(1) Field of the Invention
The invention relates to a method to reduce Joule heating in an integrated circuit device, and, more particularly, to a method to detect Joule heating problems in signal routes and to eliminate these problems through re-routing or through a heat sink.
(2) Description of the Prior Art
Integrated circuit devices typically contain a large number of signal wires, or lines. These signal lines are formed in a conductive layer such as a metal film. Individual signal lines are separated using dielectric films. The combination of signal lines and dielectric films creates a set of interconnect levels for an integrated circuit device. It is common for an integrated circuit device to use eight or more levels of interconnect formed in a stack and using via openings to allow connection between adjacent levels.
Referring now to FIG. 1, a simplified cross section of an integrated circuit device is shown. The cross section shows a substrate 10, a metal line 18, a first dielectric layer 14 between the metal line 18 and the substrate 10, and a second dielectric layer 22 overlying the metal line 18. Voltages and currents are transmitted by the signal line 18. In this example, the metal line 18 carries a current, ILINE. Typically, the metal line 18 comprises a low resistivity material such as aluminum or copper to minimize the resistance of the line, RLINE. In the DC case, the heat lost by the metal line 18 is given by:HEAT=I2LINE×RLINE.
However, many signals in IC devices are AC in nature. In an AC signal, the line current ILINE will flow in two directions. Further, these AC signals may comprise very high frequencies, such as in the case of clock signals. These high frequency signals transmitted in metal lines 18 can generate a significant amount of heat. This heat will be transferred through the dielectric layers 22 and 14. Some of the heat will be transferred through the second dielectric layer 22 to any overlying films or packaging. Some of the heat will be transferred through the first dielectric layer 14 to the underlying substrate. If the heat generated in the metal lines 18 exceeds the heat transferred away from the metal lines 18, then the metal lines will increase in temperature. This phenomenon is called Joule heating.
Referring now to FIG. 2, the Joule heating concept is further illustrated. It is known that the ability of a conductor to carry current is proportional to the cross sectional area of the conductor. Therefore, the line current (A) may be divided by the line cross section (cm2) to derive the current density J (A/cm2) for the signal line. An exemplary current density J(t) 70 is shown for an AC signal in an integrated circuit device. In this case, the signal is periodic with a frequency of 1/τ Hz. It is known that the Joule heating generated by a conductive line is proportional to the square of the root mean square (RMS) of the current density, or JRMS. For a periodic signal having J(t), JRMS is given by:
      J    RMS    =                    [                              (                                          ∫                0                t                            ⁢                                                                    J                    2                                    ⁡                                      (                    t                    )                                                  ⁢                                                                  ⁢                                  ⅆ                  t                                                      )                    /          τ                ]                    1        /        2              .  It is important to note that Joule heating occurs regardless of the direction of the current. This is why the RMS value of current density must be determined.
Referring again to FIG. 1, the current density in the signal line 18 generates heat. The extent to which the signal line increases in temperature due to this heating further depends on the heat transfer paths. Typically, the heat is not efficiently transferred above the metal lines 18 because the overlying materials are not primarily good thermal conductors. Therefore, most of the heat will be transferred through the first dielectric layer 14 to the underlying substrate 10. The relationship between the current density, the dielectric layer 14, and the Joule heating is given by:JRMS2αΔT kIMD/tIMD,where ΔT is the temperature rise with respect to the substrate 10, or Joule heating, in the metal line 18, kIMD is the thermal conductivity of the dielectric layer 14, and tIMD is the thickness of the dielectric layer 14.
The Joule heating in a signal line is important because of reliability concerns. As stated above, the signal line will increase in temperature ΔT if the heat generated in the signal line exceeds the heat transferred out of the signal line. If the signal line temperature becomes excessive, the metal line or the dielectric layers can be stressed to the point of failure. Typically, a maximum allowed temperature difference, ΔTMAX, is specified for the integrated circuit technology. For example, the ΔTMAX for a multiple metal level device may be about 15 degrees C. as an industry convention.
Another phenomenon that causes reliability concern for conductive lines is electromigration. Electromigration is a diffusion of metal material in a signal line caused by excessive current density. Electromigration causes the metal atoms to literally move in the direction of electron flow if an excessive current density situation persists. Electromigration can cause a metal line to become open circuits. Unlike Joule heating, however, electromigration is a directional phenomenon. That is, current flow in a first direction causes electromigration in the same direction. Meanwhile, current flow in an opposite direction causes electromigration in the opposite direction. Referring again to FIG. 2, as a result, positive current density J(t) 71 and negative current density J(t) 72 causes a significant cancellation of electromigration in the conductive line. However, as discussed above, Joule heating will occur regardless of the current direction and is not canceled by the AC current action.
In a typical IC process, the maximum allowed current density, JMAX, is determined by the electromigration effect. Layout design rules are established to prevent electromigration using this JMAX limit. However, to insure reliability, it is important that the Joule heating phenomenon also be considered in the layout of the device.
Several prior art inventions relate to Joule heating in metal lines in an integrated circuit device. U.S. Pat. No. 5,811,352 to Numata et al describes a method to reduce Joule heating in metal lines. Dummy metal lines are added to an IC layout. The dummy lines do not conduct current and are not connected to the signal path. However, the dummy lines are formed in close proximity to the signal metal lines to improve thermal dissipation. U.S. Pat. No. 5,510,293 to Numata discloses a method to reduce Joule heating in metal lines. A thermo-conductive dielectric layer, such as AlN, is deposited overlying metal lines while a low k-value dielectric material is formed between the metal lines. U.S. Pat. No. 5,858,869 to Chen et al teaches a method to form an intermetal dielectric layer with improved Joule heating performance. A thin, anisotropic plasma oxide is formed overlying metal lines. A low k-value dielectric material is then conformally deposited and polished down. A fluorinated silicate glass (FSG) layer is then deposited overlying the low-k material and the metal lines. U.S. Pat. No. 6,265,308 B1 to Bronner et al describes a method to form damascene lines and vias.