High Temperature Electronics Packaging Processes and Materials Development
Type of Degreedissertation
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Silicon carbide device technology is being developed for power electronics applications for use at high temperatures. In recent studies, SiC based electronics and sensors have also been demonstrated for extended operation at 500 and 600oC, but the level of integration is at a very low level. Silicon on insulator (SOI) technology provides the ability to realize complex electronics functions. SOI rated for 250oC is commercially available and 300oC operation has been demonstrated. To build functional systems operating at high temperatures, packaging technology must be developed to interconnect the SOI and SiC based devices. Key elements of a high temperature packaging technology include the interconnection substrate, die attach and wire bonding. New developments in each of these areas for high temperature operation are discussed in this work. A system-in-package (SiP) approach has been developed for SOI based devices, using thick film technology on Si3N4 ceramic substrates. Eutectic Au-Ge die attach was used with a Ti/Ti:W/Au backside die metallization for 300oC operation. Results of die attach reliability based on shear strength are discussed after thermal storage and thermal cycling tests. Wire bonding is a cost-effective and flexible interconnect technology for HTE packaging. SOI die typically have Al wire bond pads that are not compatible with Au thermosonic wire bonding for high temperature applications. A plating process that provides a barrier layer (electroless Ni) and a wire bondable finish (electroless Pd plus immersion Au) over the Al wire bond pads has been examined. Results of the investigation of Au wire bonding on substrate metallization and die surface metallization are also presented in this dissertation For the SiC based devices applications at 500C, this project examines off-eutectic Au-Sn as the die attach alloy in the Liquid Phase Transient (LPT) die attach process. Two different approaches that use thick foil Sn-Au-Sn preform and limited volume Au-Sn eutectic preform, have been investigated using. The substrate used in this project was PtAu thick film metallization on AlN substrates. A pure Au thick film layer was printed over the PtAu thick film layer. The SiC backside metallizations evaluated were Ti/TaSi/Pt/Au and Cr/NiCr/Au. Die shear tests were performed after aging at 500oC and after thermal cycling. The shear test results and failure surface analysis are discussed Hermetic sealing minimizes the intrusion of contamination and allow the circuit mounted inside the package to survive for a longer period of time. Two packaging approaches have been developed in this project: One path was an integrated package that uses a metalized AlN substrate as the package base. A machined AlN cavity lid was sealed to the AlN substrate to create a hermetic cavity for the die. For the second approach, a commercial off the shelf (COTS) Al2O3 package has been evaluated. In this case the metalized AlN substrate with the die attached and wire bonded could be hermetically sealed inside the ceramic package.