is intended to include modifications and variations that fall within the scope of the appended claims and their equivalents.

Claims

1. A method for forming an EUV photoresist hard mask, wherein the method is The process involves treating a metal-rich layer on a substrate with a reducing plasma in order to form a metal surface on the metal-rich layer, wherein the metal-rich layer has an upper portion comprising a metal oxide layer. Methods that include...

2. The method according to claim 1, wherein the metal-rich layer comprises one or more of tin (Sn), indium (In), gallium (Ga), zinc (Zn), tellurium (Te), antimony (Sb), nickel (Ni), titanium (Ti), aluminum (Al), tantalum (Ta), bismuth (Bi), and lead (Pb).

3. The method according to claim 1, wherein the metal oxide layer comprises one or more of the following: tin oxide (SnOx), indium oxide (InOx), gallium oxide (GaOx), zinc oxide (ZnOx), tellurium oxide (TeOx), antimony oxide (SbOx), nickel oxide (NiOx), titanium oxide (TiOx), aluminum oxide (AlOx), tantalum oxide (TaOx), bismuth oxide (BiOx), and lead oxide (PbOx).

4. The method according to claim 1, wherein the metal surface is substantially free of metal oxides.

5. The method according to claim 1, wherein the metal-rich layer contains tin (Sn) and the metal oxide layer contains tin oxide (SnOx).

6. The method according to claim 1, wherein the metal-rich layer has a thickness in the range of 10 Å to 50 Å.

7. The method according to claim 1, wherein the metal oxide layer has a thickness in the range of 20 Å to 100 Å.

8. The method according to claim 7, wherein treating the metal-rich layer with the reducing plasma reduces the thickness of the metal oxide layer by 10 Å to 50 Å.

9. The method according to claim 8, wherein, after treatment with the reducing plasma, the metal oxide layer and the metal surface have a combined thickness in the range of 20 Å to 100 Å.

10. The method according to claim 1, wherein the metal surface has a metal content of at least 40% after 7 days.

11. The method according to claim 1, wherein the reducing plasma contains 1% to 20% hydrogen.

12. The method according to claim 11, wherein the reducing plasma comprises at least 1% hydrogen and helium.

13. A method for forming an EUV photoresist hard mask, wherein the method is The method involves forming a metal-rich layer on a substrate, wherein the metal-rich layer has a thickness in the range of 10 Å to 50 Å, and the metal-rich layer has an upper portion comprising a metal oxide layer having a thickness in the range of 20 Å to 100 Å. In order to form a metal surface on the metal-rich layer, the metal-rich layer is treated with a reducing plasma. Methods that include...

14. The method according to claim 13, wherein the metal-rich layer comprises one or more of tin (Sn), indium (In), gallium (Ga), zinc (Zn), tellurium (Te), antimony (Sb), nickel (Ni), titanium (Ti), aluminum (Al), tantalum (Ta), bismuth (Bi), and lead (Pb).

15. The method according to claim 13, wherein the metal oxide layer comprises one or more of the following: tin oxide (SnOx), indium oxide (InOx), gallium oxide (GaOx), zinc oxide (ZnOx), tellurium oxide (TeOx), antimony oxide (SbOx), nickel oxide (NiOx), titanium oxide (TiOx), aluminum oxide (AlOx), tantalum oxide (TaOx), bismuth oxide (BiOx), and lead oxide (PbOx).

16. The method according to claim 13, wherein the metal surface is substantially free of metal oxides.

17. The method according to claim 13, wherein the metal-rich layer contains tin (Sn) and the metal oxide layer contains tin oxide (SnOx).

18. The method according to claim 13, wherein treating the metal-rich layer with the reducing plasma reduces the thickness of the metal oxide layer by 10 Å to 50 Å.

19. The method according to claim 18, wherein, after treatment with the reducing plasma, the metal oxide layer and the metal surface have a combined thickness in the range of 20 Å to 100 Å.

20. The method according to claim 13, wherein the reducing plasma contains 1% to 20% hydrogen.

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