Abstract:
The present invention relates generally to using at least one green sheet that is originally very thin with the help of at least one thicker green sheet. An adhesion barrier to build multi-layer ceramic laminates and process thereof is also disclosed. Basically, the present invention relates to a structure and method for forming laminated structures and more particularly to a structure and method for fabricating multi-density, multi-layer ceramic products using at least one very thin green sheet and/or at least one green sheet with very dense electrically conductive patterns on top of at least one thicker green sheet.

Description:
This application is a divisional of Ser. No. 09/198,819, filed on Nov. 23, 1998, now U.S. Pat. No. 6,245,171. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to using at least one green sheet that is originally very thin with the help of at least one thicker green sheet. An adhesion barrier to build multi-layer ceramic laminates and process thereof is also disclosed. Basically, the present invention relates to a structure and method for forming laminated structures and more particularly to a structure and method for fabricating multi-density, multi-layer ceramic products using at least one very thin green sheet and/or at least one green sheet with very dense electrically conductive patterns on top of at least one thicker green sheet. 
     BACKGROUND OF THE INVENTION 
     Multi-layer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, a MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched in between ceramic layers which act as a dielectric medium. For the purposes of interlayer interconnections, most of the ceramic layers have tiny holes or via holes. Prior to lamination, the via holes are filled with an electrically conductive paste, such as, a metallic paste, and sintered to form vias which provide the electrical connection between the layers. In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, to name a few. 
     Generally, ceramic structures are formed from ceramic green sheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After punching, metal paste screening, stacking and laminating, the green sheets are fired or sintered at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate. The present invention is directed to the screening, stacking and lamination steps of this process. 
     In the MLC packaging industry it is very common to use green sheets of various thicknesses. The thicknesses can typically vary from 6 mils to 30 mils and in general the art of punching and metallizing these layers are well known. Green sheet thicknesses below 6 mils, in general, are very scarcely used. This is due to a variety of reasons, such as, for example, handling, screening and stacking of green sheets thinner than 6 mils pose tremendous challenges. In fact the use of one to two mils thick ceramic green sheets, which are punched and screened, using traditional MLC technology does not exist. 
     Also, in the MLC packaging industry it is very common to use capacitor layers. The capacitance necessary in a package depends on the design and such capacitance is obtained by choosing proper dielectric layer thickness and metal area within a layer. The industry is always striving for higher capacitance and since the metal area is maxing out for a given substrate size it is necessary to use thinner dielectric layers between electrodes to obtain the required capacitance. For example, as a rule of thumb one could double the capacitance for a given dielectric system and electrode metal area by decreasing the dielectric layer thickness by half Additionally the number of layers needed for capacitance in a package as well has been reduced by about 50 percent. The reduction in the number of layers is desirable, as it reduces the cost and the process of making the substrate. 
     U.S. Pat. Nos. 5,254,191 and 5,474,741 (Mikeska) teaches the use of flexible constraining layers to reduce X-Y shrinkage during firing of green ceramic bodies. But these flexible constraining layers, among other things, do not act as an adhesion barrier during lamination. 
     However, the present invention forms laminated multi-density, multi-layer ceramic structures using at least one very thin green sheet and/or at least one green sheet with very dense electrically conductive patterns on top of at least one thicker green sheet. An adhesion barrier that is useful during the lamination process is also utilized to build these multi-density, multi-layer ceramic laminates. 
     PURPOSES AND SUMMARY OF THE INVENTION 
     Bearing in mind the problems and deficiencies of the prior art it is therefore one purpose of the present invention to provide a novel method and structure for producing metallized thin green sheets, including sub-structures in multi-layer ceramic packages with novel adhesion barrier therein. 
     Another purpose of this invention is to have a structure and a method that will provide a semiconductor substrate with at least one capacitor layer or with at least one fine line patterned conductive metal layer. 
     Yet another purpose of this invention is to provide a structure and a method that will ensure multiple thin layers in a multilayer ceramic package. 
     Still another purpose of the present invention is to provide a structure and method that will ensure higher capacitance in a multi-layer ceramic package. 
     Yet another purpose of the present invention is to have a structure and a method for fine line pattern using thin green sheets in multi-layer ceramic packages. 
     Still yet another purpose of the present invention is to provide a structure and a method for metallizing a thin green sheet without any detrimental distortion. 
     Still another purpose of the present invention is to have a structure and a method that will ensure handling of thin green sheets for multi-layer ceramic packages. 
     It is another purpose of the invention to have a structure and a method that produces a multilayer ceramic package that is predictable and repeatable. 
     Another purpose of the present invention is to laminate several stacked green sheets with novel adhesion barrier to produce sub-structures. 
     Yet another purpose of the present invention is to use at least one adhesion barrier in lamination of multilayer ceramic packages. 
     Other purposes, objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
     Therefore, in one aspect this invention comprises a method of fabricating at least one multi-density semiconductor substrate comprising the steps of: 
     (a) forming at least one electrically conductive feature on at least one thick green sheet; 
     (b) providing at least one thin green sheet with at least one via hole; 
     (c) aligning and placing said thin green sheet over said thick green sheet, such that at least a portion of said electrically conductive feature is in contact with at least a portion of said via hole; 
     (d) dusting lamination surfaces with at least one inorganic adhesion barrier powder; 
     (e) tacking and bonding said at least one thin green sheet to said at least one thick green sheet, and thereby fabricating said multi-density semiconductor substrate. 
     In another aspect this invention comprises a multi-density substrate comprising at least one thin green sheet with at least one via hole in intimate contact with at least one thick green sheet, wherein at least one electrically conductive feature is sandwiched between said thin green sheet and said thick green sheet, and wherein at least a portion of said via hole is in contact with at least a portion of said at least one electrically conductive feature, and thereby forming said multi-density substrate. 
     In yet another aspect this invention comprises a multi-density substrate comprising at least one thin green sheet with at least one via hole in intimate contact with at least one thick green sheet having at least one electrically conductive material in secure contact with said thin green sheet, and wherein at least a portion of said via hole is in contact with at least a portion of said electrically conductive material, and wherein at least a portion of at least one surface of said thick and/or thin green sheet has at least one layer of at least one adhesive barrier material, and thereby forming said multi-density substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The drawings are for illustration purposes only and are not drawn to scale. Furthermore, like numbers represent like features in the drawings. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
     FIG. 1, illustrates a thin green sheet with an unfilled via hole. 
     FIG. 2, illustrates a thick green sheet which has been punched and metallized. 
     FIG. 3, illustrates the thin green sheet of FIG. 1, secured to the thick green sheet of FIG. 2, with the adhesion barrier on the lamination plates. 
     FIG. 4, illustrates the metallization of the structure shown in FIG. 3, with the adhesion barrier of the surfaces of the green sheets that came in contact with the lamination plates. 
     FIG. 5, illustrates another embodiment of this invention where at least one more layer of a thin green sheet, as shown in FIG. 1, has been secured to the structure as shown in FIG.  4 . 
     FIG. 6, illustrates the metallization of the thin green sheet of the structure shown in FIG.  5 . 
     FIG. 7, illustrates another embodiment of this invention showing the structure of FIG. 6, being used to form a multilayer ceramic package. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The structure and method of the present invention enables the handling, screening, stacking and lamination of thin ceramic layers. These thin ceramic layers are used in the semiconductor industry for a variety of purposes, such as, for example, for a capacitor structure, or for a fine line pattern structure in MLC packages, to name a few. 
     As stated earlier, that the structure and method of the present invention enables the screening, stacking and handling of very thin green sheets and/or green sheets with very dense metallized patterns in the manufacture of multi-layer ceramic packages. With the preferred embodiment, thin punched green sheets were tacked and bonded to thick punched and screened green sheets to form a sub-structure which yields excellent stability in screening and enables excellent handling and alignment in stacking. The green sheet sub-structure may have electrically conductive features within them, such as, a via, or over them, such as, a line, cap, to name a few. 
     The tacking and bonding of thin sheets onto a thicker sheet involves preferably heat and pressure. In such instances it is necessary to eliminate paste pull and ceramic damage by using a non-sticking solid polymeric material as the adhesion barrier between the green sheets and the metal surfaces that transfer heat and pressure on to the green sheet stack. The polymeric non-sticking material as is or with surface coating in general is very expensive and could be cost-prohibitive in several manufacturing applications. In order to make certain product design and associated processes viable in manufacturing, the present invention uses one of several very fine inorganic powders, preferably in submicron size, such as magnesium silicate or alumino-magnesium-silicate ceramics or oxides of aluminum-magnesium-silicon or alumina, to name a few, as an adhesion barrier. It is preferred to dust the heated metal surfaces of the pressure applying devices that contact the green sheet stack with the inorganic powders as the adhesion barrier. It is also preferable that the surfaces of the metal plate be coated with electroless nickel or titanium nitride, to name a few, to improve the surface characteristics of the lamination plates. 
     The invention also provides for a thicker ceramic green sheet that is punched and screened to have at least one adhesive barrier material on at least one side, and the thicker green sheet be used as a base or a permanent support for a thinner ceramic punched green sheet layer which has at least one adhesive barrier material on at least on one surface. 
     Additionally, the invention also provides for the thicker permanent ceramic base to act as a shrinkage and distortion restrainer when the thinner ceramic sheet is screened with conductive paste and dried. 
     Furthermore, the securing of the thinner green sheet on the thicker green sheet base has totally eliminated handling problems, such as, for example, in stacking. 
     This invention also eliminates is the use of a polymeric adhesion barrier in lamination by the introduction of the novel and inexpensive inorganic adhesion barrier. 
     FIG. 1, illustrates at least one thin green sheet  10 , such as, a thin ceramic green sheet  10 , with at least a via hole  12 . The thinness of a green sheet is a relative measure, and it means as thin as one could preform, to as thin as one could handle through via forming technique, like mechanical punching or laser hole formation or very intensive chemical technique such as photo-processing. The via hole  12 , is a punched but not a filled via hole  12 . 
     The term thin sheet or layer as used herein means that the thickness of the sheet can be anywhere from about 0.5 mil to about 6.0 mils. Furthermore, production level screening and stacking of these types of thin sheets is not possible with the current technology as the thin sheets tend to shrink a lot and they also tend to distort during the process. 
     FIG. 2, shows at least one thicker green sheet  20 , such as, a thicker ceramic green sheet  20 , with at least one punched via hole  22 , that has been metallized with at least one electrically conductive metallic material  24 . Punching of via holes  22 , in the ceramic green sheets  20 , and filling the via holes  22 , with at least one electrically conductive metallic or composite material  24 , is well known in the art. Typically, an electrically conductive paste  24 , is screened into the via hole  22 , and the green sheet  20 , is metallized with an appropriate pattern  26  and/or  28 . The patterns  26  and  28 , could be an electrically conductive line or cap, to name a few. 
     Thickness of the green sheet  20 , is again a relative measure, and it means as thick as the design warrants, to and as thick as one could cast and personalize. Because it is a thicker sheet, it is possible to punch and screen these layers with conventional technique without any detrimental pattern distortion and radial error. In general a radial error greater than about 1.2 mils is considered not good. 
     FIG. 3, illustrates a preferred embodiment of this invention where the thin green sheet  10 , of FIG. 1, is secured to the thicker green sheet  20 , of FIG.  2 . The punched thin green sheet  10 , can be secured to the punched and screened thicker ceramic green sheet  20 , by bonding or tacking using a lamination process with plates  31 , having at least one adhesion barrier  33 . It should be appreciated that the inorganic adhesion barrier dust  33 , to some extent is transferred onto the surfaces of the substructures. As stated earlier, that the via hole  12 , is a punched but not a filled via hole  12 , in the tacked or bonded thin ceramic green sheet  10 . The screened features  26  and  28 , in the thicker green sheet  20 , can be above the surface of the sheet  20 , as shown in FIG. 2, or partially or fully imbedded in the green sheet  20 , as shown in FIG.  3 . 
     It is preferred that the average particle size for the adhesion barrier material  33 , is less than about 5 microns, and preferably less than about 1 microm The average particle size as used herein means the longest dimension for the given particle, as these particles can be of any shape and/or form. And, wherein the average thickness for the adhesion barrier layer  33 , that is on the plate  31 , is less than about 20 mils, and preferably less than about 5 mils. 
     The adhesion barrier dust particles  33 , are preferably chosen of the same material as at least one of the green sheet ceramic materials, and therefore it is not necessary to remove it after the substructure formation or lamination, or prior to or after sintering. 
     The bonding and/or tacking of the thinner green sheet  10 , to the thicker green sheet  20 , can be achieved by a variety of processes, such as, for example, by a standard lamination process. It is very important that the bonding and/or tacking process used should not distort the features  26  and  28 , located on the thicker sheet  20 , and that the green sheets  10  and/or  20 , do not stick to the plates  31 . For the green sheets  10  and  20 , that were used, a lamination pressures of less than 800 psi, and a temperature of less than 90° C., was found suitable for the bonding and/or tacking operation. It is preferred that the adhesion barriers  33 , that is used should be suitable for providing a clean separation of the plates  31 , from the green sheets  10  and/or  20 . 
     After the bonding/tacking process a multi-media or multi-density sub-structure  30 , was obtained, which comprised of at least one thin ceramic layer  10 , and at least one thick ceramic layer  20 . The multi-density structure  30 , looks and behaves as a single green sheet layer  30 . The sub-structure  30 , has via hole  12 , which starts from one surface and does not go all the way through, i.e., the via hole  12 , which started as a through-hole  12 , but now is a blind via hole  12 . Furthermore, it is preferred that the metallized vias  24 , be appropriately aligned with the screened vias  22 , and non-screened via hole  12 , and thus will provide a top to bottom alignment. These unique features of this invention enable the handling of the thin ceramic sheet  10 , as a sub-structure  30 . Furthermore, the sub-structure  30 , has no other material set, other than the green sheets  10  and  20 , and the screened paste  24 , to form features  26  and  28 , which requires least processing cost and provides best yields. As stated earlier, that the inorganic adhesion barrier dust  33 , to some extent is transferred onto the surfaces of the substructures. 
     FIG. 4, illustrates the metallization of the screened sub-structure  30 , shown in FIG. 3, to form a sub-structure  40 . Here the sub-structure  30 , was screened with an electrically conductive metal or composite paste  44 , to form features  46  and  48 , on the thin ceramic sheet  10 , to form an intermediate or final structure  40 . Feature  46 , could be a via  46 , formed in the via hole  12 , while the feature  48 , could be a pattern, such as, a cap or line  48 . The structure  40 , of this invention shows features  46  and  48 , in the thin green sheet layer  10 , that makes electrical connection to via  24 , and patterns  26  and  28 , on the thick green sheet  20 . 
     FIG. 5, illustrates another embodiment of this invention where the structure  40 , as shown in FIG. 4, has been secured with another layer of a thin green sheet  10 , as shown in FIG. 1, to form a structure  50 . Basically, the screened structure  40 , that was obtained as described from FIGS. 1 through 4, was secured, such as, by tacking/bonding and by using the adhesion barrier material, to a punched thin ceramic layer  10 , as illustrated in FIG.  1 . 
     FIG. 6, illustrates the metallization of the structure  50 , as shown in FIG. 5, to form a structure  60 . The via hole  12 , is filled with at least one electrically conductive material  64 , such that, the material  64 , is in direct contact with the via material  44 , of the earlier thin ceramic layer  10 . And that metallization  66  and  68 , if needed, is in direct contact with the via  64 , of the new thin green sheet  10 . This multi-density structure  60 , can now be further processed as a ceramic material  60 . 
     Many sub-structures can be built with as many thin green sheets  10 , as necessary to build a final MLC laminate. As one can clearly see in FIG. 6, that the sub-structure  60 , has one thick green sheet  20 , and two thin green sheets  10 , and this structure  60 , has the rigidity for handling through screening and stacking. Furthermore, the dimensional stability of the screened features in thin green sheets  10 , would be far better when screened as a sub-structure compared to screened as a free standing thin green sheet  10 . 
     FIG. 7, illustrates another embodiment of this invention showing the structure of FIG. 6, being used to form a multi-layer multi-density ceramic package  70 . The package  70 , could be formed by combining, for example, two sub-structures  60 , resulting in the ceramic package  70 , which comprises of at least one thick ceramic layer  20 , and at least one thin ceramic green sheet layer  10 . The two sub-structures  60 , could be tacked/bonded and by using adhesion barrier material  33 , to each other and they could also include several thin ceramic green sheets  10 . 
     Each of the green sheets could have one or more electrically conductive features, such as, for example, cap, line, via, to name a few. These features could be made from at least one electrically conductive material. 
     The electrically conductive material used with this invention is preferably selected from a group comprising copper, molybdenum, nickel, tungsten, metal with glass frit, metal with glass grit, to name a few. However, the electrically conductive material used for the different layers and/or features could be the same material or it could be a different material. 
     The material for the green sheet  10  and/or  20 , is preferably selected from a group comprising alumina, alumina with glass frit, borosilicate glass, aluminum nitride, ceramic, glass ceramic, to name a few. 
     The tacking and/or bonding could be done in a chemical environment, and wherein the chemical is preferably selected from a group comprising water, methanol, methyl-iso-butyl ketone, isopropyl alcohol, alumina, aluminum nitride, borosilicate, glass ceramic, copper, molybdenum, tungsten, nickel, to name a few. 
     The adhesion barrier material  33 , is selected preferably from a group comprising fine particle, preferably submicron, magnesium silicate, alumino magnesium silicate ceramic, oxides of aluminum-magnesium-silicon, alumina, ceramics, to name a few. 
     Another advantage of this invention is the ability to punch, screen and stack very dense via and pattern in a package. As the via and pattern metal density increases in a green sheet (thick or thin) the feature radial error increases as well when one handles the green sheets as a free standing body. In such instances one could use the same or similar process as described and illustrated in FIGS. 1 through 7. Basically, the dense patterns are screened on the ceramic sub-structures rather than on the free standing ceramic green sheets. It has been found that the shrinkage and distortion is far smaller when sub-structures are screened than when the free standing green sheets are similarly processed. Furthermore, the sub-structures are built using the normal green sheet materials and the existing electrically conductive metal/composite pastes. 
     As stated earlier that the adhesion barrier  33 , is applied as a dust on the lamination plates  31 , basically to prevent the adhering of the green sheets  10  and/or  20 , to the lamination plates  31 . These particles  33 , are submicron in size and they get coated on the exposed surface of the substrate during lamination, and embedded between the layers upon subsequent lamination. Upon sintering of the substrate, these inorganic adhesion barrier particles  33 , become a part of the substrate micro-structure. Since the size of these dust particle  33 , is extremely small and the quantity of these dust particles  33 , is minimal, and the particle chemistry is preferably similar to one or more of the material of the green sheets  10  and/or  20 , the adhesion barrier material  33 , does not have an impact on any of the material sintering behavior or micro-structure. 
     It should also be appreciated that when the multi-density semiconductor substrate is separated from the surfaces of the laminating plate  31 , it is done without any physical or electrical or mechanical damage to any of the feature of the multi-density substrate. 
     EXAMPLES 
     The following examples are intended to further illustrate the invention and are not intended to limit the scope of the invention in any manner. 
     Example 1 
     Several samples of multi-layer ceramic sub-structures containing thin green sheets  10 , of thicknesses varying from about 0.8 mils to about 6.0 mils, were built on thick ceramic green sheets  20 , of various thicknesses ranging from about 6 mils to about 20 mils, using the method of this invention and which resulted in a structure  30 , as shown in FIG.  3 . The material for the ceramic green sheets  10  and  20 , included alumina and glass ceramic. While the electrically conductive material included molybdenum, copper and other well-known composites. The sub-structures  30 , were built at various pressures up to about 800 psi and with temperatures of up to about 90° C. and by using an adhesion barrier material  33 . In all cases the sub-structures were measured for radial error. The radial errors were found to be less than about 1.2 mils, which showed a good layer to layer contact and alignment. 
     Example 2 
     Several single thin ceramic green sheets  10 , with thicknesses ranging from about 0.8 mils to about 3.0 mils were punched and screened as a free standing sheet  10 . The material set for the green sheet  10 , included alumina and glass ceramic and the electrically conductive material, such as, the metal paste, included molybdenum, copper and other composites. In all cases the free standing screened thin ceramic layers  10 , were measured for radial errors. The measured radial errors in all cases was more than about 1.2 mils and ranged up to about 15.0 mils. It was also noticed that the freestanding screened thin layers  10 , were all wrinkled and non-usable. 
     Example 3 
     Several samples of multi-layer ceramic sub-structures containing thin ceramic green sheets  10 , of thicknesses varying from about 0.8 mils to about 6.0 mils were built with wiring density of about 3 mils on about 7 mil pitch using the method of this invention and the structures of FIG. 3, on thick green sheets  20 , of various thicknesses ranging from about 6 mils to about 8 mils. The materials for the ceramic green sheets  10  and  20 , included alumina and glass ceramic. The electrically conductive material included molybdenum, copper and composites. The sub-structures were built at various pressures up to about 800 psi and with temperatures up to about 90° C. and by using the adhesion barrier materials  33 . In all cases the sub-structures were measured for radial error. It was found that the radial errors were less than about 1.2 mils, which meant a good layer to layer contact and alignment. Also the substructures separated from the plates  31 , with no damage to ceramic body or the features. 
     Example 4 
     Several thin single green sheets  10 , with thicknesses ranging from about 0.8 mils to about 6 mils, and wiring density of about 3 mil features on about 7 mil pitch were punched and screened as a free standing thin ceramic sheet. The material for the green sheet included alumina and glass ceramic and the material for the electrically conductive metal paste included molybdenum, copper and composites. In all cases the layers were measured for radial error. The measured radial errors in all cases were more than about 1.2 mils and ranged up to about 25 mils. 
     In most cases, in the above-mentioned examples, ceramic laminates that were made using the adhesion barriers, the inorganic powders had sintered and becomes a part of the ceramic substrate, and upon further analysis of the sintered substrate using ESCA (Electron Spectroscopy for Chemical Analysis), electrical test, chip join and mechanical test, was found to be good. 
     While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.