Abstract:
A method of reducing heat flow between IC chips and the resulting device are provided. Embodiments include attaching plural IC chips to an upper surface of a substrate; forming a lid over the IC chips; and forming a slit through the lid at a boundary between adjacent IC chips.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to the manufacture of semiconductor devices, such as integrated circuits (ICs). The present disclosure is particularly applicable to forming semiconductor packages with chips mounted close to each other, particularly for the 14 nanometer (nm) technology node and beyond. 
       BACKGROUND 
       [0002]    In a 2.5D package, multiple IC chips (e.g., a logic chip, a memory stack, etc.) are mounted close together to improve performance, bandwidth and/or functionality. Since multiple IC chips are close to each other, they are thermally coupled together. If one of the IC chips dissipates more power than the others, heat may flow from the high power IC chip to a lower power IC chip. The IC fabrication technology for each IC could be different, and hence the junction temperature specification can be different as well depending on technology, functionality, performance and bandwidth. The lower power IC could have a lower junction temperature specification. As a result, thermal management of heat generated in the 2.5D package becomes challenging. 
         [0003]    For example,  FIG. 1  illustrates heat flow in a package due to thermal interaction between IC chips. In  FIG. 1 , a substrate  101 , with an upper surface and a lower surface, has solder balls  103  metallurgically bonded to a solder ball receiving area on the lower surface of the substrate  101 . IC chips (e.g., a logic chip  105  and a memory stack  107 ) are attached to the upper surface of the substrate  101  by controlled collapse chip connection (C4) balls  109  via an interposer  108 . The logic chip  105  and the memory stack  107  are mounted on the interposer  108  with micro-bumps  106 . A lid  111  is formed over the logic chip  105  and the memory stack  107 . The lid  111  is thermally connected to the logic chip  105  and the memory stack  107  by a thermal interface material (TIM1)  113 . The lid  111  also includes lid feet in mechanical contact with the perimeter of the upper surface of the substrate by an adhesive  115 . A heat sink  117  is formed over the lid  111 . The heat sink  117  is thermally connected to the lid  111  by TIM2  119 . Arrows  121  represent heat flow through the package. As illustrated, though the heat sink  117  includes a plurality of fins, heat generated within the package continues to flow between the logic chip  105  and the memory stack  107 . As a result, the semiconductor devices tend to overheat or fail because of insufficient heat transfer. 
         [0004]    A need therefore exists for methodology enabling formation of a semiconductor packaged with reduced thermal interaction between the IC chips and the resulting device. 
       SUMMARY 
       [0005]    An aspect of the present disclosure is forming a slit through a semiconductor package lid at a boundary between a logic chip and a memory stack. 
         [0006]    Another aspect of the present disclosure is forming a semiconductor package with at least one vertical heat pipe in direct thermal contact with an IC chip and a heat sink. 
         [0007]    Another aspect of the present disclosure is a semiconductor package with a lid having a slit at a boundary between a logic chip and a memory stack. 
         [0008]    Another aspect of the present disclosure is a semiconductor package with at least one vertical heat pipe in direct thermal contact with an IC chip and a heat sink. 
         [0009]    Additional aspects and other features of the present disclosure will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present disclosure. The advantages of the present disclosure may be realized and obtained as particularly pointed out in the appended claims. 
         [0010]    According to the present disclosure, some technical effects may be achieved in part by a method including: attaching plural IC chips to an upper surface of a substrate; forming a lid over the IC chips; and forming a slit through the lid at a boundary between adjacent IC chips. 
         [0011]    Aspects of the present disclosure include the IC chips including: a logic chip; and at least one memory stack adjacent the logic chip. Further aspects include a method for forming the slit by: punch and die at a boundary between the logic chip and a memory stack. Other aspects include forming a slit between the logic chip and each memory stack. Additional aspects include forming a heat sink over the lid; and forming at least one vertical heat pipe through the heat sink and the lid, down to the IC chips, wherein each vertical heat pipe is in direct thermal contact with an IC chip and the heat sink. 
         [0012]    Another aspect includes a method for forming the vertical heat pipes by: forming co-axial holes in the lid and the heat sink; and inserting the vertical heat pipes through the holes for direct thermal contact with the IC and the heat sink. Further aspects include thermally connecting the lid to the IC chips by a first thermal interface material (TIM1); and thermally connecting the heat sink to the lid by a second thermal interface material (TIM2), wherein a length of the vertical heat pipes equals a sum of a thickness of TIM1, a thickness of the lid, a thickness of TIM2, and a height of the heat sink. Other aspects include the vertical heat pipes including copper (Cu). Additional aspects include a diameter of each vertical heat pipe ranging from 1 millimeter (mm) to a width of the IC chip with which it is in thermal contact. Further aspects include the vertical heat pipes having conductivity in only one direction. 
         [0013]    A further aspect of the present disclosure is a device including: plural IC chips attached to an upper surface of a substrate; a lid over the IC chips; and a slit through the lid at a boundary between adjacent IC chips. 
         [0014]    Aspects of the device include the IC chips including: a logic chip; and at least one memory stack adjacent the logic chip. Other aspects include the slit formed at a boundary between the logic chip and a memory stack. Further aspects include a slit formed between the logic chip and each memory stack. Another aspect includes a heat sink over the lid; and at least one vertical heat pipe through the heat sink and the lid, down to the IC chips, wherein each vertical heat pipe is in direct thermal contact with an IC chip and the heat sink. Additional aspect includes a TIM1 between the lid and the IC chips; and a TIM2 between the heat sink and the lid, wherein a length of the vertical heat pipes equals a sum of a thickness of TIM1, a thickness of the lid, a thickness of TIM2, and a height of the heat sink. Further aspects include the vertical heat pipes including Cu. Another aspect includes a diameter of each vertical heat pipe ranging from 1 mm to a width of the IC chip with which it is in thermal contact. Additional aspect includes the vertical heat pipes having conductivity in only one direction. 
         [0015]    Another aspect of the present disclosure is a method including: attaching IC chips, including a logic chip and at least one memory stack adjacent the logic chip, to an upper surface of a substrate; thermally connecting a lid to an upper surface of the IC chips by a TIM1; forming a slit through the lid by punch and die at a boundary between the logic chip and each memory stack; thermally connecting a heat sink to the lid by a TIM2; forming at least one co-axial hole in the lid and the heat sink; and inserting a vertical heat pipe through each hole for direct thermal contact with an IC chip and the heat sink, wherein a length of the vertical heat pipes equals a sum of a thickness of TIM1, a thickness of the lid, a thickness of TIM2, and a height of the heat sink, a diameter of each vertical heat pipe ranges from 1 mm to a width of the IC chip with which it is in thermal contact, and the vertical heat pipes have conductivity in only one direction. 
         [0016]    Additional aspects and technical effects of the present disclosure will become readily apparent to those skilled in the art from the following detailed description wherein embodiments of the present disclosure are described simply by way of illustration of the best mode contemplated to carry out the present disclosure. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
           [0018]      FIG. 1  schematically illustrates heat flow in a semiconductor package with adjacent IC chips; 
           [0019]      FIG. 2  schematically illustrates a semiconductor package with a slit at a boundary between adjacent IC chips, in accordance with an exemplary embodiment; 
           [0020]      FIG. 3  schematically illustrates a semiconductor package with both a slit at a boundary between adjacent IC chips and a vertical heat pipe in direct thermal contact with an IC chip and a heat sink, in accordance with an exemplary embodiment; and 
           [0021]      FIG. 4  schematically illustrates a top down view of a 2.5D package of  FIG. 3 , in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments. It should be apparent, however, that exemplary embodiments may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring exemplary embodiments. In addition, unless otherwise indicated, all numbers expressing quantities, ratios, and numerical properties of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” 
         [0023]    The present disclosure addresses and solves the current problem of thermal interaction between IC chips attendant upon forming a 2.5D semiconductor package with memory stacks close to a logic chip. In accordance with embodiments of the present disclosure, thermal interaction between IC chips can be avoided by forming a slit in the module lid at IC chip boundaries to stop heat flow from one chip to another through the lid. Correspondingly, heat flow from one IC chip to another through a heat sink base can be avoided by forming at least one vertical heat pipe that is inserted through co-axial holes in the heat sink and the lid for direct thermal contact with an IC chip. 
         [0024]    Methodology in accordance with embodiments of the present disclosure includes attaching plural IC chips to an upper surface of a substrate and forming a lid over the IC chips. A slit is formed through the lid at a boundary between adjacent IC chips. 
         [0025]    Still other aspects, features, and technical effects will be readily apparent to those skilled in this art from the following detailed description, wherein preferred embodiments are shown and described, simply by way of illustration of the best mode contemplated. The disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
         [0026]      FIG. 2  schematically illustrates a semiconductor package with a slit at a boundary between adjacent IC chips, in accordance with an exemplary embodiment. Similar to the conventional semiconductor package of  FIG. 1 ,  FIG. 2  includes a substrate  201  with an upper surface and a lower surface. Solder balls  203  are metallurgically bonded to solder ball receiving areas in the lower surface of the substrate  201 . A logic chip  205  and a memory stack  207  are attached to the upper surface of the substrate  201  by C4 balls  209  via an interposer  208 . The logic chip  205  and the memory stack  207  are mounted on the interposer  208  with micro-bumps  206 . A lid  211  is formed over the logic chip  205  and the memory stack  207 . The lid  211  is thermally connected to the logic chip  205  and the memory stack  207  by TIM1  213 . The TIM1  213  is separated at the boundary right under the slit to isolate the thermal path. The lid  211  also includes lid feet in mechanical contact with a perimeter of the upper surface of the substrate by an adhesive  215 .  FIG. 2  differs from  FIG. 1  in that a slit  221  is formed through the lid  211  at a boundary between the logic chip  205  and the memory stack  207 , for example by punch and die. The punch and die is either integrated in the lid  211  fabrication tooling or in a separate tooling. Although only one memory stack is illustrated in  FIG. 2  for illustrative convenience, a second memory stack (e.g., a hybrid memory cube (HMC) or a high bandwidth memory (HBM)) could be present on the other side of the custom logic die (e.g., an application-specific integrated circuit (ASIC)). A similar slit in the lid may be formed between the logic chip and any additional memory stack. The width of the slit  221  is at least as large as the physical space separation between the IC chips such that there is no direct thermal conduction at the slit location. The length of the slit  221  needs to be at least the length of the larger IC chip on the either side. As illustrated by arrows  223 , the slit  221  prevents heat flow from the logic chip  205  to the memory stack  207  and vice-versa through the lid  211 . In such manner, the slit  221  avoids thermal interaction between the logic chip  205  and the memory stack  207 . In addition, a heat sink  217  is formed over the lid  211 , and is thermally connected to the lid  211  by TIM2  219 . 
         [0027]      FIG. 3  schematically illustrates a semiconductor package with both a slit at a boundary between adjacent IC chips and a vertical heat pipe in direct thermal contact with an IC chip and a heat sink, in accordance with an exemplary embodiment.  FIG. 3  includes the same elements as in  FIG. 2 . Specifically, solder balls  303  are metallurgically bonded to the solder balls receiving areas in the lower surface of substrate  301 . A logic chip  305  and a memory stack  307  are attached to the upper surface of the substrate  301  by C4 balls  309  via an interposer  308 . The logic chip  305  and the memory stack  307  are mounted on the interposer  308  with micro-bumps  306 . A lid  311  is formed over the logic chip  305  and the memory stack  307  and is thermally connected by TIM1  313 . The lid  311  also includes lid feet in mechanical contact with a perimeter of the upper surface of the substrate  301  by an adhesive  315 . A slit  321  is formed through the lid at a boundary between the logic chip  305  and the memory stack  307 , e.g., by punch and die. The punch and die is either integrated in the lid  311  fabrication tooling or in a separate tooling. A heat sink  317  is formed over the lid  311 , and is thermally connected to the lid  311  by TIM2  319 . The difference between  FIG. 3  and  FIG. 2  is that co-axial holes are formed through the lid  311  and the heat sink  317 . Then, prefabricated vertical heat pipes  323  are inserted through the co-axial holes for direct thermal contact between the heat sink  317  and both the logic chip  305  and the memory stack  307 . The vertical heat pipes  323  prevent heat flow from the logic chip  305  to the memory stack  307  and vice-versa through the heat sink base. The vertical heat pipes  323  are formed of Cu or any other metal. The length of the vertical heat pipes  323  equals a sum of the thickness of TIM1, the thickness of the lid, the thickness of TIM2, and the height of the heat sink. The diameter of each vertical heat pipe  323  ranges from 1 mm to a width of the IC chip with which it is in thermal contact. The vertical heat pipes  323  have conductivity in only one direction. As shown by arrows  325 , heat flows from the logic chip away from the memory stack. 
         [0028]      FIG. 4  schematically illustrates a top down view of the 2.5D package of  FIG. 3 , in accordance with an exemplary embodiment. In  FIG. 4 , the number of the vertical heat pipes  407  shown thermally connected to a logic chip  403  is higher than the number of the vertical heat pipes  407  shown thermally connected to a memory stack  405 . However, the number of the vertical heat pipes  407  on each of the logic chip  403  and the memory stack  405  is based on the diameter of the vertical heat pipes, the size of the IC chips and the spacing rules for hole fabrication technology. The thermal conductivity of the vertical heat pipes  407  is 10,000 times higher than Cu. In addition, the vertical heat pipes  407  have conductivity in only one direction; therefore the end of the vertical heat pipes  407  at the logic chip  403  and the memory stack  405  have a higher temperature, and the other end is at a lower temperature. Further, the inherent very high thermal conductivity of the vertical heat pipes  407  allows effective heat dissipation. Accordingly, the heat from the logic chip  403  and the memory stack  405  flows to the top of the heat sink quicker and is dissipated speedily as the temperature delta between the heat sink top and ambient air is sufficiently large. With the slit and vertical heat pipes in direct thermal contact with both the IC chips and the heat sink, the thermal interaction between IC chips is reduced. A carefully designed vertical heat pipe arrangement can reduce the thermal solution size by almost 25%. 
         [0029]    The embodiments of the present disclosure can achieve several technical effects, such as, reduced thermal interaction between IC chips, smaller and lighter thermal solution size, faster heat flow from IC chips to the top of heat sink, faster heat dissipation, etc. Devices formed in accordance with embodiments of the present disclosure enjoy utility in various industrial applications, e.g., microprocessors, smart phones, mobile phones, cellular handsets, set-top boxes, DVD recorders and players, automotive navigation, printers and peripherals, networking and telecom equipment, gaming systems, and digital cameras. The present disclosure therefore enjoys industrial applicability in any of various types of highly integrated semiconductor devices, particularly for the 14 nm technology node and beyond. 
         [0030]    In the preceding description, the present disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not as restrictive. It is understood that the present disclosure is capable of using various other combinations and embodiments and is capable of any changes or modifications within the scope of the inventive concept as expressed herein.