Abstract:
A method for making a multichip “HDI” module includes the step of making a substrate for supporting the semiconductor or solid-state chips (or other components) by applying electrical conductor in a pattern to a first dielectric sheet, and applying encapsulating material to the electrical conductor. Apertures are made in the first dielectric sheet and encapsulant at locations at which the chips (or other components) are to be located. The components are affixed to a second dielectric sheet at locations registered with the apertures in the first sheet, and the sheets are juxtaposed with the chips extending into the apertures. This results in the formation of gaps between the components and the edges of the apertures, which gaps are then filled with hardenable or curable material. Electrical connection is made to the pads of the chips by means of a multilayer structure of dielectric sheets with conductor patterns, interconnected by means of plated-through vias.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to and claims the benefit of the earlier filing date under 35 U.S.C. 119, of U.S. Provisional Patent Application, Ser. No. 60/339,968 entitled “METHOD FOR MAKING MULTICHIP MODULE SUBSTRATES BY ENCAPSULATING ELECTRICAL CONDUCTORS AND FILLING GAPS,” filed on Oct. 31, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to high density interconnect (HDI) arrangements for electronic modules, and more particularly to methods for fabrication of such modules. 
     BACKGROUND OF THE INVENTION 
     High density interconnect assemblages such as those described in U.S. Pat. No. 4,783,695, issued Nov. 8, 1988 in the name of Eichelberger et al., and in numerous other patents, are finding increased usage. In one type of HDI assemblage, a dielectric substrate such as alumina has a planar surface defining one or more wells or depressions. Each well or depression extends below the planar surface by the dimension of a component which is to become part of the HDI assemblage. The component is typically an integrated circuit, having its electrical connections or contacts on an upper surface, but other components can be used. Each component is mounted in a well dimensioned to accommodate the component with its electrical contacts in substantially the same plane as the planar surface of the substrate. The components are typically held in place in their wells or depressions by an epoxy adhesive. A layer of dielectric material such as Kapton polyimide film, manufactured by DuPont of Wilmington, Del., is laminated to the devices using ULTEM polyetherimide thermoplastic adhesive, manufactured by General Electric Plastic, Pittsfield, Mass., which is then heat-cured at about 260° to 300° C. in order to set the adhesive. The polyetherimide adhesive is advantageous in that it bonds effectively to a number of metallurgies, can be applied in a layer as thin as 12 micrometers (μm) without formation of voids, and is a thermoplastic material, so that later removal of the polyimide film from the components is possible for purposes of repair by heating the structure to the plastic transition temperature of the polyetherimide while putting tension on the polyimide film. 
     Following the curing of the ULTEM adhesive layer holding the first sheet of dielectric film onto the components, through via apertures are laser-drilled through the dielectric film and its adhesive layer at the locations of at least some of the electrical connections. A patterned layer of titanium/copper/titanium electrical conductors is applied in a pattern to the exposed surface of the polyimide film, into the through vias, and onto the contacts of the components. This completes the formation of a first layer of electrical connections to the components. One or more additional thin sheets of polyimide dielectric material are layered onto the upper surfaces using silicone polyimide epoxy adhesive (SPIE). The SPIE is a thermoset material such as OXYSIM 600, manufactured by Occidental Chemical Corporation, Grand Island, N.Y., which is then cured at temperatures below 200°. Once set, the SPIE cannot be softened by heating. Each additional layer of polyimide film has its own pattern of through vias drilled as far as the upper titanium surface of a lower layer of titanium/copper/titanium conductor, followed by its own layer of titanium/copper/titanium deposition. 
     Alternative methods for making HDI modules include the “chip-on-flex” method, in which the solid-state chips are applied, electrode-side-down, onto an adhesive-faced dielectric layer. The chips are then encapsulated in a rigid molding material, which in one embodiment is Plaskon, an epoxy material, to form a rigid molded-chip-plus-dielectric-sheet piece. The flexible, multilayer interconnect sheet is then placed over the bottom of the dielectric sheet, which is to say on the side remote from the molding material, and the interconnections are made by means of laser-drilled vias. 
     SUMMARY OF THE INVENTION 
     Thus, a method according to an aspect of the invention is for generating a multi-chip module. The method comprises the steps of procuring a dielectric sheet defining a surface and tensioning the dielectric sheet, as by use of a frame, to provide a measure of rigidity to the surface. One or more electrical conductors is applied to the surface of the dielectric sheet in a predetermined pattern. The electrical conductors have a predetermined thickness. In one embodiment of the invention, the thickness is 40 thousandths of an inch, and the surface of the dielectric sheet is coated with adhesive to retain the conductors. Encapsulating material is applied to the surface of the dielectric sheet in a thickness sufficient to encapsulate the electrical conductors, to thereby generate a rigid substrate element. Wells or apertures, which may be through apertures, are fabricated, formed or defined in the rigid substrate element at predetermined locations at which semiconductor or solid-state chips are to be placed in or on the multi-chip module. The semiconductor or solid-state chips are placed on a second dielectric sheet or substrate at locations registered with the apertures or through apertures, with electrical pads, electrodes, or interconnects of the chips facing in a particular direction. In a particular embodiment of the invention, the second dielectric sheet has adhesive on one of its surfaces, and that side of the semiconductor or solid-state chips having electrical connection pads or electrodes of the semiconductor or solid-state chips are placed on the adhesive of the second sheet. The rigid substrate element with apertures is affixed to the second dielectric sheet with the semiconductor or solid-state chips extending into or through the wells or apertures. A flexible multilayer dielectric interconnection sheet carrying interconnection conductor patterns is formed on, andor applied over, at least the electrical connection pads or electrodes of some of the semiconductor or solid-state chips, for making connections between at least some of the interconnection conductor patterns of the interconnection sheet and some of the electrical connection pads. In a particular embodiment of the invention, the connections are made with the aid of plated-through vias. 
     In one variant of the method, a layer of encapsulant material is removed, shaved or ground from at least one surface of the rigid substrate element before the step of affixing the rigid substrate element to the second dielectric sheet. In another variant, the step of applying to the surface of the dielectric sheet, in a predetermined pattern, one or more electrical conductors having a predetermined thickness includes the step of applying adhesive to the surface of the dielectric sheet, and applying the one or more electrical conductors to the adhesive. In another mode of the method of the invention, an electrically conductive plate is affixed to the rigid substrate element on that side of the multichip module remote from the flexible multilayer dielectric interconnection sheet. 
     According to an aspect of the invention, the gaps between the rigid substrate element and the semiconductor or solid-state chips or other components and the associated wells or apertures are filled with a hardenable (curable) filler material. The filling may be accomplished by means of manually operated tools or by means of automatic or programmed dispensing machines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a simplified perspective or isometric view of a tensioned dielectric sheet onto which conductive slug material has been affixed, and FIG. 1 b  is a cross-section of the structure of FIG. 1 a  looking in the direction  1   b — 1   b;    
     FIG. 2 is a simplified cross-section of the structure of FIGS. 1 a  and  1   b  after encapsulation; 
     FIG. 3 is a simplified cross-section of the structure of FIG. 2 after removal of excess encapsulation and dielectric sheet; 
     FIG. 4 a  is a simplified cross-sectional view, and FIG. 4 b  a simplified perspective or isometric view, of the structure of FIG. 3 after the defining of apertures for placement of chips; 
     FIG. 5 is a simplified perspective or isometric view of another dielectric sheet with semiconductor or solid-state chips mounted thereon in a pattern registered with the apertures of the structure of FIGS. 4 a  and  4   b;    
     FIG. 6 is a simplified cross-sectional view of the structure of FIGS. 4 a  and  4   b  juxtaposed with that of FIG. 5; 
     FIG. 7 is a simplified cross-sectional view of the structure of FIG. 6, with the addition of through vias and circuit metallizations or depositions; 
     FIG. 8 is a simplified cross-sectional view of a completed multi-chip module including a ground/thermal coupling plate; 
     FIG. 9 is a perspective or isometric view of the upper surface of the partially completed structure of FIG. 6, illustrating gaps in the structure; and 
     FIG. 10 is similar to FIG. 9, but involves the use of an automatic or programmable dispensing apparatus for filling certain gaps. 
    
    
     DESCRIPTION OF THE INVENTION 
     In FIGS. 1 a  and  1   b , a tensioned dielectric sheet  10  defines an upper surface  10   us . The tensioning may be applied by way of a frame, as known in the art, to produce a radial outward force indicated by arrows f. The upper surface  10   us  may be coated with adhesive. A layer of electrically conductive material  12  is affixed to the upper surface  10   us  of dielectric sheet  10 , as for example by application of a layer  14  of adhesive to the upper surface  10   us.  Conductive layer  12  may have any thickness T, but in one embodiment of the invention, has a thickness of 0.040 inch. Such a thickness of material may possibly be better fabricated by stamping rather than by deposition, but any method will do, including machining from a block of conductive metal. As illustrated, the conductive pattern is in the form of an open rectangle or surrounding wall. Such a pattern can be useful in the context of electrically shielding components lying within the enclosed portion. Such a shape may also be useful for grounding electrical circuits, especially if the electrically conductive piece  12  is itself connected to an external ground. 
     In FIG. 2, the structure of FIG. 1 b  has been covered with a layer of encapsulating or fill material  210 . In one embodiment, the encapsulating material is the abovementioned Plaskon material. Once the encapsulating material is hardened, the layer becomes rigid to thereby define a rigid substrate element  200 , although the thickness of the element is such that it may be somewhat flexible overall. As illustrated in FIG. 2, the encapsulating material  210  fills the region between the exposed portions of the electrically conductive material  12 . As illustrated in FIG. 2, the layer  210  of encapsulant material may be thick enough to extend over the electrically conductive portions  12 . 
     FIG. 3 is a simplified cross-sectional view of the structure of FIG. 2 after the step of grinding or lapping both upper and lower surfaces of the structure to thereby expose the electrically conductive portions  12  at both surfaces. 
     FIGS. 4 a  and  4   b  illustrate the result of forming apertures  410   a  and  410   b  within the region which is electrically shielded by the presence of electrically conductive slug  12 . The apertures are dimensioned to accommodate the various semiconductor or solid-state chips (chips) which are intended for mounting therein. 
     FIG. 5 illustrates a structure  500  including a sheet  510  of dielectric material on which a plurality of semiconductor or solid-state chips, two of which are designated  512   a  and  512   b , are mounted. The mounting of the chips  512   a  and  512   b  is such that the electrical connections or pads of the chips, some of which are designated  512   a   1 ,  512   a   2 ,  512   a   3 ,  512   b   1 ,  512   b   2 , and  512   b   3 , are adjacent the dielectric sheet  510 . The mounting may be accomplished by applying adhesive to either the electrical connection sides of the chips or to the dielectric sheet  510 , and bringing the chips into contact with the dielectric sheet  510 . The locations of the chips are selected to be registered with each other and with the apertures  410   a  and  410   b  in structure  400  of FIGS. 4 a  and  4   b.    
     FIG. 6 is a cross-sectional view of the combined structures  400  of FIG. 4 with  500  of FIG.  5 . In FIG. 6, the semiconductor or solid-state chip  512   a  lies within aperture  410   a , and chip  512   b  lies within aperture  410   b . The resulting structure is designated  600 . 
     FIG. 7 illustrates the structure  600  of FIG. 6, turned over for convenience in understanding, with layer  510  of dielectric material lying above the remaining structure. As illustrated in FIG. 7, through vias  712   a ,  712   b , and  712   c  are made in the conventional manner through dielectric material  510  at the locations of the conductive slugs  12  and at the location of a contact pad  512   a   1  of semiconductor or solid-state chip  512   a . Metallizations  714   a  and  714   b  overlie the locations of electrically conductive slugs  12 , while metallization  714   c  overlies one of the electrical contacts or pads of semiconductor or solid-state chip  512   a.    
     FIG. 8 is a cross-sectional view of a structure  800  built up from structure  700  of FIG.  7 . Structure  800  includes a further heat-sink layer  810  affixed to the bottom of structure  700 , and thermally coupled at least to the lower surfaces of semiconductor or solid-state chips  512   a  and  512   b , for aiding in carrying away heat therefrom. Alternatively, or in addition, the heat sink layer  810  can be electrically conductive, and be in galvanic contact with the electrically conductive slugs  12 . In addition, a further dielectric interconnect layer  812  is affixed to the upper surface of layer  510 . Interconnect layer  812  includes further through vias and metallizations, for making other connections. More particularly, dielectric interconnect layer  812  has through vias and metallizations  814   a ,  814   b , and  814   c  made therethrough at locations of an intermediate-level connection pad  816 , and at the locations of contact pads  512   b   2  and  512   a   2 . 
     FIG. 9 is a simplified perspective or isometric view of the upper portion of structure  600  of FIG. 6, illustrating the dielectric sheet  510 , the electrical conductor  12  encapsulated in molding material  210 , and chips  512   a ,  512   b , and  512   c  extending into apertures  410   a ,  410   b , . . . defined in the molding material  210 . Due to the necessity for reliable insertion of the chips  512   a ,  512   b , . . . into the apertures  410   a ,  410   b , . . . , the apertures must necessarily be at least slightly oversized, to accommodate tolerance build-up. This tends to result in a gap on at least one side of each chip. The gaps are designated  912   a ,  912   b , and  912   c  in FIG.  9 . 
     It has been discovered that the gaps lying between the semiconductor chips and the surrounding encapsulating material may not be of uniform thickness, and in some locations may be totally absent. In those cases in which the gap is absent, there may be pressure on the chip from the adjacent encapsulating material, which puts pressure on the chip, which in turn tends to move the connection surface of the chip away from the desired common plane condition. This may adversely affect the connection steps of the method. In addition, the presence of a gap may cause the interconnection layer(s) to be deformed in the region of the gap, which also adversely affects the ability to make reliable connections. 
     According to an aspect of the invention, the gap(s)  912   a ,  912   b ,  912   c , . . . between the semiconductor chips  512   am    412   b ,  512   c , . . . and the surrounding encapsulating material  210  of FIG. 9 is filled with a dielectric material to prevent distortion of the interconnect layers. In the simplest method, a syringe filled with hardenable material is used to manually fill the gap around each of the semiconductor chips, as suggested by the syringe  920  of FIG. 9, dispensing material  922 . The material  922  may be encapsulating material corresponding to that of of substrate  210 , or it may be an elastomeric material such as a silicone elastomer, or it may be a low-density but rigid material such as a foamed polymer. 
     When high-precision equipment is used to fabricate the basic structure of assemblage  600  of FIG. 6, the width of the gap surrounding each semiconductor chip may be of uniform width, so that an automatic preprogrammed dispenser can be used to perform the filling automatically. In FIG. 10, the structure of FIG. 9 is associated with an automatic preprogrammed dispenser designated generally as  1000 , which includes a dispensing nozzle  1010  moved under by a manipulator/dispenser  1012  under the control of a controller  1014 . Such dispensing machines are known in the art, and require no further explanation. As in the case of the manually controlled syringe dispensing, the hardenable material may be the same as the encapsulating material, if appropriate for syringe deposition, or it may be another material selected to take up strains due to stress. Such a material might be a silicone encapsulant. For some situations, a foamed material might be appropriate, since it would tend to have better compliance than the equivalent non-foamed material. In one embodiment of the invention in which Plastikon was used as the substrate or encapsulant material, the fill material was silicone polyimide (SPI) epoxy, and in another embodiment the material was Hysol Dexter epoxy. 
     Thus, a method according to an aspect of the invention is for generating a multi-chip module ( 800 ). The method comprises the steps of procuring a dielectric sheet ( 10 ) defining a surface ( 10   us ) and tensioning (f) the dielectric sheet, as by use of a frame, to provide a measure of rigidity to the surface ( 10   us ). One or more electrical conductors ( 12 ) is applied to the surface ( 10   us ) of the dielectric sheet ( 10 ) in a predetermined pattern. The electrical conductors ( 12 ) have a predetermined thickness. In one embodiment of the invention, the thickness is 40 thousandths of an inch, and the surface of the dielectric sheet is coated with adhesive ( 14 ) to retain the conductors ( 12 ). Encapsulating material ( 210 ) is applied to the surface of the dielectric sheet in a thickness sufficient to encapsulate the electrical conductors ( 12 ), to thereby generate a rigid substrate element ( 200 ). Wells or apertures, which may be through apertures ( 410   a ,  410   b ), are fabricated, formed or defined in the rigid substrate element ( 200 ) at predetermined locations at which semiconductor or solid-state chips ( 512   a ,  512   b ) are to be placed in or on the multi-chip module ( 800 ). The semiconductor or solid-state chips ( 410   a ,  410   b ) are placed on a second dielectric sheet or substrate ( 510 ) at locations registered with the apertures or through apertures ( 410   a ,  410   b ), with electrical pads, electrodes, or interconnects ( 512   ap ,  512   ap   2 ,  512   bp ) of the chips ( 410   a ,  410   b ) facing in a particular direction. In a particular embodiment of the invention, the second dielectric sheet ( 510 ) has adhesive ( 510   a ) on one of its surfaces, and that side of the semiconductor or solid-state chips ( 410   a ,  410   b ) having electrical connection pads or electrodes ( 512   ap ,  512   ap   2 ,  512   bp ) of the semiconductor or solid-state chips ( 410   a ,  410   b ) are placed on the adhesive ( 510   a ) of the second sheet ( 510 ). The rigid substrate element with apertures ( 400 ) is affixed to the second dielectric sheet ( 510 ) with the semiconductor or solid-state chips ( 410   a ,  410   b ) extending into or through the wells or apertures ( 410   a ,  410   b ). A flexible multilayer dielectric interconnection sheet carrying interconnection conductor patterns ( 510 ,  812 ) is formed on, andor applied over, at least the electrical connection pads or electrodes ( 512   ap ,  512   ap   2 ,  512   bp ) of some of the semiconductor or solid-state chips ( 410   a ,  410   b ), for making connections between at least some of the interconnection conductor patterns ( 714   a ,  714   b ,  714   c ,  814   a ,  814   b ,  814   c ) of the interconnection sheet ( 510 ,  812 ) and some of the electrical connection pads ( 512   ap ,  512   ap   2 ,  512   bp ). In a particular embodiment of the invention, the connections are made with the aid of plated-through vias ( 712   a ,  712   b ,  712   c ,  814   a ,  814   b ,  814   c ). 
     In one variant of the method, a layer of encapsulant material is removed, shaved or ground from at least one surface of the rigid substrate element ( 200 ) before the step of affixing the rigid substrate element to the second dielectric sheet. In another variant, the step of applying to the surface ( 10   us ) of the dielectric sheet  910 ), in a predetermined pattern, one or more electrical conductors ( 12 ) having a predetermined thickness includes the step of applying adhesive ( 14 ) to the surface ( 10   us ) of the dielectric sheet ( 10 ), and applying the one or more electrical conductors ( 12 ) to the adhesive ( 14 ). In another mode of the method of the invention, an electrically conductive plate ( 810 ) is affixed to the rigid substrate element ( 300 ) on that side of the multichip module ( 800 ) remote from the flexible multilayer dielectric interconnection sheet ( 510 ,  812 ). 
     According to an aspect of the invention, the gaps ( 912   a ,  912   b ,  912   c , . . . ) between the rigid substrate element ( 200 ) and the semiconductor or solid-state chips ( 512   a ,  512   b ) or other components and the associated wells or apertures ( 410   a ,  410   b ,  410   c , . . . ) are filled with a hardenable (curable) filler material ( 922 ). The filling may be accomplished by means of manually operated tools ( 920 ) or by means of automatic or programmed dispensing machines ( 1000 ).