Wide-bandgap (WBG) semiconductor materials are expected to be widely utilized in next-generation power devices as modules to replace Si-based devices owing to their higher voltage-blocking capability, higher temperature operation, and switching frequencies, as well as their reduced power loss [1][2]. In WBG devices, die-attached materials that bond WBG dies to substrates are crucial for maintaining high performances in environments reaching 250 °C [3]. Conventional die bonding materials, such as Pb-5Sn solder [4], Au-Sn solder [5], and Bi-based solder [6], are not suitable for use in next-generation packaging because cracks can form at the intermetallic solder interface in high-temperature environments . One of the promising alternative approaches is Ag sinter joining technology, which allows for a sintering temperature below 300 °C at low pressure. As the melting point of Ag is higher than 900 °C, Ag sinter joining can be used in high-temperature environments exceeding 250°C .
The quality of die-attach by Ag sinter joining depends on the bond-line cohesion from Ag sintering and interfacial adhesion from atomic inter-diffusion. Therefore, the bonding quality depends on the surface finish such as its chemistry and microstructure, on both device and substrate . Usually, electro-plated Ag, electroless-plated Ni(P)/Ag, electro- and electroless-plated Ni(P)/Au is used as surface finish layer for sinter Ag joining. On the other hand, the electro- and electroless-plated Ni(P) technology was well developed and used widely, which can achieve a sufficient resin adhesion resulting from the chemical interaction between the base metal Ni and the resins. However, this additional metallization layer increase costs due to the additional raw materials and processing steps. Ag paste-based, die-attachments on a bare the direct bonded copper (DBC) substrate or direct bonded aluminum (DBA) seems an ideal option due to reduced cost and a simplified process .
Here we report a robust joint by Ag sinter joining technology for different metal interface (Au, Ag, Ni, Cu, Al) in wide band gap power modules. Micron-scale Ag flakes (AgC239, Fukuda Metal Foil, and Powder Co. Ltd, Japan) were used as the Ag fillers. The thermal behaviors and morphology of the Ag paste sintered at different temperatures were firstly studied to understand the sintering behavior of the prepared Ag paste. Sinter Ag joint structure for different metal interface was implemented by a sintering process under temperatures of 250 °C in air without pressure. In addition, each sinter Ag joint structure was investigated to obtain a comprehensive understanding for different metal interface bonding. A possible mechanism was proposed based on the SEM and TEM observation of the cross-sectional part of each sinter Ag joint structure.
Fig. 1 shows the sinter Ag joint structure for Ag metallization metal interface, and the die shear strength of sinter Ag joint structure for different metal interface. Sintered Ag shows a microporous network structure and bonding well with the Ag metallization layer at the both SiC chip and substrate side. The die shear strength of each sinter Ag joint structure is larger than 25 MPa which comparable with the value of traditional Sn–Pb solders (19~24MPa. This study helps to understand the Ag sinter joining for different metal metallization interface, enlarger the Ag sinter joining for wide band-gap power modules in high temperature applications.

ACKNOWLEDGMENTS

This work was supported by the JST Advanced Low Carbon Technology Research and Development Program (ALCA) project “Development of a high frequency GaN power module package technology” (Grant No. JPMJAL1610). The author is thankful to the Network Joint Research Centre for Materials and Devices and Dynamic Alliance for Open Innovation Bridging Human, Environment and Material.

References
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