Process for the preparation of halogenated oligosilanes from silicon and tetrachlorosilane

Abstract

Process for the preparation of halogenated oligosilanes Si n X 2n+2 with n = 2 to n = 6 as a pure compound or mixture of compounds whose substituents comprise chlorine or chlorine and hydrogen, characterized in that (a) the starting materials for the process comprise elemental silicon, SiCl4 and elemental hydrogen in at least a catalytic amount, b) the HCl gas produced in the process is recycled to more than 5%, preferably more than 20%, particularly preferably more than 50%, in particular more than 80%, into the process, c) the process comprises a reaction of elemental silicon with HCl as catalyst to form chlorinated oligosilanes, the reaction being carried out by a two-stage catalytic reaction step, d) the process comprises a plasma-chemical synthesis step which is operated in a pressure range of 0.1 hPa to 100 hPa, preferably 0.5 hPa to 30 hPa, particularly preferably 1 hPa to 10 hPa, in particular 1.5 hPa to 5 hPa, e) at least one further silane is used as starting material and / or intermediate, selected from the group of hydrogenated silanes, which includes HSiCl3, HSi2Cl5 and H2SiCl2, f) the reactants or the reactant mixture of the plasma-chemical synthesis step contain less than 1 atom% of C as such or in the form of its compounds and g) the process comprises at least one further step selected from a group comprising distillation, chlorination and thermolysis.

Description

[0001] The present invention relates to a process for the preparation of halogenated oligosilanes as a pure compound or mixture of compounds each having at least one direct Si-Si bond, the substituents of which consist exclusively of halogen or of halogen and hydrogen and in whose composition the atomic ratio of substituent to silicon is at least 1:1. State of the art

[0002] Other processes for the preparation of halogenated oligosilanes are known in the art: [Reference: M. Schmeisser, P. Voss “On silicon chloride [SiCl2] x", Z. anorg. allg. Chem. (1964) 334, 50-56 (Schmeisser, 1964)]. On the one hand, they can be produced by purely thermal reactions (Schmeisser, 1964) by heating vaporous chlorosilanes with or without reducing agents to high temperatures (over 1000°C). In this case, chlorinated polysilanes (PCS) are obtained, which have a color ranging from pale greenish-yellow to yellowish-light brown (Schmeisser, 1964; "glassy, ​​highly polymeric").

[0003] GB 702 349 A discloses a process whereby a mixture of chlorinated polysilanes is condensed from the gas stream during the reaction of silicon alloys with chlorine gas at 190-250°C.

[0004] DE 31 26 240 C2 describes the wet-chemical production of chlorinated polysilanes from Si2Cl6 by reaction with a catalyst. The resulting mixtures still contain the catalyst and are therefore washed with organic solvents.

[0005] DE 10 2005 024 041 A1 describes the production of silicon in a two-step process. First, a polysilane is generated using plasma chemistry, which is then decomposed into high-purity silicon by thermolysis.

[0006] WO 2008 / 009473 A1 describes a process for the production of silicon or hydrogenated or organically substituted oligosilanes, wherein the desired precursors are selectively produced in a plasma-chemical step and immediately subjected to further processing by distillation, hydrogenation or methylation. In DE 10 2008 025 261 B4

[0007] Halogenated polysilanes and a plasma-chemical process for their preparation are described. The polysilanes described have an average chain length of n = 9 to 20 and are described with regard to their spectroscopic and other physical properties.

[0008] Furthermore, DE 10 2008 025 260 B4 describes halogenated polysilanes and a thermal process for their preparation. The described polysilanes have an average chain length of n = 3 to 9 and are described with regard to their spectroscopic and other physical properties. The resulting halogenated polysilane mixtures are liquid and have an average chain length of n = 3 to 9. Due to their preparation from Si and SiX4, they are intrinsically free of hydrogen.

[0009] Furthermore, US 2010 / 0 080 746 A1 describes the production of Si2Cl6 and Si3Cl8 by plasma-chemical reaction of SiCl4 with a monosilane containing hydrogen in bound form, e.g., as a Si-H group or as a methyl group. Elemental hydrogen is explicitly excluded as a reducing agent. US 2012 / 0 189 501 A1, in turn, describes the production of polycrystalline silicon in essentially closed-loop processes and systems. The processes and systems generally comprise the disproportionation of trichlorosilane to produce silane or dichlorosilane and the thermal decomposition of silane or dichlorosilane to produce polycrystalline silicon. Disadvantages of the state of the art

[0010] For the OCS produced in [Schmeisser 1964], spectroscopic studies have shown that such purely thermally produced polysilanes contain a high proportion of short-chain, branched, and cyclic molecules. Furthermore, the resulting mixture is heavily contaminated with AlCl3 due to the manufacturing process (very high temperatures). Furthermore, the yields per unit time are very low because the process is carried out under high vacuum. Due to the very high temperatures, the resulting polysilanes are also heavily contaminated with disiloxanes, which are formed by wall reactions between the silicon and the ceramic tubes. Due to the polymeric structure of the PCS, purification by distillation is not possible at this stage, which is why all impurities are carried over into any subsequent reactions and products.Even if Si2Cl6 is produced as the final product, volatile contaminants such as hexachlorodisiloxane Si2Cl6O and AlCl3 are very difficult to remove due to their relatively high boiling point (144°C). The Al content must be reduced to the ppbw range, for example, for semiconductor applications.

[0011] The process described in GB 702,349 has the disadvantage that the average molecular weight of these mixtures is relatively low, since distillation yields only 2% of the silanes with n greater than 6. Therefore, the main disadvantage of this process is the low yield of the desired products.

[0012] The mixtures obtained from DE3126240C2 still contain the catalyst and are therefore washed with organic solvents, leaving traces of these solvents and the catalyst. Furthermore, the PCS obtained in this way are highly branched. This publication is mentioned only for the sake of completeness, since the products according to the invention only appear as reactants here, i.e., the objective is the formation of long-chain PCS and are therefore different. It would make little (economic) sense to decompose the PCS obtained in this way back into short-chain OCS, since Si2Cl6 would be far too expensive as a precursor.

[0013] The process from DE102005024041A1 also serves only as an overview, since the objective here is the production of high-purity silicon for semiconductor applications. While the intermediate PCS could be used to produce the OCS according to the invention, this would be considerably more expensive. First, stoichiometric amounts of SiCl4 and hydrogen are used as reactants. Second, the SiCl4 must be purchased in high purity for the reasons mentioned above. Purification of the product would be very complex, while this is practically impossible at the PCS stage.

[0014] The process of WO2008009473 also requires stoichiometric amounts of highly pure hydrogen and SiCl4 as starting compounds. Furthermore, this process is unfavorable for the production of chlorinated oligosilanes, since, although it is mentioned that these can be separated from the mixture, it is not intended to specifically convert the mixture into these compounds. Rather, WO2008009473 envisages distillations and derivatizations, but not the conversion of the medium- or high-molecular-weight fractions and bottoms into chlorinated oligosilanes.

[0015] The focus of this application is therefore on the production of silicon, hydrogenated polysilanes and organically functionalized polysilanes.

[0016] The process described in DE102008025261B4 also requires stoichiometric amounts of highly pure hydrogen and SiCl4 as starting compounds. The production of chlorinated oligosilanes is not mentioned. If any are obtained in the described process, they are only byproducts, since the average chain length of the mixture is longer (n=9 to 20) than would be expected for oligosilanes (n=2 to 8).

[0017] The process of DE102008025260B4 utilizes the direct synproportionation reaction between silicon and SiX4 to produce halogenated polysilanes. Due to the high temperatures (>700°C), wall reactions are unavoidable, leading to contamination in the resulting polysilane mixtures. Examples of such reactions include metal compounds or oxygen compounds of silicon (siloxanes), which originate from the oxide layer always present on the silicon used or, if oxidic reactor materials (e.g., quartz glass) are used, are formed by reaction with the latter. A further disadvantage of this process is that the intermediately formed dihalosilylene SiX2 easily decomposes again in the reverse of the formation reaction to form silicon and SiX4, so that the product vapor must be cooled as quickly as possible to below the decomposition temperature (approx.280°C) to avoid significant yield losses, which presents significant technical problems, since the temperature zone in which the reverse reaction takes place can be reduced, but it can never be completely eliminated. The process of US2010080746A1 has the disadvantage that at least two high-purity chlorosilanes are consumed stoichiometrically, which leads to high costs on the reactant side. This is due in particular to the fact that elemental hydrogen is used as a reducing agent in the plasma-chemical step. Table 1: Abbreviations and synonyms Abbreviations reactants Starting compounds for a chemical reaction Starting materials Starting compounds for a chemical reaction Precursors Starting material for the process under consideration OCS chlorinated oligosilanes PCS chlorinated polysilanes Si2Cl6 Hexachlorodisilane (HCDS) Si2Cl6O Hexachlorodisiloxane (HCDSO) ppbw Parts per billion by weight (10 -9 )) Sputter effect Material removal by (charged) high-energy particles HCl Hydrochloric acid gas hPa Hectopascal≡1mbar sccm Standard cubic centimeter MHz Megahertz

[0018] should be avoided. Furthermore, the use of the preferred methylated chlorosilanes such as CH3SiCl3 as reducing agents leads to contamination of the products with traces of organic substances, which is particularly critical for potential applications in the semiconductor sector. Task

[0019] The aim of the process is to provide a method for producing halogenated oligosilanes that achieves very high yields of OCS with high energy efficiency while using minimal material and cost. At the same time, the process should enable the production of halogenated oligosilanes of very high purity, particularly with regard to contamination by metal compounds and other elements harmful for semiconductor applications. Solution to the problems of the state of the art

[0020] This object is achieved by the features listed in claim 1 Process for the preparation of halogenated oligosilanes Si n X 2n+2 with n=2 to n=6 as a pure compound or mixture of compounds whose substituents comprise chlorine or chlorine and hydrogen, characterized in that (a) the starting materials for the process comprise elemental silicon, SiCl4 and elemental hydrogen in at least a catalytic amount, b) the HCl gas produced in the process is recycled to more than 5%, preferably more than 20%, particularly preferably more than 50%, in particular more than 80%, into the process, c) the process comprises a reaction of elemental silicon with HCl as catalyst to form chlorinated oligosilanes, the reaction being carried out by a two-stage catalytic reaction step, d) the process comprises a plasma-chemical synthesis step which is operated in a pressure range of 0.1 hPa to 100 hPa, preferably 0.5 hPa to 30 hPa, particularly preferably 1 hPa to 10 hPa, in particular 1.5 hPa to 5 hPa, e) at least one further silane is used as starting material and / or intermediate, selected from the group of hydrogenated silanes, which includes HSiCl3, HSi2Cl5 and H2SiCl2, f) the reactants or the reactant mixture of the plasma-chemical synthesis step contain less than 1 atom% of C as such or in the form of its compounds, g) the process comprises at least one further step selected from a group comprising distillation, chlorination and thermolysis, solved.

[0021] The advantages achieved with the process according to the invention for the synthesis of chlorinated oligosilanes consist in particular in the fact that the synproportionation reaction of silicon and SiCl4 is utilized for their production. In the prior art, temperatures of approximately 1200°C are required for this purpose, whereas in the present process, through the development of a suitable HCl-catalyzed reaction cycle, this reaction can be carried out at approximately 300°C, while at the same time, a sufficiently rapid conversion to the products according to the invention is enabled. This new catalytic reaction cycle enables comparatively very high yields of OCS with overall high energy efficiency. This is achieved, among other things, byThis is made possible because the use of the catalytic cycle avoids the direct thermal reduction of SiCl4 with silicon, which is an equilibrium reaction due to the very high reaction temperatures required. This also largely excludes the reverse reaction to the reactants, which is noticeable, among other things, in a significant increase in yield. Furthermore, the mild reaction conditions maintained throughout the process avoid contamination of the product. Mild conditions here primarily mean comparatively low temperatures (<400°C) and low electron energies, whereby mobilization and / or carry-over of contamination from the reactor materials and / or reactants can be largely excluded. In particular, sputtering effects can be largely excluded by using low electron energies in the process steps according to the invention.The process according to the invention is described by the following idealized overall reaction equation:. n / 2 SiCl4 + n / 2 Si + Cl2 → Si n Cl 2n+2

[0022] The specified chlorine gas is not essential for the process, but increases Si depending on the desired product n Cl 2n+2 the yield of the overall reaction. Therefore, it is stated as the ideal case of the process according to the invention, but is not intended to be limited to this. Furthermore, it is also possible to use other chlorinating compounds instead of chlorine for the stated purpose without departing from the disclosed process principle of the process according to the invention. Operating at low temperatures below 400°C has the further advantage that inexpensive and low-contamination reactor materials such as quartz glass or even laboratory glass can be used, while still achieving long reactor service lives.

[0023] This is achieved in the process according to the invention by the fact that the actual redox process, in which according to the simplified reaction equation n / 2 SiCl4 + n / 2 Si → Si n Cl 2n chlorinated oligosilanes of the formula Si n Cl 2nformed, is replaced by a two-stage catalytic reaction step. While the direct reaction of Si with SiCl4 requires very high temperatures of over 1000°C to achieve a technically viable reaction rate, the catalytic step using HCl as catalyst means that the process proceeds quickly and with high yields at temperatures below 400°C, preferably below 300°C, since the activation energy for the reaction of SiCl4 with Si is formally greatly reduced. Technically, this is achieved by not reacting the silicon directly with SiCl4, but first with the catalyst HCl gas, which reacts the silicon at typically 250 to 300°C according to the following idealized reaction equation. 2 Si + 7HCl -> HSiCl3 + SiCl4 + 3H2

[0024] As can be seen from the equation above, hydrogen is also produced in this sub-step, which is then consumed again in the next sub-step (see below). The first sub-step above can also be carried out at temperatures different from these without losing the essence of the catalytic effect. However, an increase in temperature above 400°C is generally not desirable, as this reduces reaction efficiency and increases the risk of contamination in the product. For example, wall reactions with the reactor material or mobilization of contaminants from the silicon used increase with increasing temperature, and these can be carried over into the final product. It is therefore advisable to carry out this catalytic step at the lowest possible temperature so that rapid conversion is just about guaranteed.The HCl catalyst is then regenerated from the intermediate mixture of HSiCl3, SiCl4, and H2 in a plasma-chemical step, releasing the desired chlorinated oligosilanes. The simplified reaction equations for SiCl4 are: n SiCl4 + (2n-2) / 2 H2 → Si n Cl 2n+2 + (2n-2) HCl, and for HSiCl3 (n-1) HSiCl3 + SiCl4 → Si n Cl 2n+2 + (n-1) HCl.

[0025] In order to ensure the highest possible efficiency of the overall process, the aim is to keep the HCl in the catalytic cycle as much as possible so that it can be reused in the first sub-step. However, certain technically induced losses may have to be accepted if, for example, the separation of the chlorinated oligosilanes according to the invention is not complete, as this would be too complex from an industrial perspective. In this plasma-chemical step, the hydrogen produced further up from sub-step one is used up again. This means that the hydrogen also acts as a catalyst in the present process and can be reused almost entirely in the catalytic reaction cycle. However, in technical implementation, certain losses may have to be accepted, particularly in the case of hydrogen, since 100% recycling requires a certain amount of effort due to its very high volatility.The exemplary embodiments present a variant of the process according to the invention in which almost complete recycling of the hydrogen (and HCl) is possible. Those skilled in the art will readily recognize that even with only partial use of the HCl and H2 catalysts in the catalytic cycle, e.g., due to a non-optimized reaction device, the essence of the process according to the invention is nevertheless retained.

[0026] The process according to the invention provides a further sub-step for extracting the desired halogenated oligosilanes for commercial use from the mixture of halogenated oligosilanes. This third step of the process comprises at least one distillation, or preferably a thermolysis and distillation, or particularly preferably a chlorination and distillation. By correctly selecting the third process step, the yield of the desired halogenated oligosilane can be significantly increased again. Furthermore, it is determined which halogenated oligosilane or oligosilane mixture is to be obtained. The silanes SiCl4 and HSiCl3, which arise as minor by-products in this final process step, are expediently reintroduced directly into the cyclic process so that almost complete conversion of the reactants SiCl4 and silicon can be achieved.Technically induced losses of the H2 and HCl catalysts must be compensated by adding catalytic amounts of H2. This is conveniently done in sub-step two (plasma-chemical step), but can also be done elsewhere, as a cyclic process is involved. The second HCl catalyst is then automatically generated in the amount appropriate for the process. Control or dosing of the total HCl quantity in the cyclic process is therefore unnecessary.

[0027] Furthermore, the starting materials used should not contain compounds such as methylsilanes (e.g., MeSiCl3, etc.) of elements such as carbon, which are harmful for use in the electronics industry, or contain them in significant quantities. "Significant" here refers to quantities exceeding the single-digit percentage range.

[0028] The establishment of the catalytic cycle in the process according to the invention is also particularly important with regard to the efficiency of the overall process, as it promotes the formation of the intermediate HSiCl3, which is formed in step one of the cyclic process. This intermediate, as H-silane, in turn supports the formation of SiCl2 in the plasma-chemical step two of the process. As described above, SiCl2 is the actual (highly reactive) primary product in the synproportionation of SiCl4 and silicon, whereby its formation formally closes the catalytic cycle (SiCl4 + Si + cat. → 2 [SiCl2]). The SiCl2 then undergoes oligomerization and insertion reactions, as illustrated below with the formation of hexachlorodisilane. SiCl4 + SiCl2 → Si2Cl6 to the desired end products or product mixtures. Those skilled in the art will readily recognize that HSiCl3 and other H-silanes such as H2SiCl2 can also be added externally to the cyclic process as reactants together with SiCl4, and can also partially replace the latter without altering the nature of the process according to the invention. Such partial replacement of SiCl4 by other silanes is generally undesirable, since SiCl4 is usually the cheapest chlorosilane, meaning that (partial) replacement would only be economically viable if the alternative chlorosilane were available at such low cost that a cost advantage could be achieved with respect to the overall process.

[0029] An advantageous embodiment is given by claim 2: Method according to claim 1, characterized in that X in Si n X 2n+2} is more than 95 atom% chlorine, preferably more than 98 atom% and / or the hydrogen content in Si nX 2n+2 less than 5 atom%, preferably less than 2 atom%, particularly preferably less than 1 atom%. By restricting the substituents to chlorine atoms, products are obtained that are already partially commercially available and used today (e.g., Si2Cl6, Si3Cl8).

[0030] A further advantageous embodiment is given by claim 3: A process according to claim 1, characterized in that the reaction of elemental silicon with HCl takes place at 200 to 800°C, preferably at 230 to 500°C, particularly preferably at 250 to 350°C, in particular at 270 to 300°C. A particular advantage of the process according to the invention results from the fact that no high temperatures (>1000°C) occur during the entire process. This leads to significantly reduced corrosion of the reactor materials and thus to longer service lives and lower contamination in the products. As the person skilled in the art will readily recognize, metallurgical silicon can also be replaced in this reaction step by other types of silicon, such as ferrosilicon or semiconductor silicon, or by other compounds that can function as silicon sources under the reaction conditions according to the invention, such as silicides, without departing from the essence of the invention. Furthermore, other catalysts can be admixed with the silicon source used, such asCopper or nickel.

[0031] A further advantageous embodiment is given by claim 4: A process according to claim 1, characterized in that the process comprises a chlorination step with elemental chlorine. The third sub-step of the process according to the invention can advantageously be designed as a chlorination with subsequent distillation. Depending on the desired product, this may significantly increase the yield again, for example if longer-chain OCS are cleaved to the desired shorter-chain OCS. If longer-chain OCS (e.g., SisCl 12 ) are desired, it may be advisable to use thermolysis with distillation or, if necessary, only distillation as a third step and, if necessary, further purification steps. A further advantageous embodiment is

[0032] Claim 5 given: Method according to claim 1, characterized in that the plasma-chemical step comprises elemental hydrogen as a reducing agent and that the hydrogen gas produced in the method is returned to the process to an extent of more than 5%, preferably to an extent of more than 20%, particularly preferably to an extent of more than 50%, in particular to an extent of more than 80%.

[0033] Particularly important with regard to product purity is that a reducing agent is used in the third plasma-chemical step that is inexpensive, high in purity, and that also does not release any elements critical for semiconductors from its molecular structure. Furthermore, it must be suitable for releasing the HCl catalyst from the intermediate products. Therefore, elemental hydrogen is the best choice for this task, especially since it is formed anyway in the first step and therefore no additional compounds are introduced into the cyclic process. Bound hydrogen, e.g., in the form of organic substances or molecular groups, is less suitable, as contamination from carbon or organic residues in the product is to be expected.

[0034] Furthermore, the process according to the invention opens up the possibility for the first time of establishing a closed cycle process, in the sense that, in principle, in addition to the introduction of the reactants, only products need to be discharged from the cycle process, while the reactants can be completely converted into products if the basic idea is consistently implemented. This is possible because the principle of the present process functions without any by-products or co-products, but rather because the by-products commonly found in the prior art, such as SiCl4, metal chlorides or HCl, do not occur as such. HCl acts, as described above, as a catalyst and is ideally repeatedly recycled, so that its disposal can be practically completely eliminated. In technical implementation, one mayOne might not opt ​​for complete recycling of the HCl or H2, meaning that a certain portion still has to be disposed of. This, however, still results in a cost advantage on the disposal side, since only a corresponding fraction, rather than the entire amount, has to be disposed of, as is the case with the state of the art. This offers a further cost advantage on the reactant side, since H2 does not have to be added in stoichiometric amounts, but only to the extent necessary to compensate for the HCl losses. A further advantageous embodiment is

[0035] Claim 6 given: A process according to claim 1, characterized in that the average chain length of the oligosilanes obtained in the plasma-chemical sub-step is greater than n=3. In contrast to some competing processes in which OCS is synthesized by chlorination of silicon or silicides and primarily SiCl4 and Si2Cl6 are formed, the process according to the invention directly yields product mixtures from OCS which have longer chains, with the crude mixture having an average chain length greater than n=3. This is advantageous because the desired compounds can be obtained from this mixture by targeted chain degradation, whereas chain synthesis would not be technically worthwhile, since this would require coupling reactions (Wurtz coupling, etc.), which are expensive and would in turn introduce contamination into the product. A further advantageous embodiment is

[0036] Claim 7 given: Method according to claim 1, characterized in that the plasma chemical step is carried out using continuous electromagnetic radiation.

[0037] From an economic perspective, it is important that the excitation method used in the plasma-chemical step can be ensured cost-effectively using commercially available standard equipment, while also avoiding any licensing or shielding issues regarding emissions. Continuous electromagnetic radiation generators produce radiation of a defined frequency with high efficiency and high power, to which the necessary shielding can be easily adapted, thus ensuring compliance with emission limits. Pulsed radiation would be more difficult to shield, especially at the very high instantaneous power levels (at least several kW), since a broad band of many frequencies is emitted, especially with short pulses. Non-sinusoidal pulses, such as triangular or rectangular pulses, in particular, contain a high proportion of higher-frequency oscillations, which are increasingly difficult to shield.Furthermore, the efficiency of the process suffers because the large number of frequencies involved also results in frequencies that are less suitable for the optimal excitation of the gas mixture with regard to the formation of the desired products. A further advantageous embodiment is

[0038] Claim 8 given: Method according to claim 7, characterized in that the frequency of the electromagnetic radiation is in the range from 1 MHz to 100 MHz, preferably this is selected from a group comprising the frequency bands in the range of 13.56 MHz, 27.12 MHz and 40.68 MHz, particularly preferably 27.12 MHz and 40.68 MHz, in particular 27.12 MHz.

[0039] The frequencies listed here belong to frequency bands for which devices are available as standard in high-frequency technology. This saves costs on the investment side compared to custom-made products. A further advantageous embodiment is provided by claim 9: Method according to claim 1, characterized in that metallurgical silicon is used as the reactant in the first sub-step. This, in turn, is an advantageous embodiment for the cost-effectiveness of the method. From the point of view of possible contamination, high-purity silicon, as is used, for example,for semiconductor applications, is more suitable as a starting material, but the special process control of the catalytic cycle allows the use of metallurgical silicon, since this is not reacted directly with SiCl4 to form OCS at high temperatures as in the prior art, which results in low-volatility products that cannot be distilled, but rather the low reaction temperatures according to the invention and the reaction with the catalyst HCl initially produce intermediate products that are vaporous under the production conditions, so that impurities such as metal salts automatically remain behind due to their lower volatility and are therefore easy to separate.

[0040] A further advantageous embodiment is provided by the following features (not claimed): Apparatus for producing halogenated oligosilanes according to claim 1, comprising 1. a dosing device for adding solid silicon, 2. a reactor furnace for silicon which can be heated to at least 270°C, 3. a separation and removal device for spent silicon, 4. a pressure measurement for corrosive atmospheres, 5. an evaporator for chlorosilanes, 6. a dosing device for chlorosilane vapors, 7. a dosing device for elemental hydrogen, 8. a vacuum-compatible plasma reactor suitable for reactions at a pressure of less than 5 hPa, 9. a high-frequency generator for continuous irradiation of at least one electromagnetic frequency in the frequency range from 1 MHz to 100 MHz, 10. a device for evacuating at least the plasma reactor to below 100 hPa, 11. a discharge opening for OCS product mixtures, 12. a device for distilling at least Si2Cl6, 13. a device for circulating reaction gases between the reactor furnace and the plasma reactor.

[0041] This describes a device with the minimum components required to carry out the disclosed process. Depending on the process variant (see exemplary embodiments), the cyclic process can be closed either by means of a vacuum pump, such as a Roots pump, which circulates the reaction gases, or the material flows are circulated by temperature oscillation of the cold traps and appropriate valve settings, so that at least a portion of the reaction gases alternately pass through the plasma reactor and the reactor furnace. Industrial applicability

[0042] The process according to the invention for the preparation of halogenated oligosilanes offers a significant economic advantage, since both the yield of products and the energy efficiency can be considerably increased by establishing the catalytic cycle process according to the invention compared to the prior art.

[0043] Furthermore, an economic advantage within a process for the production of chlorinated oligosilanes (OCS) can be achieved by • Use of inexpensive precursors. Therefore, SiCl4 is a suitable commercial precursor, as it is a by-product of numerous processes in the silicon industry and silane chemistry and can therefore be purchased relatively inexpensively. Alternatively, SiCl4 can be obtained inexpensively by carbochlorination from SiO2-containing reactants, such as sand, carbon, and chlorine or HCl, according to the following idealized reaction equations. SiO2 + 2 C + 2 Cl2 → SiCl4 + 2 CO ↑ SiO2 + 2 C + 4 HCl → SiCl4 + 2 CO ↑ + 2 H2 ↑ • a reduction in the amount of SiCl4 used. This is achieved in the process according to the invention by ensuring that, in the optimal case of complete HCl recycling, half of the amount of silicon in the final products comes from the (cheap) metallurgical silicon used, thus allowing a significant saving of more expensive SiCl4.

[0044] Various solutions are possible for the technical implementation of the method, of which non-exclusive variants for the problem at hand are listed in the exemplary embodiments listed below. Example 1

[0045] The reactor of cycle 3 from Figure 1 is evacuated and then filled with H2 to a pressure of 2 hPa. 130 sccm of SiCl4 vapor (2) are then introduced into the plasma-chemical sub-step (8). A plasma is maintained by irradiating it with 1 kW of RF power at a frequency of 13.56 MHz, forming a chlorinated polysilane mixture (9) with the release of the HCl catalyst (5). In the thermal sub-step (7), the HCl gas (5) is passed through a bed of 500 g of metallurgical silicon at 300°C. The resulting mixture of the intermediate products (6) SiCl4, HSiCl3, and the H2 catalyst is then fed to the plasma-chemical sub-step 8, where it is refreshed with 130 sccm of SiCl4 vapor (2). Any pressure losses are compensated by metering in the H2 catalyst (4). In plasma-chemical sub-step 8, the catalyst HCl is released again, which starts the cycle again.After 50 h, 3.06 kg of the chlorinated oligosilane mixture 9 is isolated. From this mixture, 2.6 kg of product (Si2Cl6) (12) are isolated by chlorination and subsequent distillation (10). The resulting byproduct SiCl4 (11) is recycled into the catalytic cycle (3). Example 2

[0046] The reactor of cycle 3 from Figure 2 is evacuated (14) and then filled with H2 to a pressure of 2 hPa. The cold trap 15 is first cooled with liquid nitrogen. 1000 sccm of SiCl4 vapor (2) and 500 sccm of H2 are then introduced into the plasma-chemical sub-step (8). A plasma is maintained by irradiating it with 1 kW of RF power at a frequency of 13.56 MHz, forming a chlorinated polysilane mixture (9) with the release of the HCl catalyst (5). The HCl gas (5) and unreacted silanes are frozen out in the cold trap 15, while the non-condensable hydrogen is pumped out via the vacuum pump 14 so that a pressure of 2.5 hPa is not exceeded. After 8 hours, the gas supply is stopped and the system is closed. Then the cold trap 15 is thawed and the outgassing HCl gas is passed through a bed of 500g of metallurgical silicon (7) at 300°C in the thermal sub-step (7).The resulting mixture of intermediate products (6) (SiCl4, HSiCl3) is condensed in cooler 13, with any existing overpressure (H2) being discharged through the pressure relief valve 16. Subsequently, the condensate, together with the liquid components from the cold trap 15, is returned in vapor form to the plasma-chemical sub-step 8 after the cold trap 15 has been refrozen and the cycle has been evacuated by means of the vacuum pump 14. It is refreshed with SiCl4 vapor (2) such that 1000 sccm of chlorosilane flow over a period of 8 hours. The H2 flow is readjusted to 500 sccm. Upon restarting the plasma-chemical sub-step 8, the cycle begins again and is repeated three times. Subsequently, 1.4 kg of the chlorinated oligosilane mixture 9 is isolated. From this, 1.1 kg of product (Si2Cl6) (12) is isolated by chlorination and subsequent distillation (10).The resulting by-product SiCl4 (11) is reintroduced into the catalytic cycle (3) in the area of ​​the cooler 13. List of reference symbols 1 Starting material: metallurgical silicon 2 Starting material SiCl4 3 Catalytic cycle 4 Catalyst H2 5 Catalyst HCl 6 Intermediate products of the cycle 7 Thermal sub-step 1 of the cycle 8 Plasma chemical sub-step 2 of the cycle process 9 Chlorinated oligosilane crude mixture 10 Substep 3 of the procedure 11 Recycled SiCl4 12 Chlorinated oligosilane end products 13 coolers for chlorosilanes 14 Vacuum pump 15 Cold trap for chlorosilanes and HCl 16 Pressure relief valve

Claims

[1] Process for the preparation of halogenated oligosilanes Si n X 2n+2 with n = 2 to n = 6 as a pure compound or mixture of compounds whose substituents include chlorine or chlorine and hydrogen, characterized by , that (a) the starting materials for the process comprise elemental silicon, SiCl4 and elemental hydrogen in at least a catalytic amount, b) the HCl gas produced in the process is recycled to more than 5%, preferably more than 20%, particularly preferably more than 50%, in particular more than 80%, into the process, c) the process comprises a reaction of elemental silicon with HCl as catalyst to form chlorinated oligosilanes, the reaction being carried out by a two-stage catalytic reaction step, d) the process comprises a plasma-chemical synthesis step which is operated in a pressure range of 0.1 hPa to 100 hPa, preferably 0.5 hPa to 30 hPa, particularly preferably 1 hPa to 10 hPa, in particular 1.5 hPa to 5 hPa, e) at least one further silane is used as starting material and / or intermediate, selected from the group of hydrogenated silanes, which includes HSiCl3, HSi2Cl5 and H2SiCl2, f) the reactants or the reactant mixture of the plasma-chemical synthesis step contain less than 1 atom% of C as such or in the form of its compounds and g) the process comprises at least one further step selected from a group comprising distillation, chlorination and thermolysis. [2] Method according to claim 1, characterized by that X in Si n X 2n+2 more than 95 atom% chlorine, preferably more than 98 atom% and / or the hydrogen content in Si n X 2n+2less than 5 atom%, preferably less than 2 atom%, particularly preferably less than 1 atom%. [3] Method according to claim 1, characterized by that the reaction of elemental silicon with HCl takes place at 200 to 800°C, preferably at 230 to 500°C, particularly preferably at 250 to 350°C, in particular at 270 to 300°C. [4] Method according to claim 1, characterized by that the process comprises a chlorination step with elemental chlorine. [5] Method according to claim 1, characterized by that the plasma-chemical step comprises elemental hydrogen as a reducing agent and that the hydrogen gas produced in the process is recycled to the process to an extent of more than 5%, preferably to an extent of more than 20%, particularly preferably to an extent of more than 50%, in particular to an extent of more than 80%. [6] Method according to claim 1, characterized by that the average chain length of the oligosilanes obtained in the plasma chemical sub-step is greater than n = 3. [7] Method according to claim 1, characterized by that the plasma chemical step is carried out using continuous electromagnetic radiation. [8] Method according to claim 7, characterized by that the frequency of the electromagnetic radiation is in the range from 1 MHz to 100 MHz, preferably this is selected from a group comprising the frequency bands in the range of 13.56 MHz, 27.12 MHz and 40.68 MHz, particularly preferably 27.12 MHz and 40.68 MHz, in particular 27.12 MHz. [9] Method according to claim 1, characterized by that metallurgical silicon is used as a starting material in the first step.

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