Increasing thermal conductivity of thermal interface using carbon nanotubes and CVD

[0001] An integrated circuit package provides increased thermal conductivity to a thermal interface between a circuit die and a thermal management solution such as a heat spreader or a heat sink by forming a chemical vapor deposited diamond surface on the thermal management solution and growing an array of carbon nanotubes on the surface of the CVDD layer or the circuit die.

[0011] The present invention relates to a structure and a process of forming an integrated circuit package that utilizes a thermal interface material layer having an aligned array of carbon nanotubes projecting from a surface thereof.

[0012] In order to cool electronic packages such as electronic dies, a bare silicon die is covered with an integrated heat spreader which is formed from a thermally conductive material such as copper and serves to laterally distribute the thermal load provided by hot spots on the die corresponding, for example, to the areas of highest transistor activity.

[0013] In FIG. 1, there is shown an integrated circuit package 10 which has an interposer or substrate 12 on which a die 14 is located adjacent the substrate 12 and thermally coupled thereto through solder balls 16 which are bonded to a surface of die 14 which adjacent to substrate 12. The space between adjacent solder balls 16 and between the surface of die 14 and substrate 12 is generally filled with a thermally conductive gel 18. The combination of the thermal conductivity of solder balls 16 and thermally conductive gel 18 provides a cooling path for a portion of the heat generated by die 14.

[0014] As shown in FIGS. 1 and 2, a copper integrated heat spreader 20 is positioned adjacent to a further surface of die 14 which is opposite to the surface of die 14 which is adjacent to substrate 12. An inner surface 22 of heat spreader 20 is, in prior art packages such as the one shown in FIGS. 1 and 2, coupled to the surface of die 14 by a first thermal interface material layer 24 which is a thermally conductive material such as thermal grease or some similar material. Finally, the heat spreader 20 is in thermal contact with a copper heat sink 26 through a second thermal interface 28. Ambient air 30 flows across heat sink 26 to cool it.

[0015] In the prior art it has also been known to provide a chemical vapor deposited diamond (CVDD) layer 32 between the die and the integrated spreader 20 to improve the lateral thermal conductivity of either the spreader 20 or the silicon die 14. CVDD layer 32 is illustrated in FIGS. 2 and 3 applied to the inner surface 22 of integrated heat spreader 20.

[0016] The thermal conductivity of the copper of the integrated heat spreader is about 395 W/mK while that of the silicon die is about 100 W/mK. The thermal conductivity of a CVDD a layer is expected to exceed 1000 W/mK. It has been found through simulations that CVDD layers having a thickness of about 450 microns can provide thermal benefits, measured as a decrease in the total junction-to-ambient thermal resistance, are approximately 10% of total for CVDD layers applied to the integrated heat spreader, 20% for the CVDD layer applied to the die and approximately 30% for combined CVDD layers on both the die and the integrated heat spreader.

[0017] One of the difficulties present with the use of CVDD layers applied to the die and the heat spreader are the roughness that is inherent with the unpolished deposited diamond surface 34 which is characterized by asperities 36 or projections that have sizes measured in microns. Although it may be possible to reduce the roughness somewhat by performing a polishing operation after the CVD process, such a polishing operation is expected to be a major contributor to the cost of manufacture of the product, particularly as the cost of the CVDD process itself decreases. Unfortunately, it also appears to be true that the roughness of the CVDD layer increases with increasing thickness of the CVDD layer despite the fact that increasing the thickness of the CVDD layer increases the thermal effectiveness of the layers. In the prior art, accommodation of the uneven surface of the die 14 or the inner surface 22 of the integrated heat spreader 20 as well as the lack of smoothness of the CVDD layer, whether it is applied to the die or to the integral heat spreader; a void filling thermal interface material 38 is inserted between the two.

[0018] Addition of aligned nanotubes to the thermal interface, whether the nanotubes are single wall or double wall, increases the thermal conductivity between the die and integral heat spreader while allowing the elimination of the polishing step from the CVDD process. Described below are embodiments of thermal solutions based upon the addition of nanotubes to the thermal interface between the die and the integrated heat spreader.

[0019] FIG. 4 shows an embodiment of the invention where the single or double wall carbon nanotubes 40 are grown on die 14 by a plasma discharge deposition process available in the art. Once the nanotubes 40 are grown on the surface of die 14, one end of nanotubes in the array is firmly affixed to the die 14. In this embodiment, the thermal conductivity of the nanotubes 40 is in a range of about 1000 to 6000 W/m-K as compared to the conductivity of the typical thermal grease thermal interface material of about 1-7 W/m-K. In the embodiment shown, the length of the nanotubes would typically be about 100 microns or less so that the thermal resistance of the layer would be negligible. The growing of the nanotubes 40 on the surface of die 14 fills the surface roughness valleys to provide a nearly uniform layer for the interface. A limited crush of the nanotubes 40 allows for an excellent thermal connection between the surface of die 14 and the CVDD coating 32 on inner surface 22 of heat spreader 20. In one embodiment a thermal grease may be applied to the array of nanotubes 40. In another embodiment, thermal grease is not used. In another embodiment, CVDD coating 32 may be formed on the surface of a heat sink 26 instead of a spreader 20.

[0020] FIG. 5 illustrates a further embodiment of the invention in which a structure has nanotubes 40 which are grown from both the CVDD surface of heat spreader 20 and the surface of die 14 using a plasma discharge method. In this embodiment the nanotubes 40 grown from and affixed to the opposed surfaces intermesh as the surfaces are mated to provide a good thermal interface, even without the use of thermal grease 38. In a further embodiment thermal grease 38 can be used in combination with the carbon nanotubes 40.

[0021] FIG. 6 shows an embodiment of the invention where single or double wall carbon nanotubes 40 are grown on and affixed to the CVDD layer 32 of the integrated heat spreader 20 using a plasma discharge process. Again, the grown nanotubes 40 fill in the surface roughness valleys and asperities 36 and compensate for them. The projecting nanotubes 40 are subject to limited crushing of the nanotubes 40 into the peak roughness features 42 of the die 14. Again, in one embodiment, no grease 38 is necessary to form the thermal interface. In a further embodiment, thermal grease 38 is provided in combination with the nanotubes 40.

[0022] FIG. 7 is a flow chart of an embodiment of the inventive process for enhancing heat flow from a circuit die to a thermal management aid such as a heat spreader or a heat sink. The first stage 72 of the process is forming a CVDD layer on a surface of thermal management aid. After the CVDD layer is formed, a further stage 74 an array of substantially aligned nanotubes is grown on one surface of the circuit die or on the CVDD layer on the thermal management aid. In stage 76 the thermal management aid is mounted to the die with the layer of nanotubes thermally coupling the surface of the circuit die to the CVDD layer of the thermal management aid.

[0023] FIG. 8 is a flow diagram of another embodiment of the inventive process for thermally coupling a circuit die to a thermal management aid such as a heat spreader or a heat sink. In an initial stage 82 a CVDD layer is formed on the thermal management aid. In stage 84 a layer of nanotubes is grown on the CVDD layer. In stage 86 the thermal management aid is mounted to the die with the layer of nanotubes thermally coupling the surface of the die to the CVDD layer of the thermal management aid.

[0024] It will be readily understood to those skilled in the art that various changes in the details, material, and arrangements of the parts and method stages which have been described and illustrated in order to explain the nature of this invention may be made without departing from the principles and scope of the invention as expressed in the subjoined claims.