Fabrication of semiconductor packages

Abstract: A heat-conductive honeycomb ceramic spacer (11) having an array of apertures therein for facilitating assembly of a semiconductor device including a plurality of semiconductor stacks (13) using a low-profile contact comprising a foil (12) with raised portions corresponding to the locations of apertures in the ceramic spacer for forming contacts with the semiconductor stacks when the spacer and the stacks are sandwiched between the foil and another conductive sheet (14). Use of such a foil also allows disconnection of defective stacks in the device which includes extra stacks to compensate therefore, according to an n - x philosophy. Solder preforms may be included on the stacks and enhanced connections made to the foil of conductive sheet by causing reflux of the solder preforms. The invention may also be applied to multi-layer device constructions.

Description: FABRICATION OF SEMICONDUCTOR PACKAGES This invention generally relates to a structure for a multi-chip semiconductor device. The growth of the electronics industry worldwide has caused a need for a great variety of semiconductor products. The packaging of the so-called semiconductor stack (hereinafter, simply "stack", defined as the produced or purchased semiconductor device of whatever desired function may be desired, without connections, protective packaging or heat-sinking structure) to produce finished devices to be used in electronics products has been a matter of great importance in the semiconductor industry. Considerations involved in the choice or design of packaging for the semiconductor device will include the number of stacks which will be required in the device, the number of connections to be applied thereto, the intended apparatus within which the package is to be used, heat dissipation requirements and the like. It is also the practice, where a power semiconductor device is required, to place a plurality of stacks within a single package for the purpose, on the one hand, of improving device reliability and specifications and, on the other hand, to provide economies in construction of the overall electronics product by minimizing the number of discrete components which must be assembled. For the semiconductor manufacturer, the question of fabrication yield is also of major concern. First, the stack fabrication process will invariably produce some imperfect or non-functional stacks. The percentage of stacks produced which are functionally acceptable is referred to as the yield. When a plurality of such stacks are included in a single semiconductor device, the yield of the manufactured device may be far less than the yield of the stack fabrication process. This is because the inclusion of only one or a few defective stacks within a device would render the entire device functionally unacceptable. Since the present state of semiconductor technology will generally provide extremely high yields in the stack fabrication process, particularly where the design of the stack is simple and contains relatively few components for relatively low current usage, the problem of reduced device yields has recently been mostly limited to the fabrication of power semiconductor devices where the number of identical, parallel-connected stacks in a single device will be particularly large. However, where chip complexity reduces stack fabrication yields, even a few stacks per device may result in unacceptably low device yields. To avoid reduced device fabrication yields a so-called (n-x) or (n-1) design philosophy has often been adopted. This means that in a device which contains n stacks of a similar type and similarly connected, x (typically 1) of the provided stacks may be defective within the performance specifications of the device even though such a defective stack or stacks may be required to be functionally disabled or removed from the circuit during fabrication. As an alternative or an addition to the (n-x) design rules, provision may also be made for the replacement of a defective stack within the device. Such an arrangement is shown for a single stack device in IBM Technical Disclosure Bulletin, Vol. 29, No. 3, August 1986, by J. A. Miraglia and J. H. Spreen, pp. 1071 - 1072. Document WO86/01031 discloses a solid state switch assembly used in a high power system. This prior art device includes a spacer made of an insulating material, with apertures for lodging semiconductor stacks, and a spider-like conductor having a central base and legs extending radially outwardly. An additional example of a structure for supporting a plurality of semiconductor stacks is illustrated in document WO86/05030, which mainly deals with the problem of compensating for dimensional differences occurring in the length of adjacent stacks. When stacks of relatively simple design are used in power semiconductor devices, however, the cost of assembling a large number of stacks and other parts tends to increase costs and tending to defeat the potential efficiencies to be obtained in the assembly of a smaller number of devices in a particular product, as indicated above. Therefore, a need exists to simplify the construction of semiconductor devices having a large number of semiconductor stacks included therein. In addition to the provision of a large plurality of parallel-connected stacks in the same semiconductor device, many special purpose devices require that the semiconductor stacks be accurately arrayed. Such applications include particle detectors, radiation sensors, microwave power receiving antennas and the like. Also, as miniaturization of power devices has increased, the arraying of the stacks has also become important to accommodate the increased criticality of heat dissipation from each respective stacks and the heat dissipation and distribution throughout the multi-stack device. Accordingly the present invention provides a structure as disclosed in claim 1 for supporting and locating an array of semiconductor stacks, which includes: a ceramic spacer having a predetermined pattern of apertures for receiving said stacks, said apertures defining locations for individual ones of said semiconductor stacks; a plurality of semiconductor stacks positioned within said apertures of said ceramic spacer; a conductive foil; a common conductor plate having an area coextensive with at least a portion of said array of said semiconductor stacks. The structure is characterised in that said conductive foil is made of a resilient material and has dimples corresponding to at least some of said apertures on said ceramic spacer, whereby each of said dimples urges a corresponding one of said semiconductor stacks into contact with said common conductor plate. In order that the invention may be fully understood preferred embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a section through a semiconductor structure according to the invention; Figure 2 is an exploded view of the structure of Figure 1, with some parts omitted for clarity; Figures 3A and 3B are views of a portion of the structure of Figure 1, showing a preferred technique of disconnection of defective stacks; Figure 4 is a sectional view of a semiconductor structure according to the invention; Figure 5 and Figure 6 are plan views of two embodiments of a ceramic spacer; Figure 7 is a perspective view of a multi-layer structure; and Figure 8 is a partial plan view of the ceramic spacer arrangement of Figure 7. Referring now to the drawings, and more particularly to Figure 1, there is shown a perspective view of a cut-away portion 10 of a semiconductor structure according to tie invention. In the section, it is seen that the device as assembled includes a heat-conductive ceramic honeycomb spacer 11 with apertures 19 therein for locating semiconductor stacks 13. The ceramic spacer and the stacks are sandwiched between a formed resilient foil 12 and a common conductor plate 14. A further plate 15, which may be conductive to provide a strong mechanical contact to the foil and for external connections may also be provided. Common conductor plate 14 may have an extension, as shown, to provide for a similar mechanically strong connection to the device. A hole 17 and a compression fastener may also be provided to assure a mechanically prestressed construction of the device. Alternatively, surface 11a of the ceramic spacer can be formed with a rough texture or roughened after forming to provide a good surface for compression bonding of the layers of the device with, for instance, epoxy or a reflowed solder paste. The structure is therefore suitable to both hermetic sealing and compression sealing. As shown more clearly in Figure 2, the formed foil member 12 is fabricated with dimples or other suitably formed raised portions 12a in a pattern corresponding to the pattern of apertures in the ceramic spacer. These raised portions 12a will also preferably be formed with small holes 12b in order to release gas pressure during fabrication of the device and to provide greater bonding area if connections to the stack are to be enhanced by the use of solder preforms, as will be discussed below. The foil is preferably of copper and should be of a thickness which is sufficient for the raised dimples to retain their shape and exert a substantial spring force against the stack within the aperture which, in turn urges the stack against the common conductor plate 14. The shape of the dimples in the foil also provide thermal stress relief. The foil may also be plated, if desired with silver, gold or the like to enhance the connection and reduce contact resistance. As further shown in Figure 2, the stack, itself may include one or more contact structures 13a and 13b of a material such as Molybdenum. Alternatively, as shown in Figure 1, the stack may be simply a silicon slug which has a desired semiconductor structure formed therein. The stack may also have one or more solder preforms 20a and 20b which may be separate, as illustrated, or formed on the stack. Since the semiconductor stack may be subjected to relatively high temperatures and thermal cycling, either of which may reduce the spring-like function of the raised portions, it is deemed desirable to use such solder preforms and to cause them to be reflowed during assembly of the device to provide a mechanically secure contact of low resistivity which is substantially less subject to thermal aging than the spring contact of the foil alone. As can also be seen in the exploded view of Figure 2, the apertures in the ceramic spacer are arranged in a predetermined pattern which will assist in the thermal management of the device as will be further discussed below. The apertures themselves also have a groove 18 which assists in the further working of the foil during disconnection of defective devices as will now be described in connection with Figures 3A and 3B. In Figure 3A, it is assumed that after assembly, the device is tested and the illustrated stack is found to be defective. Defects are typically of the form where the stack is either shorted or open-circuited. Such defects are easily located by the application of current in both directions across the device and the defective stack located by a distinct thermal response. The use of a thermally sensitive material, such as liquid crystal, will allow detection of the defective stack by inspection in a manner well-known in the art. The technique of defective stack detection, itself, is therefore not considered to form a part of the present invention. Once a defective stack has been located, it can be disconnected from the device by forming a cut 31 with a suitable cutter such as a die or even manually. In either case, groove 18 provides assistance as a guide and enhances a clean cut against the edge of the ceramic spacer surface. Due to the spring-like performance of the raised dimple in the foil, after the cut is made, the cut portion 32 will be raised above the surface of foil layer 12 and can be easily removed by suitable mechanical means or even manually, leaving only residue 33 of the solder preform, if used. Figure 4 is a sectional view of the finished device similar to Figure 1. In this case, stack 13c having been found defective and the foil cutting and removal operations described above having been done, no raised portion of the foil structure appears in the centrally illustrated aperture of the device. It is possible to leave the raised portion of the foil in place after cutting but this is not preferred since, unless material is removed to form a kerf (e.g. performing the cut by sawing), the distance between foil portions will be small and made smaller by compression of the device. Connection could then be possibly reestablished by solder reflux or mechanical deformation of the raised portion of the foil. The construction discussed above can be implemented with equal ease for any desired array of stacks. For example, Figure 5 shows a rectangular configuration and Figure 6 shows a circular configuration. Stack locations are indicated at 19 of both Figures. If desired, holes for compression devices 51, 61 can also be formed in the spacer either before or during assembly to provide a relatively even compression force to enhance heat transfer. However, it may be advantageous in the circular arrangement of Figure 6 to omit the central stack 19a and to apply compression only at location 62. Separation of stack locations 19 is a matter of thermal design and well within the level of one skilled in the art in view of the present disclosure. In this embodiment the heat flow takes cognizance of the fact that the heat flow within the stacks takes places toward both the anode and cathode of each stack. The heat flow may also be influenced by the orientation of the stack and the device containing the stack. When cooling of the device can take place from both sides of the device, the thermal conductivity of the ceramic is of less importance in comparison to the heat conductivity of the metal foil or common electrode, which will rapidly assume a relatively isothermal condition and from which heat can be readily removed. Therefore, in such a case, it is not necessary that the heat conductive ceramic spacer be directly in the thermal path for cooling of the stack anodes or cathodes. However, when heat can only be removed from one side of the device due to physical requirements such as combination with other electrical structures or, more particularly, a second embodiment of the invention to be described in detail below and with reference to Figure 7, the ceramic provides a thermal path for heat from the uncooled side of the array to the cooled side of the array. The thermal design of the device is also for the purpose of maintaining all stacks at approximately the same temperature since the conduction properties of semiconductors changes significantly with temperature although this temperature equalization is also largely a function of the copper or other metal in the conductors. Therefore, temperature equalization can usually be adequately achieved by fairly regular spacing and assuring that the area of the apertures does not exceed the remaining area of the ceramic. The ratio of aperture area to the remaining area of the ceramic spacer should preferably be adjusted in consideration of the thermal conductivity of the ceramic, the thickness of the spacer and any asymmetry of heat dissipation in the stacks. The preferred material for the ceramic spacer is alumina which is electrically insulative, exhibits good thermal conductivity and is relatively inexpensive. Other more expensive materials with a much higher thermal conductivity, such as aluminium nitride, are also usable and are preferred for some implementations. Consider an implementation where the stack array was formed with another electrical device such as a transformer or another stack array as in the embodiment of Figure 7, which is also a source of heat transferred to the device through common bus 15. If cooling for the combination device could only be provided at common electrode 14, the use of a ceramic of higher thermal conductivity, such as aluminium nitride, mentioned above, would allow formation of a thermal path including, in sequence, the heat source, the ceramic spacer of the device and a heat sink, thereby permitting heat sinking of that heat source through the stack array. Figure 7 shows a variation of the device. In this embodiment, the device is symmetrically constructed as a multi-layer device. As illustrated, a conductive compression plate 15 is used as a central common anode contact while common conductor plates 14 are arranged on the outside of the device. This configuration could be reversed with common conductor plate 14 forming the central cathode connection. However, the configuration shown is deemed preferable to improve heat dissipation from the stacks. Since the central member is conductive, insulators must be provided around compression members such as 16 in Figure 1, if used. Alternatively, a clip-like compression device 70 could be used. In either case, the compression device can also be used to provide a connection between the outside conductors. It should be noted with regard to the embodiments of both Figure 1 and Figure 7 that the common conductor 14 can also be formed as a foil with raised portions and that the stacks can be oriented in either direction as long as the direction remains consistent among the stacks which are connected in parallel. However, heat path characteristics of the stacks may require some asymmetry of design. It should also be noted that conductors 12, 14 and 15 need not be unitary and any or all of them may be applied to the device as a plurality of strips 83, 84 or other geometrical configurations, as may be desired or found to be advantageous. Some of the strips can also be used as control electrodes, allowing the invention to be readily applied to transistors, Silicon Controlled Rectifiers, thyristors and other devices as well as diode stacks, either throughout the device or in combination, such as by inclusion of a series voltage regulator with power rectifiers. If necessary to provide proper electrical connections, additional thermally conductive insulators can be provided as layers in the device such as at location 85 between conductor 15 and foil 12. Either or both of conductor 15 and foil 12 can be patterned as may be desired to form the required connections. Finally, the staggered locations of the stacks in the layers of the embodiment of Figure 7 allows good thermal management in a particularly compact form of the overall device. Figure 8 shows a plan view of this staggering of stack locations; the solid circles 19′ indicating stack locations in one layer and the dashed circles 19″ indicating stack locations in the other layer. To form a single high power rectifier, the relative orientations of the stacks is shown by the + and o designations in any one of the four quadrants illustrated. In this regard, the construction according to the invention can also be used to form a circuit of several devices, each comprising a plurality of stacks, within the same device. As illustrated in Figure 8, if the conductors are suitably divided at 81, 82, 83 and 84 and the polarities of the stacks correspondingly reversed, a bridge rectifier can easily be fabricated. In this case, the relative orientations of the stacks would be as illustrated in the four adjacent quadrants of Figure 8. Conductors 15a and 15b are provided to connect two stack groups in each layer, respectively, and divided common electrodes 14a, 14b and 14c, 14d are respectively connected in a manner such as conductor 70. Of course, a much greater number of stacks in each quadrant could be used, as required to provide a device of the desired specifications, which the present invention makes economically and technically feasible. Other structures and circuit configurations are similarly possible. The structure may be fabricated by placing or otherwise affixing (e.g. with an insulative adhesive) the stacks either within the honeycomb spacer or on the common conductor and then assembling the spacer and the common conductor. The assembly order is not critical and several different orders of assembly can be implemented. The foil member is then located on the spacer and the assembly is compressed at approximately 100 psi to form a gas-tight seal and all parts other than compression plate 15 are either bonded or fastened. Reflux of the solder preforms is also done at this time and the entire assembly and bonding process can be accomplished in a single pass. At this point, the device is sufficiently complete for burn-in and testing. Defective stacks are located and removal from the circuit is done as discussed above. Then the compression plate is bonded of fastened into place, external connections made, for instance, by a lead frame and/or welding and final encapsulation is performed. In view of the above, in at least a preferred embodiment a structure has been provided which provides for economical assembly and good thermal management in a device having a plurality of semiconductor stacks and which may be conveniently implemented in a variety of forms to produce unitary devices or circuits having high performance at a high manufacturing yield. Therefore, in at least a preferred embodiment there has been described a structure and method for facilitating fabrication of a semiconductor device including a plurality of semiconductor stacks and the disconnection of defective semiconductor stacks. The structure allows accurate arraying of semiconductor stacks within the semiconductor device, and provides means for thermally managing the semiconductor stacks during operation. The stack array structure is of sufficient thermal conductivity that other sources of heat such as parasitic heating from other components electrically or physically connected to the stack array can be cooled by forming a thermal path and heat sinking such other sources of heat through the stack array.

Claims: A structure (10) for supporting and locating an array of semiconductor stacks (13), including: a ceramic spacer (11) having a predetermined pattern of apertures (19) for receiving said stacks, said apertures defining locations for individual ones of said semiconductor stacks; a plurality of semiconductor stacks positioned within said apertures of said ceramic spacer; a conductive foil (12) a common conductor plate (14) having an area coextensive with at least a portion of said array of said semiconductor stacks, characterised in that said conductive foil (12) is made of a resilient material and has dimples (12a) corresponding to at least some of said apertures (19) on said ceramic spacer (11), whereby each of said dimples urges a corresponding one of said semiconductor stacks (13) into contact with said common conductor plate. A structure as claimed in claim 1, wherein at least one of said common conductor plate (14) and said resilient conductive foil (12) is bonded to said ceramic spacer. A structure as claimed in claim 1 or 2, wherein at least one of said semiconductor stacks (13) includes at least one solder preform (20a, 20b), used to form a connection between said semiconductor stack (13) and at least one of said common conductor (14) and said resilient conductive foil (12). A structure as claimed in claim 3, further including a further array of stacks (13); a further ceramic spacer (11) having a predetermined pattern of apertures (19) therein for receiving said further stacks (13); at least one further resilient conductive foil (12) having dimples (12a) coincident with at least some of said apertures (19) of said further ceramic spacer (11); and a plurality of semiconductor stacks (13) positioned within said apertures (19) of said further ceramic spacer (11). A structure as claimed in claim 4, further including a further common conductor plate (14) having an area substantially coextensive with at least a portion of said further array of stacks (13). A structure as claimed in claim 5, further including at least one electrical connection between selected ones of said resilient conductive foil (12), said further resilient conductive foil (12), said common conductive plate (14) and said further common conductive plate (14). A structure as claimed in claim 6, wherein at least one of said semiconductor stacks (13) includes at least one solder preform (20a, 20b) for forming a connection between said semiconductor stack (13) and at least one of said common conductor plate (14) and said resilient conductive foil (12).

Inventors: MELNICK PAUL JOHN, MENNELLA ANTHONY JOSEPH JR, MEYER HERMAN PAUL, MELNICK, PAUL JOHN, MENNELLA, ANTHONY JOSEPH, JR., MEYER, HERMAN PAUL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: H01L25
IPC codes: H01L21, H01L23, H01L25