EP3634922A1

EP3634922A1 - Method for producing layers of silicon carbide

Info

Publication number: EP3634922A1
Application number: EP18730683.2A
Authority: EP
Inventors: Siegmund Greulich-Weber; Rüdiger SCHLEICHER-TAPPESER
Original assignee: PSC Technologies GmbH
Current assignee: PSC Technologies GmbH
Priority date: 2017-06-09
Filing date: 2018-06-04
Publication date: 2020-04-15
Also published as: WO2018224438A1; DE102017112756A1; US20200165171A1; CN110719900A; JP2020523267A

Abstract

The invention relates to a method for producing thin layers of silicon carbide by means of a solution or dispersion containing carbon and silicon.

Description

Method for producing layers of silicon carbide

The present invention relates to the technical field of semiconductor technology. In particular, the present invention relates to a method for producing thin layers of silicon carbide.

Furthermore, the present invention relates to a composition, in particular a silicon carbide Precursorsol, for producing thin layers of Siliciumcar- 10 bid.

Finally, the present invention relates to thin layers of silicon carbide.

Silicon carbide with the formula SiC is an extremely interesting and versatile material that can be used in semiconductor technology and for the position of ceramic materials. Due to the high hardness and the high melting point, silicon carbide is also called carborundum and is often used as an abrasive or as an insulator in high-temperature reactors. In addition, silicon carbide incorporates alloys or alloy-like compounds having a variety of elements and compounds which have a variety of advantageous material properties, such as, for example: As a high hardness, high resistance, low weight and a low sensitivity to oxidation, even at high temperatures.

However, semiconductor applications are of particular importance, since silicon carbide 25 is extremely resistant mechanically and thermally, and the electronic properties can be tailored by suitable doping for the particular application. While pure silicon carbide is an insulator, but due to the good thermal conductivity is suitable as a substrate for semiconductor structures, by suitable doping, in particular with the elements boron, aluminum, nitrogen and phosphorus excellent semiconductor materials can be provided which at temperatures of up to about 500 ° C can be used.

Semiconductor applications generally require monocrystalline silicon carbide structures, in particular thin layers of monocrystalline silicon carbide. Although a number of processes are currently known for producing thin silicon carbide layers, the processes are often very expensive and costly. tenintensiv, which is why the use of silicon carbide in semiconductor technology is limited to a few special applications.

Thus, DE 27 27 557 A1 describes a method for forming monocrystalline silicon carbide on a monocrystalline silicon substrate, wherein at least a portion of the monocrystalline silicon substrate is converted to porous silicon by anodic treatment in an aqueous hydrofluoric acid solution and the resulting structure in an atmosphere containing a hydrocarbon gas heating a temperature in the range of 1050 ° C to 1250 ° C so that the porous silicon reacts with the gas fractions to form a monocrystalline silicon carbide layer and a polycrystalline silicon carbide overlying layer thereof.

Scientific Publications N. Singh, K. Singh, A. Pandey and D. Kaur, "Improved electric transport properties in high quality nanocrystalline silicon carbides (nc-SiC)", Materials Letters, 164, 2016, pages 28 to 31, concerns the production of thin layers of nanocrystalline silicon carbide, which are applied by sputtering. Further, the scientific paper relates to H. Shen, T. Wu, Y. Pan, L. Zhang, B. Cheng and Z. Yue, "Structural and optical properties of nc-3C films synthesized by hot wire chemical vapor deposition from SiH ₄ -C ₂ H ₂ -H ₂ mixture ", Thin Solid Films, 522, 2012, pages 36 to 39, the deposition of nanocrystalline silicon carbide films by CVD (Chemical Vapor Deposition, CVD).

However, the methods described above are all very expensive and can only be used on very costly crystalline substrates if defined silicon carbide layers with a small number of defects are to be produced.

WO 96/24709 A1 describes a method for producing monocrystalline thin layers of silicon carbide, wherein a substrate to be coated is coated with a carbon-containing polysilane, the adhering layer is pyrolyzed under protective gas and the amorphous silicon carbide layer thus produced by maintaining a high temperature of crystallized above 700 ° C. The method described is a further development of the previously described methods, since in this way relatively thick layers can be produced within a short time, However, the preparation of the corresponding polysilanes is very expensive and as substrates can also be used only crystalline substrates.

In particular, the use of exclusively crystalline substrates is problematic, since monocrystalline substrates on which the silicon carbide layer is to be produced are very expensive to produce and the material costs for the substrate far exceed the costs for carrying out the actual process. This is particularly unfavorable, in particular, when the substrate is not to be used further, but is used only as a carrier material in the production of silicon carbide layers.

The state of the art therefore still lacks a simple and cost-effective process for producing monocrystalline silicon carbide-containing layers or monocrystalline bodies containing silicon carbide.

It is thus an object of the present invention to avoid, but at least to mitigate, the disadvantages and problems described above associated with the prior art. A further object of the present invention is to provide suitable substances, in particular precursors, which can be reproducibly converted to silicon carbide monocrystals rapidly and on an industrial scale. It is further an object of the present invention to provide a method and suitable starting materials which make it possible to produce layers of silicon carbide on non-crystalline substrates, in particular non-monocrystalline substrates. The present invention according to a first aspect of the present invention is a process for producing silicon carbide thin films according to claim 1; Further, advantageous embodiments of this invention aspect are the subject of the relevant subclaims. Another object of the present invention according to a second aspect of the present invention is a composition according to claim 16; Further, advantageous embodiments of this invention aspect are the subject of the relevant subclaims. Finally, another object of the present invention is a silicon carbide layer according to claim 21. It goes without saying that the following, special embodiments, in particular special embodiments or the like, which are described only in connection with an aspect of the invention, also apply correspondingly in relation to the other aspects of the invention, without this requiring an explicit mention.

Furthermore, it should be noted in all of the below-mentioned relative or percentage, in particular weight-based quantities, that they are to be selected within the scope of the present invention by a person skilled in the art such that the sum of the ingredients, additives or auxiliaries or the like is always 100% or 100% Wt .-% result. This is understood by the skilled person but by itself.

Incidentally, it is the case that the skilled person can deviate from the numbers, ranges or quantities given below, based on the application and due to individual cases, without departing from the scope of the present invention.

In addition, it is true that all the parameter information or the like mentioned below can in principle be determined or determined using standardized or explicitly stated determination methods or else determination methods familiar to the person skilled in the art.

Given this, the subject matter of the present invention will be explained in more detail below.

The subject of the present invention - according to one aspect of the present invention - is thus a process for producing thin layers of silicon carbide, wherein

(A) in a first process step, a liquid carbon and silicon-containing solution or dispersion, in particular a SiC precursor sol, is applied to a substrate and

(b) in a second process step following the first process step (a), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is converted into silicon carbide. As the Applicant has surprisingly found out, by using carbon- and silicon-containing solutions or dispersions, in particular of SiC precursor sols, thin layers of doped or undoped silicon carbide can be produced, which have an extremely low number of defects and are outstanding Are suitable for semiconductor applications.

Moreover, the method of the invention can produce thin layers of silicon carbide on almost all substrates, not just single-crystal substrates such as silicon carbide or silicon wafers. In particular, in the context of the present invention, it is also possible to remove the substrate again after the thin layer of silicon carbide has been produced.

By repeated applications of the method according to the invention, in particular, it is also possible to produce a plurality of layers of monocrystalline silicon carbide so that finally silicon carbide wafers can also be obtained by the method according to the invention.

The method according to the invention enables the simple, cost-effective and reproducible production of thin layers of silicon carbide or silicon carbide bodies with a flat surface.

In the context of the present invention, it is generally provided that the layer of silicon carbide has a thickness of 0.1 to 1 .mu.m, in particular 0.1 to 500 .mu.m, preferably 0.1 to 300 .mu.m, preferably 0.1 to 10 μιη, has.

In the context of the present invention, a solution containing carbon and silicon is to be understood as meaning a solution or dispersion, in particular a precursor sol, which contains carbon- and silicon-containing chemical compounds, the individual compounds being carbon and / or May have silicon. The compounds which have carbon and silicon are preferably suitable as precursors for the target compounds to be prepared. In the context of the present invention, a precursor is to be understood as meaning a chemical compound or a mixture of chemical compounds which react by chemical reaction and / or under the action of energy to one or more target compounds. In the context of the present invention, a precursor sol is a solution or dispersion of precursor substances, in particular starting compounds, preferably precursors, which react to form the desired target compounds. In the precursor sols, the chemical compounds or mixtures of chemical compounds are no longer necessarily present in the form of the chemical compounds originally used, but instead, for example, as hydrolysis, condensation or other reaction or intermediate products. However, this is made clear in particular by the expression of the "sol". In the context of sol-gel processes, inorganic materials are usually converted under hydro- or solvolysis into reactive intermediates or agglomerates and particles, the so-called sol, which then age in particular by condensation reactions to give a gel, larger particles and agglomerates in the Solution or dispersion arise.

In the context of the present invention, a SiC precursor sol is a sol, in particular a solution or dispersion, which contains chemical compounds or their reaction products from which silicon carbide can be obtained under process conditions.

In the context of the present invention, a solution is to be understood as meaning a usually liquid single-phase system in which at least one substance, in particular a compound or its components, such as ions, are homogeneously distributed in another substance, the so-called solvent. In the context of the present invention, a dispersion is to be understood as meaning an at least two-phase system in which a first phase, namely the dispersed phase, is distributed in a second phase, the continuous phase. The continuous phase is also referred to as dispersion medium or dispersant; In the context of the present invention, the continuous phase is usually in the form of a liquid, and dispersions in the context of the present invention are generally solid-in-liquid dispersions. However, in particular in the case of sols or even polymeric compounds, the transition from a solution to a dispersion is often fluid and it is no longer possible to distinguish clearly between a solution and a dispersion.

In the context of the present invention, a layer is to be understood as the distribution of material, in particular in the form of a monocrystal, having a certain layer thickness in a plane, in particular on a surface of a substrate. The plane does not have to be completely covered with the material. Usually, however, at least one surface of the substrate is provided over its entire surface with the layer of silicon carbide or with a layer of the carbon- and silicon-containing solution or dispersion.

In the context of the present invention, a substrate is to be understood as meaning the material to which the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is applied. In particular, a substrate in the context of the present invention is a three-dimensional or even a nearly two-dimensional structure having at least one preferably planar surface onto which the carbon- and silicon-containing solution or dispersion is applied. The substrate is thus preferably a carrier material for producing the layer of silicon carbide from the informal carbon- and silicon-containing solution.

The silicon carbide produced in the context of the present invention is, as stated above, either doped silicon carbide or undoped silicon carbide, the silicon carbide preferably being in monocrystalline form. In particular, single crystals are suitable for use in semiconductor technology.

A doped silicon carbide is to be understood as meaning a silicon carbide which is admixed, in particular doped, with further elements, in particular from the 13th and 15th group of the Periodic Table of the Elements, in small amounts. Preferably, the silicon carbide has at least one doping element in the parts per million (ppm) or parts per billion (ppb) range. The doping of the silicon carbides in particular has a decisive influence on the electrical properties of the silicon carbides, so that doped silicon carbides are particularly suitable for applications in semiconductor technology. Dopings with doping elements which have more than 4 valence electrons are referred to as n-type dopants, while dopants with doping elements having less than 4 valence electrons are referred to as p-type dopants.

Typically, the silicon carbide is doped with an element selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, boron, aluminum, gallium, indium and mixtures thereof. Preferably, the silicon carbide is doped with elements of the 13th and 15th group of the periodic table of the elements, whereby in particular the electrical properties of the silicon carbide are specifically manipulated and adjusted. could be made. Such doped silicon carbides are particularly suitable for applications in semiconductor technology.

If the silicon carbide is doped in the context of the present invention, it has proven useful if the doped silicon carbide contains the doping element in amounts of from 0.000001 to 0.0005% by weight, in particular 0.000001 to 0.0001% by weight. , preferably 0.000005 to 0.0001 wt .-%, preferably 0.000005 to 0.00005 wt .-%, based on the doped silicon carbide containing. For the purposeful adjustment of the electrical properties of the silicon carbide, therefore, extremely small amounts of doping elements are completely sufficient.

If the silicon carbide is to be doped with nitrogen, then, for example, nitric acid, ammonium chloride or melamine can be used as doping reagents. Moreover, in the case of nitrogen, it is also possible to carry out the process for producing the silicon carbide in a nitrogen atmosphere, wherein doping with nitrogen can also be achieved, but which are less accurate.

In addition, it is also possible, for example, to achieve doping with alkali metal nitrates, but less preference is given to these because of the alkali metals which remain in the precursor granules and such doping.

If doping with phosphorus is to take place, it has proven useful if doping with phosphoric acid takes place.

If doped with arsenic or antimony, so it has proven useful if the doping reagent is selected from arsenic trichloride, antimony chloride, arsenic oxide or antimony oxide. If aluminum is to be used as doping reagent, aluminum powder can be used as doping, in particular in the case of acidic or basic pH. In addition, it is also possible to use aluminum chlorides. When using metals as doping element, it is generally possible to use the chlorides, nitrates, acetates, acetylacetonates, formates, alkoxides and hydroxides, with absorption of sparingly soluble hydroxides.

If boron is used as the doping element, then the doping element is usually boric acid. If indium is used as the doping element, then the doping reagent is usually selected from indium halides, in particular indium trichloride (lnCl ₃ ).

If gallium is used as the doping element, the doping reagent is usually selected from gallium halogynides, in particular GaCU.

In the context of the present invention, it is generally provided that in the first process step (a) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is applied to the substrate as a layer, in particular as a homogeneous layer. By applying the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, in the form of a layer on the substrate, it is possible to obtain homogeneous monocrystalline layers of silicon carbide.

The carbon- and silicon-containing solution or dispersion may be applied to the substrate by any suitable method. However, it has proven useful if in process step (a) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, by a coating process, in particular by dipping, also called dipcoating, spin coating, spraying, rolling or rolling is applied to the substrate. Particularly good results are obtained when the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is applied to the substrate by dipping, spin coating or spraying, preferably by dipping.

As far as the layer thickness is concerned, with which the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is applied to the substrate, this can vary widely depending on the particular intended use of the silicon carbide and the respective chemical composition of the silicon carbide Ranges vary. Usually, in process step (a), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, having a layer thickness in the range of 0.1 to 1 .mu.m, in particular 0.1 to 500 μΐη, preferably 0, 1 to 300 μΐη, preferably 0, 1 to 10 μιη, is applied to the substrate.

Furthermore, it can be provided in the context of the present invention that the carbon- and silicon-containing solution or dispersion, in particular the SiC- Precursorsol, a dynamic Brookfield viscosity at 25 ° C in the range of 3 to 500 mPas, in particular 4 to 200 mPas, preferably 5 to 100 mPas having. If the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, has dynamic viscosities in the abovementioned ranges, the layer thicknesses with which the silicon- and carbon-containing solution is applied to the substrate can be varied within wide ranges. In particular, very high layer thicknesses can be achieved with a single application of the silicon- and carbon-containing solution, which are advantageous, for example, in the production of silicon carbide wafers, since the wafer is accessible in only a few operations.

According to a preferred embodiment of the present invention, the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, contains

(A) at least one silicon-containing compound,

(B) at least one carbonaceous compound and

(C) at least one solvent or dispersant.

In the context of the present invention, the silicon- and carbon-containing solution or dispersion, in particular the SiC precursor sol, receives special precursors which release silicon under process conditions and special precursors which release carbon under process conditions. In this way, the ratio of carbon to silicon in the carbon- and silicon-containing solution or dispersions can be easily varied and tailored to the respective applications. Particularly good results are obtained in the context of the present invention, when the silicon-containing compound is selected from silanes, Silanhydroly- saten, orthosilicic acid and mixtures thereof. Most preferably, the silicon-containing compound is a silane. Likewise, it has been found in the present invention, when the carbon-containing compound is selected from the group of sugars, in particular sucrose, glucose, fructose, invert sugar, maltose; Strength; Starch derivatives; organic polymers, in particular phenol-formaldehyde resin and resorcinol-formaldehyde resin, and mixtures thereof. As far as the ratio of silicon to carbon in the carbon- and silicon-containing solution or dispersion is concerned, this can of course vary within wide ranges. However, it has proven useful if the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, has a weight-based ratio of silicon to carbon in the range from 1: 1 to 1:10, in particular 1: 2 to 1: 7 , preferably 1: 3 to 1: 5, preferably 1: 3.5 to 1: 4.5. Particularly good results are obtained in the context of the present invention, when the weight ratio of silicon to carbon in the carbon- and silicon-containing solution or dispersion, in particular in the SiC Precursorsol, 1: 4. With the aforementioned ratios of silicon to carbon, silicon carbide single crystals, in particular monocrystalline silicon carbide layers, can be produced specifically and reproducibly in a subsequent thermal treatment, in particular in a subsequent thermal treatment.

In addition, it can be provided in the context of the present invention that the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, doping reagents. Particularly for semiconductor applications, doping of silicon carbide with elements of the 13th and 15th group of the Periodic Table of the Elements is common to produce semiconductor properties in the silicon carbide material, as previously described. However, pure silicon carbide can be used as an insulator, for example.

As regards the substrate to which the silicon- and carbon-containing solution or dispersion, in particular the SiC precursor sol, is applied, this can be selected from a large number of suitable materials. In the context of the present invention, it is possible that the substrate is selected from crystalline and amorphous substrates. According to a preferred embodiment of the present invention, the substrate is an amorphous substrate. It is a peculiarity of the present invention that the preparation of the silicon carbide layers does not have to be carried out exclusively on crystalline, in particular monocrystalline, substrates, but that significantly cheaper amorphous substrates can also be used. As regards the material of which the substrate is made, particularly good results are obtained when the material is selected from carbon, in particular graphite, and ceramic materials, in particular silicon carbide, silica, alumina and metals and mixtures thereof. However, particularly good results are obtained in the context of the present invention if the material of the substrate is carbon, in particular graphite. In particular, by the use of graphite substrates, thin layers of silicon carbide or silicon carbide wafer can be produced in a particularly simple and cost-effective manner using the method according to the invention. In addition, further suitable materials and substrate materials are, for example, silicon oxide, in particular silicon dioxide wafers, aluminum oxide, for example in the form of sapphire, and metals or metallized surfaces which consist of monocrystalline materials, in particular silicon carbide or silicon dioxide wafers, onto which a metal , For example, platinum, is vapor-deposited.

In general, in process step (b) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, after application to the substrate is subjected to a thermal treatment, in particular a multi-stage thermal treatment. The thermal treatment, in particular the multi-stage thermal treatment, turns the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, into a monocrystalline silicon carbide layer.

According to a preferred embodiment of the present invention, during the thermal treatment

(I) in a first thermal process step, the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, to temperatures in the range of 800 to 1 .200 ° C, in particular 900 to 1 .100 ° C, preferably 950 to 1. 050 ° C, heated and

(ii) in a second thermal process stage following the first thermal process stage (i), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is heated to temperatures of 1,800 ° C. or higher.

By at least two-stage thermal treatment in process step (b), the carbon- and silicon-containing solution or dispersion is particularly gently and completely converted into monocrystalline silicon carbide, which has only an extremely small number of defects. Usually, in the first thermal process stage (i), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is converted into a glass. By heating in the first thermal process stage, in particular the solvents and dispersants and other volatile substances are removed and the non-volatile constituents of the carbon- and silicon-containing solutions or dispersions are pyrolyzed. In the pyrolysis, preferably a glass remains, in which preferably silicon and carbon are present in high concentration. In the context of the present invention, a glass is to be understood as meaning an amorphous solid which has a near but not a long distance order. A glass is in particular a solidified melt.

As far as the period in which in process step (i) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is heated, this naturally can vary within wide ranges. However, it has proven useful in the context of the present invention if, in the first thermal process stage (i), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, for a period of less than 15 minutes, in particular less than 10 minutes, preferably less than 7 minutes, preferably less than 5 minutes.

Similarly, it may be provided in the context of the present invention that in process step (i) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, for a period of 0.5 to 15 minutes, especially 1 to 10 minutes, preferably 1, 5 to 7 minutes, preferably 2 to 5 minutes, is heated. Due to the relatively short heating times in the context of the present invention in the thermal process step (i) sufficient pyrolysis of the starting compounds, in particular the SiC precursors, but still no crystallization to silicon carbide is achieved. Usually, in process step (ii), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, preferably the glass obtained in process step (i), is converted into crystalline silicon carbide, preferably monocrystalline silicon carbide. It has proved to be advantageous in the context of the present invention if, in a first thermal process stage (i), the carbon- and silicon-containing solution or dispersion is pyrolyzed and converted into a glass and in a subsequent separate process stage, in particular in the second thermal process step (ii), the crystallization and production of Siliziumcar bideinkristalle takes place. In this way, it is possible to obtain particularly pure silicon carbide crystals having small fractions of defect structures. In particular, by suitably selecting the temperature during the second thermal process step (ii), it is possible to adjust the polytype of the silicon carbide. Thus, for example, at a temperature in the second process stage (ii) of 1800 ° C., the polytype 3C-SiC is obtained, and at temperatures above 2100 ° C. in the second thermal process stage (ii) the hexagonal SiC polytypes, namely 4H-SiC and 6H-SiC.

As far as the period for which the carbon- and silicon-containing solution or dispersion, in particular that glass obtained in process step (i), in the second process stage (ii) is heated, this can vary within wide ranges. Usually, in process step (ii), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, preferably the glass obtained in process step (i), for a period of more than 10 minutes, in particular over 15 minutes, preferably over 20 Minutes, preferably over 25 minutes, heated. Likewise, it may be provided in the context of the present invention that in process step (ii) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, preferably the glass obtained in process step (i), for a period of 10 to 50 minutes , in particular 15 to 40 minutes, preferably 20 to 35 minutes, preferably 25 to 35 minutes, is heated. The aforementioned periods are sufficient to obtain complete conversion of the precursors into silicon carbide and production of silicon carbide single crystals, but sufficiently short to prevent excessive sublimation of silicon carbide. According to a preferred embodiment of the present invention, it may be provided that, following process step (i) and before carrying out process step (ii), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, preferably in process step (i). obtained glass, cooled, in particular quenched. By cooling, in particular quenching of the resulting glass, the amorphous glass state is obtained and / or frozen. In particular, it is possible in this way to obtain an ideal starting condition for the subsequent conversion to monocrystalline silicon carbide. As far as the cooling rate in this context is concerned, this can of course vary within wide limits. However, it has proved useful in the context of the present invention if the carbon-containing and silicon-containing solution or dispersion, in particular the SiC precursor sol, preferably the glass obtained in process step (i), with a cooling rate of more than 50 ° C / min , in particular more than 70 ° C / min, preferably more than 100 ° C / min, is cooled.

Usually, in the context of the present invention, at least the process stages (i) and (ii) are carried out in a protective gas atmosphere, in particular an inert gas atmosphere. According to a preferred embodiment of the present invention, in particular the entire process step (b) is carried out in a protective gas atmosphere, in particular an inert gas atmosphere, wherein it is particularly preferred if both process steps (a) and (b) are carried out in a protective gas atmosphere, in particular an inert gas atmosphere become. In the context of the present invention, a protective gas is to be understood as meaning a gas which effectively prevents the oxidation of the components of the carbon- and silicon-containing solution or dispersion by, in particular, atmospheric oxygen, while an inert gas in the context of the present invention is a gas which is compatible with the constituents of the carbon- and silicon-containing solution or dispersion undergoes no reaction under process conditions. For example, although nitrogen is to be used as protective gas in the context of the present invention, it is not an inert gas, since gaseous nitrogen, in particular in the form of nitrides, can be incorporated into the silicon carbide structure. However, if doping with nitrogen is desired, it is also possible to carry out the process according to the invention in a nitrogen atmosphere. In the context of the present invention, the protective gas is usually selected from noble gases and nitrogen and mixtures thereof, in particular argon and nitrogen and mixtures thereof. In the context of the present invention, it is particularly preferred for the protective gas to be argon.

In the context of the present invention, moreover, provision may likewise be made for the substrate to be removed after the thermal treatment, in particular following process step (b). If the substrate is removed in the context of the present invention, it has proven useful if the substrate is removed by oxidation. The substrate is usually removed thermally or chemically, in particular by thermal or chemical oxidative removal. In this connection, particularly good results are obtained if the substrate is removed in an oxygen atmosphere, by means of ozone and / or by the action of aqueous hydrogen peroxide solution. In the oxidative removal in an oxygen atmosphere, the substrate is burned, so to speak, which is particularly suitable for substrates based on graphite.

The removal of the substrate in particular allows the production of nearly two-dimensional silicon carbide bodies or of silicon carbide wafers.

According to a preferred embodiment of the present invention, the present invention is a process for producing thin layers of silicon carbide, in particular as described above, wherein

(a) a liquid carbon- and silicon-containing solution or dispersion, in particular a SiC precursor sol, is applied to a substrate in a first method step,

(b) in a second process step following the first process step (a), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is converted into silicon carbide, in particular subjected to a thermal treatment, wherein

(I) in a first thermal process step, the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, to temperatures in the range of 800 to 1 .200 ° C, in particular 900 to 1 .100 ° C, preferably 950 to 1. 050 ° C, is heated and

(ii) in a second process stage subsequent to the first thermal process stage (i), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is heated to temperatures of 1,800 ° C. or higher, and

(c) the substrate is removed in a third method step following the second method step (b). In general, the thickness of the wafer over the layer thickness of the liquid carbon- and silicon-containing solution or dispersion, in particular of the SiC precursor sol, is determined in this context. For producing silicon carbide wafers with particularly large thicknesses, the process steps (a) and (b) are repeated until a wafer of the desired thickness is obtained. In this way, single crystal silicon carbide wafers of almost any thickness can be easily obtained. According to a preferred embodiment of the present invention, the method steps (a) and (b) are repeated, wherein in each passage different carbon- and silicon-containing solutions or dispersions, in particular the SiC precursor sols, preferably with different doping reagents and / or different concentrations of doping reagents, be used. By using different carbon- and silicon-containing solutions or dispersions, in particular the SiC precursor sols, in the respective performance of process steps (a) and (b) semiconductor materials with layer-by-layer different electronic properties can be obtained, which can be used as base materials for electronic components , For example, layer sequences with pn doping for diodes and pnp or npn dopings can be used as base materials for bipolar transistors.

In this preferred embodiment of the present invention, all advantages, features and particular and preferred embodiments of the present invention can also be applied.

Another object of the present invention - in accordance with an aspect of the present invention - is a composition, in particular a SiC precursor sol, in the form of a solution or dispersion

(A) at least one silicon-containing compound,

(B) at least one carbon-containing compound,

(C) at least one solvent or dispersant; and

(D) optionally doping reagents.

The composition according to the invention is particularly suitable for use as a carbon- and silicon-containing solution or dispersion, in particular SiC Precursorsolen, in the inventive method for producing thin layers of silicon carbide.

As far as the selection of the solvent or dispersant in the composition according to the invention is concerned, this can be selected from all suitable solvents or dispersants. Usually, however, the solvent or dispersing agent is selected from water and organic solvents and mixtures thereof. In particular in the case of mixtures which comprise water, the starting compounds, which are generally hydrolyzable or sovolyzable, are converted to inorganic hydroxides, in particular metal hydroxides and silicic acids, which subsequently condense, so that precursors suitable for pyrolysis and crystallization are obtained.

The compounds used should moreover have sufficiently high solubilities in the solvent used, in particular in ethanol and / or water, in order to be able to form finely divided dispersions of the solutions, in particular sols, and must not be mixed with other constituents of the solution or dispersion during the preparation process. especially the sol, react to insoluble compounds.

In addition, the reaction rate of the individual reactions occurring must be coordinated, since the hydrolysis, condensation and optionally gelation of the composition according to the invention should, if possible, proceed undisturbed in order to obtain the most homogeneous possible distribution of the individual constituents in the sol.

Furthermore, the formed reaction products must not be sensitive to oxidation and, moreover, should not be volatile. In the context of the present invention, it may be provided that the organic solvent is selected from alcohols, in particular methanol, ethanol, 2-propanol, acetone, ethyl acetate and mixtures thereof. In this context, it is particularly preferred if the organic solvent is selected from methanol, ethanol, 2-propanol and mixtures thereof, with ethanol in particular being preferred. The abovementioned organic solvents are miscible with water over a wide range and, in particular, are also suitable for dispersing or for dissolving polar inorganic substances, for example metal salts. As stated above, in the context of the present invention, mixtures of water and at least one organic solvent, in particular mixtures of water and ethanol, are preferably used as solvents or dispersants. In this connection it is preferred if the solvent or dispersing agent has a weight-related ratio of water to organic solvent of 1:10 to 20: 1, in particular 1: 5 to 15: 1, preferably 1: 2 to 10: 1, preferably 1 : 1 to 5: 1, more preferably 1: 3. On the one hand, the rate of hydrolysis, in particular of the silicon-containing compound and of the doping reagents, can be adjusted by the ratio of water to organic solvents; on the other hand, the solubility and reaction rate of the carbon-containing compound, in particular of the carbonaceous precursor compound, such as, for example, sugar, can be adjusted.

The amount in which the composition contains the solvent or dispersant may vary widely depending on the particular application conditions and the nature of the doped or undoped silicon carbide to be produced. Usually, however, the composition comprises the solvent or dispersant in amounts of 10 to 80 wt .-%, in particular 15 to 75 wt .-%, preferably 20 to 70 wt .-%, preferably 20 to 65 wt .-%, based on the composition, on.

In the context of the present invention, it is usually provided that the composition has a weight-related ratio of silicon to carbon, in particular in the form of the silicon-containing compound and the carbon-containing compound, in the range from 1: 1 to 1:10, in particular 1: 2 to 1: 7 , preferably 1: 3 to 1: 5, preferably 1: 3.5 to 1: 4.5. Particularly good results are obtained in this connection if the composition has a silicon-to-carbon weight ratio, in particular of silicon-containing compound to carbonaceous compound, of 1: 4.

As far as the silicon-containing compound is concerned, it is preferred if the silicon-containing compound is selected from silanes, silane hydrolyzates, orthosilicic acid and mixtures thereof, in particular silanes. Orthosilicic acid As their condensation products can be obtained in the context of the present invention, for example, from alkali metal silicates whose alkali metal ions have been exchanged by ion exchange for protons. In the context of the present invention, alkali metal compounds are as far as possible not used in the composition since they are also incorporated into the silicon carbide-containing compound. However, alkali metal doping is generally undesirable in the context of the present invention. However, if desired, suitable alkali metal salts, for example, the silicon-containing compounds or alkali phosphates, may be used.

When a silane is used as the silicon-containing compound in the context of the present invention, it has proven useful if the silane is selected from silanes of the general formula I.

R ₄ -nSiX _n (I)

With

R = alkyl, in particular C ₁ - to C 6 -alkyl, preferably C _{1 to} C ₃ -alkyl, preferably C ₁ - and / or C ₂ -alkyl;

Aryl, in particular C ₆ - to C ₂ o-aryl, preferably C ₆ - to C ₅ -aryl, is preferred

Olefin, in particular terminal olefin, preferably C ₂ - to C-io-olefin, preferably C ₂ - to Cs-olefin, more preferably C ₂ - to C ₅ olefin, most preferably C ₂ - and / or C3-olefin, particularly preferably vinyl; Amine, in particular C ₂ - to do-amine, preferably C ₂ - to Cs-amine, preferably C ₂ - to C ₅ -amine, more preferably C ₂ - and / or C ₃ -amine;

Carboxylic acid, in particular C ₂ - to Cm-carboxylic acid, preferably C ₂ - to Cs-carboxylic acid, preferably C ₂ - to C ₅ carboxylic acid, particularly preferably C ₂ - and / or C ₃ -carboxylic acid;

Alcohol, in particular C ₂ - to C 6 -alcohol, preferably C ₂ - to Cs-alcohol, preferably C ₂ - to C ₅ -alcohol, more preferably C ₂ - and / or C ₃ -alcohol;

X = halide, in particular chloride and / or bromide;

Alkoxy, in particular Cr to C6-alkoxy, particularly preferably Cr to C ₄ - alkoxy, very particularly preferably Cr and / or C ₂ -alkoxy; and

n = 1 - 4, preferably 3 or 4.

However, particularly good results are obtained when the silane is selected from silanes of the general formula Ia R ₄ -nSiX _n (la) with

R = C ₁ to C ₃ alkyl, in particular C ₁ and / or C ₂ alkyl;

C ₆ - to cis-aryl, in particular C ₆ - to C 10 -aryl;

C ₂ - and / or C ₃ -olefin, in particular vinyl;

X = alkoxy, in particular C 1 -C 6 -alkoxy, particularly preferably C 1 -C ₄ -alkoxy, very particularly preferably C ₁ and / or C ₂ -alkoxy; and

n = 3 or 4.

By hydrolysis and subsequent condensation reaction of the silanes mentioned above can be obtained in the present invention in a simple manner condensed ortho silicic acids or siloxanes, which have only very small particle sizes, with other elements, in particular metal hydroxides can be incorporated into the skeleton.

Through the use of carbon- and silicon-containing solutions or dispersions, in particular of SiC precursor sols, it is possible within the scope of the present invention to arrange the components of the silicon carbide to be produced as homogeneously as possible adjacent to each other in a homogeneous and fine distribution, so that when energy is applied individual constituents of the silicon carbide-containing target compound are in the immediate vicinity of one another and do not have to diffuse for comparatively long distances.

Particularly good results are obtained in the context of the present invention, when the silicon-containing compound is selected from tetraalkoxysilanes, trialkoxysilanes and mixtures thereof, preferably tetraethoxysilane, tetramethoxysilane or triethoxymethylsilane and mixtures thereof.

As far as the amounts in which the composition contains the silicon-containing compound, this may also vary widely depending on the particular conditions of use. Usually, however, the composition contains the silicon-containing compound in amounts of 1 to 80% by weight, in particular 2 to 70% by weight, preferably 5 to 60% by weight, preferably 10 to 60% by weight, based on the composition , on.

As stated above, the composition according to the invention contains at least one carbon-containing compound. Come as a carbonaceous compound all compounds into consideration, which can either dissolve in the solvents used or at least finely dispersed and can release solid carbon in the pyrolysis. The carbonaceous compound is likewise preferably capable of reducing metal hydroxides to elemental metal under process conditions.

It has been found in the context of the present invention, when the carbon-containing compound is selected from the group of sugars, in particular sucrose, glucose, fructose, invert sugar, maltose; Strength; Starch derivatives; organic polymers, in particular phenol-formaldehyde resin and resorcinol-formaldehyde resin, and mixtures thereof.

Particularly good results are obtained in the context of the present invention, when the carbon-containing compound is selected from the group of sugars; Starch, starch derivatives and mixtures thereof, preferably sugars, since the viscosity of the composition can be specifically adjusted in particular by the use of sugars and starch or starch derivatives.

As far as the amount in which the composition contains the carbon-containing compound, this may also vary widely depending on the respective application and application conditions or the target compounds to be prepared. Usually, however, the composition contains the carbonaceous compound in amounts of 5 to 50 wt .-%, in particular 10 to 40 wt .-%, preferably 10 to 35 wt .-%, preferably 12 to 30 wt .-%, based on the composition ,

In the context of the present invention, the composition optionally has a doping reagent. When the composition comprises a doping reagent, the composition usually has the doping reagent in amounts of 0.000001 to 15% by weight, in particular 0.000001 to 10% by weight, preferably 0.000005 to 5% by weight, more preferably 0.00001 to 1 wt .-%, based on the solution or dispersion on. The addition of doping reagents can decisively change the properties of the resulting silicon carbide. If the silicon carbide is to be doped with nitrogen, then, for example, nitric acid, ammonium chloride or melamine can be used as doping reagents. In the case of nitrogen, moreover, it is also possible to carry out the additive manufacturing process in a nitrogen atmosphere, wherein also doping with nitrogen can be achieved, but which are less accurate to adjust. Further doping reagents are mentioned in particular in connection with the description of the method according to the invention.

For further details of the composition according to the invention, to avoid unnecessary repetition, reference may be made to the above statements regarding the method according to the invention, which apply correspondingly with regard to the composition according to the invention.

Finally, a further subject matter of the present invention, according to one aspect of the present invention, is a silicon carbide layer, in particular a monocrystalline silicon carbide layer, obtainable by the method described above and / or using the composition described above.

For further details on the silicon carbide layer according to the invention, reference may be made to the above statements on the further aspects of the invention, which apply correspondingly with regard to the silicon carbide layer according to the invention.

Claims

claims:

Method for producing thin layers of silicon carbide,

characterized,

that

(b) in a second process step following the first process step (a), the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is converted into silicon carbide.

2. The method according to claim 1, characterized in that in the first process step (a) the carbon- and silicon-containing solution or

Dispersion, in particular the SiC precursor sol, as a layer, in particular as a homogeneous layer, is applied to the substrate.

3. The method according to claim 1 or 2, characterized in that in process step (a) the carbon- and silicon-containing solution or dispersion, in particular the SiC Precursorsol, by a coating process, in particular by dipping (dipcoating), spin coating, Spraying, rolling or rolling is applied to the substrate, preferably by dipping, spin coating or spraying, preferably by dipping.

4. The method according to any one of the preceding claims, characterized in that in process step (a) the carbon- and silicon-containing solution or dispersion, in particular the SiC Precursorsol, with a layer thickness in the range of 0.1 to 1, 000 μιη μιη, in particular 0 , 5 to 500 μιη, preferably 0.8 to 300 μιη, preferably 1 to 100 μιη, is applied to the substrate. Method according to one of the preceding claims, characterized in that the carbon- and silicon-containing solution or dispersion, in particular the SiC Precursorsol, a dynamic viscosity according to Brookfield at 25 ° C in the range of 3 to 500 mPas, in particular 4 to 200 mPas, preferably 5 to 100 mPas.

Method according to one of the preceding claims, characterized in that the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol,

(A) at least one silicon-containing compound,

(B) at least one carbonaceous compound and

(C) at least one solvent or dispersant,

contains.

Method according to one of the preceding claims, characterized in that substrate is selected from crystalline and amorphous substrates, in particular amorphous substrates.

Method according to one of the preceding claims, characterized in that the material of the substrate is selected from carbon, in particular graphite, and ceramic materials, in particular silicon carbide, silicon dioxide, aluminum oxide and metals and mixtures thereof.

Method according to one of the preceding claims, characterized in that in step (b) the carbon- and silicon-containing solution or dispersion, in particular the SiC Precursorsol, after application to the substrate of a thermal treatment, in particular a multi-stage thermal treatment is subjected.

A method according to claim 9, characterized in that during thermal treatment

(i) in a first thermal process stage, the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, at temperatures in the range from 800 to 1,200 ° C., in particular 900 to 1 .100 ° C, preferably 950 to 1 .050 ° C, is heated and

A method according to claim 10, characterized in that in process step (i) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, is transferred into a glass.

A method according to claim 10 or 1 1, characterized in that in process step (ii) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, preferably the glass obtained in process step (i), is converted into crystalline silicon carbide.

Method according to one of claims 10 to 12, characterized in that subsequent to process step (i) and before carrying out process step (ii) the carbon- and silicon-containing solution or dispersion, in particular the SiC precursor sol, preferably in process step (i) obtained glass, cooled, in particular quenched.

Method according to one of the preceding claims, characterized in that following the thermal treatment, in particular following the step (b), the substrate is removed.

Method according to one of the preceding claims, characterized in that the method steps (a) and (b) are repeated, in particular wherein in each passage different carbon and silicon-containing solutions or dispersions, in particular SiC precursor sols, preferably with different doping reagents and / or different Concentrations of doping reagents are used.

16. Composition, in particular SiC precursor sol, in the form of a solution or dispersion containing

(A) at least one silicon-containing compound,

(B) at least one carbon-containing compound,

(C) at least one solvent or dispersant; and

(D) optionally doping reagents.

17. The composition according to claim 16, characterized in that the solvent or dispersant is selected from water and organic solvents and mixtures thereof.

18. Composition according to claim 16 or 17, characterized in that the composition has a weight-related ratio of silicon to carbon in the range of 1: 1 to 1:10, in particular 1: 2 to 1: 7, preferably 1: 3 to 1 : 5, preferably 1: 3.5 to 1: 4.5.

19. The composition according to any one of claims 16 to 18, characterized in that the silicon-containing compound is selected from silanes, silane hydrolysates, orthosilicic acid and mixtures thereof, in particular silanes.

20. Composition according to one of the preceding claims, characterized in that the carbon-containing compound is selected from the group of sugars, in particular sucrose, glucose, fructose, invert sugar, maltose; Strength; Starch derivatives; organic polymers, in particular phenol-formaldehyde resin and resorcinol

Formaldehyde resin, and mixtures thereof.

21. Silicon carbide layer, in particular monocrystalline silicon carbide layer, obtainable by a process according to one of Claims 1 to 15 and / or using a composition according to one of Claims 1 to 15

Claims 16 to 20.