Abstract:
A system and method are provided in which a first chip in a stacked multi-chip module configuration is affixed via one or more adhesion layers to a first portion of a partitioned interposer unit. Planar partitions of the interposer are physically bonded via multiple solder “bumps,” which possess high tensile strength but low resistance to horizontal shear force or torque. A second chip is affixed via one or more adhesion layers to the second portion of the partitioned interposer. The chips may thus be separated by horizontally and oppositely shearing or twisting the first and second portions of the partitioned interposer away from one another.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a divisional of U.S. application Ser. No. 12/432,672 filed Apr. 29, 2009, the contents of which are incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention was made with United States Government support under Contract No. 03-C-0216. The United States Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention relate generally to integrated circuit device packaging, and more specifically to multichip module packages that may have stacked chip arrangements. 
     BACKGROUND 
     When it comes to chip or integrated circuit device packaging, it is often desirable and sometimes imperative to have a relatively high device packaging density. Device packaging density can be defined as the number of devices per unit package volume. To such end, multichip module (MCM) packages are increasingly attractive for a variety of reasons. For example, MCM packages, which contain more than one chip per package, provide increased functionality of a given package, and decrease the interconnection length among chips in the package, thereby reducing signal delays and access times among chips. 
     One common MCM package is the three-dimensional “stacked” MCM package, in which one chip is disposed on a substrate and one or more other chips are stacked successively on top of one another and the first chip. Interconnections among chips and conductive traces on the common substrate are electrically made via bond wires. 
       FIG. 1  shows a cross-sectional view of a stacked multichip module (MCM) package  110  according to a known configuration. As shown, the MCM package  110  includes a substrate  111 , a first chip  112  and a second chip  113 . First chip  112  includes a bondable surface  121  and an active surface  122 . Bondable surface  121  is adhered to substrate  111  by means of an adhesive, such as an epoxy, thermoplastic material, tape, tapes coated with thermoplastic materials, etc. Active surface  122  includes an active circuit area (not shown) typically in the center of first chip  112 , and multiple bonding pads  112   a  located peripheral to the active circuit area. Similarly, second chip  113  includes a bondable surface  123  and an active surface  124 . Active surface  124  also includes an active circuit area (not shown) typically in the center of second chip  113 , and multiple bonding pads  113   a  located peripheral to the active circuit area. 
     The active circuit area of first chip  112  is covered by a passivation layer  125 . An adhesive layer  126  is interposed between and connects passivation layer  125  and an interposer  127 . Interposer  127  is often made of a material similar in properties to first chip  112  and second chip  113  in order to avoid thermal expansion mismatch over temperature variations. For example, if first chip  112  and second chip  113  are made of bulk silicon, interposer  127  should also be made of silicon. Interposer  127  has a thickness sufficient to allow clearance and access to the bond pads  112   a  along the edges of first chip  112 . Interposer  127  also serves as a pedestal for supporting second chip  113 . An adhesive layer  128  is disposed between and connects interposer  127  and bondable surface  123  of second chip  113 . 
     Several bond wires  114  are bonded to and between respective bonding pads  112   a  on first chip  112  and substrate  111 . Similarly, several bond wires  116  are bonded to and between respective bonding pads  113   a  on second chip  113  and substrate  111 . 
     One application in which stacked MCM packages are commonly used is space applications, or applications in other environments wherein physical space is limited and tolerance to high levels of radiation required. Packages with such tolerance to high levels of radiation are referred to as “hardened” packages. The chips in such hardened packages are also typically “hardened” through the addition of redundant circuitry and/or error detection and correction circuitry so that the chips function properly in high radiation environments like space. Due to the hardened nature, of the chips used in such environments, manufacturing costs for these chips can be inordinately expensive—often tens or even hundreds of times more expensive than counterparts of equivalent complexity used in consumer applications. For example, a hardened microprocessor could cost $10,000. 
     Unfortunately, due to current methods for manufacturing an MCM package in a stacked configuration, each chip or die in an MCM is so securely affixed to those above and below it that separation of that chip from the body of the MCM requires processes that are expensive, require high amounts of heat, or both. Thus, reworking or replacing a chip or die that has failed within a stacked MCM package often results in the destruction of one or more chips immediately above or below it. This naturally multiplies the cost of the original chip failure. 
     Consequently, it is desirable to provide an improved system and method for both manufacturing stacked MCM packages, and for reworking specific failed chips or dies within such packages. 
     SUMMARY 
     A need still exists, therefore, for providing an MCM package that allows the removal and/or replacement of one or more failed chips or dies without inflicting concomitant damage or destruction to nearby chips that would otherwise still be functional. In particular, there is a need to provide such functionality that is low in cost and allows for the removal or replacement to occur at room temperature. 
     According to certain embodiments of the present invention, a system and method are provided in which a first chip in a stacked MCM configuration is affixed via one or more adhesion layers to a first portion of a partitioned interposer unit. Planar partitions of the interposer are physically bonded via multiple solder balls or “bumps,” which possess high tensile strength but low resistance to horizontal shear force or torque. A second chip is affixed via one or more adhesion layers to the second portion of the partitioned interposer. The chips may thus be separated by horizontally and oppositely shearing or twisting the first and second portions of the partitioned interposer away from one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings are semi-diagrammatic not necessarily to scale and, particularly, some of the dimensions may be for clarity of presentation and shown greatly exaggerated. Similarly, although the views in the drawings, for ease of description, generally show similar orientations, this depiction is arbitrary for the most part. Generally, embodiments of the invention can be operated in any orientation. In addition, where multiple embodiments are disclosed and, described having some features in common, for clarity and ease of illustration, description, and comprehension thereof, similar and like features one to another will ordinarily be described with like reference numerals. 
         FIG. 1  is a cross-sectional view of a stacked multichip module (MCM) package according to a known configuration. 
         FIG. 2  is a cross-sectional schematic representation of a two-part interposer unit in accordance with an embodiment of the present invention. 
         FIG. 2A  illustrates horizontal shear force or rotational torque about the vertical dimension applied to the interposer unit of  FIG. 2 . 
         FIG. 3  shows a cross-sectional view of a stacked MCM package in accordance with another embodiment of the present invention. 
         FIG. 4  shows a cross-sectional partial view of a stacked MCM package according to an embodiment of the present invention with an illustrated force vector for die removal. 
         FIG. 5  shows a cross-sectional view of a stacked MCM package according to an embodiment of the present invention after the top portion of a two-part interposer unit has been separated from the bottom portion. 
         FIG. 6  is a cross-sectional view of a stacked MCM package showing a subsequent replacement chip and new interposer unit according to another embodiment of the present invention. 
         FIG. 7  shows a cross-sectional partial view of a stacked MCM package according to another embodiment of the present invention. 
         FIG. 8  is a block diagram of an electronic system which includes a stacked MCM package in accord with certain embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain details are set forth in conjunction with the described embodiments of the present invention to provide a sufficient understanding of the invention. One skilled in the art will appreciate, however, that the invention may be practiced without these particular details. Furthermore, one skilled in the art will appreciate that the example embodiments described below do not limit the scope of the present invention, and will also understand that various modifications, equivalents, and combinations of the disclosed embodiments and components of such embodiments are within the scope of the present invention. Embodiments including fewer than all the components of any of the respective described embodiments may also be within the scope of the present invention although not expressly described in detail below. Finally, the operation of well-known components and/or processes has not been shown or described in detail below to avoid unnecessarily obscuring the present invention. 
       FIG. 2  is a cross-sectional schematic representation of a two-part interposer unit  200  in accordance with an embodiment of the present invention. The interposer unit  200  comprises a top partition  250  and a bottom partition  255 . Because these interposer partitions are to be affixed to integrated circuit dies or chips, they are ideally constructed of material similar in properties to those chips in order to avoid thermal expansion mismatch over temperature variations. For example, if the integrated circuit chips are made of bulk silicon, interposer partitions  250  and  255  should also typically be made of silicon. Solder bumps  260  are disposed between partitions  250  and  255  and possess a high tensile strength, enabling the partitions  250  and  255  to be firmly affixed to one another along the vertical dimension indicated by arrow  256 . It may require over ten pounds of force to break the bonds created by solder bumps  260  in the vertical dimension  256 . However, the bumps are susceptible to relatively low horizontal shear force  254  or rotational torque  257  about the vertical dimension  256  as illustrated in  FIG. 2A . This rotational torque  257  is often less than a pound of such force. Thus, by applying rotational torque  257  to rotate the top partition  250  relative to the bottom partition  255 , the partitions can be separated. This enables interposer partitions  250  and  255  to be separated without excessive force and without the use of a high-heat environment, either of which could cause the chips in the MCM package to become damaged or destroyed. 
       FIG. 3  is a cross-sectional view of an MCM package  310  including the partitioned interposer unit  200  of  FIG. 2  in accordance with an embodiment of the present invention. MCM package  310  includes a substrate  111 , a first chip  112  and a second chip  113 . Substrate  111  is adhered to first chip  112 , which includes an active surface  122  on its top side that is covered by a passivation layer  125  and adhered to that passivation layer via adhesive layer  126 . Similarly, second chip  113  includes a bondable surface  123  along its lower side and an active surface  124  on its upper side. An adhesive layer  128  is bonded to bondable surface  123 . First and second chips  112  and  113  are respectively wire-bonded to substrate  111  by bond wires  114  and  116 . 
     The upper interposer partition  250  is connected to second chip  113  via adhesive layer  128  and also soldered to lower interposer partition  255  via solder bumps  260 . Solder bumps  260 , as discussed above with respect to  FIG. 2 , provide a high tensile strength for vertically bonding the upper and lower interposer partitions  250  and  255 , but are easily broken via horizontal shear force  254  or rotational torque  257 . Lower interposer partition  255  is, in turn, adhered to passivation layer  125  via adhesive layer  126 . 
       FIG. 4  shows a cross-sectional view of another embodiment, presenting a possible torque vector for separating upper and lower interposer partitions  450 ,  455  of an interposer  449  of MCM package  400 . Substrate  411  is affixed below first chip  412  via adhesive layer  425 . The top of first chip  412  is in turn affixed to lower interposer partition  455  via adhesive layer  430 , and lower interposer partition  455  is affixed to upper interposer partition  450  via a plurality of solder bumps  460 . The upper interposer partition  450  is affixed to second chip  413  via adhesive layer  440 . No bond wires appear in  FIG. 4 , although in practice both chips in the MCM package are still wire-bonded to substrate  411 . 
     To examine the MCM package  400  of  FIG. 4  in operation of the interposer  449 , assume that second chip  413  has been shown to be nonoperational. This type of failure can occur at the time of the chip manufacture or through the course of operation over time. In either case, it is advantageous to be able to remove (and possibly replace) the non-operational chip. 
     Force line AA shows a line substantially bisecting the solder bumps  460 . The vertical placement of line AA is arbitrary with respect to the vertical extension of the bumps. To separate upper and lower interposer partitions  350  and  355 , a technician or end-user collectively creates torsion in solder bumps  460  by applying equal but opposing shear forces to upper and lower interposer partitions  350 ,  355  respectively, parallel to line AA and the plane of substrate  411 . The shear force thus provided to each solder bump  460  causes the bonds formed by those solder bumps to break and allows the separation of MCM package  400  along line AA. Because the force required to effectuate the removal of second chip  413  is so low, it can be accomplished without damaging first chip  412 . Furthermore, because the chip removal can be done at room temperature, it may be performed in the same area where modules are tested, allowing immediate confirmation that the first chip has not been damaged. 
       FIG. 5  shows the MCM package  400  of  FIG. 4  after separation. Solder bumps  460  have each been sheared away at line AA, leaving the lower interposer partition  455  separated from upper interposer partition  450  and second chip  413 . If desired, the remainder of solder bumps  460  may be mechanically or chemo-mechanically removed so that a new chip and interposer unit may replace the non-operational (and now removed) chip  413  (not shown), as discussed in more detail below. 
       FIG. 6  shows an embodiment of the present invention in which a replacement chip  613  has been affixed (via adhesion layer  640 ) to replacement upper interposer partition  650 . Replacement upper interposer partition  650  is affixed to replacement lower interposer partition  655  via solder bumps  660 . Finally, replacement lower interposer partition  655  is affixed to the existing lower interposer partition  455  via new adhesive layer  630 . In this way, the lower interposer partition does not need to be removed from the existing first chip  412 ; a new interposer unit (comprising replacement upper interposer partition  650 , replacement lower interposer partition  655 , and solder bumps  660 ) is simply adhered to the lower partition of the pre-existing interposer unit from  FIG. 4 . 
     In certain embodiments, the surface of the dies or of the interposer partitions are constructed with such shape or footprint as may easily integrate with a rotational tool, such as a wrench, to enable faster and more precise breaking of the bonds between the upper and lower interposer partitions. In other embodiments, a technician or other user may simply separate the interposer partitions by applying the needed horizontal shear force or torque by hand. 
       FIG. 7  shows a cross-sectional partial view of a stacked MCM package  700  according to another embodiment of the present invention, in which an encapsulant  702  has been additionally disposed between the upper and lower interposer partitions  450  and  455 . In some circumstances, the weak horizontal, shear resilience of the solder bumps may not be sufficient to provide the strength or stability desired in the MCM package as a whole. By forming an encapsulant such as an epoxy compound around the solder bumps  460 , the MCM package  700  is strengthened and stabilized while retaining the ability to cheaply and safely remove a particular nonoperational chip. Typically, the encapsulant  702  is disposed around the solder bumps  460  and between upper and lower interposer partitions  450  and  455  after the chips  412  and  413  have already been operationally tested. 
       FIG. 8  is a block diagram of an electronic system  800 , an exemplary instance of which may be a satellite system, including electronic circuitry  810  and the multi-chip module package (MCM)  300  of  FIG. 3 . Typically, the electronic circuitry  810  and MCM package  300  are coupled to a memory  802 . Also typically, the electronic circuitry  810  is coupled through address, data, and control buses to the MCM package  300  to provide for writing data to and reading data from the MCM package. The electronic circuitry  810  includes circuitry for performing various computing functions, such as executing specific software to perform specific calculations or tasks. In addition, the electronic system  800  includes one or more input devices  804 , such as a keyboard or mouse for local input or receivers for receiving input from remote or ground locations, coupled to the electronic circuitry  810  to allow an operator to interface with the electronic system. Typically, the electronic system  800  also includes one or more output devices  806  coupled to the electronic circuitry  810 , such output devices typically including transmitters (for relaying information to remote or ground locations) and display devices. One or more data storage devices  808  are also typically coupled to the electronic circuitry  810  to store data or retrieve data from external storage media (not shown). Examples of typical storage devices  808  include hard and floppy disks, tape cassettes, compact disc read-only (CD-ROMs) and compact disc read-write (CD-RW) memories, and digital video discs (DVDs). 
     It is to be understood that even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only, and changes may be made in detail, and yet remain within the broad principles of the invention. For example, variations on certain embodiments described above or depicted in the drawings may include three or more chips in a single MCM package, many or all of which may be separated by interposer units in accord with the present invention. As another example, certain embodiments may include additional structures affixed within the MCM package between one or more chips and a respective interposer unit. Therefore, the present invention is to be limited only by the appended claims.