As very large scale integrated circuits reach faster switching speeds with more output driver circuits, the reduction of electrical noise associated with the simultaneous switching of many off-chip drivers is becoming an important consideration for attaining good performance .

One source of electrical noise is associated with the power supply lines and the distribution system that supplies voltage to- individual circuit elements. This problem is reduced by placing a capacitor between the power line and ground so current demands can be met by local capacitive energy storage. However, the inductance associated with the decoupling capacitor and its leads must be low enough to allow current to be supplied at a sufficient rate. In order to accommodate faster switching speeds by minimizing the associated noise levels, the distance of the inductance pathways is reduced by moving the capacitor as close to the semiconducting chip as possible. The advent of small sized ceramic multilayer capacitors has allowed construction alternatives to decrease these distances, yet as the. current switching rate continues to increase there is a limit to the number of circuits within a structure that can be switched simultaneously. Therefore, there is greater

demand for additional decreases in the noise level of the circuit. Recent electronic circuitry designs consider the elimination of discrete capacitors by producing a basic integrated circuit that includes the decoupling capacitor as an integral part of the package. Other designs bury the decoupling capacitor into the substrate and access them through vias. These designs minimize lead lengths, hence decrease inductance to increase system speed. In addition, elimination of discrete components from the surface of the board provides additional space for higher integrated circuit density.

Various types of polymer films are currently being used in the processing of circuit board capacitors, however, the dielectric constant of these polymers is limited and does not satisfy requirements for certain decoupling capacitor applications. In an attempt to increase the dielectric constant of these polymers, high dielectric constant inorganic fillers are dispersed within the polymer. Although in some cases the dielectric constant is increased by this method, to date this approach has. not resulted in dielectric constant values and processing characteristics meeting the demands of the electronic packaging industry. The dielectric constant of a composite material depends on the quantity of each phase present as well as the dielectric constant of the individual phases. When considering a composite composed of-materials with vastly different dielectric constants, such as BaTiθ3 and a polymer, the arrangement of the phases also plays a key role in dictating the composite dielectric constant. The two most basic phase configurations can be described as analogous to the series and parallel case of electrical circuits. If slabs of each phase are alternately stacked and the electrodes are placed

parallel to the plane of the slab, a series configuration results. In this configuration, the dielectric constant of the resulting composite is suppressed by the dielectric constant of the low dielectric constant phase. On the other hand, when the electrodes are placed perpendicular to the plane of the slabs, a parallel analog results. If one is attempting to maximize the functional dielectric properties, it is preferable to have this latter arrangement of phases because the composite dielectric constant will now be directly proportional to the dielectric constant and volume fractions of both phases.

U.S. patent 4,908,258 (Hernandez) discloses a method of producing a flexible composite film of high dielectric constant that utilizes this latter parallel analog type of phase configuration. A composite film is disclosed which is comprised of a layer of high dielectric constant sintered chips or pellets in a planar array within a polymer matrix. Because the high dielectric constant chips are fully sintered and arranged in a parallel manner with respect to the electrodes, the resulting effective dielectric constant is as high as 1000. This process demonstrates the importance of configuring the high dielectric constant phase in a parallel array with respect to the electrodes. However, in the Hernandez disclosure, the high dielectric constant pellets are separated by areas of polymer causing the dielectric constant to be inhomogeneous across the sheet. In another approach, a high dielectric constant inorganic filler may be dispersed in a resin or epoxy, resulting in a more complicated arrangement of phases. Generally, in this mixing approach, a combination of point contacts or particles will be separated by a thin polymer layer. Consequently, attaining composites with

dielectric constants above 20-40 is difficult, and requires a high volume loading of the inorganic phase. The composite becomes brittle with high volume loadings, and handling and adhesion properties of the polymer are sacrificed. Thus, admixing of high dielectric constant powders with polymers is not an effective method of fabricating high dielectric constant composite materials.

Japanese patent 53,088,198 is based on this type cf inorganic filled polymer system. This patent discloses a process for producing a dielectric paste that is coated on a pair of electrodes to form a printed capacitor for circuit boards. This was accomplished by dispersing ceramic powders, preferably BaTiθ3 having an average grain size of 2 microns in a polybutadiene resin.

Other Japanese patents disclose composite materials of inorganic filled polymer films used as part of electronic circuit boards, but do not contemplate use of these composites as capacitors. For example, Japanese patent 63270133 (Mitsubishi Denki KK) discloses a process for producing a low dielectric constant, high dielectric breakdown strength circuit board which comprises a heat-conductive metal substrate, an adhesive layer and an electrically insulating polyimide layer that contains at least 30 wt. % of a ceramic material. This composite utilizes AI2O3, Siθ2, BN, SiN, mica and MgO but does not include high dielectric constant materials such as BaTiθ3 as a filler material. Japanese patent 86306721 (Matsushita Elec. Works) discloses a thermosetting polyphenylene oxide resin composition for use in printed circuit boards that contains inorganic fillers such as titanates or lead zirconate.

Other references discuss the use of BaTiθ3 and other high dielectric constant powders dispersed in polyvinylidene (PVDF) for piezoelectric applications. For example, Yamada et al. in "Piezoelectricity of a High-Content Lead Zirconate Titanate/Polymer Composite", J.Appl.Phys., 1982, discuss the dielectric properties of a lead zirconate titanate/PVDF composite and disclose a sample containing 67 vol. % ceramic having a dielectric constant of 152. Muralidhar et al. in "Pyroelectric Behavior in Barium Titante/Polyvinylidene Fluoride Composites", (IEEE Transactions on Electrical Insulation 1986) , disclose mixing BaTiθ3 powder with PVDF which yields a composite with a dielectric constant of 133 for a sample containing 70 wt. % BaTiθ3. Another approach in obtaining polymer matrix composites involves the formation of a porous ceramic body which can be impregnated with a polymer. This results in a composite material wherein the polymer and the ceramic phases are arranged partially in parallel and partially in series. These composites have a "3-3 connectivity" pattern, denoting the number of orthogonal directions in which each phase is self- connected.

This technique is used by Rittenmyer, et al. in "Piezoelectric 3 - 3 Composites", Ferroelectrics, 1982, to process lead-zirconate titanate (PZT)/polymer composites. In this process, PZT powder is mixed with plastic spheres and an organic binder. Subsequent heat treatment of the mixture allows decomposition of the binder and plastic spheres, and sintering of the powder. The heat treatment is done such that a porous skeleton of PZT results, which is later backfilled with either silicone or epoxy to form a composite. These composites, which necessarily contain lead, are intended for hydrophone applications where high piezoelectric

coefficient and minimum dielectric permittivity are desired. The disclosure provided by this reference is limited solely to piezoelectric applications.

Other references which discuss 3-3 connectivity patterns for distinct applications include "Flexible Composite Transducers," D. P. Skinner et al., Mat. Res. Bull. Vol. 13, pp. 599-607, 1978; and "Simplified Fabrication of PCT/Polymer Composites," T. R. Shrout, et al., Mat. Res. Bull. Vol. 14, pp. 1553-1559, 1979. In the present invention, partial sintering of a high dielectric constant powder has been employed to fabricate a composite with a 3-3 connectivity pattern. The partial sintering step allows surprisingly high composite dielectric constants to be achieved. This approach to composite fabrication results in much higher values of dielectric constant than the dispersion of similar powder in polymer.

U.S. Patent 4,882,455 (Sato et al,) discloses a process for producing an electronic circuit substrate which is comprised of a porous ceramic sintered body that is subsequently filled with an epoxy resin. Sato describes this process as a means for producing circuit board substrates having a low dielectric constant, and discloses no contemplation that such substrates could provide utility as capacitors. Further, because the importance of connectivity in particle contact is not an issue for establishing low dielectric constant material, this patent demonstrates no recognition of a means to employ partial sintering process to achieve composites with a hiσh dielectric constant. Since both the ceramic and epoxy used in the disclosed Sato composite process have low dielectric constants, the resulting effective dielectric constant will be insensitive to changes in connectivity of ^' the ceramic phase.

Accordingly then, due to the above noted deficiencies in the art relating to a means of achieving ceramic/polymer composites with high dielectric constants useful for integral and buried decoupling capacitor applications, a clear need exists in this area. The instant invention fills this need by providing a high dielectric constant partially sintered flexible composite having properties suitable for providing capacitance as decoupling capacitors in electronic circuit packages.

SUMMARY OF THE INVENTION The present invention comprises a flexible, high dielectric constant composite comprising partially sintered porous ceramic and polymer resin, wherein the dielectric constant of the composite is at least about 150.

The invention further comprises an electronic circuit comprising a flexible, high dielectric composite comprising partially sintered porous ceramic and polymer resin, wherein upon at least one surface of said composite an electrically conductive circuit has been patterned..

The invention further provides a method of providing capacitance in electronic circuit packages comprising integrating composite material of the instant invention within an electronic circuit package. The invention further provides a method of providing capacitance in electronic.circuit packages comprising integrating composite material of the instant invention within an electronic circuit package, such that said composite is integrated within the substrate level of the electronic circuit package and is a decoupling capacitor accessible through vias to external microchips.

The invention further provides a method of providing capacitance in electronic circuit packages comprising integrating composite material of the instant invention within an electronic circuit package, such that said composite is integrated within the microchip carrier as a discrete decoupling capacitor for said microchip.

DETAILED DESCRIPTION OF THE INVENTION

It is the object of this invention to provide a high dielectric constant composite material comprised of partially sintered porous ferroelectric ceramic impregnated with polymer, such that the material may be used to provide capacitance in electronic circuit packages. Further, it is contemplated that the composite of the invention has utility to function as a decoupling capacitor which, because of ^" its electrical characteristics, unique flexibility and processability, can be fashioned as an integral part of the package. See, for example "Improved Electrical Performance Required for Future MOS Packaging" L.W. Schaper et al., IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-6, No.3, Sept. 1983, (herein incorporated by reference) which on pages 3-4, at figures 6-8, serves to illustrate aspects of Applicants'contemplated use of the ceramic polymer of the instant invention. These figures illustrate some of the approaches currently under investigation which consider integrating discrete decoupling capacitors as part of the chip carrier package. Applicants also contemplate that the composite of the present invention is uniquely suitable to function as a decoupling capacitor buried within the substrate level of an integrated package accessible through vias to one or more externally located chips.

The instant invention comprises a partially sintered, porous ceramic of high dielectric constant, wherein the pores of the ceramic are impregnated with a resin which imparts to the composite flexibility and processing characteristics necessary in electronic circuit applications.

Applicants have found, surprisingly, that high values of dielectric constant in composite materials can be achieved without fully sintering the ceramic phase of the composite. It is known that the dielectric constant of BaTiθ3 and other such ferroelectric materials is significantly higher in the dense polycrystalline sintered state than in powder form. Applicants have used a fine starting particle size powder which is uniformly dispersed in polymer binder solution. The material is then heated so that fusing of the particles results to form "necks" without completely removing the porosity between grains. The resulting voids have an average diameter smaller than the average particle size of the initial powder and usually below 1 micron. In addition, very narrow distribution of pore size is achieved enabling homogeneous composite properties. _. This process of partially sintering the material is also used to provide additional strength and machinability, good dimensional control, and thermal expansion coefficients similar to that of silicon.

Although composites exist in the art which consist of partially sintered porous ceramic impregnated with polymer resin, the purpose of such previous art was to obtain composites for use as low dielectric constant electronic substrates. Applicants have accomplished in the composite of the present invention a nonobvious beneficial electrical property— igh dielectric constant— from such partial sintering processes; and

have recognized unique utility for such composites, a flexible decoupling capacitors, for example.

In describing aspects of the instant invention, Applicants refer to the term "dielectric constant", meaning the ratio of the capacitance provided by a specified dielectric material to the capacitance of vacuum (or air) . More specifically, applicants inten by the term "high dielectric" that the dielectric constant is at least about 150; suitable for enabling the composite to function as a decoupling capacitor. Capacitor refers to a device that can store electrical charge in the presence of a voltage gradie Applicants use the term "flexible" to describe t relative capability of being physically bent, deforme or flexed. The characteristic of flexibility is measured by Applicants in Tables 1 and 2 of the "Examples" section by indicating the Average Flexure Strength (MPa) and Average Elastic Modulus (MPa) of t various sintered ceramics and composites presented. The term "additive" is used to describe any of t class of commonly employed fluxes, sintering aids, dopants, etc., which are routinely used by those skil in the art of ceramics to effect sintering temperatur and other characteristics of ceramic articles. As commonly employed in this art, additives are used at levels less than weight 5% of starting material.

Decoupling capacitor refers to those capacitors which are distributed in a circuit loop and are electrically coupled to the semiconductor chip to decrease electrical noise associated with simultaneous switching of relatively large number of off chip drivers.

Substrate, as relating to electronic circuit package applications, means a base material onto which

electrical components comprising the electrical circuits are located.

Electronic circuit packages refer to an enclosure comprising a single element, integrated circuit, or a hybrid circuit that is the first level of interconnection electrically connected to the device through the use of package terminals.

Integrated circuit is a microcircuit of interconnected elements inseparably associated and formed within a single substrate to perform an electronic circuit function.

Vias refer to a plated through hole used as a through connection.

The terms chip, microchip, or semiconducting microchip all refer to an individual semiconductor element or integrated circuit after it has been cut from a silicone semiconductor wafer.

Sintering is the transfer of matter to reduce the surface area within a particle compact by heating a material to approximately 3/4 of it's melting temperature. A completely sintered material is one in which all residual porosity is eliminated; however, the sintering process can be interupted at any stage prior to complete densification in which case the material is considered partially sintered.

According to the present invention, the porous ceramic sintered body is comprised of a high dielectric constant ferroelectric ceramic material including, but not limited to, lead zirconate, barium zirconate, barium titanate and titanates of strontium, lead, calcium, magnesium and neodymium and solid solutions thereof.

By the term ceramic "solid solution", Applicants mean a two or more ceramic component system in which the ceramic components are miscible in each other. The dielectric constant of BaTiθ3 is strongly dependent on

temperature and increases from room temperature to it's ferroelectric Curie point of 120 °C. Above this temperature, the structure of BaTiθ ₃ changes from tetragonal to cubic and the material no longer possesses ferroelelectric characteristics. Further, the Curie temperature can be adjusted by the addition of small amounts (less than 5%) of additives. For example, by substituting Sr ⁺ ions for Ba2 ⁺ ions, the Curie temperature drops, allowing higher values of dielectric constant at lower temperatures. Other additives can also be added to BaTiθ3 in order to flatten the Curie peak to eliminate the dependence of the dielectric constant on temperature. The temperature at which BaTiGβ sinters (>1300 °C) can also be reduced by the addition of a small amount of certain sintering aids and fluxes to the composition. Therefore, in addition to the high- dielectric ferroelectric ceramic materials described above, it is also contemplated by Applicants that the porous ceramic body of the invention may be fabricated by ferroelectric ceramic materials modified by additives including, but not limited to Zrθ2, Bi2θ3 and b2θ5.

The permeable porosity of the high dielectric constant material can be adjusted by changing the ratio of ceramic powder to polymer binder solution in the green state. In addition, the degree of porosity is dependent on the temperature to which the green sheets are fired, and therefore the heat treatment temperature can be used as an additional method of attaining ceramic sheets of various porosities. The permeable porosity of the ceramic of the instant invention should be in the range of approximately 18 to 60 volume percent of theoretical density. When the porosity is less than approximately 18 volume percent, there is not a sufficient amount of polymer to impart a useful degree of flexibility. When the permeable porosity exceeds

approximately 60 volume percent, the mechanical integrity of the porous sintered body is lost and normal methods of handling the material become impractical. Because the ceramic phase of the instant invention is not completely sintered, the inherent hardness value associated with fully dense ceramics is not realized and the porous body is easily machined. In addition, presence of the polymer in the permeable pores of the porous body increases strength, decreases stiffness and renders the composite impermeable to air and moisture. The porous sintered ceramic can be processed into the form of a sheet, laminate or film. The ceramic powder is first formed into a green body by a molding process such as pressure molding or tape casting. Because the capacitance of the layer is indirectly proportional to it's thickness, the dielectric layer should be kept as thin as possible with uniform thickness. Tape casting is known in the industry for forming large-area, thin ceramic parts of uniform thickness, thus it is the preferred molding process. In the tape casting process the BaTiθ ₃ powder is dispersed into a slip which evenly coats a moving surface by the use of a scraping blade or "doctor blade". The slip is comprised of a polymer binder, a plasticizer, and a solvent. Dispersants are also added to the slip to prevent agglomeration of the ceramic powder. The thickness of the green sheets can be controlled by the height of the doctor blade in the case of single layer parts and if significantly thicker parts are desired, individual sheets can be laminated by applying pressure at a suitable temperature. Parts of desired size are cut in the green state and placed onto an porous alumina setter for firing. The polymer binder is removed by slowly heating the compacts through the decomposition temperature of the organics in an air atmosphere. The

compacts are subsequently heated to higher temperatures suitable for self fusing or bonding of the ceramic particles by a sintering process. The extent of permeable porosity in the resulting compacts (which translates to the volume % BaTiOβ in the final composite) can be controlled by the heat treatment employed.

In order to fill the permeable pores with polymer, a resin can be impregnated in it's monomer state and converted to a polymer by subsequent heat treatment. The polymer can also be melted by heating and then impregnated into the porous body. In addition, resin in solid form can be dissolved in a solvent or dispersion medium, impregnated into the porous ceramic, dried for solvent removal and heated to melt or cure the resin. Polymer or monomer impregnation can be achieved by immersing the porous compact in a resin solution under vacuum or with the aid of externally applied pressure.

Suitable resins to be filled in the permeable pores of the porous ceramic sintered body include but are net limited to: epoxies, polyamides, polyimides, polyetherimides, polyamide-imides, fluoropolymers, polyacrylates, polyetherketones, polyetherketoneketones, polysulfones, polyphenylene sulfides, bismaleimide resins, phenolic resins, polyesters, polybutadienes, polyetheretherketones or cyanate esters. Preferred resins include epoxy, cyanate esters, and polyimides.

In another aspect of the present invention, an electrically conducting metal foil may be adhered to the composite material by lamination under pressure at sufficient temperatures to fabricate electric capacitors. If there is not sufficient adhesion between the metal foil and the polymer phase of the composite material, a polymer adhesive layer can be used, however, it has been found that the presence of even a thin layer

of adhesive severely degrades the effective dielectric constant of the laminate. Applicants have therefore employed a conductive adhesive to fabricate the laminate which yields an effective dielectric constant that more closely resembles that of the high dielectric constant composite material. Therefore, application of a conducting adhesive is the preferred method for adhering the conducting metal foil. The conductive adhesive may be in the form of a film or paste, and the conductive particles include, but are not limited to silver, copper, nickel, or gold. Other methods for metallizing the composite sheets include forming a thin layer of metal directly onto the composite surface by electroless plating, vapor deposition, or sputtering. The thin metal layers can be subsequently plated up using electoplating techniques.

EXAMP E 1 In this example, tape casting was employed as a method to produce thin sheets of BaTiθ3. To 100 parts by weight of high purity BaTiθ3 powder (HPBT-1, Fuji Titanium Industry Co., Ltd., Hiratsuka City, Japan) of 0.6 micron average particle size was added 58 parts by weight of trichlorethane (J. T. Baker Chemical Co., Phillipsburg, NJ) , 27 parts by weight of acrylic binder resin (5200 MLC ceramic binder polymer, Du Pont Co., Wilmington, DE) , 18 parts by weight of methyl ether ketone (J.T. Baker Chemical Co.), and 18 parts by weight of dioctyl phthalate (Aldrich Chemical Co. Inc., Milwaukee, WI) . The solution was mixed in a ball mill for 17 hours and coated onto a Mylar™ carrier film using a .020" doctor blade. The resulting tape had a thickness of .004" after solvent removal. Squares with sides of 2" were cut from the green tape and 4 layers were collated and laminated at 50 °C under a pressure of 2000 psi for 5 minutes. The resulting laminates were

fired on a porous alumina substrate at the temperatures listed in Table 1 in an air atmosphere for 1 hour. Electrodes were applied to the porous parts using silver paint and the dielectric properties were analyzed using a parallel plate capacitance technique. Additional porous ceramic parts were then soaked for 10 minutes in an electronic grade resin (Quatrex 5010, Dow Chemical USA, Midland, MI) consisting of 56 % solids dissolved in methyl ethyl ketone. Following the soaking process the samples were heated to 60 °C for 30 minutes for solvent removal. The preconditioned composites were then heated to 177 °C under vacuum for 90 minutes. Polymer rich regions were removed from the composite surfaces by polishing to expose material that was representative of the bulk. Electrodes were made on opposite surfaces of the sheet using conductive silver paint and the dielectric constant was measured at 1 KHz. Sample density and dielectric constants are included in Table 1 below. The composite bodies exhibited dielectric constants of 317 and 908 for samples comprised of 52 and 67 volume % BaTiθ3, respectively. The dielectric constant of the composite material is dependent on the firing temperature of the porous compact. The utility of this material to provide a broad range of capacitance is demonstrated.

EXAMPLE 2 1 oz. copper foil was adhered to opposite surfaces of the polymer impregnated samples fabricated in Example 1 by means of a silver filled adhesive (QL3410 Ditac® IC adhesive, Du Pont Company, Wilmington, DE) by hot pressing at a temperature of 150°C for 30 minutes. The dielectric properties were measured and the results are also included in Table 1 below. Although the presence of the silver filled adhesive decreases the effective dielectric constant of the composite, it is an

effective method for adhering copper foil when moderate dielectric constants are required .

Heat Treatment Density Temp . (C) (g/cc)

1100 3.15

1200 4 .47

The same procedure as described in Example 1 was repeated to make a 4 layer green laminate from tape cast BaTiθ ₃ . The laminate was heated to 500 °C for 5 hours and the density of the resulting porous compact was 3.06 g/cc. ^• The part was then soaked in an electronic grade epoxy (Quatrex 5010, as in Example 1) and heated to 60 °C for 30 minutes. The preconditioned laminate was then heated to 177 °C and held under vacuum for 90 minutes. A surface polymer rich region was polished off and the density of the impregnated part was 3.76 g/cc, corresponding to the epoxy resin occupying 96% of the pores-. Silver paint was applied to opposing surfaces of the sample and the dielectric constant (IKHz) of the resulting composite was 54. This example, where heating was insufficient to allow sintering to occur, illustrates the importance of partial sintering of the ceramic powder prior to polymer impregnation in order to achieve significantly higher dielectric constants.

EXAMPLE 4 The procedure of Example 1 was repeated to produce green laminates of .020" thickness. A series of

specimens were fired on a porous alumina substrate in an air atmosphere under atmospheric pressure at temperatures included in Table 2 for 1 hour. The partially sintered porous compacts were then impregnated with a cyanate ester monomer (AroCy™ L-10, Hi-Tek

Polymers, Jeffersontown, Ky) under vacuum and heated to 250 °C in an air atmosphere for 2 hours. Dielectric measurements were conducted at 1 KHz and the results are included in Table 2. Flexure strengths and elastic moduli were measured in 4 point bending and the results are also shown in Table 2. Table 2 also includes data for the porous specimens prior to polymer impregnation.

This example demonstrates the ability to change the dielectric constant and mechanical properties of the composite through variations in firing temperature of the precursor porous ceramic compact.

TART.F, 7

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Average

Heat Average Permeable Flexure Treatment Density Porosity Dielectric Strength Temp. (C) (g/cc) (Vol. %) Constant (MPa)

1000 3.3 45.9 506 11.4 1100 3.4 43.6 791 17.3 1200 3.8 37.3 1802 29.1 1230 4.0 33.2 1859 33.2 1275 4.8 20.3 3043 55.5