Process for the plasma-chemical production of halogenated oligosilanes from 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, where (a) the starting materials for the process comprise SiCl4 and at least a catalytic amount of elemental hydrogen, (b) 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, (c) at least one further silane is used as starting material, selected from the group of hydrogenated monosilanes, which comprises HSiCl3 and H2SiCl2, characterized in that (d) halogenated disiloxanes are added at the beginning of the process, wherein the halogenated disiloxane is selected from the group comprising Cl3SiOSiCl3, HCl2SiOSiCl3 and Cl3SiO(Si2Cl5), and (e) the process comprises at least one further step selected from a group comprising distillation, chlorination and thermolysis, and (f) the reactor is filled with HCl gas for product removal.

Description

[0001] The present invention relates to a process for the plasma-chemical production 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: [References: M. Schmeisser, P. Voss "Über das Siliciumchlorid SiCl2", Z. anorg. allg. Chem. (1964) 334, 50-56 (Schmeisser, 1964)]. They can be prepared, on the one hand, by purely thermal reactions (Schmeisser, 1964) by heating vaporous chlorosilanes with or without a reducing agent to high temperatures (over 1000°C). In this case, chlorinated polysilanes (PCS) are obtained, which range in color 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.

[0007] DE 10 2008 025 261 B4 describes halogenated polysilanes and a plasma-chemical process for their production. 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 polysilanes described have an average chain length of n = 3 to 9 and are evaluated 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 / 080746 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.

[0010] DE 10 2007 007 874 A1 discloses a process for producing higher silanes—more specifically, the production of dimeric and / or trimeric silicon compounds, in particular silicon-halogen compounds. Furthermore, the process described therein is also suitable for producing corresponding germanium compounds. Furthermore, the invention relates to an apparatus for carrying out the process and the use of the resulting silicon compounds.

[0011] WO 2011 / 067 416 A1 describes a process for the production of halogenated polysilanes - these as a pure compound or mixture of compounds which has a particular purity with regard to, among other things, boron-containing compounds. Disadvantages of the state of the art

[0012] 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.

[0013] The process described in GB 702 349 A 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.

[0014] The mixtures obtained from DE 31 26 240 C2 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.

[0015] The process described in DE 10 2005 024 041 A1 also serves only as an overview, since the objective here is the production of high-purity silicon for semiconductor applications. Intermediate PCS could be used to produce the OCS according to the invention, but this would be considerably more expensive since only stoichiometric amounts of SiCl4 and hydrogen are used as starting materials, which means that relatively low yields are to be expected.

[0016] The process of WO 2008 / 009473 A1 also requires stoichiometric amounts of highly pure hydrogen and SiCl4 as starting compounds. Furthermore, this process is unfavorable with regard to 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, WO 2008 / 009473 A1 envisages distillations and derivatizations, but not the conversion of the medium- or high-molecular-weight fractions and bottoms into chlorinated oligosilanes. The focus of this application is therefore on the production of silicon, hydrogenated polysilanes, and organically functionalized polysilanes.

[0017] The process described in DE 10 2008 025 261 B4 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).

[0018] The process of DE 10 2008 025 260 B4 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 this include metal compounds. A further disadvantage of this process is that the intermediately formed dihalosilylene SiX2 readily decomposes to silicon and SiX4 in the reverse reaction. Therefore, the product vapor must be cooled (quenched) as quickly as possible below the decomposition temperature (approx. ~280°C) after leaving the silicon bed to avoid significant yield losses. This poses significant technical problems, since the temperature zone in which the reverse reaction takes place can be reduced, but it can never be completely eliminated.

[0019] The process described in US 2010 / 080746 A1 has the disadvantage that at least two high-purity chlorosilanes are consumed stoichiometrically, leading to high costs on the reactant side. This is particularly due to the need to avoid elemental hydrogen as a reducing agent in the plasma-chemical step. 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. 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 = 1 m bar sccm Standard cubic centimeter Task

[0020] 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 dopants, metal compounds, and other elements harmful for semiconductor applications. Solution to the problems of the state of the art

[0021] 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, where (a) the starting materials for the process comprise SiCl4 and at least a catalytic amount of elemental hydrogen, (b) 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, (c) at least one further silane is used as starting material, selected from the group of hydrogenated monosilanes, which comprises HSiCl3 and H2SiCl2, characterized in that (d) halogenated disiloxanes are added at the beginning of the process, wherein the halogenated disiloxane is selected from the group comprising Cl3SiOSiCl3, HCl2SiOSiCls and Cl3SiO(Si2Cl5), and (e) the process comprises at least one further step selected from a group comprising distillation, chlorination and thermolysis, and (f) the reactor is filled with HCl gas for product removal.

[0022] The advantages achieved with the process according to the invention for the synthesis of chlorinated oligosilanes consist, in particular, in the fact that the autocatalytic effect of halogenated oligosilanes on the product formation mechanism in the plasma-chemical step of the synthesis is utilized for their production. In the prior art, either temperatures of approximately 1200°C are required, whereas in the present process, by using a plasma in combination with the development of a suitable catalyst, this reaction can be carried out efficiently at temperatures below approximately 300°C. Or, although the reactions also take place using plasmas, they do not utilize the catalytic effect of halogenated oligosilanes and / or hydrogenated monosilanes to achieve higher yields. This new catalytic reaction procedure enables comparatively high yields of OCS with overall high energy efficiency.Furthermore, the mild reaction conditions maintained throughout the process prevent product contamination. Mild conditions here primarily refer to comparatively low temperatures (<400°C) and low electron energies, which largely eliminate the mobilization and / or carryover of contaminants from the reactor materials and / or reactants. In particular, sputtering effects can be largely eliminated by using low electron energies in the process steps according to the invention.

[0023] The process according to the invention is described by the following idealized overall reaction equation: nSiCl4 + nH2 + Cl2 → Si n Cl 2n+2 + 2nHCl

[0024] The specified chlorine gas is not essential for the process, but increases Si depending on the desired product n Cl 2n+2the yield of the overall reaction. Therefore, it is presented as the ideal case of the process according to the invention, but is not intended to limit it to this. Furthermore, it is also possible to use other chlorinating compounds instead of chlorine for the stated purpose without departing from the essence of the process according to the invention. Working 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. For SiCl4: 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] The process according to the invention provides a further process variant in which halogenated disiloxanes are added to the reactants in order to achieve a purification effect during the entire process with regard to metal compounds and dopants such as boron and phosphorus. During the various process steps, which usually comprise a plasma-chemical step, a chlorination or thermolysis step and at least one distillation step, the disiloxanes bind a significant portion of the metal and dopant contamination in the form of oxides or mixed oxides, which contain, for example, Si-O groups, whereby these can be separated off as low-volatile residues in the distillation steps according to the invention. Advantageously, both the hydrogenated monosilanes and the halogenated oligosilanes as well as the disiloxanes are mixed in at the beginning of the process, whereby these, for example,They can be initially dissolved in SiCl4, or can be added to the reactant stream in liquid or, preferably, gaseous form. As a result, both the catalytic effect and the purification effect exerted by these components are already developed in the plasma-chemical step, as envisaged by the invention. However, the disiloxanes in particular can also be added to the process according to the invention at a later point in time, in which case a certain purification effect continues to be exerted. As the skilled person will readily recognize, this does not deviate from the disclosed principle of the process according to the invention.

[0026] The yield-enhancing effect of both hydrogenated monosilanes and halogenated oligosilanes is explained by the fact that both substance classes are readily converted under plasma conditions into catalytically active radicals, which in turn generate new radicals and thus support the radical chain reaction, and / or readily react to form chain formers such as SiCl2, which can, among other things, oligomerize directly to the desired oligosilanes. Furthermore, insertion reactions, as shown below for the formation of hexachlorodisilane, are possible. 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 to the process as reactants together with SiCl4 in larger amounts than usual for catalysts, and they can also partially replace the latter without altering the nature of the process according to the invention.

[0027] An advantageous embodiment is as follows:

[0028] Method according to claim 1, characterized in 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+2 less than 5 atom%, preferably less than 2 atom%, particularly preferably less than 1 atom%.

[0029] By restricting the substituents to chlorine atoms, products are obtained that are already partly commercially available and used today (e.g. Si2Cl6, Si3Cl8).

[0030] A further advantageous embodiment is given by claim 3: Process according to claim 1, characterized in that the proportion of hydrogenated monosilanes is at least 0.01% by mass, preferably at least 0.1% by mass, particularly preferably at least 1% by mass, in particular at least 20% by mass.

[0031] By using hydrogenated monosilanes as additives in low to moderate amounts, a reaction-accelerating effect can be achieved without incurring high additional costs for the additive. Depending on the type of additive and its price, a cost / benefit optimization must be carried out for the overall process.

[0032] A further advantageous embodiment is given by claim 4: Process according to claim 1, characterized in that the proportion of halogenated disiloxanes is at least 0.001% by mass, preferably at least 0.01% by mass, particularly preferably at least 0.1% by mass, in particular at least 1% by mass.

[0033] The amount of purification additive depends on the type and amount of contaminants to be removed. A larger amount of disiloxanes tends to result in better purification. However, the disiloxanes are also entrained in the process depending on their vapor pressure and can interfere with the final product. Therefore, they must be largely removed, preferably in the final step, e.g., by distillation. This is more successful the less contaminants are present initially, so a compromise is generally sought between purification effectiveness and a tolerable residual content in the final product. Depending on the boiling point of the desired final product, it is advantageous to select a disiloxane with a boiling point that is as different as possible from that of the desired final product, so as not to complicate the final purification step.

[0034] A further advantageous embodiment is given by claim 5: Process according to claim 1, characterized in that the process comprises a chlorination step with elemental chlorine.

[0035] This is generally a sensible design, since elemental chlorine is usually the cheapest chlorinating agent.

[0036] A further advantageous embodiment is given by claim 6: Method according to claim 1, characterized in that the plasma-chemical step comprises elemental hydrogen as a reducing agent.

[0037] What is particularly important with regard to the purity of the products is that in the third plasma-chemical step, a reducing agent is used which is available inexpensively in high purity and which cannot release any elements critical for semiconductors from its molecular structure.

[0038] Therefore, elemental hydrogen is the best choice for this task. 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.

[0039] A further advantageous embodiment is as follows: Process according to claim 1, characterized in that the average chain length of the oligosilanes obtained in this step is greater than n=3.

[0040] In contrast to some competing processes, in which OCS is synthesized by chlorination of silicon or silicides and primarily produces SiCl4 and Si2Cl6, the process according to the invention directly yields product mixtures of OCS that 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 through targeted chain degradation, whereas chain synthesis would not be technically worthwhile, as this would require coupling reactions (Wurtz coupling, etc.), which are expensive and would in turn introduce contamination into the product.

[0041] A further advantageous embodiment is as follows: Method according to claim 1, characterized in that for product removal in the plasma chemical step the reactor is filled with HCl gas.

[0042] This design has the advantage that no permanent gases are introduced into the system, which would hinder the rapid achievement of the desired working vacuum and promote the entrainment of halosilanes into the exhaust system.

[0043] A further advantageous embodiment is given by claim 7: Method according to claim 1, characterized in that the plasma chemical step is carried out using continuous electromagnetic radiation.

[0044] 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.

[0045] A further advantageous embodiment is given by claim 8: 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.

[0046] The frequencies listed here belong to frequency bands for which standard radio frequency devices are available. This saves investment costs compared to custom-made devices. Industrial applicability

[0047] 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 adding the catalytic additives according to the invention compared to the prior art.

[0048] Furthermore, the purity of the final products is increased by adding the purification agents according to the invention, which leads to an improved position of the product on the market, whereby economic advantages can be achieved within a process for the production of chlorinated oligosilanes (OCS).

Claims

[1] Process for the production 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, wherein (a) the starting materials for the process comprise SiCl4 and at least a catalytic amount of elemental hydrogen, (b) 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, (c) uses at least one other silane as a starting material, selected from the group of hydrogenated monosilanes which includes HSiCl3 and H2SiCl2, characterized by , that (d) halogenated disiloxanes are added at the beginning of the process, wherein the halogenated disiloxane is selected from the group comprising Cl3SiOSiCl3, HCl2SiOSiCl3 and Cl3SiO(Si2Cl5), and (e) the process includes at least one further step selected from a group which includes distillation, chlorination and thermolysis, and (f) for product extraction the reactor is filled with HCl gas. [2] Method according to claim 1, characterized by that the halogenated disiloxane is also added at a later time in the plasma chemical step, in the chlorination or thermolysis step or in the distillation step. [3] Method according to claim 1, characterized by that the proportion of hydrogenated monosilanes is at least 0.01% by mass, preferably at least 0.1% by mass, particularly preferably at least 1% by mass, and in particular at least 20% by mass. [4] Method according to claim 1, characterized bythat the proportion of halogenated disiloxanes is at least 0.001% by mass, preferably at least 0.01% by mass, particularly preferably at least 0.1% by mass, and in particular at least 1% by mass. [5] Method according to claim 1, characterized by that the process includes a chlorination step with elemental chlorine. [6] Method according to claim 1, characterized by that the plasma chemical step includes elemental hydrogen as a reducing agent. [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 of 1 MHz to 100 MHz, preferably 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, especially 27.12 MHz.