Method for producing a pure silicon coating and substrate coating composite structure, wafer

Abstract

The application relates to the technical field of semiconductor processing, and provides a method for preparing a pure silicon coating, a substrate coating composite structure and a wafer, wherein a two-dimensional semiconductor material layer is formed on the surface of a substrate; the surface of the two-dimensional semiconductor material layer is immersed in a liquid environment with a silicon source; the two-dimensional semiconductor material layer is used for adsorbing the silicon source, so that a pure silicon coating is formed on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment. The introduction of the two-dimensional semiconductor material can fill the pits on the surface of the substrate and significantly reduce the number of twin crystals on the surface of the pure silicon coating, thereby improving the coating quality.

Description

Technical Field

[0001] This invention relates to the field of semiconductor processing technology, specifically to a method for preparing a pure silicon coating, a substrate-coating composite structure, and a wafer. Background Technology

[0002] In recent years, the field of materials science has made tremendous progress, particularly in semiconductor materials and coating technologies. Semiconductor materials have always been a key component in various applications such as electronic devices and solar cells. Silicon carbide (SiC) is a semiconductor material widely used in high-temperature, high-frequency, and high-power applications. Compared with traditional silicon materials, silicon carbide has higher thermal stability and better electronic properties, making it highly favored in many fields. However, despite its excellent performance in many applications, the surface quality and the difficulty in etching its interfaces have become key issues limiting its application. To address these issues, researchers have been exploring different coating technologies to improve the surface properties of silicon carbide. Among these, the preparation of pure silicon coatings is considered an effective method for improving the performance of silicon carbide.

[0003] The preparation of pure silicon coatings is of great significance in many applications. First, pure silicon coatings can improve the surface quality of silicon carbide, reduce surface corrosion pits and surface energy barriers, thereby improving the performance of semiconductor devices. Second, pure silicon coatings can be used as auxiliary materials for solar cells to improve energy conversion efficiency. Furthermore, pure silicon coatings can also be used in the preparation of conductive ceramics, optical lenses, and optical coatings.

[0004] However, existing methods for preparing pure silicon-based coatings have some problems. Traditional coating techniques typically involve chemical vapor deposition or physical vapor deposition, which require high temperature and high vacuum conditions, and the coating quality is difficult to control. Therefore, it is necessary to develop a new and more efficient method to prepare high-quality pure silicon-based coatings on silicon carbide wafers. Summary of the Invention

[0005] The purpose of this invention is to overcome the problem that existing methods for preparing pure silicon coatings are not effective, and to provide a method for preparing pure silicon coatings, as well as a substrate coating composite structure and a wafer.

[0006] To achieve the above objectives, the present invention provides a method for preparing a pure silicon coating on a substrate surface, comprising:

[0007] Provide a base;

[0008] A two-dimensional semiconductor material layer is formed on the surface of the substrate;

[0009] By adsorbing a silicon source using the two-dimensional semiconductor material layer, a pure silicon coating is formed on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment.

[0010] As one possible implementation, the step of forming a two-dimensional semiconductor material layer on the substrate surface includes:

[0011] Provides dispersions of two-dimensional semiconductor materials;

[0012] The two-dimensional semiconductor material dispersion is coated onto the substrate surface and then dried.

[0013] Repeat the coating and drying process described above until a single or double layer of two-dimensional semiconductor material is formed on the substrate surface.

[0014] As one possible implementation, the step of providing a two-dimensional semiconductor material dispersion includes:

[0015] Provide two-dimensional semiconductor materials and organic solvents;

[0016] The two-dimensional semiconductor material is added to the organic solvent to obtain a mixed solution;

[0017] The mixed solution is stirred and / or ultrasonically dispersed to obtain a uniformly dispersed two-dimensional semiconductor material dispersion.

[0018] As one possible implementation, the molar ratio of the two-dimensional semiconductor material to the organic solvent is determined based on the solubility of the two-dimensional semiconductor material.

[0019] As one possible implementation method, the coating method of the two-dimensional semiconductor material dispersion is one or more of spin coating, dip coating, and dip coating.

[0020] As one possible implementation, the two-dimensional semiconductor material in the two-dimensional semiconductor material layer is a two-dimensional transition metal sulfide, which includes one or more of MoS2, MoSe2, HfS2, HfSe2, HfTe2, VS2, VSe2, VTe2, NbS2, NbSe2, NbTe2, TaS2, TaSe2, TaTe2, and PtSe2.

[0021] As one possible implementation method, a pure silicon coating is formed on the surface of the two-dimensional semiconductor material layer by chemical hydrothermal synthesis.

[0022] As one possible implementation, the step of forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer using a chemical hydrothermal synthesis method includes:

[0023] Provide sealed containers and silicon source synthesis solution;

[0024] A composite substrate structure consisting of a two-dimensional semiconductor material layer and a substrate is fixed in the sealed container, and a silicon source synthesis liquid is poured into the sealed container to immerse the composite substrate structure.

[0025] The sealed container is sealed, and the sealed container is heated at a predetermined temperature for a predetermined time to react, thereby forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer.

[0026] The sealed container is cooled, and then the composite substrate structure containing the pure silicon coating is taken out for cleaning and drying.

[0027] As one possible implementation, the silicon source synthesis solution comprises a silicon source, H2O, and a structure directing agent. The silicon source includes at least one of tetraethyl orthosilicate and silicon dioxide, and the structure directing agent includes at least one of TPAOH, TMAOH, and TEAOH.

[0028] As one possible implementation, when the silicon source is tetraethyl orthosilicate and the structure directing agent is TPAOH, the molar ratio of tetraethyl orthosilicate:H2O:TPAOH is in the range of 1:150-175:0.2-0.5, and the reaction time for forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer is 2h-8h.

[0029] Accordingly, the present invention also provides a substrate coating composite structure, comprising:

[0030] Base;

[0031] A two-dimensional semiconductor material layer located on the surface of the substrate;

[0032] A pure silicon coating located on the surface of the two-dimensional semiconductor material layer;

[0033] In the process of forming a pure silicon coating, the surface of the two-dimensional semiconductor material layer is immersed in a liquid environment containing a silicon source. The two-dimensional semiconductor material layer adsorbs the silicon source in the liquid, thereby forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment.

[0034] Accordingly, the present invention also provides a silicon carbide wafer, comprising:

[0035] Base;

[0036] A two-dimensional semiconductor material layer located on the surface of the substrate;

[0037] A pure silicon coating located on the surface of the two-dimensional semiconductor material layer;

[0038] In the process of forming a pure silicon coating, the surface of the two-dimensional semiconductor material layer is immersed in a liquid environment containing a silicon source. The two-dimensional semiconductor material layer adsorbs the silicon source in the liquid, thereby forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment.

[0039] The beneficial effects of this invention are:

[0040] This invention provides a method for preparing a pure silicon coating, a substrate-coating composite structure, and a wafer. The method involves forming a two-dimensional semiconductor material layer on the substrate surface; utilizing this two-dimensional semiconductor material layer to adsorb a silicon source, thereby forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment. The high specific surface area and thin thickness of the two-dimensional semiconductor material facilitate its adsorption onto the substrate surface due to intermolecular forces. Furthermore, atmospheric pressure during removal further strengthens the bond between the material and the substrate. When uniformly dispersed in an organic solvent and then adsorbed onto the substrate surface using physical methods such as dip-coating, spin-coating, or dip-coating, it can fill pits on the substrate surface and lay the foundation for subsequent hydrothermal synthesis to prepare a pure silicon-based coating, significantly reducing the number of twins on the pure silicon coating surface and improving coating quality. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the method steps for preparing a pure silicon coating on a substrate surface according to some embodiments of the present invention;

[0042] Figure 2 A schematic diagram of the surface of a silicon carbide wafer without two-dimensional semiconductor material coating;

[0043] Figure 3 A schematic diagram of the surface of a silicon carbide wafer coated with a two-dimensional semiconductor material;

[0044] Figure 4 This is a schematic diagram of a pure silicon coating surface formed directly on the surface of a silicon carbide wafer using existing technology.

[0045] Figure 5 This is a schematic diagram of a pure silicon coating grown on a silicon carbide wafer surface using two-dimensional semiconductor materials, according to an embodiment of the present invention. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] See Figure 1 This embodiment provides a method for preparing a pure silicon coating on a substrate surface, comprising:

[0048] Step S100: Provide a substrate;

[0049] Step S200: A two-dimensional semiconductor material layer is formed on the surface of the substrate;

[0050] Step S300: Immerse the surface of the two-dimensional semiconductor material layer in a liquid environment containing a silicon source, and utilize the two-dimensional semiconductor material layer to adsorb the silicon source in the liquid, so that a pure silicon coating is formed on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment.

[0051] In some embodiments, the introduction of the two-dimensional semiconductor material layer can, on the one hand, fill pits on the substrate surface. See also Figure 2 The image shows a raw high-magnification SEM image of the surface of a silicon carbide wafer without a two-dimensional semiconductor coating. Pits can be seen on the surface of the silicon carbide wafer. (See also...) Figure 3 The high-magnification SEM image of the silicon carbide wafer after coating with two-dimensional semiconductor material shows that the pits on the surface of the silicon carbide wafer have been basically filled.

[0052] On the other hand, the introduction of the two-dimensional semiconductor material layer can significantly reduce the number of twins on the coating surface and improve the coating quality. See also Figure 4 The original high-magnification SEM image of the pure silicon coating formed directly on the surface of a silicon carbide wafer shows that the pure silicon coating contains many twins; see also... Figure 5 A high-magnification SEM image of a pure silicon coating grown on the surface of a silicon carbide wafer for two-dimensional semiconductor materials shows that the number of twins in the pure silicon coating is significantly reduced.

[0053] By selecting a suitable two-dimensional semiconductor material to generate a two-dimensional confinement effect, the two-dimensional semiconductor material on the substrate surface can better adsorb the silicon source in the solution. This is mainly achieved by utilizing van der Waals forces between molecules to enrich the substrate surface environment with silicon. During the growth of the pure silicon coating, this silicon-rich environment allows the coating surface to maintain good uniformity and reduces the number of twins, further improving the quality of the pure silicon coating and providing a better foundation for subsequent device fabrication. Furthermore, the method provided in this embodiment is expected to be applicable to other coating systems.

[0054] In existing technologies, seeds (nuclei) need to be generated by the silicon source synthesis liquid itself before they can be adsorbed onto the substrate surface and slowly become a coating. However, in some embodiments provided by this invention, the two-dimensional semiconductor material on the substrate surface can provide effective nucleation sites for the subsequent film preparation, promote rapid nucleation on the crystal surface, and thus improve the coating formation speed.

[0055] In some embodiments, two-dimensional semiconductor materials possess excellent electronic properties and thermal stability, which can be used to improve the performance of semiconductor materials. In some embodiments, the preparation of pure silicon-based coatings using two-dimensional semiconductor materials can be achieved under lower temperature and pressure conditions, thereby reducing preparation costs while also improving the uniformity and quality of the coating.

[0056] In step S100, in some embodiments, the substrate can be a silicon carbide substrate, and the crystal form of the silicon carbide substrate can be one of 3C-SiC, 4H-SiC, and 6H-SiC. The shape of the silicon carbide substrate can be one of plate-like, hollow fiber-like, tubular, and multi-channel-like. However, in other embodiments, the substrate can also be a silicon substrate, a sapphire substrate, or a silicon nitride substrate, etc.

[0057] In some embodiments, step S200, forming a two-dimensional semiconductor material layer on the substrate surface, includes:

[0058] A two-dimensional semiconductor material dispersion is provided; the two-dimensional semiconductor material dispersion is coated on the surface of the substrate and then dried; the above coating and drying process is repeated until a single or double layer of two-dimensional semiconductor material is formed on the surface of the substrate.

[0059] In some embodiments, the step of providing a two-dimensional semiconductor material dispersion includes: providing a two-dimensional semiconductor material and an organic solvent; adding the two-dimensional semiconductor material to the organic solvent to obtain a mixed solution; and performing stirring and ultrasonic dispersion treatment on the mixed solution to achieve dispersion at both the macroscopic and microscopic levels, thereby obtaining a uniformly dispersed two-dimensional semiconductor material dispersion.

[0060] In some embodiments, to achieve the desired effect, a suitable two-dimensional semiconductor material selected for the two-dimensional semiconductor material layer is a two-dimensional transition metal sulfide, which allows for better adsorption of the silicon source in the solution. However, in other embodiments, other two-dimensional semiconductor materials that can achieve the desired effect may also be used. The two-dimensional transition metal sulfide includes one or more of MoS2, MoSe2, HfS2, HfSe2, HfTe2, VS2, VSe2, VTe2, NbS2, NbSe2, NbTe2, TaS2, TaSe2, TaTe2, and PtSe2. In some embodiments, the organic solvent may be one or more of methanol, ethanol, propanol, butanol, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone. In some embodiments, the molar ratio of the two-dimensional semiconductor material to the organic solvent may be determined based on the solubility of the two-dimensional semiconductor material to prevent the two-dimensional semiconductor material from failing to disperse and agglomerating, thus preventing it from spreading evenly on the substrate surface. Specifically, the ratio may be 1:100-10000. In some embodiments, the mixed solution can be stirred first, followed by ultrasonic dispersion; wherein the stirring time ranges from 3h to 12h, and the ultrasonic dispersion time ranges from 0.5h to 4h. In some embodiments, for example, 1.0mg of molybdenum disulfide can be added to a container containing 500mL of N-methylpyrrolidone, stirred at room temperature for 2h, and then sonicated in a cell sonicator for 4h to obtain a uniformly dispersed molybdenum disulfide dispersion. In some embodiments, 1.0mg of vanadium disulfide can be added to a container containing 250mL of acetone, stirred at room temperature for 0.5h, and then sonicated in a cell sonicator for 3h to obtain a uniformly dispersed vanadium disulfide dispersion.

[0061] In some embodiments, during the process of coating the two-dimensional semiconductor material dispersion onto the substrate surface and then drying, the coating method is one or more of spin coating, dip coating, and coating time can be 1 second to 360 seconds. The coating method is simple and easy to operate. In some embodiments, since the two-dimensional semiconductor material itself is not uniform in thickness and size, and is relatively light due to its sheet-like shape, the obtained two-dimensional semiconductor dispersion can be allowed to stand for 1 to 7 weeks to allow large, non-uniform materials to deposit to the bottom of the solution. Then, 10 mL of the supernatant is taken and sprayed at 1000 rpm. -1A spin-coating speed is used to spin-coat a substrate containing a two-dimensional semiconductor material layer on the surface of a 4-inch 4H-SiC wafer. The substrate is then dried at 50°C to prevent the two-dimensional semiconductor material from easily curling or cracking during the drying process. In some embodiments, to increase the content of the two-dimensional semiconductor material on the substrate surface and to prevent excessive detachment, the coating and drying process can be repeated multiple times to ensure that the substrate surface is essentially covered with the two-dimensional semiconductor material and that the material is not easily detached. For example, experiments have shown that when the two-dimensional semiconductor material layer exceeds 10 nm in thickness, the adhesion decreases, potentially leading to localized detachment. Therefore, a single-layer or double-layer two-dimensional semiconductor material layer can be formed on the substrate surface; for example, the thickness of a single layer can be 3 nm-5 nm, exhibiting ultra-thin characteristics.

[0062] In step S300, in some embodiments, a pure silicon coating can be formed on the surface of the two-dimensional semiconductor material layer using a chemical hydrothermal synthesis method. Compared with conventional physical vapor deposition and chemical vapor deposition methods, the chemical hydrothermal synthesis method of the present invention has advantages such as simple process, high coating repeatability, and low cost.

[0063] Specifically, the steps for forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer using a chemical hydrothermal synthesis method include: providing a sealed container and a silicon source synthesis solution; fixing a composite substrate structure composed of a two-dimensional semiconductor material layer and a substrate in the sealed container; pouring the silicon source synthesis solution into the sealed container to immerse the composite substrate structure; sealing the sealed container; heating the sealed container at a predetermined temperature for a predetermined time to react, thereby forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer; water cooling the sealed container; and then removing the composite substrate structure containing the pure silicon coating for cleaning and drying.

[0064] In some embodiments, the silicon source synthesis solution comprises a silicon source, H2O, and a structure directing agent. The silicon source includes at least one of tetraethyl orthosilicate and silicon dioxide, and the structure directing agent includes at least one of TPAOH, TMAOH, and TEAOH. When the silicon source is tetraethyl orthosilicate and the structure directing agent is TPAOH, the molar ratio of tetraethyl orthosilicate:H2O:TPAOH ranges from 1:150-175:0.2-0.5, and the reaction time for forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer is 2-8 hours. In some embodiments, for example, when the two-dimensional semiconductor material layer is a molybdenum disulfide layer prepared by the above molybdenum disulfide dispersion, the synthesis solution can be prepared according to a ratio of tetraethyl orthosilicate:H2O:TPAOH = 1:150:0.3. Specifically, 11.5 g of TPAOH solution is added to 114.0 g of ultrapure water and stirred continuously. 10.0 g of TEOS is added dropwise to the solution, and the mixture is aged at room temperature for 4 hours. In some embodiments, for example when the two-dimensional semiconductor material layer is a vanadium disulfide layer prepared by the above-mentioned vanadium disulfide dispersion, the synthesis solution can be prepared according to the ratio of TEOS:H2O:TPAOH = 1:200:0.4. The specific procedure is as follows: Take 15.3g of TPAOH solution and add it to 182.9g of ultrapure water while stirring continuously. Add 10.0g of TEOS dropwise to the above solution, allowing the TEOS to hydrolyze under the action of TPAOH, and then age it at room temperature for 3 hours.

[0065] In some embodiments, the microstructure of the pure silicon coating is a silicon-oxygen framework, and the structure directing agent is mainly used to promote the bonding between silicon-oxygen atoms. The content of the structure directing agent and the amount of water vary for different two-dimensional semiconductor materials, depending on the polarity of the two-dimensional semiconductor material, so that the appropriate structure directing agent can be selected according to different two-dimensional semiconductor materials.

[0066] In some embodiments, the sealed container can be a reactor. The obtained composite substrate structure containing a two-dimensional semiconductor material layer and a substrate can be vertically fixed in the high-pressure reactor. Then, the silicon source synthesis solution is slowly added to the reactor along the inner wall. After sealing, it is placed in an oven at a certain temperature for 4 hours. After the synthesis is completed, the reactor is quickly cooled with water; otherwise, the temperature of the reactor will still be high, and the reaction will continue, which is equivalent to indirectly increasing the reaction time. Afterward, the obtained composite substrate structure containing a pure silicon coating is taken out, washed with ultrapure water, and dried in a clean room.

[0067] Accordingly, some embodiments of the present invention also provide a substrate coating composite structure, comprising:

[0068] A substrate; a two-dimensional semiconductor material layer located on the surface of the substrate; a pure silicon coating located on the surface of the two-dimensional semiconductor material layer; wherein the two-dimensional semiconductor material layer is used to adsorb a silicon source, so that a pure silicon coating is formed on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment.

[0069] Accordingly, some embodiments of the present invention also provide a silicon carbide wafer, comprising:

[0070] A substrate; a two-dimensional semiconductor material layer located on the surface of the substrate; a pure silicon coating located on the surface of the two-dimensional semiconductor material layer; wherein, during the formation of the pure silicon coating, the surface of the two-dimensional semiconductor material layer is immersed in a liquid environment containing a silicon source, and the two-dimensional semiconductor material layer adsorbs the silicon source in the liquid, so that a pure silicon coating is formed on the surface of the two-dimensional semiconductor material layer in a silicon-rich environment.

[0071] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for preparing a pure silicon coating on a substrate surface, characterized in that, include: Provide a base; The step of forming a two-dimensional semiconductor material layer on the substrate surface includes: providing a two-dimensional semiconductor material dispersion; coating the two-dimensional semiconductor material dispersion on the substrate surface and then drying it; repeating the above coating and drying process until a single-layer or double-layer two-dimensional semiconductor material layer is formed on the substrate surface. A pure silicon coating is formed on the surface of the two-dimensional semiconductor material layer using a chemical hydrothermal synthesis method. The steps of forming the pure silicon coating on the surface of the two-dimensional semiconductor material layer using a chemical hydrothermal synthesis method include: A sealed container and a silicon source synthesis solution are provided. The silicon source synthesis solution contains a silicon source, H2O, and a structure directing agent. A composite substrate structure consisting of a two-dimensional semiconductor material layer and a substrate is fixed in the sealed container. The silicon source synthesis solution is poured into the sealed container to immerse the composite substrate structure. The sealed container is sealed, and the sealed container is heated at a predetermined temperature for a predetermined time to react, thereby forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer. The sealed container is cooled, and then the composite substrate structure containing the pure silicon coating is removed for cleaning and drying.

2. The method for preparing a pure silicon coating on a substrate surface according to claim 1, characterized in that, The steps for providing a two-dimensional semiconductor material dispersion include: Provide two-dimensional semiconductor materials and organic solvents; The two-dimensional semiconductor material is added to the organic solvent to obtain a mixed solution; The mixed solution is stirred and / or ultrasonically dispersed to obtain a uniformly dispersed two-dimensional semiconductor material dispersion.

3. The method for preparing a pure silicon coating on a substrate surface according to claim 2, characterized in that, The molar ratio of the two-dimensional semiconductor material to the organic solvent is determined based on the solubility of the two-dimensional semiconductor material.

4. The method for preparing a pure silicon coating on a substrate surface according to claim 1, characterized in that, The two-dimensional semiconductor material in the two-dimensional semiconductor material layer is a two-dimensional transition metal sulfide, which includes one or more of MoS2, MoSe2, HfS2, HfSe2, HfTe2, VS2, VSe2, VTe2, NbS2, NbSe2, NbTe2, TaS2, TaSe2, TaTe2, and PtSe2.

5. The method for preparing a pure silicon coating on a substrate surface according to claim 1, characterized in that, The silicon source includes at least one of tetraethyl orthosilicate and silicon dioxide, and the structure directing agent includes at least one of TPAOH, TMAOH and TEAOH.

6. The method for preparing a pure silicon coating on a substrate surface according to claim 5, characterized in that, When the silicon source is tetraethyl orthosilicate and the structure directing agent is TPAOH, the molar ratio of tetraethyl orthosilicate:H2O:TPAOH is in the range of 1:150-175:0.2-0.5, and the reaction time for forming a pure silicon coating on the surface of the two-dimensional semiconductor material layer is 2h-8h.

7. A substrate coating composite structure, characterized in that, include: Base; A two-dimensional semiconductor material layer located on the surface of the substrate; A pure silicon coating located on the surface of the two-dimensional semiconductor material layer; The two-dimensional semiconductor material layer and the pure silicon coating are formed using the method described in any one of claims 1-6.

8. A silicon carbide wafer, characterized in that, include: Base; A two-dimensional semiconductor material layer located on the surface of the substrate; A pure silicon coating located on the surface of the two-dimensional semiconductor material layer; The two-dimensional semiconductor material layer and the pure silicon coating are formed using the method described in any one of claims 1-6.

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