PROCESS FOR PREPARATION OF POLYSILSESQUIOXANE

Abstract: A process for preparation of polysilsesquioxane having a general formula wherein R is CH₃ or C₂H₅ , and n is an integer equal to about 50 to about 10,000, prepared by (a) dissolving methyl or ethyl trifunctional silane in an organic solvent at a temperature of -20°C to -50°C to form an organic solution thereof; (b) hydrolyzing the trifunc­ tional silane by dropping water into the organic solu­ tion at a temperature of -20°C to -50°C under an inert gas pressurized at 1,000 to 3,000 Pa; and (c) gradually heating the organic solution together with a water phase lying therebeneath up to a temperature of 60°C to 100°C under an inert gas pressurized at 1,000 to 3,000 Pa.

Description: The present invention relates to a process for preparing methyl or ethyl polysilsesquioxane useful for preparing an insulating layer of silylated ethyl or methyl polysilsesquioxane on an electronic circuit board. Insulating layers are commonly formed on electronic circuit boards having electronic elements, particularly integrated circuits, large scale integrated circuits, and magnetic bubble memory devices. The surface of these circuit boards exhibit stepwise differences in height due to the micropatterns formed thereon. Therefore, the insulating layers must exhibit flat step coverage. In addition, they must exhibit a thermal resistance to prevent thermal decomposition, a coefficient of expansion similar to that of the underlying material to prevent cracks, and a dense structure to prevent penetration of external gases or moisture, which would lead to deterioration of the performance of the electronic elements. Insulating layers are usually made of inorganic oxides, e.g, phosphosilicate glass (PSG) and/or organic resins. Inorganic layers have the advantage of a relatively dense structure, however, in certain cases display insufficient step coverage. In addition, such layers produced by plasma chemical vapor deposition or sputtering have the disadvantage of not forming a desirable thickness in an acceptable duration of operation. On the other hand, organic resins, e.g., polyimide, exhibit fair step coverage and, consequently, a flat surface profile. However, they have the demerits of a coarse structure, consequently, high hygroscopic property, lack of resistance against thermal decomposition and thermal shock. Furthermore, organic resins have to be readily dissolvable in an organic solvent to allow them to be applied to the electronic circuit board. An organosilicone resin has been noted to exhibit combined features of organic and inorganic materials. Particularly, organopolysilsesquioxanes having a so-called ladder structure, i.e., a two-dimensional structure, are more resistant against thermal decomposition than organopolysiloxanes having a linear structure. It is known that phenyl polysilsesquioxane suffers from cracking in the process of curing, and that methyl polysilsesquioxane exhibits reduced cracking. Brown et al report in U.S. Patent No. 3,017,386 that phenyl polysilsesquioxane exhibits fair resistance against thermal decomposition. Takiguchi et al, however, report in Japanese Unexamined Patent Publication (Kokai) No. 50-111198 that methyl polysilsesquioxane produced by a process in which methyl trichlorosilane reacts mildly with water has the demerit of insolubility in an organic solvent. This insolubility derives from the formation of a three-dimensional structure due to cross-linking at hydroxide groups, which are produced by hydrolyzation of methyl groups and not spent for polycondensation of polysilsesquioxane. Therefore, it is essential to conduct hydrolyzation of methyl trichlorosilane in extraordinarily mild conditions. Brown et al to General Electric Co. disclose in U.S. Patent No. 3,017,386 a process for preparing an organopolysilsesquioxane. A phenyl prepolymer is formed by reacting phenyl trichlorosilane in an organic solvent with alkaline water at a temperature of about 75°C to 175°C, washing the hydrolyzate with water, and removing essentially all of the water by azeotropic distillation. The resulting solution of the prepolymer is then treated at a temperature of about 250°C to obtain an organopolysilsesquioxane having a polymerization degree of several hundred to several thousand or more. Takiguchi et al to Shin-etsu Kagaku Kogyo Ltd. disclose in Japanese Unexamined Patent Publication No. 50-111198 a process for preparing a lower alkyl polysilsesquioxane. First, a prepolymer of a number average molecular weight of 9,000 to 10,000 is formed by dissolving methyl trichlorosilane together with triethyl amine in a ketone-furan solvent and hydrolyzing ice-cooled methyl trichlorosilane with dropping water under normal pressure. Second, the prepolymer is precipitated by adding methyl alcohol to the hydrolyzed solution and then, dissolved again in a ketone-furan solvent and heated at 90°C for 4 hours to obtain a polymer of a number average molecular weight of about 10,000 to about 100,000. Sumie et al to Nihon Gosei Gomu Ltd. disclose in Japanese Unexamined Patent Publication No. 53-88099 a process similar to that of Takiguchi et al, Sumie et al found that the obtained methyl polysilsesquioxane became insoluble after further standing at room temperature, although the polymer has a relatively low number average molecular weight of about 5,000. Sumie et al treated the once separated prepolymer with an ammonium salt dissolved in water or methyl alcohol at about 90°C for 4 hours to obtain methyl polysilsesquioxane of a number average molecular weight of about 20,000 to about 100,000. They found that the obtained polymer did not become insoluble after standing. In regard to organopolysilsesquioxanes adapted for use as an insulating layer, as we previously stated, phenyl polysilsesquioxane exhibits a fair resistance against thermal decomposition but not against thermal cracking and that a low alkyl polysilsesquioxane exhibits a resistance reverse to that of phenyl polysilsesquioxane. Kamiya et al disclose in Yogyo Kyokai Shi, Vol. 92, No. 5, pages 242 to 247, 1984 a process to improve the thermal resistance of a diorganosiloxane polymer by replacing the active hydroxide end radicals with trimethyl silyl groups. However, such a siloxane polymer has not been applied for insulating layers of an electronic circuit board. Sudo et al to Hitachi Ltd. disclose in Japanese Unexamined Patent Publication No. 55-50645 an organopolysilsesquioxane layer formed on and near the surface of a semiconductor device. Most of the organic groups are selected from the groups of phenyl and methyl. Part of the organic groups are selected from the groups of C2 to C6 alkyl, vinyl, halogenized phenyl, hydroxyl alkyl, mercaptoalkyl, alkyl phenyl, and aminophenyl. Shoji et al to Hitachi Ltd. disclose in Japanese Unexamined Patent Publication No. 55-145355 a layer insulating layer of organopolysilsesquioxane in which organic radicals are selected from the groups of methyl, ethyl, propyl, butyl, phenyl, halogenophenyl, halogenomethyl phenyl, and amyl hexyl phenyl ethyl. Tokunaga et al to Fujitsu Ltd. disclose in Japanese Unexamined Patent Publication No. 56-125857 a process for producing a semiconductor device in which methyl polysilsesquioxane is used to flatten the stepwise lower portion between underlying micropatterns of conductors and an inorganic insulating layer is formed thereon. However, Tokunaga et al do not refer at all to the active end groups of the methyl polysilsesquioxane. Neither Sudo et al, Shoji et al, nor Tokunaga et al teach or suggest to replace the active end radicals of the organopolysilsesquioxane with trialkyl silyl groups. According to the present invention, there is provided a process for preparing polysilsesquioxane having a general formula wherein R is CH₃ or C₂H₅ , and n is an integer of 50 to 10,000, comprising the steps of: (a) dissolving methyl or ethyl trifunctional silane, e.g., methyl or ethyl trihalogenosilane, preferably methyl or ethyl trichlorosilane, or trimethoxy or triethoxy silane in an organic solvent at a temperature of -20°C to -50°C to form an organic solution thereof; (b) hydrolyzing the methyl or ethyl trifunctional silane, preferably methyl or ethyl trichlorosilane, or trimethoxy or triethoxy silane by dropping water into the organic solution at a temperature of -20°C to -50°C under an inert gas pressurized at 1,000 to 3,000 Pa; and (c) gradually heating the organic solution together with a water phase lying therebeneath up to a temperature of 60°C to 100°C under an inert gas pressurized at 1,000 to 3,000 Pa. By the invention it is possible to prepare a methyl or ethyl polysilsesquioxane that can be resistant against thermal decomposition, that can be resistant against cracking, and that can be soluble in an organic solvent even after prolonged standing. The process involves a single reaction system wherein methyl or ethyl trichlorosilane-containing organic phase co-exists with an underlying water phase. One preferred product is methyl polysilsesquioxane, the weight average molecular weight of whichis 6.7 x 10³ to 1.34 x 10⁶. Another preferred product is ethyl polysilsesquioxane, the weight average molecular weight of which is 8.2 x 10³ to 1.62 x 10⁶. Figure 1 is a diagram showing the relationship between the weight average molecular weight of methyl polysilsesquioxane and the square root of polymerization time at various pressures; Fig. 2 is a diagram showing the relationship between infrared transmittance through methyl polysilsesquioxane and the wave number; Fig. 3 is a diagram showing the relationship between weight average molecular weight and the melting point of trimethyl silylated methyl polysilsesquioxane; Fig. 4 is a diagram showing the relationship between rate of residual weight of silylated and nonsilylated methyl polysilsesquioxane and polyimide. According to the present invention, due to the treatment at a very low temperature in the stages of dissolving methyl or ethyl trichlorosilane in an organic solvent and of dropping water into the resulting solution of methyl or ethyl trichlorosilane, it is possible to control the reaction of methyl or ethyl trichlorosilane with a trace of water, which may exist in the organic solvent, and with dropping water, thus to hydrolyze very gradually methyl or ethyl trichlorosilane to form a prepolymer, thereby preventing the prepolymer from further polycondensating to have a three-dimensional structure. The product therefore exhibits effective solubility in an organic solvent. Owing to the pressurized inert gas atmosphere in the stages of dropping water into methyl or ethyl trichlorosilane-containing organic phase and of heating the organic phase with a water phase lying therebeneath, it is possible to control the reaction of methyl or ethyl trichlorosilane with water, while water is inhibited from evaporating to form microbubbles of water vapor which might come up into the organic phase where methyl or ethyl trichlorosilane might react with water. Thus, the pressurized inert gas is one of the most characteristic features of the present invention. The inert gas is pressurized at 1,000 to 3,000 Pa. As shown in Fig. 1, the weight average molecular weight of the polymer does not increase with the increased duration of polymerization at normal pressures, however, the ratio of increasing the degree of polymerization can be enhanced by increasing the pressure of the inert gas from +1,000 Pa to +3,000 Pa. It is surprising that such a minor pressure is sufficient to control the polymerization degree of the polymer and inhibit the cross-linking of the polymer. Thus, according to the present invention, formation of methyl or ethyl polysilsesquioxane is carried out in one reaction system, i.e., in the presence of a water phase underlying the organic phase. The obtained polymer exhibits desirable features adapted for use as an insulating layer of semiconductor devices, i.e., a high molecular weight, which may be easily modified by changing the temperature and duration of the reaction, and a fair solubility in an organic solvent, even after standing storage over a few months. The polysilsesquioxane may be methyl polysilsesquioxane. It may be silated, and the resultant compound may be used for forming an insulating layer, as described in parent application EP-A-0198976 Thus the final compound can be methyl polysilsesquioxane having trimethyl silyl end groups. The following examples refer to the preparation of methyl polysilsesquioxane. However, ethyl polysilsesquioxane can be similarly obtained according to the present invention. Example 1 An 84 ml amount of triethylamine and 540 ml amount of methyl isobutyl ketone were poured into a flask provided with a reflux condenser, a dropping funnel, a nitrogen-blowing pipe, and a stirrer. The flask was purged with nitrogen. The flask was cooled at -30°C to -40°C by means of a dry-ice-cooled ethyl alcohol bath. A 78 ml amount of methyl trichlorosilane was added to the solution while stirring, to react with the trace of water dissolved in the solution and to generate hydrogen chloride, which reacted with triethyl amine (C₂H₅)₃N to form a white precipitate of triethyl amine hydrogen chloride (C₂H₅)₃N·HCl. Nitrogen pressurized at 1,000 Pa was introduced into the flask. About 100 ml of water was added drop by drop for 90 minutes to form triethyl amine hydrogen chloride, which was soluble in an excess of water to promote hydrolyzation of methyl trichlorosilane and to form a prepolymer of methyl polysilsesquioxane. With the organic phase and the water phase in the flask kept under nitrogen pressurized at 1,000 Pa, the dry ice cooling was stopped. Thus, the temperature of the flask rose gradually up to room temperature. Then, the flask was heated to reflux on an oil bath, still under pressurized nitrogen, up to 90°C ± 2°C for 10 hours to achieve the polycondensation for obtaining high molecular methyl polysilsesquioxane. The organic phase was separated from the water phase and washed with water until triethyl amine hydrogen chloride was removed and the washing water became neutral. After removing the washing water, the organic phase was distilled to concentrate the same. Then, ethyl alcohol was added to the organic phase to precipitate high molecular methyl polysilsesquioxane. The precipitate was washed with ethyl alcohol and dried in vacuum. The obtained polymer was found to have a weight average molecular weight of 100,000, which corresponds to a polymerization degree of about 750, by means of gel permeation chromatographic analysis, and to have peak infrared absorption spectra of 3,000 cm⁻¹ (ν C-H ), 780 cm⁻¹ (ν Si-CH ), 1,125 cm⁻¹, and 1,040 cm⁻¹, the latter two recognized to be ν asym Si-O-Si as described in J. Polym, Sci., Vol. C-1, p.83 (1963). The infrared absorption spectra are shown in Fig. 2. Thus, the polymer was identified to be methyl polysilsesquioxane having a so-called ladder structure. In addition, the obtained polymer was soluble in toluene or methyl isobutyl ketone solvent, even after standing 3 months at room temperature. Example 2 The same procedure was carried out as in Example 1, except that the nitrogen was pressurized at 3,000 Pa and that the reflux heating was continued for 4 hours. The obtained methyl polysilsesquioxane was found to have a weight average molecular weight of 600,000, which corresponds to a polymerization degree of about 4,500. Example 3 Methyl polysilsesquioxane was prepared by a procedure similar to Example 2, except that the reflux heating was continued for 8 hours. The obtained methyl polysilsesquioxane was found to have a weight average molecular weight of 1,000,000, which corresponds to a polymerization degree of about 7450. Comparative Example 1 The same procedures were carried out as in Example 1, except that the nitrogen was kept at normal pressure. The prepolymer formation was conducted at a temperature as low as that of Example 1. The reflux heating was prolonged to 10 hours at 90°C ± 2°C. The obtained polymer was found to have a weight average molecular weight of 40,000, which corresponds to a polymerization degree of as low as about 300. Example 4 The same procedure was carried out as in Example 2, except that the reflux heating was continued for 2 hours. The obtained methyl polysilsesquioxane, which had a polymerization degree of about 220, dissolved in a solvent mixture of 600 ml of methyl isobutyl ketone and 100 ml of pyridine. An 80 ml amount of trimethyl chlorosilane (CH₃)₃SiCl was added drop by drop to the solution at 50°C to 60°C to react with metyl polysilsesquioxane for 2 to 3 hours, whereby trimethyl silylated methyl polysilsesquioxane was obtained. An insulating layer of polymer formed on aluminum patterns was subjected to thermal analysis up to 900°C, and the ratio of residual weight of the polymers was determined. As shown in Fig. 4, the silylated polymer (curve A) exhibits substantially no weight decrease up to 700°C, while nonsilylated polymer (curve B) begins to lose weight at 450°C. A polyimide resin (curve C), shown for comparison, decreases far more rapidly. Table 1 indicates the results of a comparative test of the resistance properties against crack generation of silylated methyl polysilsesquioxane and nonsilylated methyl polysilsesquioxane. The test was carried out by heating samples in which the two kind of polymers were spun on a substrate, having a stepwise difference between the conductive layer and insulating layer formed on a silicon substrate. The samples were heated at a temperature of 500°C for 1 hour under a nitrogen atmosphere. As seen from Table 1 above, non-silylated methyl polysilsesquioxane has insufficient resistance against crack generation. On the other hand, the silylated methyl polysilsesquioxane of the present invention has a good resistance against crack generation although the thickness of the polymers is about 4 µm, Silylated methyl polysilsesquioxane, obtained by silylating the residual active hydrogen in OH group in the polysilsesquioxane, can retain a molecular structure able to relax mechanical strain after heating and be melted to be flat by suppressing the proportion of the thermal cross-linkage. Consequently, the silylated methyl polysilsesquioxane of the invention has superior resistance against crack generation.

Claims: A process for preparing a polysilsesquioxane having a general formula wherein R is methyl or ethyl and n is an integer equal to 50 to 10,000, comprising hydrolyzing methyl or ethyl trifunctional silane with water, characterized in that the process comprises: (a) dissolving said trifunctional silane in an organic solvent at a temperature of -20°C to -50°C to form an organic solution thereof; (b) hydrolyzing said trifunctional silane by dropping water into said organic solution at a temperature of -20°C to -50°C under an inert gas pressurized at 1,000 to 3,000 Pa; and (c) gradually heating said organic solution together with a water phase lying therebeneath up to a temperature of 60°C to 100°C under an inert gas pressurized at 1,000 to 3,000 Pa. A process according to claim 2, wherein said polysilsesquioxane is methyl polysilsesquioxane, the weight average molecular weight of which is 6.7 x 10³ to 1.34 x 10⁶. A process according to claim 2, wherein said polysilsesquioxane is ethyl polysilsesquioxane, the weight average molecular weight of which is 8.2 x 10³ to 1.62 x 10⁶.

Inventors: FUKUYAMA, SHUN-ICHI, FUJITSU ATSUSGI-RYO, 3-10, MATSUURA, AZUMA, MIYAGAWA, MASASHI, NISHII, KOTA, YONEDA, YASUHIRO
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: C08G77
IPC codes: H01L23, H01B3, H01L21, H05K1, C08G77