Abstract:
A method of making a microelectronic component by providing a conductive element, providing a resist at selected locations on said conductive element and electrophoretically depositing an uncured dielectric material on the conductive element, wherein the uncured material will be deposited on the conductive element except at locations covered by the resist. The deposited material is cured to form a dielectric layer and the resist is removed so that the dielectric layer has openings extending to the conductive element at locations the locations which were covered by the resist.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 08/695,642, filed on Aug. 12, 1996, which is in turn a division of U.S. patent application Ser. No. 08/277,336, filed on Jul. 19, 1994, now U.S. Pat. No. 5,590,460. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of electrical circuitry, and more particularly relates to layered circuit structures such as multi-layer circuit boards, to components and methods utilized in fabrication of such structures and to methods of making the same. 
     BACKGROUND OF THE INVENTION 
     Electrical components are commonly mounted on circuit panel structures such as printed circuit boards. Circuit panels ordinarily include a generally flat sheet of dielectric material with electrical conductors disposed on a major, flat surface of the sheet or on both major surfaces. The conductors are commonly formed from metallic materials such as copper and serve to interconnect the electrical components mounted to the board. Where the conductors are disposed on both major surfaces of the panel, the panel may have via conductors extending through the dielectric layer so as to interconnect the conductors on opposite surfaces. Multi-layer circuit panel assemblies have been made heretofore which incorporate plural, stacked circuit panels with additional layers of dielectric materials separating the conductors on mutually facing surfaces of adjacent panels in the stack. These multi-layer assemblies ordinarily incorporate interconnections extending between the conductors on the various circuit panels in the stack as necessary to provide the required electrical interconnections. 
     Electrical components which can be mounted to circuit panel structures include so-called “discrete” components and integrated circuits which include numerous transistors on a single chip. Chips of this nature can be mounted to commonly referred to as “chip carriers” which are specialized circuit panel structures. A chip carrier may be incorporated in a package which is mounted to a larger circuit board and interconnected with the remaining elements of the circuit. Alternatively, the chip can be mounted directly to the same circuit panel which carries other components of the system. This arrangement is ordinarily referred to as a “hybrid circuit”. Relatively large circuit panels are commonly made of polymeric materials, typically with reinforcement such as glass, whereas very small circuit panels such as those used as semiconductor chip carriers may be formed from ceramics, silicon or the like. 
     There have been increasing needs for circuit panel structures which provide high density, complex interconnections. These needs are addressed by multilayer circuit panel structures. The methods generally used to fabricate multi-layer panel structures have certain serious drawbacks. Multi-layer panels are commonly made by providing individual, dual sided circuit panels with appropriate conductors thereon. The panels are then laminated one atop the other with one or more layers of uncured or partially cured dielectric material, commonly referred to as “prepregs” disposed between each pair of adjacent panels. Such a stack ordinarily is cured under heat and pressure to form a unitary mass. After curing, holes are drilled through the stack at locations where electrical connections between different boards are desired. The resulting holes are then coated or filled with electrically conductive materials, typically by plating the interiors of the holes to form what is called a plated through hole. It is difficult to drill holes with a high. ratio of depth to diameter. Thus, the holes used in assemblies fabricated according to these prior methods must be relatively large and hence consume substantial amounts of space in the assembly. These holes ordinarily extend from the top or bottom of the stack. Even where interconnections are not required in the top or bottom layers, space must be provided for holes to pass through these layers so as to provide needed interconnections in the middle layers. Accordingly, substantial amounts of the-available surface area on the panels must be allocated to the holes and to accommodate tolerance zones around the holes. Moreover, the electrical interconnections formed by depositing conductive materials in such drilled holes tend to the weak. The drilling method and the general nature of the laminates used therein is described, for example in Doherty, Jr. U.S. Pat. No. 3,793,469; and Guarracini, U.S. Pat. No. 3,316,618. 
     Various alternative approaches have been proposed. Parks et al., U.S. Pat. No. 3,541,222; Crepeau, U.S. Pat. No. 4,249,032; Luttmer, U.S. Pat. No. 3,795,037; Davies et al., U.S. Pat. No. 3,862,790, Fox U.S. Pat. No. 4,954,878, and Zifcak, U.S. Pat. No. 4,793,814 all relate generally to structures which have metallic or other conductive elements arranged at relatively closely spaced locations on a dielectric sheet with the conductive elements protruding through the dielectric sheet in both directions. Such a sheet may be sandwiched between a pair of circuit boards and the circuit boards may be clamped or otherwise held together so as to provide mechanical engagement between conductive elements on the adjacent faces of the circuit boards and the conductive elements of the composite sheet. In each of these arrangements, the conductive elements, the composite sheet or both is resilient or malleable so as to provide for close engagement between the conductive elements of the composite sheet and the conductors on the circuit boards. 
     Beck, U.S. Pat. No. 3,616,532 and Dube et al., U.S. Pat. No. 3,509,270 describe variants of this approach in which resilient elements are used with a fusible solder. These elements are mounted on insulating boards which are then stacked between printed circuit layers. The assembly is heated so as to melt the solder, thereby freeing the resilient elements so that the resilient elements and solder cooperatively form an interconnection between the adjacent circuit boards. 
     Evans et al, U.S. Pat. No. 4,655,519 describes a connector with numerous strip-like contact springs disposed in holes in a flat dielectric body, together with other spring elements. The ends of the strips protrude from opposite surfaces of the body. These are adapted to compress when electronic elements are engaged with the body surfaces, so that the ends of the strips engage pads on the electronic elements. Walkup, U.S. Pat. No. 5,167,512 discloses a further system using springs disposed in holes in a dielectric body. 
     Grabbe, U.S. Pat. No. 5,228,861 describes a connector having a sheet-like dielectric body with numerous generally X-shaped spring contact elements, each having four arms, lying on a first side of the sheet. Two arms of each X-shaped element are bent inwardly toward the sheet, and extend through holes in the sheet so that the tips of these arms protrude above the second, opposite face of the sheet. The other two arms are bent away from the sheet, and hence protrude from the first surface. When the connector is placed between circuit panels, each X-shaped element is compressed between mating pads of the circuit panels, causing the bent arms to flatten and causing the tips of the arms to wipe the surfaces of the pads. After engagement, the contact is maintained by the resilience of the arms. 
     Bernarr et al, U.S. Pat. No. 4,548,451 describes a connector or interposer having a sheet-like elastomeric body with crushable protrusions extending outwardly from oppositely-directed surfaces. Tabs formed from a metal-coated flexible film extend on both surfaces of the body, and overlie the protrusions. The tabs on opposite sides are connected to one another by vias. When the interposer is engaged between circuit panels, the tabs and posts are crushed between contact pads on opposing panels, and the tabs assertedly wipe the pads for more effective contact. The tabs are maintained in engagement with the pads by the resilience of the elastomeric sheet and the posts; there is no permanent bond formed. 
     Patraw, U.S. Pat. Nos. 4,716,049; 4,902,606; and 4,924,353 describe microelectronic connection schemes using deformable contacts protruding upwardly from a substrate. Each contact has a dome-like tip and a plurality of legs extending downwardly from the tip to the substrate. These contacts are formed by selective deposition of aluminum on a pedestals of a fugitive material such as potassium chloride or a photoresist using a shaped mask. The pedestals are removed after deposition. 
     Dery et al., U.S. Pat. No. 4,729,809 discloses the use of an anisotropically conductive adhesive material disposed between opposing sublaminates, the adhesive composition having sufficient conductivity across the relatively small spaces between conductors on adjacent layers to form an electrical interconnection therebetween, but having low conductivity across the relatively large spaces between adjacent conductors on the same surface so that it does not produce an unwanted lateral interconnection along one surface. 
     Berger et al., U.S. Pat. No. 4,788,766 uses conductor bearing circuit lamina having hollow, eyelet-like via structures, each such via structure having a rim protruding vertically from the surrounding structure. Each such via structure is provided with a thin layer of a conductive bonding material. In making the multi-layer structure, dielectric bonding films are interposed between the circuit bearing lamina. The dielectric films have apertures in locations corresponding to the locations of the eyelet structures, in the adjacent circuit bearing lamina. Thus, the upstanding rims of the eyelet structures can bear upon one another when the assembly is forced together under heat and pressure. The layers of conductive bonding material on the rims of the abutting eyelets are said to form bonds between the abutting eyelet structures. 
     Other structures for forming multilayer electronic assemblies are taught in Dux et al., U.S. Pat. No. 5,224,265 and Ehrenberg et al. U.S. Pat. No. 5,232,548, which use sublaminates made by depositing a dielectric material onto a perforated metal sheet, as by vapor-phase polymerization or electrophoretic bonding, to form a dielectric sheet with vias. The vias are filled with a flowable joining material such as a metal-loaded polymer. These structures are stacked and heated to join the vias into unitary vertical conductors. 
     Other multilayer assembly systems using flowable conductive materials to join structures in stacked elements are disclosed in Bindra et al., U.S. Pat. No. 5,129,142. Still further improvements are disclosed U.S. Pat. No. 5,282,312 of Thomas H. DiStefano et al. The &#39;312 patent discloses as background certain lamination techniques or methods of making multi-layer circuit assemblies using flowable conducting materials. 
     Despite these and other efforts in the art, there are needs for still further improvement. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these needs. 
     One aspect of the present invention provides an interposer for making connections to electrical contacts on the surface of a microelectronic elements such as a circuit panel, a semiconductor chip or other element having a contact-bearing surface. The interposer includes a body having a first major surface, such that the body defines horizontal directions parallel to the first major surface and vertical directions perpendicular to the first major surface. The interposer further has a plurality of conductors in the body, such as via conductors extending in or through the body. The interposer further includes contacts at the first major surface of the body electrically connected to the conductors. Each contact extends over the first surface of the body generally radially outwardly from a central axis which is perpendicular to the first surface. Thus, each contact extends in a plurality of horizontal directions away from the axis and away from the junction of the contact with the conductor. Each contact has a periphery remote from the central axis. The contacts are adapted to deform so that the periphery of each contact will expand generally radially outwardly, away from the axis, in response to a force applied to the contact directed toward the body. In use, the contacts are engaged with corresponding connection pads of a circuit panel or other microelectronic element when the element is juxtaposed with the first surface of the interposer. The panel or other element can then be forced toward the body, so that the pads apply vertical forces to the contacts. As each contact expands outwardly in response to the vertical force, it wipes across the surface of the mating pad. The wiping action removes oxides and other contaminants from the mating surfaces to provide an effective, low-resistance electrical connection between the pads and the contacts and, in preferred embodiments, to facilitate bonding of the contacts and pads. 
     The contacts desirably are arranged to deform so that the periphery of each contact moves vertically downwardly toward the body as the periphery moves radially outwardly. In the initial, undeformed condition, the periphery of each contact may be spaced vertically above the surface of the body. Each contact may include a central portion adjacent the axis which may be in engagement the body, whereas a peripheral portion of the contact may extend radially outwardly from the central portion and vertically upwardly, away from the body. The central portion of each contact may be connected to a conductor, so that the contact extends radially outwardly, atop the surface of the body, from the conductor. Each contact may be generally in the form of a body of a revolution about the vertical central axis. Thus, the peripheral portion may be a shell in the form of an upwardly and outwardly flaring of revolution about the central axis. In this instance, the expanding action of the contact is reminiscent of the setting of a rivet. 
     Alternatively, the periphery of each contact may be non-circular. Thus, each contact may include a plurality of tabs extending generally radially away from the central axis, each such tab having a tip remote from the axis and remote from the conductor. Desirably, the tabs of each contact are disposed in a substantially symmetrical pattern about the central axis and about the junction with the conductor, so that the tabs extend generally symmetrically in radial directions away from the central axis, and away from the conductor. A preferred symmetrical pattern includes four tabs disposed at equally spaced intervals around the central axis, such that the tabs define a quatrefoil pattern. 
     When the tips of the tabs are engaged by a pad on a circuit panel or other microelectronic element and forced downwardly towards the body, the tip of each tab will bend downwardly and outwardly, away from the central axis. Thus, the tabs of the contact unfold away from one another, to provide an effective wiping action against the surface of the engaged pad. Although the present invention is not limited by any theory of operation, it is believed that this outward movement of the tab tips is produced at least in part by the bending of each tab generally around the radially inward or proximal end of the tab and around the adjacent regions of the body itself. The body surface may have depressions housing the central portions of the contacts, and may also have outwardly-flaring walls surrounding the depressions. Each tab may be arcuate, and may conform to the outwardly-flaring wall. 
     Preferably, the contacts include bonding materials adapted to facilitate the bonding of the contacts to a mating pad engaged therewith. The bonding material may be selected from a group consisting of solders, brazing alloys, diffusion bonding alloys, eutectic bonding alloys and conductive materials incorporating polymers. The contacts may be metallic and may be formed integrally with the associated conductors. The interposer body preferably is a lamellar structure, such as a sheet-like or platelike structure, and defines a second major surface facing in the opposite direction from the first major surface. At least some of the conductors may be through conductors having second ends disposed adjacent the second major surface. In this case, the interposer has second end contacts attached to the second ends of the conductors. Each such second end contact may be configured in the same manner as the first end contacts discussed above. Thus, each second end contact may extend radially outwardly away from a central axis which is perpendicular to the second surface, and may extend from a second end of a conductor connected at its first end to one of the contacts on the first surface. The contact desirably is adapted to deform so that the periphery will expand radically outwardly, away from the axis and from the conductor second end in response to a vertical force on the second end contact. Such interposers can be used, for example, between pairs of circuit panels or other microelectronic elements. 
     The contacts and the conductors may constitute portions of continuous, generally tubular vias extending through a hole in the body and flaring outwardly at both ends of the hole. The interposer body may have a heat activatable adhesive, a thermoplastic or other flowable dielectric material at the surfaces of the body, so that the interposer can form a substantially void free dielectric seal with the body of a circuit panel or other microelectronic element engaged therewith. Where the conductive bonding material on the contacts is activated by heat, the adhesive or the flowable dielectric material on the body may be arranged for activation at approximately the same temperature as the flowable conductive material. 
     A further aspect of the invention provides a microelectronic connection interposer having a body with a first horizontally extensive surface having a principal plane and a plurality of contacts disposed in an array on the body at said surface. Each contact includes a central portion and a peripheral portion extending outwardly from the central portion. The central portion may be recessed vertically from the principal plane of the surface, whereas the peripheral portion may extend at or vertically above the principal plane. Desirably, the peripheral portion of each contact is non-circular and may include tabs as aforementioned projecting away from the central portion. The body may include a plurality of indentation in the first surface, each such indentation having a central axis generally perpendicular to the first surface and having walls flaring generally radially outwardly up away from the central axis so that the indentation widens progressively from the base of the indentation to the opening of the recess at the principal plane. 
     Another aspect of the present invention provides methods of making a multi-layer circuit. A method according to this aspect of the present invention desirably includes the step of stacking a microelectronic element such as a circuit panel and an interposer so the first surface of the interposer confronts a surface of the microelectronic element. The interposer may be an interposer as described above, having a body, having conductors and having contacts joined to the conductors, each contact extending over the surface of the body and extending radially outwardly from its juncture with the conductor to a peripheral portion. The method includes the further step of compressing the stacked microelectronic element and interposer so as to forcibly engage the peripheral portions of the contacts with the pads of the microelectronic element, thereby expanding the peripheral portion of each contact radially outwardly, away from the central axis and away from the associated conductor. Thus, the peripheral portion of each contact moves horizontally with respect to the engaged pad and wipes the surface of the pad. Preferably, during the compressing step, each peripheral portion also moves vertically downwardly, i.e., toward or into the body of the interposer. 
     The stacking step may include the step of stacking a plurality of circuit panels and one or more interposers in interleaved, vertically alternating arrangements so that one interposer is disposed between each pair of circuit panels and so that oppositely directed first and second surfaces of each interposer confront the mating surfaces of the panels. In this arrangement, the conductors of each interposer desirably include through or via conductors extending through the body and having contacts at both ends of each conductor, on opposite surfaces of the interposer. The contacts on the opposite surfaces engage pads on the two neighboring circuit panels on opposite sides of the interposer. Thus, when the contacts are engaged with the pads of the circuit panels, the through conductors will interconnect the pads on the neighboring circuit panel with one another. In preferred processes according to this aspect of the present invention, the wiping action of the contacts provides for reliable electrical bonding between the circuit panels. Moreover, the vertical movement of the contact peripheral portions compensates for nonplanarity of the circuit panels and interposers. Preferred methods according to this aspect of the present invention can provide reliable interconnections and reliable multi-layer structures even with very small feature sizes. Thus, the spacings between adjacent pads on the circuit panels and between adjacent contacts on each surface of each interposer may be about 1.0 mm or less. 
     Preferably, the method includes the step of bonding the contacts of the interposers to the pads on the panels, by momentarily heating the stacked elements to activate an electrically conductive bonding material at interfaces between the contacts and contacts. Preferably, the method also includes a step of activating a flowable dielectric material such as an adhesive to flow at the interfaces between each of the interposer bodies and the adjoining microelectronic elements or panels so as to join the stacked elements into a substantially unitary mass. The flow of the dielectric material may occur concomitantly with the flow of the electrically conductive bonding material. Where the peripheral portions of the contacts overlie portions of the dielectric material of the interposer body, softening of the dielectric bonding material tends to facilitate vertical movement of the peripheral portions of the contacts. As the dielectric material softens, the peripheral portion of each contact may be displaced downwardly into the underlying dielectric material. 
     Further aspects of the present invention includes methods of making a circuit assembly including the steps of stacking microelectronic elements such as a circuit panel and an interposer in a vertically superposed arrangement so that a first horizontally extensive surface of the interposer confronts a horizontally extensive surface of the circuit panel and so that a plurality of electrically conductive contacts at the first surface of the interposer confront pads on the surface of the circuit panel. The method further includes the step of compressing the stacked microelectronic elements and the interposer vertically so as to forcibly engage the contacts with the pads and cause the contacts to deform so that at least a portion of each contact engaged with the pad moves horizontally with respect to the engaged contact and wipes the surface of the contact during the compressing step, and subsequently bonding each contact to the engaged pad, as by activating a bonding material at interfaces between the contacts and the pads by momentarily heating the stacked elements. The bonding step may include diffusion bonding, soldering, brazing, eutectic bonding, activation of a polymeric conductive adhesive or the like. The wiping action cleans the surfaces to facilitate effective bonding. 
     Still further aspects of the present invention provide methods of making microelectronic interposers. Methods according to this aspect of the present invention preferably include the steps of providing a body defining a first surface and providing a first temporary layer on the first surface so that the temporary layer has apertures therein. An electrically conductive structural material such as copper or another metal is deposited in each aperture so that a layer of the structural material extends within the aperture and extends over at least a portion of the temporary layer, thereby forming contacts. The method also includes the step of removing the temporary layer, leaving an outwardly extending peripheral portion of each contact spaced away from the surface of the body where the structural material was deposited on the temporary layer. Desirably, the body has holes extending inwardly from the first surface in registration with the apertures in the temporary layer and the structural material is deposited in the holes so that the conductors are formed integrally with the contacts. 
     Where the body has oppositely directed first and second surfaces, the holes in the body may extend from the first surface to the second surface and the method may include the step of providing a second temporary layer on the second surface with apertures in registration with the holes. Thus, the step of depositing the structural material may include the step of depositing the structural material in the apertures of the second temporary layer as well, so that the structural material extends over part of the second temporary layer. The method may further include a step of removing the second temporary layer thereby leaving contacts on the second surface integral with the conductors and with the contacts on the first surface. The holes and the apertures in the temporary layers may be formed so that walls of the holes of the body are substantially continuous with the walls of the apertures in the temporary layers. Thus, the step of forming the holes in the body may be performed after the step of applying the temporary layers with their respective apertures, as by removing material from the body using one or both of the temporary layers as masks. 
     Desirably, each aperture in each of the temporary layers has a central axis and the temporary layer defines a wall at each aperture flaring outwardly, away from the central axis of the aperture in the vertically upward and outward direction and away from the associated surface of the body. The step of depositing the structural material may include the step of depositing a thin layer of metal on these outwardly flaring walls. Thus, when the temporary layers are removed, the portions of the structural material overlying the outwardly flaring walls will be spaced from the surface of the body and will slope progressively upwardly away from the body in the radially outward direction from the central axis. 
     The step of providing the temporary layer may include the step of depositing the temporary layer material as an imperforate layer on the surface of the body and then forming the apertures by masking the imperforate layer so that the mask has openings. The masked layer may then be exposed to an isotropic etchant through the openings. Such treatment tends to form a generally cup-shaped surface. The structural material desirably is a metal such as copper, gold, nickel and the like which may be plated over the temporary layer. The temporary layer may be formed from a metal such as aluminum, tin or nickel, and the temporary layer may be removed by a process such as caustic etching, leaving the structural material intact. Alternatively, the temporary layer may include a layer of a polymeric material such as a resist, or other materials which can be removed without damaging the structural material. 
     Other aspects of the present invention provide methods of making microelectronic interposers using a body having a first surface, the first surface including a principal planar portion and a plurality of open indentations with vertical axes normal to the principal plane and with walls flaring outwardly away from the axes. Methods according to this aspect of the invention desirably include the step of depositing an electrically conductive structural material in the indentations to form a layer on the outwardly-flaring walls, and controlling the extent of the layers in horizontal directions, transverse to the axes, so that each layer forms a plurality of tabs extending generally radially from the axes. The horizontal extent of the layers may be controlled by selective deposition during the depositing step, or by selective removal, such as etching, after the depositing step. 
     The body desirably has two major surfaces as aforesaid, facing in opposite directions. The indentations on the two major surfaces desirably include pairs of coaxially aligned indentations which merge with one another to form continuous through holes or via holes extending through the entire body. The steps of providing the body may include the steps of providing a sheet having holes therein, as by etching the sheet from opposite sides to form holes tapering to narrowest points remote from the faces of the sheet, and then depositing a coating of a dielectric material on the sheet, and in the holes of the sheet, as by electrophoretic deposition. As further described below, the preferred methods according to this aspect of the invention provide unique curved tabs. 
     These and other objects have features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary, diagrammatic sectional view depicting portions of an interposer during one step of a fabrication process according to one embodiment of the invention. 
     FIGS. 2,  3  and  4  are views similar to FIG. 1 but depicting the interposer during later stages of the same process. 
     FIG. 5 is a diagrammatic perspective view H depicting a through conductor and contacts of the interposer of FIGS. 1-4. 
     FIG. 6 is a further view similar to FIGS. 1-4 but depicting the completed interposer. 
     FIG. 7 is a diagrammatic elevational view depicting stacked components of the beginning of an assembly process using the interposer of FIGS. 1-6. 
     FIGS. 8 is a fragmentary, diagrammatic sectional view depicting a portion of the components illustrated in FIG. 7 at the beginning of the assembly process. 
     FIG. 9 is a view similar to FIG. 8 but depicting the components at a later stage in the assembly process. 
     FIG. 10 is a fragmentary, diagrammatic sectional view depicting portions of an interposer during one step of a fabrication process according to another embodiment of the invention. 
     FIGS. 11 through 14 inclusive are views similar to FIG. 10 but depicting the interposer at progressively later stages of the fabrication process. 
     FIG. 15 is a diagrammatic, fragmentary plan view of the surface of the interposer shown in the sectional view in FIG.  14 . 
     FIG. 16 is a view similar to FIGS. 10-15 but depicting the completed interposer. 
     FIG. 17 is a diagrammatic perspective view depicting a through conductor and contacts of the interposer of FIGS. 10-16. 
     FIG. 18 is a fragmentary diagrammatic plan view depicting portions of stacked components during an assembly process using the interposer of FIGS. 10-17. 
     FIG. 19 is a diagrammatic sectional view taken along lines  19 — 19  in FIG.  18 . 
     FIG. 20 is a fragmentary, diagrammatic sectional view on an enlarged scale depicting the components during a later stage of the assembly process. 
     FIG. 21 is a fragmentary, diagrammatic sectional view depicting an interposer according to a further embodiment of the invention. 
     FIG. 22 is a fragmentary, diagrammatic sectional view depicting an interposer in accordance with yet another embodiment of the invention, in conjunction with other components during one stage of an assembly process. 
     FIG. 23 is a view taken along line  23 — 23  in FIG.  22 . 
     FIG. 24 is a view similar to FIG. 22 but depicting a later stage in the assembly process. 
     FIG. 25 is a view taken along line  25 — 25  in FIG.  24 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A process in accordance with one embodiment of the invention begins with a body  30  in the form of a sheet of a dielectric material having a first or top surface  32  and a second or bottom surface  34 . The dielectric material should be a material having a relatively high elastic modulus such as a polyimide material. One suitable polyimide is available under the trademark UPILEX from the Ube Corporation. Sheet  30  desirably is about 15-50 microns (“μ”) thick, and most preferably about 25μ thick. A layer  36  of a temporary layer material susceptible to etching by an etchant which does not attack body  30  is coated on top surface of the body  30 . The temporary layer material desirably is a metal such as aluminum, tin, or nickel which can be removed by etching. Temporary layer  36  may be about 15-100μ or more thick, most desirably about 50μ thick. A second temporary layer  38  similar to layer  36  is applied to the bottom surface  34  of body  30 . 
     Photoresist layers  40  and  42  are applied on the temporary layers  36  and  38  and then photographically exposed to develop openings  44  and  46 , respectively. Resist layers  40  and  42  may be formed from a conventional photoresist material. However, dry film photo resists are preferred. One suitable resist is a dry film resist about 25μ thick of the type sold under the trademark DYNACHEM by Morton Electrical Materials of Tustin, Calif. Openings  44  and  46  are circular, as viewed in directions perpendicular to surfaces  32  and  34 . Also, openings  44  and  46  are coaxial with one another on a common axis  48  perpendicular to the surfaces of body  30 . Although only a single set of aligned openings is illustrated in the fragmentary view of FIG. 1, aligned numerous sets are provided in one or more arrays extending over the surfaces of the body. The spacing or “pitch” dimension between adjacent pairs of openings is selected to match the pitch of the pads on the panel or other element which will be mated with the interposer. Preferred structures according to this embodiment of the invention can be fabricated with pitch dimensions of 1.5 mm or less, preferably about 0.5 to 1.5 mm. Preferably, each set of aligned openings  44  and  46  are coaxial with one another within a tolerance of about 10μ. This degree of alignment can be achieved by conventional film handling and exposure devices such as a conventional two-sided exposure tool. Sheet  30  typically is provided with sprocket holes (not shown) in margins remote from the area used to form the interposer for handling and aligning the sheet. Alignment based on such holes normally will be adequate for this step. 
     In the next stage of the process, the sheet with the temporary layers and photoresist layers is exposed to an etchant, such as a caustic solution, adapted to etch the temporary layers  36  and  38  isotropically. That is, the etchant will attack the temporary layers at substantially equal rates in all directions, wherever the temporary layers are exposed to the openings. The etchants attack the first or top temporary layer  36  through opening  44  progressively, until the first or top surface  32  of body  30  is exposed. At the same time, however, the etchant undercuts photoresist layer  40 , starting from the edge  50  defined by the periphery of opening  44  and the exposed surface of layer  36 . Thus, the etching process forms an aperture  52  in first temporary layer  36 , such aperture having a wall  54  substantially in the form of a surface of revolution about central axis  48 . The generator of surface  54  is substantially in the form of an arc having its center approximately at the top surface of layer  36 , i.e., at the surface of temporary layer  36  remote from body  30 . Thus, wall  54  is generally in the form of a cup or a section of a toroid. The wall flares radially outwardly, away from central axis  48  in the vertically outward or upward direction away from body  30 , i.e., upwardly as seen in FIG.  2 . Thus, the first or top temporary layer  36  includes a rim  55  surrounding each hole  52 . Rim  55  has progressively increasing thickness in the radial direction away from common central axis  48 . 
     The same etching step also forms apertures  56  in the second or bottom temporary layer  38 , these apertures being similar in configuration and diameter to apertures  52 . Thus, the wall  58  of each aperture  56  is generally in the form of a surface of revolution about central axis  48 , and flares radially outwardly, away from the central axis, in the vertically upward direction, away from body  30 . As used in this disclosure with reference to features associated with a surface of a body, the term “upward” means the direction out of such surface, away from the center of the body. Thus, with reference to the second surface  34  of body  30 , the “upward” direction is the direction toward the bottom of the drawing as seen in FIG.  2 . Second temporary layer  38  includes rims of progressively increasing thickness  39  surrounding apertures  56 . Each aperture  56  in the second layer  38  is substantially coaxial with the corresponding aperture in top or first aperture layer  36 . After conventional rinsing and deactivating steps, holes  60  are formed in body  30  by ablating the material of the body. The ablation is performed by directing radiant energy onto first surface  32  through the holes  52  in the top or first temporary layer. The radiant energy may be applied using a laser, such as a KrF excimer laser of wavelength 308 nm. Temporary layer  36 , and particularly rims  55  immediately surrounding the apertures protect body  30  from the impinging radiant energy. Thus, holes  60  are formed in the body of  30 . Each hole  60  is coaxial with the corresponding aperture  52  in a layer  36  an hence with common axis  48  and with the corresponding aperture  56  in layer  38 . The masking action of the top layer inherently adjusts the diameter of hole  60  to the diameter of the aperture at the apex of rim  55 , i.e., the diameter at the juncture of rim  55  with sheet top surface  32 . Thus, for each set of apertures, the peripheral wall  54  of the aperture in the top layer, the peripheral wall  62  of the hole  60  and the peripheral wall  58  of the apertures in second or bottom layer  38  are substantially continuous with one another. Absolute smoothness or continuity is not required. Thus, the radiant energy may spread outwardly beneath rim  55  of the first temporary layer, causing the diameter of hole  60  to vary slightly from the top surface  32  of body  2  to the bottom of surface  34 . This may yield a small ridge, less than a few microns high, at the juncture of hole surface  62  with aperture surface  58 , at the apex of rim  59 . Such ridges normally do not impede the process. 
     After formation of holes  60 , additional photoresist layers  64  and  66  are applied on the first or top temporary layer  36  and the second or bottom temporary layer  38  respectively. These layers initially bridge over the apertures in the temporary layers and thus hole  60  as well. Resist layers  64  and  66  may be formed by a dry film resist and may extend over the apertures and holes in the underlying layers without filling the same. The photoresist layers are exposed using conventional techniques to form pairs of openings  68  and  70  in the top and bottom resist layers respectively in alignment with holes  60  and hence coaxial with the apertures  52  and  56  in the temporary layers  36  and  38 . Holes  68  are circular and slightly larger than the maximum diameter of the corresponding diameter of the corresponding apertures in layer  36 , so that a small ledge  72  is formed around the periphery of each aperture. Likewise, each hole  70  is slightly larger than the maximum diameter of the corresponding aperture  56 . 
     An electrically conductive structural material such as a metal is then deposited within hole  60  and within the apertures  52  and  56  in the temporary layer. The deposition process can be performed by seeding the surfaces of the holes and apertures, as with a palladium salt or other deposition promoting agent, and then applying the metal structural material by plating over the seeded layer. The seeding is performed before application of photoresist layers  64  and  66 . The structural material may be applied as a continuous, integral layer  74  covering the peripheral surfaces  62  of the holes in the body and also covering the peripheral walls  54  and  58  of the apertures in the temporary layers. The structural material layer should extend radially outwardly from the apertures and covering ledges  72 . 
     The structural material should be relatively ductile. Metals selected from the group consisting of gold, copper, tin, nickel and alloys and combinations thereof are preferred. Gold is particularly preferred where the structural material layer includes only one metal. The structural material layer desirably is about 2 to about 20μ thick, most preferably between about 5μ and about 10μ thick. Alternatively, structural material layer  74  may include several sublayers such as a structural metal sublayer, a diffusion barrier sublayer and a corrosion resisting sublayer. Particularly useful sublayered structural metal combinations are as follows. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 STRUCTURAL 
                   
                   
               
               
                 METAL 
                 DIFFUSION BARRIER 
                 CORROSION RESIST 
               
               
                   
               
             
             
               
                 Copper 
                 Nickel 
                 Gold 
               
               
                 Nickel 
                 Cobalt 
                 Palladium 
               
               
                 Silver 
                 None 
                 Pd/Ni Alloy 
               
               
                   
               
             
          
         
       
     
     A further layer  76  of an electrically conductive bonding material is applied over structural metal layer  74  by conventional deposition processes such as electroplating. The electrically conductive bonding material may be a solder, braze, diffusion bonding material, eutectic bonding material, or polymer filled with electrically conductive particles. Preferably, the bonding material is adapted to become active and to bond at a predetermined elevated temperature. Some suitable bonding material compositions are indicated in Table 2 below. In Table 2, the entry under the heading “pad surface” refers to the preferred pad surface for mating with the particular bonding material. Also, conventional protective layers, such as a thin gold layer, may be applied over the conductive bonding material layer  76  to protect the same from corrosion prior to use. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 TYPE 
                 CONDUCTIVE BONDING MATERIAL 
                 PAD SURFACE 
               
               
                   
               
             
             
               
                 Solder 
                 Tin 95%, Silver 5% 
                 Tin 
               
               
                 Solder 
                 Tin 60%, Lead 40% 
                 Tin 
               
               
                 Solder 
                 Tin 63%, Lead 35%, Silver 2% 
                 Gold 
               
               
                 Braze 
                 Tin 94%, Gold 6% 
                 Gold 
               
               
                 Diffusion 
                 Gold 80%, Germanium 20% 
                 Gold 
               
               
                 Diffusion 
                 Gold 80%, Tin 20% 
                 Gold 
               
               
                 Diffusion 
                 Tin 90%, Silver 10% 
                 Gold 
               
               
                 Polymer 
                 Silver 50% in Epoxy 
                 Gold 
               
               
                   
               
             
          
         
       
     
     Structural metal  74  and bonding material  76  thus form integral vias, each such integral via including a through conductor  78  extending through layer  30 , a contact  80  on the first side of body  30  and a further contact  82  on the opposite side of the body. One such via, is depicted in FIG.  5 . The first or top-surface contact  80  is generally cup-shaped, and extends radially outwardly, in horizontal directions away from central axis  48 . The contact thus extends from the first end  86  of conductor  78  to a periphery  84  remote from the conductor and remote from central axis  48 . The generally cup-shaped contact  80  defines an opening  88  facing upwardly, vertically outwardly, away from conductor  78  and hence away from the dielectric body  30  (FIG.  4 ). In this embodiment, the entire contact is a body of revolution around the central axis  48 . Contact  82  at the second or bottom end  90  of conductor  78  has a similar configuration, but opens downwardly, in the opposite vertically outward or upward direction away from the second surface  34  of the body. 
     After the vias are formed, they are optionally filled with a flowable conductive fill material  92 , as by stenciling or squeegee application over the temporary layers and vias. Fill material  92  desirably is arranged to remain solid at temperatures below the activation temperature of the conductive bonding material  76 . Suitable fill materials include polymers and polymer precursors with conductive particles dispersed therein. The polymeric material may be a thermosetting material such as a B-staged or partially cured epoxy or a thermoplastic such as a polyimide siloxane. After filling, temporary layer  36  and  38  are removed by etching with a caustic solution. The etching process also removes any surface debris left from prior steps. Removal of the temporary layers leaves portions of the contacts spaced vertically from the surfaces of the body. Thus, cup-shaped contact  80  is spaced vertically above first surface  32  of body  30 . The spacing increases progressively from zero at the juncture of the contact with conductor  78  to a maximum at the periphery  84  of the contact. Similarly, contact  82  is spaced in the opposite vertically outward direction away from second surface  34 , the spacing increasing from zero at the juncture of the contact with conductor  78  to a maximum at the periphery of the contact. 
     After removal of the temporary layers, layers  94  and  96  of a flowable dielectric adhesive are applied on the surfaces of the body. The flowable dielectric material or adhesive is also selected to flow and form a bond with adjacent dielectric materials at a preselected activation temperature which is approximately equal to the activation temperature of conductive bonding material  76 . The adhesive may be applied by methods such as screen printing, application of a dry film preform and squeegee application. Desirably, the thickness of the adhesive layers is less than the vertical extent of the contacts  80  and  82 , so that the contacts protrude vertically beyond the adhesive layers. The adhesive may be a material commonly referred to as a “snap-cure adhesive”. In this condition, the interposer is now ready for use. 
     In a joining process according to a further embodiment of the invention, a plurality of interposers  95 , such as the interposers made in the process of FIGS. 1-6, are stacked in interleaved, alternating arrangements with a plurality of circuit panels  98  (FIG.  7 ) so that one interposer is disposed between each pair of adjacent circuit panels and so that the circuit panels and interposers are superposed on one another. The circuit panels are generally lamellar structures, i.e., platelike or sheet-like structures, having major surfaces with contact pads  100  thereon. Panels  98  also have conductors  102  (FIG. 8) extending in the horizontal directions (the directions to the left and the right and into and out of the plane of the drawing in FIGS.  7  and  8 ), as well as internal through conductors or vias (not shown) interconnecting the pads  100  on oppositely facing major faces of each panel  98  with one another. The stacking step serves to align the pads  100  with the contacts  80  and  82 , so that each pad confronts and bears on the associated contact adjacent to the periphery thereof. Each pad  100  thus bears on the associated contact  80  at locations remote from its central axis  48  and at locations where the contact is spaced vertically from body  30 . 
     In the next stage of the process, the stack is compressed and heated, as by squeezing the stack between heated platens (not shown). The compression forcibly engages the contacts of the interposer with the pads of the circuit panels and compresses the contacts  80  and  82  vertically and axially in directions parallel to the central axis  48  and towards the medial plane of the interposer body. As each contact is compressed axially it expands radially outwardly, in horizontal directions, away from central axis  48 . Thus, the cup-shaped contacts  80  and  82  tend to bend downwardly, toward the medial plane of the body, and outwardly. The action is generally similar to heading or clinching of a rivet. As each contact expands outwardly, it wipes across the surface of the associated pad  100  on the mating panel. This wiping action is particularly effective because it occurs under substantial axial or vertical loading during the compressing step. 
     This action continues as the panels move closer to one another during the compressing step, until the assemblage reaches the condition illustrated in FIG.  9 . During this process, the flowable dielectric material or adhesive in layers  94  and  96  fills the spaces between the body  30  of each interposer and the major surfaces of the adjacent circuit panels. The dielectric material flows to fill the spaces between the conductors  102  on the surfaces of the panels. This forms a substantially unitary, multi-layered structure substantially free of voids. At the same time, the electrically conductive bonding material  76  on the contacts of the interposers forms a metallurgical bond with the pads  100  of the circuit panels. The metallurgical bonding action is greatly enhanced by the wiping action which occurs during the compressing step. The fill material  92  within each contact or via structure flows into engagement with the pads  100  engaged with such via structure. The flow of this fill material is effectively constrained by the surrounding walls of the contacts. Accordingly, the fill material will not contaminate adjacent features and cause shorting. 
     An interposer fabrication process according to a further embodiment of the invention begins with a conductive, preferably metallic sheet  200  (FIG.  10 ), such as 25% hard rolled and annealed copper about 30-70μ thick, typically about 50μ thick. The metal sheet is cleaned using conventional methods such as sulfuric peroxide etch, rinsed and then passivated or protected by washing in a passivating agent such as ethylene diazol. Resist layers  202  and  204  are applied on the surfaces of the sheet, as by laminating the layers as dry films. The resist layers desirably are about  50  microns thick. The resist is cured, as by baking, and then developed in a pattern including matched pairs of cylindrical holes  206  and  208  in the top and bottom layers  202  and  204  respectively. The holes of each pair are co-axial, on a common central axis,  210 . The axes  210  are distributed in a regular grid pattern, at spacings as described above, typically 0.5 to 1.5 mm. In the next step, the sheet  200 , is etched from both sides using an isotropic etching solution. Where the sheet is formed from copper or a copper alloy, the etching solution may be a solution of CuCl and HCl. As the etch attacks sheet  200  through the holes in the resist layers it perforates the sheet to form holes  212  centered on axes  210 . Each hole  212  has a narrowest point or throat  214  adjacent the medial plane  215  of sheet  200 , i.e, the plane equidistant from the top and bottom surfaces of the sheet. Each hole  212  has walls sloping radially outwardly from throat  214  to the surfaces of the sheet. A relatively slow etch rate, using moderate HCl concentration, is preferred for forming the sloping wall configuration. Each hole  212  may have a minimum diameter of about 100 to about 250 microns, preferably about 125 microns. The maximum diameters of each hole, at the surfaces of the sheet, typically are about 125 to about 300 microns. 
     After the holes are formed, resist layers  202  and  204  are removed. Dots  216  of a further resist are applied the top surface of sheet  200 . Each such dot may be about 50 microns to about 100 microns in diameter. Resist  216  may have the same composition as resist layers  202  and  204 , and may be applied by a similar process. Dots  216  are applied in a regular grid pattern offset from the grid pattern of holes  212  and axes  210 . As further discussed below, the places covered by dots  216  will ultimately form contacts connected to sheet  200 , which is used as a plane conductor, such as a ground plane or power plane in the finished structure. Accordingly, dots  216  are positioned in a grid matching the ground or power plane connection pads on the elements with which the interposer will be employed. 
     After application of resist dots  216 , a conformal coating of a dielectric material  218  is applied over sheet  200 . The conformal coating covers the sheet  200 , including the surfaces within holes  212 . The coating has a substantially uniform thickness on the surfaces of the sheet and within the holes. Desirably, the coating is between about 20μ and about 50μ thick, preferably about 38μ. The dielectric material preferably is applied by electrophoretic deposition. In the electrophoretic deposition process, sheet  200  is electrically connected to a potential source  220  and immersed with a counter-electrode in a bath of liquid electrophoretic deposition mixture. The potential applied by source  220  deposits solid material from the electrophoretic deposition mixture. Preferred electrophoretic deposition mixtures include materials supplied under the designation Powercron cationic acrylic (700-900 series), or Powercron cationic epoxy (400-600) by the PPG Company. Preferably, the conductive sheet  200  and the counterelectrode (not shown) extend horizontally in the bath, and are disposed at a uniform spacing from one another, typically about 2 cm. The counterelectrode should be larger than the sheet, so that the counterelectrode extends beyond the edges of the sheet. The current density during the electrophoretic deposition step preferably is maintained below about 1 milliampere per square centimeter so as to minimize bubble formation in the deposited coating. Desirably, the current is maintained constant during the deposition process. The potential applied may be about 100V , and the process typically takes about 4 minutes. After the electrophoretic deposition process, the coated sheet or body is removed from the bath, washed to remove clinging undeposited solution and then baked to cure the deposited coating. 
     The deposited dielectric body  218  has a first or top surface  222  and a second or bottom surface  224 , and has holes  226  extending entirely through the body from the first surface to the second surface. Holes  226  extend through the holes  212  of sheet  200  and have walls in the form of surfaces of revolution about central axis  210 . Each hole  226  has a throat or smallest diameter point adjacent the medial plane of body  218 , i.e., approximately at the medial plane  215  of sheet  200 , and approximately midway between first surface  222  and second surface  224 . The walls of each hole  226  flare radially outwardly from the central axis  210  in the axially outward directions, away from the medial plane  215  so that the diameter of each hole  226  gradually increases to a maximum at first surface  222  and second surface  224 . Stated another way, each surface  222  and  224  includes a principal, planar portion and the holes form indentations  227  in surfaces  222  and  224 , the indentations merging with one another at medial plane  215 . Resist  216  is removed after deposition of the dielectric material, typically before the dielectric material is cured. Removal of resist  216  leaves additional indentations  228  extending inwardly from the first surface of  222  to the dielectric body to the metallic sheet  200 . 
     In the next stage of the process, a structural material, such as the structural materials discussed above is plated on all of the surfaces of the dielectric body  218 , including the interior surfaces of holes  226  and the indentations  228 . The deposition process may include the steps of seeding the surfaces, as by exposure to a liquid seeding process or by sputter deposition. For example, an adhesion-promoting layer of chromium, typically about 0.1 microns thick may be applied followed by a 1 micron layer of copper. The seed layer  230  contacts metallic sheet  200  in indentations  228  and hence is electrically connected to the metallic sheet. The remainder of the structural material is then electroplated over seed layer  230  as by connecting an electroplating potential source  231  to metal sheet  200  and immersing the assembly in an electroplating bath with a counterelectrode (not shown). The structural material conforms to the shape of the dielectric body. As the structural material is deposited, it completely fills the throats or narrowest points of holes  226  so as to form a solid conductor  233  extending through each hole  226 . However, the structural material does not completely fill those portions of the holes or indentations adjacent surfaces  222  and  224  of the body. Instead, the structural material is deposited as a layer on the outwardly curving walls of holes  226  so as to form an outwardly flaring, horn-shaped shell  232 , integral with conductors  233 , within each indentation  227  at surfaces  222  and  224 . Similarly, indentations  228  are partially filled by the deposited material, which forms a similar outwardly flaring shell  234  at the opening of each such indentation on first surface  222 . The structural metal also electrically connects each of the outwardly flaring shells  234  to the plane conductor or sheet  200 . The structural metal additionally forms planar structures  236  covering the principal planar portions of surfaces  222  and  224  of the dielectric body. 
     In the next stage of the process, an additional resist  238  is applied over the structural metal layer. The structural metal also electrically connects each of the outwardly flaring shells  234  to sheet  200 . Once again, an electrical potential may be applied through sheet  200 , and hence applied to the structural metal by way of the shells of layers  234 . For example, this resist may be deposited to a thickness of about 25 microns. One suitable resist is sold under the designation EAGLE electrophoretic resist by the Shipley company of Marlborough, Mass., and can be deposited to the desired thickness in about 3 minutes at about 200 volts. This resist is then cured by application of a cellulosic coating, drying and baking. Resist  238  is then exposed and developed in the conventional manner, using photographic exposure on both sides. 
     The photographic patterning is arranged to provide generally X-shaped openings  240  (FIG. 15) in resist layers  238 . These openings are in registration with the shells  232  at through conductors  233  and in registration with the potential plane shells or vias  234 . The openings  240  at each shell  232  is symmetrical about the central axis  210  of the shell. Each opening  240  includes a central zone  241  adjacent the central axis  210  and further includes slots  242  extending radially outwardly from central region  241 , away from the central axis  210 . The resist layer thus has projections  244  extending inwardly towards central axis  210 . These projections start at locations on the flat metallic sheet  236  and extend radially inwardly to locations within the shells  232 . Thus, the resist tabs  244  extend axially or vertically downwardly towards the medial plane  215  of the interposer body. Similar X-shaped openings  240  are provided at shells  234 . As best appreciated with reference to FIG. 15, the through holes in the dielectric body, and thus the central axes  210 , are disposed in a regular, rectilinear grid pattern of rows and columns. Slots  242  are arranged on diagonals of this grid. An electrically conductive bonding material, such as those discussed above is then plated onto the structure, within openings  240 . The bonding material thus covers portions of shells  232  and  234  and also covers minor portions of the planar structures  236 . At this stage of the process, all portions of the structural metal are still electrically continuous with sheet  200 , and hence an electrical potential for plating can be applied through sheet  200  for electroplating. 
     Resist  238  is then stripped and the structural metal is etched. During this etching step, the bonding material  246  acts as an etching mask. Thus, the portions of the structural metal which were previously covered by resist layers  238  are removed. This exposes the planar parts of first surface  222  and second surface  224  of the dielectric body  218 , and leaves contacts  250  having shapes corresponding to the X-shapes of windows  238 . Contacts  250  are provided at both ends of each through conductor  233 . As best seen in FIG. 17, each contact  250  includes a generally cup-shaped central portion  252 , substantially in the form of a body of revolution about central axis  210 , having a rim or outer edge at radius Rc from the central axis. The contact further includes four tabs  254  projecting radially outwardly from the rim of the cup-shaped central portion, each such tab having a tip  256  at radius R t  from axis  210 . For contacts disposed in a rectilinear grids at a pitch or contact-to-contact spacing along the rows and columns of the grid between about 0.5 and 1.0 mm (about 20 to 40 mils), R t  typically is between about 100μ and about 230μ. Stated another way, the maximum diameter of each contact, measured across the tips of oppositely-directed tabs, is less than about 0.5 mm and desirably less than about 400 μm. Preferably, R c  is about 40 to about 100μ. The width W t  of each tab, typically is about 25 to about 50μ and increases towards the central portion of the contact. The central portion  252  of each contact  250  is disposed within one of the indentations  227  in the surfaces  222  and  224  of the dielectric body, whereas the tabs  254  project vertically or axially outwardly from the central portions, and extend outwardly from the indentations in the surfaces to the surrounding planar portions of body surfaces. Tabs  254  extend along the diagonals to the rectilinear grid of axes  210 . The tabs and central portions include a continuous structural metal layer  258 , including portions originally formed as shells  232  and other portions originally included in the planar structures  236 . The structural metal layer  258  of each contact  250  is continuous with the structural metal of the associated through conductor  233 . The tabs and the central, cup-like portions slope radially outwardly, away from axis  210  in the vertically or axially outward direction, away from the medial plane of the body. In the same manner, the shells  232  are converted into plane-conductor contacts  252  (FIG.  16 ). Each such plane-conductor contact has substantially the same configuration as the through conductor contacts  250 , but is metallurgically bonded to the plane conductor or sheet  200 . 
     The interposer of FIGS. 10-17 may be assembled with circuit panels or other microelectronic components in an assembly procedure similar to that described above. In the assembly process, one of the interposers is stacked between each pair of circuit panels  260 . Circuit panels  260  have internal plane conductors  261 . Panels  260  also have signal pads  262  disposed in a regular, rectilinear grid pattern corresponding to the rectilinear grid of through conductors  233 , contact axes  210  and contacts  250  in the interposer, and also have plane conductor connection pads  264 , such as ground or power plane connection pads, at certain locations between signal pads  262 . Pads  262  and  264  are substantially square. The sides of the pads extend in the row and column direction of the array of pads and hence the diagonals are aligned with the diagonals of the array. 
     In the stacking procedure, the signal pads  262  are brought into registration with the signal contacts  250 , whereas the potential plane pads  264  are registered with plane conductor contacts  252 . When the pads are brought into registration with the contacts  250 , the tabs  254  of the contacts are aligned with the diagonals of the square pads  262 . A contact  250  is illustrated in the broken lines in FIG. 18 superposed on one pad  262 . All of the other signal contacts  250  and plane conductor contacts  252  are similarly oriented on the associated pads. Although only two panels and one interposer are illustrated in FIG. 19, the stacking step may be performed with any number of panels and interposers interleaved with one another. 
     In this process as well, the stacked elements are compressed and heated. Desirably, a vacuum is applied during the compressing step to remove air from between the panels and interposers. As the stacked elements are compressed, the pads  262  of the circuit panels bear on the peripheries of the contacts  250 , i.e., on the tabs  254  adjacent to tips thereof. As best illustrated in FIG. 20, this action tends to bend the tabs downwardly towards the medial plane  215  of the interposer, and tends to force each tab  254  into the adjacent surface  222  or  224  of the dielectric body  218 . The heat applied during this procedure softens dielectric  218  and hence facilitates bending of the tabs. As the tab bends downwardly, the tip of the tab moves radially outwardly form the central axis  210  of the contact. This again causes the surface of the tab to wipe over the surface of the engaged pad. As pointed out above, the wiping action promotes formation of an effective electrical connection. Because the tabs are separated from one other at their tips, the tabs can be bent radially outwardly in this fashion without tearing or substantially cracking the structural material. Further, the individual tab tips have relatively small surface areas, and can be pushed into the underlying dielectric  218 . The cup-shaped central portion  251  of each contact tends to be more rigid than the tabs. However, the central portion is disposed in an indentation in the body surface, so that the central portion recessed relative to the tips of the tabs and relative to the principal plane of the body surface. The central portion therefore does not engage the pad and does not substantially impede bending of the tabs. The plane conductor contacts  252  operate in the same manner. The heat applied during the process also activates the conductive bonding material  246  so as to form a permanent connection between the tabs of the contact and the associated pad. 
     The dielectric material on the interposer desirably flows to an extent sufficient to fill spaces between the interposers and panels, including spaces around the raised conductors on the panel surfaces. The dielectric material of the interposer preferably also adheres to the materials of the circuit panel. To promote adhesion, the dielectric material may be only partially cured during the interposer fabrication step. Curing of the dielectric material may be completed during the compression and heating steps of the assembly process. 
     Numerous variations and combinations of the features described above can be utilized without departing from the present invention as defined by the claims. In one such variant, the surfaces of the contacts which engage the pads may be provided with small bumps or asperities to provide a scraping action during the wiping motion. Alternatively or additionally, asperities can be provided on the contact pads of the circuit panels. The asperities can be formed from the structural material, or preferably, from the electrically conductive bonding material on the surfaces of these elements. 
     Although the interposers typically are used in engagement with circuit panels to form multi-layer circuitry, they can also be employed with other microelectronic elements, such as semiconductor chips, active or passive components, flat flexible cables and the like. The components incorporating the contacts are referred to herein as “interposers” because the components commonly used between other elements, but the interposers need not be configured for such use. For example, an interposer may have only one surface bearing contacts, and the contacts may be linked to a plane conductor, to through conductors, or other conductors on or within the interposer. The interposer body can be integral with the body of a microelectronic component such as a semiconductor chip, a printed circuit board, a ceramic or other multi-chip module base, a flex cable, a chip capacitor or other microelectronic components. For example, as illustrated in FIG. 21, an interposer body  300  includes a dielectric layer  302  deposited on the front surface  304  of a semiconductor chip  306 . Dielectric sheet  302  is provided with holes  306  in alignment with the connection pads  308  of the chip. Contacts  310 , similar to contacts  250  discussed above with reference to FIGS. 10-20, are provided at holes  306 . Each contact  310  is permanently connected to the associated connection pad  308  of the chip, and hence connected to the internal electrical conductor  312  of the chip. For example, the contact may be metallurgically bonded to the connection pad of the chip during a deposition process used to form the contact. The exposed surface  314  of dielectric layer  302  constitutes the first or contact-bearing surface of this single-sided interposer. In use, this first surface is juxtaposed with a surface of a substrate or other electrical component, and the contacts are engaged with connection pads on the substrate and bonded thereto using a process substantially as discussed above. The contacts in this structure as well may incorporate electrically conductive bonding material, and the dielectric layer  302  may incorporate flowable dielectric materials, adhesives or the like. 
     In other variants of the invention, contacts having outwardly projecting tabs may include more than four or fewer than four tabs. Also, the tips of the tabs are positioned above the surface of the interposer body, with a gap therebetween to facilitate downward bending of the tabs. 
     The structures illustrated in FIGS. 22-25 have contacts  425  incorporating such gaps. Each contact  425  includes a central portion  451 . Each such central portion is generally in the form of a hollow body of revolution having a central axis  410  perpendicular to the surface  422  of the interposer and a generator curving away from the central axis. The central portions of the contacts merge with tubular through conductors  480  adjacent the medial plane  415  of the interposer body. A small rim  429  extends radially outwardly, away from axis  410 , at the top end of the central portion, remote from the medial plane. 
     Four tabs  454  are connected to the rim of each contact. The tabs extend radially outwardly, away from the central axis. Each tab has a small bump or asperity  461  adjacent its radially outermost tip  456 . As best seen in FIG. 23, the tips of tabs  454  are rounded as seen in plan view. The edges of each tab slope away from one another in the radially inward direction, away from the tip  456  of the tab, so that each tab widens and joins smoothly with rim  429  of the central portion and with the next adjacent tab. Thus, the tabs cooperatively define a structure in the form of a rounded and smooth X-shape, with curved edges at junctures between adjacent tabs, i.e., at the junctures of the tabs and rim  429 . The peripheral portions of each contact, including at least the tips of tabs  454  are spaced vertically above the surfaces of the interposer body, so that there are gaps  455  below the tips of the tabs. 
     The interposer body includes a metallic sheet or plane conductor  400  similar to the plane conductors discussed above, together with a composite dielectric layer. The composite includes a first layer  418  of an electrophoretically deposited polymer, desirably a thermosetting polymer such as the epoxy compositions discussed above, in contact with the plane conductor or sheet  400 . A second layer of a flowable dielectric material, such as a relatively heat-resistant thermoplastic material, overlies the first layer. One such flowable dielectric material which may be used is a polyetherimide sold under the registered trademark ULTEM. 
     Structures according to this arrangement can be fabricated using techniques similar to those discussed above. Thus, the interposer body can be formed using deposition techniques similar to those used to form the interposer body of FIGS. 10-20. A temporary layer similar to those discussed above with reference to FIGS. 1-9 is then applied on the surfaces-of the body and removed after deposition of the structural material of the contacts and through conductors. As in the processes discussed above, removal of the temporary layer leaves the peripheral portions of the contact, in this case including tabs  454 , spaced vertically above the surfaces of the interposer body. 
     In an assembly process, the interposer is stacked with circuit panels other microelectronic devices, and then compressed. One such panel or element  460  is juxtaposed with each surface of the interposer. The condition of the assembly immediately after stacking but before compression is shown in FIGS. 22 and 23. As discussed above, the panels have square connection pads  462 , and tabs  454  lie along the diagonals of the square connection pads. When the stacked elements are compressed, the contacts deform as indicated schematically in FIGS. 24 and 25. The tabs  454  deform radially outwardly, away from the central axis  410 , and also bend downwardly, into engagement with the adjacent surfaces  422  of the body. In this action, the tips  454  of the interposer and asperities  461  on the tabs wipe radially outwardly over the surfaces of connection pads  462 . The joining material on the contacts thus can form an effective bond between the tabs and the connection pads. The downward or axial movement of the contact periphery, particularly of tabs  454  also compensates for vertical tolerances in the assembly. That is, if the connection pads  462  of the mating elements are not perfectly coplanar, or if the contacts themselves vary slightly in height, the differences will be taken up by differences in the amount of deformation in the tabs. This contributes to the reliability of the assembly. 
     The dielectric materials of the interposer, particularly the flowable dielectric adhesive  419 , fill any minor spaces beneath the contacts and around the contacts and the connection pads. Here again, the dielectric materials also fill spaces around irregularities on the surfaces of the circuit panels. For example, where the circuit panels have conductors on their exposed surfaces, the dielectric materials desirably fill the spaces between such conductors so as to provide a substantially void-free interface. 
     In still further variants, the fill material used in the embodiment of FIGS. 1-9 may be omitted or, conversely, such a fill material can be included in the structure of FIGS. 10-20, as by positioning the fill material within the cup-shaped central portions  251  of the contacts. Also, the dielectric interposer body can be fabricated by other techniques. For example, the dielectric material of the body can be molded, as by injection molding, compression molding, plastisol techniques, solvent casting or other known molding techniques. The molded part may have holes and indentations shaped as discussed above with reference to FIGS. 10-20. A plane conductor may be incorporated in such a molded part, as by insert molding. As these and other variations and combinations of the features discussed above can be utilized without departing from the present invention, the foregoing description of the preferred embodiments should be taken by way of illustration, rather than by of limitation of the invention described in the claims.