IMPROVED COMPOSITE DIELECTRIC

Abstract: A low-temperature-sintering, ceramic-filled-glass composite dielectric for microelectronic packaging has been developed that consists of 30-76 % (by volume) of a low dielectric constant, refractory ceramic filler or mixture of fillers, and 24-70 % of non-crystallizing, reduction resistant, alkaline-earth alumino-borate glass.

Description: IMPROVED COMPOSITE DIELECTRIC FIEIiP OF THE IN EN IO This invention relates to improved composites for microelectronic packaging. Specifically, a low- temperature-sintering, ceramic-filled-glass (CFG) composite consisting essentially of a silicate and/or nitride ceramic filler, dispersed in a continuous matrix of non-crystallizing, reduction resistant, alkaline- earth alumino-borate (AEAB) glass has been developed. BACKGROUND OF INVENTION Current developments in microelectronic packaging are being driven by the need for improved performance and reliability. To address these needs, new, low dielectric constant, low loss, thermally conductive, mechanically sound packaging materials having a thermal coefficient of expansion (TCE) approximately equal to that of the semiconductor chip have to be developed. There is also an economic incentive to lower production costs by lowering processing temperatures, reducing the number of processing steps, and utilizing less expensive metallurgies (e.g., Cu) . It is desirable to sinter a dielectric packaging material to >95% of theoretical density at <1000°C to allow single-stage co- firing with high electrical conductivity metallurgies (e.g., Cu, Au, Ag, or Ag/Pd alloys). By adding a low-temperature-sintering glass to a refractory ceramic, it is possible to produce a ceramic- filled-glass (CFG) composite dielectric that can be co- fired at <1000°C with high electrical conductivity metallurgies (see Steinberg, U.S. Patent 4,654,095, Ushifusa et al. U.S. Patent 4,598,167, and Kokubu et al. U.S. Patent 4,598,167) . Alumina is commonly used as the refractory ceramic filler in conventional CFG composite dielectrics; however, alumina is neither an exceptional dielectric insulator nor an exceptional thermal conductor, both of which are desirable for a state-of-the-art electronic packaging material. Typically, a Siθ 2 ~based glass serves as the sintering flux and the high temperature inorganic binder in a CFG composite dielectric. Siθ 2 -based glasses currently used in microelectronic packaging are typically modified with high polarizability alkali ions that adversely affect the glass dielectric properties. Additionally, Pb or Bi-oxides, which are unstable in a reducing atmosphere, are often added to Siθ 2 ~based glasses used in microelectronic packaging to lower the glass transition (T g ) and dilatometric softening ( ^) temperatures, and promote lower temperature densification. In addition to their undesirable chemical constituents, Siθ 2 -based glasses used in microelectronic packaging typically have moderate to high wetting angles on oxide ceramics, and high viscosities at 800-1000°C. Additionally, more than 50% (by volume) , and typically, 70% glass is required to produce a dense, CFG composite dielectric at <1000°C with Siθ 2 ~based glasses. In a CFG composite, both thermal conductivity and mechanical strength degrade with increasing glass concentration; thus, to fabricate a low-temperature- sintering, CFG composite with optimum thermal conductivity and strength, it is desirable to minimize the glass concentration in the composite. Dimensional stability and reproducible densification behavior during sintering also degrade with increasing glass concentration in a CFG composite; consequently it is desirable to limit the concentration of glass in a CFG composite to improve processing reproducibility and reliability. In addition to improving thermal and mechanical properties, and processing reproducibility, minimizing the concentration of glass in a CFG composite enhances the potential for tailoring the TCE of the composite by increasing the latitude of control available over the concentration and chemistry of the ceramic filler(s) in the composite. In microelectronic packaging, it is desirable to control the TCE of a CFG composite dielectric to minimize the TCE mismatch and resultant mechanical stress that can develop between a dielectric substrate and an attached semiconductor chip. CFG composite densification is enhanced with a non- crystallizable glass with a low wetting angle on the ceramic filler, and a low viscosity at the sintering temperature; thus to minimize glass concentration in a CFG composite and produce improved CFG composite dielectrics, it is desirable to utilize a non- crystallizable, good wetting, low viscosity glass. SUMMARY OF THE INVENTION Improved CFG composite dielectrics for microelectronic packaging can be produced by substituting silicate and/or nitride ceramic fillers for alumina, and substituting B 2 θ 3 ~based, alkaline-earth alumino-borate (AEAB) glass for Siθ2~based glasses. Silicate and nitride ceramics offer a desirable alternative to alumina as fillers in CFG composite dielectrics as they provide the potential for producing dielectrics with lower dielectric constants and losses, and higher thermal conductivities, respectively. B 2 O 3 - based, AEAB glasses offer a desirable alternative to the Siθ2~based glasses typically used to make CFG composite dielectrics as they are good low-temperature-deformable, non-crystallizing (see Park et al., U.S. Patent 4,341,849), reduction resistant (see Burn, U.S. Patent 4,101,952), glasses that do not contain undesirable alkali modifiers, or Pb or Bi-oxide fluxes. Additionally, B 2 θ 3 ~based, AEAB glasses have excellent wetting characteristics on oxide ceramics, and have low viscosities at 800-1000°C, making it possible to produce dense, low-temperature-sintering, CFG composite dielectrics with significantly lower glass concentrations than with the Siθ 2 ~based glasses currently used in CFG composite dielectrics(see Mattox et al., Ceram. Eng. Scl . Proc , 9 [11-12] pp 1567-78 (1988) ) . B 2 θ 3 ~based, AEAB glasses also have high electrical resistivities (see Owen, Phys. Che . Glasses, 2 [3] June (1961)), low dielectric constants and losses (see Hoffman, U.S. Patent 3,544,330), and resist corrosion in hot, humid air (see atkins, SAND87- 0393-UC-13 Sandia Report, NTIS (1987)), which are also desirable properties for microelectronic packaging materials. Silicate and/or nitride ceramic-filled, AEAB glass composites offer a desirable alternative to conventional alumina-filled, Siθ 2 ~based glass composites for microelectronic packaging as they provide improved potential for designing and developing novel, low- temperature-sintering, CFG composite dielectrics with superior dielectric, thermal, and mechanical properties, and improved dimensional stability during sintering. Additionally, the properties of silicate and/or nitride ceramic-filled, AEAB glass composites can be tailored to meet the specific performance requirements of a broad range of microelectronic packaging applications. More specifically, the invention relates to a ceramic-filled-glass (CFG) composite dielectric consisting essentially of: A) 40-76% (by volume) silica, a silica containing ceramic compound, a silica coated ceramic, or a mixture thereof, or B) 30-60% of a nitride or mixture of nitride ceramics, or C) 30-76% of a mixture of A) and B) , with D) 24-70% of non-crystallizing, reduction resistant, alkaline-earth alumino-borate (AEAB) glass of composition (in mole %) 15-40% RO • 5-25% AI2O3 • 0-15% Siθ 2 • 40-75% B 2 O 3 , where R is an alkaline-earth metal or combination of alkaline-earth metals, preferably, Ba, Ca, Mg, or a mixture thereof. The ceramic fillers described in A) and B) should have limited solubility in, and should not promote devitrification of the glass described in D) . The CFG composite dielectric consists of preferably, crystalline, but not excluding amorphous, ceramic filler(s) of A), B) or C) dispersed within a continuous glass matrix. Preferably, 50-75% (by volume) of powdered silica (i.e., Siθ 2 ) , a silica containing ceramic compound, and/or a silica coated ceramic compound, and/or 35-50% (by volume) of a powdered nitride ceramic compound is dispersed in a continuous matrix of AEAB glass of preferred composition (in mole %) 20-30% ([X] BaO • [1-X] CaO) • 5-20% AI 2 O 3 • 0-10% Siθ 2 • 50-70% B 2 O 3 , where X ranges from 0.1-0.9, preferably 0.2-0.8, and most preferably 0.67. Conventional powder mixing techniques are used to prepare the composite dielectric powders, and conventional powder pressing and tape casting techniques are used to produce green, CFG composite dielectric bodies. Dense (>95% of theoretical density) , low dielectric constant (<9, and preferable <5) , low loss (<0.003, and preferably <0.001), mechanically sound, thermally conductive (>0.5 W/m-K, and preferably >3 W/m«K) , CFG composite dielectrics having a tunable, room temperature to 300°C TCE ranging from 3.5 to 6.7 μm/m/°C (to match silicon or alumina, respectively) are produced by sintering at <1000°C in air or nitrogen for <2 hr. The AEAB glass serves as the flux during sintering, making it possible to densify the CFG composite dielectric at ≤1000°C, and serves as the high- temperature inorganic binder that holds the CFG composite dielectric together after sintering. The CFG composite dielectrics of this invention can be co-fired with high electrical conductivity metallurgies (e.g., Cu, Au, Ag, or Ag/Pd alloys). DETAILS OF THE INVENTION CERAMIC FILLERS Silicate and/or nitride ceramics offer a desirable alternative to alumina as fillers in CFG composite dielectrics. In contrast to alumina, which is the filler typically used in low-temperature-sintering CFG composite dielectrics, lower dielectric constant (<9, and preferably 2-6) silica, silica containing ceramic compounds, and/or silica coated ceramics, or higher thermal conductivity (>36 W/m«k, and preferably, >100 /m«k) nitride ceramic compounds are used to produce the low-temperature-sintering CFG composite dielectrics of this invention. Low-temperature-sintering CFG composites have been produced with crystalline and amorphous Siθ 2 (quartz and fused silica, respectively) and mixtures thereof, as well as Al 2 θ 3 /Siθ 2 microspheres, anorthite, mullite, cordierite, silica coated alumina, cubic and hexagonal BN, Si 3 N 4 , and A1N ceramic fillers. Silicate and/or nitride ceramic fillers that have limited solubility in, and do not promote devitrification of the matrix, AEAB glass are suitable filler materials. The ceramic filler(s) used should have a narrow particle size distribution about a median particle size of 0.1-10.0 μm, and preferably 1.0-3.0 μm. Equiaxed ceramic filler particles are preferred; however, it should be possible to produce low-temperature-sintering, CFG composites with ceramic whiskers and fibers of the aforementioned compositions as well. ALKALINE-EARTH ALUMINO-BORATE (AEAB) GLASS B 2 θ 3 ~based, AEAB glasses offer a desirable alternative to the Siθ2~based glasses typically used in CFG composite dielectrics as they are low-temperature- deformable, non-crystallizing, reduction resistant glasses that do not contain undesirable alkali modifiers, or Pb or Bi-oxide fluxes. Additionally AEAB glasses exhibit excellent wetting characteristics on oxide and nitride ceramics, and have exceptionally low viscosities at 800-1000°C. AEAB glasses also have high electrical resistivities, low dielectric constants and losses, and resist corrosion in hot, humid air. AEAB glasses can be formed over a wide range of compositions; consequently, their thermal and dielectric properties, their resistance to corrosion, and their resistance to devitrification can be modified by modifying the glass chemistry. Dielectric constant decreases with decreasing RO:B 2 θ 3 ratio, and/or BaO:CaO ratio. TCE decreases with decreasing RO:B 2 θ 3 ratio, and/or BaO:CaO. T g and T^ both increase with decreasing RO:B2θ 3 ratio, and/or BaO:CaO. To produce a low dielectric constant (<7.5), low loss (<0.003), intermediate/low TCE (5.5-10.0 μm/m/°C) glass that sinters at <1000°C, that has a high enough T (>600°C) to allow complete organic burnout prior to composite densification, a glass composition of (in mole %) 20-30% ([X] BaO • [1-X] CaO) • 5-20% AI2O 3 • 0-10% Siθ 2 • 50-70% B 2 O 3 is preferred, where X ranges f om 0.1-.9, and is preferably 0.67. MgO behaves similarly to, and can be substituted for CaO; however, CaO is preferred as it has a broader glass forming range. Al 2 θ 3f Siθ 2 , and RO, where R can be Ba, Ca, Mg, or a mixture thereof, increase the chemical durability (i.e., in a hot, humid environment) and stability (i.e., to devitrification) of B 2 θ 3 ~based glasses. Up to 25 mole % AI 2 O 3 and 30 mole % Siθ 2 can be added to an alkaline-earth modified, B 2 θ 3 ~based glass before devitrification occurs; however, glass wetting angle and viscosity also increase with increasing AI 2 O 3 and Siθ2 concentration; consequently, 20 mole % AI 2 O 3 and <10 mole % Siθ 2 are preferred. Impurities and common glass nucleating agents such as Zrθ 2 , Tiθ 2 , and ZnO that promote devitrification should specifically be avoided. AEAB glasses are prepared using conventional glass melting techniques whereby the respective glass precursors (e.g., nitrates, acetates, carbonates, and/or oxides) are mixed together and reacted in a platinum crucible at a suitable temperature (1000-1100°C) to produce a sufficiently fluid liquid to allow complete mixing and fining in ≤l hr without excessive volatilization of boron. The glass melt is fritted by quenching in water, followed by drying, and subsequently, ball milling to produce <40.0 μ particles. A narrow particle size distribution about a median particle size of 1.0-10.0 μm is preferred for CFG composite fabrication (Example 1A) . CERAMIC-FILLED-GLASS (CFG) COMPOSITE DIELECTRICS CFG composite dielectrics are produced by mixing the requisite amounts of silicate and/or nitride ceramic filler(s) and AEAB glass using conventional powder mixing techniques (see Example IB) , forming the desired size and shape green body using conventional ceramic forming techniques, and sintering. Green CFG composite dielectric bodies can be formed by dry pressing with or without an organic binder; however, for microelectronic packaging, tape casting and thick film forming processes are preferred. Green tape slurries and thick film pastes are prepared using conventional techniques, whereby silicate and/or nitride ceramic filler(s) and AEAB glass are dispersed in an appropriate organic binder and vehicle to produce a slurry or paste having suitable properties for tape casting (Examples 1C and 2) or screen printing (Example 3), respectively. Dense CFG composite dielectrics are produced by sintering the green composite bodies at 2-50°C/min to 800-1000°C, soaking for 0.17-2 hr in air or nitrogen, and cooling to room temperature at <50°C/min. The B 2 O 3 - based, AEAB glass serves as the sintering flux that enables densification to occur at <1000°C, and affords high temperature, inorganic binder that holds the CFG composite dielectric together after sintering. CFG composite densification occurs by a combination of glass infiltration, ceramic filler particle rearrangement, and viscous flow in a process described as non-reactive, liquid-phase sintering. In addition to a low glass wetting angle and viscosity at the soak temperature of 800-1000°C, chemical compatibility between the ceramic filler and the glass is imperative for good sintering. In particular, the ceramic filler should have limited solubility in the glass, and should not promote devitrification of the glass during sintering. The silicate and/or nitride ceramic-filled, AEAB glass composite dielectrics of this invention that meet the above conditions, exhibit excellent sinterability. To produce a low-temperature-sintering (<1000°C) , low dielectric constant (<6) , low loss (<0.003), CFG composite dielectric, a combination of crystalline cordierite, crystalline quartz, and/or fused silica filler(s) dispersed in AEAB glass of composition (in mole %) 20-30% ([X] BaO • [1-X] CaO) • 5-20% Al 2 0 3 • 0- 10% Siθ 2 • 50-70% B 2 O 3 is preferred, where X ranges from 0.1-.9, is preferred. To produce a low-temperature- sintering, CFG composite dielectric whose room temperature to 300°C TCE is tunable between 3.5 and 6.7 μm/m/°C (to match silicon or alumina, respectively) , a combination of crystalline quartz and fused silica ceramic fillers dispersed in AEAB glass is preferred. To produce a thermally conductive (>3 /m«K) , low- temperature-sintering, CFG composite dielectric, a combination of crystalline BN and/or crystalline A1N ceramic filler(s) dispersed in AEAB glass is preferred. EXAMPLE 1 A AEAB GLASS PREPARATION A 200 g batch of AEAB glass of composition (in mole %) 20% BaO • 10% CaO • 10% AI 2 O 3 • 60% B 2 O 3 was prepared by combining 89.6 g of BaC0 3 , 53.8 g of Ca( θ 3 ) 2 • 4 H 2 O, 170.8 g of Al(Nθ 3 ) 2 • 9 H2O, and 94.6 g of anhydrous B 2 O 3 in a 0.5 L glass jar, and agitating to form a uniform mixture. The 200 g powder mixture was transferred to a 150 mL platinum crucible, heated at ca. 100°C/min to 1000°C, soaked for ca. 30 min, and then fritted. Fritting was performed by pouring the molten, fully reacted AEAB glass from the platinum crucible into a 7.6 L stainless steel bucket of ambient temperature water. The water was drained, and the fritted glass was washed in methanol, and vacuum dried at 60°C to remove the remaining water. The dried glass was crushed to <20 mesh using a steel mortar and pestle, and the crushed glass was dry ball-milled for 44 hr in a #00, Norton ceramic ball mill half full of 0.95 cm, high-density- alumina, milling media to obtain a median size of 1.9 μm. B: Green CFG COMPOSITE DIELECTRIC POWDER A 100 g CFG composite dielectric green powder mixture was prepared by combining 70% (by volume) of powdered, crystalline quartz filler and 30% of powdered, AEAB glass of the composition described in Example 1A. 50 g batches of the composite powder mixture were combined with ca. 15 mL of isopropyl alcohol and ca. 5 0.635 cm diameter Lucite™ beads in a 60 mL glass jar, and Spex® milled for 10 min. To minimize segregation, the resultant dispersion was rapidly dried in Teflon™ coated aluminum foil boats on hot plates. £ LOW DIELECTRIC CONSTANT CFG COMPOSITE DIELECTRIC 1A££ An 80 g (solids) CFG composite dielectric green powder mixture was prepared by combining 52 g of powdered, fused silica and 28 g of powdered, AEAB glass of the composition described in Example 1A with isopropyl alcohol, Spex® milling, and drying as described in Example IB. The green CFG composite dielectric powder was combined with 6.25 g of Elvacite 2010 acrylic resin, 6.25 g of Santizer 160 plasticizer, 0.5 g of a developmental plasticizer, and 150 mL of trichloroethylene solvent in a 250 mL flask and mixed for 4 hr using a solution blending technique. The resultant slurry was tape cast on a polyester film using a 25 mil doctor blade. The cast tape was dried for 1 hr at 250°C, and then stripped from the polyester film. Six 5.1 cm squares were cut from the green tape, stacked, and laminated at 70°C and 69 MPa. The multilayer, CFG composite dielectric was sintered for 2 hr at 950°C in air to produce a dense ceramic substrate having a 1 MHz dielectric constant and loss of 4.6 and 0.001, respectively. EXAMPLE 2: LOW DIELECTRIC CONSTANT CFG COMPOSITE DIELECTRIC TAPE An 80 g (solids) CFG composite dielectric green powder mixture was prepared by combining 52 g of powdered, crystalline quartz and 28 g of powdered, AEAB glass of the composition described in Example 1A with isopropyl alcohol, Spex® milling, and drying as described in Example IB. The green CFG composite dielectric powder was combined with 6.25 g of an acrylic resin, 6.25 g of Santizer 160 plasticizer, 0.5 g of a plasticizer, and 50 mL of trichloro-ethylene solvent in a #000 Norton high-alumina ball mill, and milled for 24 hr. The resultant slurry was tape cast on a polyester film using a 25 mil doctor blade. The cast tape was dried for 1 hr at 250°C, and then stripped from the polyester film. Six 5.1 cm squares were cut from the green tape, stacked, and laminated at 70°C and 69 MPa. The multilayer, CFG composite dielectric was sintered for 2 hr at 950°C in air to produce a dense ceramic substrate with a 1 MHz dielectric constant and loss of 4.7 and 0.001, respectively. EXAMPLE 3; LOW DIELECTRIC CONSTANT CFG COMPOSITE DIELECTRIC THICK FILM A screen-printable CFG composite dielectric thick film paste was prepared by combining three parts of a mixture of 60% (by volume) crystalline quartz and 35% AEAB glass of the composition described in Example 1A with one part acrylic resin solution. A homogeneous dispersion was produced by Hoover mulling a total of five passes. The resultant thick film paste was screen printed on silver electroded, 2.5 cm square, 96% alumina substrates. A 325 mesh stainless steel screen containing a 25 μm bottom emulsion was used for screen printing. To produce a sintered film thickness of 38- 127 μ , 1-4 prints were screened, respectively. Between prints, the screened films were air dried at 80°C. Thick films produced with ≥3 prints were sintered at 10°C/min to 900°C, and soaked for 10 min in air to produce dense, CFG composite dielectric films having a 1 MHz dielectric constant and loss of ca. 4.5 and <0.001, respectivel . EXAMPLE 4; THERMALLY CONDUCTIVE CFG. COMPOSITE DIELECTRIC A 5 g CFG composite dielectric green powder mixture was prepared by combining 60% (by volume) cubic BN and 40% AEAB glass of the composition described in Example 1A with isopropyl alcohol, Spex® milling, and drying as described in Example IB. A ca. 100% theoretical density CFG composite dielectric with a thermal conductivity of ca. 7.5 W/m«K was produced by vacuum hot pressing ca. 2.25 g of the dried powder in a 2.5 cm diameter graphite die at 800°C and 35 MPa for 30 min.

Claims: CLAIMS. 1. A ceramic-filled-glass (CFG) composite dielectric consisting essentially of a low dielectric constant, refractory, ceramic filler selected from: A) 40-76 percent (by volume) silica, a silica containing ceramic compound, a silica coated ceramic, or a mixture thereof, or B) 30-60 percent by volume of a nitride or mixture of nitride ceramics, or C) 30-76 percent by volume of a mixture of A) and B) , with 24-70 percent by volume of non- crystallizing, reduction resistant, alkaline-earth alumino-borate (AEAB) glass comprising in molar percent 15-40% RO, 5-25% A1 2 0 3 , 0-15% Si0 2 , 40-75% B 2 0 3 , where R is an alkaline-earth metal or mixture thereof. 2. The composite of Claim 1 wherein the ceramic film is 40-76 percent (by volume) silica, a silica containing ceramic compound, a silica coated ceramic, or a mixture thereof. 3. The composite of Claim 1 wherein the ceramic film is 30-60 percent by volume of a nitride or mixture of nitride ceramics. 4. The composite of Claim 1 wherein the ceramic is 30-76 percent by volume of a mixture of A) and B) , with 24-70 percent by volume of non-crystallizing, reduction resistant, alkaline-earth alumino-borate (AEAB) glass comprising in molar percent 15-40% RO, 5-25% AI2O3, 0-15% Siθ2, 40-75% B2O3, where R is an alkaline-earth metal or mixture thereof. 5. The composite of Claims 1, 2, 3 or 4 wherein the AEAB glass in mole percent is 20-30% ( [X] BaO • [1-X] CaO), 5-20% AI 2 O 3 , 0-10% Siθ 2 , 50-70% B 2 O 3 , where X is 0.1-0.9. 6. The composite of Claim 5 wherein X is 0.67. 7. The CFG composite dielectric of Claim 2 consisting of 25-50% AEAB glass and 50-75% ceramic filler. 8. The CFG composite dielectric of Claim 3 consisting of 35-50% ceramic filler and 50-65% AEAB glass. 9. The CFG composite dielectric of Claim 4 consisting of 30-76% by volume ceramic filler and 24-70% AEAB glass. 10. The CFG composite dielectric of Claim 1 that can be sintered to >95% of theoretical density at <1000°C in <2 hr in air or nitrogen. 11. The CFG composite dielectric of Claim 1 wherein the dielectric constant and loss are <5 and <0.001, respectively. 12. The CFG composite dielectric of Claim 1 wherein the thermal conductivity is >3 W/m«K. 13. The CFG composite dielectric of Claim 1 wherein the room temperature to 300°C thermal coefficient of expansion can be tuned from 3.5 to 6.7 μm/m/°C. 14. The CFG composite of Claims 1, 2, 3 or 4 containing in addition Cu, Au, Ag, or Ag/Pd alloys. 15. The CFG composite of Claim 5 containing in addition, Cu, Au, Ag or Ag/Pd alloys.

Inventors: EWSUK KEVIN GREGORY, HARRISON LARRY WAYNE, EWSUK, KEVIN, GREGORY, HARRISON, LARRY, WAYNE
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1990
Main classification: H01B3
IPC codes: C03C14, H01B3, H01L23, H05K1