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Silicon-on-insulator (SOI) integrated circuits have been demonstrated for use at temperatures up to 300 °C. However, to build functional electronics, multiple devices must be interconnected to provide the desired functionality. A system-in-package approach has been developed using thick-film technology on Si3N4 ceramic substrates. Si3N4 has a near coefficient-of-thermal-expansion match to Si and a higher flexural modulus than Al2O3, which is commonly used for thick-film applications. The conductor metallization is Au. For 300 °C operation, eutectic Au-Ge die attach was used with a Ti/Ti:W/Au backside die metallization. After 3000 h at 325 °C, the mean die shear strength decreased from 3.96 to 3.33 kg/mm2, a decrease of only 16%. Formation of Au-Si-Ge ternary eutectic (melting point 326 °C) was observed and limits the use of Au-Ge die attach to 300 °C. SOI dies typically have Al wire bond pads that are not compatible with Au thermosonic wire bonding for high-temperature applications. Two plating processes have been examined: electroless Ni/immersion Au/electroless Au and electroless Ni/electroless Pd/immersion Au. The plating processes provide a barrier layer (Ni) and a wire-bondable finish (Au or Pd-Au) over the Al wire bond pads. After 10 000 h at 300 °C, the wire pull force for the Ni/Au samples decreased by ~30% due to annealing of the Au wire, while the ball shear force increased by ~35%. The daisy-chain electrical resistance remained relatively constant. For the Ni-Pd-Au samples, after 2000 h at 320 °C, the ball shear force remained constant or increased slightly, the wire pull force decreased by ~25% due to annealing of the Au wire, and the daisy-chain resistance remained relatively constant. After 3000 h, however, cratering of the Si wire bond pad was observed corresponding to some first bond pad lifts and - - increased daisy-chain resistance. Optimization of the wire-bonding parameters for bonding to the harder Ni/Pd/Au bond pad is required to eliminate cratering.
Date of Publication: July 2011