A Review of Eutectic Au-Ge Solder Joints
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INTRODUCTION
RESEARCH on semiconductor materials and devices in the 1940s was primarily made on the semiconducting materials germanium (Ge) and silicon (Si). Gold was deposited onto germanium to solve a contact issue with an early prototype amplifier device (point contact transistor) by a research team led by William Bradford Shockley Jr. and Stanley Morgan at Bell Labs in 1947.[1] This device led the research team to the discovery of the transistor effect. William Bradford Shockley, John Bardeen, and Walter Houser Brattain were awarded the Nobel prize in physics in 1956 for their work on the transistor and the discovery of the transistor effect.[2] Thus, the Au-Ge contact was a central part of the birth of modern electronics. The first types of semiconducting transistors that became commercially available in the 1950s were also made from germanium. The more stable silicon replaced germanium in the 1960s, which changed the role of Au-Ge from providing a contact surface for interconnects (wire bonds) to a die-attach material joining dies to substrates. Eutectic Au-Ge bonding as a die-attach method was patented already in the early 1960s.[3,4] In the 1970s and 1980s, research on Au-Ge bonding focused more
ANDREAS LARSSON is with the Department of Applied Physics, TECHNI AS, 3184, Borre, Norway and also with the Department of Materials and Micro-integration, University of South-Eastern Norway (USN), 3184, Borre, Norway. Contact e-mails: [email protected], [email protected], and [email protected] TORLEIF A. TOLLEFSEN is with the TEGma AS, 3015, Drammen, Norway. OLE MARTIN LØVVIK is with the Department of Sustainable Energy Technology, SINTEF Industry, 0373, Oslo, Norway. KNUT E. AASMUNDTVEIT is with the Department of Materials and Micro-integration, University of South-Eastern Norway (USN). Manuscript submitted February 8, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
towards ohmic contacts for gallium arsenide devices.[5–7] In the 1990s and especially post 2000, the high-temperature compatibility of Au-Ge joints has been thoroughly explored.[8–22] Environmental demands such as the RoHS directive have also lead to investigations on the Au-Ge system as a replacement for lead-based solders.[23,24] The high material cost has most likely limited its applicability into volume mainstream, low-end electronics devices. The great majority of electronic devices and uses comprising Au-Ge joining technology has been explored with the utilization of a eutectic (or near eutectic) composition of the Au-Ge system. These alloys are today commonly used as a high-reliability, high-temperature compatible die-attach technology. The main motivation for this seems to be its high melting temperature at 361 °C[25] and the stable properties, corrosion, and thermal fatigue resistance, combined with excellent joint strength of the final joint. Typically, high-temperature applications with Au-Ge have aimed for use up to around 300 °C.[8,9,11,12,17,19,20,22,26–33] But, joints have also been explored for cryogenic temperatures down to around 170
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