Abstract:
The present invention relates generally to a new ceramic structure and process thereof. 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-layer ceramic products using very thin green sheets and/or green sheets with very dense electrically conductive patterns on top of a stronger support sheet. 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. The thin green sheets were tacked/bonded to thicker and stronger support sheets to form a sub-structure which had excellent stability in screening and enabled further processing. The sheets are anchored or pinned in such a way as to allow the processing of the green sheet with the subsequent easy removal of the support sheet.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 09/587,580 filed May 25, 2000, now U.S. Pat. No. 6,607,780. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a new ceramic structure and process thereof. 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-layer ceramic products using very thin green sheets and/or green sheets with very dense electrically conductive patterns on top of a stronger support 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, conventional 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 in the prior art. 
     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. 
     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. Production level screening and stacking 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. 
     U.S. Pat. No. 5,176,772 (Ohtaki) and U.S. Pat. No. 5,300,163 (Ohtaki), addresses a method of forming a thin sintered ceramic board by laminating punched and screened green tapes on a presintered ceramic body and sintering the assembly to obtain a flat board. This method essentially enables one to obtain a flat laminate. But, with ceramic substrates with materials like alumina and aluminum nitride there is typically a 14 to 20 percent X-Y shrinkage. So there will be delamination and distortion involved in sintering due to presintered base. Furthermore, one needs to handle these sheets through screening in free standing state. 
     U.S. Pat. No. 5,368,667 (Minh), teaches preparing a multi-layer capacitor which is one to two mils thick by extruding a thick dielectric layer and a metal containing layer through a roll laminator. This approach is good for using a thin layer having the blanket metallurgy. But in almost all multi-layer ceramic packages, one needs to have vias for electrical connectivity. However, this patent does not address the problem of handling the thin sheets in via filling and stacking. 
     U.S. Pat. No. 5,480,503 (Casey), teaches releasably-supporting the thin green sheets on a temporary carrier support having an ablatable release layer over a patterned conductive layer, and filling the vias with conductive metal paste, whereby the thin green sheets are supported against warpage and distortion. The supported green sheets are formed as single layers, pairs and stacks thereof, and separated from temporary support for use. The suggested temporary support is a glass plate. The metallization technique is CVD type plating and in the process has to use non-ablatable and ablatable films on the green sheet. These films inherently will distort the green sheet during temperature and pressure processing. Furthermore, the non-ablatable film stays with every single green sheet layer and will create delamination and density difference in sintering. 
     U.S. Pat. No. 5,976,286 (Natarajan), assigned to International Business Machines Corporation, Armonk, N.Y., USA, the disclosure of which is incorporated herein by reference, teaches the use of at least one thin green sheet and a thick green sheet for fabricating a multi-density sub-structure. 
     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 green sheets were tacked and bonded to a support sheet, such as, a thin metal or a like carrier, to form a sub-structure which yields excellent stability in screening and enables excellent handling and alignment in stacking. The green sheet may have electrically conductive features within them, such as, a via, or over them, such as, a line, cap, to name a few. 
     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 as capacitor layers or with fine line patterned conductive metal layers 
     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 to produce sub-structures. 
     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 for fabricating a ceramic substrate comprising the steps of: 
     (a) applying at least one thin photosensitive polymer layer over a support sheet, 
     (b) forming at least one opening in said thin photosensitive polymer layer and said support sheet, 
     (c) filling said at least one opening with at least one electrically non-conductive paste, 
     (d) placing at least one green sheet over said at least one photosensitive polymer layer and said paste, 
     (e) opening at least one through-hole through said green sheet, such that at least a portion of said electrically non-conductive paste remains adhered to at least a portion of the side-wall, 
     (f) filling said opening with at least one electrically conductive paste, and thereby fabricating said ceramic substrate. 
     In another aspect this invention comprises a ceramic substrate comprising at least one green sheet with at least one via hole having at least one electrically conductive material in intimate contact with at least one carrier support sheet, and wherein said carrier support sheet has at least one via opening which is filled with an electrically conductive material and an electrically non-conductive material and wherein at least a portion of said at least one electrically conductive material is surrounded by at least a portion of at least one electrically non-conductive material, and thereby forming said ceramic substrate. 
     In yet another aspect this invention comprises a ceramic substrate comprising at least one green sheet with at least one via hole having at least one electrically conductive material in intimate contact with at least one metal carrier support sheet, and wherein said metal carrier support sheet has at least one via opening which is filled with an electrically conductive material and an electrically non-conductive material and wherein at least a portion of said at least one electrically conductive material is surrounded by at least a portion of at least one electrically non-conductive material, and thereby forming said ceramic 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 photosensitive polymer layer over a thin support sheet. 
     FIG. 2, illustrates openings in the structure of FIG.  1 . 
     FIG. 3, illustrates an optional step of adhering a layer in the open areas. 
     FIG. 4, illustrates the insertion of paste in the openings. 
     FIG. 5, illustrates the adhering of a green sheet over the structure shown in FIG.  4 . 
     FIG. 6, illustrates the formation of openings in the structure shown in FIG.  5 . 
     FIG. 7, illustrates the filling of the openings in the structure of FIG.  6 . 
     FIG. 8, illustrates the joining of two structures of FIG.  7 . 
     FIG. 9, illustrates the joined structures of FIG. 8, after the removal of the thin support sheet. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The structure and method of the present invention enables the handling, screening and stacking 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. Basically, the invention is a novel structure and method where a support sheet, such as, a thick or a thin metal sheet, is used as a base or a support for a thinner ceramic green sheet layer. Additionally, the support sheet or the base acts as a shrinkage and distortion restrainer when the thinner ceramic sheet is screened with conductive paste and dried. Furthermore, the thinner green sheet on the thin support base totally eliminates handling problems, such as, for example, in stacking. 
     FIG. 1, illustrates at least one thin photosensitive polymer layer  12 , over a support sheet  10 . The photosensitive polymer layer  12 , can be on one side or both sides of the support sheet  10 . The support sheet  10 , can be a thin metal sheet or a polymer/plastic sheet  10 . It is preferred that the metal sheet  10 , be selected from a group comprising stainless steel, nickel, aluminum, molybdenum, permalloy (alloy of Ni and Mo), to name a few. It is preferred that the support sheet  10 , be selected from a group comprising polymer, plastic, polyester, PEN (Polyethylene Naphthalate), to name a few. However, for the support sheet  10 , one could even use paper, such as, a coated or uncoated paper, for example, wax paper, silicone or acrylic coated paper, to name a few. 
     For the preferred embodiment one would use a cold rolled thin metal sheet  10 , as shown in FIG. 1, and form openings  22 , through the photosensitive polymer  12 , as illustrated in FIG.  2 . 
     Using conventional photo-processing techniques one would then form at least one through-hole  22 , in the thin metal sheet  10 , as shown in FIG.  2 . The holes or openings  22 , are on a grid, i.e., essentially the thin metal sheet  10 , will have holes  22 , for every via that is going to be formed in every layer of the MLC structure of this invention. 
     FIG. 3, also illustrates an optional step of adhering a layer  30 , in the open areas of the thin metallic sheet  10 . The metal sheet  10 , can be treated with the layer  30 , so that structure could have a firmer or better bond with a green sheet. The treatment can be a chemical or a mechanical process. A chemical process could include oxidation of the surface, whereas a mechanical process could include spraying, coating, etc. of an adhesive onto the surface. It should be noted that the treatment could be on one or more selected surfaces, and does not have to be on all the exposed surfaces. Through-hole  32 , in FIG. 3, is the same as hole  22 , except that it has the optional treatment  30 . 
     FIG. 4, illustrates the insertion of at least one paste  40 , in the opening  22 , to form a personalized carrier  45 . The bonding paste  40 , could be screened or extruded into the hole  22 , in the thin metal sheet  10 . The bonding paste  40 , can be a fugitive paste (i.e. that it contains only organic materials) or a paste  40 , that contains dielectric inorganic or conductive polymers. The basic purpose of the paste  40 , includes bonding of the thin metal sheet  10 , to a green sheet  50 , in areas where vias in the green sheet  50 , may be formed, as more clearly illustrated in FIG.  5 . 
     As shown in FIG. 5, the sub-lamination of the green sheet  50 , to the personalized carrier  45 , minimizes the radial movement of a via in green sheets  50 , when the green sheets  50 , are subsequently personalized. This is due to the fact that the bonding paste  40 , anchors or pins the green sheet  50 , to the support sheet  10 , and this anchoring or pinning substantially reduces and/or eliminates any radial movement between the green sheet  50 , and the support sheet  10 . The green sheet  50 , can be of any thickness, however, it is preferred that the green sheet  50 , be less than 6 mils, as that would define the green sheet  50 , to be a thin sheet. 
     The term thin green sheet as used herein 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 bonding and/or tacking of the thinner green sheet  50 , to the carrier  45 , can be achieved by a variety of processes, such as, for example, a lamination or sub-lamination process. 
     The sub-lamination process is a very low pressure lamination, typically less than about 1000 psi, at temperatures of less than about 90° C., for a time period of less than about 5 minutes. The sub-lamination process besides bonding the carrier  45 , to a green sheet  50 , also provides a very flat and smooth surface which is very helpful for the subsequent operations. 
     FIG. 6, illustrates the formation of via openings  62 , in the structure shown in FIG.  5 . The hole  22  , in the metal support sheet  10 , may be slightly larger in diameter than the via holes  62 , in the green sheet  50 . Since there are several thousand vias  62 , in a given MLC, the bonding or location pinning of vias  62 , are very effective for stability control. 
     The via openings  62 , could be formed by methods well known in the art, such as, punching (mechanical, laser, etc.) or maybe by forming via openings in the green sheet  50 , if the green sheet  50 , is thick enough to handle via openings without having any degrading effect. Even though the metal support sheet  10 , has vias on all grids, the required vias  62 , in the green sheet  50 , are opened using the corresponding filled via  22  as a guide. The via openings  62 , in the green sheet  50 , are smaller compared to the via  22 , which now has been filled with bonding paste  60 , in the side-walls in the thin metal support sheet  10 . Thus the via positional integrity and the bonding of the green sheet  50 , to the metal carrier  10 , are maintained. 
     FIG. 7, illustrates the filling of the via openings  62 , in the structure of FIG. 6, using at least one metal paste  70 , to form an inventive structure  75 , of this invention. The metal paste  70 , would be used to fill at least one via opening  62 . It is preferred that the via opening  62 , is filled from the side of the green sheet  50 , and not from the side of the metal  10 . Typically, an electrically conductive paste  70 , would be screened into the via hole  62 , and the green sheet  50  would be metallized with an appropriate pattern  73  and  74 . The patterns  73  and  74 , could be an electrically conductive line  73 , or a cap  74 , thus personalizing the green sheet  50 . 
     FIG. 8, illustrates the joining of two structures  75 , of FIG. 7, to form a multi-layer ceramic (MLC) structure  85 . However, several layers of structure  75 , can be bonded thus to form the MLC structure  85 . 
     It is very important that the bonding and/or tacking process used to form the MLC structure  85 , should not distort the features  73  and  74 , located on the green sheets  50 . A lamination pressure of less than 800 psi and a temperature of less than 90° C. was found suitable for the bonding and/or tacking operation. 
     After the bonding/tacking process a multi-media or multi-density sub-structure  85 , was obtained, which comprises of at least two ceramic layers  50 , with at least two carriers  45 . The structure  75 , looks and behaves as a single green sheet layer  50 . The sub-structure  75 , has metallized vias formed by filling via opening  62 . Furthermore, the metallized vias are very well aligned with patterns  73 , and  74 , and thus enable top to bottom alignment when stacked. These unique features of this invention enable the handling of the thin ceramic sheet  10 , as a sub-structure  75 . 
     Many sub-structures  75 , can be built with as many thin green sheets  50 , as necessary to build a final MLC laminate  85  or  95 . As one can clearly see in FIG. 7, that the sub-structure  75 , has one thin green sheet  50 , and this structure  75 , has the rigidity for handling through screening and stacking. Furthermore, the dimensional stability of the screened features in thin sheets  50 , would be far better when screened as a sub-structure compared to screened as a free standing thin sheet  50 . 
     FIG. 9, illustrates the joined structures of FIG. 8, after the removal of the metallic supports. Basically, the MLC structure  85 , is stacked and the carrier sheet  45 , is separated. For most applications the carrier sheet  45 , can be removed by a simple X-Y twist and/or peeling. This results in a MLC substrate  95 , having a green sheet  90 , with embedded vias  92 , and pattern  73 , and pattern  74 . 
     As shown in FIG. 9, the multi-layer multi-density ceramic package  95 , could be formed by combining, for example, two sub-structures  75 , resulting in the ceramic package  85 , comprising of at least one thick ceramic layer  50 , and at least one thin or thick ceramic layer  50 . The two sub-structures  75 , could then be tacked/bonded to each other and processed to form the ceramic package  95 . 
     The green sheet 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. 
     The material for the ceramic green sheet is preferably selected from a group comprising alumina, alumina with glass frit, borosilicate glass, aluminum nitride, 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. 
     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. 
     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.