Wafer-Level Au—Sn Solid-Liquid Interdiffusion Bonding for Microwave Applications

Abstract

Future compact integration of microwave modules will require intermetallic bonding with pitch connections lower than 10 μm. Solid-liquid interdiffusion (SLID) bonding is a potential bonding method for close heterogeneous integration because of its bonding temperature in the range of solder bonding, its ability to withstand higher temperaturs than bonding and its moderate compatibility with the joining of rough surfaces. This paper presents the results of an investigation of the Au—Sn system for integration of A3B5 chips on wafer level using SLID bonding. A sequence of five layers of Au and Sn with weight ratios of about 3:2 was deposited on a sapphire substrate. The structure and phases of the obtained system were investigated before and after annealing. The initial system has five distinctive alternating layers with an overall thickness of around 4.5 μm. After heat treatment at 320 °C with a holding time of 10 minutes, the Au—Sn system transforms into one layer that consists of only one phase ζ (Au0.853Sn0.147). The homogenization process is completed in less than 10 minutes. The ζ phase, of which the system is composed after annealing, has a melting point of 522 °C, which ensures the high temperature stability of the bonding layer. Bonding using the formed system was conducted. Morphology, elemental composition, and the initial mechanical properties of bonded structures have been studied. There are two types of defects in the joint's cross section: voids and the presence of local interfaces between two surfaces (an unbonded-like line). EDX analysis of the cross section of the joint between two crystals shows that the average ratio of Sn to Au over the cross section of the joint is 10.7 %. The complex of studies conducted suggests that the bonding area mostly includes the ζ phase, which will provide the best mechanical strength characteristics among the phases with a high Au concentration. The average bond shear strength is 32 MPa. Initial robustness results of the bonding are promising for space applications.