Abstract:
Semiconductor packages are provided to prevent a chip, such as a central processing unit (CPU) chip, from being degraded due to hot spot heat generated during the operation of the chip and absorbs thermomechanical stresses in interfaces between the chip, a thermal interface material (TIM) and a lid. The chip is electrically connected, e.g., flip-chip bonded, to a package substrate. The lid is thermally connected to and disposed over a back surface of the chip with the TIM interposed therebetween. A heat dissipation means adjacent the TIM is also located between the lid and the chip to prevent the hot spot effect.

Description:
This is a divisional of application Ser. No. 10/245,228 filed Sep. 16, 2002, now U.S. Pat. No. 6,952,050. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of semiconductor packages and, more particularly, to a semiconductor chip package. 
   2. Description of the Related Art 
   Wire bonding is used to make electrical connections between central processing unit (CPU) chips and semiconductor packages. Flip-chip technologies have been employed to package high-speed semiconductor devices. There are two types of semiconductor package structures formed using the flip-chip technologies; a lid type and a non-lid type. The lid type structure is typically used in chip packages that include a high-frequency CPU chip that generates a large quantity of heat. The non-lid type structure is generally used in chip packages that have a low-frequency CPU chip that generates a relatively small quantity of heat. 
     FIGS. 1 and 2  show a conventional semiconductor chip package  100  having a lid  40 . Electrode bumps  24  of a CPU chip  20  are attached to the upper surface  12  of a substrate  10  using flip-chip technology. The CPU chip  20  is covered with a lid  40 . A plurality of external connection pins  30 , are electrically connected to the CPU chip  20 , extend from the lower surface  14  of the substrate  10 . An epoxy resin  52  fills an area between the CPU chip  20  and the substrate  10  to provide an under-fill adhesive. 
   The lid  40  is made of a material having a good heat emissive capacity. In order to maximize the heat emissive capacity through the lid  40 , a thermal interface material (TIM)  56  is interposed between a bottom surface  42  of the lid  40  and a back surface of the CPU chip  20 . A non-conductive adhesive  54 , (e.g., a non-conductive thermosetting silicone adhesive) is used as a sealant for attaching the lid  40  to the upper surface  12  of the substrate  10 . After applying the non-conductive adhesive  54  to the periphery of the substrate  10 , the lid  40  is attached and the non-conductive adhesive  54  is cured (hardened). Thus, the space on which the CPU chip  20  is mounted is encapsulated. 
   The TIM can be a thermal grease type material, or a rigid type material (such as epoxy or solder). The thermal grease type has a thermal conductivity of 1 to 6 W/mk. Epoxy has a thermal conductivity of 10 to 25 W/mk and solder has a thermal conductivity of 25 to 80 W/mk. 
   In a conventional semiconductor package  100 , the TIM  56  is interposed between the lid  40  and the CPU chip  20 . In this arrangement, damage may occur depending on the type of TIM  56  used. In a CPU chip, a single chip type cache SRAM is recently employed to improve interface speed in the system. In this case, a localized area of thermal stresses, such as a hot spot, may occur. The term “Hot spot,” as used herein, refers to a local area where excessive heat is generated. As device power increases, the hot spot increases in size and/or number. When power reaches a predetermined level, the hot spot effects are greater than the other thermal stress factors. Thus, such a hot spot can degrade the performance of the CPU chip  20 . In order to prevent the CPU chip  20  from being degraded, heat generated from the hot spot should be dissipated uniformly over the CPU chip  20  and emitted away from the CPU. However, the conventional TIM  56  does not have enough heat dissipation capability sufficient to dissipate the heat to a level as required above. 
   The thermal grease type TIM absorbs the thermomechanical stresses between the lid  40  and the CPU chip  20 , but has a poor heat emissive capacity. On the other hand, the rigid type TIM, such as solder, has a good heat emissive capacity, but is less capable of absorbing the thermomechanical stresses between the lid  40  and the CPU chip  20 . As a result, cracks can occur in the TIM  56  itself or in the CPU chip  20 . Thermomechanical stresses arise due to differences in the coefficients of thermal expansion (CTE) between the lid  40 , the CPU chip  20  and TIM  56 . These CTE differences are commonly referred to as a “CTE mismatch.” 
   Accordingly, a need arises for a semiconductor package that has a good heat emissive capacity and has an improved structure for absorbing thermomechanical stress. 
   SUMMARY OF THE INVENTION 
   The present invention provides a semiconductor package that uniformly dissipates heat over a chip that generates a large quantity of heat. 
   The present invention also provides a semiconductor package that absorbs thermal and mechanical stresses generated in interfaces between a chip, a thermal interface material and a lid. 
   According to one embodiment of the present invention, a semiconductor package includes a heat dissipation means located between a lid and a chip to prevent the hot spot effect during the operation of the chip. The semiconductor package has the lid disposed over and thermally connected to the back surface of the chip. A thermal interface material (TIM) is also located between the lid and the chip. The TIM can be formed directly under the lid over the heat dissipation means. Alternatively, for example, the TIM can be directly under the heat dissipating means and above the chip. 
   The heat dissipation means has a thermal conductivity of 100 W/mk or more and has a coefficient of thermal expansion of 4.0 or less. 
   According to yet another embodiment of the present invention, the heat dissipation means may be formed of a heat dissipation cover formed around the outer surface of the chip except the active surface of the chip, or may be formed of a heat dissipation layer formed on the bottom surface of the lid opposite to the TIM. 
   A filling material may fill a space between the lid and the substrate for dissipating the thermal and mechanical stresses generated in the chip. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be readily understood with reference to the following detailed description thereof provided in conjunction with the accompanying drawings, wherein like reference numerals designate the structural elements, and, in which: 
       FIG. 1  is a perspective view of a conventional semiconductor package having a lid; 
       FIG. 2  is a cross-sectional view taken along the line I—I in  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of a semiconductor package in accordance with a first embodiment of the present invention, showing a heat dissipation cover between a lid and a CPU chip; 
       FIG. 4  is a partially cutaway perspective view of the heat dissipation cover in  FIG. 3 , in which the heat dissipation cover is formed along the periphery of the CPU chip; 
       FIG. 5  is a cross-sectional view of a semiconductor package in accordance with a second embodiment of the present invention, showing a heat dissipation layer formed on an inner surface of the lid; 
       FIG. 6  is a partially cutaway perspective view of the heat dissipation layer in  FIG. 5 ; 
       FIG. 7  is a cross-sectional view of a semiconductor package in accordance with a third embodiment of the present invention, showing the heat dissipation layer formed on an inner surface of the lid; 
       FIG. 8  is a partially cutaway perspective view of the heat dissipation layer in  FIG. 7 ; and 
       FIG. 9  is a cross-sectional view of a semiconductor package in accordance with a forth embodiment of the present invention, in which the heat dissipation layer is formed on an inner surface of the lid, showing filling material being filled a space between the lid and the substrate. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 3  is a cross-sectional view of a semiconductor package  200  in accordance with an embodiment of the present invention, showing a heat dissipation cover  160  between a lid  140  and a CPU chip  120 .  FIG. 4  is a partially cutaway perspective view of the heat dissipation cover  160  shown in  FIG. 3 , in which the heat dissipation cover  160  is formed along the periphery of the CPU chip  120 . 
   With reference to  FIGS. 3 and 4 , the CPU chip  120  is attached to an upper surface  112  of a substrate  110  using a flip chip bonding method or any other suitable methods and covered with a lid  140 . A thermal interface material (TIM)  156  is located between the lid  140  and the CPU chip  120 . A plurality of external connection pins  130 , electrically connected to the CPU chip  120 , extend from a lower surface  114  of the substrate  110 . An epoxy region  152  fills an area between the CPU chip  120  and the substrate  10  to provide an under-fill adhesive. 
   According to an embodiment of the present invention, a semiconductor package  200  includes the CPU chip  120  covered with a heat dissipation cover  160  along the periphery of the CPU chip  120  except the active surface of the CPU chip  120 . The heat dissipation cover  160  uniformly dissipates and carries away heat generated in a hot spot over the CPU chip  120 , thereby preventing the CPU chip  120  from being degraded due to the hot spot. Preferably, the heat dissipation cover  160  has a thermal conductivity of 1,000 W/mk or more, and can be made of diamond, graphite or synthetic silicon, which has a coefficient of thermal expansion of 4.0 or less. The heat dissipation cover  160  may be formed by a sintering method, an injection molding method or a die casting method, for instance. 
   The CPU chip  120  is flip-chip bonded to the substrate  110 . The CPU chip  120  having the heat dissipation cover  160  is mounted on a substrate pad  116  of the upper surface  112  of the substrate  110 . Then, a reflow process is carried out, for example, at a temperature of 350 to 360° C. for approximately 100 seconds. The space between the CPU chip  120  and the substrate  110  is filled with a liquid epoxy resin  152  at a predetermined temperature using an under-filling method. The epoxy resin  152  is then hardened at a predetermined temperature. 
   The substrate  110  is a circuit wiring board having wiring patterns, and can be a printed circuit substrate, a ceramic substrate, a tape wiring substrate and so on. The substrate pad  116 , which is electrically connected to electrode bumps  124  of the CPU chip  120 , is formed on the upper surface  112  of the substrate  110 . The substrate  110  includes wiring patterns (not shown) that connect the substrate pad  116  to external connection pins  130 . On the lower surface  114  of the substrate  110  are external connection pins  130 . The pin type external connection terminals may be replaced by ball type external connection terminals or any other connection terminals suitable for implementing the principles of the present invention. 
   The lid  140  is made of a conductive metal having a good heat emissive capacity, for example Cu, Al, CuW, AlSiC, AlN or BeO as a basic material, coated with a conductive material, for example Ni, Au, Ag, Sn or Cr on the surface thereof. The lid  140  has a chip mounting space  148  inside thereof in order to receive the CPU chip  120 . The portion of the outer wall of the lid  148  is attached to the upper surface  112  of the substrate  110 . 
   Contacts with the TIM  156  and the lid  140  may include a passivation layer comprising (Ti, Cr)/Ni/(Au, Ag) to prevent oxidation and contamination. The contacts may be formed by an anodizing process. In this application, multi-layered metal layers are described as “A(B)/(C)/D,” where “/” represents a metal between the layers. A(B) means that the layer A is formed but B may be replaced with A. (C) means that C may be formed or may not be formed. 
   In order to maximize the heat emissive capacity through the lid  140 , the TIM  156  is interposed between a bottom surface  142  of the lid  142  and a back surface of the CPU chip  120 . The TIM  156  may be a thermal grease type or a rigid type that forms a coating using a dispensing method. If solder is used as a TIM, based on Pb, Sn, In, Ag, Bi, Sb or Au as a basic material, an under barrier metal (UBM) such as Ti (Cr)/VNi/Au (Ag) is preferably formed on the bottom surface  142  of the lid  140  and on the upper surface of the heat dissipation cover  160  for good bondability with the solder. On the other hand, in the case of a thermal grease type, such an UBM may not be needed. 
   The lid  140  is attached to the upper surface  112  of the substrate  110  with the non-conductive adhesive  154  so that the CPU chip  120  may be included in the chip mounting space  148 . The non-conductive adhesive  154  is preferably a non-conductive thermosetting silicone adhesive. That is, the non-conductive adhesive  154  is applied to the area to which the lid  140  is to be attached. The lid  140  is attached and the adhesive is hardened. Thus, the CPU chip mounted area is hermetically sealed. The hardening process of the non-conductive adhesive  154  can be carried out at the temperature of 100 to 150° C. for approximately one hour, for instance. 
   According to one embodiment of the present invention, the heat dissipation cover  160  surrounds the periphery of the CPU chip  120 , due to excellent thermal conductivity of the heat dissipation cover  160 , dissipates heat from the hot spot, which is generated during the operation of the CPU chip  120 . This prevents the defects that might result from the hot spot. Further, the heat dissipation cover  160  comprises a material having a low coefficient of thermal expansion, and capable of absorbing the thermomechanical stresses between the TIM  156  and the lid  140 . 
   Although embodiments of the present invention preferably use the lid  140  having the chip mounting space  148 , a lid having a plate shape without the chip mounting space  148  may be also used. The plate-shaped lid is arranged on a stiffener ring and the back surface of the CPU chip  120 , after the stiffener ring or pedestal is placed along the periphery of the substrate  110 . 
   Although the above-described embodiment of the present invention discloses, as illustrated in  FIGS. 3 and 4 , the heat dissipation cover  160  formed around the outside surface of the CPU chip  120  except the active surface of the CPU chip  120  to prevent the hot spot effect, a heat dissipation layer  260  may also be formed on the bottom surface  242  of the lid  240 , as shown in  FIGS. 5 and 6 . 
   Referring to  FIGS. 5 and 6 , a CPU chip  220  is electrically connected, for example, flip-chip bonded to an upper surface  212  of a substrate  210  and covered with the lid  240 . A TIM  256  is formed between the lid  240  and a back surface of the CPU chip  220 . A heat dissipation layer  260  is formed on a bottom surface  242  of the lid  240  and is in contact with the TIM  256 . 
   According to another embodiment of the present invention, the heat dissipation layer  260  is substantially identical to the heat dissipation cover  160  of the above-described embodiment in that the heat from hot spot is dissipated and emitted away from the CPU chip  220 , thereby preventing the CPU chip  220  from being degraded. Elements  214 ,  216 ,  224 ,  230 ,  248 ,  252 , and  254  have generally the same structure and function as the corresponding elements having the same last two digits in semiconductor package  200 . The heat dissipation layer  260  may be made of the same material as the heat dissipation cover  160 . In order to form the heat dissipation layer  260  on the lid  240 , an under barrier metal (UBM)  262  is preferably formed before the formation of the heat dissipation layer  260 . The UBM  262  is preferably formed by, for example, an anodizing method, a plating method, a sputtering method or an evaporation method, depending on the material used to form the lid  240 . For example, if the lid  240  is made of aluminum, the UBM  262  is preferably formed by an anodizing method. If the lid  240  is made of Cu, CuW, AlSiC or CuMo, the UBM  262  is preferably formed by a plating method. And, if the lid  240  is made of Si, SiO 2 , Al 2 O 3 , AlN or BeO, the UBM  262  is preferably formed by a sputtering or an evaporation method. The UBM  262  in this embodiment may be identical to the UBM used in the conventional solder or gold bump, for example, Cr (Ti)/(V7Ni93)/Au(Ag, Pd), Cr/Ni/Cu/Ag(Au, Pd), or TiW/Vni/Au(Ag) or Ni/Au(Ag, Pd). 
   A portion of the lid  240  exposed through the heat dissipation layer  260  preferably has a passivation layer such as Ni/(Au, Ag) or (Ti, Cr)/Ni/(Au, Ag) formed thereon to prevent oxidation and contamination. 
   If solder, based on Pb, Sn, In, Ag, Bi, Sb or Au, is used as a TIM, an UBM such as Ti (Cr)/VNi/Au (Ag) is preferably formed between the heat dissipation layer  260  of the lid  240  and the back surface of the CPU chip  220  for good bondability. 
   Although this embodiment of the present invention discloses the heat dissipation layer  260  formed on an existing lid  240 , the heat dissipation layer  260  may be alternatively formed during the manufacture of the lid  240 , as shown in  FIGS. 7 and 8 . 
   According to yet another embodiment of the present invention, a heat dissipation layer  360  is formed integral with a lid  340  during the formation of the lid  340  using a method such as a sintering method, an injection molding or a die casting method. Elements  314 ,  316 ,  324 ,  330 ,  348 ,  352 , and  354  have generally the same structure and function as the corresponding elements having the same last two digits in semiconductor package  200 . The structure of a semiconductor package  400  in this embodiment is substantially identical to the above-described embodiment. Thus, detailed description thereof is omitted. 
   Although the present invention discloses a lid-type semiconductor package having the chip mounting space, a lid  440  having a plate-shape may be also used as shown in  FIG. 9 . 
   Referring to the  FIG. 9 , a CPU chip  420  is electrically connected, for example, flip-chip bonded to an upper surface  412  of a substrate  410  and covered with a lid  440 . A thermal interface material (TIM)  456  is interposed between a lid  440  and the CPU chip  420 . The lid  440  has a plate shape and is formed with a heat dissipation layer  460 . The heat dissipation layer  460  is formed on the bottom surface  442  of the lid  440  and is in contact with the TIM  456 . Elements  414 ,  416 ,  424 ,  430 ,  448 ,  452 , and  454  have generally the same structure and function as the corresponding elements having the same last two digits in semiconductor package  200 . 
   A space between the lid  440  and the substrate  420  is filled with a filling material  470 . The filling material may be epoxy molding compound (EMC), underfill epoxy or silicon. The filling material dissipates the thermal and mechanical stresses applied on the CPU chip  420 . 
   The space between the lid  440  and the substrate  410  may be directly filled with the filling material  470  without an under-filling process, the space between the lid  440  and the substrate  410  may be alternatively filled with the filling material after the under-filling process. 
   Although the preferred embodiments of the present invention have been described in detail hereinabove, it should be understood that many variations and/or modifications of the basic inventive concepts herein taught, which may appear to those skilled in the art, will still fall within the spirit and scope of the present invention as defined in the appended claims. For example, the CPU chip having the heat dissipation cover may be attached to the substrate by a flip-chip bonding method or other suitable methods. The plate-shaped lid may be covered. Then, a filling material fills a space between the lid and the substrate. Further, the lid having the heat dissipation layer on the bottom surface of the plate-shaped lid may be used in the semiconductor package. In addition, although the above-described embodiments are described in connection with the CPU chip, a person skilled in the art will appreciate that the principles of the present invention can be applied in others types of semiconductor chips that generate a large quantity of heat.