This invention relates to improved methods and techniques for preparing polysilanes in increased yields, higher molecular weights and/or lower polydispersities, and for controlling the polymerization of silanes to form polysilanes.
Interest in polysilanes, a "new" class of polymers with a backbone of silicon atoms bonded to various organic substituents, (Scheme 1), is growing rapidly as a result of an increasing number of emerging technological applications for the materials. These include uses as: precursors to SiC fibers, S. Yajima, Am. Ceram. Soc. Bull., 62, 893 (1983) and references therein; ceramic reinforcements, K. Mazdyasni, et al., J. Am. Ceram. Soc., 61, 504 (1978); vinyl polymerization catalysts, A. R. Wolff, R. West, and D. C. Peterson, 17th Organosilicon Symposium, Fargo, ND (1983); conductive polymers for batteries, mid-UV solvent-developed photoresists, D. C. Hofer, R. D. Miller and C. G. Willson, SPIE Adv. Resist Tech., 469, 16 (1984); imageable etch barriers in bilayer microlithography, H. Hiraoka, et al., U.S. Pat. No. 4,464,480 (1984); contrast enhancement layers, D. C. Hofer, R. D. Miller, C. G. Willson, A. Neureuther, SPIE Adv. Resist Tech., 469, 108 (1984); self-developing deep UV photoresists and imageable etch barriers, etc., J. M. Zeigler, L. A. Harrah, and A. W. Johnson, SPIE Adv. Resist Tech., 539, 166 (1985). Several of these are covered in U.S. Ser. Nos. 597,005 and 616,148, both now allowed, and both of which disclosures are entirely incorporated by reference herein. ##STR1##
Despite the technological potential of these materials, well-controlled and reproducible methods for making them in high yield, high molecular weight, and with narrow molecular weight distributions have not been reported. Since these qualities are of major importance for many applications, particularly those in the microelectronics processing area, a need exists for improved synthetic methods and routes to polysilanes.
Polysilanes are prepared by the reductive Wurtz-type coupling of the corresponding dichlorosilanes with an alkali metal, typically sodium (Scheme 1). Copolymers can be synthesized by using a mixture of two (or more) dichlorosilane monomers. This method generally gives a mixture of linear polymer and cyclic low molecular weight oligomers. For most of the aforementioned applications, the cyclics fraction is of little or no value. It is desirable to shift the course of the reaction to production of as much linear polymer as possible at the expense of the cyclics. In methods known to the prior art, this reaction normally produces a molecular weight distribution in the linear polymer fraction which is highly polydisperse (Mw/Mn&gt;25) and has at least two (and often more) molecular weight modes. This extremely high polydispersity is particularly detrimental to the proposed application of polysilanes as photoresists or imageable oxygen reactive ion etch barriers, since it can lead to low apparent photospeed, degradation of physical properties, or both. Thus, it would be highly desirable to have an improved route to polysilanes which give higher yields of linear polymer having decreased breadth (polydispersity) of the molecular weight distribution. Polysilanes have been prepared by the reaction in Scheme 1 since the first preparation of the intractable (Ph.sub.2 Si).sub.n by Kipping, J. Chem. Soc., 125, 2291 (1924); ibid., 119, 830 (1921). Burkhard, (U.S. Pat. No. 2,554,976 (1951); C. A. Burkhard), J. Am. Chem. Soc., 71, 963 (1949), teaches the preparation of insoluble, infusible (Me.sub.2 Si).sub.n (where Me=CH.sub.3) of MW about 3200 by the above reaction carried out in aromatic or aliphatic hydrocarbons in an autoclave at a temperature above the melting point of the sodium metal reductant. No differentiation is provided regarding order of reactant addition and no specific studies of the effect of varying the reaction solvent are reported. Clark (U.S. Pat. No. 2,606,879 (1952)) describes the preparation of various silane homopolymers as greasy or wax-like mixtures by essentially the same method using Na metal but without the autoclave. No specific molecular weight characterization is provided. A related disclosure is Jap. Kokai Tokyo Koho JP No. 58,185,628. More recently, Yajima, et al., (U.S. Pat. No. 4,052,430 (1977)), disclosed the preparation of low molecular weight branched and/or polycyclic polycarbosilanes (--Si--C--Si-- main chain polymers) from (Me.sub.2 Si).sub.n prepared essentially by the method of Burkhard. A similar preparation of (Me.sub.2 Si).sub.n as a precursor to polycarbosilanes is reported by Iwai, et al., (U.S. Pat. No. 4,377,677 (1983)). In neither of these latter cases is information provided regarding the MW distribution of the (Me.sub.2 Si).sub.n prepared, but one must assume that it is similar to that reported by Burkhard since the synthetic method is the same.
In a series of three papers, Wesson and Williams report the synthesis of (Me.sub.2 Si).sub.n from highly purified Me.sub.2 SiCl.sub.2, J. P. Wesson and T. C. Williams, J. Polym. Sci., Polym. Chem. Ed., 2833 (1979), the synthesis of copolymers of the Me.sub.2 Si subunit with EtMeSi or n-propyl MeSi subunits, ibid., 959 (1980), and, finally, the preparation of block copolymers of the --(--Ph.sub.2 Si).sub.5 --block with blocks of random length containing Me.sub.2 Si, EtMeSi, or Et.sub.2 Si subunits, respectively ibid., 65 (1981)). In the Wesson and Williams preparation of (Me.sub.2 Si).sub.n, highly purified Me.sub.2 SiCl.sub.2 is added in a single portion to an octane dispersion of Na metal, whereupon the mixture is then heated above the melting point of the Na (98.degree. C.). At this point, a very vigorous reaction occurs. This method is claimed to give an 80% yield of (Me.sub.2 Si).sub.n where n, on the average, is claimed to be 637. Again, no data on the molecular weight distribution are available due to the insolubility of (Me.sub.2 Si).sub.n. This method is essentially comparable to that of Burkhard, (U.S. Pat. No. 2,554,976 (1951)), J. Am. Chem. Soc., 71, 963 (1949). The same procedure was utilized for the copolymers with yields ranging from 20-90% for the total polymer product. Since various fractions of these copolymers were obtained which were soluble to varying extents, it is clear that they are fairly polydisperse although no MW distributions were reported. It was claimed, however, that the higher polymers had Mn of about 25,000-100,000. The route to block copolymers involves entirely different chemistry as shown in Scheme 2 and is not directly germane to the coupling chemistry illustrated in Scheme 1. ##STR2##
A German patent teaches chemistry related to that in Scheme 2, Farbenfab. Bayer A. G., DE No. 1,236,506 (1967). In this patent, various (PhRSi).sub.n polymers, where R=aryl, alkyl, cycloalkyl, are prepared by coupling the corresponding dichlorosilanes with Na metal essentially as in Burkhard and Clark. The resulting polysilanes are then cleaved with K metal to give K--(--PhRSi).sub.x --K which are the subject of the claims. Thus, the preparation of high molecular weight polysilanes is only incidental to the main subject of the patent.
A method for the preparation of a copolymer containing both PhMeSi and Me.sub.2 Si units is described in a patent by West, et al., (U.S. Pat. No. 4,324,901 (1982)), and a paper, J. Am. Chem. Soc., 103, 7352 (1981). The copolymer is obtained by cocondensation of the corresponding monomers according to Scheme 1 using toluene as solvent. Approximately a 60% yield of polysilane copolymer having an extremely broad bimodal molecular weight distribution was obtained. Similarly, Trujillo has reported (J. Organomet. Chem., 198, C27 (1980)) that approximately a 60% yield of the (PhMeSi).sub.n homopolymer can be obtained by adding the monomer to a refluxing dodecane slurry of finely dispersed Na. This method again provides a broad bimodal MW distribution and considerable amounts of insoluble polymer are formed due to crosslinking.
Several papers describe the synthesis of various silane homo and copolymers by the addition of the appropriate monomer(s) to a refluxing slurry of sodium dispersion in toluene. P. Trefonas, et al., J. Polym. Sci., Polym. Lett. Ed., 21, 819 (1983); X. H. Zhang and R. West, J. Polym. Sci., Polym. Chem. Ed., 22, 159 (1984); ibid., 225 (1984). Broad MW distributions and high polydispersities are again obtained as in previous cases where this method is used. Yields range from 20-60% but the polymer obtained is contaminated with large quantities of low MW material.
The use of mixed solvents for the coupling reaction has been examined by two groups. A group at IBM has reported that mixtures of toluene with diglyme provide improved yields of (cyclohexyl MeSi).sub.n R. D. Miller, et al., in Materials for Microlithoqraphy, L. F. Thompson, C. G. Willson; J. M. J. Frechet, Eds., American Chemical Society, Washington, D.C. 1984, pp. 294-296; and McKean et al., IBM Tech. Discl. Bull., 27, (3), 1984. (from 10 up to approx. 30%). This modification has two serious defects: (1) the MW distribution becomes bimodal and each mode is broadened; and (2) the molecular weight modes are both below the critical molecular weight for chain entanglement at about 30,000 daltons. This latter point is important because polymers below this critical value show rheological and structural properties which resemble monomeric compounds rather than high polymers. In a series of several papers, reports, and patent applications, Schilling, et al., have disclosed the use of mixtures of various ethers with alkanes or arenes as reaction solvents for the cocondensation of vinyl dichlorosilanes with various methyl silanes to give partially soluble, vinyl-containing polysilanes for use as precursors to SiC. C. L. Schilling, Jr. and T. C. Williams, Polym. Prepr., 25(1), 1 (1984); C. L. Schilling, Eur. Pat. Appln. No. 123,834; C. L. Schilling, Technical Report 83-4 and 83-4, contracts #N-00014-75-C-1024 and N-00014-81-C-0682 (Office of Naval Research). Although Schilling found that addition of these cosolvents reduces crosslinking during synthesis, the polymer formed was of very low MW (&lt;5000 daltons).
A patent by Peterson, et al. (U.S. Pat. No. 4,276,424 (1981)), discloses an unusual variant of the Na mediated coupling of dichlorosilanes shown in Scheme 1. In this case, the reduction is carried out in tetrahydrofuran using Li metal as reductant and a monochlorosilane, R.sub.1 R.sub.2 SiHCl, as the silicon monomer. In this reaction LiCl and H.sub.2 gas are produced as byproducts. This method is claimed to be particularly useful for producing cyclic oligomeric silanes rather than high polymer. It is, thus, not applicable to the production of linear high polysilanes.
A Yajima, et al., patent (U.S. Pat. No. 4,159,259) involves various procedures for the preparation of polycarbosilanes and polycarbosilanes containing various metallic species which are useful as precursors to silicon carbide. Several of the examples demonstrate the use of various cyclic or linear polysilanes as precursor materials. In the examples, two different synthesis procedures for (Me.sub.2 Si).sub.n are given. In example 8, sodium and Me.sub.2 SiCl.sub.2 are reacted in an unspecified solvent to give (Me.sub.2 Si).sub.n as an insoluble solid of unspecified molecular weight. In example 16, sodium is heated in "decalin anhydride" and Me.sub.2 SiCl.sub.2 "added dropwise." This provided (Me.sub.2 Si).sub.n again as a precipitate. Although the experimental details are incomplete, both procedures appear to fall within the scope of Burkhard's original patent (U.S. Pat. No. 2,554,976). Neither provides any information regarding the molecular weight distribution of the (Me.sub.2 Si).sub.n obtained.
A patent to Nitzsche, et al., (U.S. Pat. No. 3,830,780) discloses a process for condensing linear, hydroxy-terminated polysiloxanes to higher polysiloxanes by heating in the presence of an optional aluminum catalyst. No polysilane structures are mentioned and the chemistry of the condensation process yields Si-O-Si bonds in the polymer backbone.
In Gordon and Clark (U.S. Pat. No. 2,696,480), methods for the preparation of polymers having Si--Si, Si--C.sub.6 H.sub.4 --Si--O, and --C.sub.6 H.sub.4 --Si--C.sub.6 H.sub.4 --Si--O-- backbone structures are disclosed. Although the polymers prepared in this patent contain Si--Si linkages like polysilanes, these constitute a relatively small fraction of the backbone bonds. Thus, this patent does not involve polysilane synthesis per se. No guidance is provided regarding optimum procedures for producing Si-Si backbone polymers with high molecular weights and low polydispersities.
Baney, et al., (U.S. Pat. No. Re. 31,447) describe the preparation and use of alkoxylated or phenoxylated polysilanes and precursors to silicon carbide. The polysilanes in this patent are actually oligosilanes (Mn=632, vide col. 7, lines 51-53) formed by the equilibration of direct process residue (a mixture of various di, tri, and tetrachlorodisilanes) with various phosphonium chlorides. Reaction of the equilibrated chloroterminated oligosilanes in situ with alcohols provides the subject silicon carbide precursor materials. The Baney, et al. processes provide only low molecular weight products by an equilibration process of disilanes rather than by Na-mediated condensation of dichlorosilanes. A publication of the data in this patent (Baney, et al., Organometallics, 2, 859 (1983)) suggests that the products of the Baney, et al. process are polycyclic polysilanes rather than high molecular weight linear polysilanes.
A patent to Schilling, Jr., et al. (U.S. Pat. No. 4,472,591) describes methods for the preparation of hydrosilyl modified polycarbosilanes. Even though some Si-Si backbone bonds are formed when the various silanes are condensed with alkali metals by the procedures described in this patent, the branched polycarbosilanes obtained are only distantly related to linear high molecular weight polysilanes.
Clark (U.S. Pat. No. 2,563,005) and West (U.S. Pat. No. 4,260,780) deal with the synthesis of various polysilanes from dichlorosilanes by condensation with alkali metals in various hydrocarbon and non-hydrocarbon solvents. The only molecular weight data in either patent is for a (PhMeSi).sub.n polymer prepared by condensation with Na metal in toluene. A molecular weight of 30,000 is claimed for this particular material but no information regarding the nature of the distribution is given. The procedures utilized in this patent upon reproduction provide materials which are of low molecular weight (Mw&lt;30,000) and high polydispersity M.sub.w /M.sub.n &gt;20).
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