PROCESS FOR FORMATION OF INSULATING LAYER OF SILYLATED POLYSILSESQUIOXANE ON ELECTRONIC CIRCUIT BOARD

Abstract: A lower alkyl polysilsesquioxane having a general for­ mula wherein R is lower alkyl, preferably CH₃ or C₂H₅, and n is an integer equal to about 50 to about 10,000, prepared by (a) dissolving a lower alkyl trifunctional silane in an organic solvent at a temperature of -20°C to -50°C to form an organic solution thereof: (b) hydrolysing the lower alkyl trifunctional silane by dropping water into the organic solution at a temperature of -20°C to -50°C under an inert gas pressurised 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 pressurised at 1,000 to 3,000 Pa. Also, a flat surfaced insulating layer of a silylated organopolysilsesquioxane having a general formula wherein R¹ and R² are each selected from alkyl and phenyl (preferably methyl or ethyl), and n is an integer equal to about 50 to about 2,000, preferably about 50 to about 500, formed on a circuit board having stepwise differences in height thereon, by applying an organic solution of the polymer; evaporating the solvent; and melting the polymer to flatten the surface of the polymer and curing the polymer.

Description: The present invention relates to a process for forming an insulating layer which flattens stepwise differences in height due to micropatterns of conductors 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 to General Electric Co. report in U.S. Patent No. 3,017,386 that phenyl polysilsesquioxane exhibits fair resistance against thermal decomposition. Takiguchi et al to Shin-etsu Kagaku Kogyo Ltd. 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. 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 methyl or 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 forming a flat surfaced insulating layer comprising the steps of: applying a layer of an organic solution of a silylated organopolysilsesquioxane having a general formula wherein R¹ and R² are methyl, ethyl or phenyl group, and n is an integer equal to 50 to 2,000, preferably 50 to 500, on an electronic circuit board having stepwise differences in height thereon; then evaporating the solvent; and melting the silylated organopolysilsesquioxane to flatten the surface and curing the polymer. By the invention, it is possible to produce an insulating layer which flattens stepwise differences in height due to micropatterns of conductors on an electronic circuit board. Preferably R¹ and R² are methyl or ethyl. Suitable silylated organopolysilsesquioxanes include materials made by silylating the active hydrogens in the hydroxyl groups in organopolysilsesquioxanes made by the method described in divisional application publication no. EP 406911. One preferred process utilises methyl polysilsesquioxane having trimethyl silyl end groups, the weight average molecular weight of which is 7.0 x 10³ to 2.7 x 10⁵ preferably 7.0 x 10³ to 6.7 x 10⁴. Another preferred process utilises ethyl polysilsesquioxane having triethyl silylated end groups, the weight average moleculr weight of which is 8.4 x 10³ to 3.2 x 10⁵, preferably 8.4 x 10³ to 8.1 x 10⁴. In the accompanying drawings: Figures 1 (a) to (f) are sectional views showing the process of producing an insulating layer on an electronic device; Figure 2 is a diagram showing the relationship between weight average molecular weight and the melting point of trimethyl silylated methyl polysilsesquioxane; Figure 3 is a diagram showing the relationship between the difference in height and thickness of silylated methyl polysilsesquioxane before and after heating; Figure 4 is a diagram showing the relationship between rate of residual weight of silylated and non-silylated methyl polysilsesquioxane and polyimide; and Figure 5 is a diagram showing the relationship betwen voltage and leakage current through insulating layers: a double layer of silylated methyl polysilsesquioxane and PSG, and a single layer of PSG. The insulating layer may be an interlayer insulating layer preferably having an insulating layer of an inorganic material thereon, thereby forming a double layered insulating layer. In another embodiment, the insulating layer made in the invention may be a passivation layer formed on the upper surface of the circuit board. The following examples refer to forming an insulating layer of silylated methyl polysilsesquioxane. However, a layer of silylated ethyl or phenyl polysilsesquioxane may be similarly obtained. Example 1 Methyl polysilsesquioxane, which had a polymerisation degree of about 220, was made by the process of Example 2 of divisional application publication no. EP 406911. except that the reflux heating was continued for 2 hours. It was dissolved in a solvent mixture of 600ml of methyl isobutyl ketone and 100ml of pyridine. An 80ml amount of trimethyl chlorosilane (CH₃)₃SiCl was added drop by drop to the solution at 50°C to 60°C to react with methyl polysilsesquioxane for 2 to 3 hours, whereby trimethyl silylated methyl polysilsesquioxane was obtained. As shown in Fig. 1 (a), a silicon substrate 1 having an insulating layer 1', e.g., SiO₂ layer, was provided with patterns of aluminum 2 of 0.9 µm thickness, 3 µm minimum width, and 2 µm minimum spacing. A toluene solution of 20% by weight trimethyl silylated polymethylsilsesquioxane having a weight average molecular weight of 30,000 which corresponds to a polymerization degree of about 220 was spun to coat the substrate 1 at 2,500 rpm for 40 seconds. The solvent was removed by heating at 80°C for 30 minutes, then the polymer was melted to be flat and cured by heating under a nitrogen stream at 350°C for 60 minutes. The heating temperature is preferable 350°C to 400°C. Because the molecular weight of trimethyl silylated methyl polysilsesquioxane is similar to that of nonsilylated polymer, its melting point can be estimated from the diagram of Fig. 2. The melting point of trimethyl silylated methyl polysilsesquioxane is linearly proportional to the weight average molecular weight, as shown in Fig. 2. Thus, the silylated polymer can melt at about 300°C. The curing condition was determined in order that the cured silylated polymer does not decrease in its thickness after immersing for 5 minutes in solvents. Temp. (°C) Time (min) Thickness (µm) After in acetone (µm) After in xylene (µm) 200 30 1.00 0.35 0.10 200 60 1.00 0.40 0.25 300 30 0.95 0.70 0.40 300 60 0.95 0.85 0.45 350 30 0.95 0.90 0.65 350 60 0.90 0.90 0.90 400 30 0.85 0.85 0.85 Thus, the condition was determined at 350°C for 60 minutes. Because trimethyl silylated methyl polysilsesquioxane has no reactive hydroxy end radicals, thermal crosslinking of methyl groups may solely occur, consequently, a low density of crosslinking is secured after curing. Thus, the thickness of an insulating layer can be as thick as 3.5 µm on patterns of aluminum, which has a relatively large coefficient of thermal expansion, without suffering from cracking. The thickness of the polymer 3 was about 0.6 µm on the aluminum and about 1.5 µm on the area between the aluminum. Thus, the difference in height was almost completely eliminated, with at most 0.05 µm remaining. The layered circuit structure provided with, as shown in Fig. 1 (b), trimethyl silylated methyl polysilsesquioxane as a passivation layer, was subjected to a pressure-cooker test at 120°C under 2 atmospheres for 48 hours. No corrosion of aluminum was observed. Figure 3 shows the step coverage relationship between the thickness of the silylated methyl polysilsesquioxane layers coated over a substrate having a stepwise difference in height of 1 µm. One curve indicates the step coverage of the spun coated silylated methyl polysilsesquioxane over the substrate before melting of the polymer. The other curve indicates the step coverage of the silylated methyl polysilsesquioxane by melting and flowing to be flat. The step coverage can be improved by melting the silylated polymer. The leveling property of the silylated methyl polysilsesquioxane was evaluated as the thickness 1.2 µm of the spun coated polymer layer over the substrate having stepwise differences in height of 1 µm, to flatten or to decrease the stepwise differences less than 0.1 µm. On the other hand, that of the nonsilylated methyl polysilsesquioxane was more than 3.0 µm. 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 2 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. The stepwise difference in height was 1 µm. The thicknesses of the coated polymers on the substrate were varied in the range of from 0.5 µm to 4.0 µm. The conductive or insulating pattern had a thickness of 1 µm. Aluminum was used for conductive patterns, and SiO₂ was used for insulating patterns. Thickness of polymers (µm) Polymers Silylated Non-ed on Al Silylated Non-ed on SiO₂ 0.5 o ▵ o o 1.0 o x o ▵ 1.5 o x o x 2.0 o x o x 2.5 o x o x 3.0 o x o x 3.5 ▵ x o x 4.0 x x ▵ x o - No cracks ▵ - Cracks occurred on part of the surface x - Cracks occurred over all the surface As seen from Table 2 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 low alkyl 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. Example 2 A silicon substrate having an insulating layer on the surface thereof was provided with patterns of aluminum. A 4-methyl-2-pentanone (MIBK) solution of trimethyl silylated polymethylsilsesquioxane having a weight average molecular weight of about 30,000, which corresponds to a polymerization degree of about 220, was spun to coat the substrate. The solvent was removed by evaporation, then the polymer was melted to be flat and cured by heating under a nitrogen stream at 350°C for 60 minutes. The thickness of the polymer 3 was about 2 µm. Thus formed polymer layer can be used for the passivation layer, and the difference in height was almost completely eliminated. The silicon circuit structure provided with trimethyl silylated methyl polysilsesquioxane as a passivation layer was subjected to a pressure-cooker test at 120°C under 2 atmospheres for 48 hours. No corrosion of aluminum was observed. Comparative Example 1 A bipolar semiconductor element was produced similar to Example 2, except that nonsilylated methyl polysilsesquioxane was coated instead of the silylated methyl polysilsesquioxane. A toluene solution of 20% by weight of methyl polysilsesquioxane was spun to coat the substrate at 2500 rpm for 40 seconds. The solvent was removed by heating at 80°C for 30 minutes, then the polymer was heat treated under a nitrogen atmosphere at 350°C for 60 minutes. Cracks were observed in the formed methyl polysilsesquioxane polymer layer on the aluminum layer. The difference in height remained at 0.2 µm, when the original difference in height was 1 µm. Comparative Example 2 A silicon substrate having an SiO₂ insulating layer on the surface thereof was provided with patterns of aluminum. The SiO₂ passivation layer of 1 µm thickness was formed by plasma chemical vapor deposition. The length of time needed for forming the passivation layer of 1 µm thickness was about 5 to 10 minutes and the step coverage was not good. The silicon circuit structure provided with this plasma CVD SiO₂ as a passivation layer was subjected to a pressure-cooker test at 120°C under 2 atmosphere for 10 hours corrosion of aluminum was observed. Example 3 As shown in Fig. 1 (a), a silicon substrate 1 having an insulating layer 1', e.g., SiO₂ layer was provided with patterns of aluminum 2 of 1 µm thickness. A methyl isobutyl ketone solution of 20% by weight of trimethyl silylated methyl polysilsesquioxane having a weight average molecular weight (MW) of 30,000, which corresponds to a polymerization degree of about 220, and a ratio of the weight average molecular weight to a number average molecular weight (MW/MN), i.e., degree of dispersion, of 1.2 was spun to coat the substrate 1 at 4,000 rpm. The solvent was removed by heating at 120°C for 10 minutes, then the polymer was melted to be flat and cured by heating at 350°C for 60 minutes. The thus formed trimethyl silylated methyl polysilsesquioxane layer having a thickness of 1.2 to 1.4 µm enabled flattening of the stepwise differences between the substrate and aluminum layer. The thickness of the polymer 3 was about 1.2 to 1.4 µm on the area between the aluminum. Thus, the difference in height was almost completely eliminated, with at most 0.05 µm remaining. Next, a PSG layer 4 of 1.0 µm thickness was formed by chemical vapor deposition on the flat surface of the trimethyl silylated methyl polysilsesquioxane layer 3, as shown in Fig. 1 (c). Through holes 5 were opened by CF₄ plasma etching of the insulating layers 3 and 4 using a mask 6 of photoresist registered tradename "AZ 1350". Thereafter, the photoresist 6 was eliminated by O₂ plasma etching, and then second aluminum layer 7 of 1 µm thickness was deposited to connect to the first aluminum 2 via the through holes 5, as shown in Fig. 1 (d) to Fig. 1 (f). The resultant multilayer circuit structure was subjected to a electric leakage test. No electric leakage was observed, as shown in curve D of Fig. 5, by applying 25 volts. Furthermore, the multilayer circuit structure was subjected to a reliability test, i.e., first to heating at 250°C for 500 hours and second to thermal shock of -65°C to 150°C for 500 hours, in which one cycle continued one and a half hours. Thereafter, no electric leakage was observed by applying 25 volts. Comparative Example 3 A multilayer circuit board was produced similar to Example 2 except that no trimethyl silylated methyl polysilsesquioxane layer 3 was formed, in other words, only an inorganic insulating layer was formed. The obtained multilayer circuit structure exhibited leakage of approximately 1 mA upon applying 13 volts, as shown in Curve E of Fig. 5, in an electric leakage test similar to that of Example 3. Example 4 A trimethyl silylated methyl polysilsesquioxane layer was formed by a procedure similar to Example 3, except that the trimethyl silylated methyl polysilsesquioxane was applied on a magnetic bubble memory having a layer of nickel-iron. That is, a 4-methyl-2-pentanone solution of 20% by weight of trimethyl silylated methyl polysilsesquioxane having a weight average molecular weight of about 31,000 which corresponds to a polymerization degree of about 230 was spun to coat a magnetic bubble memory having a layer of a nickel-iron alloy, registered tradename "Permalloy". After removing the solvent, the polymer melted to be flat and was cured by heating under nitrogen at 350°C for 1 hour. In this case, the passivation layer of the polymer had a thickness of 2 µm. The magnetic bubble memory provided with trimethyl silylated methyl polysilsesquioxane as a passivation layer was subjected to a pressure-cooker test at 120°C under 2 atmospheres for 48 hours. No corrosion of the nickel-iron alloy was observed. Comparative Example 4 A 4-methyl-2-pentanone solution of 20% by weight of nonsilylated methyl polysilsesquioxane having a polymerization degree of about 230 was spun to coat a magnetic bubble memory having a layer of a nickel-iron alloy, registered tradename "Permalloy". After removing the solvent, the polymer was cured by heating under nitrogen at 350°C for 1 hour. In this case, the passivation layer of the polymer had a thickness of 1 µm. Because this nonsilylated methyl polysilsesquioxane layer generated cracks after heating when the thickness was greater than 1 µm, the step coverage was insufficient. In addition, after a pressure cooker test at 120°C under 2 atmospheres for 30 hours, corrosion of the nickel-iron alloy was observed.

Claims: A process for forming a flat surfaced insulating layer of a polysilsesquioxane on a circuit board having stepwise differences in height thereon, characterised in that it comprises: applying onto the board an organic solution of silylated polysilsesquioxane having a general formula wherein R¹ and R² are each selected from methyl, ethyl and phenyl groups, and n is an integer equal to 50 to 2,000; evaporating the solvent; and melting said silylated polysilsesquioxane to flatten the surface of said polymer and curing the polymer. A process according to claim 1 wherein n is an integer equal to 50 to 500. A process according to claims 1 or 2 in which the groups R¹ and R² are each selected from methyl and ethyl. A process according to claim 3 wherein said silylated polysilsesquioxane is methyl polysilsesquioxane having trimethyl silyl end groups, the weight average molecular weight of which is 7.0 x 10³ to 2.7 x 10⁵. A process according to claim 4 wherein said silylated polysilsesquioxane is methyl polysilsesquioxane having trimethyl silyl end groups, the weight average molecular weight of which is 7.0 x 10³ to 6.7 x 10⁴. A process according to claim 3 wherein said silylated polysilsesquioxane is ethyl polysilsesquioxane having triethyl silylated end groups, the weight average molecular weight of which is 8.4 x 10³ to 3.2 x 10⁵. A process according to claim 6 wherein said silyated polysilsesquioxane is ethyl polysilsesquioxane having triethyl silylated end groups, the weight average molecular weight of which is 8.4 x 10³ to 8.1 x 10⁴. A process according to any of claims 1 to 7 wherein said insulating layer is a passivation film. A process according to any of claims 1 to 8 wherein said insulating layer is an interlayer insulating layer, having an insulating layer of an inorganic material thereon, thereby forming a double layered insulating layer.

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