Abstract:
A method and apparatus are provided for implementing an enhanced three dimensional (3D) semiconductor stack. A chip carrier has an aperture of a first length and first width. A first chip has at least one of a second length greater than the first length or a second width greater than the first width; a second chip attached to the first chip, the second chip having at least one of a third length less than the first length or a third width less than the first width; the first chip attached to the chip carrier by connections in an overlap region defined by at least one of the first and second lengths or the first and second widths; the second chip extending into the aperture; and a heat spreader attached to the chip carrier and in thermal contact with the first chip for dissipating heat from both the first chip and second chip.

Description:
[0001]    This application is a divisional application of Ser. No. 14/184,868 filed Feb. 20, 2014. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with Government support under Contract No. B601996 awarded by the United States Department of Energy. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to the data-processing field, and more particularly, to a method and apparatus for implementing an enhanced, three-dimensional (3D) semiconductor stack. 
       DESCRIPTION OF THE RELATED ART 
       [0004]    Three-dimensional (3D) semiconductor stacking including 3D semiconductor memory stacking is an emerging technology. A 3D semiconductor memory stack advantageously can include a processor die, also known as the logic die or master die in a stack with a slave die, such as a slave DRAM stack. 
         [0005]    Micron Technology has recently proposed a hybrid memory cube (HMC) in which four to eight dynamic random-access memory (DRAM) die are stacked one above the other using through-silicon-via (TSV) technology. This space- and energy-efficient group of DRAMs is then connected to a controller device, forming either a five-chip or nine-chip stack. A Hybrid Memory Cube Consortium (HMCC) is backed by several major technology companies. 
         [0006]    The HMC is a very-high-bandwidth device, and promises to be a compelling technology, although it is a low-memory-capacity replacement for a hub-chip memory, Dual Inline Memory Module (DIMM) with a controlled module, for example IBM&#39;s SuperNova DIMM. 
         [0007]    A High-Bandwidth DRAM (HBM) is a similar stack of DRAMs being developed at JEDEC, an independent semiconductor engineering trade organization and standardization body. Although high bandwidth, the HBM is also a low-memory-capacity replacement for a buffered DIMM, or DIMM with address, data, and clock redrive. 
         [0008]    Both the HMC and the HBM are expected to be placed adjacent to a processor on a substrate having the capacity for high-density wiring. Both approaches to 3D stacking of memory require a high-wiring-density substrate between the data storage DRAM and the data processing, to carry the many signals expected between the processor and memory stack. 
         [0009]    U.S. Pat. No. 8,343,804, issued Jan. 1, 2013 to Paul W. Coteus et al. and assigned to the present assignee, discloses a method and structure for implementing multiple different types of dies for memory stacking. In  FIGS. 1A , and  1 B, a master-slave structure comprises a printed circuit board (PCB), a master die, and a plurality of slave dies. For example, in the illustrated prior-art master-slave, only the bottom die, labeled “master” in the stacked package, communicates to the outside of the package, thereby to save standby power by allowing the shutting down of circuitry in other dies that are not required to operate.  FIG. 1B  illustrates the bottom master die, which includes a plurality of arrays and a periphery segment centrally located between the arrays. Multiple through-silicon-vias (TSVs) are placed within the periphery segment. 
         [0010]    U.S. Pat. No. 8,343,804, issued Aug. 20, 2013 to Paul W. Coteus et al. and assigned to the present assignee, discloses a method and circuit for implementing stacking to distribute a logical function over multiple dies in through-silicon-via stacked semiconductor devices. Each die in the die stack includes predefined functional logic for implementing a respective predefined function. The respective predefined function is executed in each respective die and a respective functional result is provided to an adjacent die in the die stack. Each die in the die stack includes logic for providing die identification. An operational die signature is formed by combining a plurality of selected signals on each die. A die signature is coupled to an adjacent die using TSV interconnections where it is combined with that die signature. 
         [0011]    A need exists for an efficient and effective method and apparatus for implementing an enhanced three dimensional (3D) semiconductor stack. It is desirable that a master die be connected directly to a chip-carrier substrate that provides power and carries interface signals. It is desirable to provide such a 3D semiconductor stack structure that preserves a simple stacked DRAM without needing to pass power connections, signal connections, or heat from an associated master die through the DRAM stack, or to impose area or power limitations on the master die. 
       SUMMARY OF THE INVENTION 
       [0012]    A principal aspect of the present invention is to provide a method and apparatus for implementing an enhanced three dimensional (3D) semiconductor stack. Other important aspects of the present invention are to provide such method and apparatus substantially without negative effects, and that overcome many of the disadvantages of prior-art arrangements. 
         [0013]    In brief, a method and apparatus are provided for implementing an enhanced, three-dimensional (3D) semiconductor stack. A chip carrier has an aperture of a first length and first width. A first chip has at least one of a second length greater than the first length or a second width greater than the first width; a second chip attached to the first chip, the second chip having at least one of a third length less than the first length or a third width less than the first width; the first chip attached to the chip carrier by connections in an overlap region defined by at least one of the first and second lengths or the first and second widths; the second chip extending into the aperture; and a heat spreader attached to the chip carrier and in thermal contact with the first chip for dissipating heat from both the first chip and second chip. 
         [0014]    In accordance with features of the invention, the first chip includes a master die of an inverted master-slave 3D semiconductor stack. A processor or master die is connected directly to the chip carrier substrate that provides power and carries interface signals to a slave DRAM stack. 
         [0015]    In accordance with features of the invention, the master die is placed on a wiring substrate of the chip carrier, which provides the power and all signal connections. 
         [0016]    In accordance with features of the invention, the second chip extending through a hole in the chip carrier includes a DRAM stack that is attached directly to the master die. A thermal path exists between the master die and DRAM stack, and the DRAM stack is effectively cooled through the master die. 
         [0017]    In accordance with features of the invention, the chip carrier aperture optionally includes a blind hole with the DRAM stack attached directly to the master die before assembly with the chip carrier. The DRAM stack is set in the blind hole cavity in the chip carrier which protects it. This also allows the lower wiring layers of the chip carrier to carry signals to an area array of contacts covering substantially the entire bottom surface thereof. 
         [0018]    In accordance with features of the invention, the 3D semiconductor stack includes the master die, which is oversized on all four sides, thereby eliminating the need for through-silicon-vias. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein: 
           [0020]      FIG. 1  is an exploded perspective view not to scale of example structures for implementing an enhanced three dimensional (3D) semiconductor stack in accordance with preferred embodiments; 
           [0021]      FIGS. 2A ,  2 B,  2 C, and  2 D show an example assembly sequence for implementing the example enhanced three dimensional (3D) semiconductor stack of  FIG. 1  in accordance with preferred embodiments; 
           [0022]      FIGS. 3A ,  3 B,  3 C, and  3 D show another example assembly sequence for implementing the example enhanced three dimensional (3D) semiconductor stack of  FIG. 1  in accordance with preferred embodiments; 
           [0023]      FIG. 4  is an exploded perspective view not to scale of example structures for implementing a second enhanced three dimensional (3D) semiconductor stack in accordance with preferred embodiments; 
           [0024]      FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E show an example assembly sequence for implementing the example second enhanced three dimensional (3D) semiconductor stack of  FIG. 4  in accordance with preferred embodiments; and 
           [0025]      FIG. 6  illustrates a temperature distribution of example enhanced three dimensional (3D) semiconductor stacks in accordance with preferred embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    In the following detailed description of embodiments of the invention, reference is made to the accompanying drawings, which illustrate example embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. 
         [0027]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0028]    In accordance with features of the invention, a method, and structures are provided for implementing enhanced three dimensional (3D) semiconductor stacks. 
         [0029]    Having reference now to the drawings,  FIG. 1  is an exploded perspective view not to scale of example structures for implementing an enhanced three dimensional (3D) semiconductor stack generally designated by the reference character  100  in accordance with preferred embodiments. As shown, an imaginary Cartesian coordinate system  102  defines x, y, and z directions. 
         [0030]    The 3D semiconductor stack  100  includes a lid or heat spreader  104 , a processor die  106  that is also called a logic die or a master die, a chip carrier or substrate  108  having a through hole  112 , and a DRAM stack  110 . Chip carrier  108 , which provides all power and signal connections, may be a conventional, low-cost, thin-core or coreless organic substrate with additive wiring layers, a known technology, such as practiced by Kyocera. 
         [0031]    Through hole  112  in chip carrier  108  is slightly larger in the x and y directions than DRAM stack  110 , so that DRAM stack  110  may nest in through-hole  112 . Through-hole  112  optionally includes one or more corner reliefs  114  to minimize stress concentration. 
         [0032]    Master die  106  is not thinned in the z direction, thereby eliminating the cost of a thinning process. In the x and y directions, master die  106  is provided intentionally larger than the through hole  112 , so that the periphery of master die  106  may engage arrays  116 ,  118 ,  120 , and  122  of connection elements that are fabricated on the positive-z-facing surface of chip carrier substrate  108 , around the edges of through-hole  112 . The positive-z-facing surface of chip carrier  108  comprises a plurality of arrays of connection elements around the periphery of through-hole  112 , such as connection arrays  116 ,  118 ,  120 , and  122 , to which master die  106  will be attached. 
         [0033]    Decoupling capacitors  124  will also be attached to chip carrier  108 , using pads (not shown). The positive-z-facing surface of DRAM stack  110  comprises an array of connection elements  126  that is used to attach DRAM stack  110  to the negative-z-facing surface of master die  106 . 
         [0034]    Referring to  FIGS. 2A ,  2 B,  2 C, and  2 D, there is shown an example assembly sequence for implementing the example enhanced three dimensional (3D) semiconductor stack  100  of  FIG. 1  in accordance with preferred embodiments. 
         [0035]    Referring now to  FIG. 2A , the 3D semiconductor stack  100  is shown in a different view illustrating additional features generally designated by the reference character  200 . As shown in  FIG. 2A , a plurality of arrays  202 ,  204 ,  206 , and  208  of connection elements are fabricated on the periphery of negative-z-facing surface of master die  106 , which includes a central area  210 . The negative-z-facing surface of heat spreader  104  comprises a central area  212  in which master die  106  and decoupling capacitors  122  nest, and a peripheral area  214  that will, in the assembly process shown presently, be attached to the positive-z-facing surface of chip carrier  108 . 
         [0036]    The negative-z-facing surface of the chip carrier substrate  108  comprises an array of connection elements  211 , such as gold-plated pads for use with a land-grid-array connector that connects the package  100  to a main circuit board (not shown). The thickness of chip carrier  108  must be large enough so that, after assembly, the negative-z-facing surface  216  of DRAM stack  110  does not protrude beyond the negative-z-facing surface of chip carrier  108 . 
         [0037]    Referring now to  FIG. 2B , a first step generally designated by the reference character  220  in the first assembly sequence of the 3D semiconductor stack  100  is shown. The first step  220  in the first assembly sequence is to solder-attach the master die  106  and the decoupling capacitors  122  to the chip carrier substrate  108 . Specifically, the arrays  116 ,  118 ,  120 , and  122  of connection elements on the positive-z-facing surface of chip carrier substrate  108  are solder-attached to the corresponding arrays  202 ,  204 ,  206 , and  208  of connection elements on the negative-z-facing surface of master die  106 . 
         [0038]    Referring now to  FIG. 2C , a second step generally designated by the reference character  230  in the first assembly sequence of the 3D semiconductor stack  100  is shown. The second step  230  of the first assembly sequence is to solder-attach the DRAM stack  110  to the master die  106 , as shown in  FIG. 2C , with the array  126  of connection elements on the plus-z-facing surface of the DRAM stack  110  (see  FIG. 1 ) being solder-attached to the array  211  of connections elements on the negative-z-facing surface of the master die  106 . 
         [0039]    Referring now to  FIG. 2D , there is shown a next or final step generally designated by the reference character  240  in the first assembly sequence of the 3D semiconductor stack  100 . The final step  240  of the first assembly sequence is to attach the heat spreader  104  to the chip carrier  108 , by conventional techniques well known in the art. 
         [0040]    Referring to  FIGS. 3A ,  3 B,  3 C, and  3 D, there is shown a second example assembly sequence for implementing the example enhanced three dimensional (3D) semiconductor stack of  FIG. 1  in accordance with preferred embodiments. As compared to the first assembly sequence of  FIGS. 2A ,  2 B,  2 C, and  2 D, the second assembly sequence reverses the order of the first and second steps. 
         [0041]    Referring now to  FIG. 3A , the 3D semiconductor stack  100  is shown in an exploded view starting position generally designated by the reference character  300  for the example second assembly sequence. 
         [0042]    Referring now to  FIG. 3B , there is shown a first step generally designated by the reference character  310  in the second assembly sequence of the 3D semiconductor stack  100 . The first step  310  in the second assembly sequence is to solder-attach DRAM stack  110  to master die  106 . Specifically, array  126  of connection elements on the plus-z-facing surface of DRAM stack  110  (see  FIG. 1 ) is solder-attached to array  210  of connections elements on the negative-z-facing surface of the master die  106 . This step  310  creates a master-die/DRAM assembly, as shown in  FIG. 3B . 
         [0043]    Referring now to  FIG. 3C , there is shown a second step generally designated by the reference character  330  in the second assembly sequence of the 3D semiconductor stack  100 . The second step in the second assembly sequence is to solder-attach the master-die/DRAM assembly to chip carrier  108 . Specifically, arrays  116 ,  118 ,  120 , and  122  of connection elements, located on the positive-z-facing surface of chip carrier  108 , visible in  FIG. 3B , are solder-attached to the corresponding arrays  202 ,  204 ,  206 , and  208  of connection elements located on the negative-z-facing surface of master die  106 . 
         [0044]    Referring now to  FIG. 3D , there is shown a final step generally designated by the reference character  340  in the second assembly sequence of the 3D semiconductor stack  100 . The final step  340  of the second assembly sequence is to attach heat spreader  104  to chip carrier  108 , where the result is identical to that shown in  FIG. 2D . 
         [0045]    Referring now to  FIG. 4 , there is shown a second enhanced three dimensional (3D) semiconductor stack generally designated by the reference character  400  in accordance with preferred embodiments. In  FIG. 4  and  FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E the same reference numbers are used for substantially similar or identical components of the second enhanced 3D semiconductor stack  400  as compared to the enhanced 3D semiconductor stack  100 . 
         [0046]    As shown in  FIG. 4 , the second enhanced 3D semiconductor stack  400  as compared to the enhanced 3D semiconductor stack  100  includes a chip carrier  402  having a blind-hole cavity  404 , instead of the chip carrier  108  having the through-hole  112 . For example, the blind-hole cavity  404  is drilled, milled or reamed to a set depth without breaking through to the other side of chip carrier  402 . 
         [0047]    Referring also to  FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E, a second example assembly sequence for implementing the example second enhanced three dimensional (3D) semiconductor stack  400  in accordance with preferred embodiments. Because the chip carrier  402  comprises blind hole  404  rather than through hole  112 , the first assembly sequence described with respect to  FIGS. 2A ,  2 B,  2 C, and  2 D for the first 3D semiconductor stack  100  is not possible for the second first 3D semiconductor stack  400 . 
         [0048]    Referring to  FIG. 5A , the 3D semiconductor stack  400  is shown in an exploded view starting position generally designated by the reference character  500  for the example second assembly sequence. The negative-z-facing surface of substrate  402  advantageously, but optionally, is populated with a full rectangular array of connections elements  502 , whereas the first 3D semiconductor stack  100  may be populated with only a partial array of connections elements  210 , due to the presence of the through hole  112 , for example as shown in  FIG. 2A . All other components are identical in the 3D semiconductor stack  100  and the 3D semiconductor stack  400 . 
         [0049]    Referring now to  FIGS. 5B and 5C , there are shown two perspective views of a first step in the second assembly sequence of the 3D semiconductor stack  100 , the first step being generally designated by the reference character  510 . The first step  510  in the second assembly sequence is to solder-attach the DRAM stack  110  to the master die  106 . This step  510  creates a master-die/DRAM assembly, visible in  FIG. 5B . 
         [0050]    Referring now to  FIG. 5D , there is shown a second step generally designated by the reference character  540  in the second assembly sequence of the 3D semiconductor stack  100 . The second step  540  in the second assembly sequence is to solder-attach the master-die/DRAM assembly to chip carrier  402 . Specifically, arrays  116 ,  118 ,  120 , and  122  of connection elements on the positive-z-facing surface of chip carrier  402 , visible in  FIG. 5C , are solder-attached to the corresponding arrays  202 ,  204 ,  206 , and  208  of connection elements on the negative-z-facing surface of master die  106 , visible in  FIG. 5B . 
         [0051]    Referring now to  FIG. 5E , there is shown a final step generally designated by the reference character  550  in the second assembly sequence of the 3D semiconductor stack  100 . The final step  550  of the second assembly sequence is to attach heat spreader  104  to chip carrier  402 . 
         [0052]    Although the second 3D semiconductor stack  400  rules out the first assembly sequence, the 3D semiconductor stack  400  has two advantages over the first 3D semiconductor stack  100 . First, blind hole  404  protects the rear surface of DRAM stack  106 ; and second, the full array  502  of connections elements in semiconductor stack  400  comprises a greater number of connections elements than the partial array  210  of connection elements in the 3D semiconductor stack  100 . Such a large number of connection elements may be required to carry a large number of signals to and from the semiconductor stack  400 . Consequently, if the second assembly sequence is viable, then the second 3D semiconductor stack  400  is preferred. However, if the second assembly sequence is not viable, then the first 3D semiconductor stack  100  is preferred. 
         [0053]    In the first and second enhanced three dimensional (3D) semiconductor stack  100 ,  400 , power enters the chip stack through the peripheral portion of the master die that overhangs the through-hole  112  or blind-hole  402 . Consequently, power to DRAM stack  110  and any logic of master die  106  under DRAM stack  110  is fed horizontally, parallel to the plane of master die  106 , which adds undesirable inductance and resistance to the wiring. The power-delivery problem has two potential solutions. The first solution makes use of so-called thick metal layers on master die  106 . Although thick wiring layers are usually not employed on the DRAM layers, thick metal layers can be used on master die  106  for power distribution. Such thick metal layers are sufficient to deliver sufficient power to the interior of the stack  100 ,  400  to power both significant master die logic, as well as the DRAM stack  110 . The second solution makes use of on-die voltage-regulation techniques, such as that used in IBM&#39;s Power? processor. Such on-die voltage-regulation techniques create a very well-regulated voltage, offsetting the undesirable effects of the lateral wiring inductance and resistance. 
         [0054]    Both the first enhanced 3D semiconductor stack  100  and the second enhanced 3D semiconductor stack  400  allow for the backside of master die  106  to be connected directly to heat spreader  104  or other cooling means, such that heat generated in the DRAM stack  110  travels through master die  106  to the cooling means. This cooling arrangement is preferred to the opposite situation in the prior art, where heat generated in the master die travels through the DRAM stack to the cooling means, because the master die generates more power than the DRAM stack. Consequently, thermal performance is better for the 3D semiconductor stack  100  and the 3D semiconductor stack  400 , as illustrated by the thermal simulation illustrated in  FIG. 6 . Although DRAM temperature is slightly increased as compared to prior-art arrangements, the master-die temperature is dramatically reduced. 
         [0055]    Referring to  FIG. 6  there is shown an example temperature distribution for the 3D semiconductor stack  100  and the 3D semiconductor stack  400  in accordance with preferred embodiments. Effective thermal performance is provided, as illustrated by the temperature distribution  600  for the 3D semiconductor stack  100  and the 3D semiconductor stack  400 , providing improvement over the prior-art cooling arrangements. 
         [0056]    While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.