DE102014007766A1 - Process for the plasma-chemical preparation of halogenated oligosilanes from tetrachlorosilane - Google Patents

Abstract

The present invention relates to a method and an apparatus for producing halogenated oligosilanes from silicon and tetrachlorosilane. In the prior art, halogenated oligosilanes such as Si2Cl6 are obtained by chlorination of silicon or calcium silicide with chlorine gas or as a by-product in the conversion of HSiCl3 to semiconductor silicon ("Siemens process"). These reactions primarily provide SiCl4 as the product and the desired halogenated oligosilanes only as by-products. Furthermore, accumulating contaminants from the end products due to similar boiling points can be separated only with great effort. The new process is intended to deliver the desired products in high yield and purity. The synthesis takes place in a catalytic cycle with HCl as catalyst. The reaction takes place in the first step with silicon at about 270 ° C and in the second step in the cold plasma. Partial chlorination completes the synthesis. The yield of this process is above that of the known processes. The process is suitable for the preparation of halogenated oligosilanes such as Si2Cl6, Si3Cl8, Si4Cl10 and higher homologs. These find z. As in the deposition of silicon nitride layers in CVD processes use.

Description

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

State of the art

Other processes for preparing halogenated oligosilanes are known in the art:
[Lit .: M. Schmeisser, P. Voss "On the silicon chloride [SiCl 2] x", Z. anorg. General Chem. (1964) 334, 50-56 (Schmeisser, 1964) ]. On the one hand, they can be prepared by pure thermal reactions (Schmeisser, 1964) by heating vaporous chlorosilanes with or without reducing agent to high temperatures (above 1000 ° C.). In this case, chlorinated polysilanes (PCS) are obtained, which have a color from pale green-yellow to yellowish-light-brown (Schmeisser, 1964, "glassy, high polymer").

In GB702,349 discloses a method that is condensed in the reaction of silicon alloys with chlorine gas at 190-250 ° C, a mixture of chlorinated polysilanes from the gas stream.

DE3126240C2 describes the wet-chemical preparation of chlorinated polysilanes from Si ₂ Cl ₆ by reaction with a catalyst. The resulting mixtures still contain the catalyst and are therefore washed with organic solvents.

In DE 10 2005 024 041 A1 describes the production of silicon in a two-step process. For this purpose, a polysilane is first plasma-chemically generated, which is then decomposed by thermolysis to high-purity silicon.

The WO2008009473 describes a process for the preparation of silicon or hydrogenated or organically substituted oligosilanes, wherein the desired precursors are selectively produced in a plasmachemischen step and immediately subjected to further processing by distillation, hydrogenation or methylation.

In the DE 10 2008 025 261 B4 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 in terms of their spectroscopic and other physical properties.

Furthermore, the describes DE 10 2008 025 260 B4 halogenated polysilanes and a thermal process for their preparation. The polysilanes described have an average chain length of n = 3 to 9 and are described in terms of 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 the preparation of Si and SiX ₄ , they are intrinsically free of hydrogen.

Furthermore, the describes US2010080746A1 the production of Si ₂ Cl ₆ and Si ₃ Cl ₈ by plasma-chemical reaction of SiCl ₄ with a monosilane, which hydrogen in bound form, z. B. as Si-H group or as a methyl group. Elementary hydrogen is explicitly excluded as a reducing agent.

Disadvantages of the prior art

For in [ Schmeisser 1964 ] spectroscopic studies have shown that such purely thermally produced polysilanes have a high content of short-chain, branched and cyclic molecules. Furthermore, the mixture obtained is due to its production (very high temperatures) heavily contaminated with AlCl ₃ . Furthermore, the yields per time are very low, since working under high vacuum. By working at very high temperatures, the resulting polysilanes are also heavily contaminated with disiloxanes formed by wall reactions of the silicon with the ceramic tubes. Due to the polymeric structure of the PCS, no purification by distillation can take place at this stage, which is why all impurities are carried off into possible subsequent reactions and products. The procedure from the GB 702,349 has the disadvantage that the average molecular weight of these mixtures is relatively low, since in a distillation only 2% of the silanes with n greater than 6 are obtained. Therefore, the drawback of this process is, above all, the low yield of desired products.

The resulting mixtures of the DE3126240C2 still contain the catalyst and are therefore washed with organic solvents, leaving traces of these solvents and the catalyst. In addition, PCSs obtained in this way are highly branched. This publication is only mentioned for the sake of completeness, since the products of the invention occur here only as starting materials, ie, the objective is the construction of long-chain PCS and thus different. It would make little sense (economically) to decompose the resulting PCS into short-chain OCS again.

The procedure from the DE 10 2005 024 041 A1 It also serves as an overview, since the objective here is the production of high-purity silicon for semiconductor applications. Although it would be possible to use the intermediate PCS for the preparation of the OCS according to the invention, this would be considerably more expensive, since only stoichiometric amounts of SiCl ₄ and hydrogen are used as starting materials, as a result of which relatively low yields are to be expected. The procedure of WO2008009473 also requires in stoichiometric amount of highly pure hydrogen and SiCl ₄ as starting compounds. Furthermore, this process is unfavorable in relation to the production of chlorinated oligosilanes, since although it is mentioned that they can be separated from the mixture, but is not intended to implement the mixture targeted to them. The WO2008009473 rather, provides for distillations and derivatizations, but no conversion of medium or high molecular weight fractions and bottoms to chlorinated oligosilanes. The focus of this application is therefore on the production of silicon, hydrogenated polysilanes and organically functionalized polysilanes. Also the procedure of DE 10 2008 025 261 B4 again requires in stoichiometric amount of high purity hydrogen and SiCl ₄ as starting compounds. The production of chlorinated oligosilanes is not mentioned. If such processes are obtained in the process described, they are only by-products since the mean chain length of the mixture is greater (n = 9 to 20) than would be expected for oligosilanes (n = 2 to 8).

The procedure of DE 10 2008 025 260 B4 uses the direct synproportionation reaction between silicon and SiX ₄ to produce the halogenated polysilanes. Due to working at high temperatures (> 700 ° C) wall reactions are unavoidable, resulting in contamination in the polysilane mixtures obtained. Here are z. B. to call metal compounds. A further disadvantage of this process is that the dihalosilylene SiX ₂ formed as an intermediate readily decomposes again into a reversal of the formation reaction to silicon and SiX ₄ , so that the product vapor, after leaving the silicon bed, is cooled as rapidly as possible below the decomposition temperature (about 280 ° C.) (Quenched) in order to avoid larger yield losses, which involves considerable technical problems, because you can shrink the temperature zone, in which the reaction takes place, but this is never completely rid of.

The procedure of US2010080746A1 has the disadvantage that at least two chlorosilanes are consumed stoichiometrically in high purity, which leads to high costs on the educt side. This is particularly due to the fact that elemental hydrogen is to be avoided as a reducing agent in the plasma chemical step. Furthermore, the use of the preferred methylated chlorosilanes such as CH ₃ SiCl ₃ as a reducing agent leads to a contamination of the products with traces of organic substances, which is particularly critical for the potential application in the semiconductor field.

task

The aim of the process is to provide a process for the preparation of halogenated oligosilanes, which achieves very high yields of OCS with high energy efficiency at low material and cost. At the same time Table 1: Abbreviations and synonyms

Methods make it possible to produce halogenated oligosilanes of very high purity, in particular with regard to contamination by dopants and metal compounds and other elements which are harmful for use in the semiconductor sector.

Solving the problems of the prior art

• 1. die Ausgangsstoffe für das Verfahren SiCl₄ und in mindestens katalytischer Menge elementaren Wasserstoff umfassen,
• 2. das Verfahren einen plasmachemischen Syntheseschritt umfasst, welcher in einem Druckbereich von 0,1 hPa bis 100 hPa, bevorzugt 0,5 hPa bis 30 hPa, besonders bevorzugt 1 hPa bis 10 hPa, insbesondere 1,5 hPa bis 5 hPa betrieben wird,
• 3. als Ausgangsstoff und/oder Zwischenprodukt mindestens ein weiteres Silan Verwendung findet, ausgewählt aus der Gruppe der hydrierten Monosilane, welche HSiCl₃ und H₂SiCl₂ umfasst,
• 4. als weiterer Ausgangsstoff und/oder Zwischenprodukt mindestens ein weiteres Silan Verwendung findet, ausgewählt aus der Gruppe der halogenierten Oligosilane, welche HSi₂Cl₅, Si₂Cl₆ und Si₃Cl₈ umfasst, und/oder ausgewählt aus der Gruppe der halogenierten Disiloxane, welche Cl₃SiOSiCl₃, HCl₂SiOSiCl₃, Cl₃SiO(Si₂Cl₅) und Cl₃SiOSiCl₂OSiCl₃ umfasst,
• 5. das Verfahren mindestens einen weiteren Schritt aufweist, ausgewählt aus einer Gruppe, welche Destillation, Chlorierung und Thermolyse umfasst.

gelöst.This object is achieved by the features listed in claim 1 method for preparing halogenated oligosilanes Si _n X _{2n + 2} with n = 2 to n = 6 as a pure compound or mixture of compounds whose substituents chlorine or chlorine and hydrogen, characterized in that

• 1. comprise the starting materials for the process SiCl ₄ and at least catalytic amount of elemental hydrogen,
• 2. the process comprises a plasma-chemical synthesis step which is operated in a pressure range from 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,
• 3. at least one further silane is used as starting material and / or intermediate, selected from the group of the hydrogenated monosilanes, which comprises HSiCl ₃ and H ₂ SiCl ₂ ,
• 4. at least one further silane is used as further starting material and / or intermediate, selected from the group of the halogenated oligosilanes, which comprises HSi ₂ Cl ₅ , Si ₂ Cl ₆ and Si ₃ Cl ₈ , and / or selected from the group of the halogenated Disiloxanes comprising Cl ₃ SiOSiCl ₃ , HCl ₂ SiOSiCl ₃ , Cl ₃ SiO (Si ₂ Cl ₅ ) and Cl ₃ SiOSiCl ₂ OSiCl ₃ ,
• 5. the process comprises at least one further step selected from a group comprising distillation, chlorination and thermolysis.

solved.

The advantages achieved by the process according to the invention for the synthesis of chlorinated oligosilanes are, in particular, that the autocatalytic action of halogenated oligosilanes on the product formation mechanism in the plasma-chemical step of the synthesis is used for their production. In the prior art either temperatures of about 1200 ° C are necessary, while in the present process by applying a plasma in combination with the development of a suitable catalyst, this reaction can be carried out efficiently already below about 300 ° C, or find the reactions Although using plasmas instead, do not use the catalytic effect of halogenated oligosilanes and / or hydrogenated monosilanes in order to achieve higher yields. This new catalytic reaction allows comparatively high yields of OCS with overall high energy efficiency. Furthermore, contamination of the product is avoided by the mild reaction conditions maintained throughout the process. Under mild conditions, especially comparatively low temperatures (<400 ° C) and low electron energies understood here, whereby a mobilization and / or carryover of contaminants from the reactor materials and / or reactants can be largely excluded. In particular, sputtering effects due to the use of low electron energies in the process steps according to the invention can be largely excluded. The process according to the invention is described by the following idealized overall reaction equation: nSiCl ₄ + nH ₂ + Cl ₂ → Si _n Cl _{2n + 2} + 2nHCl

The specified chlorine gas is not absolutely necessary for the process, but increases depending on the desired product Si _n Cl _{2n + 2,} the yield of the overall reaction. Therefore, it is given as an ideal case of the method according to the invention, but should not be limited thereto. Moreover, it is also possible to use chlorine other chlorinated compounds for the stated purpose, without departing from the essence of the method 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 service lives of the reactors. for SiCl ₄ : nSiCl ₄ + (2n - 2) / 2H ₂ → Si _n Cl _{2n + 2} + (2n - 2) HCl, and for HSiCl ₃ : (n - 1) HSiCl ₃ + SiCl ₄ → Si _n Cl _{2n + 2} + (n - 1) HCl.

The process according to the invention provides a further process variant in which halogenated disiloxanes are added to the educts in order to achieve a purification action in the course of the overall process with regard to metal compounds and dopants such as boron and phosphorus. The disiloxanes in this case bind during the various process steps, which usually comprise a plasmachemic step, a chlorination or thermolysis step and at least one distillation step, a considerable part of the metal and dopant contaminations in the form of oxides or mixed oxides which, for. B. Si-O groups, whereby they can be separated in the distillation steps according to the invention as low-volatile residues. Conveniently, both the hydrogenated monosilanes and the halogenated oligosilanes and the disiloxanes are admixed at the beginning of the process, these z. B. can be presented dissolved in SiCl ₄ , or liquid or preferably gaseous can be added to the reactant stream. As a result, the catalytic effect as well as the purifying effect that these components cause already in the plasma chemical step, as provided for by the invention. However, especially the disiloxanes can also be added to the process according to the invention at a later point in time, with a certain purifying effect still being exhibited. As the skilled artisan easily recognizes, this does not abandon the disclosed principle of the method according to the invention.

The yield-enhancing effect of both the hydrogenated monosilanes and the halogenated oligosilanes is explained by the fact that both classes of substances under plasma conditions are easily converted to catalytically active radicals, which in turn generate new radicals and thus support the radical chain reaction, and / or easily to chain formers such as SiCl ₂ , which can, inter alia, oligomerize directly to the desired oligosilanes. Furthermore, insertion reactions are, as shown below with reference to the formation of Hexachlordisilans, SiCl ₄ + SiCl ₂ → Si ₂ Cl ₆ possible to the desired end products or product mixtures. The skilled artisan readily recognizes that HSiCl ₃ and other H-silanes such as H ₂ SiCl _{2 can} also be added to the process as starting materials together with SiCl ₄ in a larger amount than is usual for catalysts, although they can also partially replace this without the essence of the invention change process according to the invention.

An advantageous embodiment is given by claim 2:
A process according to claim 1, characterized in that X in Si _n X _{2n + 2} is more than 95 atomic% chlorine, preferably more than 98 atomic% and / or the hydrogen content in Si _n X _{2n + 2 is} less than 5 atomic%, is preferably less than 2 atom%, particularly preferably less than 1 atom%.

Due to the restriction to chlorine atoms as substituents, products are obtained which are already partially commercially available and utilized today (eg Si ₂ Cl ₆ , Si ₃ Cl ₈ ).

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

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

A further advantageous embodiment is given by claim 4:
A method according to claim 1, characterized in that the proportion of halogenated oligosilanes and / or halogenated disiloxanes at least 0.001% by mass, preferably at least 0.01 mass%, more preferably at least 0.1 mass%, in particular at least 1 mass%.

The amount of cleaning 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 depending on their vapor pressure in the process and can interfere in the final product, which is why they are preferably in the last step, z. B. must be largely removed by distillation. This is all the better, the less was present from the outset, so that one will usually choose a compromise between cleaning effect and tolerable residual content in the final product here. It is advantageous, depending on the boiling point of the desired end product, to choose a disiloxane which has a boiling point which is as different as possible in order not to complicate the final purification step.

A further advantageous embodiment is given by claim 5. A method according to claim 1, characterized in that the method comprises a chlorination step with elemental chlorine.

This is usually a useful design, since elemental chlorine is usually the cheapest chlorinating agent.

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

Particularly important with regard to the purity of the products is that in the plasma-chemical third substep a reducing agent is used which is inexpensive available in high purity and also can not release from its molecular structure any elements critical for semiconductors. Therefore, elemental hydrogen is the best choice for this task. Bound hydrogen, eg. B. in the form of organic substances or molecular groups, is not very suitable, since in this case contamination by carbon or organic radicals in the product can be expected.

A further advantageous embodiment is given by claim 7:
A method according to claim 1, characterized in that the average chain length of the oligosilanes obtained in this step is greater than n = 3.

In contrast to some competitive methods in which OCS is built up by chlorination of silicon or silicides and primarily SiCl ₄ and Si ₂ Cl ₆ arise, the inventive method provides directly product mixtures of OCS, which have longer chains, the crude mixture on average a Chain length greater than n = 3 has. This is advantageous since the desired compounds can be obtained from this mixture by targeted chain degradation, while a chain construction would not be technically worthwhile since this would require coupling reactions (Wurtz coupling, etc.), which would be expensive and in turn bring contamination into the product.

A further advantageous embodiment is given by claim 8:
A method according to claim 1, characterized in that for product removal in plasmachemischen step, the reactor is filled with HCl gas. This embodiment has the advantage that no permanent gases are introduced into the system, which would hinder the rapid achievement of the desired working vacuum and would promote the carryover of halosilanes into the exhaust system.

A further advantageous embodiment is given by claim 9:
A method according to claim 1, characterized in that the plasma-chemical step is carried out using continuous electromagnetic radiation.

From an economical point of view, it is important that the type of excitation used in the plasmachemical step can on the one hand be ensured cost-effectively by commercially available standard equipment and, on the other hand, that no approval or shielding problems are generated on the emission side. Generators of continuous electromagnetic radiation generate with high efficiency and high power radiation of defined frequency to which the necessary shielding can be easily adapted, so that the emission limits can be easily met. Pulsed radiation would be more difficult to shield, especially with the then very high instantaneous power (at least a few kW), since a wide band of very many frequencies is emitted, especially with short pulses. Especially non-sinusoidal pulses such as triangular or rectangular pulses have a high proportion of higher-frequency oscillations, which are becoming increasingly difficult to screen.

A further advantageous embodiment is given by claim 10:
A method according to claim 9, characterized in that the frequency of the electromagnetic radiation in the range of 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, more preferably 27.12 MHz and 40.68 MHz, in particular 27.12 MHz.

The frequencies listed here belong to frequency bands for which equipment is standard available in high-frequency technology. This saves on the investment side costs compared to custom-made products.

Industrial Applicability

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 of the invention over the prior art. Furthermore, the purity of the end 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 advantage can be achieved within a process for producing chlorinated oligosilanes (OCS).

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• GB 702349 [0003, 0010]
• DE 3126240 C2 [0004, 0011]
• DE 102005024041 A1 [0005, 0012]
• WO 2008009473 [0006, 0012, 0012]
• DE 102008025261 B4 [0007, 0012]
• DE 102008025260 B4 [0008, 0013]
• US 2010080746 A1 [0009, 0014]

Cited non-patent literature

• M. Schmeisser, P. Voss "On the silicon chloride [SiCl 2] x", Z. anorg. Gen. Chem. (1964) 334, 50-56 (Schmeisser, 1964). [0002]
• Schmeisser 1964 [0010]

Claims (10)

Process for the preparation of halogenated oligosilanes Si _n X _{2n + 2} where n = 2 to n = 6 as pure compound or mixture of compounds whose substituents include chlorine or chlorine and hydrogen, characterized in that (a) the starting materials for the process SiCl ₄ and in at least catalytic amount of elemental hydrogen, (b) the method comprises a plasmachemischen synthesis step, which in a pressure range from 0.1 hPa to 100 hPa, preferably 0.5 hPa to 30 hPa, particularly preferably 1 hPa to 10 hPa, in particular (C) as starting material and / or intermediate, at least one further silane is used, selected from the group of hydrogenated monosilanes which comprises HSiCl ₃ and H ₂ SiCl ₂ , (d) as further starting material and / or intermediate at least one further silane is used, selected from the group of halogenated oligosilanes, which comprises HSi ₂ Cl ₅ , Si ₂ Cl ₆ and Si ₃ Cl ₈ , and / or a selected from the group of halogenated disiloxanes comprising Cl ₃ SiOSiCl ₃ , HCl ₂ SiOSiCl ₃ , Cl ₃ SiO (Si ₂ Cl ₅ ) and Cl ₃ SiOSiCl ₂ OSiCl ₃ , (e) the process has at least one further step selected from a group comprising distillation, chlorination and thermolysis. A process according to claim 1, characterized in that X in Si _n X _{2n + 2} is more than 95 atomic% chlorine, preferably more than 98 atomic% and / or the hydrogen content in Si _n X _{2n + 2 is} less than 5 atomic%, is preferably less than 2 atom%, particularly preferably less than 1 atom%. A method according to claim 1, characterized in that the proportion of hydrogenated monosilanes at least 0.01 mass%, preferably at least 0.1 mass%, more preferably at least 1 mass%, in particular at least 20 mass%. A method according to claim 1, characterized in that the proportion of halogenated oligosilanes and / or halogenated disiloxanes at least 0.001% by mass, preferably at least 0.01 mass%, more preferably at least 0.1 mass%, in particular at least 1 mass%. A method according to claim 1, characterized in that the method comprises a chlorination step with elemental chlorine. A method according to claim 1, characterized in that the plasma-chemical step comprises elemental hydrogen as a reducing agent. A method according to claim 1, characterized in that the average chain length of the oligosilanes obtained in this step is greater than n = 3. A method according to claim 1, characterized in that for the product removal, the reactor is filled with HCl gas. A method according to claim 1, characterized in that the plasma-chemical step is carried out using continuous electromagnetic radiation. A method according to claim 9, characterized in that the frequency of the electromagnetic radiation in the range of 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, more preferably 27.12 MHz and 40.68 MHz, in particular 27.12 MHz.

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