Abstract:
This invention relates generally to a new method of forming semiconductor substrates with defect-free surface metallurgical features. In particular, the invention related to a method for providing surface protected ceramic green sheet laminates using at least one thermally depolymerizable surface layer. More particularly, the invention encompasses a method for fabricating semiconductor substrates wherein a thermally depolymerizable/decomposable surface film is placed over a ceramic green sheet stack or assembly prior to lamination and caused to conform to the surface topography of the green sheet during lamination. The invention also encompasses a method for fabricating surface protected green sheet laminates which can be sized or diced without causing process related defects on the ceramic surface. After lamination the thermally depolymerizable/decomposable film is conveniently and cleanly removed due to thermal depolymerization and burn-off of volatile species during the sintering process, thus providing surface defect-free ceramic substrates.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method and a structure for forming semiconductor substrates, and particularly relates to improvement in ceramic manufacturing process and structure for the production of ceramic packages using at least one thermally depolymerizable surface layer. More particularly, this invention relates to ceramic green laminate processing and structure using at least one protective overlay that adheres to at least a portion of the green ceramic surface to provide reduction/elimination of defects caused during processing, such as, handling and sizing/dicing operations. 
     BACKGROUND OF THE INVENTION 
     Semiconductor substrates and devices are becoming smaller and more dense with the evolution of new technologies. However, increase in circuit density produce a corresponding increase in overall manufacturing problems. These manufacturing problems must however be kept to a minimum in order for the semiconductor manufacturer to remain competitive. The semiconductor manufacturers are therefore constantly being challenged to improve the quality of their products by identifying and eliminating defects which produce defective parts or components. Whereas significant improvements are being made to eliminate systematic defects by reducing process variability, process improvements alone are not sufficient to eliminate all the random defects which effect both yield and reliability. Historically, screening techniques have been employed to improve product failure rates to acceptable levels by culling out many of these random defects. 
     In their desire to improve their products, the semiconductor manufacturers are constantly finding new ways and new techniques to improve or provide new products. Semiconductor substrates or ceramic chip carriers generally are made with a planar surface having a flat chip site for mounting the semiconductor device. The substrates are often referred to as modules which can be made from a single ceramic layer or green sheet forming a single layer ceramic module or a plurality of ceramic layers could be used to form a multi-layer ceramic (MLC) module. 
     While the remaining discussion will be directed to MLC modules, it should be understood that the teachings of the present invention can be equally applicable to single layer modules. 
     MLC modules are normally used in the electronic industry to package high performance integrated circuits or chips (hereafter just chips). These high performance chips have a large number of external inputs/outputs (I/Os), such as pads or solder balls, to name a few, and these chips have a very high power dissipation requirement. In order to accommodate such high performance chips, the MLC module also has to provide a high number of external I/Os, such as pads, pins, solder balls, to name a few, and also be able to handle the very high power dissipation both from the module as well as the chip. 
     Standard process for ceramic substrate production involves green sheet casting of ceramic slurry comprising ceramic filler, organic binder, solvent vehicle and plasticizer dispersion, blanking, via punch, circuit personalization or screening of conductive paste through a screen mask, singulation/sizing, inspect/collate/register, stack and lamination, binder burn-off and sintering operations. In the handling of green laminates especially sizing and dicing operations, flying ceramic debris generally falls on the surface layer which gets embedded in the ceramic during subsequent processing, such as, the sintering process. Such defects in the ceramic substrates are undesirable and thus significant number of sintered substrates have to be reworked or discarded causing additional expenses and/or product yield losses. 
     The present invention, however, solves this problem by the use of a thermally depolymerizable/decomposable surface film or layer that protects the green ceramic surface during lamination, and also serves as a sacrificial protective layer against any flying ceramic debris during handling and sizing/dicing operations. However, the protective layer is preferably made from a material that decomposes and burns off during the sintering cycle. 
     PURPOSES AND SUMMARY OF THE INVENTION 
     The present invention is a novel method and structure for providing at least one sacrificial protective polymer adhesive layer over a green sheet laminate in order to minimize and/or eliminate processing defects. The surface protective film preferably comprises of at least one thermally depolymerizable polymer which cleanly burns off during the sintering cycle. 
     Therefore, it is a purpose of the present invention to provide an improved method for semiconductor substrate production without causing damage or defects to the ceramic body or causing paste pull-outs. 
     Another purpose of the present invention is to provide a thermally decomposable surface film, which film avoids any damage to the ceramic body and/or causes any paste pull-outs. 
     Yet another purpose of this invention is to provide a novel thermally depolymerizable/decomposable adherent surface film as a protective layer in the ceramic green laminates production to reduce and/or eliminate processing defects. 
     Still yet another purpose of the invention is to provide a novel thermally depolymerizable/decomposable adherent surface film as a protective layer in the ceramic green laminates production to reduce and/or eliminate processing defects caused during lamination, handling, and/or sizing/dicing operations. 
     Therefore, in one aspect this invention comprises a method of protecting an unsintered structure having at least one defect-free surface feature, comprising: 
     (a) placing at least a portion of at least one thermally depolymerizable/decomposable layer over said surface feature on said unsintered structure, wherein said thermally depolymerizable/decomposable layer has the characteristics of being thermally bondable to at least a portion of said unsintered structure and to at least a portion of said surface feature on said unsintered structure during lamination and being thermally decomposable without leaving any carbonaceous residue during subsequent sintering; 
     (b) applying pressure onto at least a portion of said thermally depolymerizable/decomposable layer, such that at least a portion of said thermally depolymerizable/decomposable layer conforms to at least a portion of the surface topography of said surface feature and said unsintered structure, and wherein at least a portion of said thermally depolymerizable/decomposable layer further adheres to at least a portion of the surface of said unsintered structure, and thereby protects said unsintered structure. 
     In another aspect this invention comprises an unsintered structure comprising at least one semiconductor substrate having at least one metallized feature and at least a portion of at least one thermally depolymerizable/decomposable layer conformably secured to a portion of said metallized feature and said at least one semiconductor 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 the problem of debris being fused on to an unsintered green ceramic laminate during processing. 
     FIG. 2, illustrates a plurality of unsintered ceramic green sheet with multiple substrate pattern punched and screened. 
     FIG. 3, illustrates a cross-sectional view taken along  3 — 3 , of FIG. 2, showing the stack of unsintered ceramic green sheets, which have been metallized, stacked or assembled, aligned and laminated. 
     FIG. 4, illustrates the inventive laminate protection layer readied for positioning on to the unprotected ceramic green sheet stack. 
     FIG. 5, illustrates the embodiment of this invention where at least one layer of a thin laminate protective layer is laminated on to the green ceramic laminate. 
     FIG. 6, illustrates the inventive protective layer after lamination being adhered to and protecting the surface of the laminate and eliminating the debris fusion problem on green laminate during operation but prior to sintering. 
     FIG. 7, illustrates the sintered ceramic substrate free of fused ceramic defects due to the inventive material and process of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, a sintered or unsintered substrate  15 , typically a ceramic substrate  15 , comprises one or more individual ceramic sheets or layers  10 , having a top surface  11 , and a bottom surface  13 . The sheets  10 , typically have a frame-like area  12 , which in most cases is made from the same material as the material for the sheets  10 . The area  12 , in most cases is an extension of the sheets  10 , and the area  12 , may be used for non-functional activity or features. It has been discovered that when the sheets  10 , are processed, particles or debris  14 ,  16  and/or  18 , gets lodged onto the surface of the substrate  15 . As shown in FIG. 1, the debris  14 , is lodged inside the ceramic sheets  10 , the particle  16 , is both below and above the surface  11 , while particle  18 , is on the surface  11 , of the substrate  15 . Because some particles may be loosely lodged into the surface  11 , of the substrate  15 , it might be easier to dislodge and blow-away the particles, such as, particle  18 , by means well known in the art. However, other particles, such as, particles  14  and  16 , may be a little more difficult to dislodge or remove from the surface  11 , as they could be fully or partially embedded into the surface of the ceramic layer  10 . One way to remove the particles  14 ,  16  and/or  18 , could be through any of the known polishing techniques, however, that would add additional processing steps and may affect the thickness of the layer  10  or substrate  15 . 
     The debris  14 ,  16  and/or  18 , could have been lodged on top of the surface  11  and  13 , for a variety of reasons. For example, it is a common practice in MLC industry to green size laminates, such as, by using a diamond saw. During such sizing operation, generally ceramic debris is generated which debris settles and adheres to the top of the surfaces  11  and  13 , of the substrate  15 . The green sizing process is such that the debris at the moment of production is hot due to high speed cutting of the green ceramic body  15 . The temperature of such debris  14 ,  16  and  18 , at the moment of being ejected is far greater than the glass transition temperature of the polymer binder in the green sheet stack  10 . Therefore, the debris  14 ,  16  and/or  18 , upon settling on the substrate  15 , would tend to fuse with the material on the surfaces  11  and/or  13 , of the substrate  15 . In such instances the debris  14 ,  16  and/or  18 , gets sintered and results in as fused ceramic particles  14 ,  16  and/or  18 , on the surface  11  and/or  13 , as well as on top of any metallurgy that may be present. This fusion of the particles  14 ,  16  and/or  18 , causes serious problem in post-sinter operations, such as, plating, surface mounting, etc. These substrates  15 , with fused material  14 ,  16  and  18 , would of course be rejected as they would directly impact the product yields and production cost. 
     Now referring to FIGS. 2 and 3, where FIG. 2, illustrates an unsintered substrate  25 , that has one or more ceramic green sheets  20 . As illustrated at least one green sheet  20 , such as, thin or thick ceramic green sheet  20 , could be further divided into one or more green sheets  24 , having a frame or non-functional area  22 , and at least one punched and/or metallized area  26 , comprising electrically conductive material  26 . The substrate  25 , may also contain non-functional and/or non-metallized area  22 , in-between the metallized area  26 . Each of the metallized area  26 , may be similar or different in design. The green ceramic substrate  25 , may also contain at least one feature  28 ,  29 , such as, for example, a location hole  28 , an alignment marking  29 , to name a few. The patterns that are formed in the metallized area  26 , are well known to a person skilled in the art. 
     FIG. 3, illustrates a cross-sectional view taken along section  3 — 3  of FIG. 2, showing a stack of at least one unsintered ceramic green sheet  20 , which have been metallized, aligned, stacked or assembled, and then laminated into a unsintered substrate  25 . Basically, the stack of green sheets  20 , comprises of at least one green sheet  24 , with exposed top surfaces  21  and  23 , and with a non-metallized area  22 . After the stacking of the green sheets  20 , they can be laminated using conventional lamination techniques, such as, uniaxial lamination. 
     FIG. 4, illustrates the first step of the inventive process using the inventive material to eliminate the problem associated with the fusing of the debris  14 ,  16  and/or  18 . The unsintered green sheet  25 , comprising of at least one green sheet  25 , with exposed surfaces  21  and  23 , with metallized area  26 , and with or without a non-metallized area  22 , in-between. The green sheet stack  25 , is then protected by at least one top thermally decomposable polymer protective layer  41 , and at least one bottom thermally decomposable polymer protective layer  43 . It is preferred that both the top protective layer  41 , and the bottom protective layer  43 , cover at least a portion of the metallized area  26 , of the ceramic green sheet  20 , in order to protect the same from any debris in subsequent processing. It should be noted that in most cases the debris on the frame or non-metallized area  22 , is not fatal or detrimental to the subsequent processing of the substrate  25 , however, it is desired that all the surfaces be clear of any debris. 
     FIG. 5, illustrates the laminated structure  50 , of stacked green sheets  20 , with the top and bottom protective layers  41  and  43 . It is preferred that both the top protective layer  41 , and the bottom protective layer  43 , adhere to the surface  21  and  23 , respectively, of the substrate  25 , so that the layer  41 and/or layer  43 , does not delaminate from the metallized area  26 , of the ceramic green sheet  20 , and continue to protect the same from any debris in subsequent processing. As one can clearly see that the top surface of the protective layer  41 , and bottom surface of the protective layer  43 , are now exposed in post-lamination operation. 
     FIG. 6, illustrates the green sized substrates  65 , that were obtained from the laminate  50 . During green sizing of the laminate  50 , the debris formed and debris  64  and  66 , are debris which have settled or lodged onto the surface of the protective layer  41  and/or  43 . The debris  64  and/or  66 , can now be easily removed from the top surface of the decomposable protective layer  41  and/or  43 , by any mechanical means such as, brushing, scraping, machining, etc. However, any debris  64  and/or  66 , that cannot be easily removed by the mechanical means can be left behind and any such debris  64  and/or  66 , will either get sintered or get dislodged during the subsequent sintering cycle. 
     FIG. 7, illustrates the sintered substrates  75 , that is obtained after the protective layer  41  and/or  43 , have been thermally decomposed during the sintering cycle in the furnace and/or densifying of the ceramic substrate  75 . The non-metallized area or frame  22 , would sinter into area  72 , which area could be removed or left in place during subsequent operations. As one can fully appreciate that the metallized area  26 , was fully protected during the various processes. 
     The material for the green sheet  10  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. Preparation of ceramic slurry and green sheet casting is carried out by methods well known in the art. 
     The electrically conductive material  26 , that is used for screening the metallurgical patterns  26 , is preferably selected from a group comprising copper, molybdenum, nickel, tungsten, metal with glass frit, to name a few. However, the electrically conductive material  26 , used for the different layers and/or features could be the same material or it could be a different material. Electrically conductive screening material or paste  26 , comprising conductive filler, polymer binder, and solvent carrier could be used and prepared according to methods well known in the art. 
     The assembled green sheet stack  50 , with top and bottom thermally decomposable protective layers  41  and/or  43 , should be subjected to the lamination process according to techniques well known in the art, such as, using isostatic pressure, or hydrodynamic or hydraulic pressure. In the case of isostatic pressure lamination, the assembly shown in FIG. 5, may be transferred to an enclosure, and isostatic pressure may be applied by placing the enclosure in a fluid, such as, water or gas (e.g. air or nitrogen). However, care should be taken that the fluid employed does not interact or degrade the enclosure material and/or its contents. Such methods and apparatus are well known in the art, see for example, U.S. Pat. No. 5,746,874 (Natarajan, et al.) and U.S. Pat. No. 5,785,800 (Natarajan, et al.), assigned to International Business Machines Corporations, Armonk, N.Y., and the disclosure of which is incorporated herein by reference. In some lamination processes, the assembly or apparatus could also be heated to say about 75° C., to help facilitate the lamination process. 
     After the lamination and sizing operations, the laminated structure  65 , with protective top and bottom layer  41  and/or  43 , would go through a sintering cycle. The thermally decomposable protective layer  41  and/or  43 , will be depolymerized and burnt-off early in the sintering cycle, i.e., before the on-set of densification. 
     The choice of material for the thermally decomposable polymer layer  41  and/or  43 , is very important to the present invention. It is preferred that the protective layer  41  and/or  43 , be flexible, soft and moldable, and have thermoplastic characteristics with glass transition temperature preferably below the lamination temperature. It is also preferred that the protective layer  41  and/or  43 , have a high percentage of Eb (elongation-at-break) and be adherent to the green ceramic surface under the lamination conditions of pressure and temperature. Furthermore, the decomposable layer  41  and/or  43 , should be completely depolymerizable thermally below about 1100° C. and preferably below about 550° C. to form low molecular weight species which burn-off without leaving any carbonaceous residue or other detrimental by-products. This decomposable layer  41  and/or  43 , should also be tough and tear-resistant during lamination, i.e., it distributes the lamination stress and allows for uniform pressure distribution and not suffer any deformation in the process. 
     Suitable polymer materials for the surface film  41  and/or  43 , as protective and conformable overlay according to this invention, are acrylic polymer derived thermoplastic films comprised of homopolymers, co-polymers, terpolymers, or physical mixtures of polymers derived from methyl acrylate, ethyl acrylate, ethyl methacrylate, butyl methacrylate, acrylic acid, methacrylic acid, isobutylmethacrylate, fluorobutylmethacrylate, and the like. Preferred homopolymers, co-polymers, terpolymers, or physical mixtures thereof according to this inventions have Tg less than about 70° C., with tensile modulus in the range of between about 300 and about 800 N/mm 2 . The protective layers  41  and/or  43 , should have a high tensile strength such that there is no problem of tearing or distortion at the lamination temperature and pressure conditions, including lamination at higher pressures above about 2500 psi and any lamination temperatures from between about 45° C. to about 90° C. It is preferred that the protective layers  41  and/or  43 , have a thermal coefficient of expansion (TCE) between about 100 and about 200 ppm/° C. It is also preferred that the protective layers  41  and/or  43 , depolymerize and burn-off cleanly during the sintering process and leaves no carbonaceous residue in the sintered ceramic. The acrylic polymer derived films as protective and conformable overlay according to this invention are partially cured (B-stage) or fully cured (C-stage) flexible and thermoplastics films with excellent adhesive bonding characteristics to green sheet under the lamination conditions, and having non-stick surface for flying debris during sizing/dicing operations. 
     Furthermore, the thickness of the protective layers  41  and/or  43 , is also important. It is preferred that the maximum thickness for the thermally decomposable material  41  and/or  43 , should be less than about 10 mil, to have high yield in lamination process and high throughput in the sinter furnace. 
     It should be appreciated that the preferred material(s) for the thermally depolymerizable polymer layer  41  and/or  43 , should be acrylate polymers comprising homopolymers, co-polymers, terpolymers, or physical mixtures thereof, derived from the monomers or mixtures thereof, of the type methyl acrylate, ethyl acrylate, ethyl methacrylate, butyl methacrylate, acrylic acid, methacrylic acid, isobutylmethacylate, fluorobutylmethacrylate, and the like. Among these, preferred homopolymers, co-polymers, terpolymers, or physical mixtures thereof useful for the purpose of this invention can be formed from any set or sets of monomers so long as the thermoplastic films derived therefrom have Tg less than about 70° C., preferably in the range of between about 25 to about 65° C., and tensile modulus of between about 300 to about 700 N/mm 2 , preferably in the range of between about 350 and about 600 N/mm 2 . 
     The thermoplastic acrylic polymer films according to this invention have the unique characteristics of 
     (a) adhesive bonding to ceramic laminates during high pressure lamination process which bonding is maintained during sizing and dicing operations providing excellent protection for ceramic and thus minimizing yield loss which otherwise occurs due to defects caused by flying debris landing on the surface of ceramic; 
     (b) undergo facile depolymerization at high temperature and cleanly burns-off during sintering process conditions leaving no carbonaceous residue on sintered ceramic substrate surface; 
     (c) acrylic polymer overlay causes no perturbation of the paste screened features in the contacting green sheet with no paste spreading or paste-pull problem; 
     (d) acrylic polymer films are pre-processed to B-stage or C-stage curing which require no further solvent processing for applications according to this invention; 
     (e) provide perfectly conformable thermoplastic mask for ceramic green sheets under the lamination pressure and temperature conditions in the production of MLC substrates. 
     Representative homopolymers, co-polymers, and terpolymers derived from the acrylic monomers according to this invention include: poly(ethyl methacrylate-co-methyl acrylate-co-butyl methacrylate); poly(methyl acrylate-co-methacrylicacid-co-ethyl methacrylate); poly(butyl-methacrylate); poly(ethylmethacrylate-methacrylic acid-co-methacrylate); poly(ethylmethacrylate); poly(ethyl acrylate-co-ethylmethacylate-co-methacrylic acid), and the like. 
     It has been found that the acrylic polymer based adhesive films, typically the adhesive films of the type QL adhesives series and the Pyralux products LF0100, LF0200 and related series of preformed polymer films which are commercially available from DuPont, are among the representative protective and conformable overlay films for the purpose of this invention. 
     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 the multilayer ceramic structures having defect-free surface metallurgical features were fabricated using the process of this invention. 
     In one sample a stack containing metallized ceramic layers  24 , was placed in a lamination frame (not shown). A polyacrylate based thermoplastic adhesive film QL3500 having thickness of about 1-2 mil, was then placed over the top and bottom surface of the stack. The protective adhesive thermoplastic film  41  and/or  43 , which had a thickness of about 1-2 mil, was obtained from DuPont and was found to have an elongation of about 200 percent at room temperature. 
     This assembly was then placed inside a 3 mil thick polyurethane bag (durometer 30 shore A), evacuated and heat sealed. The polyurethane bag, was obtained from Stevens Urethane Film &amp; Sheet, Northampton, Mass., USA. The bag, with its contents were then laminated under pressure to about 5000 psi and at a temperature to about 90° C. in an isostatic press. 
     After the lamination, the substrate was removed from the bag, and inspected. The protective adhesive film  41  and  43 , formed a well bonded surface protective layer on the unfired substrate  50 . Significantly, due to the elongation/conformability and the nature of the base material surface, there was no paste-pull/separation under these severe lamination conditions. Furthermore, subsequent sizing operation left no debris bonded onto the protective adhesive film  41  and  43 . 
     The green laminate  50 , was then sintered in a sintering furnace by methods well known in the art. During the pyrolysis and binder removal segments of the sinter cycle the acrylic polymer protective layer  41  and/or  43 , decomposed completely without leaving any residue in the ceramic body and there was no surface damage to the ceramic substrate  75 . Surface analysis by x-ray-photoelectron spectroscopy (XPS) showed absence of any carbon residue attributable to the polymer overlay. Also, Leco carbon analysis was done to test for the carbon residue which showed no evidence of any extraneous residue. 
     Example 2 
     In another set of samples the assembly of ceramic layers stack  25 , was laminated in a manner described in Example 1, but no thermally depolymerizable surface layer was used. These samples had embedded surface debris and bonded green sizing debris and showed severe paste and ceramic pull by the bag material and also by the lamination plate. This demonstrated the need for the sacrificial protective interface layer  41  and/or  43 , and the process of this invention. 
     Example 3 
     In another series of samples, different type of polyacrylate based thermally decomposable surface films, LF0100 and LF0200 films of 1-2 mil thickness, commercially available from DuPont as a family of Pyralux products were used. Basically, the assembly of metallized ceramic layers were stacked and placed in the lamination frame, as described in Example 1. 
     Other polyacrylates derived thermally depolymerizable/ decomposable films tried were made of homopolymers, co-polymers, terpolymers, or physical mixtures thereof, of derived from acrylic monomers. The desired characteristics of these decomposable thermoplastic films include a thickness in the range of about 1 mil to about 5 mils and elongation varying between 150 percent to about 550 percent, with elastic modulus being about 0.3 GPa. 
     The rest of the assembly and the procedure followed was similar to the one explained in Example 1. Again, there was no evidence of fused debris on the metallurgical features and no paste pull-outs was seen, thus it showed that the adhered surface film, protected the screened features and the ceramic. Also, all of the thermally depolymerizable/decomposable layers or films left no residue in subsequent sintering operation. 
     Example 4 
     In this example, an assembly of ceramic layers with the protective layers  41  and/or  43 , were stacked and laminated in a manner described in Example 1 except that the lamination pressure applied was from a hydraulic press. Lamination occurred at a pressure of about 4,500 psi and at a temperature of about 75° C. Inspection of the laminated structure showed the screened features and the ceramic was protected by the adhered surface film  41  and/or  43 , both in lamination and in subsequent green sizing operation. 
     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.