MANUFACTURING MULTILAYER CERAMIC SUBSTRATE

Abstract: Green sheet having a carrier film made of a polymer film which has a glass transition point of 90 DEG C or higher, and a thermal expansion coefficient of 6.0X10<-5>cm/cm DEG C or less, and has a high organic solvent durability and a high humidity durability, and production of a multilayer printed circuit substrate using the same. Damages during the step of producing green sheets for ceramics multilayer substrate can be decreased and the step of producing the multilayer substrate can be simplified.

Description: MANUFACTURING MULTILAYER CERAMIC SUBSTRATE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to unsintered ceramics sheets (green sheets) for use in ceramics multilayer substrates for electronic circuits, a method for the production of the same and a method for the production of ceramics multilayer substrates using the green sheets. Description of Related Arts As is well known in the art, there has been a keen demand for reduction in size, increase in the reliability and in the operational speed of electronic apparatus for processing a large amount of information such as computers, transmission devices, industrial apparatus, and electronic exchange, and in order to cope with this demand it is necessary to increase the actual density of electronic circuits used therein. In light of this, it becomes important to increase the actual density of electronic circuits by using circuits in the form of modules. In order to achieve this, it is appreciated that higher densities are difficult to be attained by the conventional method in which IC chips are mounted on a substrate after being housed in respective packages, and nowadays, various new technologies for the actual application of IC chips have been developed IC chips to replace thereby the conventional method. As an example thereof, there has been put into practice as an effective approach a high density actual application technology in which is a number of IC's and LSI's in the form of chips are mounted directly on a multilayer wired substrate. As for the substrate for use in multilayer wiring, there has been used usually ceramics substrates with which it is easy to achieve higher densities and which have a high heat dissipation. While the most widely used material for such ceramics multilayer wired substrate is an aluminum substrate, other materials are also provided for the purpose which are highly toxic and hence have been used less widely but have high electric conductivity, such as beryllia, new silicon carbide materials, and aluminum nitride. Furthermore, there have recently been proposed ceramics materials which can be sintered at low temperatures such as crystallized glass ceramics having crystallization temperatures lower than the melting temperatures of copper. In these days such ceramics substrates are produced exclusively by a green tape method, in which an unsintered ceramics sheet called "green sheet" is used. The green sheet is produced as follows. For example, in the case of alumina substrates, it is produced by mixing alumina powder with a flux component such as Si0 2 , CaO or MgO as a sintering aid, adding thereto an organic binder, a plasticizer, and a solvent, wet mixing the resulting mixture in a ball mill sufficiently to form a slip in the form of a paste, casting the paste on a carrier tape, scraping the paste with a doctor blade to form a sheet having a uniform thickness, and then drying to evaporate the solvent and form a unsintered ceramics sheet having a flexibility (this being called a "green sheet") . The green sheet is soft and is easy to be processed as described above, and therefore is subjected to machining before sintering. For example, the green sheet is slit to a predetermined size and peeled off of the carrier film therefrom. When a multilayer substrate is produced, the green sheet thus processed is provided with via holes as by punching, and via hole filling printing with a conductor paste is carried out using the screen 3 having the pattern corresponding to the via hole 2 in the green sheet 1 as shown in Fig. 1. Thereafter, a circuit is printed on the surface of the green sheet with a conductor paste. This is dried (120°C, 5 minutes) and the thus obtained green sheet is laminated with an insulating layer and a desired number of other sheets. The laminate is then sintered (l,500 β C _ 1,650°C) to obtain a multilayer substrate. The following publications are available on the technologies relative to the above-described green sheets. U. S. Patent No. 4,153,491 to Swiss et al. discloses a green ceramics sheet material comprising a high alumina and glass frit composition dispersed in a binder made of an organic substance. In U. S. Patent No. 3,717,487, Hurleys et al. disclose among others a ceramics slip concentrate comprising A1 2 0 3 dispersed in a slip containing a polymethacrylate binder, a solvent and a dispersing agent. In U. S. Patent No. 3,857,923, Gardner et al. disclose a ceramics green tape comprising mullite dispersed in a binder such as poly(vinyl butyral) . In U. S. Patent No. 3,962,162, Schmank discloses a molding solution for preparing green ceramics sheets, which solution comprises a polyester solution having dispersed therein powder of a refractory substance such as A1 2 0 3 , a crosslinking monomer, a free radical initiator and a releasing agent. U. S. Patent No. 3,988,405 to Smith et al. discloses a composition for molding, comprising an acrylic copolymer latex one of whose comonomers is a polymerizable carboxylic acid, having dispersed therein a ceramics substance, particularly glass ceramics. U. S. Patent Nos. 4,080,914 and 4,104,345 to Anderson et al. relate to ceramics green sheets prepared from a solution of molding containing both a solvent and a non-solvent to an organic binder. U. S. Patent No. 4,272,500 to Eggerding et al. relates to a ceramics green tape comprising a mixture of mullite and A1 2 0 3 dispersed in a poly(vinyl butyral) binder. U. S. Patent No. 4,183,991 to Smiley discloses a solution for molding for preparing a thin filled polymer sheet of 0.1 inch (0.25 cm), comprising a dispersion of inert filler particles in a polymer-in-monomer solution. U. S. Patent No. 4,301,329 to Kumar et al. discloses a ceramics green tape in which the ceramics substance is b-geikielite or cordielite. The aforementioned green sheet and method for the production of the same have a problem that cracks tend to occur in the sheet because the green sheet is thin, thus decreasing workability. As shown in Fig. 1, upon the conductor filling printing of the via hole 2 in the green sheet 1, the corresponding portion 3a of the screen 3 which corresponds to the via hole 2 must be registered with the via hole 2. However, this operation is difficult and a problem arises that mass production is difficult. As for the workability, it may be considered to carry out the process by attaching the green sheet to a guide frame. However, this method leads to increased loss of the green sheet and therefore is not suitable from in view of cost. SUMMARY OF THE INVENTION In order to solve the aforementioned problems, the present inventors have made extensive investigations and have now obtained the following knowledge. That is, in the step of producing ceramics substrates, damages to green sheets can be decreased to prevent the occurrence of cracks in the sheet by preparing a green sheet, slitting it to a predetermined size, providing it with via holes without peeling off a carrier film unlike the conventional method, filling the via holes with a conductor paste by screen printing using the carrier film as a screen, printing a circuit on the side of the ceramics, drying, and then peeling off the carrier film for the first time, followed by the same subsequent procedures as in the conventional method. In the step of filling the via holes, because the carrier film is provided with holes of the same size as those in the ceramics sheet, printing can be carried out using the carrier film as a substitute for the screen, resulting in that registering of the screen is no longer.necessary at the time of the via hole filling printing, which makes it possible to simplify the production steps. Upon examination of characteristics required for the carrier film, it has revealed that it must have the following characteristics: i) Good via punching property. ii) Glass transition point (Tg) of 90°C or higher. iii) Good thermal stability at 90°C or lower. IV) Good organic solvent durability and good humidity durability. As a result of extensive experimental research with view to finding films having the aforementioned characteristics, it has now revealed that polymer films selected from polyimide films, polyetherimide films, polyphenylene sulfide films, and polyethersulfone films are suitable. The present invention has been completed based on the above-described discovery. That is, the green sheet of the present invention is characterized by having a carrier film made of a polymer film which has a glass transition point of 90°C or higher, and a thermal expansion coefficient of 6.0 x 10" 5 cm/cm °C or less, and has a high organic solvent durability and a high humidity durability. More specifically, the polymer film is any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyethersulfone film. The method for the production of green sheet according to the present invention comprises mixing ceramics powder with a sintering aid, adding thereto an organic binder, a plasticizer and a solvent, wet mixing the resulting mixture to from a slip in the form of a paste, casting the paste on a carrier tape serving as a carrier film to form a sheet having a uniform thickness, and then drying to evaporate the solvent and form a green sheet, wherein said carrier film is a polymer film which has a glass transition point of 90 β C or higher, and a thermal expansion coefficient of 6.0 x 10 ~5 cm/cm "C or less, and has a high organic solvent durability and a high humidity durability. More specifically, as the polymer film, there is used any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyethersulfone film. Further, the method for the production of a ceramics multilayer substrate according to the present invention comprises mixing ceramics powder with a sintering aid, adding thereto an organic binder, a plasticizer and a solvent, wet mixing the resulting mixture to form a slip in the form of a paste, casting the paste on a carrier tape serving as a carrier film, said carrier film being a polymer film which has a glass transition point of 90°C or higher, and a thermal expansion coefficient of 6.0 x 10 ~5 cm/cm °C or less, and has a high organic solvent durability and a high humidity durability, to form a sheet having a uniform thickness, and then drying the sheet to evaporate the solvent and form a green sheet having a flexibility, slitting the green sheet to a predetermined size, providing the green sheet thus obtained with via holes at predetermined positions, performing via hole filling printing with a conductor paste on a side of the carrier film of the green sheet with the via holes using the carrier film as a screen, printing a circuit on a side of the green sheet on which the ceramics is present, drying, peeling off the carrier film, laminating the green sheet after the peeling off of the carrier film with one or more other sheets to form a laminate, and sintering the resulting laminate. In this method for the production of a ceramics multilayer substrate, as the polymer film, there is used specifically any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyethersulfone film. According to the present invention, damages of the green sheet in the step of producing green sheets for ceramics multilayer substrates can be decreased and the steps for the production of the ceramics multilayer substrates can be simplified. The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 is a cross-sectional view showing the via hole filling printing step in the conventional method for the production of a ceramics multilayer substrate. Fig. 2 is a cross-sectional view showing the via hole filling printing step in the method for the production of a ceramics multilayer substrate according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the production of ceramics multilayer substrate, the green sheet of the present invention undergoes the piercing of not only the ceramics sheet itself but also the carrier film simultaneously when the via holes are punched. Because the via holes are used as a screen mask at the time of the via hole filling printing, poor punching property, that is, presence of failure such as burr in the carrier film which makes it difficult to perform the via hole filling printing after the punching causes a fatal problem in the production of substrates. In view of this, the via punching property is one of the most important characteristics among the conditions met by the carrier film of the green sheet used in the step of producing substrates. Also, in the production of substrates, the drying of the green sheet after the via hole filling printing and the circuit printing is carried out in a state in which the carrier film is still attached thereto, and hence the thermal stability of the carrier film is important. The drying temperature of the conductor paste for the green tape (6142D, 6199D, 6191D, 6193D and the like) is suitably 80 to 90 P C in the step in which the carrier film is attached, and the carrier film must not undergo glass transition within this temperature range. Therefore, the carrier film is required to have a glass transition temperature Tg of 90°C or higher. Even when the film has a glass transition temperature of 90 β C or higher, that having a high thermal expansion coefficient at 90°C or lower tends to be peeled off from the green sheet during drying the conductor paste due to difference in size change therebetween, and care must be taken on this point. On this occasion, the thermal expansion coefficient must be 6.0 X 10" 5 cm/cm β C or lower. In light of the aforementioned point, polyethylene terephthalate (PET) film conventionally used as a carrier film is poor in via punching property, and often causes failure such as burr after the punching step. Because of its glass transition point being as low as at most 70°C, PET is unsuitable for the step of producing a green sheet in which the green sheet with a carrier film is processed, which is aimed at by the present invention. As polymer film which would meet the requirements necessary for the carrier film as described above, there were prepared eight types of films, i.e., a polyimide film (PI) of 75 urn thick, a polyetherimide film (PEI) of 75 urn, a polycarbonate (PC) film of 75 um, a polyphenylene sulfide (PPS) film of 75 um, a polyethersulfone (PES) film of 75 um, a polyacrylate (PAR) of 75 um, a polysulfone (PSF) film of 70 um, a polyethylene terephthalate (PET) film of 75 u , and the following performance tests were carried out. (a) Via hole piercing ability (V. P.): Diameters of 100 um, 150 um, and 200 u . (b) Thermal expansion coefficient (T. E. C.) (c) Glass transition temperature (Tg) (d) Solvent durability (S. D.): Against 1,1,1- trichloroethane (e) Humidity durability (H. D. S.) : Catalogue values Tables 1 and 2 show the results of the tests obtained. As will be apparent from Tables 1 and 2, the polymer film suitable as a carrier film used in the green sheet of the present invention is any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film, and a polyethersulfone film. Table 1 V. P. Failured Ratio [%] Film 200 um 150 um 100 um 15 um* 15 um* 15 um* PEI 3.0 5.8 9.0 PI Almost the same as the above PEI PC 9.3 6.5 2.8 PPS 1.3 1.8 2.3 PES 3.5 3.0 18.3 PAR 9.5 13.8 12.3 PSF 3.5 5.8 6.3 PET 52.5 82.8 3.0 Allowance Next, the method for the production of a green sheet according to the present invention will be explained. As the ceramics material, there can be used alumina, beryllia, new silicon carbide materials, aluminum nitride as in the conventional method, and in addition thereto ceramics materials which can be sintered at low temperatures such as crystallized glass ceramics. Upon slipping it, the powder raw material of the ceramics material is usually kneaded together with an organic binder, a plasticizer, a solvent and one or more other additives to prepare a slip. The organic binder is preferably an organic polymer binder. As the organic polymer binder, there can be used, for example, vinyl type polymers such as polyvinyl butyral, polyvinyl acetate, and polyvinyl alcohol; cellulose type polymers such as methylcellulose, ethylcellulose, hydroxyethylcellulose, and methylhydroxyethylcellulose; atactic polypropylene; polyethylene; silicone type polymers such as polymethylsiloxane, and polymethylphenylsiloxane; polystyrene; butadiene/styrene copolymer; polyvinylpyrrolidone; polya ide; high molecular weight polyether; ethylene oxide/propylene oxide copolymer; and the like. In addition, there can be used acrylic type polymers such as sodium polyacrylate, polyalkyl acrylate, polyalkyl methacrylate, alkyl allylate/alkyl methacrylate copolymer, ethyl methacrylate/methyl acrylate copolymer, and ethyl acrylate/methyl methacrylate/methacrylie acid terpolymer. As a matter of course, monomers, oligomers and low molecular weight polymers of the aforementioned polymers can be added in order to improve the properties of the polymers as an organic polymer binder. As for the plasticizer, there can be used, for example, diethyl phthalate, dibutyl phthlate, butyl benzyl phthalate, dibenzyl phthalate, alkyl phosphates, polyalkylene glycols, polyethylene oxide, hydroxyethylated alkyl phenols, tricresyl phosphate, triethylene glycol diacetate, polyester type plasticizers singly or as mixtures of two or more thereof depending on the polymer for which the plasticizer is used. Examples of the solvent which can be used include acetone, xylene, ethanol, ethanol, isopropanol, methyl ethyl ketone, 1,1,1-trichloroethane, tetrachloroethylene, amyl acetate, 2,2,9-triethylpentanediol-l,3- monoisobutyrate, toluene, methylene chloride, fluorocarbon solvents and the like. These can be used singly or as mixtures of two or more thereof. The other additives include a dispersing agent, a flocculating agent, a wetting agent, a releasing agent, a defoaming agent, a leveling accelerator, a pin hole inhibitor, and the like. These can be used singly or as mixtures of two or more thereof depending on the purpose for which they are used. The composition of the slip is not limited to the aforementioned materials. The kneading of the slip can be performed by applying usual milling methods such as using ball mill, sand mill, bead mill, oscillating mill. The particle diameter of the inorganic powder is generally within the range of 1 to 10 mm as average particle diameter but is not limited to this range. It should be noted that if the particle diameter is larger the density of the sintered product is difficult to be increased, resulting in that the surface smoothness of the product is poor after the sintering. On the other hand, if the particle diameter is too small, the viscosity increases to make it difficult to prepare the slip or a larger amount of the organic polymer binder is needed, resulting in that sintering failure occurs or the sintered product fails to have an increased density. Therefore, the average particle diameter must be selected properly. As for the method for coating the slip, there may be selected a proper method from those usually used, for example, curtain coating, air knife coating, blade coating, extrusion coating, roll coating and the like, depending on the viscosity of the slip or the thickness of the coating film. In view of the aforementioned characteristics such as via hole punching property, and dimension stability,organic solvent durability, there is used any one polymer film selected from a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyestersulfone film as a carrier film in the coating. After the coating, drying is performed to evaporate the solvent in the same manner as in the conventional method to obtain the green sheet of the present invention. Next, explanation will be made on the method for the production of the ceramics multilayer substrate using the aforementioned green sheet according to the present invention. First, a sintering aid is mixed with ceramics powder, and an organic binder, a plasticizer and a solvent are added thereto, followed by wet mixing the resulting mixture to form a slip in the form of a paste. As described above, the slip in the form of a paste is cast on a carrier tape made of any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyethersulfone film to form a sheet having a uniform thickness, which is dried to evaporate the solvent to prepare a flexible green sheet. The green sheet thus obtained is slit to a predetermined size and is provided with via holes at predetermined positions thereof by punching with the carrier film remaining attached thereto. As a result, as shown in Fig. 2, the green sheet 10 is formed with via holes 11, and at the same time the carrier film 12 attached to the green sheet is provided with the holes 12a of the same size as the via holes 11. Then, as shown in Fig. 2, on the side of the carrier film 12 of the green sheet 10 is provided with the screen 13 of such a size as enough to cover the circumference of the green sheet 10, and via hole filling printing is carried out with a conductor paste from the side of the green sheet 10 provided with via holes 11 on which the carrier film 12 is present, using the carrier film 12 as a via mask. Subsequently, a circuit is printed with the same conductor paste as used conventionally on the ceramics surface side of the green sheet, and after being dried, the carrier film 12 is peeled off from the green sheet 10. The green sheet 10 after the peeling of the carrier film is laminated with an insulating sheet or one or more other sheets on which a circuit is printed, and the resulting laminate is sintered to obtain a ceramics multilayer substrate. Because the carrier film used in the green sheet of the present invention has the following characteristics, i) Good via punching property. ii) Glass transition point (Tg) of 90°C or higher, iii) Good thermal stability at 90 β C or lower. IV) Good organic solvent durability and good humidity durability, good via holes can be pierced without peeling off the carrier film unlike the conventional method after preparing the green sheet and slitting it to a predetermined size. Thereafter, the via holes can be filled with a conductor paste by screen printing using the carrier film as a screen. Subsequently, a circuit is printed on the ceramics side the green sheet, which is then dried and for the first time peeled off of the carrier film therefrom, followed by the same treatments as in the conventional methods so that in the step of producing ceramics substrates, damages to the green sheet can be decreased, and the occurrence of cracks or other failure in the sheet can be prevented. Further, as described above, in the step of filling the via holes, printing can be performed using the carrier film as a substitute for a screen because not only the ceramics sheet but also the carrier film is provided holes of the same size as those formed in the ceramics sheet. As a result, upon the via hole filling printing, it is unnecessary to perform the registering of the screen, which can simplify the production step. The invention has been described in detail with respect to embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the invention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.

Claims: CLAIMS WHAT IS CLAIMED IS: 1. A green sheet for a ceramics multilayer substrate having a carrier tape made of a carrier film and an unsintered ceramics sheet having a uniform thickness formed on said carrier tape, wherein said carrier film comprises a polymer film which has a glass transition point of 90°C or higher, and a thermal expansion coefficient of 6.0 X 10 ~5 cm/cm °C or less, and has a high organic solvent durability and a high humidity durability. 2. The green sheet for a ceramics multilayer substrate as claimed in claim 1, wherein said polymer film is any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyethersulfone film. 3. A method for the production of a green sheet for a ceramics multilayer substrate by mixing ceramics powder with a sintering aid, adding thereto an organic binder, a plasticizer and a solvent, wet mixing the resulting mixture to from a slip in the form of a paste, casting the paste on a carrier tape serving as a carrier film to form a sheet having a uniform thickness, and then drying to evaporate the solvent and form a flexible green sheet, wherein said carrier film, a polymer film which has a glass transition point of 90 β C or higher, and a thermal expansion coefficient of 6.0 x 10 ~5 cm/cm "C or less, and has a high organic solvent durability and a high humidity durability. 4. The method for the production of a green sheet for a ceramics multilayer substrate as claimed in Claim 3, wherein said polymer film is any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyethersulfone film. 5. A method for the production of a ceramics multilayer substrate, comprising the steps of mixing ceramics powder with a sintering aid, adding thereto an organic binder, a plasticizer and a solvent, and wet mixing the resulting mixture to form a slip in the form of a paste; casting the paste on a carrier tape serving as a carrier film, said carrier film being a polymer film which has a glass transition point of 90 ° c or higher, and a thermal expansion coefficient of 6.0 X 10~ 5 cm/cm β C or less, and has a high organic solvent durability and a high humidity durability, to form a sheet having a uniform thickness; drying said sheet to evaporate the solvent and form a flexible green sheet; slitting the green sheet to a predetermined size, and providing said green sheet thus obtained with via holes at predetermined positions; and performing via hole filling printing with a conductor paste on a side of said carrier film of said green sheet with the via holes using said carrier film as a screen, printing a circuit on a side of said green sheet on which the ceramics is present, drying, peeling off said carrier film, laminating said green sheet after the peeling off of said carrier film with one or more other sheets to form a laminate, and sintering the resulting laminate. 6. The method for the production of a green sheet for a ceramics multilayer substrate as claimed in claim 5, wherein said polymer film is any one of a polyimide film, a polyetherimide film, a polyphenylene sulfide film and a polyethersulfone film.

Inventors: HARA HIROYUKI, HARA, HIROYUKI
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1992
Main classification: C04B35
IPC codes: B28B1, B32B18, B32B27, C04B35, H01B3, H01L23, H05K1, H05K3