Low dielectric, low temperature fired glass ceramics

Abstract: A low dielectric, low temperature fired glass ceramic produced by firing at about 800 - 900°C a mixture comprising at least one glass consisting of, by weight percent, 0 to 50% B₂O₃, 35 to 65% CaO, and 10 to 65% SiO₂, and selected from the glasses of CaO-B₂O₃-SiO₂ glass system. The glass ceramic is light weight, exhibits a dielectric constant of less than about 7.9, has adequate mechanical strength and thermal conductivity, and is compatible with precious metal conductors. The glass ceramic is useful in the manufacture of integrated circuits and any other electronic or mechanical components where a glass ceramic exhibiting this particular combination of characteristics is suitable.

Description: Background Of The Invention Technical Field This invention relates to low dielectric constant, low temperature fired glass ceramics especially useful in the manufacture of multilayer integrated circuits, thick film hybrid circuits and other electronic components, and comprising two glassses from the CaO-B 2 O 3 -SiO 2 glass system. Description Of The Related Art Cofired multilayer circuits (CMC) are known in the art. Advances in information processing technology have resulted in ever increasing numbers of circuit elements per unit area. A concurrent need for CMC substrates satisfying certain performance criteria has arisen. The desirable characteristics that a CMC substrate should exhibit are a low dielectric constant, a thermal expansion coefficient (CTE) matching that of the chip, high thermal conductivity and amenability to be processed into cofired multilayer circuits at low temperatures. Multilayer circuits can be prepared from endless tapes containing dielectric material dispersed in a binder-solvent solution and cast onto an endless flexible film by techniques known in the art. Typical dielectric materials have included Al 2 O 3 as a refractory filler or crystalline phase in a noncrystalline glass (glass/ceramic composite systems). Composite ceramics incorporating alumina have been used as substrates for a long time. Current conductor, resistor and dielectric systems have properties, such as the coefficient of thermal expansion, matched to that of alumina. Early ceramics employing alumina required temperatures of 1400 - 1500°C for sintering. Such high temperatures presented problems well-known in the art, such as limiting conductor metallurgy to refractory metals such as molybdenum and tungsten, the use of which presented the further problem of oxidizing at sintering temperatures. The oxidation of the conductor metals necessitated firing in a neutral H 2 atmosphere. These difficulties lead to attempts at developing CMC substrate materials that could be fired at temperatures below about 950°C where silver and copper can be used. Glass ceramic composites were considered good candidates for CMC substrate materials because of low dielectric constants and low sintering temperatures. Glass ceramic composites, however, suffer the drawback of exhibiting low thermal conductivities. The current art, then, is comprised of composite materials that are the result of judicious compromises among the various requirements. The speed of signal transmission in hybrid microelectronic circuits is affected by the dielectric constant of the insulating layer. Attempts at fabricating lower dielectric materials for use in thick film hybrid microcircuits have included polymeric dielectric materials, porous dielectric materials, and glass-ceramic composite dielectric materials. The glass-ceramic composite materials have also been good candidates for thick film substrates because of their superior mechanical, electrical, and hermetic properties compared to porous dielectrics. They also have superior mechanical, physicochemical, and refractory properties compared to polymeric dielectrics. The desirable characteristics that a glass-ceramic for use in thick film pastes should exhibit are a low dielectric constant, a thermal expansion coefficient (CTE) compatible with that of the substrate, excellent hermetic and insulating properties, compatibility with precious metal pastes, and amenability to be processed into thick film hybrid circuits at temperatures below about 950°C. Thick film hybrid circuits can be prepared from inorganic powders made up into pastes with an organic vehicle; the pastes are applied to a suitable substrate, such as alumina, and then fired. Typical dielectric materials have included Al 2 O 3 as a refractory filler or crystalline phase in a noncrystalline glass (noncrystallizing glass/ceramic composites). U.S. Patent No. 4,654,095 teaches such a dielectric composition for use in tapes in the fabrication of multilayer circuits wherein selected noncrystallizing glass having certain deformation and softening temperatures are mixed with a refractory filler that is insoluble in the glass, and then fired at 825-900°C. U.S. Patent No. 4,643,148 describes a multilayer system requiring a two-stage firing in which the substrate is composed of alumina, a crystallizable glass such as crystallizing spodumene or cordierite, and a noncrystallizing soft glass such as borosilicate or aluminosilicate glass. U.S. Patent No. 4,621,066 teaches low temperature fired (800°C to 1000°C) composite ceramics for use in electronic components wherein noncrystalline glass from the CaO-SiO 2 -Al 2 O 3 system is mixed with 35 to 50% alumina. U.S. Patent No. 4,598,167 teaches composite ceramic substrate materials that are mixtures of a low temperature softening glass and various forms of SiO 2 , and sinterable at temperatures below about 900°C. The dielectric constants are reported to be in the range of 4-6, and the CTE in the range of 4.5 to 7.0 ppm/°C. U.S. Patent Nos. 4,413,061 and 4,301,324 describe two glass ceramic systems having spodumene and cordierite as the main crystallizing phase. The dielectric constants are reported as in the range of 5.0 to 6.5, and the CTE as in the range of 2.0 to 8.3 ppm/°C. U.S. Patent No. 3,816,172 teaches a partially crystallized dielectric comprising glass crystals in a glassy matrix. U.S. Patent No. 4,714,687 teaches a low k glass ceramic suitable for dielectric substrates and comprising willemite and cordierite. WO-A-91/02363 describes a dielectric composition comprising at least one crystallized glass (A), and at least one non-crystallized glass (B), wherein, in preferred embodiments, (A) contains SiO 2 20-30%, CaO 50-60%, and B 2 O 3 15-25%, and (B) includes SiO 2 60-80%. Summary Of the Invention The invention provides a glass composition for use in producing a glass ceramic dielectric composition, said composition comprising: (A) a hard crystallizing glass from the CaO-B 2 O 3 -SiO 2 glass system having a composition in weight percent which is: an amount not exceeding about 55% B 2 O 3 , from about 35 % to about 55 % CaO, and from about 10% to about 65 % SiO 2 ; and (B) a soft crystallizing glass from the CaO-B 2 O 3 -SiO 2 glass system having a composition in weight percent which is: an amount not exceeding about 55% B 2 O 3 , from about 35% to about 55% CaO, and from about 10% to about 65% SiO 2 . It also provides a ceramic article prepared by firing such a composition, preferably a low dielectric constant, low temperature fired glass ceramic produced by firing at about 800 to 950°C. The CMC glass ceramics exhibit dielectric constant values of less than about 7.0. The relative proportions of the glasses affect the final properties of the ceramic, such as the thermal expansion coefficient. The glass ceramics of the present invention are useful in any electronic or mechanical components where a glass ceramic exhibiting the particular combinations of characteristics is desirable. As used in the specification and the claims, the terms "hard" and "soft", when applied to a pair of glasses in the CaO-B 2 O 3 -SiO 2 glass system, are used in a relative sense and with reference to the CaO-B 2 O 3 -SiO 2 glass system phase diagram, a hard glass being farther from the eutectic than its paired soft glass. Another relationship between a hard and a soft glass is that the hard glass has a liquidus temperature above that of the soft glass. The glass ceramic of the present invention is to be distinguished from noncrystallizing glass and ceramic (refractory) filler preparations of the prior art. Unlike glass/ceramic composites wherein a refractory filler (ceramic) is mixed with a noncrystallizing glass in which it is insoluble to form a glassy matrix surrounding a refractory filler, the material of the present invention comprises a glass ceramic formed by the in situ crystallization of one or more crystallizable glasses from the same glass system. A noncrystallizing glass/glass ceramic dielectric material which is a dielectric material wherein a ceramic phase is dispersed in a glassy matrix where the ceramic is formed by the crystallization of the glass ceramic is envisaged. Unlike glass/ceramic composites wherein a refractory filler (ceramic) is mixed with a noncrystallizing glass in which it is insoluble to form a glassy matrix, a material comprising a glass ceramic formed in situ by the crystallization of one glass in a noncrystallized matrix of another glass can be envisaged. The glass ceramic of the present invention is advantageous for use in cofired multilayer substrates because it does not require a refractory ceramic component that can contribute to an increase in the dielectric constant. Additionally, the glass ceramic of the present invention, being composed of two crystallizing glasses from the CaO-B 2 O 3 -SiO 2 glass system, is free from problems caused by the interactions between refractory filler and glass. Yet when a refractory filler is needed or desired, the glasses are amenable to accepting such a filler. Importantly, when two glasses are used to make a two-glass ceramic, the use of a hard glass and a soft glass from the same system means total compatibility, so that fine adjustments to the composition of the ceramic can be made during processing by simply adding more of one or the other of the glasses. The fact that both glasses in the two-glass ceramic are crystallizing results in a ceramic that is stable after the first fire. The use of two crystallizing glasses gives many options in the fabrication of the ceramics, such as using different grain sizes to achieve different rates of crystallization so as to change the amount of residual glass. Also, small amounts of lower dielectric constant fillers such as fused SiO 2 , mullite, cordierite, fosterite and the like are easily tolerated by the system, yet allow for further manipulation of the properties of the final ceramic without encountering compatibility problems. The binary glass ceramic of the present invention simulates the use of refractory filler without the drawbacks associated with a glass/ceramic system. The two-glass ceramic of the present invention is lightweight, having a density lower than glass and alumina composite ceramic. The thermal conductivity of the glass ceramics is adequate for use in microcircuits, and the thermal expansion matches that of alumina to a degree where available inks usually used for alumina CMC substrates may also be used with these ceramics. The present glass ceramics enjoy compatibility with noble metals, so that silver, gold, and palladium, as well as copper conductors can be employed. Upon firing, the present ceramics exhibit adequate dimensional stability, with XY shrinkage in the 12% range. Additionally, when made up as thick films, they can be fired using a typical thick film firing profile. CMC substrates prepared from two glasses from the same system comprise up to about 50% by weight soft glass and about 50% by weight hard glass. The preferred composition comprises 40% of the eutectic glass, as on Table II, below, and 60% of the glass designated as CBS-1 on Table II and comprising 10% B 2 O 3 , 40% CaO, and 50% SiO 2 . The glasses are in the form of powders, preferably having an average particle size of about 1 to 10 µm, and most preferably, 3 to 6 µm. Up to about 5% by weight aluminum, silicon, or magnesium oxide may be also present in the composition, these incidental amounts of these oxides being picked up in the milling process. Still other features and advantages and a full understanding of the invention will become apparent to those skilled in the art from the following description of the preferred embodiments. Brief Description Of The Drawing Figure 1A is a phase diagram of the CaO-B 2 O 3 -SiO 2 system showing the composition range of the four CMC substrates. Figure 1B is the phase diagram of the CaO-B 2 O 3 -SiO 2 system showing the eutectic range at 12. Description of The Preferred Embodiments 1. Tapes For Cofired Multilayer Circuits Referring to Figure 1A, there is shown a phase diagram of the CaO-B 2 O 3 -SiO 2 system showing the composition range of the glasses for the CMC substrates of the present invention. (The eutectic for the glass system is marked on Figure 1B at 12.) For incorporation into dielectric compositions for use in tapes for cofired multilayer circuits, two glasses having a composition lying on the phase diagram of Figure 1 and bounded by the limits of O to 50% B 2 O 3 , 35 to 65% CaO, and 10 to 65% SiO 2 are milled to an appropriate size powder, about 1 to 10 µm average size, preferably in the range of 2 to 6 µm average size. The powdered glass, with or without further refractory filler, is mixed with the binder and then cast into a tape of desired thickness using a doctor blade, all by techniques known in the art. The binder may include a solvent, surfactant, plasticizer. Typical binders include acrylic binders and polyvinyl compounds. Plasticizers may include any of the phthalates. Additionally, viscosity modifiers, anti-skinning agents and the like can be used, as is known in the art. Table I lists the properties of two glass dielectric composition cofired multilayer tapes. The composition of the glasses in the glass system column (1) of Table I are identified in Table II. The glasses of Table I were prepared as in Example 1. EXAMPLE 1 The glass melting and milling procedure was of a type known in the art where glass was melted in a platinum crucible at 1400-1500°C for about one hour until it was fully melted and homogenized. The glass was poured through a set of rolls and rolled into flakes. The flakes were milled by known techniques to an average powder size of 2 to 6 µm. A binder solution, was prepared from, by weight, acrylic binder 11.33%; phthalate plasticizer 3.97%; and a solvent recommended by the suppliers, in this case a mixture of methyl ethyl ketone/cyclohexane solvent, 84.7%. The powder was added to the binder so that the ratio of solids to liquid was about 60/40. The mixture was milled together in a ball mill with alumina media until a homogeneous slurry was obtained, that is for 2 to 24 hours. The resulting mixture was then degassed and cast into a tape using a doctor blade. The cast tape was dried and blanked. Blanked pieces were laminated in a laminating die at a temperature of about 70°C for 10 minutes at 3000 psi (21 MPa). The binder was burnt out at 350°C for 12 hours. The laminate was fired on a setter in the belt furnace in a standard thick film profile of 850°C/10 minutes on an alumina substrate. The resultant substrate was then subjected to the measurements recorded in Table I. System CaO B 2 O 3 SiO 2 646/0 45.00 15.00 40.99 P12-z K 47.72 8.34 43.95 A 47.52 11.12 41.36 B 47.33 13.90 38.78 C 47.13 16.68 36.19 D 46.94 19.46 33.61 E 46.74 22.24 31.02 F 46.55 25.02 28.44 G 46.35 27.80 25.85 N 51.00 21.00 28.00 O 54.00 18.00 28.00 T 55.00 20.00 25.00 AA 50.00 11.00 39.00 AB 50.00 15.00 35.00 AC 50.00 18.00 32.00 AD 60.00 11.00 29.00 AE 64.00 14.00 22.00 20D 40.00 20.00 40.00 20E 45.00 20.00 35.00 20F 50.00 20.00 30.00 20G 55.00 20.00 25.00 30B 40.00 30.00 30.00 30C 45.00 30.00 25.00 30D 50.00 30.00 20.00 30E 55.00 30.00 15.00 30F 60.00 30.00 10.00 40C 40.00 40.00 20.00 40D 45.00 40.00 15.00 40E 50.00 40.00 10.00 AS1 36.30 36.10 26.10 AS2 37.30 36.10 27.60 AS3 36.30 36.10 27.60 15A 40.00 15.00 45.00 25A 40.00 25.00 35.00 22.5A 40.00 22.50 37.50 EUTI12 39.00 32.00 29.00 CBS-1 40.00 10.00 50.00 CBS-2 46.00 10.00 44.00 CBS-3 37.78 15.00 47.22 CBS-4 43.44 15.00 41.56 For each substrate in Table I, the appearance was qualitatively assessed, and the assessment is listed in column (2). A gray appearance may indicate that binder burn off is incomplete or that a residue is left. Edge definition is listed in column (3) and is also a qualitative assessment. The coefficient of thermal expansion was measured as the samples were heated from room temperature to 300°C, 500°C and 800°C, respectively. The value of the coefficient is a reflection of the compatibility of inks and the like used for alumina technology and transferable to the present invention. For alumina, CTE300 is about 64, and CTE800 is about 78 to 80. The K value in column (5) is the dielectric constant, and the D in column (6) is the dissipation factor or dielectric loss, both measured at 10 megahertz by known techniques. Porosity in column (7) was qualitatively assessed. Sx and Sy in column (8) are shrinkage measurements made in the x and y directions of the CMC substrate. For isotropic shrinkage, Sx should be as close to Sy as possible. Substrates in Table I were prepared from two glasses selected from the CaO-B 2 O 3 -SiO 2 system bounded by the same limits. Each of the glasses of the pair is a crystallizing glass. It is contemplated that other noncrystallizing glasses, both hard and soft, from other glass systems will be found useful in combination with glasses from the CaO-B 2 O 3 -SiO 2 system. Such glasses can alter the speed of crystallization in a known manner. Also, the incorporation of crystallizing agents is contemplated to affect the degree of crystallization of a particular substrate. It is contemplated that low dielectric constant fillers, such as mullite, cordierite and fosterite, and the like, will be satisfactorily incorporated into the dielectric material in a manner and with results known in the art.

Claims: A glass composition for use in producing a glass ceramic dielectric composition, said composition comprising: (A) a hard crystallizing glass from the CaO-B 2 O 3 -SiO 2 glass system having a composition in weight percent which is: an amount not exceeding about 55% B 2 O 3 , from about 35 % to about 55% CaO, and from about 10% to about 65% SiO 2 ; and (B) a soft crystallizing glass from the CaO-B 2 O 3 -SiO 2 glass system having a composition in weight percent which is: an amount not exceeding about 55 % B 2 O 3 , from about 35 % to about 55 % CaO, and from about 10% to about 65% SiO 2. A glass composition as set forth in claim 1 further including a non-crystallizing glass. A method of preparing a ceramic which comprises firing a composition according to claim 1 or 2. A ceramic prepared by firing a composition according to claim 1 or 2.

Inventors: HANKEY DANA L, LEANDRI DAVID J, MURALIDHAR S K, ROBERTS GORDON J, SHAIKH AZIZ S, VLACH THOMAS J, HANKEY,DANA L., LEANDRI, DAVID J., MURALIDHAR,S.K., ROBERTS,GORDON J., SHAIKH,AZIZ S., VLACH,THOMAS J.
Applicants: FERRO CORP, FERRO CORPORATION
Application year: 1992
Main classification: H01L23
IPC codes: H01B3, C03C14, C03C10, H01L23, H05K1, C03C8