On the Microstructure of Off-Eutectic Au-Ge Joints: A High-Temperature Joint
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MANY industries and applications require high-temperature compatible electronic systems, including down-hole instrumentation in oil, gas, and geothermal wells, power electronics in automotive (electric vehicle (EV) and hybrid electric vehicle (HEV)), and thermoelectric energy harvesting. In recent years many high-temperature compatible components have been developed and demonstrated. Devices made from widebandgap materials have been demonstrated to operate at high temperatures.[1] In particular, silicon carbide (SiC) and gallium nitride (GaN) have demonstrated their potential in commercial devices.[2] They offer high-temperature stability with low losses due to fast switching speeds. This reduces the need for costly and bulky cooling systems.[1] To enable the integration of high-temperature compatible components into electronic systems require high-temperature stable joints. Joints formed with a process temperature that is lower
ANDREAS LARSSON is with the Department of Applied Physics, TECHNI AS, Borre, Norway and also with the Department of Materials and Micro-Integration, University of South-Eastern Norway (USN), Borre, Norway. Contact e-mail: [email protected]. KNUT E. AAMUNDTVEIT is with the Department of Materials and Micro-Integration, University of South-Eastern Norway (USN). Manuscript submitted June 23, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
than the final operation temperature of the joints have been evaluated for high-temperature applications. Such joining technologies include transient liquid phase (TLP) bonding,[3–7] solid–liquid interdiffusion (SLID) bonding,[8–10] and sintering.[11–13] TLP, SLID and sintering are often limited by long process times or require a high bond line pressure during fabrication. Another joining technology that can be used with high-temperature compatible devices is soldering, which require relatively high process temperatures. Unlike TLP, SLID, and sintering, soldering is typically quick with limited requirement for applied bond line pressure. Eutectic gold (Au) based joints are a feasible option for high-temperature applications up to around 300 C,[14–16] e.g., gold-silicon (Au-Si, eutectic temperature at 363 C[17]), gold–germanium (Au-Ge, eutectic temperature at 361 C[18]), and gold-indium (Au-In, eutectic temperature at 450 C to 496 C[19]). In a recent study, we have demonstrated that Au-rich off-eutectic Au-Ge joints have significant shear strength capacity, (39 ± 9) MPa at 410 C,[20] i.e., 50 C above the eutectic temperature at 361 C. In other studies, we have demonstrated an increased effective melting point, > 600 C, of similar Au-rich Au-Ge joints.[21–23] These joints seem to have the same benefits as regular eutectic joints, i.e., fast and low bond line pressure while sharing the attractive feature of TLP, SLID and sintering, offering a lower process temperature than the final operation temperature. For off-eutectic joints to be used
near, or above the eutectic melting point, it is crucial that the kinetics of microstructure is stable or pr
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