Abstract:
The present invention relates to dissipating heat from an interconnect formed in a low thermal conductivity dielectric in an integrated circuit apparatus. The integrated circuit apparatus includes integrated circuit devices interconnected by conductive interconnection metallurgy and input/output pads subject to electrostatic discharge events. At least one latent heat of transformation absorber is associated with at least one of the input/output pads for preventing the energy generated by an electrostatic discharge event from damaging the conductive interconnection metallurgy.

Description:
BACKGROUND OF INVENTION 
     Field of the Invention 
     The field of the present invention is semiconductor integrated circuits. More specifically, the present invention relates to dissipating heat in semiconductor integrated circuits. 
     Integrated circuits formed on a semiconductor substrate such as, for example, silicon, silicon-on-insulator (SOI), or silicon germanium (SiGe), include devices such as transistors, capacitors, resistors and inductors which are connected by interconnects (e.g. wires). Interconnects are typically located in Back-End-of-Line (BEOL) levels above the Front-End-of-Line (FEOL) devices of the integrated circuit. Interconnects and vias provide an electrical path between devices within an integrated circuit so that desired electrical connections are formed between devices. Interconnects can also provide electrical connection from devices to input/output pads for connection to an external signal (i.e. power supply, ground, or signal line). Since interconnects are used to provide electrical connection, interconnects are typically formed of an electrically conductive material such as, for example, a metal. Metals such as, for example, aluminum or copper are commonly used due to their relatively low electrical resistance. 
     Forming conductive interconnects and vias on a semiconductor substrate can be achieved by a variety of methods. For example, damascene and dual damascene processes can be used to form interconnects and vias. For a given wiring level, a dielectric layer is deposited, patterned and etched to form a trough for interconnects (i.e. damascene process) or interconnects and vias (i.e. dual damascene process). Metal is deposited to fill the trough and any metal overlying the dielectric layer is removed typically by a chemical mechanical polish (CMP). The dielectric layer electrically isolates interconnects from each other. In addition, as heat is generated due to electrical current flowing through an interconnect, the dielectric layer provides a thermal conduction path so that heat can be dissipated away from the interconnect. 
     In order to increase the speed of integrated circuits, the size of devices and interconnects that form integrated circuits must be reduced. Two effects limiting the speed of integrated circuits are the electrical resistance of the interconnect (i.e. line resistance) and resistive-capacitive (RC) coupling induced time delay due to higher wiring density and closer spacing of interconnects. As the distance between adjacent interconnects decreases, RC coupling between adjacent interconnects increases. To reduce line resistance, aluminum interconnects are being replaced with copper interconnects since copper has a lower electrical resistance than aluminum. RC coupling induced time delay is being addressed by the use of low dielectric constant (low-k) dielectrics such as, for example, polyimide nanofoams (also known by tradenames such as, for example, SiLK which is manufactured by Dow Chemical Co., Midland, Mich.), to replace conventional dielectrics (i.e. silicon oxide). By utilizing a low-k dielectric, the capacitance between adjacent interconnects is reduced, thus reducing the RC coupling induced time delay. 
     Due to lower electrical interconnect resistance and lower RC coupling induced time delay afforded by the use of copper interconnects and low-k dielectrics, increased current flow can be applied to the interconnects in order to provide higher performance integrated circuits (i.e. higher operating frequency). Also, interconnects are exposed to undesirable current flow such as high current, short time duration pulses caused by, for example, an electrostatic discharge (ESD) event or a power up/down event. High current flow through an interconnect results in the generation of a large amount of heat which must be effectively dissipated. 
     The combination of copper interconnects with a low-k dielectric creates a thermal dissipation problem in an integrated circuit. Low-k dielectrics characteristically do not provide heat dissipation as well as silicon oxide dielectrics. Additionally, some low-k dielectrics, especially types formed of organic foams, will degrade both structurally and electrically at temperatures exceeding about 350° C. If heat is not adequately dissipated from the interconnect, the temperature of the interconnect increases and the electrical resistance of the interconnect also increases, thus degrading the performance of the integrated circuit. If enough heat is generated, the interconnect can melt leading to reliability issues and/or failure of the interconnect. The migration of interconnects from aluminum to copper has improved the thermal robustness of interconnects since the melting temperature of copper is higher than aluminum. However, continued scaling of copper interconnects to smaller dimensions to meet increased integrated circuit requirements will expose copper interconnects to temperatures that exceed the melting temperature of copper. 
     Dissipating heat from an interconnect formed in a low thermal conductivity dielectric is desired. 
     SUMMARY OF INVENTION 
     It is thus an object of the present invention to dissipate heat from an interconnect formed in a low thermal conductivity dielectric. 
     The foregoing and other objects of the invention are realized, in a first aspect, by an integrated circuit apparatus including integrated circuit devices interconnected by conductive interconnection metallurgy and input/output pads subject to electrostatic discharge events. At least one latent heat of transformation absorber is associated with at least one of the input/output pads for preventing the energy generated by an electrostatic discharge event from damaging the conductive interconnection metallurgy. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other features of the invention will become more apparent upon review of the detailed description of the invention as rendered below. In the description to follow, reference will be made to the several figures of the accompanying Drawing, in which: 
     FIGS. 1A-E show the formation of a latent heat of transformation absorber according to a first embodiment of the present invention. 
     FIGS. 2A-B show the formation of a latent heat of transformation absorber according to a second embodiment of the present invention. 
     FIG. 3 shows the formation of a latent heat of transformation absorber according to a third embodiment of the present invention. 
     FIG. 4 shows the formation of a latent heat of transformation absorber according to a fourth embodiment of the present invention. 
     FIGS. 5A-B are graphs illustrating heat dissipation from an interconnect according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention describes integrating a latent heat of transformation absorber in the BEOL metallurgy of an integrated circuit. For BEOL metallurgy including, for example, copper interconnects and low-k inter-level dielectrics, a primary function of the latent heat of transformation absorber is to remove heat away from copper interconnects and to dissipate it more efficiently than the low-k dielectric surroundings. The latent heat of transformation absorber takes advantage of the constant temperature that is maintained as a material undergoes a phase change such as, for example, from solid to liquid phase. Thus, additional heat from a copper interconnect can be absorbed resulting in increased heat dissipation away from the interconnect. The latent heat of transformation absorber includes a heat transient layer having a high thermal conductivity and a lower melting point temperature than the melting point temperature of the interconnect. The heat transient layer can be formed from a material such as, for example aluminum, tin or lead or their alloys for integration into copper BEOL metallurgy. 
     According to a first embodiment of the present invention, FIG. 1A shows integrated circuit  10  formed on substrate  20  such as, for example, a silicon wafer. FEOL devices  25  such as, for example, transistors and capacitors, are formed on substrate  20 . BEOL level  30  includes interconnects and vias for connecting FEOL devices  25  to form integrated circuit  10 . It should be noted that although only one BEOL level  30  is shown throughout FIGS. 1-4, one or more BEOL levels  30  can be incorporated into integrated circuit  10 . BEOL level  30  includes inter-level dielectric  35 , vias  40   a ,  40   c  and interconnect  40   b . Vias  40   a ,  40   c  and interconnect  40   b  can be formed by processes known in the art such as damascene (i.e. via  40   a ) or dual damascene (i.e. interconnect  40   b  and via  40   c ). Barrier layer  45  comprising a dielectric such as, for example, silicon nitride having a preferred thickness of about 0.02 um to 0.2 um, more preferably from about 0.025 to 0.035 um, is formed on BEOL level  30  and insulator layer  50  comprising a dielectric such as, for example, silicon oxide having a preferred thickness of about 0.2 um to 2.0 um, more preferably from about 0.3 um to 0.4 um, is formed on silicon nitride layer  45 . 
     Vias  40   a ,  40   c  and interconnect  40   b  are formed of a low electrical resistance metal such as, for example, copper. Inter-level dielectric  35  comprises an electrically insulating material such as, for example, a low-k dielectric (i.e. SiLK). The present invention will be described with reference to copper interconnects formed in a SiLK inter-level dielectric. It should be noted, however, that the present invention is not limited to copper interconnects formed in a SiLK inter-level dielectric. As will be described hereinafter, the present invention can be applied to interconnects formed of other metals such as, for example, aluminum, and inter-level dielectrics formed of other insulating materials such as, for example, silicon oxide. 
     Silicon oxide layer  50  is patterned and etched to form trench  52  as illustrated in FIG.  1 B. Trench  52  can be formed to any desired depth up to the thickness of silicon oxide layer  50 , a preferred depth being about 0.2 um. A heat transient layer  55  comprising a material having a lower melting point temperature than the melting point temperature of the interconnect (i.e. copper) is formed in trench  52 . Heat transient layer  55  can be formed from a material such as, for example, aluminum, lead, and tin. A layer of aluminum is formed by a known process such as, for example, chemical vapor deposition (CVD), sputter deposition or evaporation on silicon oxide layer  50  and fills trench  52 . Chemical-mechanical polishing (CMP) is performed to remove aluminum which over-fills trenches  52  resulting in aluminum heat transient layer  55 . Aluminum heat transient layer  55  can include a thin liner film (not shown) formed of a material such as, for example, titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride TaN) or tungsten (W). It should be understood that although the thin liner film is not shown as a discrete layer in the figures, the thin liner film is included with reference to a respective aluminum heat transient layer. 
     Barrier layer  60  comprising a dielectric such as, for example, silicon nitride having a preferred thickness of about 0.02 um to 0.2 um, more preferably from about 0.025 to 0.035 um, is formed on silicon oxide layer  50  and aluminum heat transient layer  55  as shown in FIG.  1 C. Silicon nitride layer  60  electrically isolates aluminum heat transient layer  55  from a subsequently formed copper interconnect. Further, silicon nitride layer  60  prevents a reaction (i.e. chemical reaction) between aluminum heat transient layer  55  and a subsequently formed copper interconnect. Inter-level dielectric layer  65  comprising a low-k dielectric such as, for example, SiLK having a preferred thickness of about 0.2 um to 2.0 um, more preferably from about 0.3 to 0.4 um, is formed on silicon nitride layer  60 . Referring to FIG. 1D, copper interconnect  70  is formed in SILK layer  65  by a known process such as a damascene process. Copper interconnect  70  can include a thin liner film such as, for example, tantalum nitride (TaN) (not shown) for preventing corrosion of copper interconnect  70 . It should be understood that although the thin liner film is not shown as a discrete layer in the figures, the thin liner film is included with reference to a respective copper interconnect. One or more of copper interconnects  70  can be connected to an input/output pad  72  for providing electrical connection to an external source such as, for example, power, ground, or a signal line. 
     As described herein above, aluminum heat transient layer  55  is formed in close proximity to a lower surface of interconnect  70 . Optionally, an additional heat transient layer can be formed in close proximity to an upper surface of copper interconnect  70  as shown in FIG.  1 E. Process steps used to form lower heat transient layer  55  are repeated to form upper heat transient layer. Barrier layer  75  (i.e. silicon nitride) having a preferred thickness of about 0.02 um to 0.2 um, more preferably from about 0.025 to 0.035 um, is formed on SiLK layer  65  and copper interconnect  70 . Dielectric layer  80  (i.e. silicon oxide) having a preferred thickness of about 0.2 um to 2.0 um, more preferably from about 0.3 to 0.4 um, is formed on silicon nitride layer  75 . Heat transient layer  85  comprising aluminum is then formed as was described with reference to aluminum heat transient layer  55 . It should be noted that heat transient layer  85  does not have to be the same material as aluminum heat transient layer  55 . That is, heat transient layer  85  can be formed from a different material such as, for example, lead or tin. 
     According to a second embodiment of the present invention, a heat transient layer is formed in close proximity to one or more sides of an interconnect. Although the formation of a heat transient layer in such a manner is effective in thermal dissipation of heat away from the interconnect, it may not provide maximum heat removal since the area of the thermal conduction path between the heat transient layer and the interconnect is determined by the height and length of the interconnect. For an interconnect which has a height that is less than its width (i.e. a thin interconnect), thermal dissipation of heat by a heat transient layer formed in close proximity to a side of the interconnect will be less than that from a heat transient layer formed in close proximity to a lower (or upper) surface of the interconnect. However, design or performance requirements may require the formation of a heat transient layer in close proximity to a side of an interconnect such as in the case where it is not possible to form a heat transient layer in close proximity to a lower or upper surface of the interconnect. 
     Referring to FIGS. 2A-B, a heat transient layer according to a second embodiment of the present invention will be described. It should be noted that structures shown in FIGS. 2A-B that are the same structures as those described in FIGS. 1A-E are denoted by the same numerals. Referring to FIG. 2A, integrated circuit  10  includes copper interconnect  100  that is formed in inter-level dielectric  50 ′(i.e. SiLK) having a preferred thickness of about 0.2 um to 2.0 um, more preferably from about 0.3 to 0.4 um, by a known process such as, for example, damascene or dual damascene. One or more of copper interconnects  100  can be connected to an input/output pad  72  for providing electrical connection to an external source such as, for example, power, ground, or a signal line. 
     Trench  105  is formed adjacent copper interconnect  100  as shown in FIG.  2 B. Trench  105  can be formed by patterning and removing SILK layer  50 ′ by known photolithographic and etch (e.g. oxygen reactive ion etch) techniques. Liner  110  comprising a dielectric such as, for example, silicon nitride is formed along exposed surfaces of trench  105 . Silicon nitride liner  110  electrically and physically isolates a subsequently formed heat transient layer from an interconnect. Liner  110  can also be formed of an electrically conducting material such as, for example, Ti, TiN, Ta, TaN or W resulting in subsequently formed heat transient layer  115  electrically connected to an interconnect. The main function of liner  110  is to physically isolate heat transient layer  115  and the adjacent interconnect in order to prevent any chemical reaction between aluminum and copper. Heat transient layer  115  comprising aluminum, lead, or tin is then formed in trench  105 . Aluminum heat transient layer  115  is formed by depositing (i.e. chemical vapor deposition) a layer of aluminum to fill trench  105  and removing excess aluminum by a process such as, for example, CMP. Barrier layer  120  comprising a dielectric such as, for example, silicon nitride having a preferred thickness of about 0.02 um to 0.2 um, more preferably from about 0.025 to 0.035 um, is formed to encapsulate copper interconnect  100  and aluminum heat transient layer  115 . 
     Although FIG. 2B shows aluminum heat transient layer  115  formed in close proximity to the sides of multiple copper interconnects  100 , it should be noted that since trench  105  is defined by a photolithography process, aluminum heat transient layer  115  may be formed adjacent all copper interconnects  100 , or formed adjacent one or more selected copper interconnects  100  (i.e. interconnects that are known to require a high rate of heat dissipation). Further, aluminum heat transient layer  115  may be formed adjacent only one side of copper interconnect  100  rather than on both sides as shown in FIG.  2 B. 
     According to a third embodiment of the present invention, an additional heat transient layer as was described with reference to FIG. 1E can be formed in conjunction with aluminum heat transient layer  115  (see FIG.  2 B). Referring to FIG. 3, aluminum heat transient layer  85  is formed in close proximity to an upper surface of copper interconnect  100 . Aluminum heat transient layer  85  is formed in silicon oxide layer  80  oxide as was described herein above with reference to FIG.  1 E. Heat transient layer  85  can be formed of the same material as heat transient layer  115  (i.e. aluminum) or a different material such as, for example, tin or lead. Heat transient layer  85  provides for additional heat dissipation away from copper interconnect  100 . 
     According to a fourth embodiment of the present invention, a copper interconnect is surrounded by multiple heat transient layers as shown in FIG.  4 . By forming aluminum heat transient layers  55 ,  115  and  85  as described herein above with reference to FIGS. 1D,  2 C, and  1 E, respectively, copper interconnect  100  can be surrounded to provide a significantly increased amount of heat dissipation. It should be noted that each of heat transient layers  55 ,  115  and  85  can be formed of the same or different materials. For example, heat transient layers  55  and  85  can be formed of the same material such as aluminum while heat transient layer  115  can be formed of a different material such as tin or lead. 
     Although various configurations of heat transient layers have been described with reference to FIGS. 1-4, the present invention also encompasses additional configurations such as, for example, forming a heat transient layer adjacent an upper surface of an interconnect (i.e. heat transient layer  85  shown in FIG. 1E) without forming a heat transient layer adjacent a lower surface of the interconnect. Likewise, heat transient layer can be formed having various widths, lengths and heights according to design and heat dissipation requirements for a given interconnect. 
     An interconnect which is connected to an input/output pad is especially exposed to high electric currents from sources such as, for example, an ESD event. An ESD event can result in, for example, an electric current of about at least  100  mA for a short time duration pulse of about 1×10E-9 seconds (e.g. nanoseconds) being generated from an external source (i.e. static charge from a human being) that is transmitted via the input/out pad to the respective interconnect within the integrated circuit. The high current pulse from such an ESD event results in a large amount of heat being generated in the interconnect which must be dissipated away from the interconnect in order to avoid detrimental effects such as melting of the interconnect. 
     The heat transient layer of the present invention significantly enhances the dissipation of heat away from an interconnect such that a high current pulse from an ESD event can be withstood so that the occurrence of detrimental effects such as melting of the interconnect can be reduced. The heat transient layer is selected to have a melting point temperature which is below the melting point temperature of the interconnect. As the local temperature generated by current flow through the interconnect exceeds the melting point temperature of the heat transient layer, the heat transient layer undergoes an endothermic phase change from a solid to a liquid. Therefore, as the heat transient layer undergoes the phase change from solid to liquid, additional heat can be absorbed away from the interconnect by the heat transient layer without an increase in temperature of the heat transient layer. The present invention takes advantage of the latent heat of transformation required to convert a solid to a liquid. Once the melting point temperature of the solid heat transient layer is achieved, the melting point temperature is maintained as additional heat that is absorbed is used to convert the solid heat transient layer to a liquid heat transient layer. It should be noted that the phase change is a fully-reversible reaction so that the heat transient layer returns to a solid once the local temperature decreases below the melting point temperature of the heat transient layer. The additional heat that can be absorbed during the solid to liquid phase change of the heat transient layer provides increased protection of a copper interconnect especially, for example, during an ESD event since higher currents can be withstood by the copper interconnect. 
     The present invention also provides for heat transient layers associated with an interconnect to be customized (see FIGS. 1-4) to further enhance heat dissipation. For example, an interconnect that is known to be susceptible to high current can be surrounded by multiple heat transient layers as shown in FIG.  4 . The heat transient layer also dissipates heat away from an adjacent interconnect by solid phase thermal conduction. That is, heat generated by the interconnect creates a local temperature that is below the melting point temperature of the heat transient layer so the heat transient layer remains in the solid phase while dissipating heat away from the interconnect. 
     The advantages of a heat transient layer integrated into the BEOL metallurgy of an integrated circuit can be calculated by using a model where heat is dissipated away from a copper interconnect by aluminum heat transient layers adjacent an upper and lower surface of the copper interconnect (see FIG.  1 E). The aluminum heat transient layers are modeled as having the same height, width, and length as the copper interconnect. Referring to FIG. 5A, curve A represents the temperature of a copper interconnect associated with the aluminum heat transient layers. Curve B represents the temperature of a copper interconnect with no aluminum heat transient layer. FIG. 5A shows the relatively rapid heat dissipation away from the copper interconnect achieved in the presence of aluminum heat transient layers for a given electrical current. FIG. 5B shows the increase in copper interconnect temperature due to power input. Curve A represents the temperature of a copper interconnect with aluminum heat transfer layers adjacent upper and lower surfaces of the copper interconnect (see FIG.  1 E). Curve B represents the temperature of a copper interconnect with no aluminum heat transient layer. Use of heat transient layers adjacent to upper and lower surfaces of a copper interconnect allow for increased current flow in the copper interconnect for a given temperature compared to a copper interconnect without a heat transient layer. 
     While the invention has been described above with reference to the preferred embodiments thereof, it is to be understood that the spirit and scope of the invention is not limited thereby. Rather, various modifications may be made to the invention as described above without departing from the overall scope of the invention as described above and as set forth in the several claims appended hereto. For example, heat transient layer can be connected to other non-electrical thermal conducting structures such as, for example, a heat sink in order to further enhance heat dissipation from an interconnect. Also, an interconnect can be formed of other metals such as a refractory metal, for example, tungsten, and the corresponding heat transient layer formed of another metal such as copper, where copper has a lower melting point temperature than refractory metals so that copper would undergo a solid to liquid phase transformation at a temperature below the melting point temperature of the refractory metal. The invention can also take advantage of other latent heat of transformation phase changes such as, for example, a latent heat of transformation of heat transient layer to a gaseous phase.