Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyVIII. THERMAL SIMULATIONS AND RESULTSthe chip by the surface area of all the combined HBT C-BAfter having achieved a full-chip 3D mesh that was junctions, the calculated average power density wasfeasible for a thermal solution on a single PC, we applied 1.37×10 8 W/m 2 . This value was then used for subsequentappropriate heat sources, as described earlier in Section VI. full-chip thermal simulation of IC2.Using the above described mesh with 196K cells, achieving Similarly as for IC1, we used the overall 3D modela full thermal solution took about 2 minutes of CPU time on generation method (directly from imported GDSII layout ofa 2.4GHz Pentium machine. The peak memory (RAM) IC2) as well as the mesh coarsening approach described inrequirements was 259 MB for this case. Sample results are previous section, ultimately utilizing the “k eff -Bond”shown in Figure 10. With the reference temperature T = 300 structures. This allowed us to build a new full-chip 3DK applied at the bottom boundary of bump bonds (simulated model and mesh quickly and in automated manner. The bintreemesh for entire IC2 chip contained 389K cells – the sizeheat sink), the maximum computed temperature was T =303.2 K, that is, ∆T = 3.2°C above the heat sink. The that is easy to solve on a PC. At the same time, the IC2temperature gradient across the entire chip is very small, but model and mesh were also able to resolve a singlelocal hot spots appear around the active transistor junctions transistor’s C-B junction (0.5 μm voxel resolution was used).which are heat sources in this case - see Figure 10. Close-up Using this 3D mesh, achieving a full thermal solution forview to one such area is shown in Figure 11.IC2 took about 4 minutes on a 2.4GHz Pentium machine.The peak memory (RAM) requirement was 506 MB for thiscase. Sample thermal results are shown in Figure 12.ΔT max= 3.2 deg K or C(over heat sink)ΔT max= 7.7 deg K or C (over heat sink)IC1Bump Bonds(keff-Bonds)Bump Bonds(keff-Bonds)Mesh: 196K cellsPeak RAM: 259 MBCPU time: 2 min.(2.4GHz Pentium)Heat Sink(at the bottom of Bonds)= Isothermal BC (T = 300 K)* ILD = Inter Layer Dielectric (SiO2) not visible, for better viewingFigure 10. Full chip 3D thermal results for the IC1.Heat Sink– at the bottomof Bump Bonds= Isothermal BC(T = 300 K)Mesh: 389K cellsPeak RAM: 506 MBCPU time: 4 min.(2.4GHz Pentium)IC2* ILD = Inter Layer Dielectric (SiO2) not visible, for better viewingFigure 12. Full chip 3D thermal results for the IC2.NPN HBT Devices(C-B Junctions)Figure 11. Full-chip 3D thermal results for IC1 – close-up view oftemperature distribution near active HBT devices.IC2 Test ChipAs another verification and demonstration, we also usedour new modeling approach, procedures and tools on adifferent RF chip design, called IC2. In IC2, there are 248active HBT devices, which are sparsely located throughoutthe chip. After having divided the total dissipated power ofIX. EXPERIMENTAL VERIFICATIONVerification of the modeling results has been conductedusing the Infrascope III from Quantum Focus Instruments(QFI) located at Wright-Patterson Air Force Base, Ohio. TheInfraScope detects and measures infrared light emanatingdirectly from the sample [8]. It uses an Indium Antimonidedetector cooled with liquid nitrogen and a max spatialresolution of 2.8 um. QFI uses patented algorithms andproprietary software to measure and compensate for theemissivity of the material being tested in order to generate anaccurate, calibrated thermal image. This eliminates the needof the circuit surface to be coated with expensive anddestructive inks to accurately calculate the surface emissivityand they have also been shown to induce heat spreadingunderestimating hot spots.The InfraScope operates in the mid-wave infrared(MWIR) band, which has a spectral response of 2 to 5.4microns. Since undoped silicon is transmissive at thiswavelength, it is quite easy to see through the backside ofthe wafer for backside inspection of this flip-chippedpackage.©EDA Publishing/THERMINIC 2008 68ISBN: 978-2-35500-008-9