NON-CONDUCTIVE END LAYER FOR INTEGRATED STACK OF IC CHIPS

Abstract: An integrated stack of layers (22) incorporating a plurality of IC chip layers (26) has an end layer (30) which is formed of dielectric material (or covered with such material). The outer surface (42) of the end layer (30) provides a substantial area for the spaced location of a multiplicity of lead-out terminals (50), to which exterior circuitry can be readily connected. In the preferred embodiment, each lead-out terminal (50) on the outer surface (42) of the end layer (30) is connected to IC circuitry embedded in the stack by means of conducting material in a hole (48) through the end layer (30), and a conductor (trace) (44) on the inner surface (40) of the end layer (30) which extends from the hole to the edge of the end layer, where it is connected by a T-connect to metallization on the access plane face of the stack (22).

Description: Non-Conductive End Layer For Integrated Stack of IC Chips Background of the Invention This invention relates to the fabrication of three- dimensional electronic packages in which a multiplicity of individual integrated circuit (IC) chips are secured together in a stack which provides a very high density electronic package. As stated in a corresponding, common assignee application (Attorney File No. ISC-30) , which is incorporated herein by reference to provide more detailed disclosure of certain process steps, the assignee of this application pioneered the use of IC chip stacks, first as modules providing photo-detector focal plane circuitry, and then as units suitable for computer memories and the like. Patents 4,525,921 and 4,646,128 relate to the stacks designed for general use as memory devices and other non-focal-plane packages. The methods used for fabricating such three dimensional (3D) IC chip stacks have become increasingly sophisticated. The three dimensional approach has been applied to both SRAM and DRAM memory chips with satisfactory results. Stacking of memory chips has reached density levels of seventy chips in a .220 inch x .520 inch x .520 inch stack, each chip having a 1 megabit memory. One problem, which applies to stacks used as memory devices and also to other non-focal-plane packages, is the difficulty of connecting exterior circuitry to the large number of conductors on the access plane of the completed stack. Focal plane chip stack modules incorporate multiplexer circuitry, which greatly reduces the number of module output connections. However, providing output connections for memory devices is a much greater challenge. There are two acknowledged orientations which represent the structural relationship of the stacked IC chips in a module to the lead-out plate, or substrate, which makes outside electrical circuitry available for connection to the multiplicity of electrical leads (terminals) which are formed on the access plane face of the module, and which lead to the IC circuitry embedded in the module. In one arrangement, the layers of the module extend in planes perpendicular to the plane of the lead-out plate, or substrate. And in the other arrangement, the layers of the module extend in planes parallel to the plane of the lead-out plate, or substrate. The lead-out plates, or substrates, may be located below, above, or along the side of, the stacked chip module. The two most common structures are described as a "sliced bread" stack, or as a "pancake" stack. Common assignee Patent No. 4,706,166 discloses a "sliced bread" stack, in which the IC chips in the stacked module are in planes perpendicular to a stack-supporting substrate. The substrate carries electrical conductors, which lead to external circuitry. The access plane of the stack faces the supporting substrate. And the electrical connections between the stack face and substrate are formed by bonding aligned solder bumps on the facing surfaces, a process which may be referred to as surface mount technology. In such a construction, the lead-out terminals are necessarily located very close to one another, a fact which creates difficulties in obtaining satisfactory lead-out connections. "Pancake" stacks comprise IC chips which are in planes parallel to a supporting substrate. The electrical leads from the many terminals on the access plane of the stack preferably are brought either to the bottom or to the top of the stack, in order to be connected to external circuitry. Such "pancake" stacks are disclosed in copending common assignee application (Attorney File NO. ISC-30) . "Pancake" stacks, as 0 distinguished from "sliced bread" stacks, are more likely to be used where a smaller number of IC chips are included in the stacked layer module, either because fewer chips are needed for a particular module, or because of limited "headroom", i.e., limited available space in which the module is located. The present invention deals primarily with the problem of connecting the circuitry of IC chips in pancake stacks with suitable lead-out terminals, which are used in connecting to external circuitry. Summary of the Invention This invention utilizes a non-conducting end, or cap, layer, which is an integral part of the stack, and which provides means for interconnecting the circuitry inside the stack with exterior circuitry. A significant result of this invention is a major increase in the real estate available for locating lead-out terminals. The inner, non-exposed surface of the cap layer carries electrical lines (traces) which lead from metalization formed on the face of the stack to openings extending through the cap layer. Such openings provide conductive paths to the outer, exposed surface of the cap layer, where terminals are formed to connect to exterior circuitry. A cap layer can be used as the top layer of the stack, the bottom layer of the stack, or as both the top and bottom layers of the stack. The cap is an insulated layer, which may be formed of dielectric material, or may be covered with dielectric material. The preferred cap layer is formed of dielectric material having a thermal expansion coefficient similar to that of the semiconductor material which constitutes the IC chip layers of the stack. Brief Description of the Drawings Figure 1, which generally corresponds to Figure 2 of the cross-referenced application (Attorney File No. ISC- 30) , is a front view of the access plane of a large stack of layers, containing a plurality of small stacks, each of which includes a cap layer and a plurality of IC chip layers; Figure 2, which generally corresponds to Figure 4 of the cross-referenced application, is an isometric view of a short stack, which has been separated from the large stack shown in Figure 1; Figure 3 is a close-up of the metalization at the center of the short stack in Figure 2; Figure 4 is a plan view of the inner surface of the cap layer; Figure 5 is a plan view of the outer surface of the cap layer; and Figure 6 is a cross-section through the cap layer showing the metalization in the holes which extend through the cap layer. Detailed Description of Preferred Embodiment The present invention is relevant to either large stacks of chips, or small stacks of chips. However, large stacks are more likely to be mounted in the "sliced bread" configuration than in the "pancake" configuration. Although small stacks might be individually fabricated, they normally will be fabricated by segmenting large stacks. The copending application (Attorney File No. ISC-30) is directed to the method of forming small stacks by creating a large stack, processing the access plane surface of the large stack, and then segmenting the large stack into a plurality of smaller stacks. In Figure 1, a large stack 20 has been prepared and processed, and is ready for segmentation. A separation tool is used on the large stack 20 to remove the short stack 22 shown in Figure 2. The front plane of the stacks, as seen in Figures 1 and 2, is the access plane used for connection to external circuitry. The short stack in Figure 2 has the shape of a rectangular parallelepiped. It contains four lead- carrying surfaces 24 formed on four active (IC) chip layers 26. Two inactive layers 28 and 30 are located at the bottom and top, respectively, of the short stack. They allow the active layers to be isolated from exterior circuitry, except for the metalization formed on the access plane. A reason for using inactive end layers is to permit T-connect engagement between each lead from the interior of the stack and metalization formed on the access plane of the stack. The large stack 20 in Figure 1 provides 9 short stacks 22, each having four active chips. The number of chips in the large stack and in the small stack can be varied to suit particular stack fabrication needs. It is assumed, however, that each of the multiple short stacks formed from a given large stack will normally be identical in size. As seen in Figures 1 and 2, the access plane of the large and small stacks has electrically conductive metalization formed thereon, in the form of numerous vertically extending buses 32, and pads 33 permitting separate access to each chip. These buses and pads are electrically connected to the circuitry embedded in the stacks, by means of a multiplicity of T-connects of the type disclosed in prior common assignee patents and applications. The term "T-connect" means that each flat lead coming from the IC portion of the layer to the layer edge is in electrical contact with a metal strip or pad which extends beyond the lead in both directions (both upwardly and downwardly in Figures 1-3) . Experience has demonstrated the value of T-connects where the more common techniques of solder bonding are not feasible. Note that the T-connect metalization strips extend onto the lower portion of the top (inactive) layer 30. Stacks of layers which include active IC chip layers, and also inactive end layers, are assembled in a stacking fixture, with a thin layer of adhesive placed between adjacent layers. The stacked layers and fixture are then placed in an oven and baked at the curing temperature for a given time. The "face" of the stack is sanded and lapped, then plasma etched to expose the metal leads on each active chip. The leads are of the order of 1 micrometer thick and 125 micrometers wide. After sufficient etching, several layers of polyimide are deposited over the stack face, covering to a depth somewhat greater than the length of the exposed metal leads. This polyimide serves as a passivating layer between the silicon chips and the metal pads/buslines deposited later in the process. After curing the polyimide layers, the face of the stack is thinly lapped to clear cured polyimide from the metal lead ends. Using photolithography (either lift-off or etch- back) and sputter deposition of metal, the pads and buslines are formed on the stack face. The metal lead interconnects that are formed are preferably T-connects. In order to segment the large stack shown in Figure 1 into a plurality of the small stacks shown in Figure 2, means must be devised to divide the full stack into several short stacks. This has been accomplished using a thermoplastic adhesive on the plane between the short stacks, heating to the adhesive 1 s softening temperature, and applying a shear force to slide one short stack off another. The adhesive bonding of the layers within the short stack is of course still rigid and unaffected by the shear force. In the short stack of Figure 2, the four active chip layers 26 are usually formed of silicon, which is a semiconductor material. The inactive upper layer, or cap, 30 is preferably formed of a non-conducting (dielectric) material. If desired, the bottom layer 28 may also be formed of the same non-conducting (dielectric) material. The cap layer 30 could be formed of the same material as that of the IC chips, and then covered with insulating material. The advantage of such an insulation-covered silicon layer would be an identical coefficient of thermal expansion (CTE) . However, it is considered preferable to use a dielectric material as the cap layer 30, selecting a material which has a CTE very similar to that to which of the IC chip material. Having similar CTE values is very important in avoiding stresses due to temperature variations to which the stacked layer module is subjected. The preferred material for cap layer 30 is aluminum nitride, which has a CTE of 4.6 ppm. This is close to the CTE of the semiconductor silicon, which is generally used as the IC chip material. The CTE of silicon is approximately 4.0 ppm. Aluminum nitride also provides good mechanical strength, and it is available in the desired layer thickness. Because the aluminum nitride (ceramic) cap layer is an integral part of the stacked layer module, it is necessary to pre-form that layer, and to pre-process the circuitry on that layer, before it is inserted into the stack, which thereafter is integrated and then subjected to various processing steps. The interconnection between the internal short stack circuitry (IC chips) and the external circuitry is made through the cap layer 30, starting with T-connects on the edge of the inner (lower) surface of the cap layer. These T-connects join the buses 32 and pads 33 to lead lines (traces) formed on the inner surface of the cap layer. Figure 4 shows the inner surface 40 of cap layer 30; and Figure 5 shows the outer surface 42 of cap layer 30. Parallel traces, or conductors, 44 on inner surface 40 each extend from edge 46, where the T-connects are formed, to one of the holes 48, which extend through to the outer surface 42. On outer surface 42, each hole 48 is surrounded by a terminal pad 50, which will be subsequently connected to exterior circuitry, e.g., by wire bonding. A significant advantage of the present invention is the substantial increase in real estate available for the lead-out terminals 50. The entire area of outer surface 42 of layer 30 is available for spacing the locations of the terminals. This means that lead-out connections, such as wire bonds, can be readily formed, without risk of creating any short circuits. Conductive connections through the holes 48 are provided by material, preferably metal, which is suitably deposited in the holes. This material must maintain electrical continuity between pads 50 and traces 44, regardless of thermal changes or other stresses. The presently preferred method of providing the conductors 52 (see Figure 6) which extend through the holes 48 is by deposition of a suitable metal on the inner walls of the holes. Providing continuous electrical conduction from the access plane T-connects, along the traces 44, and along the conductors 52 in holes 48, to the exterior terminals 50, may be accomplished by any suitable method. One such method is electroplating. Assuming top layer 30 is formed of aluminum nitride (a ceramic) , a preliminary "electroless" process must be performed, in order to form a very thin metallic "seeding" layer on the ceramic material. The seeded metal is then available as the cathode in an electroplating (electrolytic) process which is used to thicken the metal conductors to the desired extent. The traces 44, the metal conductors on the walls of holes 48, and the pads 50, may be simultaneously formed by the seeding and electroplating steps. Another possible method for providing the continuous electrical conduction on and through layer 30 is metal sputtering. That process is feasible if the layer 30 is thin enough and if the holes 48 are large enough. Before sputtering on the primary conducting metal, such as gold or copper, an adhesion metal layer, such as titanium- tungsten would be sputtered on the dielectric surface. Yet another possibility would be filing the holes 48 with conducting material. As previously stated, the use of holes 48, combined with traces 44, permits the access plane connections to be T-connects, which are considered preferable as reliable conductor intersections. However, it would be possible to extend the metalization strips on the access plane of the stack to the outer surface 42 of cap layer 30, and then use wrap-around connections leading directly to traces on the outer surface 42, thereby eliminating the need for holes 48. Such an electrical conduction method would retain the important real estate advantage of the present invention, in which the full area of outer surface 42 is available for locating lead-out terminals. If the surface having the lead-out terminals 50 is exposed, as it is on the top of the pancake stack shown in Figure 2, access of external circuitry to the pads 50 is easily obtained, e.g., by wire bonding. If external circuitry is to be connected at the bottom of the stack, surface mount technology, such as direct solder bonding, may be used, similar to that of Patent No. 4,706,166. Lead-out conductors could be provided at both the top and bottom cap layers. Forming lead-out terminals on the non-exposed end surface of the stack would have the same real estate advantage as forming such terminals on the exposed surface. Even where the exterior circuitry access is solely at the top of the stack, it may be desirable to use a ceramic layer also as the bottom layer of the stacked chip module. One of the process steps performed on the large stack (see ISC-30) is etching to remove some silicon semiconductor material from the access plane, prior to passivating the access plane with a material such as polyimide. The silicon etching process does not significantly etch the aluminum nitride material. If aluminum nitride layers are located at both the top and bottom of each short stack, a symmetrical access plane surface will remain after the etching process. Another advantage might result from the fact that separation of adjacent small stacks from a large stack would involve separation of layers formed of the same material. It should be understood that the ceramic layer or layers will not project from the access plane of the stack at the time of forming the metalization thereon, because of the planarization of the access plane accomplished by the final lapping process. From the foregoing description, it will be apparent that the method and structure disclosed in this application will provide the significant functional benefits summarized in the introductory portion of the specification. The following claims are intended not only to cover the specific embodiments disclosed, but also to cover the inventive concepts explained herein with the maximum breadth and comprehensiveness permitted by the prior art.

Claims: What Is Claimed Is; 1. An integrated stack of layers which form a rectangular parallelepiped having an access plane, and containing embedded integrated circuitry (IC) , comprising: a plurality of layers provided by IC chips, each having a multiplicity of electrical leads at the access plane; a cap layer at one end of the stack, which is formed of a dielectric material, and which has a plurality of holes extending between its inner surface and its outer surface; a plurality of traces on the inner surface of the cap layer, each leading from a hole to the access plane; a plurality of terminals on the outer surface of the cap layer each located at one of the holes; conducting material extending through each hole to connect a trace to a terminal; and conducting material on the access plane connecting each trace to an electrical lead one or more IC layers. 2. The integrated stack of layers of claim 1, in which the cap layer material has a coefficient of thermal expansion similar to the coefficient of thermal expansion of the material which forms the IC chip layers. 3. The integrated stack of layers of claim 2, in which the material forming the IC chip layers is silicon, and the material forming the cap layer is aluminum nitride. 4. The integrated stack of layers of claim 1, which also comprises: a second cap layer, which is located at the other end of the stack from the first cap layer, so that both ends of the stack are covered by layers composed of dielectric material. 5. The integrated stack of layers of claim 1, in which: continuous metalization is provided by each trace, its through-the-hole conductor, and its terminal; and such continuous metalization is formed on the cap layer by means of an electroplating process. 6. The integrated stack of chips of claim 5, in which the electroplating process comprises: electroless forming of a thin metallic seeding layer on the dielectric material; and a subsequent electrolytic plating to bring the metalization to the desired thickness. 7. An integrated stack of layers which form a rectangular parallelepiped having an access plane, and containing embedded integrated circuitry (IC) , comprising: a plurality of layers provided by IC chips, each having a multiplicity of electrical leads at the access plane; a cap layer having dielectric inner and outer surfaces; terminals formed on the outer surface of the cap layer which are adapted to be connected to external circuitry; conductors formed on the access plane and connected to the electrical leads on the IC chip layers; and conductors connecting the conductors on the access plane to the respective terminals on the outer surface of -the cap layer. 8. The integrated stack of layers of claim 7 in which: the cap layer has a plurality of holes extending from its inner surface to the respective terminals on its outer surface; a plurality of conductors formed on the inner surface of the cap layer, each of which leads from one of the holes to a conductor on the access plane of the stack; and conductors which extend through the holes in the cap layer, each connecting one on the conductors formed on the inner surface of the cap layer to one of the terminals formed on its outer surface. 9. The method of fabricating an electronic package which comprises: stacking a plurality of layers which are formed from IC chips, each having embedded IC circuitry and each having leads which extend to the layer edge; the lead-carrying edges of the layers constituting a stack access plane; forming an end layer of dielectric material having a flat inner surface and a flat outer surface; forming holes extending through the end layer; forming conductors on the inner surface of the end layer, each of which connects one of the holes to the access plane of the stack; forming terminals on the outer surface of the end layer, each of which is located at one of the holes; forming conductors which extend through the holes in the end layer, in order to provide continuous electrical conduction from each terminal to the access plane of the stack; adding the end layer to the plurality of IC chip layers; and forming an integrated stack of layers which includes the end layer.

Inventors: MIYAKE MICHAEL K, MIYAKE, MICHAEL, K.
Applicants: IRVINE SENSORS CORP, IRVINE SENSORS CORPORATION
Application year: 1993
Main classification: H01L21
IPC codes: H01L23, H01L25