Abstract:
A storage material for obtaining H-silanes which is present in the form of a hydrogenated polysilane (HPS), as a pure compound or as a mixture of compounds having on average at least six direct Si—Si bonds, the substituenis of which predominantly consist of hydrogen and in the composition of which the atomic ratio of sabstitueot to silicon is at least 1:1.

Description:
RELATED APPLICATIONS 
       [0001]    This is a §371 of international Application No. PCT/EP2011/003232, with an international filing date of Jun. 30, 2011 (WO 2012/000673 A1 published Jan. 5, 2012), which is based on German Patent Application No. 10 2010 025 710.9 filed Jun. 30, 2010, the subject matter of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to a storage material and to a method for obtaining H-silanes therefrom. 
       BACKGROUND 
       [0003]    Gaseous or vaporizable, i.e., short-chain, H-silanes Si n H 2n+2  are important starting materials for processes in which silicon is to be deposited on surfaces, for example, is CVD processes or for preparation of solutions (for ink-jet processes, for example). One decisive disadvantage of short-chain H-silanes is that the lower representatives (n=1,2) are gaseous and therefore only handleable in pressurized gas flasks. Further, all H-silanes up to a chain length of n=6 are without exception pyrophoric and therefore require appreciable safety precautions to be stored in a comparatively large amount. Yet the high vapor pressure of short-chain H-silanes is advantageous for gas phase processes since it ensures high concentrations of the silanes in the gas phase. 
         [0004]    There is accordingly a need for a safe form of storing short-chain H-silanes and for a suitable method of releasing them as or when required. 
         [0005]    It is known from DE 2139155, for example, that polysilanes Si n H 2n+2  having a chain length of n=7 or more are nonpyrophoric in air. 
         [0006]    There are several known methods of releasing short-chain H-silanes from longer-chain polysilanes: 
       a) Thermal Decomposition 
       [0007]    Polysilanes decompose into silicon and hydrogen at high temperature. However, that thermolysis starts at as low a temperature as close to 300° C. and then leads to hydrogen, short-chain silanes Si n H 2n+2  and also polymeric products (SiH &lt;2 ) x , as known, for example, from R. Schwarz, F. Heinrich, Zeitschrift für anorganische und allgemeine Chemie 1935 (221) 277. The disadvantage with that method is the low yield of short-chain silanes obtained from the starting material since the thermolysis generates silicon-containing residues. 
       b) Catalytic Polymerization 
       [0008]    It is know from R. C. Kennedy, L. P. Freeman, A. P. Fox, M. A. Ring, Journal of inorganic and nuclear chemistry 1966 (28) 1373, for example, that silanes having at least one Si—Si bond will in the presence of suitable catalysts such as lithium salts polymerize at comparatively low temperatures to form higher polysilanes (SiH 2 ) x  by elimination of SiH 4 . The disadvantage with that method is that only one molecule of SiH 4  is formed per starting molecule Si n H 2n+2  during polymerization and the yield of short-chain material for safe storage substances with n&gt;6 remains small. F. Feher, F. Ocklenburg, D. Skrodzki, Zeitschrift für Naturforschung 1980 (35b) 869 report that higher oligosilanes react with approximately equimolar amounts of AlCl 3  on heating to eliminate SiH 4  and a little di- and trisilane and form a yellow polymer having a composition of SiH 0.98 . A slowed polymerization is observed even at lower amounts of AlCl 3  in aromatic solvents and at temperatures of not more than 85° C. Although the polymer contains less hydrogen, scarcely more than 50% of the starting silicon is released as short-chain silanes in the case of silanes where n&gt;6. 
         [0000]    c) Reaction of Polyfluorosilane with Hydrofluoric Acid 
         [0009]    P. L. Timms, R. A. Kent, T. C. Ehlert, J. L. Margrave, Journal of the American Chemical Society 1965 (87) 2824 discloses a method that leads to formation of polysilanes from polyfluorosilanes (F 2 Si) x . The material is admixed with hydrofluoric acid and silanes having a chain length n up to 6 can be isolated in that only fully hydrogenated compounds Si n H 2n+2  are formed. The disadvantage is again the low yield of silanes since, during the reaction, H +  from the acid is formally reduced to hydride while at the same time SiO 2  is formed, for example: 
         [0000]      4/ x (F 2 Si) x +6H 2 O−&gt;SiH 4 +3SiO 2 +8HF.
 
         [0010]    None of the cited processes demonstrate an efficient production of H-silanes in usable yields. The known methods also lack a storage material (storable form) that can be provided in the amounts needed. 
         [0011]    It could therefore be helpful to provide a safe storable form for H-silanes and a method for recovering these. 
       SUMMARY 
       [0012]    We provide a storage material for obtaining hydrogenated silanes (H-silanes) including hydrogenated polysilane as a pure compound or mixture of compounds having on average at least six direct Si—Si bonds, substituents of which consist predominantly of hydrogen and are in a composition of which atomic ratio for substituent:silicon is at least 1:1. 
     
    
     DETAILED DESCRIPTION 
       [0013]    Hydrogenated silanes (H-silanes) can be present as a pure compound or as a mixture of compounds and are obtainable from the storage material by degradation or further reaction in that preferentially H-silanes Si n H 2n+2  and/or Si n H 2n  where n=1-6 are generated. 
         [0014]    The hydrogenated polysilane (HPS) used as storage material is by virtue of its chain length nonpyrophoric, merely oxidizing slowly on contact with air. The HPS in its use as a storage material is in solid and/or liquid form. Hydrogenated polysilanes that are solids in pure form may be wholly or partly dissolved in liquid hydrogenated polysilanes. 
         [0015]    Especially solid HPS can further be processed into shaped articles, for example, pellets or ingots, for improved handling and/or to reduce the oxidation sensitivity of solid polysiloxane mixtures. Production of shaped articles can be combined with addition of additives to further cohere the shaped articles and/or reduce the oxidation sensitivity of polysilanes present therein. 
         [0016]    Shaped articles can be produced at up to temperatures at which the room temperature solid polysilane mixtures melt wholly or partly, but do not yet decompose. 
         [0017]    The shaped articles can also be subsequently overcoated with a functional layer, for example, to reduce attrition, make the shaped articles cohere better and/or reduce their sensitivity to oxidation. 
         [0018]    Advantageously, the hydrogenated polysilane HPS may be kept in a transport container before further use, the surface of which does not transmit light and the interior of which is blanketed with an inert gas. Further advantageously, this transport container may be equipped with a withdrawal device with which the HPS can be withdrawn in metered fashion and fed to the further use. 
         [0019]    Possible storage materials show their significant vibration bands in IR molecular vibration spectra in the region below 2400 wavenumbers, the measurement having been carried out on an FT/IR-420 spectrometer from Jasco Corp. as KBr compact. 
         [0020]    Possible storage materials show their significant vibration bands in Raman molecular vibration spectra in the region below 2300 wavenumbers and preferably in the range of 2000-2200 wavenumbers, the measurement having been carried out on an XY 800 spectrometer from Dilor with tunable laser excitation (T-sapphire laser, pumped by Ar-ion laser) and confocal Raman and luminescence microscope, liquid nitrogen cooled CCD detector, measuring temperature equal to room temperature, excitation wavelengths in the visible spectrum, inter alia 514.53 nm and 750 nm. 
         [0021]    Possible storage materials display chemical shifts in the  29 Si NMR spectra from −50 ppm to −170 ppm, preferably from −70 ppm to −130 ppm, the  29 Si NMR spectra having been recorded on a 250 MHz instrument of the Bruker DPX 250 type with the pulse sequence zg30 and referenced against tetramethylsilane (TMS) as external standard [δ( 29 Si)=0.0]. The acquisition parameters are: TO=32 k, AQ=1,652 s, D1=10s, NS=2400, O1P=−40, SW=400. 
         [0022]    A particularly suitable storage material is characterized by the following features: a) has its significant vibration bands in IR molecular vibration spectra in a region below 2300 wavenumbers, b) has a significant vibration band in RAMAN molecular vibration spectra in a region of 2000-2200 wavenumbers and c) has its significant product signals in  29 Si NMR spectra in a chemical shift range of −70 ppm to −130 ppm. 
         [0023]    Average chain length of the hydrogenated polysilane is preferably greater than 6 and less than 100, more preferably greater than 10 and less than 50 and even more preferably greater than 15 and less than 30. The hydrogenated polysilane is colorless to light yellow and advantageously has a metal content of less than 1%. It preferably contains virtually no short-chain branched chain or ring compounds while the level of branching points in the short-chain fraction is preferably less than 2%, based on the product mixture as a whole. 
         [0024]    Particularly preferably, the substituents of the hydrogenated polysilane consist exclusively of hydrogen. 
         [0025]    The hydrogenated polysilane preferably has predominantly linear long chains, is viscid to solid and soluble in suitable inert solvents. It more particularly contains less than 1 atom % of halogen. 
         [0026]    The method for obtaining H-silanes is characterized in that these are obtained from a storage material of the type described above. 
         [0027]    This method releases H-silanes, especially short-chain H-silanes from the storage material which represents a safe storable form, as and when required. 
         [0028]    The storage material may be produced in a first step and H-silanes are obtained therefrom in a second step. 
         [0029]    Preferably, the H-silanes are obtained from the storage material by one or more steps of synthesis as follows: a) thermolysis of hydrogenated polysilanes (HPS), at a pressure of 0.1-1013 hPa; b) ring-opening polymerization of cyclosilanes (Si n H 2n ), where n=4,5,6, and at a pressure of 0.1-1013 hPa; c) catalytic chain extension conducted at a pressure of 1-300 bar; d) treating the hydrogenated polysilane (HPS) with activated hydrogen species at a pressure of 0.1-1013 hPa;e) basic hydrolysis of the hydrogenated polysilane (HPS) in presence of low concentrations of strong bases or substoichiometric amounts of base or in presence of weak bases at a pressure of 0.1-1013 hPa; f) admixing the hydrogenated polysilane (HPS) with_chemical hydrogenating agents and thermal treatment, at a pressure of 1-300 bar; g) admixing the hydrogenated polysilane (HPS) with one or more transition metals and applying hydrogen, at a pressure of 1 -300 bar; and h) reaction of hydrogenated polysilane (HPS) with hydrofluoric acid or water-containing HF or HF gas or liquid HF at a pressure range of 0.1-1013 hPa. The individual aspects are reproduced hereinbelow: 
         [0030]    The H-silanes may be obtained by thermolysis of HPS. The pressure can be reduced for augmentation, for example, so that H-silanes of lower vapor pressure may be obtained. This may further serve to improve the conversion of the storage material into H-silanes. It is preferable to employ a pressure range of 0.1-1013 hPa. 
         [0031]    The HPS may be converted into H-silanes by ring-opening polymerization of cyclo-silanes (Si n H 2n ), where n is=4 to 10, in the HPS. It is preferable to employ a pressure range of 0.1-1013 hPa. 
         [0032]    The HPS may be converted into the desired H-silanes by catalytic chain extension, which is preferably done in a pressure range of 1-300 bar. What may be concerned here is a dehydrogenating polycondensation with transition metal catalysts, for example. 
         [0033]    The HPS may be treated with activated hydrogen species. These are obtainable, for example, by reaction of a strong electromagnetic alternating field or of an electric discharge on hydrogen gas. Activated hydrogen species are hydrogen molecules in an excited electronic state; hydrogen atoms; and also ionized hydrogen molecules or hydrogen ions. It is preferable to employ a pressure of 0.1-1013 hPa. 
         [0034]    The HPS may be subjected to a basic hydrolysis under conditions which lead to the release of silanes, especially by the presence of low concentrations of strong bases, substoichiometric amounts of a base or use of weak bases, for which it is preferable to employ a pressure of 0.1-1013 hPa. Water and alcohols are suitable hydrolysis reagents. Alkali metal hydroxides are examples of strong bases and organic amines are examples of weak bases. A low concentration of strong bases is a concentration which produces a pH of 8 to 13 in aqueous solution. A substoichiometric amount of a base for the purposes of the invention is a base quantity insufficient to convert the HPS into silicate salts according to the exemplary reaction equation 
         [0000]      1/ x (H 2 Si) x +2OH − +H 2 O−&gt;3H 2 +SiO 3   2− .
 
         [0035]    Chemical hydrogenating agents such as metal hydrides may be added to the HPS and a hydrogenation of Si—Si bonds may be brought about by thermal treatment. Hydrides of transition metals or aluminum hydride are examples of suitable metal hydrides. The reaction can also be carried out in inert solvents. It can be augmented with coordinating solvents such as, for example, dialkyl ethers or polyethers. It is preferable to employ a pressure of 1-300 bar. 
         [0036]    The HPS may be admixed with one or more suitable transition metals, and hydrogen may be applied to cause the HPS to split into short-chain H-silanes. Suitable transition metals are preferably Pt, Pd, Ni, Ti. The reaction with hydrogen can take place in inert solvents. A preferred method of admixing the HPS with transition metals is mechanical alloying by conjointly milling the HPS with the added metal. The transition metals can also be added in the form of compounds, for example, organic complexes, halide salts or hydrides. The hydrogen is preferably applied at a pressure of 1-300 bar. 
         [0037]    HPS may be reacted with an aqueous solution of HF (hydrofluoric acid), water-containing HF or dry HF in the liquid or gaseous state. The reaction with hydrofluoric acid can be represented for example by the following idealized reaction equation: 
         [0000]      4/ x (H 2 Si) x +2H 2 O−&gt;3SiH 4 +SiO 2 .
 
         [0038]    The yield of silanes is appreciably improved compared with the use of polyfluorosilane by avoiding the H +  reduction to hydride. In the absence of water, the following exemplary idealized reaction equation can be formulated: 
         [0000]      4/ x (H 2 Si) x +4HF−&gt;3SiH 4 +SiF 4 .
 
         [0039]    The resultant mixture of H-silanes and fluorosilanes can be purified by selective absorption of SiF 4 . The reaction can also be carried out in the presence of inert solvents. The resultant H-silanes can be freed of water fractions via suitable drying methods e.g., adsorption methods. It is preferable to employ a pressure of 0.1-1013 hPa. 
         [0040]    In general, in methods for releasing the H-silanes from the storage material by one or more steps of synthesis, the reactor parts in which the HPS is converted are maintained at a temperature of −70° C. to 300° C., especially −20° C. to 280° C. The temperature of the reatants is maintained at −70° C. to 300° C., especially at −20° C. to 280° C. 
         [0041]    The polysilane and/or the H-silanes preferably meet the requirements for uses in the manufacture of semiconductors. 
         [0042]    Herein the terms “short-chain” and “short chain length” relate to those compounds where n is=1 to 6. 
         [0043]    “Virtually no” is to be understood as meaning that there is less than 2% in the mixture. “Predominantly” is to be understood as meaning that the constituent referred to is present in the mixture at more than 50%. 
         [0044]    “Inert solvents” is to be understood as meaning solvents which, under standard conditions, do not react spontaneously with the hydrogenated polysilanes (hereinafter also referred to as “polysilane”), examples being SiCl 4 , benzene, toluene, paraffin, and the like. 
         [0045]    Obtaining a storage material will now be described with reference to an example: 
         [0046]    25.7 g of a substantially chlorinated polysilane mixture obtained by plasma-chemical methods are dissolved in 350 ml of benzene and are added dropwise with stirring at 0° C. in 180 mL of a 1 M LiAlH 4  solution in diethyl ether. This is followed by gradual warming to room temperature. After altogether 24 h, the solid precipitate is filtered off and washed with benzene. The residue is extracted with altogether 400 mL of absolute ethanol at room temperature to remove lithium salts and, as residue, 3.85 g of solid polysilane mixture are isolated as storage material.