LOW DIELECTRIC CONSTANT SUBSTRATE AND METHOD OF MAKING

Abstract: A low dielectric constant substrate which includes ceramic particles uniformly coated with a borosilicate glass. The substrate preferably has a dielectric constant of about 4 or less. There is also disclosed a method of making the low dielectric constant substrate which includes as an important element therein of coating the ceramic particles with the borosilicate glass by a sol-gel process.

Description: LO DIELECTRIC CONSTANT SUBSTRATE AND METHOD OF MAKING BACKGROUND OF THE INVENTION This invention relates to glass and/or ceramic (hereinafter just ceramic) substrates, and more particularly relates to ceramic substrates useful for electronics packaging and to a method for making such substrates. Ceramic structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering will become a sintered dense metal interconnection of metal-based conductor. In general, conventional ceramic structures are formed from ceramic green sheets which are prepared by mixing a ceramic particulate, a catalyst (e.g., such as that disclosed in Herron et al. U.S. Patent 4,627,160), a thermoplastic polymeric binder, -2- plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After blanking, stacking, and laminating, the green sheets are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate. The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. It is more desirable, however, to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof. Present state-of-the-art ceramic substrates are made from cordierite glass-ceramic particulate materials such as that disclosed in Kumar et al. U.S. Patent 4,301,324. These substrates exhibit a dielectric constant of about 5 or more and a thermal coefficient of expansion (TCE) that closely matches that of silicon. It is desirable to fabricate substrates out of low dielectric constant materials so as to increase signal propagation speed, which varies with the inverse square root of the dielectric constant. Prior to the cordierite glass-ceramic materials, alumina for a number of years had been an adequate dielectric material for microelectronic packaging. Alumina, however, has a dielectric constant approaching 10 which causes high signal propagation delay and low signal-to-noise ratio. Further, alumina has a TCE about twice as high as- silicon which impacts the silicon chip to ceramic thermal fatigue resistance. The transition from alumina to the cordierite glass-ceramics represented a leap of technology. It is anticipated that future requirements of electronic packaging will require substrates having improved properties, particularly improved (i.e., decreased) dielectric constant, over the cordierite glass- ceramics. Porous silica films have been formed which have a dielectric constant as low as 1. This material is not expected to be suitable for electronic packaging because of shrinkage on firing, insufficient mechanical integrity of the resulting structures, and thermal expansion mismatch with silicon. A possible successor material of interest is a ceramic comprised of silica and borosilicate glass. For example, Tosaki et al. U.S. Patent 4,547,625 and Kokuleu et al. U. S. Patent 4,624,934 disclose mixtures of a borosilicate glass and silica glass or refractory particles. These mixtures are made by ball-milling the components together. The resulting products have dielectric constants of 4.05 or greater. The conventional method of fabricating the raw ceramic material by simply ball-milling silica and borosilicate glass or glass-forming powders leads to inhomogenous microstructures. During sintering, the structures formed from such materials tend to experience dimensional distortion. Various authors, however, have proposed forming silica and borosilicate glass ceramics by a sol-gel process. For example, Kumar, Mat. Res. Bull., 19, pp. 331-338 (1984) and Noga i et al., J. of Non-Crystalline Solids, 8, pp. 359-366 (1982) have disclosed the sol-gel synthesis of silica and borosilicate glass ceramics. Tohge et al., Yogyo-Kyokai Shi, 95_, pp. 182-185 (1987) have proposed the fabrication of silica and borosilicate glass films on glass substrates. Tohge et al., J. of Non-Crystalline Solids, 100, pp. 501-505 (1988) have proposed the patterning of silica and borosilicate glass films on glass substrates. The films are patterned by a mechanical stamper while the films are in the gel state. Barringer et al. U.S. Patent 4,788,046 have gone further and made substrates for electronic packaging from such ceramic materials. Barringer fabricates his ceramic materials by a sol gel process. The borosilicate glasses disclosed include, in addition to silica and boron oxide, magnesia, calcia, and alumina. These additional components are added to the glass for a number of reasons. Included among these reasons are the need for chemical stability of the glass and the desire that the ceramic particles not react excessively with the glass. Unfortunately, adding magnesia, calcia, and alumina also raises the dielectric constant of the glass. As noted in the Table following Example 3 in the reference, nearly all the glass ceramic materials have dielectric constants greater than 5.3. The one exception is the glass ceramic material consisting of quartz and a borosilicate glass which has dielectric constants of 4.5-5.0, but this material suffers from a TCE much higher than that of silicon. Notwithstanding the efforts of Barringer et al. and others in this field, there still remains a need for a ceramic material that has a low dielectric constant, a TCE compatible with silicon, mechanical integrity, and manufacturability. Accordingly, it is an object of the present invention to have an improved ceramic material containing a borosilicate glass that meets or exceeds the above electrical and mechanical requirements. It is another object of the invention to have a method of making such an improved ceramic material. These and other objects of the invention will become more apparent after reference to the following detailed description of the invention considered in conjunction with the accompanying Figures. BRIEF SUMMARY OF THE INVENTION in accordance with the present invention, the objects of the invention have been achieved by providing a low dielectric constant substrate which comprises ceramic particles uniformly coated with a borosilicate glass. In order to achieve the uniform coating of the ceramic particles, it is necessary that they be coated by a sol-gel process. The resulting ceramic substrate preferably has a dielectric constant of about 4 or less. -6- BRIEF DESCRIPTION OF THE FIGURE Figures 1A and IB are schematical representations before and after sintering, respectively, of a borosilicate glass/ceramic particle composition prepared by a conventional ball-milling technique. Figures 2A and 2B are schematical representations before and after sintering, respectively, of a borosilicate glass/ceramic particle composition prepared by a sol-gel technique. Figure 3 is a graph of percent theoretical density versus percent borosilicate glass coating on silica ceramic particles. DETAILED DESCRIPTION OF THE INVENTION According to the invention, there is disclosed a low dielectric constant substrate comprising ceramic particles uniformly coated with a borosilicate glass where the substrate has a dielectric constant of about 4 or less. As will be shown hereafter, the dielectric constant is a function of the composition of the borosilicate glass, the percent of borosilicate glass coating the ceramic particles, and the composition of the ceramic particles. The uniform coating of the ceramic particles is a distinguishing characteristic resulting from their prior sol-gel processing. The fact that ceramic particles coated by a sol-gel process can be so readily identified is made apparent by a comparison of Figures 1 and 2. In Figure 1, a ceramic mixture is schematically illustrated after being prepared by a conventional ball-milling technique. Thus, in Figure 1A, the ceramic particles are mixed with borosilicate glass particles prior to sintering. After sintering, as shown in Figure IB, the borosilicate glass has formed irregularly shaped globules cementing the ceramic particles together. Note that there is little, if any, coating of the ceramic particles by the glass. Figures 2A and 2B also schematically illustrate ceramic mixtures prior to and after sintering, respectively. In Figures 2, however, the ceramic mixture prior to sintering has been prepared by a sol-gel process. In Figure 2A, the ceramic particles have been coated with a borosilicate glass. The coating appears uniform which shall mean that the coating is evenly distributed around the periphery of the ceramic particles. Then, in Figure 2B, the borosilicate glass coating on the ceramic particles appears intact and uniform even where there is particle to particle bonding. Note the absence of borosilicate globules which can cause distortion. It is preferred that the glass comprises 10 to 50 volume percent of the substrate with the remainder being made up by the ceramic particles. In a most preferred embodiment of the invention, the borosilicate glass is made up of 10 to 30 weight percent boron oxide with the remainder being silica (Si0 2 ). Other than minor impurities or other components not materially affecting the electrical or mechanical properties of the glass, no additional compounds should be added to the boron oxide and silica. It has been found that the best properties are obtained when boron oxide and silica are the essential components of the borosilicate glass. It is, of course, possible that applications of the present invention may require additional properties not important here, for example, lowered melting point. In this situation, it may be desirable or necessary to trade-off a slightly-raised dielectric constant for the lowered melting point by adding small amounts of magnesia, calcia, and/or alumina and/or other components. These additional components should amount to no more than about 20 weight percent of the glass, more preferably about 10 weight percent or less, while the amount of boron oxide should remain in the range of 10 to 30 weight percent. Further, while the preferred ceramic particles are silica and cordierite for lowest dielectric constant, other ceramic particles may be added to or instead of the silica and/or cordierite to the ceramic mixture to obtain the desired properties. For purposes of illustration and not limitation, these other ceramic particles may include alumina, spodumene, mullite, enstatite, forsterite, spinel, beta-eucryptite, anorthite, aluminum nitride, silicon nitride, and mixtures thereof. Also contemplated within the scope of the invention is the replacement of a portion of the ceramic particles with hollow spheres, preferably silica, or fibers or whiskers. It is generally preferred that the fibers or whiskers be of a different material than the ceramic particles. To this end, the fibers or whiskers may consist of, for example, silicon nitride or silicon carbide. An important aspect of the present invention is the ability to make substrates which are less than fully dense. Porous substrates are advantageous in that they have a lower dielectric constant than fully dense substrates . As discovered with the porous silica substrates mentioned previously, porous substrates often suffer from poor mechanical integrity. The present inventors have found, however, that mechanically strong, porous substrates may be made by regulating the amount of -borosilicate glass that coats the ceramic particles. The substrates according to the invention are made by the following process: (a) coating the ceramic particles with- a borosilicate glass by a sol-gel process. The sol-gel process consists of hydrolyzing a boron oxide precursor with a silica precursor in a solvent or solvent mixture, mixing this solution with the ceramic particles and then stripping the solvent or solvent mixture from the ceramic particles. A convenient stripping process is drying the particles in an oven or on a hot plate at low temperature. The glass mixture and ceramic particles chosen are selected to meet the electrical and mechanical requirements of the resulting substrate. Often paramount in the consideration is a low dielectric constant, preferably about 4 or less; (b) forming the coated particles into a substrate. A preferred method of forming 5 the substrate is by the tape casting method. According to this method, the coated particles are mixed with a suitable solvent or solvents and a binder material to form a slurry. Common binders include 10 polyvinyl butyral (PVB) or polymethylmethacrylate (PMMA) plus additional ingredients such as particle -dispersants, plasticizers, and flow control agents. The slurry is then cast into a 15 plurality of green sheets. Wiring lines and vias may then be formed on the green sheets with a conductive paste. Thereafter, the green sheets are stacked and laminated to form the green 20 (unsintered) substrate; and (c) sintering the substrate. The green substrate is placed in an oven with a suitable atmosphere and sintered for the desired amount of time. If the substrate 25 has copper (preferred because of its low resistivity) wiring patterns, then a protective atmosphere has to be used to sinter the substrate and burn out the organic residues while avoiding oxidation 30 of the copper. Additionally, because copper melts at 1083°C, the substrate should be able to be sintered at a temperature less than about 1000°C. The advantages of the present invention will become more apparent after referring to the following examples. EXAMPLES EXAMPLE I A ceramic material consisting of 10 weight percent borosilicate glass (20 weight percent B 2 0 3 , 80 weight percent Si0 2 ) and 90 weight percent silica particles was made in the following manner: 278.5 grams of Aesar amorphous Si0 2 spheres (average diameter 3.24μm) was slurried in 1800 milliliters of denatured, undried ethanol in a 5 liter, 3-neck, round-bottomed flask fitted with a motor stirrer. 89 milliliters of tetraethoxysilane (TEOS) (0.40 mole) was added to this slurry, to which was added further a mixture of 200 milliliters of water with 1 milliliter of 38% HCl. The slurry was stirred for 4 hours to allow the TEOS to hydrolyze. Then 34 milliliters (0.2 mole) of triethylborate was added to the slurry, which was further stirred overnight. The slurry was then poured into a flat tray and warmed gently on a hot plate to drive off all of the solvents. The powder was subsequently broken up in a Pulverisette 2 grinding mill. One hundred (100) grams of the powder were then added to a PMMA binder solution consisting of the PMMA resin and solvents of acetone, ethyl alcohol, and isopropyl alcohol. A 5-layer laminate was pressed and fired in air. This sample was used for -12- dielectric constant measurements. The dielectric constant as measured was 2.4. Another sample was prepared by pressing the dry powder into pellets and heated to 1000°C in a dilatometer. Less than 1% shrinkage was observed. The sintered product was about 70% dense. EXAMPLES II Additional pellet samples having 20 weight percent B 2 0 3 in the borosilicate glass were prepared. in these samples, the percent of the glass coating on the silica particles was varied. For two of the ceramics, the dielectric constant was calculated. In a similar manner, pellet samples for ceramics having 10 weight percent and 30 weight percent B 2 0 3 in the borosilicate glass were prepared. With these samples as well, the percent of the glass coating on the silica particles was varied. All the samples were measured for percent theoretical density obtained during sintering versus the weight percent of the borosilicate glass in the ceramic. The results are tabulated in Table I below and are illustrated in Figure 3. Also included in Table I and Figure 3 are the results from Examples I. TABLE I 20/80 10/90 70 2.4+ 20/80 25/75 77 2.9* 20/80 50/50 90 3.5* 30/70 20/80 74 30/70 40/60 83 ♦dielectric constant was calculated +dielectric constant was measured Reviewing the results and Figure 3, it can be seen that minimum densification occurs with the borosilicate glass having 10 weight percent B 2 0 3 , no matter how much of the borosilicate glass is added to the silica particles. On the other hand, the borosilicate glasses having 20 and 30 weight percent B 2 0 3 provide increasing densification with increasing amounts of borosilicate glass. For those desiring a porous substrate, 50% glass is the maximum that would be desired. Additionally, the higher the percentage of glass, the higher is the dielectric constant. All the samples in Examples I and II exhibited sufficient strength so that the samples could be handled without significant damage to the samples. It is apparent, then, that the objects of the invention have been achieved by the compositions according to the invention. It will be obvious to those skilled in the art having regard to this disclosure that other modifications of this invention beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.

Claims: ■ 15- CLAIMS : 1. A low dielectric constant substrate comprising ceramic particles uniformly coated with a borosilicate glass, said substrate having a dielectric constant of about 4 or less. 2. The substrate of claim 1 wherein said glass comprises 10-50 volume percent of said substrate, the remainder ceramic particles. 3. The substrate of claim 1 wherein said glass comprises 10-30 weight percent B 2 0 3 , remainder Si0 2 plus other additives comprising MgO, CaO, and/or A1 2 0 3 . 4. The substrate of claim 3 wherein said other additives of MgO, CaO, and/or A1 2 0 3 comprise about 20 weight percent or less of said glass. 5. The substrate of claim 3 wherein said other additives of MgO, CaO, and/or A1 2 0 3 comprise about 10 weight percent or less of said glass. 6. The substrate of claim 1 wherein said glass consists essentially of 10-30 weight percent B 2 0 3 , remainder Si0 2 . 7. The substrate of claim 2 wherein said ceramic particles comprise hollow spheres. 8. The substrate of claim 7 wherein said hollow spheres comprise Si0 2 . 9. The substrate of claim 2 wherein said ceramic particles comprise fibers or whiskers. 10. The substrate of claim 9 wherein said fibers or whiskers comprise a material selected from the group consisting of Si 3 N 4 and SiC. 11. The substrate of claim 10 wherein said ceramic particles comprise Si0 2 . 12. The substrate of claim 1 wherein said substrate has a density of about 90% or less of theoretical. 13. The substrate of claim 1 wherein said ceramic particles are selected from the group consisting of silica and cordierite and mixtures thereof. 14. The substrate of claim 13 wherein said ceramic particles are silica. 15. A method of forming a low dielectric substrate comprising the steps of: (a) coating by a sol-gel process ceramic particles with a borosilicate glass, said borosilicate glass is chosen in conjunction with said ceramic particles so that a resulting substrate will have a dielectric constant of about 4 or less; (b) forming said coated particles into a substrate; and (c) sintering said substrate. 16. The method of claim 15 wherein said coating by a sol-gel process step comprises the steps of: (a) hydrolyzing a boron oxide precursor with a silica precursor in a solvent or solvent mixture; (b) mixing the solution of (a) with ceramic particles; and (c) stripping from said mixture said solvent or solvent mixture. 17. The method of claim 15 wherein said forming step comprises the steps of: (a) mixing said coated particles with at least one solvent and a binder material to form a slurry; (b) casting said slurry into a plurality of green sheets; and (c) stacking and laminating said green sheets to form a multilayer substrate. 18. The method of claim 15 wherein the step of sintering comprises heating said substrate to a temperature less than about 1000°C. 19. The method of claim 15 wherein said glass comprises 10-50 volume percent of said substrate, the remainder ceramic particles. -18- 20. The method of claim 15 wherein said glass comprises 10-30 weight percent B 2 0 3 , remainder Si0 2 plus other additives comprising MgO, CaO, and/or A1 2 0 3 . 21. The method of claim 20 wherein said other additives of MgO, CaO, and/or A1 2 0 3 comprise about 20 weight percent or less of said glass. 22. The method of claim 20 wherein said additives of MgO, CaO, and/or A1 2 0 3 comprise about 10 weight percent or less of said glass. 23. The method of claim 15 wherein said glass consists essentially of 10-30 weight percent B 2 0 3 , remainder Si0 2 . 24. The method of claim 16 wherein said ceramic particles comprise hollow spheres. 25. The method of claim 24 wherein said hollow spheres comprise Si0 2 . 26. The method of claim 16 wherein said ceramic particles comprise fibers or whiskers. 27. The method of claim 26 wherein said fibers or whiskers comprise a material selected from the group consisting of Si 3 N 4 and SiC. 28. The method of claim 27 wherein said ceramic particles comprise Si0 2 . 29. The method of claim 15 wherein said ceramic particles are selected from the group consisting of silica and cordierite and mixtures thereof. 30. The method of claim 29 wherein said ceramic particles are silica. 31. A low dielectric constant substrate comprising ceramic particles uniformly coated with a borosilicate glass comprising 10-30 weight percent B 2 0 3 , remainder Si0 2 . 32. The substrate of claim 31 wherein said glass comprises 10-50 volume percent of said substrate, the remainder ceramic particles. 33. The substrate of claim 31 wherein said ' glass further comprises about 20 weight percent or less of MgO, CaO, and/or A1 2 0 3 . 34. The substrate of claim 31 wherein said glass further comprises about 10 weight percent or less of MgO, CaO, and/or A1 2 0 3 . 35. The substrate of claim 31 wherein the composition of said glass consists essentially of 10-30 weight percent B 2 0 3 , remainder Si0 2 . -20-- 36. The substrate of claim 32 wherein said ceramic particles comprise hollow spheres. 37. The substrate of claim 36 wherein said hollow spheres comprise Si0 2 . 38. The substrate of claim 32 wherein said ceramic particles comprise fibers or whiskers. 39. The substrate of claim 38 wherein said fibers or whiskers comprise a material selected from the group consisting of Si 3 N 4 and SiC. 40. The substrate of claim 39 wherein said ceramic particles comprise Si0 2 . 41. The substrate of claim 31 wherein said substrate has a density of about 90% or less of theoretical. 42. The substrate of claim 31 wherein said ceramic particles are selected from the group consisting of silica, alumina, cordierite, spodumene, mullite, enstatite, forsterite, spinel, beta-eucryptite, anorthite, aluminum nitride, silicon nitride, and mixtures thereof. 43. The substrate of claim 42 wherein said ceramic particles are silica. 44. A method of forming a low dielectric substrate comprising the steps of: -21- (a) coating by a sol-gel process ceramic particles with a borosilicate glass comprising 10-30 weight percent B 2 0 3 , remainder Si0 2 ; (b) forming said coated particles into a substrate; and (c) sintering said substrate. 45. The method of claim 44 wherein said coating by a sol-gel process step comprises the steps of: (a) hydrolyzing a boron oxide precursor with a silica precursor in a solvent or solvent mixture; (b) mixing the solution of (a) with ceramic particles; and (c) stripping from said mixture said solvent or solvent mixture. 46. The method of claim 44 wherein said forming step comprises the steps of: (a) mixing said coated particles with at least one solvent and a binder material to form a slurry; (b) casting said slurry into a plurality of green sheets; and (c) stacking and laminating said green sheets to form a multilayer substrate. 47. The method of claim 44 wherein the step of sintering comprises heating said substrate to a temperature less than about 1000°C. -22- 48. The method of claim 44 wherein said glass comprises 10-50 volume percent of said substrate, the remainder ceramic particles. 49. The method of claim 44 wherein said glass further comprises about 20 weight percent or less of MgO, CaO, and/or A1 2 0 3 . 50. The method of claim 44 wherein said glass further comprises about 10 weight percent or less of MgO, CaO, and/or A1 2 0 3 . 51. The method of claim 44 wherein the composition of said glass consists essentially of 10-30 weight percent B 2 0 3 , remainder Si0 2 . 52. The method of claim 45 wherein said ceramic particles comprise hollow spheres. 53. The method of claim 52 wherein said hollow spheres comprise Si0 2 . 54. The method of claim 45 wherein said ceramic particles comprise fibers or whiskers. 55. The method of claim 54 wherein said fibers or whiskers comprise a material selected from the group consisting of Si 3 N 4 and SiC. 56. The method of claim 55 wherein said ceramic particles comprise Si0 2 . 57. The method of claim 55 wherein said ceramic particles are selected from the group consisting of silica, alumina, cordierite, spodumene, mullite, enstatite, forsterite, spinel, beta-eucryptite, anorthite, aluminum nitride, silicon nitride, and mixtures thereof. 58. The method of claim 57 wherein said ceramic particles are silica.

Inventors: DAVID LAWRENCE DANIEL, KNICKERBOCKER SARAH HUFFSMITH, DAVID, LAWRENCE, DANIEL, KNICKERBOCKER, SARAH, HUFFSMITH
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: H01L23
IPC codes: C03C14, C04B41, H01L23, H05K1